# Insights into the influence of the Ag loading on Al2O3 in the H2-assisted C3H6-SCR of NOx

Tesnim Chaieb, Laurent Delannoy, Guylène Costentin, Catherine Louis, Sandra Casale, Ruth L. Chantry, Z.Y. Li, Cyril Thomas

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Tesnim Chaieb, Laurent Delannoy, Guylène Costentin, Catherine Louis, Sandra Casale, et al. Insights into the influence of the Ag loading on Al2O3 in the H2-assisted C3H6-SCR of NOx. Applied Catalysis B: Environmental, 2014, 156-157, pp.192-201. 10.1016/j.apcatb.2014.03.025. hal-00979536

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# Insights into the influence of the Ag loading on Al₂O₃ in the H₂-assisted C₃H₆-SCR of NOₓ

<figure><img src="image_3.png" /></figure>

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Tesnim Chaieb<sup>a,b</sup>, Laurent Delannoy<sup>a,b</sup>, Guylène Costentin<sup>a,b</sup>, Catherine Louis<sup>a,b</sup>, Sandra Casale<sup>a,b</sup>, Ruth L. Chantry<sup>c</sup>, Z.Y. Li<sup>c</sup>, Cyril Thomas<sup>a,b,*</sup>

## ARTICLE INFO

**Article history:**

Received 8 January 2014

Received in revised form 7 March 2014

Accepted 11 March 2014

Available online 20 March 2014

**Keywords:**

Ag/Al₂O₃

H₂ effect

Selective catalytic reduction

Kinetics

Compensation phenomenon

## ABSTRACT

The addition of H₂ has been reported to promote drastically the selective catalytic reduction of NOₓ by hydrocarbons (HC-SCR). Yet, the influence of the Ag loading on the H₂-promoted HC-SCR has been the subject of a very limited number of investigations. The H₂-HC-SCR earlier studies reported mostly on Ag/Al₂O₃ samples containing about 2 wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H₂. The present study highlights for the first time that the H₂-C₃H₆-SCR catalytic performances of Ag/Al₂O₃ samples improved in the 150–550 °C temperature domain as the Ag loading (Ag surface density: x (Ag/nm²Al₂O₃)) decreased well below 2 wt%. A detailed kinetic study of H₂-C₃H₆-SCR was performed in which the reaction orders in NO, C₃H₆ and H₂, and the apparent activation energies were determined for the reduction of NOₓ to N₂ on a Ag(x)/Al₂O₃ catalysts series, for which Ag was found to be in a highly dispersed state by TEM and HAADF-STEM. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to 0.7 Ag/nm²Al₂O₃ (Ag loading of 2.2 wt%) coinciding with the changes observed earlier in the NOₓ uptakes of the Al₂O₃ supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H₂-C₃H₆-SCR reaction proceeds via the activation of H₂ and C₃H₆ on Ag species and their further reaction with NOₓ adspecies activated on the Al₂O₃ support. The unexpected higher catalytic performances of the Ag samples with the lower Ag surface densities was attributed to the higher concentration of active sites on the Al₂O₃ supporting oxide able to chemisorb NOₓ species, in agreement with the NOₓ uptake data. The kinetic data obtained for Ag surface densities lower than 0.7 Ag/nm²Al₂O₃ also suggest that the interaction between NOₓ and C₃H₆ adspecies would be rate determining in the C₃H₆-SCR process.

© 2014 Elsevier B.V. All rights reserved.

## 1. Introduction

Since the late 1970s, the preservation of the air quality has become one of the major concerns for the OECD (Organization for Economic Co-operation and Development) countries in order to minimize the impact of air pollutants on environment and health. In Europe in particular, the Euro 6 standards, expected to enter into force in January 2014, will mainly further reduce the NOₓ (NO + NO₂) emissions from diesel automotives, from 180 to

80 mg/km [1]. To meet these ever more stringent standards, further improvements in the efficiency of the catalytic converters are required.

As emphasized earlier [2], the catalytic reduction of NOₓ to N₂ in a strongly oxidizing medium is not trivial. The selective catalytic reduction of NOₓ by the hydrocarbons (HC-SCR) would be an elegant alternative to the selective catalytic reduction of NOₓ by NH₃ (NH₃-SCR) and the lean NOₓ trap (LNT) technologies, which both exhibit intrinsic shortcomings such as the NH₃ slip and CO₂ penalties, respectively. Among the catalytic formulations evaluated to date, Ag/Al₂O₃ has been reported to be the most promising catalyst in the pioneering work by Miyadera [3]. In this study, it was shown that NOₓ could be efficiently and selectively reduced to N₂ by various hydrocarbons in the presence of water and in a large excess of O₂. Yet the use of such Ag/Al₂O₃ catalysts is limited because of

<footnote><sup>a</sup> Sorbonne Universités, UPMC Univ Paris 06, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, F-75005 Paris, France</footnote>

<footnote><sup>b</sup> CNRS, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, F-75005 Paris, France</footnote>

<footnote><sup>c</sup> Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom</footnote>

<footnote>* Corresponding author at: Sorbonne Universités, UPMC Univ Paris 06, UMR 7197, Laboratoire de Réactivité de Surface, 4 Place Jussieu, Case 178, F-75005 Paris, France. Tel.: +33 1 44 27 36 30; fax: +33 1 44 27 60 33.</footnote>

<footnote>E-mail address: cyril.thomas@upmc.fr (C. Thomas).</footnote>
the restricted operating temperature window (300–500 °C) within which these materials efficiently catalyze the HC-SCR reaction [2,3], as exhausts temperatures as low as 150 °C are typically encountered in the catalytic converters of diesel cars [4]. Improved HC-SCR efficiencies at the lower temperatures have been obtained with higher hydrocarbons [5,6] or ethanol [7], but, despite the observed broadening of the operating temperature window with the higher hydrocarbons, the Ag/Al₂O₃ catalysts do not meet the SCR efficiencies needed at the lower temperatures yet [8].

The discovery of a low-temperature promoting effect of H₂ on the HC-SCR of NOₓ by Satokawa et al. for C₁–C₄ hydrocarbons [9,10], which was later confirmed for higher hydrocarbons [2,11,12], is undoubtedly a major breakthrough for alumina-supported silver catalysts. Interestingly, the H₂ effect has been shown to be specific to Ag/ZSM-5 [13] and Ag/Al₂O₃ [10], as Ag/SiO₂, Ag/TiO₂ and Ag/ZrO₂ were found to be inactive in the presence of H₂ in the C₃H₈-SCR feed [10]. This prompted the importance of aluminum species and the potential contribution of Al₂O₃ in the HC-SCR process as outlined by several groups [14,15].

