Micro-Structure Engineering in Pd-InOx Catalysts and Mechanism Studies for CO2 Hydrogenation to Methanol

Significant interest has emerged for the application of Pd-In2O3 catalysts as high-performance catalysts for CO2 hydrogenation to CH3OH. However, precise active site control in these catalysts and understanding their reaction mechanisms remain major challenges. In this investigation, a series of Pd-InOx catalysts were synthesized, revealing three distinct types of active sites: In-O, Pd-O(H)-In, and Pd2In3. Lower Pd loadings exhibited Pd-O(H)-In sites, while higher loadings resulted in Pd2In3 intermetallic compounds. These variations impacted catalytic performance, with Pd-O(H)-In catalysts showing heightened activity at lower temperatures due to the enhanced CO2 adsorption and H2 activation, and Pd2In3 catalysts performing better at elevated temperatures due to the further enhanced H2 activation. In situ DRIFTS studies revealed an alteration in key intermediates from *HCOO over In-O bonds to *COOH over Pd-O(H)-In and Pd2In3 sites, leading to a shift in the main reaction pathway transition and product distribution. Our findings underscore the importance of active site engineering for optimizing catalytic performance and offer valuable insights for the rational design of efficient CO2 conversion catalysts.


Introduction
The hydrogenation of carbon dioxide (CO 2 ) to methanol (CH 3 OH) represents a crucial catalytic process, pivotal in addressing environmental concerns and fostering sustainable chemical production.This process provides a viable solution for utilizing CO 2 , a significant greenhouse gas [1], while also contributing to the production of CH 3 OH, a versatile chemical feedstock with a wide range of industrial applications [2,3].However, this process presents formidable challenges, considering the ultra-stable C=O bond in CO 2 [4][5][6].Meanwhile, the reverse water-gas shift reaction (RWGS), wherein CO 2 reacts with hydrogen for producing water, redirects the reaction towards the undesired production of carbon monoxide (CO) rather than CH 3 OH.
To address the above issue, indium oxide (In 2 O 3 ) based catalysts have recently garnered widespread attention due to their exceptional productivity for CH 3 OH [7].Nonetheless, pure In 2 O 3 exhibits insufficient adsorption capacity for reaction gases, leading to suboptimal reaction efficiency [2].Additionally, the instability of the In 2 O 3 structure results Molecules 2024, 29, 3715 2 of 12 in a rapid deactivation of catalysts.Consequently, numerous researchers have explored surface modifications of In 2 O 3 to enhance its activity and stability by altering its active sites.For instance, Liu et al. demonstrated the introduction of new active sites through nitrogen doping, resulting in a great enhancement of reaction activity and catalytic stability of In 2 O 3 [8].Recent discoveries have revealed that the interface sites between Pd and In 2 O 3 catalysts are more active for the activation of H 2 [9].Ge et al. reported the notable improvement for CO 2 hydrogenation to CH 3 OH by coupling highly dispersed Pd nanoparticles with In 2 O 3 , delivering a methanol productivity of 0.89 g MeOH •h −1 •g cat −1 [10].However, it is noteworthy that various catalytic sites may co-exist in addition to the Pd-In 2 O 3 interfaces in the Pd/In 2 O 3 system.For example, metal Pd might tend to form large particles, exacerbating the competitive RWGS reaction, and leading to the increase in CO byproduct [11].
Meanwhile, Pd can also form alloys with other metals, resulting in the tailored active site for CO 2 hydrogenation reactions.Bowker et al. reported that ZnO-supported Pd-Zn alloys with optimized particle size and surface structure demonstrated a high performance to activate CO 2 with CO 2 conversion of 11% and methanol selectivity of 60%, at conditions of 250 • C and 20 bar [12].Song et al. used SiO 2 as support for Pd-Cu alloy particles.With the optimization of Pd/(Pd + Cu) atomic ratios, it was found that Pd(0.34)-Cu/SiO 2 was most active with CH 3 OH yield of 0.31 µmol•gcat −1 •s −1 [13].Additionally, Pd-In alloy may be simultaneously created, which may also shine for CO 2 hydrogenation to CH 3 OH [14].These complex multi-structures might complicate the reaction route, incubating different intermediates, which makes it difficult to achieve a clear understanding of the reaction mechanism [15][16][17].Thus, engineering specific active sites and exploring the structure-function relationship is crucial, albeit challenging for the development of advanced Pd/In 2 O 3 catalyst [14,18].
