# Deciphering the improvement in (H2-)C3H6-SCR performance of Ag/Al2O3 catalysts prepared from warm-water-treated alumina: A NMR-assisted identification of the Ag anchoring sites of the alumina support

Yannick Millot, Guylène Costentin, Clémence Rodigue, Thomas Onfroy, Cyril Thomas

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Yannick Millot, Guylène Costentin, Clémence Rodigue, Thomas Onfroy, Cyril Thomas. Deciphering the improvement in (H2-)C3H6-SCR performance of Ag/Al2O3 catalysts prepared from warm-water-treated alumina: A NMR-assisted identification of the Ag anchoring sites of the alumina support. Applied Catalysis B: Environmental, 2024, 351, pp.123975. 10.1016/j.apcatb.2024.123975. hal-04529323

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# Deciphering the improvement in (H₂-)C₃H₆-SCR performance of Ag/Al₂O₃ catalysts prepared from warm-water-treated alumina: A NMR-assisted identification of the Ag anchoring sites of the alumina support

Yannick Millot<sup>a, *, 1</sup>, Guylène Costentin<sup>a, 2</sup>, Clémence Rodrigue<sup>a, b</sup>, Thomas Onfroy<sup>a, 3</sup>, Cyril Thomas<sup>a, *, 4</sup>

## ARTICLE INFO

### Keywords:

Catalysis

Ag/Al₂O₃

NOₓ-TPD

FTIR

Selective Catalytic Reduction by Propene

## ABSTRACT

Lean exhausts aftertreatment has been the subject of numerous investigations due to ever stringent regulations on pollutants emissions such as that of NOₓ. Among aftertreatment technologies, the selective catalytic reduction of NOₓ by the hydrocarbons (HC-SCR) has been studied intensively, in particular by using Ag/Al₂O₃ materials. The present work highlights that pretreatment of pristine Al₂O₃ in warm water prior to Ag deposition allows to prepare catalysts of significantly higher (H₂-)C₃H₆-SCR performance compared to conventional Ag/Al₂O₃ catalysts. In addition, NOₓ-TPD-C₃H₆-SCR structure-activity correlations indicate that optimum composition of the Ag/Al₂O₃ catalysts can be improved by about 50% in terms of Ag content for the first time. Unprecedented in-depth NMR investigations allow for the identification of the alumina sites of importance in the anchoring of Ag. These sites are µ<fnref n="1" />-OH groups located near the (100)/(110)₁ edge and then near the (110)₃/((100) edge of Al₂O₃ crystallites and bonded to octahedrally coordinated Al species.

## 1. Introduction

Air pollution has become a growing health concern for human being [1] since the industrial revolution in the nineteen's century. Air quality is daily monitored worldwide [2] and the European Union acted on a reduction of certain atmospheric pollutants such as Sulfur dioxide (SO₂), Nitrogen oxides (NO₂ and NO), Non-Methane Volatil Organic Compounds (NMCOV), Ammonia (NH₃) and Particle Matter (PM) through the National Emission Ceilings Directive (NECD) [3]. While SO₂ emissions have been decreased by more than 80% from 1990 to 2011, mainly by using low sulfur content fuels [4], those of NOₓ have decreased by only about 60% on a much longer period of time (from 1990 to 2017–2021) [5,6]. The reasons for this lower decrease in these latter atmospheric pollutants lie both on the fact that NOₓ species are essentially formed by recombination of N₂ and O₂ in the flames of the

combustion chambers, which can be hardly avoided unless by improving the combustion processes to a certain limit, and that transport is a significant contributor to NOₓ emissions [7]. The mitigation of the NOₓ emissions from light and heavy duty vehicles (LDVs and HDVs, respectively) has been achieved by the implementation of catalytic converters in Europe in 1977 for LDVs and 1988 for HDVs [7]. Whereas Three-Way Catalysts (TWC) have been shown to be very efficient in the concomitant remediation of CO, unburnt hydrocarbons and NOₓ from stoichiometric exhausts of gasoline engines [8], such catalytic devices have been found to be unsuccessful in the reduction of NOₓ from lean exhausts of gasoline and diesel engines [9]. As stated by Burch, promoting a reduction reaction, such as that of NOₓ reduction to N₂, in a strongly oxidizing environment, namely lean mixtures, appears to be extremely challenging from a chemistry viewpoint [9]. To overcome such a difficulty, the use of catalysts using ammonia as a reductant (NH₃-SCR) [10,11]

<footnote><sup>a</sup> Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, Paris 75005, France</footnote>

<footnote><sup>b</sup> CESI, 93 boulevard de la Seine, Nanterre 92000, France</footnote>

<footnote>* Correspondence to: Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, UMR CNRS 7197, 4 Place Jussieu, Tour 43-53, 3ème étage, Case 178, Paris 75252, France.</footnote>

<footnote>E-mail addresses: yannick.millot@sorbonne-universite.fr (Y. Millot), cyril.thomas@sorbonne-universite.fr (C. Thomas).</footnote>

<footnote n="1">¹ orcid.org/0009-0005-1044-6369</footnote>

<footnote n="2">² orcid.org/0000-0003-1559-6890</footnote>

<footnote n="3">³ orcid.org/0009-0006-1621-4566</footnote>

<footnote n="4">⁴ orcid.org/0000-0003-4224-6095</footnote>
and Lean NOₓ Trap (LNT) [12] have been developed to catalyze the reduction of NOₓ to N₂ in lean exhausts. Yet the ammonia slip in NH₃-SCR, an atmospheric regulated pollutant [3], and CO₂ penalties, due to the short incursions in rich mixtures needed to proceed to the reduction of the NOₓ adsorbed species [12], are inherent drawbacks of these aftertreatment technologies [13] from air pollution and global warming viewpoints.

Many studies have been devoted to the selective catalytic reduction of NOₓ by Hydrocarbons (HC-SCR) [9,14,15], as an elegant potential alternative of the NH₃-SCR and LNT technologies [9], since the pioneering remarkable discoveries of the Ag/Al₂O₃ catalyst for C₃H₆-SCR by Miyadera [16] and for low-temperature promoting effect H₂ in H₂-C₁-₄-SCR by Satokawa and co-workers [17,18]. Not only light hydrocarbons have been reported to be efficient reductants of NOₓ over Ag/Al₂O₃, either in the presence or in the absence of H₂, but also long chain alkanes [9,14,19–22] and alcohols [9,14,23–26], or a mixture of both [27]. Overall, the studies performed to date have essentially concluded to the elevated dispersion of Ag on Al₂O₃ and that oxidized Ag species, such as electron-deficient clusters (Ag<sup>δ+</sup><sub>n</sub>) [28,29], would be the active catalytic species [9,15,30]. In HC-SCR, the formation of N₂ would result from the interaction of NOₓ, either in an adsorbed state [30] or not, with the hydrocarbons to form various intermediates [15, 31], for which isocyanates have been claimed to be among the most important intermediates leading ultimately to nitrogen [32–35]. It has also been reported that the best catalytic performance in HC-SCR was obtained with an optimum loading of 2 wt% Ag on alumina [16,30, 36–39] and that such an optimum Ag loading would also stand for H₂-promoted HC-SCR [21,28]. Based upon a disruptive approach, namely the challenging characterization of the alumina support of the Ag/Al₂O₃ catalysts by adsorption-desorption of NOₓ (NOₓ-TPD method) developed initially for tungstated-zirconia materials [40,41], NOₓ-TPD-(H₂-)C₃H₆-SCR structure-activity relationships led some of us to conclude that (i) the optimum Ag loading of 2 wt% could be attributed to the maximum content of Ag for which this metal remained in a highly-dispersed state on Al₂O₃ [42] and (ii) that this optimum loading did not stand for H₂-C₃H₆-SCR as the reduction of NOₓ to N₂ was found to be favored as the Ag loading decreased [43]. Earlier literature reports attributed the H₂ promoting effect to (i) changes in the speciation of the Ag species [21,28,34,44], (ii) enhancement of hydrocarbon activation to surface oxygenated compounds [22,28,34,45], (iii) the formation of reactive oxygen species such as peroxides [28,34,46], (iv) changes in the surface concentration of NOₓ adsorbed species [34,44,45,47,48], and/or (v) the modification of chemical reactions [9,49,50]. In addition, Burch and co-workers have claimed that the formation of organo-NOₓ intermediates, resulting from the interaction of NOₓ and activated hydrocarbons species, would be rate determining for both HC-SCR [15] and H₂-HC-SCR [51]. In the case of HC-SCR, Thomas and co-workers reported that the temperature at which NOₓ adsorbed species interacted with hydrocarbons may be valubably monitored by exposing preadsorbed NOₓ species to hydrocarbon-containing feeds under temperature-transient conditions [52,53]. By using a similar approach with C₃H₆-containing feeds of increasing complexity, Thomas suggested that H₂ could also promote the formation-decomposition-reaction of organo-NOₓ species at the lowest temperatures based on the temperature-transient reactivity of preadsorbed nitrites [54].