Recently, the characterization of the Al₂O₃ supporting oxide of a series of Ag/Al₂O₃ samples, via a newly-developed characterization method: namely the temperature-programmed desorption of NOₓ [16,17], allowed us to provide further insights into the origin of the optimum loading of Ag on Al₂O₃ for the C₃H₆-SCR of NOₓ [18], reported to be about 2 wt% in most studies [3,19–24]. We came to the conclusion that this particular Ag loading resulted from the maximum loading of silver per unit surface area of Al₂O₃ (Ag surface density concept) for which Ag₂O clusters remain highly dispersed on freshly calcined samples [18]. To our knowledge, the influence of the Ag loading on the H₂-promoted HC-SCR of NOₓ has been the subject of a very limited number of investigations [24,25], most of the works in this field reporting on Ag/Al₂O₃ samples with a nominal Ag loading close to 2 wt% [2,6]. Sadokhina et al. [24] and Shimizu et al. [25] concluded to an optimum Ag loading of 2 wt% in the H₂-C₆H₁₄-SCR and H₂-C₃H₈-SCR reactions in the presence of 0.1% and 0.5% H₂, respectively. In these works the aliquots of catalysts evaluated in the H₂-HC-SCR reactions were either kept constant [24] or varied to maintain conversions below 30% [25] so that the amounts of Ag changed in the experiments performed. This brings about additional complexity in the interpretation of the catalytic data, especially when those are expressed as NOₓ conversions [24] or rates of NO reduction [25] per g of Ag/Al₂O₃ sample. Additionally, Shimizu et al. reported on the NO reduction rates as a function of the Ag loading of the Ag/Al₂O₃ catalysts at a given temperature (300 °C) [25], whereas Sadokhina et al. showed that the catalytic activity of the investigated Ag/Al₂O₃ samples was influenced significantly by the reaction temperature and preferred to provide comparison of the catalysts over a wide range of reaction temperatures (100–550 °C) [24].

The aim of the present work is to gain further understanding on the influence of the Ag loading of Ag/Al₂O₃ samples in the H₂-assisted C₃H₆-SCR of NOₓ (H₂-C₃H₆-SCR). For this purpose, and as done in our earlier study on the C₃H₆-SCR of NOₓ [18], the amount of Ag in the aliquots of samples evaluated in H₂-C₃H₆-SCR was kept constant via dilution of the Ag/Al₂O₃ catalysts with the bare Al₂O₃ oxide. This study also reports on original findings in the influence of Ag loading on the kinetics of H₂-C₃H₆-SCR that help explaining the observed catalytic trend in the H₂-promoted C₃H₆-SCR reaction.

## 2. Experimental

### 2.1. Catalyst synthesis and characterization

The series of Ag/Al₂O₃ samples investigated in the catalysis study corresponds to that used and characterized earlier in the

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** NOₓ uptakes as a function of the Ag surface density/Ag loading: (●) newly synthesized Ag/Al₂O₃ samples and (○) data extracted from [18].</figcaption></figure>

C₃H₆-SCR study [18]. The γ-Al₂O₃ support (Procatalyse, 180 m²/g) was ground and sieved, and the fraction between 0.200 and 0.315 mm was used to prepare the Ag-promoted samples. The deposition of Ag was performed by incipient wetness impregnation of the bare Al₂O₃ support (0.7 cm³/g porous volume) with aqueous solutions of AgNO₃ (Aldrich, >99%) to achieve silver loadings varying from 0.5 to 4.3 wt%, which were ascertained by inducively coupled plasma atomic emission spectroscopy (ICP-AES, CNRS-Solaize). After impregnation, the Ag-loaded samples were aged for 4 h under ambient conditions and subsequently dried at 100 °C overnight. Finally, the Ag-loaded samples were calcined at 600 °C (3 °C/min) for 4 h in a muffle furnace. From here on, the samples will be denoted as Ag(x)/Al₂O₃, where x represents the Ag surface density expressed as the number of Ag atoms per nm² of support (Ag/nm²) [18]. The Ag loadings of Ag(0.0)/, Ag(0.1)/, Ag(0.3)/, Ag(0.4)/, Ag(0.6)/, Ag(0.7)/, Ag(0.8)/, Ag(0.9)/, Ag(1.1)/and Ag(1.3)/Al₂O₃ amounted to 0.0, 0.5, 0.9, 1.3, 1.8, 2.2, 2.6, 3.1, 3.5 and 4.3 wt%, respectively [18].

Ag(0.6)/, Ag(0.7)/, Ag(0.8)/Ag(0.9)/and Ag(1.1)/Al₂O₃ samples were newly prepared according to the above-mentioned procedure to be investigated by electron microscopy techniques after being characterized by the NOₓ-TPD method described in Refs. [16–18]. The Ag loading and the BET surface area of the newly-prepared samples were measured using an X-ray fluorescence (XRF) spectrometer XEPOS HE (AMETEK) and a Belsorp max (Bell Japan) equipments, respectively. The characterization of these newly-prepared samples by the NOₓ-TPD method was in excellent agreement with that reported earlier [18] within the limits of accuracy of the technique (Fig. 1), thus attesting for the reproducibility of the preparation method. These particular silver loadings were selected on the basis of the earlier NOₓ-TPD results which suggested that Ag remained in an optimum dispersed state on freshly calcined samples up to an Ag surface density of about 0.7 Ag/nm² [18].

Bright field TEM and energy dispersive X-ray spectroscopy (EDS) (PGT detector) characterization of Ag(0.6)/ and Ag(1.1)/Al₂O₃ was performed using a JEOL 2010 microscope operating at 200 kV equipped with an Orius CCD camera (Gatan). Aberration-corrected scanning transmission electron microscopy (STEM) imaging was carried out at the University of Birmingham using a 200 kV JEOL 2100F microscope, fitted with a high angle annular dark field (HAADF) detector and a Bruker EDS detector. For the purposes of imaging the samples were dropped, in dry powder form, onto amorphous carbon-coated copper TEM grids. For comparison purposes, the Ag(0.7)/Al₂O₃ sample was also deposited on a TEM grid after being dispersed in ethanol.

### 2.2. H₂-C₃H₆-SCR runs

The steady state catalytic H₂-C₃H₆-SCR experiments were carried out in a U-type quartz reactor (12 mm i.d.). It is important
to note that unless specified otherwise and in contrast with most studies published to date, the amount of silver introduced in the catalyst beds remained essentially constant. The samples were held on plugs of quartz wool and consisted in 0.38 g of mechanical mixtures of Ag(x)/Al₂O₃ and Al₂O₃ of the same grain sizes in which the amount of Ag was equal to 30.9 ± 1.2 µmol. The temperature of the tubular furnace was set by a Eurotherm 2408 temperature controller using a K type thermocouple. Prior to the H₂-assisted 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 mLₙₘₚ/min. After cooling down to 150 °C, the samples were submitted to a C₃H₆-SCR experiment from 150 to 550 °C [18]. The samples were subsequently exposed to the H₂-C₃H₆-SCR feed at 150 °C. 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). Typically, the composition of the H₂-NOₓ-C₃H₆-O₂-He feed was: 0.21% H₂, 385 ppm NOₓ (~96% NO), 400 ppm C₃H₆ and 8% O₂ in He, and the total flow rate was 230 mLₙₘₚ/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 5A molecular sieve column (80 °C, 150 kPa He. 200 ms injection time, 30 s backflush time), was used to separate H₂, N₂, O₂ and CO. 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-HT) allowed the simultaneous detection of both NO and NO₂. NOₓ conversions to N₂ and N₂O were calculated as follows:

$$X_{\text{NO}_x \text{ to } N_2}(\%) = \left( \frac{2 \times [N_2]}{[\text{NO}_x]_{\text{inlet}}} \right) \times 100 \quad (1)$$

$$X_{\text{NO}_x \text{ to } N_2 \text{O}}(\%) = \left( \frac{2 \times [N_2 \text{O}]}{[\text{NO}_x]_{\text{inlet}}} \right) \times 100 \quad (2)$$

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. C₃H₆ conversions were calculated on the basis of the COₓ (CO + CO₂) products formed:

$$X_{\text{C}_3\text{H}_6}(\%) = \left( \frac{[\text{CO}] + [\text{CO}_2]}{[\text{C}_3\text{H}_6]_{\text{inlet}}} \right) \times 100 \quad (3)$$

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 performances 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).