In this study, Pd-InO x catalysts with tailor-orchestrated structures were precisely designed and synthesized.The cryogenic activity has great advantages over existing work.Combined (in situ) characterizations were systematically conducted to shed light on the reaction intermediates and reaction pathways.These findings provide valuable insights into the catalytic mechanism of InO x -based catalysts, offering guidance for the rational design and development of catalysts for CO 2 hydrogenation to CH 3 OH.

Structure Characterization of Pd-InO x
The synthesis of InO x with varying Pd contents was achieved using a two-step method.Analysis by ICP-AES revealed the actual Pd content in 0.4Pd-InO x and 1.2Pd-InO x are 0.40 wt.% and 1.28 wt.%, respectively (Table S1), consistent with the theoretical values.N 2 adsorption-desorption isotherms in Figure 1a indicate that the specific surface area of all samples is below 11 m 2 •g −1 , as summarized in Table S1.XRD analysis depicted the presence of the h-In 2 O 3 phase (JCPDS 04-005-4422) in all InO x and Pd-InO x samples before calcination (Figure 1b).Upon subsequent calcination and reduction of these samples, a coexistence of h-In 2 O 3 and c-In 2 O 3 phases (JCPDS 97-001-4388) was observed over pure InO x and 0.4Pd-InO x samples, indicating a crystal transition of In 2 O 3 (Figure 1c) while only the h-In 2 O 3 phase was observed in 1.2Pd-InO x .Moreover, no diffraction peaks associated with Pd were discernible in the 0.4Pd-InO x sample, indicating highly dispersed Pd on the InO x substrate [19].In contrast, diffraction peaks corresponding to the Pd 2 In 3 intermetallic compound were evident (JCPDS 97-064-0231) in the 1.2Pd-InO x sample.The interaction between Pd and In in Pd 2 In 3 might prevent the crystal transition of h-In 2 O 3 into the c-In 2 O 3 phase [20,21].SEM images revealed a bulky shape for the catalysts, while EDS mapping demonstrated the uniform distribution of In, O and Pd atoms in all xPd−InOx samples (Figures S1 and S2).We further investigated the elemental distribution in the material using STEM−EDX.The distribution of elements in InOx (Figure S3) and 0.4Pd−InOx (Figure S4) is uniform, with no large aggregates observed, which confirms the evenly dispersed state of Pd in 0.4Pd−InOx, while for the sample of 1.2Pd−InOx, distinct Pd heterogeneous particles are observed (Figure S5).Combined with the XRD results, it can be inferred that Pd2In3 is formed for the sample of 1.2Pd−InOx.(HR−TEM) analysis revealed inter−planar distances of 0.27 nm and 0.29 nm over InOx (Figure 1d) and 0.4Pd−InOx (Figure 1e), corresponding to the (110) facet of h−In2O3 and (222) facet of c−In2O3, respectively.These characterizations underscore that InOx and 0.4Pd−InOx constitute a homogeneous structure composed of c−In2O3 and h−In2O3.Additionally, a distinct inter−planar distance of 0.31 nm was observed in 1.2Pd−InOx, corresponding to the (011) crystal plane of Pd2In3 (Figure 1f).It suggests that higher loading of Pd leads to some Pd interacting with In to form Pd2In3 intermetallic compounds.The SAED pattern of Pd−InOx is shown in Figure S6, consisting of polycrystalline diffraction rings.From the innermost to the outermost ring, they are indexed as (011), (−130) of Pd2In3, respectively.The SAED values are in good agreement with XRD results, further confirming the presence of Pd2In3 in 1.2Pd−InOx.