Despite the tremendous number of investigations performed in this particular field to date, the limited activity of the Ag/Al₂O₃ catalysts over the temperature of interest of the catalytic converters prevented their implementation [55], as exhausts temperatures as low as 150 °C are commonly met in these devices [56]. Hence many works have been devoted to the modification of the silver alumina system by promoters. For instance, the addition of limited amounts of chlorides has been claimed recently to promote (H₂-)C₃H₆-SCR mainly at high Ag loadings because of the stabilization of electron-deficient Ag clusters (Ag<sup>δ+</sup><sub>n</sub>) in greater proportion compared to that in the Cl-free catalyst [57]. Likewise, the introduction of tungstates on Al₂O₃ has been reported to

promote C₂H₅OH-SCR [58]. Such a promotional effect of tungstates for the Ag/Al₂O₃ system contrasts with the detrimental effect of tungstates that one of us observed on n-C₁₀-SCR [59], however, which led us to confirm the importance of the close proximity between the silver species and the alumina carrier. The importance of Al₂O₃ as a supporting oxide of Ag for H₂-HC-SCR, as deduced from the lack of reactivity of Ag/SiO₂, Ag/TiO₂ and Ag/ZrO₂ for H₂-C₃H₈-SCR [18] and also indicated by the positive reaction order with respect NO up to a silver loading of about 2 wt% on Al₂O₃ in H₂-C₃H₆-SCR [43] -, therefore makes the promotion of Ag/Al₂O₃ extremely risky as the introduced promoters may also compete with the alumina sites involved in H₂-HC-SCR. Several studies have aimed at investigating the influence of the nature of Al₂O₃ via the use of its precursors [60,61] or its morphology [62–64]. The claim of Yang et al. regarding the higher catalytic C₃H₈-SCR performance of a silver alumina sample prepared from Al₂O₃ with a flake morphology compared to that prepared with a sphere morphology must be taken with caution, however, as the synthesis of the latter support included sulfates, whereas such anions were not used in the synthesis of Al₂O₃--flake [64]. Not only the presence of sulphates on Al₂O₃ has been shown to be detrimental to the NOₓ adsorption capacity [64,65] but also to C₃H₈-SCR performance [65]. Several studies have also reported on the use of newly developed synthesis methods to improve the efficiency of the silver alumina catalysts in (H₂-)HC-SCR. Petitto and Delahaye reported that the heating of a mechanical mixture made of Ag₂O and Al₂O₃ in a wet atmosphere above 600 °C led to more active catalysts in n-C₁₀-SCR compared to those prepared conventionally by impregnation of alumina by a silver nitrate aqueous solution [66]. This peculiarity may be linked to the agglomeration/oxidative dispersion of Ag nanoparticles in the presence of a wet oxidizing atmosphere at high temperature [67] and the importance of the surface hydroxyls of γ-Al₂O₃ for such processes [68]. The use of silver alumina catalysts prepared by ball milling have also showed promising n-C₈-SCR at temperature much lower than those usually reported in the field over samples prepared conventionally [69], whereas such a beneficial effect of ball milling could not be observed in C₂H₅OH-SCR [70]. Yet it must be highlighted that the speed with which the ball-milled samples were prepared has been reported to be critical in making efficient Ag/Al₂O₃ catalysts, as the use of high-speed ball milling appeared to be detrimental to SCR reactions [71]. The greater effectiveness of ball-milled-prepared silver alumina catalysts compared to conventionally prepared samples has also been illustrated in H₂-n-C₈-SCR and assigned to modifications in the affinity of water and hydrocarbons with the Al₂O₃ surface [51] and the concentration/rate of formation of isocyanate intermediates [51,71]. Hence these latter studies have prompted on the importance of the Al₂O₃ supporting oxide for the SCR reactions. Recently, it has been elegantly shown that Ag dispersion capacity, i.e. the amount of Ag anchored onto Al₂O₃ in a highly dispersed state, was strongly correlated to the surface OH content of such a supporting oxide [68]. In an earlier study, Thomas and co-workers have reported that H₂-C₃H₆-SCR could be increased drastically over an alumina support that was pretreated in warm water prior to deposition of Ag by conventional impregnation [72]. Such a peculiarity was inferred to the preferential anchoring of Ag on OH groups that were not present in the pristine Al₂O₃ support, but created during its treatment in warm water, and hence the preservation of a higher NOₓ coverage [43] in the warm-water-treated-Al₂O₃-supported Ag sample compared to the conventionally prepared Ag/Al₂O₃ catalyst [72]. To our knowledge, enhanced hydroxylation of Al₂O₃ to favor metal anchoring has been reported scarcely in the case of supported Pt catalysts [73,74] only.

Whereas NMR [61,63,64,70,75,76] and FTIR [63,75–77] spectroscopies are known to be powerful techniques to monitor protons from the OH groups and/or the Al species of Al₂O₃, very few investigations have been reported on their use to monitor the changes brought about by the introduction of Ag on Al₂O₃. 1D ¹H [63,70] and ²⁷Al [61,63,64,75,76] NMR have been essentially used to characterize Ag/γ-Al₂O₃ samples, whereas in-depth characterization of γ-alumina has been provided
rather recently with 2D $^1$H-$^1$H DQ [78–81] and $^1$H-$^{27}$Al D-HMQC [78–82] sequences.

Firstly, the present work aims at providing further insights into the influence of a modification of γ-Al₂O₃ in warm water prior to deposition of Ag, following a procedure similar to that reported earlier [72] but over a series of Ag/γ-Al₂O₃ catalysts with Ag loadings varying from about 1 to 9 wt%, on the catalytic performance in C₃H₆-SCR promoted or not by hydrogen ((H₂)-)C₃H₆-SCR) so as to establish NOₓ-TPD-(H₂)-)C₃H₆-SCR structure-activity correlations. Secondly, FTIR and NMR spectroscopic investigations are performed on this series of Ag/γ-Al₂O₃ catalysts to unravel the preferential Ag anchoring sites of γ-Al₂O₃, which would pave the way to the design of alumina supports of improved efficiency not only in the dispersion of Ag species of interest in the C₃H₆-SCR field but also for the deposition of any other transition metals on such a widely used supporting oxide.

## 2. Experimental

### 2.1. Catalyst preparation

As indicated in earlier literature reports, Ag/Al₂O₃ catalysts are most often prepared by incipient wetness impregnation of the alumina support by an aqueous solution of AgNO₃ [42,43]. In the present work, a series of Ag/Al₂O₃-H₂O samples was prepared by incipient wetness impregnation after the γ-Al₂O₃ support (Procatalyse, 125–200 µm, 40 g) had been treated in 400 mL of warm water (80 °C) for 66 h without any stirring. After filtering and drying under vacuum for 16 h at RT, such a warm-water-treated alumina support was calcined at 600 °C (3 °C/min heating rate from RT to 600 °C) for 4 h. The as-modified support, denoted as Al₂O₃-H₂O, was used to prepare the Ag(x)/Al₂O₃-H₂O series of samples, where x denotes the Ag content (wt%), determined by ICP-OES, that was deposited on Al₂O₃-H₂O by incipient wetness impregnation of aqueous solutions of AgNO₃. After ageing under vacuum at RT for 4 h, the samples were dried at 100 °C for 18 h in a laboratory oven. Finally, the Ag(x)/Al₂O₃-H₂O samples were calcined at 600 °C (3 °C/min heating rate from RT to 600 °C) for 4 h. This procedure appears to differ substantially from that reported in our earlier study in which the γ-Al₂O₃ support was vigorously stirred in warm water (80 °C) for 24 h, which resulted in the loss of most of the 125–200 µm fraction due to severe attrition of the grains, and for which Ag was deposited on a vacuum-dried (not calcined) Al₂O₃-OH support [72]. For comparison, where necessary, Ag(x)/Al₂O₃ samples were also prepared following the procedure described earlier [42,43], namely incipient wetness impregnation of pristine γ-Al₂O₃ by aqueous AgNO₃ solutions. Figure S1 provides a schematic summary of the synthesis conditions used for both series of silver alumina samples.

### 2.2. Catalyst characterization

Ag loadings were determined by ICP-OES (CREALINS, Lyon).

XRD patterns of Al₂O₃-H₂O (vacuum-dried for 16 h and calcined at 600 °C for 4 h) were recorded in the 2θ range of 10–90° with 0.01° steps and 0.5 s/step with a Bruker D8 ADVANCE diffractometer equipped with a copper source (λCu-Kα1 = 1.54056 Å and λCu-Kα2 = 1.54439 Å) and a LynxEye detector.

N₂-sorption measurements were carried out on a Belsorp MAX Instrument (Bel Japan) at 77 K after evacuation of the samples at 300 °C for 3 h. The specific surface area of the samples was determined by using the BET method in the 0.05 < *p*/*p*⁰ < 0.30 domain.

SEM-FEG imaging of the supporting oxides was done with a Hitachi SU-70 field emission gun scanning electron microscope at magnifications varying from × 300 to × 10,000. The 125–200 µm powder samples were deposited as is onto carbon adhesives taped on an aluminum SEM support. Imaging was performed without any metallization at 1 kV and a working distance of about 15 mm, hence by an “in lens” secondary electron detector (SE-Upper).

NOₓ Temperature-Programmed Desorption (NOₓ-TPD) experiments were carried out in a U-type quartz reactor (12 mm i.d.). The samples (about 0.200 g) were held on a plug of quartz wool and the temperature was set by a Eurotherm 2408 temperature controller using a K-type thermocouple. Reactant gases (Air Liquide) were used as received and fed from independent gas cylinders by means of mass flow controllers (Brooks 5850TR) with a total flow rate of 230 mLₙTPD/min. Prior to the NOₓ-TPD experiments, the samples were calcined at 500 °C (3 °C/min) for 2 h under a flow rate of 100 mLₙTPD/min of 20% O₂/He. The reactor outlet was continuously monitored by a chemiluminescence NOₓ analyzer (Thermo Environmental Instruments 42C-HL) that allowed the simultaneous detection of both NO and NO₂. The samples were exposed to the adsorption mixture (Typically 385 ppm NOₓ and 8% O₂ in He with a total flow rate of 230 mLₙTPD/min) for 4 h at RT. This latter parameter was set to ensure that saturation coverage was reached for all of the samples investigated under the present experimental conditions, as the time after which no change was observed in the gas phase NOₓ concentrations was found to be dependent on the Ag loadings of the silver alumina samples. Before NOₓ-TPD, the samples were flushed under a flow of 230 mLₙTPD/min of O₂ 8%/He at RT for about 1 h to remove weakly chemisorbed species until the NO and NO₂ concentrations detected at the outlet of the reactor were found to be negligible. NOₓ-TPD experiments were carried out from RT to 500 °C at a heating rate of 3 °C/min under a flow of 230 mLₙTPD/min of O₂ 8%/He.

FTIR spectra of the Ag/γ-Al₂O₃ samples were collected in transmission mode on a Bruker Vertex 70 FTIR spectrometer equipped with a liquid N₂-cooled MCT detector and a data acquisition station. 64 scans were averaged with a spectral resolution of 4 cm⁻¹. The samples were pressed into self-supporting wafers of 6–8 mg/cm² (12–16 mg for wafers of 1.6 cm diameter). The wafers were loaded into a moveable glass sample holder, equipped on top with an iron magnet, and inserted in a conventional quartz glass cell (CaF₂ windows) connected to a vacuum system. The iron magnet allowed for the transfer of the catalyst sample from the oven-heated region to the infrared light beam. Before spectrum recording, the catalysts were submitted to a dynamic (20 mL/min) oxidizing pretreatment (O₂, Air Liquide, 99.999%) at 500 °C for 2 h at atmospheric pressure. The samples were then evacuated (5 × 10⁻⁶ mbar) at 400 °C for 1 h. Finally, the temperature was decreased to RT under dynamic vacuum.

