# Enhancing and understanding the stability of Ni catalysts via In-promotion for the steam reforming of oxygenates: An in-depth operando XRD-XAS and modeling investigation

Martina Fracchia, Thantip Roongcharoen, Mauro Coduri, Luca Sementa, Soroosh Saeedi, Xuan Trung Nguyen, Dragos Constantin Stoian, Emanuela Pitzalis, Beatrice Campanella, Claudio Evangelisti, et al.

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Martina Fracchia, Thantip Roongcharoen, Mauro Coduri, Luca Sementa, Soroosh Saeedi, et al. Enhancing and understanding the stability of Ni catalysts via In-promotion for the steam reforming of oxygenates: An in-depth operando XRD-XAS and modeling investigation. Applied Catalysis B: Environmental, 2025, 366, pp.125074. 10.1016/j.apcatb.2025.125074. hal-05099687

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# Enhancing and understanding the stability of Ni catalysts via In-promotion for the steam reforming of oxygenates: An in-depth *operando* XRD-XAS and modeling investigation

Martina Fracchia<sup>a,b,c,1</sup>①, Thantip Roongcharoen<sup>d,1</sup>, Mauro Coduri<sup>a,b,c</sup>①, Luca Sementa<sup>e</sup>,
Soroosh Saeedi<sup>c,f</sup>, Xuan Trung Nguyen<sup>d</sup>①, Dragos Constantin Stoian<sup>g</sup>①, Emanuela Pitzalis<sup>d</sup>,
Beatrice Campanella<sup>d</sup>, Claudio Evangelisti<sup>d</sup>, Alessandro Fortunelli<sup>d</sup>①, Vladimiro Dal Santo<sup>c</sup>,
Filippo Bossola<sup>c,e</sup>*

## ARTICLE INFO

**Keywords:**

Hydrogen
Steam reforming
Nickel-indium
Quasi-simultaneous XRD/XAS
Modeling
Operando

## ABSTRACT

Indium is a well-known stability promoter for high-temperature dry reforming reactions but has not yet been applied to other reforming reactions for hydrogen production, such as those of oxygenated compounds which suffer from rapid deactivation. In addition, little is known about the stabilization mechanism and the limited insights mostly come from *ex-situ* characterizations. Here, we developed an In-modified Ni-based catalyst by synthesizing a Mg-Al-In mixed oxide support and by depositing Ni via a urea-assisted deposition procedure. The catalyst was evaluated in the steam reforming reaction of acetic acid showing stable activity for more than 24 h at 700 °C and S/C = 3 corresponding to double the hydrogen productivity than the unpromoted catalyst which, on the contrary, was not stable. Superior stability was achieved also with glycerol. We then investigated the catalysts under *operando* conditions performing a quasi-simultaneous XAS and XRD experiment using synchrotron light, assisted by computational modeling. After reduction, the surface of the Ni nanoparticles is enriched with In, as confirmed by TEM-EDS, in the form of a shell consisting of a Ni₃In-like phase. The presence of In in the surface layers competes with C insertion thus protecting the Ni particles from coke deposition. As the reaction proceeds, the intermetallic phase is consumed resulting in Ni nanoparticles that are more stable, although less active, than those of the unpromoted catalyst. This may be due to a different faceting caused by the dealloying of the Ni-In phase, which once again disfavors the coke formation.

## 1. Introduction

Steam reforming reaction of oxygenated molecules derived from bio-oils is a strategic alternative to classical methane reforming for hydrogen production but, to date, the process lacks the necessary stability to become a viable industrial reality [1]. Only expensive noble metal-based catalysts have shown satisfactory stability whereas base metals such as

Ni still suffer from quick and severe deactivation [2–9]. Among the many solutions explored to extend the catalysts’ lifetime, the use of In as a promoter has gained increasing attention in the last years [10–14]. It has proven particularly effective with Ni-based catalysts for the dry reforming reaction, essentially by limiting coke formation. The origin of such higher coke resistance has been the object of several studies and has been linked to a suppressed C-H bond activation of methane [12,13,15,

<footnote><sup>a</sup> University of Pavia, Department of Chemistry, Viale Taramelli 12, Pavia 127100, Italy</footnote>

<footnote><sup>b</sup> INSTM, National Inter-University Consortium for Materials Science and Technology, Via G. Giusti 9, Florence 50121, Italy</footnote>

<footnote><sup>c</sup> CNR-Istituto di Scienze e Tecnologie Chimiche “Giulio Natta”, Via C. Golgi 19, Milano 20133, Italy</footnote>

<footnote><sup>d</sup> CNR-Istituto di Chimica dei Composti Organometallici, Via G. Moruzzi 1, Pisa 56124, Italy</footnote>

<footnote><sup>e</sup> CNR-Istituto per i Processi Chimico-Fisici, Via G. Moruzzi 1, Pisa 56124, Italy</footnote>

<footnote><sup>f</sup> Currently at Hydrogen for Development of Environmental Projects s.r.l., Via Colle Eghezzone 5, Lodi 26900, Italy</footnote>

<footnote><sup>g</sup> Swiss-Norwegian Beam Lines at European Synchrotron Radiation Facility, Grenoble 38000, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: filippo.bossola@cnr.it (F. Bossola).</footnote>

<footnote n="1">¹ Equally contributing authors</footnote>
[16,17]. The higher stability of the Ni nanoparticles at temperatures typical of dry reforming (>700 °C) has also been associated to the presence of In, possibly due to a not well-identified Ni-In alloy formed upon reduction [10,15,18]. Despite these encouraging results, no active and stable Ni-In catalyst has ever been reported for the steam reforming reaction of oxygenates. It must be also noted that the limited information available on Ni-In systems derives from characterizations performed on freshly reduced or spent catalysts. No insight into the actual catalytic process nor on the evolution of the catalytic systems as the reaction proceeds has ever been provided, which prevents a deep understanding of the stabilization mechanism. For example, it is not clear whether the Ni-In alloy formed after the catalyst reduction is retained under reaction conditions and which role it may play [15,19,20].

To make progress over the present scenario, here we report a com-
bined experimental and theoretical study with a two-fold purpose. First,
taking full advantage of the stabilization effect previously reported for In
in the dry reforming reaction, we develop a Ni-In/MgAlO catalyst pre-
pared through a novel route, that is, by synthetizing a Mg-Al-In mixed
oxide followed by the deposition of Ni via a urea-assisted method. The
stability of the so-prepared catalysts is evaluated in the steam reforming
reaction of oxygenates (acetic acid and glycerol as model compounds of
bio-oils [21]). Second, we shed light on the In-stabilization mechanism
by performing synchrotron-based quasi-simultaneous X-ray diffraction
(XRD) and absorption (XAS) measurements under acetic acid steam
reforming operando conditions [22]. This quasi-simultaneous mode is a
peculiarity of the current study, as generally these investigations are
performed independently, since they require different experimental
setups. Combining XAS and XRD enables us to achieve a holistic view of
the catalysts, obtaining information throughout the whole reaction on
the long-range structure and the crystallographic phases (XRD) as well
as on the oxidation states and the local structure of Ni and In (XAS).
Moreover, we complement the information gained from these charac-
terization techniques with first-principles theoretical simulations that
are uniquely able to provide a picture of the evolution at an atomistic
scale of the Ni-In systems during reforming reactions.