The Structure-Function Relationship
To further investigate the structural states of these catalysts, H2−TPR experiments were conducted on the fresh catalysts.From Figure 2a, peaks at 200-300 °C (as indicated by the red heart) is observed in the curves of all catalysts contributed to the formation of oxygen vacancy on the In2O3 surface, resulting in the reduced state of InOx.Further, compared to InOx, the area of this peak becomes stronger over 0.4Pd−InOx and 1.2Pd−InOx.It suggests the promoted surface reduction of InOx facilitated by the interaction with Pd [22].Additionally, a new peak at higher temperatures (300-400 °C) over 1.2Pd−InOx is formed.It may be attributed to the presence of Pd2In3 intermetallic compounds [23,24], which also suggested that the Pd2In3 intermetallic compounds significantly benefit the activation of hydrogen.SEM images revealed a bulky shape for the catalysts, while EDS mapping demonstrated the uniform distribution of In, O and Pd atoms in all xPd-InO x samples (Figures S1 and S2).We further investigated the elemental distribution in the material using STEM-EDX.The distribution of elements in InO x (Figure S3) and 0.4Pd-InO x (Figure S4) is uniform, with no large aggregates observed, which confirms the evenly dispersed state of Pd in 0.4Pd-InO x , while for the sample of 1.2Pd-InO x , distinct Pd heterogeneous particles are observed (Figure S5).Combined with the XRD results, it can be inferred that Pd 2 In 3 is formed for the sample of 1.2Pd-InO x .(HR-TEM) analysis revealed inter-planar distances of 0.27 nm and 0.29 nm over InO x (Figure 1d) and 0.4Pd-InO x (Figure 1e), corresponding to the (110) facet of h-In 2 O 3 and (222) facet of c-In 2 O 3 , respectively.These characterizations underscore that InO x and 0.4Pd-InO x constitute a homogeneous structure composed of c-In 2 O 3 and h-In 2 O 3 .Additionally, a distinct inter-planar distance of 0.31 nm was observed in 1.2Pd-InO x , corresponding to the (011) crystal plane of Pd 2 In 3 (Figure 1f).It suggests that higher loading of Pd leads to some Pd interacting with In to form Pd 2 In 3 intermetallic compounds.The SAED pattern of Pd-InO x is shown in Figure S6, consisting of polycrystalline diffraction rings.From the innermost to the outermost ring, they are indexed as (011), (−130) of Pd 2 In 3 , respectively.The SAED values are in good agreement with XRD results, further confirming the presence of Pd 2 In 3 in 1.2Pd-InO x .

The Structure-Function Relationship
To further investigate the structural states of these catalysts, H 2 -TPR experiments were conducted on the fresh catalysts.From Figure 2a, peaks at 200-300 • C (as indicated by the red heart) is observed in the curves of all catalysts contributed to the formation of oxygen vacancy on the In 2 O 3 surface, resulting in the reduced state of InO x .Further, compared to InO x , the area of this peak becomes stronger over 0.4Pd-InO x and 1.2Pd-InO x .It suggests the promoted surface reduction of InO x facilitated by the interaction with Pd [22].Additionally, a new peak at higher temperatures (300-400 • C) over 1.2Pd-InO x is formed.It may be attributed to the presence of Pd 2 In 3 intermetallic compounds [23,24], which also suggested that the Pd 2 In 3 intermetallic compounds significantly benefit the activation of hydrogen.XPS analysis was carried out to characterize the surface chemical information of the catalysts after reduction.The full XPS spectrum proved clearly the presence of In, O, and Pd as displayed in Figure S7.The In 3d spectra exhibit binding energies consistent with In 3+ over InOx (Figure 2b) [25,26], while Pd 3d signals could be deconvoluted into four peaks corresponding to Pd δ+ and metallic Pd 0 (Figure 2c) [27,28].Notably, compared to pure In2O3, a slight shift towards lower binding energy for In 3d XPS peak was observed for 0.4Pd/InOx, indicating a reduced valence state [29].Previous studies have indicated that the incorporation of Pd in oxides may result in the formation of Pd−O(H)−M bonds [30], while the presence of Pd δ+ species in the Pd 3d XPS spectra (Figure 2c) may offer evidence for the existence of Pd−O(H)−In structures for Pd/InOx [31].It is interesting to find the relative content of Pd δ+ decreased as the content of Pd increased to 1. CO2−TPD analysis reveals a