## 2.3. Kinetic measurements

Prior to the kinetic measurements, 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 mLₙₘₚ/min and the temperature was cooled down to 325 °C. In the H₂-C₃H₆-SCR kinetic investigations, the amount of Ag(x)/Al₂O₃ catalysts diluted in Al₂O₃, to obtain catalyst beds of 0.38 g of the corresponding mechanical mixtures, varied from 1 to 5 mg to maintain the conversions of NOₓ, C₃H₆ and H₂ below 13%, 16% and 30%, respectively. For comparison purposes, C₃H₆-SCR kinetics was also studied on a mechanical mixture of 0.15 g of Ag(0.7)/Al₂O₃ (2.2 wt% Ag) and 0.23 g of Al₂O₃ at 325 and 375 °C. In this case, the conversions of both NOₓ and C₃H₆ varied from 5 to about 30% when the temperature was increased from 325 to 375 °C. At 325 °C, the

conversions of NOₓ and C₃H₆ on 0.38 g of the bare Al₂O₃ support in the C₃H₆-SCR and H₂-C₃H₆-SCR reactions were found to be about 3% and 2%, respectively. After being pretreated, the samples were contacted with the standard reacting mixture (0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆ and 8% O₂ in He) used in the H₂-C₃H₆-SCR runs (Section 2.2) for 1–2 h at 325 °C. After reaching steady-state, the determination of the kinetic parameters (α, β, γ and Eₐ) was performed on the basis of the following power rate law equation:

$$r_{N_2} = k[\text{NO}]^\alpha [\text{C}_3\text{H}_6]^\beta [\text{H}_2]^\gamma = A \exp(-E_a/RT)[\text{NO}]^\alpha [\text{C}_3\text{H}_6]^\beta [\text{H}_2]^\gamma \quad (4)$$

where rN₂, k, [NO], [C₃H₆], [H₂], α, β, γ, A, Eₐ, R and T are the rate of reduction of NOₓ to N₂ (mol/s g), the kinetic constant, the inlet concentrations in NO, C₃H₆ and H₂, the reaction orders with respect to NO, C₃H₆ and H₂, the pre-exponential factor, the apparent activation energy (J/mol), the gas constant (8.314 J/mol K) and the reaction temperature (K), respectively.

The determination of the reaction orders with respect to NO, C₃H₆ and H₂ was performed at 325 °C by varying the concentration of one of these reactants while the concentrations of the others were kept constant. The following ranges of concentrations were used: 200–700 ppm NO, 200–700 ppm C₃H₆ and 1400–2400 ppm H₂. Note that the influence of O₂ was not investigated and its concentration was maintained to 8%. The reaction orders were determined from the slope of the straight lines obtained by plotting the logarithm of the rate of N₂ production as a function of the logarithm of the concentration of the investigated reactant. The rate of N₂ production (Eq. (5)) was proportional to the conversion of NOₓ to N₂ (Eq. (1)) and calculated as follows:

$$r_{N_2} = F_{\text{NO}_x} X_{\text{NO}_x \text{ to } N_2} / W \quad (5)$$

where FNOₓ, XNOₓ to N₂ and W are the NOₓ molar flow rate (mol/s), the conversion of NOₓ to N₂ (Eq. (1)) and the catalyst loading (g), respectively. It was also verified that the N₂ reaction rates obtained initially under the standard conditions (0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆ and 8% O₂ in He) were not affected by the changes in the concentrations in the reacting feeds used for the determination of the different reaction orders.

Apparent activation energies were estimated from the slope of the straight lines obtained in the Arrhenius-type plots under the standard feed (0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆ and 8% O₂ in He) at reaction temperatures in the 325–375 °C range with increments of 10 °C.

The presence of external and internal diffusion limitations was verified according to the criteria defined by Koros–Nowak [26]. External diffusion limitations were checked by changing the amount of Ag(0.7)/Al₂O₃ introduced in the reactor while keeping a constant flow rate to catalyst loading ratio. The H₂-C₃H₆-SCR data obtained under the standard conditions (0.15 g Ag(0.7)/Al₂O₃ + 0.23 g Al₂O₃ and a total flow rate of 230 mLₙₘₚ/min) were compared to those obtained for an H₂-C₃H₆-SCR reaction in which the amounts Ag(0.7)/Al₂O₃ and Al₂O₃, and the flow rate were half of those of the standard conditions (0.08 g Ag(0.7)/Al₂O₃ + 0.11 g Al₂O₃ and a total flow rate of 115 mLₙₘₚ/min) (Fig. 2a). Internal diffusion limitations were investigated by changing the grain size of a Ag(0.8)/Al₂O₃ catalyst from 50–125 to 200–315 µm (Fig. 2b), while keeping a constant flow rate to catalyst loading ratio (0.12 g Ag(0.8)/Al₂O₃ + 0.26 g Al₂O₃ and a total flow rate of 230 mLₙₘₚ/min). The NOₓ and C₃H₆ conversions plotted in Fig. 2 indicate the absence of both external and internal diffusion limitations under the present experimental conditions. Comparable experiments carried out for the C₃H₆-SCR reaction also suggested the absence of diffusion limitations in the absence of H₂ in the reacting feed (not shown).
<figure><img src="image_5.png" /></figure>

**Fig. 2.** Examination of the external (a) and internal (b) diffusion limitations in the reduction of NOₓ to N₂ (gray curves) and the oxidation of C₃H₆ to COₓ (black curves) in the H₂-C₃H₆-SCR reaction. Feed composition: 0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆, 8% O₂ and He balance. Mechanical mixtures of (a) 0.15 g Ag(0.7)/Al₂O₃ + 0.23 g Al₂O₃ and 230 mL NTP/min (–), and 0.08 g Ag(0.7)/Al₂O₃ + 0.11 g Al₂O₃ and 115 mL NTP/min (---), and (b) 0.12 g Ag(0.8)/Al₂O₃ + 0.26 g Al₂O₃ and 230 mL NTP/min with 50–125 and 125–200 (–), and 200–315 µm (---) particle sizes.

### 3. Results and discussion

#### 3.1. TEM and HAADF-STEM characterization of the post-NOₓ-TPD samples

A thorough TEM examination of Ag(0.6)/Al₂O₃ and Ag(1.1)/Al₂O₃, previously characterized by NOₓ-TPD, could not reveal unambiguously the presence of Ag and/or Ag₂O clusters on these samples, although the presence of Ag was confirmed in various areas on both samples by means of EDS analyses (not shown). As the poor contrast of Ag and/or Ag₂O clusters on Al₂O₃ may account for the difficulty in their observation, further characterization of the samples exhibiting Ag surface densities ranging from 0.6 to 1.1 Ag/nm² was performed by HAADF-STEM. As in the case of the TEM investigation, it was found to be extremely difficult to observe any Ag and/or Ag₂O clusters on all samples with HAADF-STEM. Ag and/or Ag₂O clusters of about 1 nm could thereby be seen on only one area of the Ag(0.6)/Al₂O₃ sample (examples of clusters are highlighted in Fig. 3a). Interestingly when the Ag(0.7)/Al₂O₃ sample was deposited onto the TEM grid from an ethanol suspension, large Ag particles (>10 nm) could be clearly seen on all areas investigated (Fig. 3b and c), whereas no comparable larger particles were observed on the samples deposited in a dry powder form. This observation is consistent with earlier studies of Sayah et al. [27] in which it was concluded that the use of ethanol to disperse Ag/Al₂O₃ samples on TEM grids should be avoided, as doing such leads to the reduction of highly dispersed oxidized Ag species by ethanol and the corresponding formation of large Ag particles which are found to be not representative of the state of Ag in the investigated samples. The larger size of these particles allowed EDS characterization, a typical example of which is shown in Fig. 3c.