Magnetic Resonance (MAS NMR) spectra were recorded at room temperature using a Bruker Advance spectrometer operating in a static field of 11.7 T. The resonance frequency of $^1$H and $^{27}$Al were 500.2 and 130.3 MHz, respectively. A Bruker 4 mm MAS probe was used to perform the experiments at a spinning speed of 14 kHz for one-dimensional (1D) spectra and 12.5 kHz for two-dimensional (2D) spectra. The $^1$H and $^{27}$Al chemical shifts were referenced to external standards of tetra-methylsilane (TMS) and Al(NO₃)₃ aqueous solutions, respectively. For all experiments, the powdered samples were first calcined at 500 °C (3 C/min) for 2 h under 100 mLₙTPD/min of O₂(20%)/He using a quartz reactor equipped with stop valves. The samples were then flushed with 100 mLₙTPD/min of He for 15 min at RT before closing of the valves of the reactor to prevent the samples from exposure to ambient atmosphere and potential rehydration. Selected samples (Al₂O₃-H₂O and Ag(1)/Al₂O₃-H₂O), were also contacted with NOₓ (96% NO + 4% NO₂) by flowing a NOₓ(385 ppm)-O₂(8%)-He mixture at RT and 230 mLₙTPD/min for 4 h before being flushed by He (100 mLₙTPD/min) for 1 h. Samples were then transferred from the reactor to the NMR rotors into a glovebox where the NMR rotors were finally closed. $^1$H MAS NMR 1D spectra were recorded with a 90° pulse duration of 2.75 µs, a recycle delay of 10 s, and a scan number of 64. The MAS equipment for rotation was carefully cleaned with ethanol before being dried to avoid spurious proton signals. From two successive experiments performed with the same recording conditions and using the same empty or filled rotor, the signals of the probe and the rotor were subtracted from the total FID. The $^1$H DQ-SQ 2D spectrum was recorded with the POST-C7 sequence for excitation and conversion block, and a RF field strength
was used during the dipolar recoupling sequence of 87.5 kHz and 60 t1 increments of 80 µs for F1 dimension acquisition. The ²⁷Al-¹H D-HMQC (heteronuclear multiple quantum coherence) experiments were performed with RF nutation frequencies of 9 and 92 kHz for ²⁷Al and ¹H, respectively. A SR4¹ method was used as the recoupling sequence to reintroduce ²⁷Al-¹H dipolar interaction with a RF field strength of 25 kHz (±2*νrot) during 960 µs. The recorded spectra result from averaging 4096 transients and a recycle delay of 0.5 s.

## 2.3. (H₂-)C₃H₆-SCR catalytic experiments

Steady state catalytic (H₂-)C₃H₆-SCR experiments were carried out in a U-type quartz reactor (12 mm i.d.). In contrast to most studies reported to date, the amount of silver introduced in the catalyst beds remained essentially constant when the silver loading of the Ag(x)/Al₂O₃-H₂O samples varied. To be compared to the catalytic data recorded earlier on silver alumina samples prepared with a non-treated Al₂O₃ support [42, 43], the samples were held on plugs of quartz wool and consisted in about 0.38 g of mechanical mixtures of Ag(x)/Al₂O₃-H₂O and Al₂O₃ of the same grain sizes in which the amount of Ag was equal to 30.9 ± 0.1 µmol. The temperature of the tubular furnace was set by a Eurotherm 2408 temperature controller using a K type thermocouple. Prior to the (H₂-)C₃H₆-SCR experiments, the samples were calcined in situ in O₂(20%)–He at 550 °C (3 °C/min) for 2 h with a flow rate of 100 mL·NTP/min. After cooling down to 150 °C, the samples were submitted to a C₃H₆-SCR experiment from 150 to 550 °C [42]. The samples were subsequently exposed to the H₂-C₃H₆-SCR feed at 150° C [43]. H₂(2%/He), NO(4000 ppm/He), C₃H₆(2000 ppm/He), O₂(100%) and He (100%) were fed from independent cylinders (Air Liquide) without any further purification via mass flow controllers (Brooks 5850TR). The composition of the H₂-NOₓ-C₃H₆-O₂-He feeds was: 0 or 0.21% H₂, 385 ppm NOₓ (~96% NO), 400 ppm C₃H₆ and 8% O₂ in He, and the total flow rate was 230 mL·NTP/min. The temperature was then increased stepwise from 150 to 550 °C with 25 °C increments and left for about 1 h at each temperature step. The reactor outflow was analyzed using a µ-GC (Agilent Technologies, CP4900) equipped with two channels. The first channel, a 5Amolecular sieve column (80 °C, 150 kPa He, 200 ms injection time, 37 s backflush time), was used to separate H₂, N₂, O₂ and CO, while the second channel, equipped with a poraplot Q column (60 °C, 150 kPa He, 200 ms injection time), was used to separate CO₂, N₂O, C₃H₆ and H₂O. A chemiluminescence NOₓ analyzer (Thermo Environmental Instruments 42C-HL) allowed the simultaneous detection of both NO and NO₂. NOₓ conversions to N₂ and N₂O were calculated as follows:

$$X_{\text{NOx to N2}}(\%) = (2 \times [\text{N}_2])/[\text{NOx}_{\text{inlet}}] \times 100, \quad (1)$$

$$X_{\text{NOx to N2O}}(\%) = (2 \times [\text{N}_2\text{O}])/[\text{NOx}_{\text{inlet}}] \times 100, \quad (2)$$

where [NOₓ]inlet, [N₂] and [N₂O] were the concentrations in NOₓ measured at the inlet of the reactor and in N₂ and N₂O at the outlet of the reactor, respectively. C₃H₆ conversions were calculated on the basis of the COₓ (CO + CO₂) products formed:

$$X_{\text{C3H6}}(\%) = ([\text{CO}] + [\text{CO}_2])/([\text{C3H}_6]_{\text{inlet}} \times 3) \times 100, \quad (3)$$

where [CO], [CO₂] and [C₃H₆]inlet were the concentrations of CO and CO₂ measured at the outlet of the reactor and that of C₃H₆ measured at the inlet of the reactor, respectively. The comparison of the catalytic performances of the materials investigated in the present study was also made on the basis of an efficiency criterion (%) in the reduction of NOₓ to N₂ in the 150–550 °C range of temperatures. This criterion compares the catalytic performance of the investigated samples to those of a catalyst that would allow for the full reduction of NOₓ to N₂ from 150 to 550 °C (100% efficiency).

# 3. Results

## 3.1. Influence of a pretreatment of Al₂O₃ in warm water on the structural and textural properties of the Ag/Al₂O₃ materials

The Ag contents and the surface areas of the samples prepared in the present work are listed in Table 1. It can be first seen that exposure of the pristine Al₂O₃ support to water at 80 °C for 66 h with subsequent drying and calcination at 600 °C for 4 h led to a slight increase in its surface area (Al₂O₃-H₂O ~ 195 m²/g) compared to the starting material (Al₂O₃ ~ 180 m²/g). It must be noted that a similar surface area (195 m²/g) was also measured on dried Al₂O₃-H₂O, which indicates that the increase in surface area of this sample cannot be attributed to the calcination step. In an earlier study in which the same pristine Al₂O₃ support was exposed to water at 80 °C but for a much shorter duration, i.e. 24 h [72] compared to 66 h in the present work, the limited increase in specific area of Al₂O₃-OH (185 m²/g) compared to untreated Al₂O₃ (180 m²/g) [72] did not appear to be significant at first sight. From the present surface area data, it can be concluded that the increase in surface area was indeed significant in the earlier work [72], albeit very limited (+ 5 m²/g), however, as the increase in surface area measured in the present work (+ 15 m²/g) appears to be roughly proportional to the duration of the exposure of Al₂O₃ to water at 80 °C (from 24 h [72] to 66 h in the present work), as also reported previously by Lefèvre et al. [83]. It can be observed that the measured Ag contents are in good agreement with those targeted. Overall, the specific surface area per g of sample decreases as the Ag content increases and the associated surface area per g of Al₂O₃ remains essentially constant (Table 1).

As reported earlier [72,84], the treatment of γ-Al₂O₃ in warm water led to the formation of bayerite crystallites in the Al₂O₃-H₂O sample dried at RT under vacuum for 16 h [72], as indicated by the reflections at 18.8, 20.3, 27.9, 40.6, 53.2, 63.8, 70.8 and 78.9° (Figure S2, ● on

<caption>**Table 1**</caption>

Ag contents determined by ICP-OES and surface areas of the newly prepared silver alumina samples.

<table><thead><tr><th>Samples</th><th>Targeted Ag content (wt%)</th><th>Measured Ag content (wt%)</th><th>Surface area (m²/g)</th><th>Surface area (m²/g/Al₂O₃)</th></tr></thead><tbody><tr><td>Al₂O₃-H₂O</td><td>-</td><td>-</td><td>195</td><td>195</td></tr><tr><td>Ag(1)/ Al₂O₃- H₂O</td><td>1.00</td><td>1.11</td><td>192</td><td>194</td></tr><tr><td>Ag(2)/ Al₂O₃- H₂O</td><td>2.00</td><td>2.18</td><td>184</td><td>189</td></tr><tr><td>Ag(3)/ Al₂O₃- H₂O</td><td>3.00</td><td>3.23</td><td>187</td><td>193</td></tr><tr><td>Ag(5)/ Al₂O₃- H₂O</td><td>5.00</td><td>5.11</td><td>181</td><td>191</td></tr><tr><td>Ag(7)/ Al₂O₃- H₂O</td><td>7.00</td><td>7.13</td><td>179</td><td>193</td></tr><tr><td>Ag(9)/ Al₂O₃- H₂O</td><td>9.00</td><td>9.21</td><td>172</td><td>189</td></tr><tr><td>Al₂O₃</td><td>-</td><td>-</td><td>180</td><td>180</td></tr><tr><td>Ag(1)/ Al₂O₃</td><td>1.00</td><td>1.05</td><td>178</td><td>180</td></tr><tr><td>Ag(2)/ Al₂O₃</td><td>2.00</td><td>2.12</td><td>176</td><td>180</td></tr><tr><td>Ag(3)/ Al₂O₃</td><td>3.00</td><td>3.15</td><td>173</td><td>179</td></tr><tr><td>Ag(4)/ Al₂O₃</td><td>4.00</td><td>4.19</td><td>171</td><td>178</td></tr><tr><td>Ag(7)/ Al₂O₃</td><td>7.00</td><td>-</td><td>167</td><td>180¹</td></tr></tbody></table>

<footnote>* Surface area estimated on the basis of a theoretical Ag loading of 7 wt%.</footnote>
green trace). The longer exposure of the pristine Al₂O₃ support in water at 80 °C in the present work (66 h) compared to that in our earlier work (24 h [72]) did not seem to have a significant influence on the formed bayerite crystallites, as the (201) bayerite to (400) Al₂O₃ intensity ratio of the peaks at 40.6 and 46.0° appears to be similar in both XRD patterns (1.29 in Figure S2 compared to 1.26 in Fig. 3 of ref. [72]). Yet, the longer exposure to the pristine Al₂O₃ support to water at 80 °C induced the formation of gibbsite, as indicated by a diffraction peak of low intensity at 18.3° (Figure S2), which could hardly be observed earlier in the tail of the bayerite peak at 18.8° [72]. As both bayerite and gibbsite can transform subsequently to boehmite and γ-Al₂O₃ upon heating and/or η- and χ-Al₂O₃ transition aluminas [85], which can be hardly distinguished from γ-Al₂O₃ by XRD, the XRD pattern after calcination of Al₂O₃ pretreated in warm water (Figure S2, blue trace) therefore closely resembles that of the pristine γ-Al₂O₃ support [72]. If any surface modifications of the pristine Al₂O₃ support had been achieved by its exposure to water at 80 °C, these changes can hardly be monitored by XRD, in agreement with the inherent bulk sensitivity of such a technique.