## 2. Experimental

### 2.1. Catalyst preparation

The Mg-Al mixed oxide supports were prepared via a sol-gel pro-
cedure as follows. First, an aqueous solution (140 mL) was prepared by
dissolving Al(NO₃)₃ and Mg(NO₃)₂ (Merck) precursors in order to obtain
a Mg/Al molar ratio of 1 in the final support material. In(NO₃)₃ xH₂O
(Merck) was added according to the final loadings of 0.4, 0.8, 2.0, and
4.0 wt%. This solution was then added dropwise into another aqueous
solution of NaOH and Na₂CO₃ (2.8 mmol), followed by aging at 65 °C for
18 h. After this time, the solid was filtered and washed with deionized
water. Finally, the material was dried overnight in air at 110 °C and then
calcined at 700 °C in air for 10 h.

Ni was deposited on the calcined supports via a urea-assisted depo-
sition method. NiBr₂ xH₂O and urea (Merck) were dissolved in deionized
water at a Ni/urea molar ratio of 1/100. After raising the temperature of
the so-prepared solution to 80 °C, the support was added and stirred for
20 h. The obtained material was separated by centrifugation, washed
twice with deionized water, and finally dried overnight in air at 110 °C.
The intended Ni loading was 10 wt%. The catalysts were labelled ac-
cording to the In/Ni molar ratio after reduction as follows: Ni-In(0), Ni-
In(0.02), Ni-In(0.04), Ni-In(0.10), and Ni-In(0.20). A Ni-In(0.04) cata-
lyst was also prepared via wet impregnation procedure in water using Ni
(NO₃)₂ (Merck) as precursor followed by calcination at 500 °C in air.

### 2.2. Catalyst characterization

The metal content was determined by Inductively Coupled Plasma
Optical Emission Spectroscopy (ICP-OES) with an Optima 8000 ICP-OES

(Perkin Elmer, Waltham, MA, USA) operating at 1500 W and equipping
an autosampler S10, MiraMist® Nebulizer (Perkin Elmer) and cyclonic
chamber. Argon (420.069 nm) was used as the internal standard. Ni and
In were examined at 231.604 nm and 230.606 nm wavelengths,
respectively. 10 mg of material were treated with 3 mL HNO₃ (Fluka,
Trace Select for trace analysis >69 %) and 1 mL H₂O₂ (Sigma Aldrich,
>30 % for trace analysis) and kept at boiling temperature for three
hours, until complete digestion. After cooling, the sample was diluted
with ultrapure water. Metal content was determined by comparison with
calibration curves obtained by commercial standard solutions (Fluka
TraceCERT®) properly diluted in 2 % HNO₃.

The BET specific surface area and the BJH pore width were measured by N₂ physisorption analyses at 77 K using an ASA2020 (Micromeritics) instrument. The reduced samples were degassed prior to the analysis for 3 h at 110 °C in high vacuum.

Morphological and structural characterization by Transmission Electron Microscopy (TEM) was performed using a Talos™ F200X G2 TEM microscope (Thermo Scientific). Energy dispersive X-ray spectra (EDS) and element maps were collected along with HAADF-STEM (high angular annular dark field scanning transmission electron microscopy) micrographs and four-detector SuperX Energy Dispersive. Samples for analysis were prepared by their suspension in isopropyl alcohol and ultrasonically dispersed followed by the deposition of a drop of the suspension on a holey carbon copper grid (300 mesh).

Raman analyses were realized using a Renishaw InVia Spectrograph with a multichannel air-cooled CCD detector, equipped with a Nd-YAG laser (532 nm) operated at a nominal power lower than 0.3 mW with an Olympus 50 × objective. The spectral resolution was 5 cm⁻¹ and the spectral range analyzed between 100 and 3000 cm⁻¹. Data were accumulated over 10 acquisitions with an exposure time of 10 s per acquisition to improve the signals.

### 2.3. Catalytic tests

The catalytic tests were carried out in a home-made fixed bed test
unit described elsewhere [23] equipped with an on-line 8890 N GC-TCD
(Agilent). Quartz reactors of 9 mm internal diameter were used. Before
each run, the catalysts were reduced in pure H₂ flow by ramping up the
temperature at 10 °C/min to 700 °C and holding for 1 h followed by
purging with He. The acetic acid or glycerol solutions were delivered to
the reactor at a steam-to-carbon (S/C) molar ratio of 3. The Ni catalysts
were tested at a weight hourly space velocity (WHSV) of 30 h⁻¹,
calculated as the ratio of the mass of acetic acid or glycerol fed per hour
over the catalyst mass. The conversion to gaseous products was calcu-
lated as the molar ratio between the sum of the CO₂, CO, and CH₄
productivities over the C feeding rate. Selectivity was calculated as the
molar productivity of a C-containing gaseous product over the sum of
the CO₂, CO, and CH₄ productivities. Hydrogen Space Time Yield (STY)
is reported as moles of hydrogen per minute per moles of Ni. The bare
supports are only slightly active for a very brief period and thus are not
shown.

### 2.4. Operando XRD/XAS investigation

The Ni-In(0), Ni-In(0.10), and Ni-In(0.20) catalysts were investi-
gated through a combined operando XRD and XAS investigation at the
BM31 station of the Swiss-Norwegian Beamlines at the European Syn-
chrotron Radiation Facility (ESRF).

About 10 mg of the catalysts were loaded within a 2 mm quartz
capillary reactor, plugged with quartz wool, and hooked to the gas lines
on the BM31 sample stage. At first, the lines were purged with He, then
the atmosphere was switched to pure hydrogen and the temperature
increased at 10 °C/min up to 700°C, which was kept constant for one
hour. Afterwards, the lines were purged again with He for one hour
before delivering by an HPLC pump the aqueous solution of acetic acid
to start the reaction. The concentration was kept the same (S/C of 3) as
the tests carried out with the test unit, whereas the WHSV was 4 times higher. A heated transfer line at 140 °C ensured the vaporization of the reactants' solution. The outlet liquids were separated from the gas by means of a condenser. The gas flow was measured by a DryCal Defender 500 and analyzed online via mass spectrometry (Pfeiffer Omnistar GSD 320).

Heating was provided by a hot air blower placed a few millimeters below the reactor. The blower temperature was calibrated by reproducing the heating ramp locating a thermocouple at the place of the catalyst.

XRD patterns were collected at λ= 0.25509 Å (~50 keV) in transmission geometry using a PILATUS3 X CdTe 2 M detector. The beam size on the specimen was set to about 200 × 200 µm. The sample-to-detector distance (~70 cm) was calibrated against a LaB₆ NIST standard. Raw images were recorded with 60 s exposure time, all the images were averaged and azimuthally integrated using the pyFAI package [24].