To summarize, the characterization of our Ag/Al₂O₃ samples by electronic microscopy techniques shows that Ag is present in a highly dispersed state on Al₂O₃. Such a conclusion is consistent with earlier XAS (X-ray absorption spectroscopy) studies in which

<figure><img src="image_6.png" /></figure>

**Fig. 3.** HAADF-STEM images of post-NOₓ-TPD samples: (a) Ag(0.6)/Al₂O₃ deposited on the TEM grid in dry powder form (with some example clusters ringed) and (b) Ag(0.7)/Al₂O₃ deposited on the TEM grid with an ethanol suspension. (c) EDS elemental map overlaid onto (b, dotted square) in which the Al and Ag signals are shown respectively in red and green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

it was shown that Ag was almost atomically dispersed after calcination when supported on Al₂O₃ [25,28].

#### 3.2. Catalytic performances

The influence of the addition of H₂ on the C₃H₆-SCR catalytic performances of the Ag(x)/Al₂O₃ catalysts is illustrated in Fig. 4. In agreement with earlier data of Zhang et al. [29], the addition of
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Influence of the addition of 0.21% H₂ on the conversions of NOₓ to N₂ (—) and C₃H₆ to COₓ (---) (C₃H₆-SCR (▲, △) and H₂-C₃H₆-SCR (●, ○) reactions) on (a) Ag(0.3)/Al₂O₃, (b) Ag(0.4)/Al₂O₃, (c) Ag(0.6)/Al₂O₃, (d) Ag(0.7)/Al₂O₃, (e) Ag(0.8)/Al₂O₃ and (f) Ag(1.1)/Al₂O₃ for mechanical mixtures of Ag(x)/Al₂O₃ catalysts and Al₂O₃ for which the amount of Ag was kept essentially constant to 30.9 ± 1.2 µmol. Feed compositions: 0% or 0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆, 8% O₂ and He balance with a 230 mL NTP/min flow rate. The concentration of H₂ in the course of the H₂-C₃H₆-SCR reaction is also shown (—).</figcaption></figure>

H₂ drastically promoted the C₃H₆-SCR reaction for temperatures lower than 400 °C. Overall, the addition of 0.21% H₂ in the feed shifted the NOₓ to N₂ and C₃H₆ to COₓ conversions to temperatures approximately 150 °C lower than those obtained in the C₃H₆-SCR reaction. On Ag(0.7)/Al₂O₃ (Fig. 4d), the conversions of NOₓ and C₃H₆ increased steeply from 150 to 225 °C in the presence of H₂ in the feed, while this sample hardly catalyzed the C₃H₆-SCR reaction from 150 to 300 °C. For temperatures increasing from 225 to 400 °C, the conversions of NOₓ and C₃H₆ also increased in the presence of H₂ but to a much more limited extent than from 150 to 225 °C. The pseudo plateau observed in the conversions of NOₓ and C₃H₆ in the 225–400 °C region corresponded to a temperature domain for which H₂ was fully consumed (Fig. 4d). For temperatures higher than 400 °C, the conversions of NOₓ and C₃H₆ were no longer significantly influenced by the presence of H₂ in the feed. Comparable comments can be made on the other investigated samples (Fig. 4). On Ag(1.1)/Al₂O₃, however, the catalytic performances were found to be higher in C₃H₆-SCR compared to those in H₂-C₃H₆-SCR from 400 to 500 °C (Fig. 4f).

The catalytic performances of the Ag(x)/Al₂O₃ samples in the H₂-C₃H₆-SCR of NOₓ are compared in Fig. 5. It can be seen that the NOₓ reduction temperature window broadened to lower temperatures as the Ag surface density (Ag loading) increased (Fig. 5a). In parallel, the maximum in N₂O conversion and the conversion of C₃H₆ shifted to lower temperatures as the Ag surface density increased (Fig. 5b and c). The broadening of the NOₓ reduction activity to lower temperatures occurred, however, at the expense of the NOₓ conversions at the higher temperatures (Fig. 5a). This illustrates how complex is the comparison of

the catalytic performances of Ag/Al₂O₃ catalysts with various Ag surface densities (Ag loadings) over a broad temperature domain (150–550 °C).

Fig. 6a shows that plotting the NOₓ conversions as a function of the Ag surface density at a given reaction temperature does not allow for an easy comparison of the samples. Indeed, it can be seen in this figure that the Ag surface density providing optimum conversion of NOₓ to N₂ in the H₂-C₃H₆-SCR reaction decreases from 0.8 to 0.3 Ag/nm²Al₂O₃ as the temperature increases from 200 to 300 °C. This may be attributed to differences in the conversions of C₃H₆ and H₂, in particular, and to the fact that these conversions varied significantly on the samples investigated (Fig. 4). The comparison of the SCR performances of a series of catalysts at a given reaction temperature should be made ideally with the same level of conversions for all reactants. Note that this is particularly challenging when two reactants of very different reactivity are used, such as H₂ and hydrocarbons in the H₂-HC-SCR reaction. In a H₂-C₃H₈-SCR study performed at 300 °C, Shimizu et al. [25] suggested the existence of an optimum loading of Ag at about 2 wt%. Despite the fact that these authors reported that the corresponding H₂-C₃H₈-SCR reaction rates were measured under conditions where conversions were below 30%, it is uncertain that the term "conversions" also included that of H₂. It must be emphasized that most of the literature data in the H₂-HC-SCR field usually do not report on the conversion of H₂. In agreement with the data shown in Fig. 4, Richter et al., who were among the very few authors to provide the H₂ conversions, also observed an increase in the H₂ consumption in the H₂-C₃H₈-SCR reaction as the Ag loading increased [30].
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Influence of Ag surface density of Ag(x)/Al₂O₃ catalysts on the conversions of (a) NOₓ to N₂, (b) NOₓ to N₂O and (c) C₃H₆ to COₓ in the H₂-C₃H₆-SCR of NOₓ on Al₂O₃ (—○—), and Ag(0.3)/Al₂O₃ (black), Ag(0.4)/Al₂O₃ (gray), Ag(0.6)/Al₂O₃ (blue), Ag(0.7)/Al₂O₃ (red), Ag(0.8)/Al₂O₃ (purple) and Ag(1.1)/Al₂O₃ (green) for mechanical mixtures of Ag(x)/Al₂O₃ catalysts and Al₂O₃ for which the amount of Ag was kept essentially constant to 30.9 ± 1.2 µmol. Feed composition: 0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆, 8% O₂ and He balance with a 230 mL NTP/min flow rate. (For interpretation of references to color in this figure legend, the reader is referred to the web version of the article).</figcaption></figure>