Representative SEM images of the pristine Al₂O₃ support, and the dried and calcined warm-water-pretreated Al₂O₃ samples are displayed in Figure S3. Figure S3 shows that the surface of the grains of the pristine Al₂O₃ support are rather smooth (a,b) and the presence of small entities of ill-defined shape at elevated magnification (c). The pretreatment of Al₂O₃ in water at 80 °C for 66 h had a dramatic influence on the Al₂O₃ grains that became almost fully covered by crystallites (Figure S3d) exhibiting a platelet-like morphology (Figure S3e,f) with a wideness as high as 2–3 µm. The fact that the platelets of the present work were found to be bigger than those observed earlier by Lefèvre et al. [83] can be assigned to the much higher temperature at which Al₂O₃ was immersed in water compared to that in the earlier study of Lefèvre et al. [83], and a corresponding improvement in the kinetics of dissolution-reprecipitation of Al₂O₃ as bayerite [84]. The presence of such well-crystallized platelets after treatment of Al₂O₃ in water is consistent with the appearance of the thin XRD contributions observed in the corresponding pattern (Figure S2, green trace). Figure S3g-i shows that the peculiar morphology observed in dried warm-water-pretreated Al₂O₃ (Figure S3d-f) was preserved after calcination (Al₂O₃-H₂O). The presence of such micrometric Al₂O₃ platelets (disappearance of the bayerite diffraction peaks on the blue diffraction pattern in Figure S2) onto the surface of the Al₂O₃ grains should be increasing the roughness of the external surface of the Al₂O₃-H₂O grains and should be at the origin of the moderate, but significant, increase in the surface area of Al₂O₃-H₂O compared to Al₂O₃ (Table 1).

## 3.2. Influence of a pretreatment of Al₂O₃ in warm water and the deposition of Ag on surface hydroxyl groups

Characterization of the surface OH groups of Al₂O₃ was performed by FTIR. Figure S4a shows that pretreatment of Al₂O₃ in water at 80 °C led to changes in the absorption spectrum in the region of the hydroxyl groups. Overall the envelope of the hydroxyl groups (3800–3400 cm⁻¹) increased by about 10% in Al₂O₃-H₂O compared to Al₂O₃ and the intensity of the contributions at 3790 and 3754 cm⁻¹ decreased, whereas those at 3770, 3680, 3633 and 3602 cm⁻¹ increased and that at 3730 cm⁻¹ remained essentially constant in Al₂O₃-H₂O compared to Al₂O₃. The introduction of increasing amount of Ag on Al₂O₃-H₂O led to a vanishing of the bands at 3790 and 3754 cm⁻¹ at the lowest content of Ag, a pronounced decrease of the bands at 3770 and 3680 cm⁻¹ for Ag (1)/Al₂O₃-H₂O followed by a more moderate decrease at higher Ag loadings and an increase in that at 3730 cm⁻¹ up to Ag(3)/Al₂O₃-H₂O before decreasing at a higher Ag loading (Figure S4b). Such trends are in agreement with those reported earlier on Ag(x)/Al₂O₃ samples [63,75,77,80]. The nature of the corresponding OH groups of Al₂O₃ has been debated comprehensively [63,75,77,80,86–88] and will be discussed in relation with other findings in Section 4.2.

Before describing the NMR data, it must be recalled that the various hydroxyl groups (OH) of γ-alumina are commonly denoted as µⁿ-OH, which denotes that the hydroxyl group is bounded to n aluminum atoms [78]. The ¹H spectrum of γ-Al₂O₃ (Fig. 1, spectrum a) can be decomposed into three regions between (i) –1 and 1 ppm and (ii) 1 and 3 ppm, and (iii) for a chemical shift greater than 3 ppm. In earlier literature reports, the first region has been attributed to terminal hydroxyl groups (µ¹-OH), the second one to doubly-bridging hydroxyl groups (µ²-OH) and the last one to triply-bridging hydroxyl groups (µ³-OH) and residual water [78]. More recently, thanks to DFT calculations, a new assignment of the ¹H chemical shifts has been proposed [81]. The first region corresponds to µ¹-OH species located on the edges of the γ-Al₂O₃ crystallites, while the second region corresponds to µ¹-OH and µ²-OH species located essentially on the Al₂O₃ facets. Finally, the ¹H contributions with a chemical shift greater than 3 ppm were assigned to µ¹-OH, µ²-OH and µ³-OH species with H-bond donors. The effect of the pretreatment of Al₂O₃ in warm water on its hydroxylation can be seen in Fig. 1 (spectra a and b). The integral of the Al₂O₃-H₂O spectrum (Fig. 1, spectrum b) is about 1.6 times greater than that of the Al₂O₃ spectrum (Fig. 1, spectrum

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** ¹H MAS NMR spectra of (a) Al₂O₃ (dotted line), (b) Al₂O₃-H₂O, (c) Ag (1)/Al₂O₃-H₂O, (d), Ag(2)/Al₂O₃-H₂O, (e) Ag(3)/Al₂O₃-H₂O and (f) Ag(5)/Al₂O₃-H₂O. Signal assignment based on the earlier studies of Taufik et al. [78] and Batista et al. [81].</figcaption></figure>
a), hence showing a significant increase in the amount of protons present on the alumina surface after pretreatment in warm water.

2D double quantum-single quantum (DQSQ) correlation experiment provides useful information on the proximities (~ 5 Å) between dipolar-coupled protons, as the observed DQ coherences depend strongly on the internuclear distance. The diagonal peaks (δA, 2δA) are autocorrelation peaks resulting from the dipolar interaction of spins with identical chemical shift, while the pairs of off-diagonal peaks at (δA, δA+δB) and (δB, δA+δB) correspond to correlations between two spins with different chemical shifts. Usually, the intensity of these correlation peaks reflects the strength of the dipolar interaction between the spin pairs and therefore the distance between the different proton species. In agreement with the A-G OH terminology defined by Merle et al. [80] (Fig. 2), the ¹H-¹H DQSQ MAS NMR spectrum of Al₂O₃-H₂O (Fig. 3a) exhibits two autocorrelation peaks at – 0.10 and 2.25 ppm attributed to µ¹-OH (F) and µ²-OH (G) species, respectively. Two off-diagonal correlations between µ¹-OH and µ²-OH species are also observed, which show proximity of two µ¹-OH sites (– 0.15 ppm (C) and – 0.20 ppm (A)) with two µ²-OH species (1.90 ppm (D) and 2.50 ppm (B)), respectively. In addition, an off diagonal correlation between two µ²-OH species (1.90 ppm (D) and 1.55 ppm (E)) is visible. This experiment shows that the signal at about – 0.10 ppm (Fig. 1) assigned to µ¹-OH groups is composed of at least three different species with proton shifts at – 0.20 ppm (A), – 0.15 ppm (C) and – 0.1 ppm (F). The ¹H-¹H DQSQ MAS NMR spectrum and associated proximity information (Fig. 3a) are found to be identical to those reported by Merle et al. [80] and fairly close to those reported by Batista et al. [81] and Völker et al. [82]. The differences in the ¹H-¹H DQSQ MAS NMR spectrum and associated proximity information (Fig. 3a) with those of the latter studies may be due to differences in the morphology of the studied aluminas and/or sample pre-treatments. Finally, it should be also noted that the ¹H-¹H DQSQ MAS NMR spectrum seems to show a proximity between OH species with and without H-bond donors at about 3.50 ppm.