The XAS spectra were collected at the Ni K-edge, scanning continuously using a Si(111) double crystal monochromator between 8230 and 9300 eV with a step size of 0.5 eV, and at the In K-edge, scanning between 27830 and 28927 eV with a step size of 1 eV. The beam was calibrated using a Ni foil and an In foil assigning the first inflection point to 8333 eV and to 27940 eV, respectively. The spectra were acquired in transmission mode. Reference spectra of NiO (Ni K-edge), In₂O₃ (In K-edge), and Ni₃In (Ni and In K-edges) were acquired in transmission mode. For this purpose, Ni₃In was synthesized following the procedure described by Li et al. [19]. X-ray signal extraction and normalization were performed by means of the ATHENA code, belonging to the set of interactive programs IFEFFIT [25].

A quasi simultaneous XAS and XRD investigation was achieved by automatically switching between the Pilatus detector and the XAS-dedicated ion chambers on the transmitted X-rays. LN2 cooled Si111 double crystal monochromator (DCM) and a flat second crystal (parallel beam, unfocused) were used for XAS, while sagittal focusing second crystal (focused beam) was employed for XRD. Detectors, as specified already, are a PILATUS3 X CdTe 2 M for XRD, and ion chambers for transmission XAS measurements. Typical gas filling for the ion chambers involves inert gas mixtures that afford 20 % absorption into the monitor (I0, before the sample) and 80 % absorption into the chamber after the sample (I transmitted or I1) – at the corresponding edge energies.

A complete round of XRD and XAS data collection took about 7 min for the In-free specimen and about 12 min in the presence of In.

The schematics and pictures of the setup are shown in Figure S1.

## 2.5. Modeling

Density functional theory (DFT) calculations were performed using the Quantum ESPRESSO (QE) package [26], GBRV's ultra-soft pseudopotentials [27] along with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [28]. A plane-wave basis set with a cut-off energy of 40 Ry was used for the expansion of the wave function, while a cutoff of 200 Ry was used for expanding the charge density, and calculations were always conducted spin-polarized. Simulations were performed on a set of structural models based on slab geometries. The lattice parameters of Ni bulk were fully optimized with a k-space grid of (15 × 15 × 15). Slab models of Ni(111), Ni(100), and Ni(311) were then built. The bottom 3 layers were kept frozen, whereas Ni and In atoms were fully relaxed without any constraints. A vacuum space of 15 Å was added perpendicular (in the z direction) to the Ni and Ni-In slabs to avoid interaction between periodic images. The convergence energy threshold for the self-consistent-field calculations was set to 10–6 Ry. Relaxation of Ni-In systems and CH3COOH intermediates on the Ni surfaces were performed using a (3 × 3 × 1) k-points mesh. Reaction energy barriers and transition state configurations were predicted via DFT/NEB calculations (Nudged Elastic Band, NEB) [29], using 6 images in a first step, then refined via a Climbing Image approach [30].

To investigate the stability of Ni-In mixed models, we calculate the

mixing energy E_mix [31], defined by the equation:

$$E_{\text{mix}} = E_{\text{NiIn\_slab}} - \frac{N_{\text{Ni}}}{N_{\text{Ni}} + N_{\text{In}}} E_{\text{Ni\_slab}} - \frac{N}{N_{\text{Ni}} + N_{\text{In}}} E_{\text{In\_slab}}$$

where E_mix is the mixing energy of given Ni-In slab model, E_NiIn_slab is the total energy of this Ni-In alloy, E_Ni_slab and E_In_slab are the total energy of Ni and In slabs cut from their bulk, and N_Ni and N_In are the number of Ni and In atoms in the Ni-In model, respectively. According to this equation, more negative values of E_mix correspond to a larger stability of the Ni-In system.

To predict the stability of carbon adsorption on catalyst surfaces, we must elaborate the approach for mixing energy to take into account gas-phase entropic effects. We thus define an adsorption free energy (Gads). Under the reaction condition of Steam Reforming (SR) of CH₃COOH, carbon species derive from the CH₃COOH molecule as the chemical feedstock in the reaction. The reaction of CH₃COOH reforming to produce hydrogen gas and other products including carbon atoms is given in equation T.1 below:

$$\mathrm{CH_3COOH} \rightarrow \mathrm{CO_2} + \mathrm{C} + 2\mathrm{H_2} \quad (\mathrm{T.1})$$

The adsorption free energy (Gads) of carbon atom on Ni-based catalysts is then written as:

$$G_{\text{ads}} = G_{\text{complex}} - G_{\text{carbon}} - G_{\text{Ni\_based}} \quad (\text{T.2})$$

where Gads is the adsorption free energy of carbon on the catalyst surface, Gcomplex is the total energy of the complex made of the catalyst-slab-model plus adsorbed carbon, and GNi_based is the total energy of bare Ni-based catalyst system. The free energy (chemical potential) of carbon can be derived from Equation T.1 as:

$$G_{\mathrm{CH_3COOH}} = G_{\mathrm{CO_2}} + G_{\mathrm{C}} + 2G_{\mathrm{H_2}} \quad (\mathrm{T.3})$$

$$G = E - T\Delta S \quad (\mathrm{T.4})$$

where G and E refer to free energy and total energy obtained from optimization of each species, respectively. Thermodynamic corrections for gas-phase species were taken from frequency calculation using the Gaussian G09 package. T is the temperature that we set at: T = 900.15 K. Note that we never include vibrational contributions to free energy, so that the total energy of slab models coincides with DFT energy.

Finally, XANES spectra were simulated via the ab initio FEFF9 code [32], which calculates the self-consistent multiple scattering from atomic targets whose scattering potential is obtained by overlapping free atom densities in the muffin tin approximation, both as averages over all atoms of the same chemical element and as spectra of individual atoms.

# 3. Results and discussion

## 3.1. Stability of Ni-In catalysts

Morphological and compositional data of the catalysts are reported in Figure S2 and Table S1.

In-modified catalysts are generally prepared either by depositing both the main metal and In onto the supporting materials [14,16] or by co-precipitation of all the constituent elements [19,20]. However, this leads to the well-known effect of In burying the main metal with the consequent loss of active metal area especially at high temperatures [33]. We prepared a series of catalysts in this way, that is, by co-depositing Ni and In onto the Mg-Al mixed oxide support, but they turned out to be less catalytically active and stable than the monometallic counterpart. Instead, when In is a component of the support, the catalyst displays much higher stability, as shown later. Furthermore, catalysts prepared by depositing the Ni phase onto the support via classical wet impregnation method show poor performance both in
terms of conversion and H₂ productivity (Figure S3). We then resorted to a deposition precipitation method assisted by urea (DPU) [16], which not only does result in a monometallic Ni catalyst more active than the same prepared via wet impregnation (Figure S3A), but also in a Ni-In system in which the addition of In results in a marked enhancement of the stability (Figure S3B).

In Fig. 1A we report the H₂ space time yields (STY) obtained with the whole series of catalysts. The first thing to note is that there is a linear correlation between the H₂ productivity after about one hour TOS and the In/Ni ratio, with Ni-In(0.20) having about 15 % lower activity than the monometallic counterpart. This implies that the presence of In hampers the sheer catalytic activity of the Ni phase [13,16,18].