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. As illustrated in Fig. 6b, this criterion is defined, for a given catalyst, as the ratio of the area under the NOₓ to N₂ conversion curve between 150 and 550 °C to the area under the same curve assuming 100% conversion, for the same range of temperature (i.e. the area of the dotted square in Fig. 6b). This criterion is expressed as a percentage. Fig. 6c firstly shows that the use of such a criterion allows concluding to an optimum Ag surface density of 0.7 Ag/nm²Al₂O₃ (Ag loading of 2.2 wt%) in the C₃H₆-SCR reaction (open symbols). This is in agreement with earlier literature reports in which it was claimed that optimum NOₓ reduction activity was obtained for Ag/Al₂O₃ samples with an Ag loading close to 2 wt% [3,18–24]. Fig. 6c also clearly illustrates that the catalytic performances in the C₃H₆-SCR of the Ag(x)/Al₂O₃ samples were drastically promoted by the addition of H₂, as the NOₓ reduction to N₂ efficiencies in the H₂-C₃H₆-SCR reaction (full symbols: 55–32%) were found to be much higher than those in the C₃H₆-SCR reaction (open symbols: 20–27%). In contrast to the C₃H₆-SCR reaction, Fig. 6c allows concluding that the concept of optimum Ag surface density (Ag loading) does not apply to the H₂-C₃H₆-SCR reaction over a broad temperature domain (150–550 °C), since increasing NOₓ reduction efficiencies were obtained with decreasing Ag surface densities. This conclusion differs substantially from those reported by Sadhokina et al. [24] and Shimizu et al. [25] for whom an optimum Ag loading close to 2 wt% was identified in the H₂-promoted C₆H₁₄⁻ and C₃H₈-SCR reactions, respectively. It cannot be excluded that the nature of the reducing hydrocarbons [2] may be at the origin of the discrepancy within the results of the present work. It must also be recalled that contrary to the procedure followed in the present work (constant silver contents in the aliquots of samples tested), the data reported by Sadhokina et al. [24] and Shimizu et al. [25] were obtained with Ag/Al₂O₃ aliquots in which the amounts of Ag varied, thus bringing additional complexity in the interpretation of the data. To mimic the experimental conditions used by Sadhokina et al. [24] and Shimizu et al. [25], the catalytic performances of aliquots of samples in which the amounts of Ag(x)/Al₂O₃ catalysts (with x = 0.3 and 0.6 Ag/nm²Al₂O₃) diluted with Al₂O₃ were kept constant (0.19 g Ag(x)/Al₂O₃ + 0.19 g Al₂O₃) were investigated

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** (a) conversion of NOₓ to N₂ in the H₂-C₃H₆-SCR reaction for given reaction temperatures: 200 (—♦—), 225 (—●—), 250 (—■—), 275 (—○—) and 300 °C (—▲—), (b) description of the NOₓ reduction efficiency criterion in the H₂-C₃H₆-SCR reaction on Ag(0.6)/Al₂O₃ and (c) NOₓ reduction to N₂ efficiencies in the C₃H₆-SCR (—○—) and H₂-C₃H₆-SCR (—●—) reactions in the 150–550 °C range of temperatures as a function of the Ag surface density for 0.38 g of mechanical mixtures of Ag(x)/Al₂O₃ and Al₂O₃ for which the amount of Ag was kept essentially constant to 30.9 ± 1.2 µmol. Feed compositions: 0% or 0.21% H₂, 385 ppm NOₓ, 400 ppm C₃H₆, 8% O₂ and He balance with a 230 mL NTP/min flow rate.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Influence of the amounts of Ag introduced on the conversions of (a) NOₓ to N₂ and (b) C₃H₆ to COₓ in the H₂-C₃H₆-SCR of NOₓ on (–) 0.19 g Ag(0.3)/Al₂O₃ + 0.19 g Al₂O₃ (15.4 µmol Ag) and (––) 0.19 g Ag(0.6)/Al₂O₃ + 0.19 g Al₂O₃ (31.9 µmol Ag). Feed composition: 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>

(Fig. 7). As already noticed on aliquots of samples for which the amounts of Ag was kept essentially constant (Fig. 5a and c), Fig. 7 shows a broadening in the NOₓ reduction temperature window to lower temperatures at the expense of the NOₓ conversions at the higher temperatures and a shift in the conversion of C₃H₆ to lower temperatures as the Ag surface density (Ag loading) increased. As a consequence, the highly-loaded sample (Ag(0.6)/Al₂O₃) is the more active in the NOₓ reduction to N₂ at temperatures below 290 °C, whereas it becomes less active than the lowly-loaded sample (Ag(0.3)/Al₂O₃) at higher temperatures (Fig. 7a). Moreover, despite the fact that the amount of Ag in Ag(0.3)/Al₂O₃ (15.4 µmol) was twice lower than that in Ag(0.6)/Al₂O₃ (31.9 µmol), the NOₓ reduction to N₂ efficiency estimated for Ag(0.3)/Al₂O₃ (52%) was found to be slightly higher than that of Ag(0.6)/Al₂O₃ (48%). Thus, it appears that the Ag surface density (Ag loading) for which optimum catalytic performances in H₂-C₃H₆-SCR were achieved was strongly dependent on the selected reaction temperature (Figs. 6a and 7a).

### 3.3. Kinetic investigations

HC-SCR processes have been the subject of a very limited number of kinetic studies [5,20,30–41]. Among these studies, those that have aimed at investigating the kinetics of the H₂-promoted HC-SCR are even scarcer [30,34–36,40] although kinetics has been shown to be extremely profitable in providing unique information about the understanding of the catalytic reactions at a molecular level [26].

Kinetic measurements were performed to gain further insights into the origin of the improved H₂-C₃H₆-SCR catalytic performances obtained with Ag(x)/Al₂O₃ samples of decreasing Ag surface density (Figs. 5a and 6c (–●–)). Fig. 8a shows that the NO reaction order increased from 0.1 to about 0.5 up to a Ag surface density of 0.7 Ag/nm²<sub>Al₂O₃</sub> and then levels off at higher Ag surface densities. Fig. 8b indicates that the C₃H₆ reaction order remained essentially constant (0.4) for Ag surface densities lower than or equal to 0.7 Ag/nm²<sub>Al₂O₃</sub> and then increased before leveling off (0.8) for Ag surface densities higher than or equal to 0.9 Ag/nm²<sub>Al₂O₃</sub>. Fig. 8c shows that the H₂ reaction order (0.5) remained essentially constant for Ag surface densities lower than or equal to

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Influence of Ag surface density of Ag(x)/Al₂O₃ catalysts on the kinetic parameters (a) NO (–○–●–), (b) C₃H₆ (–Δ–·) and (c) H₂ (–□–·) reaction orders, and (d) apparent activation energies (Eₐ, –○–·) of the H₂-C₆H₆-SCR reaction. The NOₓ uptakes determined previously on the freshly calcined samples [18] are also reported (–●–). For comparison purposes, the reaction orders with respect to NO and C₃H₆ and the activation energy measured on Ag(0.7)/Al₂O₃ in the C₃H₆-SCR reaction (absence of H₂) are also reported (circled symbols) in (a), (b) and (c), respectively.</figcaption></figure>

0.8 Ag/nm²<sub>Al₂O₃</sub> and then increased up to 0.9 for an Ag surface density of 1.1 Ag/nm²<sub>Al₂O₃</sub>. Finally, Fig. 8d shows a decrease in the apparent activation energy from 61 to about 26 kJ/mol when the Ag surface density increased from 0.3 to 0.7 Ag/nm²<sub>Al₂O₃</sub> before remaining essentially constant up to a Ag surface density of 1.1 Ag/nm²<sub>Al₂O₃</sub>.

It is remarkable that changes in the reaction orders with respect to NO, C₃H₆ and H₂, and in the apparent activation energy (Eₐ) for the reduction of NOₓ to N₂ in the H₂-C₃H₆-SCR reaction occurred at an Ag surface density (Fig. 8: ~0.7 Ag/nm²<sub>Al₂O₃</sub> = 2.2 wt% Ag) coinciding with the changes observed in the NOₓ uptakes of the Al₂O₃ support of the Ag(x)/Al₂O₃ catalysts [18].