The presence of 1 wt% Ag on Al₂O₃-H₂O leads to an overall decrease in µ²-OH species (1–3 ppm), an increase in the overall amount of µ¹-OH species (– 0.1 to + 1 ppm) and the shift of some of the µ¹-OH species (from about – 0.1 ppm to about 0.4 ppm) (Fig. 1, spectra b and c). Not only such trends have already been reported by Wang et al. [63] for Ag/Al₂O₃ but also, more surprisingly, by Barrow et al. for SrO/Al₂O₃ [89]. The ¹H-¹H DQSQ spectrum of Ag(1)/Al₂O₃-H₂O (Fig. 3b), which has not been reported to date to the best of our knowledge, provides additional information about the newly formed µ¹-OH species at about 0.4 ppm disturbed by the addition of Ag on Al₂O₃-H₂O. The comparison of Fig. 3b and Fig. 3a indicates that part of the – 0.20 ppm µ¹-OH (A) and most of the – 0.10 ppm µ¹-OH (F) species are shifted to 0.35 ppm (A') and 0.40 ppm (F') µ¹-OH species, respectively, on the addition of 1 wt% Ag on Al₂O₃-H₂O. The µ¹-OH (F') species are coupled with themselves as being located on the diagonal of the spectrum shown in Fig. 3b, whereas the µ¹-OH (A') species are coupled with µ²-OH (B') species, which corresponds to µ²-OH (B) species shifted to lower chemical shift (2.10 ppm instead of 2.50 ppm). In contrast, – 0.15 ppm µ¹-OH (C), 1.90 ppm µ²-OH (D), 1.55 ppm µ²-OH (E) and 2.25 ppm µ²-OH (G) species do not

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. ¹H-¹H DQSQ MAS NMR spectra of (a) Al₂O₃-H₂O and (b) Ag(1)/Al₂O₃-H₂O.</figcaption></figure>

appear to be affected by the presence of silver atoms (same chemical shift and same correlation in Fig. 3b compared to Fig. 3a). The sample containing 2 wt% silver shows hardly any change in chemical shifts of most of the protons, but only a slight decrease in the intensity of µ²-OH (1–3 ppm and particularly at higher chemical shifts) and – 0.15 ppm

<figure><img src="image_6.png" /><figcaption>Fig. 2. Structure of the main hydroxyl groups of γ-Al₂O₃ adapted from the work of Merle et al. [80].</figcaption></figure>
µ¹-OH (C) species, as well as an increase in intensity and a narrowing of the signals corresponding to µ¹-OH (A') and (F') species around 0.4 ppm (Fig. 1, spectrum d and Figure S5). For samples with 3 and 5 wt% silver on alumina (Fig. 1, spectra e and f), a disappearance of the µ²-OH (1–3 ppm) and – 0.15 ppm µ¹-OH (C) species, and an increase in the µ¹-OH species around 0.4 ppm still occur but to a much more limited extent than those observed for Ag/Al₂O₃-H₂O samples with lower Ag loadings (Fig. 1, spectra c and d). Interestingly, disappearance of the ¹H signal at about – 0.10 ppm on the addition of increasing quantities of Ag appears to be completed for Ag(3)/Al₂O₃-H₂O, as detailed in Figure S6 that shows a zoom from – 1 to + 1 ppm of the ¹H spectra shown in Fig. 1. Finally, it must be highlighted that the amount of protons was found to remain essentially constant on the addition of increasing quantities of Ag on Al₂O₃-H₂O (Fig. 1, spectra b-f).

Figure S7 shows that exposure of Al₂O₃-H₂O, calcined at 500 °C for 2 h, to NOₓ species at RT for 4 h results in a shift of the protons' signals to higher chemical shifts and the vanishing of the µ¹-OH signal at about – 0.10 ppm suggesting the preferential chemisorption of NOₓ species in close proximity with such OH groups. A similar trend is observed for Ag (1)/Al₂O₃-H₂O for which not only the signal at about – 0.10 ppm vanishes but also that at about 0.4 ppm, resulting from the introduction of Ag (Fig. 1, spectra b and c), decreases to a significant extent with the exposure of this sample to NOₓ (Figure S7).

Information about the environment of the Al atoms close to the hy-
droxyl groups can be deduced from the ¹H–²⁷Al D-HMQC MAS NMR
spectrum (Fig. 4). Such an experiment is based on heteronuclear dipolar
interaction across space and therefore does not indicate direct bonding
of Al atoms to hydroxyl groups, which makes interpretation more deli-
cate as underlined by Völker et al. [82]. Fig. 4 shows that the µ¹-OH
groups (A, C and F) at about – 0.1 ppm correlate with aluminum atoms
with a very broad signal covering the chemical shifts of tetra- (Al⁴, ~
60 ppm), penta- (Al⁵, ~ 30 ppm) and hexa-coordinated (AlVI, ~
10 ppm) aluminum atoms. As already observed in earlier studies [80,
82], this signal is composed of two contributions, the first corresponding
to Al⁴ with a large quadrupolar coupling constant (very broad line with
low intensity at 60–20 ppm in the dashed blue rectangle), which are
bonded to all types of µ¹-OH hydroxyl groups ((Al⁴)-µ¹-OH species). The
second contribution is due to AlVI species (~7 ppm) not directly bonded
to µ¹-OH hydroxyl groups but in their proximity. µ²-OH groups are
bonded to two aluminum atoms and the four types of µ²-OH groups (B,
D, E and G) are found to correlate with Al species exhibiting chemical
shifts around 7 ppm. This suggests that, for all µ²-OH groups, one of
these two Al atoms corresponds to AlVI species. For the second Al atoms,
the µ²-OH groups (E), higher in chemical shift (1.55 ppm), could be
bonded to AlV species at about 30 ppm, while the other µ²-OH groups (D,

<figure><img src="image_7.png" /><figcaption>Fig. 4. ¹H–²⁷Al D-HMQC MAS NMR spectrum of Al₂O₃-H₂O.</figcaption></figure>

G and B) appear closer to Al<sup>IV</sup> species. From these observations, the following aluminum atom environments can be proposed for the µ<sup>2</sup>-OH species: (Al<sup>VI</sup> Al<sup>IV</sup>)-µ<sup>2</sup>-OH (B), (Al<sup>VI</sup> Al<sup>IV</sup>)-µ<sup>2</sup>-OH (D), (Al<sup>VI</sup> Al<sup>V</sup>)-µ<sup>2</sup>-OH (E), (Al<sup>VI</sup> Al<sup>IV</sup>)-µ<sup>2</sup>-OH (G). The ¹H–²⁷Al heteronuclear correlation experiment for Ag(1)/Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O (Figure S8) mainly shows changes in the environment of aluminum atoms bonded to µ<sup>1</sup>-OH species. For Ag (1)/Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O, the contribution of Al<sup>IV</sup> species correlating with µ<sup>1</sup>-OH species (A' and F') (Figures S8 and S9), appears to be more intense and narrower than that of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O (Fig. 4). This suggests a decrease in quadrupolar interaction and thus a more charge-symmetric environment of the Al atoms, probably due to the presence of the silver atoms in close proximity. For Ag(x)/Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O samples containing 2, 3 and 5 wt % of silver (Figures S9-S12), the contribution of Al<sup>IV</sup> species bonded to µ<sup>1</sup>-OH increases steadily, while the contribution of Al<sup>VI</sup> species near µ<sup>1</sup>-OH species remains essentially constant (Figure S9a). The spectra of the Ag/Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O samples containing 2–5 wt% Ag (Figure S9a) show the presence of a new signal at about 70 ppm. The environment of aluminum atoms, close to µ<sup>2</sup>-OH species, does not seem to change much with the addition of Ag, which contributes to a decrease in the overall intensity of the signals related to Al<sup>IV</sup>, Al<sup>V</sup> and Al<sup>VI</sup> species due to the decrease in the number of µ<sup>2</sup>-OH sites as the amount of silver increases (Figures S9b, S10-12).

It must be highlighted that changes in ¹H (Figure S13) and ¹H–²⁷Al D-HMQC MAS NMR spectra (Figures S14 and S15) on the addition of Ag on Al₂O₃ appear to be similar to those described in the previous sections for the addition of Ag on the warm-water-pretreated Al₂O₃ support (Al₂O₃-H₂O). Yet one may notice that the vanishing of the µ¹-OH species at about – 0.10 ppm occurs at a lower Ag loading on Al₂O₃ (2 wt%, Figure S13) than on Al₂O₃-H₂O (3 wt%, Figure 1 and Figure S6).

3.3. Adsorption-desorption of NOₓ and (H₂-)C₃H₆-SCR catalytic reactions

As reported in earlier studies [42,43,59,72], the
adsorption-desorption of NOₓ from Ag/Al₂O₃ materials have shown
unique insights into the comprehensive understanding of the HC-SCR
processes by providing unprecedented structure-activity correlations
in this field. The characterization of the Ag(x)/Al₂O₃-H₂O series and Ag
(4)/Al₂O₃, the latter material acting as a reference sample of the Ag
(x)/Al₂O₃ series, by NOₓ-TPD is compared to the data acquired previ-
ously on Ag(x)/Al₂O₃ [42,43]. In agreement with the profiles already
described in our previous studies [42,53], the Ag(x)/Al₂O₃-H₂O series
and Ag(4)/Al₂O₃ showed a two-peak desorption profile with a first
desorption peak at about 115–200 °C, made of NO mainly and NO₂, and
a second peak at about 440 °C essentially due to the release of NO₂, with
small amounts of NO (Figure S16a-c) likely due the thermodynamically
favored backward reaction of the 2 NO + O₂ = 2 NO₂ equilibrium at the
highest temperatures [54]. The introduction of Ag onto Al₂O₃-H₂O led to
progressive shift of the temperature of maximum release of NOₓ of the
low-temperature desorption peak to higher temperatures and a decrease
in the intensity of both peaks as the content of Ag increases
(Figure S16b), as also observed on the Ag(x)/Al₂O₃ series [42]. The NOₓ
uptake of Ag(4)/Al₂O₃ (O in Fig. 5a) estimated from Figure S16c was
found to be in good agreement with that expected from the earlier
measurements on an Ag(x)/Al₂O₃ series (● in Fig. 5a) [42], thus
attesting for the good reproducibility of the synthesis procedure and the
reliability of the analytical set-up, as also illustrated in ref. [43]. The
NOₓ uptakes of the Ag(x)/Al₂O₃-H₂O series appeared to be estimated
with a good reproducibility (■ and □ data in Fig. 5a) and were found to
be significantly greater than those of the Ag(x)/Al₂O₃ series (● data in
Fig. 5a) by about 15–25%. As observed for the Ag(x)/Al₂O₃ series (●
data in Fig. 5a), the NOₓ uptakes of Ag(x)/Al₂O₃-H₂O series (■ data in
Fig. 5a) decreased linearly as the Ag loading increased before levelling
off. Whereas the levelling off in the NOₓ uptake occurred at an Ag
loading close to 2 wt% for the Ag(x)/Al₂O₃ series (● data in Fig. 5a)
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) NOₓ uptakes (data from ref. [42]) on Ag(x)/Al₂O₃ (●) and recorded on Ag(4)/Al₂O₃ newly synthesized in the present work (○) and on Ag (x)/Al₂O₃-H₂O (□) with duplicate measurements (□), (b) C₃H₆-SCR (data from ref. [42]) on Ag(x)/Al₂O₃ (●) and recorded on Ag(7)/Al₂O₃ (○) and on Ag(x)/Al₂O₃-H₂O (□) newly synthesized in the present work) and (c) H₂-C₃H₆-SCR (data from ref. [42]) on Ag(x)/Al₂O₃ (●) and recorded on Ag (x)/Al₂O₃-H₂O (□) as a function of the Ag loading with an amount of Ag in the catalyst bed that was kept essentially constant to 30.9 ± 1.2 µmol for Ag (x)/Al₂O₃ in ref. [42] and 30.9 ± 0.1 µmol for Ag(x)/Al₂O₃-H₂O. Feed composition: 0 or 0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆, 8% O₂ and He balance with a 230 mL<sub>NTP</sub>/min flow rate.</figcaption></figure>