At the beginning of the reaction, Ni-In(0) is the most active catalyst but, after a brief stage of stability of about 7.5 h at conversion ~80 %, which is in line with those reported for Ni-based systems evaluated in similar reaction conditions [34,35], it starts losing catalytic activity with a simultaneous increase in CH₄ selectivity. For non-promoted Ni-based systems, the raise of CH₄ selectivity is generally associated with a progressive deactivation of the catalyst [36–38].

Despite the lower initial STY of the catalysts modified with In, the catalytic activity progressively stabilizes within 20 h TOS. In particular, the Ni-In(0.10) catalyst retains more than 97 % of its initial activity whereas the monometallic Ni catalyst loses nearly half of its activity within the same timeframe (Fig. 1A). At higher In loading, i.e., Ni-In (0.20), the stabilization is partly lost even though the catalyst still exhibits a higher stability than the unpromoted one. The conversion profiles of the catalysts reflect the very same trend as reported in Fig. 1B. It is worth pointing out that the increased CH₄ selectivity shown by all the In-modified samples is not linked to the catalyst deactivation, since the Ni-In(0.10) catalyst does not show any in the timeframe here considered. As already observed [39], the presence of In favors the CO₂ hydrogenation reaction: the CO₂ selectivity is lower in all the In-modified catalysts compared to monometallic Ni-In(0) (70 % vs 75 %, respectively).

Comparing literature results on the stabilization of the catalytic activity of Ni-based catalysts in steam reforming is not easy because authors have used very different reaction conditions and formulations of catalysts. Table S2 lists the most relevant works on Ni-based catalysts in open literature compared with our data. The most common strategies to

improve the resistance include the use of high Ni loadings up to 77 wt% [40] and reaction temperatures as high as 800 °C [9,36,41] or high S/C molar ratios of 14 [34]. Our Ni-In/MgAlO catalyst, despite featuring only 10 wt% Ni loading, is stable for up to 26 TOS at high WHSV (30 h⁻¹), which clearly marks a difference with the other solutions proposed so far. Comparable performances have been obtained only with noble metal-based catalysts [2,7,42].

Ni-In(0.10) catalyst confirms its superior stability over the unpromoted Ni catalyst also in the steam reforming of another model molecule of bio-oils—glycerol—thus expanding the effectiveness of In as stability promoter beyond acetic acid (Figure S4).

There are two aspects of the outcome of this series of tests that deserve further investigation.

The first one is that, irrespective of the In loading, all the catalysts here evaluated produce coke despite showing markedly different stability. Raman analyses on the used catalysts, however, indicate that In promotes the formation of less graphitized structures. This is particularly evident in the case of the Ni-In(0.20) catalyst, having the lowest I<sub>g</sub>/I<sub>d</sub> value of 0.22 compared to 0.58 of the unpromoted catalyst (Figure S5). This will be further supported by TEM images in the next subsection.

Another aspect is represented by the change in the shape of the reaction profiles when In is present in the catalysts (Fig. 1B). More specifically, whereas the monometallic Ni-In(0) catalyst deactivates at a constant rate and Ni-In(0.10) does not display significant changes, the Ni-In(0.20) catalytic profile can be divided into two sections. After an initial period of stability, the activity first drops, then stabilizes again for about 8 h TOS before decreasing again.

## 3.2. *Ex-situ* TEM investigation

*Ex-situ* HRTEM investigations provide a significant glimpse of the morphology and structure of both the used and freshly reduced catalysts treated in H₂ flow at 700 °C. As evidenced by the comparison between Fig. 2A and B, the presence of In plays a role in controlling the final size and shape of the metal nanoparticles. The Ni-In(0.10) sample shows well-formed roundish nanoparticles that are highly dispersed onto the support with a narrow size distribution and a mean diameter of 9.7 nm. High resolution TEM micrographs on single particles (Fig. 3A) show the

<figure><img src="image_4.png" /><figcaption>Fig. 1. Acetic acid steam reforming performances of Ni-In(0), Ni-In(0.10), and Ni-In(0.20) catalysts at 700 °C, S/C of 3 and WHSV of 30 h⁻¹. A) H₂ STY and retained H₂ STY after more than 20 h TOS. B) Conversion and selectivity (orange CO₂, green CO, and purple CH₄).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Representative TEM micrograph and particle size distribution (inset) of freshly reduced catalysts: A) Ni-In(0) and B) Ni-In(0.10).</figcaption></figure>

presence of a core-shell structure with a lower contrast shell surrounding a darker core, with a shell thickness of ca. 2 nm in the case of particles measuring around 10 nm in diameter. Lattice fringes are present in both regions, suggesting the existence of a crystalline core@shell structure with fringes of 2.01 Å in the core, which corresponds well to the fcc Ni (111) lattice spacing, and expanded fringes in the outer shell (2.13 Å), which can be tentatively ascribed to the presence of some Ni-In alloy (e. g., Ni₃In(111) measuring 2.136 Å) [43]. Few ca. 2 nm particles of light contrast with expanded lattice fringes (~2.1–2.2 Å) are also present, consistent with the formation of Ni-In alloy particles. Interestingly, no lattice fringes typical of segregated metallic In or In oxides crystalline domains can be observed. On the other hand, the Ni-In(0) catalyst exhibits particles with a non-regular shape and a larger mean diameter (11.7 nm).

HAADF-STEM/EDS element maps of the freshly reduced Ni-In(0.10) catalyst show that In is not dispersed within the support matrix but rather the signals of Ni and In overlap in the particles (Fig. 3B). The detailed analysis of the element distribution profile along a particle of about 20 nm (Figure S6) clearly shows a surface enrichment of In atoms in the nanoparticle outer shell and a core rich of Ni atoms, confirming what observed by high resolution TEM analyses. EDS analyses (Figure S7) carried out on particles having a size similar to the mean diameter (ca. 10 nm) reveal a mean In content ranging 5–10 at% with a mean value of 7.2 at%. It can therefore be concluded that the reduction step of the bimetallic system leads to a migration of In atoms from the support onto the Ni phase that results in the formation of roundish nanoparticles with a smaller size than those observed in the Ni-In(0) monometallic catalysts.

The used catalysts are markedly different. While carbon nanotubes, as expected [44], are the main form of carbonaceous material produced

in the Ni-In(0) sample with also evidences of Ni nanoparticles detachment from the support (Figure S8), in Ni-In(0.10) there are no well-defined carbonaceous structures (Figure S9). Indeed, the carbonaceous material observed in the In-doped used catalysts appears markedly different from carbon nanotubes, and rather amorphous-like. This is consistent with the lower I<sub>g</sub>/I<sub>d</sub> values from Raman spectra discussed in the previous subsection. Furthermore, in the used Ni-In(0.10) catalyst the size of the metal nanoparticles is only slightly increased (mean diameter 12.1 nm) whereas in the used Ni-In(0) sample particles as large as 100 nm can be found. Interestingly, on the used Ni-In(0.10) catalyst the interaction between the two elements is partly lost (residual In into the particles is less than 0.5 at%) and In can be found also on the support (Figure S10).