The kinetic parameters reported in earlier HC-SCR investigations performed on Ag/Al₂O₃ catalysts are summarized in Table 1 in the order of increasing carbon number in the HC reductant. For comparison purposes, the Ag loadings have also been expressed as Ag surface densities according to Ref. [18]. It can be seen from this table that these studies have mainly focused on the determination of the apparent activation energies (Eₐ) of the HC-SCR process and that the studies in which the NO and HC reactions orders have been determined are rather limited, in particular in the case of the
<table><caption>**Table 1**
Comparison of the kinetic data reported earlier in the HC-SCR and H₂-HC-SCR of NOₓ on Al₂O₃ and Ag/Al₂O₃ catalysts with those obtained in the present study.</caption><thead><tr><th rowspan="2">Ag<sup>a</sup> (wt%)</th><th rowspan="2">δ<sup>b</sup> (Ag/nm<sup>2</sup>Al<sub>2</sub>O<sub>3</sub>)</th><th rowspan="2">HC</th><th colspan="3">HC-SCR of NO<sub>x</sub></th><th colspan="3">H<sub>2</sub>-HC-SCR of NO<sub>x</sub></th><th rowspan="2">Ref.</th></tr><tr><th>E<sub>a</sub><sup>c</sup> (kJ/mol)</th><th>NO<sup>d</sup></th><th>HC<sup>d</sup></th><th>E<sub>a</sub><sup>c</sup> (kJ/mol)</th><th>NO<sup>d</sup></th><th>HC<sup>d</sup></th></tr></thead><tbody><tr><td>0.0</td><td>0.0</td><td>CH<sub>4</sub></td><td>124</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>[31]</td></tr><tr><td>0.8–21.5</td><td>0.2–4.8</td><td>CH<sub>4</sub></td><td>95–124</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>[31]</td></tr><tr><td>2.0</td><td>0.7</td><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>57 ± 3</td><td>-0.28</td><td>0.34</td><td>-</td><td>-</td><td>-</td><td>[32]</td></tr><tr><td>0.0</td><td>0.0</td><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>47</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>[33]</td></tr><tr><td>2.1</td><td>0.5</td><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>39</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>[33]</td></tr><tr><td>3.5</td><td>0.8</td><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>37</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>[33]</td></tr><tr><td>8.0</td><td>1.9</td><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>39</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>[33]</td></tr><tr><td>2.0</td><td>0.5</td><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>166</td><td>-</td><td>-</td><td>116</td><td>-</td><td>-</td><td>[34]</td></tr><tr><td>3.8</td><td>1.1</td><td>-C<sub>12</sub>H<sub>26</sub>-</td><td>123</td><td>-</td><td>-</td><td>89</td><td>-</td><td>-</td><td>[34]</td></tr><tr><td>6.2</td><td>1.9</td><td>m-xylene</td><td>83</td><td>-</td><td>-</td><td>65</td><td>-</td><td>-</td><td>[34]</td></tr><tr><td>0.0</td><td>0.0</td><td>C<sub>3</sub>H<sub>6</sub></td><td>85</td><td>0.3</td><td>0.3</td><td>-</td><td>-</td><td>-</td><td>[35]</td></tr><tr><td>1.0</td><td>0.2</td><td>C<sub>3</sub>H<sub>8</sub></td><td>147</td><td>-</td><td>-</td><td>33</td><td>-</td><td>-</td><td>[30]</td></tr><tr><td>5.0</td><td>1.1</td><td>C<sub>3</sub>H<sub>8</sub></td><td>250</td><td>-</td><td>-</td><td>30</td><td>-</td><td>-</td><td>[30]</td></tr><tr><td>2.0</td><td>0.4</td><td>C<sub>3</sub>H<sub>8</sub></td><td>224</td><td>-2.53</td><td>1.91</td><td>61</td><td>0.49</td><td>0.76</td><td>[36]</td></tr><tr><td>2.0</td><td>0.6</td><td>C<sub>6</sub>H<sub>14</sub></td><td>67</td><td>0.8</td><td>0.9</td><td>-</td><td>-</td><td>-</td><td>[20]</td></tr><tr><td>2.0</td><td>0.6</td><td>C<sub>8</sub>H<sub>18</sub></td><td>-</td><td>≥0</td><td>≥0</td><td>-</td><td>-</td><td>-</td><td>[5]</td></tr><tr><td>2.0</td><td>0.6</td><td>C<sub>8</sub>H<sub>18</sub></td><td>-</td><td>0</td><td>>1.0</td><td>-</td><td>-</td><td>-</td><td>[37]</td></tr><tr><td>2.0</td><td>0.6</td><td>C<sub>8</sub>H<sub>18</sub></td><td>-</td><td>0</td><td>1.0</td><td>-</td><td>-</td><td>-</td><td>[38]</td></tr><tr><td>1.5</td><td>1.2</td><td>C<sub>8</sub>H<sub>18</sub></td><td>-</td><td>0.5–0.7</td><td>0–1.7</td><td>-</td><td>-</td><td>-</td><td>[39]</td></tr><tr><td>1.9</td><td>0.6</td><td>C<sub>8</sub>H<sub>18</sub></td><td>-</td><td>-0.39</td><td>-</td><td>-</td><td>0.14–0.40</td><td>0.26–0.40</td><td>[40]</td></tr><tr><td>1.9</td><td>0.6</td><td>C<sub>16</sub>H<sub>34</sub></td><td>-</td><td>0.5–0.6</td><td>≥0</td><td>-</td><td>-</td><td>-</td><td>[41]</td></tr><tr><td>2.2</td><td>0.7</td><td>C<sub>3</sub>H<sub>6</sub></td><td>105</td><td>0.0</td><td>1.0</td><td>26</td><td>0.4</td><td>0.4</td><td>This study</td></tr><tr><td>0.5–4.3</td><td>0.1–1.4</td><td>C<sub>3</sub>H<sub>6</sub></td><td>-</td><td>-</td><td>-</td><td>61–23</td><td>0.1–0.5</td><td>0.4–0.8</td><td></td></tr></tbody></table>

a Ag loading in the Ag/Al₂O₃ catalysts.

b Ag surface density corrected for the content of Ag as Ag₂O following the procedure described in Ref. [18].

c Apparent activation energies.

d Reaction orders with respect to NO and HC for the production of N₂.

H₂-promoted HC-SCR. The kinetic data reported for the first time in Fig. 8 for the H₂-C₃H₆-SCR reactions over an extended range of Ag surface densities are, therefore, of the utmost interest. It can also be deduced from Table 1 that the nature of the hydrocarbons has a significant influence on the kinetic parameters of the HC-SCR reaction. Nevertheless, some common trends can be drawn from these sets of data.

In particular, Richter et al. [30], Kim et al. [34] and Shimizu et al. [36] reported on a significant decrease in Eₐ with the promotion of the HC-SCR reaction by H₂ (Table 1). This finding can also be observed in Fig. 8d where the Eₐ of the C₃H₆-SCR reaction decreases tremendously from 105 (circled full diamond) to 26 kJ/mol (open diamond) when promoted by H₂ on Ag(0.7)/Al₂O₃. Regarding the evolution of Eₐ with the Ag loading (Ag surface density), conflicting results were reported by Kim et al. in C₂H₅OH-C₁₂H₂₆-m-xylene-SCR [34] and Richter et al. in C₃H₈-SCR [30]. Kim et al. [34] concluded to a decrease in Eₐ, whereas Richter et al. [30] reported on either an increase in Eₐ in C₃H₈-SCR or constant Eₐ in H₂-C₃H₈-SCR with increasing loadings of Ag (Table 1). The data plotted in Fig. 8d indicate a decrease in Eₐ of the H₂-C₃H₆-SCR reaction with increasing Ag loadings, but in a more restricted range of Ag surface densities (0.3–0.7 Ag/nm²Al₂O₃) than that reported by Kim et al. (0.5–1.9 Ag/nm²Al₂O₃) [34].