low−temperature desorption peak for all catalysts, with the desorption temperature remaining unchanged but the peak area slightly increasing with more Pd loadings (Figure 2e) [33].This phenomenon indicates similar adsorption strength but a slightly enhanced adsorption capacity for Pd/InOx [34,35], which may be due to the beneficial effect of Pd−O(H)−In and Pd2In3 for the CO2 adsorption.It can be observed that after the formation of Pd2In3, an overlapping CO2 desorption peak appears at 171 °C, which may be attributed to the change in the CO2 desorption pattern caused by the formation of Pd2In3 [36].The H2−TPD analysis could demonstrate the catalyst's capability for dissociating and activating H2.On pure InOx, there is virtually no desorption peak, indicating a weak adsorption and activation capacity for H2 on the pure In−O bonds.As the Pd loading increases, the desorption peak area for xPd−InOx enhances at elevated XPS analysis was carried out to characterize the surface chemical information of the catalysts after reduction.The full XPS spectrum proved clearly the presence of In, O, and Pd as displayed in Figure S7.The In 3d spectra exhibit binding energies consistent with In 3+ over InO x (Figure 2b) [25,26], while Pd 3d signals could be deconvoluted into four peaks corresponding to Pd δ+ and metallic Pd 0 (Figure 2c) [27,28].Notably, compared to pure In 2 O 3 , a slight shift towards lower binding energy for In 3d XPS peak was observed for 0.4Pd/InO x , indicating a reduced valence state [29].Previous studies have indicated that the incorporation of Pd in oxides may result in the formation of Pd-O(H)-M bonds [30], while the presence of Pd δ+ species in the Pd 3d XPS spectra (Figure 2c) may offer evidence for the existence of Pd-O(H)-In structures for Pd/InO x [31].It is interesting to find the relative content of Pd δ+ decreased as the content of Pd increased to 1.2 wt.% (Figure 2d); however, the 3d 5/2 of In further shifted to a lower binding energy (444.4eV).This suggests the formation of another chemical bond over 1.2Pd/InO x , possibly attributed to the formation of Pd 2 In 3 intermetallic compound on the surface [18,32].Thus, Pd species tend to form highly dispersed Pd-O(H)-In sites on the sample surface at a low Pd loading, while partly forming Pd 2 In 3 sites at high Pd loading, resulting in a surface where Pd-O(H)-In and Pd 2 In 3 synergistically co-existence.Moreover, the presence of these active sites may alter the adsorption properties and reaction pathway, influencing the catalytic performance of CO 2 hydrogenation.Further detailed investigations are required to elucidate these effects.
CO 2 -TPD analysis reveals a low-temperature desorption peak for all catalysts, with the desorption temperature remaining unchanged but the peak area slightly increasing with more Pd loadings (Figure 2e) [33].This phenomenon indicates similar adsorption strength but a slightly enhanced adsorption capacity for Pd/InO x [34,35], which may be due to the beneficial effect of Pd-O(H)-In and Pd 2 In 3 for the CO 2 adsorption.It can be observed that after the formation of Pd 2 In 3 , an overlapping CO 2 desorption peak appears at 171 • C, which may be attributed to the change in the CO 2 desorption pattern caused by the formation of Pd 2 In 3 [36].The H 2 -TPD analysis could demonstrate the catalyst's capability for dissociating and activating H 2 .On pure InO x , there is virtually no desorption peak, indicating a weak adsorption and activation capacity for H 2 on the pure In-O bonds.As the Pd loading increases, the desorption peak area for xPd-InO x enhances at elevated temperatures (300-400 • C), as depicted in Figure 2f.In comparison, the peak area for 1.2Pd-InO x is greater than that for 0.4Pd-InO x .Considering the formation of Pd-O(H)-In and Pd 2 In 3 active sites within the Pd-InO x catalyst, this finding may corroborate that the Pd-O(H)-In site plays a more significant role in facilitating H 2 activation at lower temperature [9], whereas Pd 2 In 3 is more favorable for H 2 activation at higher temperatures [14].This is in agreement with the H 2 -TPR results.It is to be mentioned that a minor desorption peak observed at approximately 436 • C for 1.2Pd-InO x may be ascribed to the spillover of H 2 from Pd 2 In 3 to the support [37].