Mg-Al mixed oxide supports notoriously play a role in promoting the stability of Ni nanoparticles at high temperatures thanks to their high surface area and via the formation of Ni/MgAl₂O₄ spinel phases [45, 46]. Compared to the unpromoted Ni catalyst, the much more limited growth of the Ni nanoparticles observed in Ni-In(0.10) after reaction suggests that other mechanisms contribute to their stability and probably involve the presence of In. Indeed, increased stability of metal nanoparticles in In-promoted catalysts has already been reported, although it has never been actually studied [14,15].

Although these observations, particularly those regarding the formation of a Ni-In shell after the reduction stage (which is absent in the used samples), are insightful, it is hard to draw solid conclusions on the evolution of the catalytic systems and the cause of their different stability solely basing on these images.

Quasi-simultaneous XAS/XRD investigations using synchrotron light performed in operando conditions in synergy with theoretical calculations will allow us to bridge this gap, enabling a deeper understanding of
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Freshly reduced Ni-In(0.10): A) high-resolution TEM micrograph of a 10 nm representative particle; B) HAADF-STEM micrograph of a representative catalyst grain and the corresponding EDS element mapping of Ni (red), In (blue), Al (green), and Mg (yellow).</figcaption></figure>

the catalytic systems evolution during the reaction. We will focus our attention on the Ni-In(0), Ni-In(0.10), and Ni-In(0.20) catalysts as representative compositions. First, we will provide a picture of the samples as synthesized and prior to any treatments. Then, the reduction process in H₂ flow will be described to establish a starting condition of the catalysts prior to the reactions, which have been carried out until total deactivation. Theoretical modeling will be essential in this journey by complementing the overall picture of the evolution of catalytic

systems with an atomistic view and by providing key elements to explain the role of In and the other phases during the reactions.

### 3.3. Quasi-simultaneous XAS/XRD investigation in operando conditions and theoretical modeling

#### 3.3.1. Before reduction

The room temperature XRD patterns of the catalysts show that they

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Experimental XRD patterns collected during the reduction in H₂ of the three catalysts. a) temperature-resolved patterns of catalyst Ni-In(0.10); b) and c) comparison of the three catalysts at 25 °C and 700°C, respectively.</figcaption></figure>
occur in the form of hydrotalcite (HT), with a small fraction of brucite Mg(OH)₂ (Fig. 4). The best refinement of the structure (Figure S11) has been obtained with a structural model of composition (Mg₀.833Al₀.167) (OH)₂(CO₃)₀.083(H₂O)₀.75, taken from ref [47], s.g. R-3m, a= 3.07 Å, c ~ 23.98 Å. In the presence of In, the signal fingerprint of brucite becomes less evident and blurred, but no evidence of In-rich phases, such as In₂O₃, is observed. To gain further information on the electronic and structural properties of the three samples, XAS spectra have been measured both at the Ni and, for the In-containing catalysts, at the In K-edge (Figure S12). In all cases, the edge energy position of the XANES (X-ray absorption near-edge structure) at the Ni K-edge coincides with that of NiO, indicating the presence of Ni²⁺ in the pristine samples. The spectral shape is, in turn, compatible to the one expected for hydrotalcite, as previously reported in literature [48–50]. Also, the pre-edge peaks of the spectra, located at ca. 8343 eV, have the typical shape and low intensity expected for Ni²⁺ in an octahedral environment [51].

The spectra at the In K-edge share nearly the same edge energy position of In₂O₃ (shown as reference), indicating that In is trivalent in the two samples. The main peak (white line) located at ca. 27949 eV, shows the typical large intensity and shape of In³⁺ in an octahedral geometry [52,53]. The spectral shape, however, deviates significantly from that of In₂O₃, thus suggesting that In³⁺ is not segregated as oxide but rather is incorporated in the hydrotalcite structure. Indeed, the EXAFS signals of Ni-In(0.10) and Ni-In(0.20), at both the In and Ni K-edge, were extracted and fitted using the HT structural model, obtaining a very good agreement (Figure S12 and Tables S3 and S4 for the refined parameters).

### 3.3.2. Reduction

All the catalysts were reduced in H₂ flow up to 700 °C. XRD and XAS

spectra are illustrated in Fig. 4 and Fig. 5, respectively. In the Ni-In(0) catalyst, the XRD patterns show that heating in reducing atmosphere above 150°C promotes the formation of a second phase (HT') which coexists with HT up to ~300°C (Figure S13). These phases are consumed upon heating, leading to the formation of MgO above ~350°C and metallic Ni above ~530°C. This is consistent with the XAS spectra collected at the Ni K-edge. Indeed, heating promotes a different Ni²⁺-containing species, indicating the thermal decomposition of the room temperature hydrotalcite structure. The spectrum of this Ni²⁺ species is similar, although not exactly coinciding, with that of NiO. Previous works reported that the decomposition of HT in a Ni-rich system may lead to NiO and possibly to NiAl₂O₄ [48]. In the XRD patterns, only a small fraction of spinel (less than 1 %, see Figure S14) is observed and can be attributed to MgAl₂O₄, where Ni is likely substituting for Mg forming a solid solution. It should also be noted that NiO has a total miscibility in MgO, implying that a solid solution is probably formed. In the following, this species arising from the HT thermal decomposition will be simply referred to as Ni²⁺ oxide.

Both In-containing catalysts show a similar behavior as the monometallic counterpart during the reduction process. However, the formation of Ni particles is observed at higher temperature and further XRD reflections appear from 550 °C, which can be indexed with a primitive cubic cell with lattice parameter of about 3.8 Å. This is consistent with the formation of carbides, such as Ni₃MgC or Ni₃InC₀.₅ (s.g. Pm-3m). The fact that such species is observed only in the presence of In points towards the formation of Ni₃InC₀.₅, which had already been observed in Ni-In systems [54]. The presence of C can be explained by the release of CO₂ from the support and the subsequent reduction to CH₄, as testified by the mass spectra collected during the reduction process (Figure S15).

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** XAS spectra at the Ni- and the In- K edges (a and b, respectively) collected during the reduction in H₂ on the three catalysts.</figcaption></figure>
In addition, the analysis on the support (that is, MgAlO mixed oxide herein defined as In(0)) shows the formation of MgO as the only crystalline phase (Figure S16), thus ruling out the Ni₃MgC option. XRD provided no evidence of either Ni₃C (s.g. R-3c) nor Ni₃In (s.g. P6₃/mmc), not even during the reaction.

From a XAS perspective, the local structure and coordination around the photoabsorber (be it Ni or In), for Ni₃In and Ni₃InC₀.₅, is similar. The spectrum at the Ni and In K-edges of Ni₃In is therefore not expected to be significantly different from that of Ni₃InC₀.₅. Having no direct experimental information about the XANES spectra of Ni₃InC₀.₅, since none is reported in the literature, we used the experimental XANES of a Ni₃In reference specimen as a fingerprint of this Ni-In mixed phase. The validity of this assumption is confirmed by first-principles theoretical modeling (see SI, Figure S17). XAS spectra at the In K-edge can further corroborate the formation of such Ni-In mixed phase. Indeed, the decrease of the white line and the shift toward lower energies confirm the reduction of In as the temperature ramps up. The spectrum of the bare support at 700 °C (Figure S16) can be taken as a reference of In in the metallic state in the Mg-Al mixed oxide matrix (In(0)). This spectrum is markedly different from those of Ni-In(0.10) and Ni-In(0.20) collected at the same temperature. Indeed, the spectra of these two samples can be obtained by a Linear Combination Fit (LCF) of the spectrum of In(0) and Ni₃In, demonstrating that part of In is present in a mixed phase with Ni. This is further proof of the strong interaction between Ni and In in the catalysts through the formation of the intermetallic. The LCF was also carried out at the Ni K-edge, using the spectra of Ni²⁺ oxide, Ni *fcc* and Ni₃In. Again, the use of Ni₃In proves to be necessary for the quality of the fit, further confirming the presence of this species. As an example, the LCF of Ni-In(0.10) at 700 °C, obtained at both the Ni and In K-edges, is shown in Figure S18.