[42], such a levelling off was found to occur at about 3 wt% for the newly synthesized Ag(x)/Al₂O₃-H₂O series (■ data in Fig. 5a). The decrease in NOₓ uptake with increasing loadings of Ag up to 2 and 3 wt% for Ag(x)/Al₂O₃ and Ag(x)/Al₂O₃-H₂O, respectively, (Fig. 5a) is assigned to a competition of the Ag and NOₓ species for the same alumina surface sites. The levelling off of the NOₓ uptake for greater Ag loadings in both series indicates that the number of alumina surface sites for Ag anchoring must be limited on Al₂O₃ and Al₂O₃-H₂O albeit being different in these series. As reported earlier for an Ag loading of about 2 wt% [42] for the Ag(x)/Al₂O₃ series, the NOₓ uptake stabilization for Ag loadings greater than or equal to 3 wt% on the newly prepared Ag (x)/Al₂O₃-H₂O series (Fig. 5a) is attributed to the maximum Ag loading for which this metal remains in a highly dispersed state on the freshly calcined Ag(x)/Al₂O₃-H₂O samples, due to the limited number of Ag anchoring sites of the Al₂O₃-H₂O support. At higher Ag loadings than 3 wt%, the additional Ag atoms no longer bind to the Al₂O₃-H₂O surface sites also able to chemisorb NOₓ species, explaining that the NOₓ uptake

remains essentially constant above 3 wt% Ag on Al₂O₃-H₂O (Fig. 5a), and very likely contribute to a growth of the Ag clusters [43].

C₃H₆-SCR and H₂-C₃H₆-SCR catalytic data recorded on the Ag(x)/Al₂O₃-H₂O series and on Ag(7)/Al₂O₃ are shown in Fig. 6. The addition of increasing amount of silver for the Ag(x)/Al₂O₃-H₂O series leads to an increase in the conversion of NOₓ to N₂ in C₃H₆-SCR up to an Ag loading of about 3 w%, instead of about 2 wt% in most earlier studies [16,30,36–38,42], before decreasing at higher Ag loadings. Such a trend is much better illustrated in Fig. 5b, which shows the NOₓ reduction efficiency to N₂ of the various Ag(x)/Al₂O₃-H₂O samples, in other words the integration of the NOₓ to N₂ trace from 150 to 550 °C normalized to 100% conversion of NOₓ to N₂ over the same temperature range as exemplified by the dotted square region in Fig. 6g for C₃H₆-SCR on Ag (3)/Al₂O₃-H₂O. Fig. 5b also shows that pretreatment of Al₂O₃ in warm water prior to Ag deposition leads to catalysts of higher C₃H₆-SCR efficiency compared to those prepared with a non-treated alumina support for Ag loadings lower than 5 wt%. The addition of minute amounts of H₂ in C₃H₆-SCR was found to promote drastically C₃H₆-SCR with a shift to much lower temperatures of the reduction of NOₓ to N₂ by about 125–175 °C compared to C₃H₆-SCR from Ag(1)/Al₂O₃-H₂O (Fig. 6a) to Ag(3)/Al₂O₃-H₂O (Fig. 6c), respectively, in agreement with the pioneering finding of Satokawa [17] for H₂-C₃H₈-SCR and the work of Zhang et al. [90] for H₂-C₃H₆-SCR. Whereas the H₂ promoting effect is still observable at the highest silver loadings (Fig. 6e,f), it remains much less pronounced compared to that observed at the lower Ag loadings and therefore less easily quantifiable on a temperature shift basis. Fig. 5c shows that the NOₓ to N₂ reduction efficiency improved to a significant extent for the series of samples for which the alumina support had been treated in warm water prior to Ag deposition (Ag(x)/Al₂O₃-H₂O) compared to the series of samples prepared on non-pretreated alumina (Ag(x)/Al₂O₃) for Ag loadings lower than 7 wt%. As reported for Ag (x)/Al₂O₃ samples [43], and in contrast with earlier reports in this field [21,28], the Ag(x)/Al₂O₃-H₂O newly prepared series did not show any optimum Ag content and H₂-C₃H₆-SCR decreased as the Ag loading increased (Fig. 5c). Under the present experimental conditions, Ag (1)/Al₂O₃ appeared to be extremely active for NOₓ reduction and reached 100% from 375 to 450 °C so that its efficiency in NOₓ reduction to N₂ could not be ascertained reliably and reported in Fig. 5c. The catalytic performance of Ag(2)/Al₂O₃ [42,43] and Ag(2)/Al₂O₃-H₂O, which display similar Ag loadings close to 2.2 wt%, is compared for C₃H₆-SCR (Figure S17a) and H₂-C₃H₆-SCR (Figure S17b). This figure shows that the pretreatment of Al₂O₃ in warm water prior to Ag deposition has little influence on C₃H₆ conversion in C₃H₆-SCR (Figure S17a), while such a pretreatment promotes C₃H₆ conversion in H₂-C₃H₆-SCR at temperatures higher than 250 °C (Figure S17b). It can also be seen from this figure that the better NOₓ to N₂ conversion of Ag(2)/Al₂O₃-H₂O occurs at temperatures higher than 400 and 225 °C in C₃H₆-SCR (Figure S17a) and H₂-C₃H₆-SCR (Figure S17b), respectively.

# 4. Discussion

## 4.1. Exceeding the 2 wt% Ag optimum loading on Al₂O₃ by pretreating this oxide support in warm water prior to Ag deposition: towards structure-activity correlations

FTIR (Figure S4) and ¹H NMR (Fig. 1) clearly indicated that the pretreatment of alumina in warm water increased the OH content of Al₂O₃. Whereas FTIR estimates the increase in OH by about 10% after pretreatment of Al₂O₃ in warm water, this increase appears to be much higher by ¹H NMR (about 60%). This discrepancy may be attributed to (i) the fact that the extinction coefficients of the various OH species may be different, while the NMR proton response is known to be independent of the nature of the OH groups, and/or (ii) that the pretreatment of the samples used for both characterization techniques, i.e. evacuation at 400 °C in FTIR compared to calcination at 500 °C for ¹H NMR (experimental
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Influence of the addition of 0.21% H₂ on the conversions of NOₓ to N₂ (full symbols) and C₃H₆ to COₓ (open symbols) (C₃H₆-SCR (▲, △) and H₂-C₃H₆-SCR (●, ○) reactions) on (a) Ag(1)/Al₂O₃-H₂O, (b) Ag(2)/Al₂O₃-H₂O, (c) Ag(3)/Al₂O₃-H₂O, (d) Ag(5)/Al₂O₃-H₂O, (e) Ag(7)/Al₂O₃-H₂O and (f) Ag(7)/Al₂O₃ for mechanical mixtures of Ag(x)/Al₂O₃(-H₂O) catalysts and Al₂O₃ for which the amount of Ag was kept essentially constant to 30.9 ± 0.1 µmol. Illustration of the NOₓ to N₂ efficiency criterion for C₃H₆-SCR (g) and H₂-C₃H₆-SCR (h) on Ag(3)/Al₂O₃-H₂O. Feed compositions: 0 or 0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆, 8% O₂ and He balance with a 230 mL_NTP/min flowrate. The concentration of H₂ in the course of the H₂-C₃H₆-SCR reaction is also shown (— ● —).</figcaption></figure>
section), would lead to more dehydroxylated Al₂O₃ surfaces in the samples investigated by FTIR compared to those investigated by ¹H NMR. As the pretreatment used prior to recording ¹H NMR spectra appeared to be closer to that used of the samples prior to Ag deposition, it can be concluded that the pretreatment of Al₂O₃ in warm water followed by a calcination at 600 °C led to an increase in the OH groups of about 60% in Al₂O₃-H₂O compared to Al₂O₃. It is to be noted that such an increase in the amount of OH groups in Al₂O₃-H₂O₂O cannot be correlated straightforwardly to an increase in its surface area, as the surface area of Al₂O₃-H₂O (~ 195 m²/g) was found to increase by about 8% only compared to that of Al₂O₃ (~ 180 m²/g, Table 1). It must also be mentioned that the envelope of the ¹H NMR spectrum of Al₂O₃-H₂O closely resembled that of Al₂O₃ (Fig. 1), which indicates that all types of OH species were increased evenly by the pretreatment of Al₂O₃ in warm water. The changes in Al₂O₃-H₂O may be reasonably attributed to the chemical weathering of alumina [84] and the associated extraction of Al³⁺ cations followed by their precipitation as aluminum hydroxides and their transformation to transition aluminas, hardly differentiable by XRD, after a calcination step (Figure S2). The occurrence of dissolution-precipitation phenomena during the pretreatment of Al₂O₃ in warm water is supported by the changes in surface morphology of the Al₂O₃ grains observed by SEM. Figure S3 indeed shows the appearance of micrometric bayerite and Al₂O₃ platelets on the surface of the Al₂O₃ grains after pretreatment of the pristine support in warm water and further calcination, respectively.