Catalytic Performance for CO 2 Hydrogenation
Under reaction conditions of 5 MPa and a gas hourly space velocity (WHSV) of 3000 mL•gcat −1 •h −1 , CO 2 hydrogenation experiments were conducted.As shown in Figures 3a and S8, the catalytic performance varies significantly among these three catalysts.At 200 • C, the CO 2 conversion rate of InO x is only 1.28%.In contrast, for 0.4Pd-InO x and 1.2Pd-InO x , the CO 2 conversion rates at 200 • C are 3.66% and 4.04%, respectively.It implies the promoted role of Pd-O(H)-In sites and Pd 2 In 3 sites in CO 2 conversion.Interestingly, the change in the CO 2 conversion rate with the increase in reaction temperature differed obviously for these catalysts.For InO x , the CO 2 conversion rate improved from 1.28% at 200 • C to 4.20% at 300 • C, while 0.4Pd-InO x demonstrates a slight CO 2 conversion rate increase to 4.56% at 300 • C. Prominently, for 1.2Pd-InO x , its CO 2 conversion rate increased notably at elevated temperatures, reaching 6.69% at 300 • C.These differences are attributed to the varied active sites over these catalysts [5,33].
1.2Pd−InOx, the CO2 conversion rates at 200 °C are 3.66% and 4.04%, respectively.It implies the promoted role of Pd−O(H)−In sites and Pd2In3 sites in CO2 conversion.Interestingly, the change in the CO2 conversion rate with the increase in reaction temperature differed obviously for these catalysts.For InOx, the CO2 conversion rate improved from 1.28% at 200 °C to 4.20% at 300 °C, while 0.4Pd−InOx demonstrates a slight CO2 conversion rate increase to 4.56% at 300 °C.Prominently, for 1.2Pd−InOx, its CO2 conversion rate increased notably at elevated temperatures, reaching 6.69% at 300 °C.These differences are attributed to the varied active sites over these catalysts [5,33].
To obtain a more comprehensive understanding of the reaction trend, the yields of CH3OH were calculated.According to the curves, the yield change was divided into the low−temperature region (200-250 °C) and the high−temperature region (250-300 °C).The InOx catalyst, with limited activation capability for H2, yields less than 2% in the lower−temperature region (Figure 3b).With the increase in Pd loading, the yields increase to over 3% for 0.4Pd/InOx and 1.2Pd/InOx catalysts.This might be attributed to the enhanced CO2 and H2 adsorption capability, especially for the H2 adsorption capability at lower temperatures due to the formation of Pd−O(H)−In sites.However, in the higher−temperature region, the yield of 1.2Pd−InOx reached over 6%, much higher than the yields of InOx (4.25%) and 0.4Pd−InOx (4.28%) (Figure 3c).The more favorable reaction of 1.2Pd−InOx at high temperatures may be due to the formation of Pd2In3 which greatly enhances its ability to adsorb and dissociate H2 at high temperatures.These phenomena illustrate that the varied active sites might play different roles in the reaction intermediates and pathways, which will be further discussed.We provide a comparison of the performance of Pd−based catalysts in Table S2.To obtain a more comprehensive understanding of the reaction trend, the yields of CH 3 OH were calculated.According to the curves, the yield change was divided into the low-temperature region (200-250 • C) and the high-temperature region (250-300 • C).The InO x catalyst, with limited activation capability for H 2 , yields less than 2% in the lowertemperature region (Figure 3b).With the increase in Pd loading, the yields increase to over 3% for 0.4Pd/InO x and 1.2Pd/InO x catalysts.This might be attributed to the enhanced CO 2 and H 2 adsorption capability, especially for the H 2 adsorption capability at lower temperatures due to the formation of Pd-O(H)-In sites.However, in the higher-temperature region, the yield of 1.2Pd-InO x reached over 6%, much higher than the yields of InO x (4.25%) and 0.4Pd-InO x (4.28%) (Figure 3c).The more favorable reaction of 1.2Pd-InO x at high temperatures may be due to the formation of Pd 2 In 3 which greatly enhances its ability to adsorb and dissociate H 2 at high temperatures.These phenomena illustrate that the varied active sites might play different roles in the reaction intermediates and pathways, which will be further discussed.We provide a comparison of the performance of Pd-based catalysts in Table S2.