As expected considering the higher In loading, at 700 °C the fraction of the intermetallic is larger for Ni-In(0.20) compared to Ni-In(0.10). Interestingly, while almost all the In is found in the metallic state at the end of the reduction process, the extent of the Ni reduction, either as Ni *fcc* or intermetallic, depends on the catalyst and increases with the In content (Table 1). This matches well with the XRD results: higher In loadings reduce the fraction of the Ni *fcc* phase, which is counter-balanced by a larger fraction of Ni₃InC₀.₅.

Both XRD and XAS, supported as we will see later by theoretical modeling, point towards the formation during the reduction process of metallic Ni particles, accompanied, in the presence of In, by a second phase where Ni and In coexist in the metallic state. Again, no In₂O₃ has been observed. This fits with the HRTEM images of the reduced Ni-In (0.10) sample showing the formation of a Ni-In mixed phase surrounding a Ni⁰ core. Whereas no binary Ni-In alloy was detected in the XRD patterns, the formation of a small surface layer, likely disordered or not well crystalline, cannot be excluded, as it would not be spotted by XRD, or it would overlap to the other main signals.

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

Comparison of XAS and XRD results on the catalysts at the end of the reduction process at 700 °C. The percentage of Ni⁰ in the three catalysts is determined through LCF of the spectra at the at the Ni K-edge, specifying either the percentage of Ni⁰ as pure Ni *fcc*, or the overall quantity of Ni⁰ (given by the sum of Ni *fcc* and Ni involved in the Ni₃In intermetallic). The remaining part is present as Ni²⁺. The wt% of Ni *fcc* and Ni₃InC₀.₅ were determined via Rietveld refinements.

<table><thead><tr><th>Catalyst</th><th>Ni fcc (XAS) / %</th><th>Ni fcc + Ni₃In (XAS) / %</th><th>Ni fcc (XRD) / wt%</th><th>Ni₃InC₀.₅ (XRD) / wt%</th></tr></thead><tbody><tr><td>Ni-In(0)</td><td>68</td><td>68</td><td>8.7(1)</td><td>-</td></tr><tr><td>Ni-In (0.10)</td><td>36</td><td>74</td><td>6.0(1)</td><td>3.6(1)</td></tr><tr><td>Ni-In (0.20)</td><td>28</td><td>84</td><td>4.4(1)</td><td>8.7(1)</td></tr></tbody></table>

### 3.3.3. Reaction

The evolution of the species of interest during the reaction, as determined by Rietveld refinements and LCF at the Ni and In K-edges, is shown in Fig. 6 together with conversion profiles. The catalytic activity of all the catalysts is basically consistent with the previous findings after accounting for the different space velocity. In detail, the Ni-In(0) catalyst, although being the most active in terms of initial conversion (around 27 %) is the least stable and deactivates completely after only one hour on stream. Both Ni-In(0.10) and Ni-In(0.20) catalysts have an initial conversion rate lower than the unpromoted counterpart (about 23 % and 20 %, respectively), but they display activity for much longer time, although with some major differences. The former is stable up to 6 h whereas the latter only for 3 h. In both cases, the conversion profiles can be divided into an initial stage of maximum activity, followed by a second stage with reduced but ongoing activity, and ultimately ending with the catalyst's deactivation.

Some selected XRD patterns and XAS spectra collected during the reactions are reported in Fig. 7. For simplicity, here we report the experimental data collected at 700 °C under He flow just before the reaction (He in Fig. 7), followed by the second, the sixth, and the last measurement. The second corresponds to the beginning of the reaction where the acetic acid/water mixture flow is steady but the H₂ production is still limited, while the sixth has been collected in the region of highest H₂ production. The XANES spectra refer to the Ni K-edge, while those acquired at the In K-edge are reported in Figure S19. The full set of experimental XRD patterns is given in Figure S20.

For catalyst Ni-In(0), the main change in the XRD patterns during the reaction involves the appearance of a carbon signal (see below), while no major changes occur either in the nature nor in the relative proportion of MgO and Ni phases. Concerning the XANES spectra, the increase of the white line at the beginning of the reaction indicates that the Ni⁰ phase undergoes minimal oxidation. This is likely due to partially oxidizing conditions and the still low H₂ production rate. After this initial stage, Ni-In(0) quickly recovers as evidenced by the decrease of the white line intensity (Fig. 7). Interestingly, the amount of metallic and oxidized Ni remains constant throughout all the reaction until total deactivation (Fig. 6). This is consistent with XRD data, which shows no important effects during the reaction, thus implying that the cause of deactivation is not a loss of active phase per se but is rather due to the coke deposition. We recall that TEM images taken on the used catalyst, tested at lower space velocity, clearly show the formation of abundant structures, such as nanotubes, surrounding the Ni nanoparticles (Figure S8). The carbon formation can be tracked by the XRD patterns taken during the reaction, where a blurred signal appears at the position corresponding to the 002 reflection of graphite. A small shoulder, compatible with the 004 reflection, is also observed. Hereafter this signal will be defined as “carbon” since, being not fully crystalline, we cannot identify from XRD which form of carbon this is associated with. The content of carbon soon reaches a plateau, and then is consumed, reflecting the same trend as the fraction of Ni *fcc* and of the catalytic activity (Fig. 6 and S21).

Let us now consider the Ni-In(0.10) catalyst. The main event observed during the reaction is the rapid consumption of the Ni₃InC₀.₅ phase, as shown in Fig. 6, which is concomitant to the formation of carbon, as for the In-free catalyst. In this case, however, the carbon XRD signal is preserved during the reaction (Figure S20). The disappearance of this Ni-In phase is accompanied by an enrichment of Ni *fcc*, as shown by XRD and by the LCF at the Ni K-edge (Fig. 6). This finding fits well with the TEM images taken on the used catalyst, showing that In is spread all over the material and no longer localized in proximity of Ni (Figure S10). This is further confirmed at the In K-edge (Fig. 6), which shows that the fraction of the Ni₃In-like phase drastically decreases over time. Contrary to what is observed for the Ni K-edge, however, a small fraction of the intermetallic is detected even at the end of the reaction. This is only an apparent discrepancy, since the XAS measurements at the In K-edge display a larger sensitivity towards In-containing species.
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Results of the quasi-simultaneous XAS/XRD operando investigation during the steam reforming of acetic acid reaction with Ni-In(0), Ni-In(0.10), and Ni-In (0.20) catalysts. Conversion profiles and percentage of the species present during the catalytic runs until deactivation, obtained by the Rietveld refinement of the XRD patterns and by LCF at the Ni and In K-edges.</figcaption></figure>

Considering that In is only 2 wt%, any residual Ni₃In-like phase is well below the detection limit of XAS at the Ni K-edge. Its consumption is also accompanied by the expansion of the Ni fcc phase (Figure S22a), which continues well after the bulk Ni₃InC₀.₅ phase has been consumed. It is interesting to note that the cell expansion ends approximately when the catalytic activity drops, and the second catalytic stage begins.