More interestingly, it was found that the pretreatment of Al₂O₃ in warm water prior to Ag deposition led to significantly more active silver alumina catalysts (Fig. 5b,c, ■ Ag(x)/Al₂O₃-H₂O) compared to those prepared with the pristine Al₂O₃ support (Fig. 5b,c, ● Ag(x)/Al₂O₃) for Ag loadings lower than 5 and 7 wt% for C₃H₆-SCR (Fig. 5b) and H₂-C₃H₆-SCR (Fig. 5c), respectively. In addition, the optimum loading of Ag for C₃H₆-SCR (Fig. 5b, ● Ag(x)/Al₂O₃), and widely accepted in the field as 2 wt% on Al₂O₃ [16,30,36–38,42], was found to be shifted to about 3 wt% for catalysts prepared from the Al₂O₃ support pretreated in warm water prior to Ag deposition (Fig. 5b, ■ Ag(x)/Al₂O₃-H₂O). To our knowledge, such a shift in the optimum Ag loading in C₃H₆-SCR due to the pretreatment of Al₂O₃ has never been reported to date. This shift in optimum Ag loading for C₃H₆-SCR can be remarkably correlated to the characterization of the Al₂O₃ support of the Ag(x)/Al₂O₃-H₂O samples by NOₓ-TPD (Fig. 5a,b, ■ Ag(x)/Al₂O₃-H₂O, blue dotted arrow) for which optimum C₃H₆-SCR catalytic activity is found to perfectly match the Ag loading at which the NOₓ uptake starts to remain essentially constant, as also reported earlier [42] in the case of a Ag(x)/Al₂O₃ (Fig. 5a,b, ● Ag(x)/Al₂O₃, red dotted arrow). Such a unique NOₓ-TPD–C₃H₆-SCR structure–activity correlation provides further support on the usefulness of the characterization of the Al₂O₃ supporting oxide of Ag/Al₂O₃ catalysts by NOₓ-TPD and can be used as a reliable descriptor of the efficiency of the silver alumina catalysts in the field of C₃H₆-SCR. One may argue that our preliminary work on the influence of the pretreatment of Al₂O₃ in warm water prior to Ag deposition would refute such a statement, as in this previous work an improvement in H₂-C₃H₆-SCR performance only was observed on Ag(1.8)/Al₂O₃-OH [72]. Despite the difference in the duration of the pretreatment Al₂O₃ in warm water, it must be highlighted that Ag was deposited on a dried pretreated sample in this earlier work [72], whereas the samples studied in the present work were prepared on a warm-water-treated support that was subsequently dried and calcined prior to Ag deposition. Hence this difference in pretreatment prior to Ag deposition may be at the origin of such apparently conflicting results. It is also important to note that the shift in optimum Ag loading from about 2 (Ag(x)/Al₂O₃) to about 3 wt% (Ag(x)/Al₂O₃-H₂O), i.e. about 50% increase, in C₃H₆-SCR performance (Fig. 5b) and NOₓ uptake stabilization (Fig. 5a) remarkably agrees with the increase in OH species quantified by ¹H NMR (Fig. 1), i.e. about 60%. In addition, the present NMR data unprecedentedly show that Ag (Fig. 1 and Figure S13) and NOₓ (Figure S7) perturbs the same µ¹-OH species of

Al₂O₃, pretreated or not in warm water, which is fully consistent with the decrease in NOₓ uptake observed as the loading of Ag increases at low Ag loadings in both silver alumina series (Fig. 5a). Identification of the adsorption sites of NOₓ on Al₂O₃ remains challenging. In addition to the potential interaction of NOₓ species with Al³⁺ and O²⁻ surface sites [91–93], earlier FTIR studies have highlighted on the disappearance of part of the OH groups of Al₂O₃ after exposure to various NOₓ atmospheres [91,93]. Szanyi et al. also concluded that most of the consumed OH groups were not restored during TPD following the NO₂ uptake experiments [93]. These observations are consistent with the OH-assisted formation of nitrates (3 NO₂ + 2 OH⁻ = 2 NO₃ + NO + H₂O) proposed earlier by Hadjiivanov and co-workers over TiO₂ [94]. Considering all of the above, the greater NOₓ uptake of Al₂O₃-H₂O₂O compared to Al₂O₃ (Fig. 5a) can be reasonably attributed, at least partly, to an increase in the OH content of Al₂O₃-H₂O₂O compared to Al₂O₃ (Fig. 1). Finally, a remarkable correlation can also be established between the vanishing of the µ¹-OH species at about ~ 0.10 ppm that occurred at about 2 and 3 wt% Ag for ¹H NMR spectra of the Ag (x)/Al₂O₃ (Figure S13) and Ag(x)/Al₂O₃-H₂O (Fig. 1) series, respectively, and the corresponding NOₓ uptakes (Fig. 5a) and C₃H₆-SCR performance (Fig. 5b). All of the above could indicate that a key parameter in the synthesis of silver alumina catalyst to preserve Ag in a highly dispersed state [42] would lie in the amount of OH species available on the Al₂O₃ supporting oxide. Such a conclusion would be consistent with the earlier statement of Zhang and Kaliaguine [60] and Wang et al. [63] on the potential exchange process between Ag⁺ ions and some protons of Al₂O₃ during the conventional impregnation step to lead to Ag-O-Al species. This justifies the works done by several groups on the use of various Al₂O₃ precursors [60,61], on the morphology of Al₂O₃ [62–64] and on ball-milled Al₂O₃ supports [69,71] to improve HC-SCR performance. This OH key parameter could also help reconcile earlier literature data for which optimum Ag loading was unexpectedly determined to be 4 wt% [95], rather than the commonly accepted loading of 2 wt% [16,30,36–38,42], for which the used Al₂O₃ may have exhibited a much greater proportion of OH groups of interest as anchoring sites of Ag.

The present series of Ag(x)/Al₂O₃-H₂O₂O was therefore subjected to in-depth NMR investigations to help unravel the Al₂O₃ sites of importance in the anchoring of Ag.

## 4.2. Unravelling of the alumina sites of importance for anchoring Ag: a NMR-assisted study

As indicated in the NMR result section, the ¹H-¹H DQSQ MAS spectrum of Al₂O₃-H₂O₂O (Fig. 3a) was found to be identical to that reported by Merle et al. [80] and therefore used for the assignation of the protons of the present study. While various combinations of protons have been proposed in the work Merle et al. [80], their preferential location on the Al₂O₃ crystallites has not been attempted. By combining NMR measurements and DFT modelling, based on revisited models for the (100), (111), basal (110)b and lateral (110)b facets of γ-Al₂O₃, Batista et al. recently provided further insights into the location of the various protons on the Al₂O₃ crystallites [81]. In their study, Batista et al. showed the proximity of µ¹-OH species (located on the edge defined by the intercept of the basal (110)b and (100) facets, and noted as (110)b/(100) edge) and µ²-OH species (located on the basal (110)b facet near the (110)b/(100) edge), which are themselves close to µ²-OH species (located on basal (110)b facets) [81]. Given that the NMR fingerprints of the OH species identified in the present work are fairly consistent with those reported earlier by Merle et al. [80] for a comparable alumina carrier, and that Batista et al. investigated the surface location of the OH species on model alumina crystallites [81], we tentatively assigned the location of the various (A-G) hydroxyl groups (Fig. 7). We propose that µ¹-OH (110)b/(100) edge, µ²-OH on the basal (110)b facet near the (110)b/(100) edge and µ²-OH (110)b facets species could be attributed to µ¹-OH (C), µ²-OH (D) and µ²-OH (E) species,
<figure><img src="image_10.png" /><figcaption>Fig. 7. Proposed location of the A-G hydroxyl species on the model Al₂O₃ crystallite reported by Batista et al. [81].</figcaption></figure>

respectively. In addition, Batista et al. predicted the presence of two µ¹-OH species located on the (100)/(110)₁ edge. The first ones are in close proximity with identical µ¹-OH species and could be attributed to µ¹-OH (F) species. The second ones are close to µ²-OH species located on the (100) facet near the (100)/(110)₁ edge [81] and could be assigned to µ¹-OH (A) and µ²-OH (B), respectively. Finally, µ²-OH species on basal (110)b and lateral (110)₁ facets of γ-Al₂O₃ close to each other could be assigned to µ²-OH (G) species.

As already stated in the preceding section, Ag anchoring has been proposed to occur on the alumina surface via substitution of a proton from an OH group and eventually leading to the formation of Al-O-Ag species [60,63,75]. From the comparison of the NMR data of Al₂O₃-H₂O and Ag(1)/Al₂O₃-H₂O (Fig. 1, Fig. 3a, Fig. 3b), the increase in the signal of the µ¹-OH species at about 0.4 ppm, and the decrease in those of µ¹-OH at about – 0.10 ppm and µ²-OH at about 2.5 ppm may be accounted for by the grafting of Ag on a µ¹-OH group, issued from the rehydroxylation of the µ²-OH species during the Ag impregnation step of Al₂O₃-H₂O, and located near the (100)/(110)₁ edge (Fig. 8c). Such a grafting leads to the shift of µ¹-OH (A) at – 0.2 ppm and (F) at – 0.1 ppm of Al₂O₃-H₂O (Fig. 8a) to µ¹-OH (A') and (F') at about 0.4 ppm due to Ag

perturbation on Ag(1)/Al₂O₃-H₂O (Fig. 8c), and the creation of new µ¹-OH (N) species on the (100) facets at the expense of those of µ²-OH (B) at 2.5 ppm. The chemical shift of this newly formed µ¹-OH (N) species on Ag(1)/Al₂O₃-H₂O should also be of about 0.4 ppm, as the intensity of this signal increases to a much greater extent than the decrease in the µ¹-OH (A) and (F), and µ²-OH (B) signals (Fig. 1). Such an Ag grafting would also account for the increase and the narrowing of the signal corresponding to (Al⁴⁻)-µ¹-OH species (Figure S9), whereas the interference caused by the presence of silver disturb the environment of Al⁵⁻ species, leading to a narrowing of the signal at about 7 ppm in the ²⁷Al dimension of D-HMQC experiment (Figure S9). For Ag(2)/Al₂O₃-H₂O, similar Ag grafting should also applies and the essentially constant µ²-OH (B) may be attributed to an overlapping of this signal with the signal at 0.4 ppm, which increases drastically when the Ag loading increases from about 1 to about 2 wt%. The fact, on the one hand, that the signal at 0.4 ppm remains essentially constant and that the signal at – 0.15 ppm disappears at Ag loadings greater than 2 wt% and on the other hand, that the signal at about 2 ppm decreases at Ag loadings greater than or equal to 3 wt% (Fig. 1, spectra e,f) suggests an additional anchoring of Ag onto µ²-OH (D) species located on the (110)b

<figure><img src="image_11.png" /><figcaption>Fig. 8. Description of the various proton environments on γ-Al₂O₃ and schematic representation of the influence of the addition of Ag on the OH groups of γ-Al₂O₃. The dotted double arrows show nuclei that could be probed by ¹H–¹H DQSQ MAS and ¹H–²⁷Al D-HMQC MAS NMR, while simple arrows show how silver influences the signals of the neighboring protons. The colors of the dotted boxes correspond to those used for the edges and planes of the Al₂O₃ crystallite shown in Fig. 7.</figcaption></figure>
facets and close the (110)$_b$/(100) edge (Fig. 8d) and a shift of the µ¹-OH (C) species at – 0.15 ppm to 0.4 ppm (µ¹-OH (C’)). The presence of Ag atoms in the proximity of (Al$^{IV}$)-µ¹-OH (A’, C’, F’ and N) species may account for by the appearance of the new Al$^{IV}$ signal at about 70 ppm (Figure S9).