Reaction Mechanisms
The activation pathways for CO 2 and H 2 to produce CH 3 OH were well established by the previous reports [38].Initially, CO 2 adsorbs onto the sample surface, forming *CO 2 , which then reacts with lattice oxygen (O lattice ) or dissociated hydrogen (*H) to produce either *CO To elucidate the effect of varied active sites on intermediate evolution, in situ DRIFTS studies were conducted on InO x (Figure 4a), 0.4Pd-InO x (Figure 4b), and 1.2Pd-InO x (Figure 4c).*CO 3 2− at 1456 and 1520 cm −1 and *HCO 3 − at 1650 cm −1 are observed on InO x [39,40].Subsequently, formate at 1559 and 1683 cm −1 , as well as *COOH at 1540 and 1637 cm −1 , are detected with time on stream [31,38], along with a small amount of *CO at 2077 cm −1 [8,41].In comparison to InO x , the initial area of CO 3 2− and HCO 3 − is significantly higher for 0.4Pd-InO x and 1.2Pd-InO x , which may be due to the enhancement of − to other intermediates (Figure 5b).
The activation pathways for CO2 and H2 to produce CH3OH were well established by the previous reports [38].Initially, CO2 adsorbs onto the sample surface, forming *CO2, which then reacts with lattice oxygen (Olattice) or dissociated hydrogen (*H) to produce either *CO3 2− or *HCO3 − .Subsequently, CH3OH is generated through two possible pathways.The first pathway involves the formation of *CO intermediate, generated via the RWGS routes involving carboxyl (*COOH) species.This intermediate is then hydrogenated to produce CH3OH.The second pathway entails the formation of formate (*HCOO) intermediates through the hydrogenation of CO2, ultimately leading to the production of CH3OH through C−O bond cleavage and *HCO or *H2CO intermediates (referred to as the formate pathway).

Preparation of InO x Catalysts
Firstly, 2.0 g of indium nitrate (99.99%,Macklin, Shanghai, China) was dissolved in a solution comprising 30 mL deionized (DI) water and 60 mL ethanol (AR, Sinopharm, Shanghai, China).Subsequently, 0.1 mL of nitric acid (AR, Sinopharm, Shanghai, China) was added.After complete dissolution, the obtained homogeneous solution was transferred into a Teflon-lined autoclave with a capacity of 200 mL and placed in an oven at 150 • C for 12 h.Upon naturally cooling down to room temperature, the resulting product was collected by centrifugation, washed with plenty of DI water and ethanol, and then dried at 60 • C overnight to obtain the InO x -UN.The InO x -UN was calcinated at 450 • C for 3 h in air to obtain the InO x .Before characterizations and the catalytic applications, InO x was reduced by H 2 at 380 • C for 120 min, except for H 2 -TPR.