No major changes were observed during the second catalytic stage. Just like in the case of Ni-In(0), the catalyst eventually fails likely due to coke formation since the Ni fcc is still detected. No clear structures such as carbon nanotubes were found on the used catalyst (Figure S9).

Ni-In(0.20) roughly undergoes the same processes involving the disappearance of Ni₃InC₀.₅ phase with the consequent enrichment of Ni fcc, although with major differences. First, the higher amount of In is reflected in a larger fraction of the Ni₃InC₀.₅ phase compared to Ni-In (0.10). However, for Ni-In(0.20), the fraction of Ni₃InC₀.₅ remains constant during the first stage of the reaction (about two hours) and eventually decreases along with the conversion. Furthermore, less carbon is produced, reflecting the lower overall catalytic activity. During this process, the cell parameter of metallic Ni and Ni₃InC₀.₅ show an opposite evolution, which suggests a possible reorganization between the two phases: whereas Ni shows a monotonic contraction during the reaction yet keeping a larger cell parameter than the other catalysts, the cell of Ni₃InC₀.₅ expands. This hints to a possible atom exchange between the two phases (Figure S22). In this context, the XRD signal of the Ni metallic phase splits and becomes asymmetric, likely due to a disordered atom distribution. Just like with Ni-In(0.10), the drop of the catalytic activity in the first stage occurs when the cell parameters are stabilized, while the catalyst deactivates as the Ni₃In-like phase fraction starts decreasing.

Summarizing, the formation of a Ni-In intermetallic phase appears to be critical to the development of a stable Ni catalyst for the steam reforming of oxygenates. However, an excessive amount of it, besides decreasing the overall catalytic activity, does not exert the same stabilizing effect and, in fact, the catalyst tends to deactivate sooner.

Operando XRD and XAS provided a clear picture of the evolution of

the catalytic systems under reaction conditions; however, they do not elucidate the specific reasons why the Ni₃In-like phase enhances the stability of the catalysts. To achieve a deeper understanding, we complemented the operando study with first-principles theoretical simulations. We created a series of slab models to investigate the most energetically favored facets of bulk fcc Ni (variously doped with In atoms), Ni₃In, and Ni₃InC₀.₅ phases, their stability, and the adsorption of carbon atoms on them, as well as some representative decomposition reaction mechanisms.

First, we considered the most energetically favored facets of pure Ni fcc: (100), (111) and (311), as well as representative Ni-In surface alloys of the same facets – schematic atomistic depictions are given in Figure S23 – and the most stable (111) facet of the ordered Ni₃In phase (Fig. 8). These models enable us to investigate In doping into Ni systems. Analysis of the corresponding energetics demonstrates that In mixes well with Ni, with mixing energies ranging around sizeable values of ≈ -5.2 eV for the most stable surface alloy configurations (Figure S23). The Ni₃In phase is also thermodynamically very stable, and, depending on the In chemical potential and mass availability, it can be predicted to be more abundant than surface alloys. However, we find that there is a barrier for the transformation from surface alloy to bulk alloy configurations, because in this transformation In atoms must go through sub-surface layer positions that are less energetically favored than surface segregated ones, as shown in Figure S24(b,c,e,f). The fact that the more favorable position of In in Ni-In surface alloy systems is in the top-most layers is due to the greater structural freedom at the surface that can better accommodate the larger size of the In atom.

Second, we studied the adsorption of carbon adatoms on slab models of facets of pure Ni, surface Ni-In alloys, and the Ni₃In phase, and a selection of the corresponding atomistic models as well as the values of carbon adsorption free energies, ΔGads, are reported in Fig. 8.

The DFT calculations on the energetics of carbon adsorption on pure Ni models, Fig. 8(c,e), are in line with the results of a previous combined experimental and theoretical study [55]. Without repeating the whole analysis, we recall the main conclusions of this previous study: the
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Selected *operando* XRD patterns (a-c) and XAS spectra (d-f) collected during the catalytic reaction.</figcaption></figure>

adsorption of carbon on pure Ni facets produces a stabilization of the (311) surface, because this facet interacts well with carbon and is able to adsorb a larger number of carbon atoms per surface area, so that it grows at the expense of the (100) facet under conditions in which carbon adatoms are produced during the reforming reaction. Then, in the super-saturation régime the (311) surface ejects the adsorbed carbon atoms, and due to geometric (stepped facet) and energetic factors, it does so in such a way that favors the formation of linear chains eventually merging into carbon nanotubes which cover and deactivate the Ni catalytic surfaces [55].

The situation drastically changes when In is introduced into the system. We find in fact that In and C atoms strongly repel and compete with each other for binding to Ni. This is immediately apparent already from carbon adsorption on Ni-In surface alloy models, Fig. 8(d,f), which exhibit strongly diminished values of $G_{ads}$, entailing a much lower density of carbon adatoms than on pure Ni facets, and reaches its extreme for carbon adsorption on the compact (100) facets of the carbon-rich $Ni_3InC_{0.5}$ phase, Fig. 8(g,i), on which $G_{ads}$ becomes positive, Fig. 8(h,j). What is critical here is that this C/In competition disfavors the (311) facet, which is not stabilized any more with respect to the (100) facet. Nanoparticle restructuring transforming facets from (100) into (311) is therefore not predicted to occur, nor in turn the formation of carbon nanotubes which eventually lead to catalyst deactivation. Assuming the strength of carbon adsorption as a descriptor of catalytic activity in reforming, the first conclusion we can draw from these results

is that In doping reduces the catalytic performance of Ni systems, but stabilizes them against coke formation. This is consistent with the experimental catalytic tests, as well as with the observation of a non-nanotube form of carbon produced by In-containing catalyst systems (Figure S5).

A further observation we can derive from DFT simulations comes from the analysis of structural rearrangement during In and C co-absorption. As shown in Fig. 8(d,f), in fact, the presence of carbon atoms forces the In atoms to protrude from the surface, in such a way that they are close to being expelled. This suggests that during the course of the reforming reaction the presence of carbon as a decomposition product can lead to In de-alloying from the surface. This is in agreement with the experimental observations of a progressive disappearance of the $Ni_3In$-like phase (Fig. 6) with the eventual spreading of In all over the catalyst (Figure S10).