In summary, the present in depth NMR study allowed us to conclude to the grafting of Ag (i) preferentially on µ¹-OH groups of Al₂O₃-H₂O, in agreement with the earlier studies [63,75], up to a loading of about 3 wt % Ag that would be located near the (100)/(110)$_i$ edge and then near the (110)$_b$/(100) edge of Al₂O₃ crystallites and (ii) on µ²-OH groups of Al₂O₃-H₂O at higher Ag loadings (Fig. 8).

The FTIR band at 3730 cm⁻¹ has been commonly assigned in earlier literature reports to the vibration of µ²-OH groups [63,80], and references therein. The observed increase in intensity of this band on the addition of Ag on Al₂O₃-H₂O (Figure S4b), together with the fact that the ¹H signal at 0.4 ppm, -unambiguously assigned to µ¹-OH species-, was found to be the only one to increase in ¹H spectra (Fig. 1) therefore question the original FTIR assignation of the band at 3730 cm⁻¹ as µ²-OH vibrators.

The finite number of µ¹-OH Ag anchoring species on the edges of Al₂O₃ may therefore be at the origin of the limited amount of Ag able to be accommodated on Al₂O₃ in a highly dispersed state to form the catalytic Ag species and/or to act their precursor in the (H₂)-C₃H₆-SCR reactions. This finite number of µ¹-OH Ag anchoring species on the edges of Al₂O₃ would be at the origin of the well-known existence of an optimum loading of Ag on Al₂O₃ for C₃H₆-SCR [42]. Li et al. recently took advantage of such a peculiarity to lower the amount of Ag introduced on Al₂O₃ by poisoning its Ag anchoring sites by copper prior to introduction of Ag [96]. This original approach was reported to provide Ag-low-loaded Ag-CuOx/Al₂O₃ catalysts with enhanced activity and selectivity for the catalytic oxidation of ammonia compared to their Ag/Al₂O₃ counterparts.

## 5. Conclusion

The present work shows that pretreatment of pristine Al₂O₃ in warm water prior to Ag deposition allows to prepare catalysts of significantly higher performance in (H₂)-C₃H₆-SCR compared to conventional Ag/Al₂O₃ catalysts. In addition, remarkable NOₓ-TPD-C₃H₆-SCR structure-activity correlations indicate that optimum composition of the Ag/Al₂O₃ catalysts can be improved by about 50% in terms of Ag content for the first time, leading to an optimum composition of about 3 wt% Ag on Al₂O₃-H₂O compared to about 2 wt% on pristine Al₂O₃. Combining ¹H-¹H DQSQ MAS and ¹H-²⁷Al D-HMQC MAS NMR in-depth unprecedented NMR investigations on Ag-Al₂O₃ samples and a thorough literature survey of most recent investigations performed on Al₂O₃ in this field, allowed us to identify the alumina sites of importance in the anchoring of Ag. These sites are proposed to be µ¹-OH groups located near the (100)/(110)$_i$ edge and then near the (110)$_b$/(100) edge of Al₂O₃ crystallites and bonded to octahedrally coordinated Al species. This study, the first of its kind, paves the way to further improvements in the comprehensive understanding of Al₂O₃ and in particular the deposition of Ag on Al₂O₃, which may be broaden to the deposition of other transition metals on such a widely used supporting oxide.

### CRediT authorship contribution statement

**Yannick Millot:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Data curation, **Clémence Rodigue:** Investigation, Data curation. **Thomas Onfroy:** Writing – review & editing, Investigation, Data curation. **Cyril Thomas:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Investigation, Data curation, Conceptualization.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data Availability

Data will be made available on request.

### Acknowledgments

The authors thank Sorbonne Université and CNRS for providing funding (Crédits récurrents) and access to the analytical equipment used in the present work. The authors also acknowledge S. Koné-Guira, Dr. J. Schnee and S. Casale for recording the N₂ sorption isotherms on about half of the samples listed in Table 1, the XRD patterns of the vacuum-dried and calcined Al₂O₃-H₂O samples (Figure S2), and the SEM images (Figure S3), respectively. The SEM microscope and NMR spectrometer used in the present work belonging to the Fédération de Chimie et Matériaux de Paris-Centre (FCMat, Fédération de Recherche 2482) were funded by UPMC, CNRS and Région Ile de France.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2024.123975.

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Supplementary Information

Deciphering the Improvement in C₃H₆-SCR
Performance Promoted by H₂ or not of
Ag/Al₂O₃ Catalysts prepared from Warm-Water-
Treated Alumina: a NMR-Assisted
Identification of the Ag Anchoring Sites of the
Alumina Support

Yannick Millot<sup>a,*</sup>, Guylène Costentin<sup>a</sup>, Clémence Rodigue<sup>a,b</sup>, Thomas Onfroy<sup>a</sup> and
Cyril Thomas<sup>a,*</sup>

Cyril Thomas<sup>a,*</sup>

<footnote><sup>a</sup>Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, F-75005 Paris, France.</footnote>

<footnote><sup>b</sup>CESI, 93 boulevard de la Seine, 92000 Nanterre, France.</footnote>

<footnote>* To whom correspondence should be addressed:</footnote>

<footnote>Drs. Cyril Thomas and Yannick Millot</footnote>

<footnote>Sorbonne Université, CNRS, Laboratoire Réactivité de Surface, LRS, UMR CNRS 7197, 4 Place</footnote>

<footnote>Jussieu, Tour 43-53, 3ème étage, Case 178, F-75252, Paris, France</footnote>

<footnote>e-mails: cyril.thomas@sorbonne-universite.fr, yannick.millot@sorbonne-universite.fr</footnote>

<footnote>Tel: + 33 1 44 27 36 30/Fax: + 33 1 44 27 60 33</footnote>
<figure><img src="image_12.png" /><figcaption>**Figure S1.** Schematic description of the synthesis of the Ag(x)/Al₂O₃ and Ag(x)/Al₂O₃-H₂O series. The green arrow in the synthesis route using warm-water-pretreated Al₂O₃ (Al₂O₃-H₂O) refers to as the same procedure as that used for Ag deposition on the pristine Al₂O₃ support (γ-Al₂O₃).</figcaption></figure>
<figure><img src="image_13.png" /></figure>

**Figure S2.** XRD patterns of Al₂O₃-H₂O (treated in water at 80 °C for 66 h) after vacuum drying at RT (green) and calcination at 600 °C for 4 h (blue). γ-Al₂O₃ (JCPDS 79-1558), Bayerite (● JCPDS 20-11) and Gibbsite (▲ JCPDS 33-18).
<figure><img src="image_14.png" /><figcaption>**Figure S3.** SEM-FEG images at various magnifications of (a-c) the pristine Al₂O₃ support (Al₂O₃), (d-f) the dried warm-water-pretreated Al₂O₃ support and (g-i) the calcined warm-water-pretreated Al₂O₃ support (Al₂O₃-H₂O).</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S4.** Absorbance FTIR spectra normalized on a 10 mg/cm² basis of (a) Al₂O₃ and Al₂O₃-H₂O and (b) Ag(x)/Al₂O₃-H₂O samples.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure S5. ¹H-¹H DQSQ MAS NMR spectrum of Ag(2)/Al₂O₃-H₂O.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure S6.** Stacking of the ¹H MAS NMR spectra of the Ag(x)/Al₂O₃-H₂O series of samples shown in **Figure 1** in the -1.0 - +1.0 ppm region.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure S7.** Influence of the exposure of Al₂O₃-H₂O and Ag(1)/Al₂O₃-H₂O to NOₓ at RT for 4 h on the ¹H MAS NMR signals.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Figure S8. ¹H-²⁷Al D-HMQC MAS NMR spectrum for Ag(1)/Al₂O₃-H₂O.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S9.** Sum of the spectra of the <sup>27</sup>Al dimension of the 2D D-HMQC spectrum between (a) 0.5 and 0.5 ppm and (b) 1 and 3 ppm.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>Figure S10. ¹H-²⁷Al D-HMQC MAS NMR spectrum of Ag(2)/Al₂O₃-H₂O.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>Figure S11. ¹H-²⁷Al D-HMQC MAS NMR spectrum of Ag(3)/Al₂O₃-H₂O.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>Figure S12. ¹H-²⁷Al D-HMQC MAS NMR spectrum of Ag(5)/Al₂O₃-H₂O.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure S13.** ¹H MAS NMR spectra of Al₂O₃, Ag(1)/Al₂O₃, Ag(2)/Al₂O₃ and Ag(3)/Al₂O₃.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>Figure S14. ¹H-²⁷Al D-HMQC MAS NMR spectrum of Al₂O₃.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>Figure S15. ¹H-²⁷Al D-HMQC MAS NMR spectrum of Ag(1)/Al₂O₃.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Figure S16.** (a) NO-(gray), NO₂-(orange) and NOₓ-TPD (blue) profiles recorded on Al₂O₃-H₂O, (b) NOₓ-TPD profiles recorded on Al₂O₃-H₂O (blue), Ag(1)/Al₂O₃-H₂O (orange), Ag(2)/Al₂O₃-H₂O (gray), Ag(3)/Al₂O₃-H₂O (yellow), Ag(5)/Al₂O₃-H₂O (red), Ag(7)/Al₂O₃-H₂O (green) and Ag(9)/Al₂O₃-H₂O (purple), and (c) NO-(dark gray), NO₂-(light gray) and NOₓ-TPD (black) profiles recorded on Ag(4)/Al₂O₃, after calcination at 500 °C for 2 h under 20% O₂/He (100 mL_NTP/min), flushing at RT for 1 h at RT under 8 % O₂/He (230 mL_NTP/min) and temperature-programmed desorption with a heating rate of 3 °C/min under 8 % O₂/He (230 mL_NTP/min).</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S17.** Comparison of C<sub>3</sub>H<sub>6</sub>-SCR (a) and H<sub>2</sub>-C<sub>3</sub>H<sub>6</sub>-SCR (b) catalytic performance of Ag(2)/Al<sub>2</sub>O<sub>3</sub> (red traces) and Ag(2)/Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O (blue traces). Feed compositions: 0 or 0.21% H<sub>2</sub>, 385 ppm NO<sub>x</sub>, 400 ppm C<sub>3</sub>H<sub>6</sub>, 8% O<sub>2</sub> and He balance with a 230 mL<sub>NTP</sub>/min flowrate.</figcaption></figure>