Regarding the reaction orders, earlier studies concluded that the promotion of the HC-SCR reaction by H₂ led to a significant increase in the NO reaction order [36,40] and a decrease in the reaction order in C₃H₈ [36] for a given Ag loading (0.4–0.6 Ag/nm²Al₂O₃) (Table 1). The data shown in Fig. 8a and b for Ag(0.7)/Al₂O₃ are consistent with these findings. The NO reaction order (Fig. 8a) increased from 0 (C₃H₆-SCR, circled full diamond) to 0.4 (H₂-C₃H₆-SCR, open diamond), whereas the C₃H₆ reaction order (Fig. 8b) decreased from 1.0 (C₃H₆-SCR, circled full triangle) to 0.4 (H₂-C₃H₆-SCR, open triangle). To our knowledge, the trends in the NO, C₃H₆ and H₂ reaction orders shown in Fig. 8a-c for an H₂-HC-SCR reaction have not been reported to date.

Before providing interpretation for the evolutions of the kinetic parameters with increasing Ag surface densities (Fig. 8), the main conclusions of the characterization of the Ag(x)/Al₂O₃ catalysts by the NOₓ-TPD method, and the associated evolution of the NOₓ uptakes of the supporting Al₂O₃ oxide (NOₓ species do not chemisorb on Ag species) [18], shall be recalled. The decrease in the NOₓ uptakes observed with increasing Ag surface densities up to an Ag density of about 0.7 Ag/nm²Al₂O₃ ([18] and Fig. 8), for which optimum C₃H₆-SCR activity was obtained (Fig. 6c, --○--), was interpreted as the maximum loading of silver per unit surface area of Al₂O₃ for which optimal Ag dispersion was preserved [18]. The observed linear decrease in the NOₓ uptake below an Ag surface density of about 0.7 Ag/nm²Al₂O₃ was also attributed to the formation of homogeneously distributed Ag species of increasing density [18]. For Ag surface densities higher than 0.7 Ag/nm²Al₂O₃, the NOₓ uptakes of the Al₂O₃ supporting oxide remained essentially constant (Fig. 8). This was assigned to an increase in the size of the Ag clusters, the Al₂O₃ surface sites onto which Ag was anchored being saturated for an Ag surface density of about 0.7 Ag/nm²Al₂O₃ [18]. Despite the NOₓ-TPD characterization of the Ag(x)/Al₂O₃ catalysts was done on calcined samples [18], the correlations obtained in the present work between the NOₓ uptakes and the changes in the kinetic parameters of the H₂-HC-SCR reaction (Fig. 8) suggest that the Ag species in the calcined samples may be taken as representative of those existing under the reaction conditions from an Ag dispersion point of view. This assumption is supported by the EXAFS studies performed by Shimizu et al. [25] and Burch et al. [28] on Ag/Al₂O₃ samples with Ag loadings of about 2 wt% (Ag surface density of 0.4 Ag/nm²Al₂O₃ in [25]) which concluded to a very limited sintering of the Ag phase under HC-SCR reaction conditions, promoted or not by H₂, and to the preservation of Ag in a highly dispersed state with the formation of Ag clusters made of 3–4 Ag atoms for this particular Ag loading. These earlier studies, together with the characterization of the newly-prepared samples by NOₓ-TPD (Fig. 1) and by electronic microscopy techniques (Section 3.1),
for which Ag was shown to remain in a highly dispersed state whatever the silver loading in the dry powder samples, provide support for the use of the Ag surface density concept in the present study.

Fig. 5c clearly demonstrates that the activation of C₃H₆ in the H₂-C₃H₆-SCR reaction occurred on Ag sites, as the bare Al₂O₃ support does not catalyze C₃H₆ oxidation at temperatures as low as those observed for the Ag(x)/Al₂O₃ samples. The much lower conversion of H₂ on Al₂O₃ (<5% at 500 °C, not shown) than those measured on the Ag(x)/Al₂O₃ catalysts (100% above 325 °C, Fig. 4) also indicates that the activation of H₂ occurred on Ag species. These conclusions are consistent with the recently published elegant study of Kim et al. in which the promotional effect of H₂ in HC-SCR was attributed to morphological and chemical changes of the Ag phases [34]. For Ag surface densities lower than or equal to 0.7 Ag/nm²<sub>Al₂O₃</sub>, the reaction orders of 0.4 in C₃H₆ (Fig. 8b) and 0.5 in H₂ (Fig. 8c) remained essentially constant and positive. Within the same range of Ag surface densities (< 0.7 Ag/nm²<sub>Al₂O₃</sub>), the NO reaction order increased from 0.1 to 0.4 (Fig. 8a). This indicates that the activated form of NO (NOₓ adsorbed species: NOₓ adspecies) does not compete for the Ag sites responsible for the activation of C₃H₆ and H₂, as such a competition should have resulted in changes in the reaction orders in C₃H₆ and H₂, which was not observed (Fig. 8b and c). As a consequence, the sites responsible for the adsorption of NOₓ are rather related to the Al₂O₃ supporting oxide. This proposal is consistent with earlier FTIR studies in which it was shown that significant amounts of NOₓ were stored on the Al₂O₃ supporting oxide [12,34,36,42,43] and it was suggested that nitrates would be reaction intermediates of HC-SCR [20].

The increase in the NO reaction order with the addition of H₂ on Ag(0.7)/Al₂O₃ (Fig. 8a), from 0 for the C₃H₆-SCR to 0.4 for the H₂-C₃H₆-SCR (Fig. 8a), could be assigned to a depletion in the coverage of the NOₓ adspecies [2,36] due to the drastic increase in the H₂-C₃H₆-SCR reaction rate compared to that of C₃H₆-SCR at 325 °C (Fig. 4d). The much lower C₃H₆-SCR reaction rate would thus allow for saturation coverage of the Al₂O₃ sites by the NOₓ adspecies which would result in the observed 0th reaction order with respect to NO. This explanation would be consistent with the decrease in nitrate coverage with the introduction of H₂ observed by Shimizu et al. [36]. Likewise, the increase in the NO reaction order as the Ag surface density increases (Fig. 8a) is attributed to a decrease in the coverage of the catalyst surface by the NOₓ adspecies which is coherent with the observed decrease in the NOₓ uptakes of the Ag(x)/Al₂O₃ samples for Ag surface densities lower than 0.7 Ag/nm²<sub>Al₂O₃</sub>. The observed leveling off of the NO reaction order for Ag surface densities higher than 0.7 Ag/nm²<sub>Al₂O₃</sub> can be associated to the constant coverage of the catalyst surface by the NOₓ adspecies, in agreement with the steadiness of the NOₓ uptakes observed in this Ag surface density domain (Fig. 8a). It would have been of the utmost interest to have been able to provide additional quantitative data supporting the suggested differences in NOₓ coverage accounting for the changes observed in the NO reaction order. One may have thought about the use of the SSITKA (Steady-State Isotopic Transient Kinetic Analysis) techniques to provide such information [44]. Recently, Burch and co-workers [45,46] put particular emphasis on the fact that conventional SSITKA should be used with an extreme caution in identifying true reaction intermediates in the H₂-C₈H₁₈-SCR reaction. These difficulties could be overcome for isocyanate intermediates by using short time on stream SSITKA (STOS-SSITKA) instead of conventional SSITKA [45,46]. While the involvement of nitrate-type species adsorbed on or close to the active Ag sites could be revealed by STOS-SSITKA in the H₂-NH₃-SCR reaction [47], no such information could be obtained in the more complex H₂-C₈H₁₈-SCR reaction [46]. These arguments therefore clearly prevent the use of STOS-SSITKA in the