Preparation of Pd-InO x Catalysts
The Pd-InO x samples were synthesized using the impregnation method.The palladium nitrate solution (Pd 18.09 wt.% in nitric acid, Macklin, Shanghai, China) was diluted with DI water to a concentration of 5.04 g•L −1 , designated as solution A. Subsequently, 0.8 g of InO x -UN (the precursor before reduction and roasting) was dispersed in 100 mL of ethanol under stirring, followed by the addition of 0.48 mL of solution A, which was stirred to dry at 70 • C. The resulting product was further dried overnight at 60 • C to obtain the 0.4Pd-InO x -UN.The samples were calcinated at 450 • C for 3 h in air, denoted as 0.4Pd-InO x (the number represented the weight percentage of Pd).1.2Pd-InO x was also prepared with a similar procedure to that of 0.4 Pd-InO x but adding 1.60 mL of palladium nitrate solution.Before characterizations and the catalytic applications, the catalysts were reduced by H 2 at 380 • C for 120 min, except for H 2 -TPR.

Catalyst Characterizations
The Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) measurements were carried out on an Avio 200 instrument (Thermo Electron Corporation, Massachusetts, USA).N 2 adsorption-desorption isotherms were measured on a Micromeritics ASAP 2460 (Micromeritics, Shanghai, China) at -196 • C. All the samples were outgassed at 150 • C overnight before the measurement.The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method.Scanning and transmission analytical electron microscopy (STEM-EDX) was performed using the FEI Talos F200X (FEI, Hillsboro, UAS).High-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) was performed using the JEM 2100F (JEOL, Japan).X-ray diffraction (XRD) patterns were collected on a Japanese Science Ultima IV (Rigaku Corporation, Japan) diffractometer equipped a Cu Kα radiation source, operated at 40 kV and 40 mA.The data were collected at a scan speed of 5 • •min −1 in the 2θ range of 15-85 • .
X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Scientific ESCALAB Xi+ XPS instrument (Thermo, Massachusetts, USA).Spectra of In 3d, C 1s, and Pd 3d were obtained, which were calibrated with the C 1s peak at 284.8 eV.To avoid the influence of air for the XPS test, the sample was protected by Ar gas first and then was vacuum packaged before transferring to the chamber of the XPS equipment.Hydrogen temperature-programmed reduction (H 2 -TPR) was performed on a ChemBET Pulsar (Quantachrome, Florida, USA).Typically, 50 mg of sample was placed in a U-pipe and purged with an argon flow (30 mL•min −1 ) at 260 • C for 60 min.The temperature was then decreased to 50 • C.Meanwhile, the gas flow was switched to a 10% H 2 /Ar mixture (30 mL•min −1 ) and the temperature was increased from 50 to 400 • C at a rate of 10 • C•min −1 .The effluent gas was monitored in real time via a thermal conductivity detector (TCD).CO 2 temperature-programmed desorption (CO 2 -TPD-MS) was performed on a lab-made reactor, and the outlet gases were analyzed by mass spectrum (HPR-20 EGA, HIDEN, Warrington, UK).In particular, the catalysts were flushed with He gas flow (30 mL•min −1 ) at room temperature for 30 min.Next, the catalysts were saturated with 10% CO 2 /He (30 mL•min −1 ) at room temperature for 30 min, followed by purging with He flow (30 mL•min −1 ) for 30 min.Then, the CO 2 -TPD experiment was measured from room temperature to 400 • C with a ramping rate of 10 • C•min −1 under He gas flow (30 mL•min −1 ).H 2 temperature-programmed desorption (H 2 -TPD) was also performed on a ChemBET Pulsar (Quantachrome, FL, USA).The catalysts were flushed with He gas flow (30 mL•min −1 ) at room temperature for 30 min.Next, the catalysts were saturated with 10% H 2 /He (30 mL•min −1 ) at room temperature for 30 min, followed by purging with He flow (30 mL•min −1 ) for 30 min.Then, the H 2 -TPD experiment was measured from room temperature to 450 • C with a ramping rate of 10 • C•min −1 under He gas flow (30 mL•min −1 ).