Simulations of reaction mechanism steps finally give us some hints to solve the experimental paradox that the de-alloyed catalyst still maintains a residual catalytic activity (although diminished) and a longer catalytic lifetime compared to the pure Ni catalyst. Initial catalytic steps on pure Ni(100) and Ni(111) are illustrated in Fig. 9 and Figure S25, respectively. On Ni(111), C-O bond breaking is thermodynamically favored and precedes C-H or C-C bond breaking. On Ni(100), instead, the first C-H breaking has a low energy barrier of 0.62 eV and is definitively favored with respect to C-O breaking, that occurs only in a second step. The Ni(100) facet is thus the most catalytically active one, but it
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** (a) Top and side views of optimized structures of slab model of the (111) facet of the hexagonal Ni₃In bulk phase, (b) depiction of 4-carbon-atom adsorption on Ni₃In(111) slabs with an average Gads of -0.98 eV/atom. (c-f) few-carbon-atom adsorption on (100) and (311) facets of pure Ni and of selected Ni-In surface alloy models, respectively. Structural rearrangements during indium and carbon co-adsorption on Ni-In (100) and Ni-In (311) with lengthening of In-Ni distances are shown in d and f, respectively. (g, i) optimized structures of Ni₃InC₀.₅(100) with Ni-In-terminated and C-Ni-In-terminated surfaces, respectively. (h, j) 1-carbon adsorption on Ni-In-terminated and C-Ni-In-terminated surfaces, respectively. Color coding: Ni in green, In in orange, C in grey.</figcaption></figure>

produces C adatoms, that stabilize the (311) facet and promote the (100)→(311) reconstruction [55]. In contrast, the (111) facet is less catalytically active, but it does not produce carbon adatoms. If the genetic process of the catalyst under reaction conditions is such that the (100) facets are masked, e.g., by amorphous carbonaceous species (not carbon nanotubes) as produced by a weakened Ni-In alloy catalyst, we can hypothesize that the so generated catalyst will exhibit a diminished activity but will be less prone to coke formation and thus will have a longer lifetime. The preferred C-O scission on the (111) facet also suggests that we can expect greater propensity toward CH₄ production, again in line with experimental observations (Fig. 1). Further characterizations and catalytic tests (not presently available to us) will be needed in the future to support or disprove this hypothesis.

The stabilization mechanism so described for the Ni-In system is different from those proposed in the literature for typical promoters such as Ce [56], La [57,58], and Mg [9,59], and surface alloys [60–62]. Specifically, in the presence of the Ni-In shell, In and C compete to bind with Ni resulting in a significantly increase resistance to coke deposition; after Ni-In de-alloy, our data suggest that Ni(111) facets are favored and, although they are less active than Ni(100), do not produce C adatoms, which are coke precursors, thus explaining the improved coke resistance of the de-alloyed system. This can in turn originate from a masking effect on the most active sites due to the deposition of amorphous

carbonaceous species [63]. This is consistent with the available experimental evidence: as discussed in subsection 3.1 (Raman) and 3.2 (TEM), there is an abundance of carbon nanotube in the spent Ni catalysts, that is replaced by amorphous carbonaceous phases in the spent In-doped catalysts. The knowledge gained with this work will be useful for the development of non-noble metal-based catalysts with enhanced stability for the steam reforming of oxygenates.

## 4. Conclusions

In this work, we synthesized a series of novel Ni-based catalysts modified with In using a urea-assisted precipitation method, aiming to enhance the stability of Ni catalysts in the steam reforming of acetic acid and glycerol—model compounds representing oxygenated molecules derived from bio-oils.

The catalytic tests carried out at 700 °C clearly indicate that the addition of In is beneficial to the overall stability of the catalysts. In detail, the unpromoted catalysts started deactivating within a few hours, whereas an In-promoted catalyst did not show any relevant sign of deactivation for about 26 h on stream, resulting in double the H₂ productivity than the unpromoted catalyst.

Ex-situ TEM and Raman analyses revealed that the presence of In promotes the formation of less structured carbon deposits. Additionally,
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** (A) Reaction energy diagram of the initial steps of CH₃COOH decomposition on the Ni(100) surface, and (B) schematic depiction of optimized structures of intermediates corresponding to the reaction energy diagram of (A). In (A), the lowest-energy path via initial C–H scission (a → b → c → d → g) is shown in black line, while other possible intermediates are shown in blue and orange colors. Color coding in (B): Ni in green, C in grey, H in white, O in red.</figcaption></figure>

it was found that a Ni-In shell formed around a Ni core in the catalyst nanoparticles after reduction, but this shell was no longer observed in the used catalysts, as In was found to be distributed throughout the entire sample.

The evolution of the crystalline phases and of the oxidation state and local structure of Ni and In during the reduction process and throughout the reaction was investigated through a quasi-simultaneous XAS and XRD operando experiment, using synchrotron radiation.

Computational modeling helped interpret the experimental findings and to unveil the origin of the improved stability, providing further insights into the different behaviors of the three catalysts.

It was found that, after reduction at 700 °C, the primary metallic phase consists of Ni with an fcc structure, while in the presence of In, a Ni₃In-like phase is also observed, in line with the TEM images. Important differences between the catalysts emerged during the steam reforming of acetic acid. The fraction of the metallic phase in the unpromoted Ni catalysts remained unchanged until deactivation, implying that the main cause of failure is coke formation in the form of nanotubes. In the In-promoted catalysts, the Ni₃In-like phase surrounding the Ni core acts as a barrier against C insertion into the Ni particles. In detail, In and C

atoms strongly repel each other and compete for binding to the Ni, thus retarding the deposition of carbonaceous materials. The (311) faceting of the Ni particles, which typically promotes the formation of carbon nanotubes, is also disfavored.

As the reaction proceeds, In is expelled and the Ni₃In-like phase is no longer observed. However, the consumption of such a phase is crucial to overall stability since the catalyst retains part of the initial activity. We speculate that the Ni (100) facets in the de-alloyed Ni nanoparticles are masked by amorphous carbonaceous materials, which are indeed detected by both TEM and Raman analyses, leaving less active, but also less prone to coke formation, Ni (111) facets exposed. This may explain the difference between the unpromoted and de-alloyed Ni nanoparticles.

## 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.

## Acknowledgments

The Swiss Norwegian Beamline (SNBL@ESRF) is acknowledged for providing of beamtime (experiment CH-6616). The BM31 setup was funded by the Swiss National Science Foundation (grant 2021_18962) and the Research Council of Norway (grant 296087).

F.B. and V.D.S. thank the financial support of the Ministero dell'Ambiente e della Sicurezza Energetica (MASE) through the project ""Mission Innovation" POA 2021-2023 - Hydrogen Demo Valley: Infrastrutture Polifunzionali Per La Sperimentazione e Dimostrazione delle Tecnologie dell'idrogeno"".

M.F. and M.C. acknowledge the support from the Ministero dell'Università e della Ricerca (MUR) and the University of Pavia through the program "Dipartimenti di Eccellenza 2023-2027".

We are grateful to the Italian CINECA supercomputing center for providing computational resources. We acknowledge the financial support from PNRR ICSC (Next Generation EU, grant number CN00000013), and from the Italian MASE POR H2 AdP project, funded by the EU PNRR, Mission 2, Component 2, Investment 3.5 "Research and development on hydrogen".

T. R., S.S., and X. T. N. thank the financial support through the BIKE project. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 813748.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2025.125074.

## Data availability

Data will be made available on request.

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