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Schematic representation of the H₂-C₃H₆-SCR reaction on Ag(x)/Al₂O₃ catalysts with increasing Ag surface densities (x).</figcaption></figure>

estimation of the NOₓ ad-species involved in the H₂-C₃H₆-SCR process of the present study. Finally, exhausts generally contain high concentrations of CO₂ and H₂O (5–12%). As the NOₓ species were deduced to be chemisorbed on Al₂O₃ in the present work, it cannot be excluded that the presence of CO₂ and H₂O in the feed would lead to a decrease in the performances of the Ag/Al₂O₃ catalysts in the SCR reactions due to their competitive adsorption with the NOₓ species. In agreement with this, the inhibiting effect of H₂O has been highlighted previously on the C₃H₆-SCR [48–50] and H₂-C₃H₈-SCR [30] performances.

The increase in the C₃H₆ and H₂ reaction orders for Ag surface densities higher than 0.7 Ag/nm²<sub>Al₂O₃</sub> (Fig. 8b and c) can be assigned to the fact that the combustion of these molecules on the larger Ag nanoparticles, present at such high Ag loadings and which oxidation capabilities have been clearly illustrated [51], prevails over their efficient use in the SCR reaction for the production of N₂ [3].

The decrease in the apparent activation energy (Eₐ) with increasing Ag surface densities up to 0.7 Ag/nm²<sub>Al₂O₃</sub>, also reported recently by Kim et al. [34], is more difficult to explain as the higher Eₐ (Fig. 8d) were found on the more active samples in H₂-C₃H₆-SCR (Fig. 5a). As recalled by Richter et al. [30] and Bond et al. [52], the rate constant k (Eq. (4)) not only varies with Eₐ but also with the pre-exponential factor (A), which itself depends on the concentration of active sites. This peculiarity is defined as the so-called compensation phenomenon in catalysis [52]. The unexpected higher activity in the H₂-C₃H₆-SCR reaction of the Ag(x)/Al₂O₃ samples with the lower Ag surface densities, associated to their higher Eₐ, might be related to the higher number of active sites on the Al₂O₃ supporting oxide able to chemisorb NOₓ species in agreement with the NOₓ uptake data (Fig. 8), hence to their higher pre-exponential factors. Yet the observed changes in the NO, C₃H₆ and H₂ reaction orders (Fig. 8a–c) prevented the estimation of the corresponding pre-exponential factors. For Ag surface densities higher than or equal to 0.7 Ag/nm²<sub>Al₂O₃</sub>, Eₐ remained essentially constant as were the NOₓ uptakes (Fig. 8d), and therefore also the number of active sites on the Al₂O₃ supporting oxide able to chemisorb NOₓ species and the corresponding pre-exponential factors. The lower catalytic performances of these samples (Fig. 4b) is thus rather assigned to the combustion of C₃H₆ on larger Ag nanoparticles [3] formed at these particularly high Ag surface densities [18].

In summary, the interpretation of the activity and kinetic data led us to conclude that the H₂-C₃H₆-SCR reaction proceeds via the activation of H₂ and C₃H₆ on Ag species which further react with NOₓ adspecies activated on the Al₂O₃ support, as schematically illustrated in Fig. 9. Such a proposal is consistent with the conclusions drawn earlier by She and Flytzani-Stephanopoulos on CH₄-SCR studies [31]. The unexpected decrease in the H₂-C₃H₆-SCR performances observed with increasing Ag surface densities (Figs. 5a and 6c) is assigned first to the decrease in the number of
active sites on the Al₂O₃ supporting oxide able to chemisorb NOₓ species [18] for Ag surface densities lower than 0.7 Ag/nm²Al₂O₃ and then to the combustion of C₃H₆ on the larger Ag nanoparticles for Ag surface densities higher than 0.7 Ag/nm²Al₂O₃ (Fig. 9). Finally, the kinetic data obtained for Ag surface densities lower than 0.7 Ag/nm²Al₂O₃, in particular, suggest that the interaction between the NOₓ and C₃H₆ would be rate determining in the C₃H₆-SCR process in agreement with the earlier proposal of Burch et al. [49].

## 4. Conclusion

The promotional effect of H₂ in the C₃H₆-SCR reaction was confirmed on a series of Ag(x)/Al₂O₃ samples exhibiting various Ag surface densities (0.3 < x < 1.1 Ag/nm²Al₂O₃). TEM and HAADF-STEM analyses of the Ag(x)/Al₂O₃ catalysts indicated that silver was in a highly dispersed state, whatever the metal loading. The introduction of H₂ resulted in (i) a broadening in the NOₓ reduction temperature window to lower temperatures at the expense of the NOₓ conversions at temperatures higher than 300 °C and (ii) a shift in the conversion of C₃H₆ to lower temperatures as the Ag surface density increased. In contrast to the C₃H₆-SCR reaction, the concept of optimum Ag surface density at 0.7 Ag/nm²Al₂O₃ (Ag loading of 2.2 wt%) [18] did not apply to the H₂-promoted C₃H₆-SCR reaction over a broad temperature domain. Indeed, the catalytic performances in the H₂-C₃H₆-SCR reaction in the 150–550 °C range of temperatures improved as the Ag surface density of the Ag(x)/Al₂O₃ samples decreased. A detailed kinetic study was performed in which the reaction orders in NO, C₃H₆ and H₂, and the apparent activation energies were determined on the Ag(x)/Al₂O₃ series. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to 0.7 Ag/nm²Al₂O₃ (Ag loading of 2.2 wt%) coinciding with the changes observed earlier in the NOₓ uptakes of the Al₂O₃ supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H₂-C₃H₆-SCR reaction proceeds via the activation of H₂ and C₃H₆ on Ag species which further react with NOₓ adspecies activated on the Al₂O₃ support. The unexpected decrease in the H₂-C₃H₆-SCR performances observed with increasing Ag surface densities was assigned to a decrease in number of active sites on the Al₂O₃ supporting oxide able to chemisorb NOₓ species [18] for Ag surface densities lower than 0.7 Ag/nm²Al₂O₃ and to the combustion of C₃H₆ on the larger Ag nanoparticles for Ag surface densities higher than 0.7 Ag/nm²Al₂O₃ (Fig. 9). Finally, the kinetic data obtained for Ag surface densities lower than 0.7 Ag/nm²Al₂O₃ also suggest that the interaction between the NOₓ and C₃H₆ would be rate determining in the C₃H₆-SCR process.

## Acknowledgments

TC gratefully acknowledges UPMC for financial support (PhD Grant No.: 322/2012). The authors would like to thank the IMPC (Institut des Matériaux de Paris Centre) through the “plateforme de microscopie électronique”. The authors also acknowledge financial support from COST Action MP0903 Nanoalloys. The aberration corrected STEM instrument used in this work was funded by the Birmingham Science City project. RLC and ZYL acknowledge the Engineering and Physical Sciences Research Council U.K. for financial support (Grant No.: EP/G070326/1). J. Olek (Engineer, Agilent

Technologies) is acknowledged for his prompt and useful assistance in the use of the µ-GC system.

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