In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) spectra were collected using a Thermo IS10 FTIR (Massachusetts, USA) spectrometer with a mercury cadmium telluride (MCT) detector cooled with liquid nitrogen.The CO 2 and H 2
2 wt.% (Figure 2d); however, the 3d5/2 of In further shifted to a lower binding energy (444.4eV).This suggests the formation of another chemical bond over 1.2Pd/InOx, possibly attributed to the formation of Pd2In3 intermetallic compound on the surface [18,32].Thus, Pd species tend to form highly dispersed Pd−O(H)−In sites on the sample surface at a low Pd loading, while partly forming Pd2In3 sites at high Pd loading, resulting in a surface where Pd−O(H)−In and Pd2In3 synergistically co−existence.Moreover, the presence of these active sites may alter the adsorption properties and reaction pathway, influencing the catalytic performance of CO2 hydrogenation.Further detailed investigations are required to elucidate these effects.

3 2 −
or *HCO 3 − .Subsequently, CH 3 OH is generated through two possible pathways.The first pathway involves the formation of *CO intermediate, generated via the RWGS routes involving carboxyl (*COOH) species.This intermediate is then hydrogenated to produce CH 3 OH.The second pathway entails the formation of formate (*HCOO) intermediates through the hydrogenation of CO 2 , ultimately leading to the production of CH 3 OH through C-O bond cleavage and *HCO or *H 2 CO intermediates (referred to as the formate pathway).
CO 2 adsorption caused by the presence of Pd-O(H)-In sites.Furthermore, a new peak corresponding to the adsorption of *COOH at the interface appears at 1758 cm −1 , and the proportion of *CO and *COOH intermediates increases, as shown in Figure•5a and Table S3.This suggests that the appearance of Pd-O(H)-In sites alters the main reaction pathway transition, initiating the conversion of the key intermediate in CH 3 OH synthesis from *HCOO to *COOH and *CO [15,29].This likely occurs because the formation of Pd-O(H)-In modulates the catalyst's activation capability concerning *H [42].H 2 undergoes rapid activation at Pd-O(H)-In sites, subsequently converting to *COOH intermediates on the surface [43].For 1.2Pd-InO x that contained Pd 2 In 3 sites, the peak intensity of *HCO 3 − initially increases and then decreases.It indicates that the modulation of H 2 adsorption and dissociation capability by Pd 2 In 3 renders the catalyst more favorable for the transition from *HCO 3 2Pd−InOx, which may be due to the enhancement of CO2 adsorption caused by the presence of Pd−O(H)−In sites.Furthermore, a new peak corresponding to the adsorption of *COOH at the interface appears at 1758 cm −1 , and the proportion of *CO and *COOH intermediates increases, as shown in Figure•5a and Table S3.This suggests that the appearance of Pd−O(H)−In sites alters the main reaction pathway transition, initiating the conversion of the key intermediate in CH3OH synthesis from *HCOO to *COOH and *CO [15,29].This likely occurs because the formation of Pd−O(H)−In modulates the catalyst's activation capability concerning *H [42].H2 undergoes rapid activation at Pd−O(H)−In sites, subsequently converting to *COOH intermediates on the surface [43].For 1.2Pd−InOx that contained Pd2In3 sites, the peak intensity of *HCO3 − initially increases and then decreases.It indicates that the modulation of H2 adsorption and dissociation capability by Pd2In3 renders the catalyst more favorable for the transition from *HCO3 − to other intermediates (Figure 5b).

Figure 4 . 12 Figure 5 .
Figure 4. (a) In situ DRIFT spectra of InO x , (b) 0.4Pd-InO x , and (c) 1.2Pd-InO x .Reaction conditions: 200 • C, CO 2 /H 2 = 1/3, 10 mL•min −1 , and 0.1 MPa.Based on the phenomenon, distinct active sites might induce different reaction pathways, as summarized in Figure 6 [24].On the InO x surface, CO 2 and H 2 were activated on In-O, generating small amounts of *CO 3 2− and *HCO 3 − , which are then hydrogenated to form *HCOO, leading to CH 3 OH production primarily through the formate pathway [2,44].With the formation of Pd-O(H)-In interfacial sites on 0.4Pd-InO x , the initial peak area of *HCO 3 − and *CO 3 2− increases.Subsequently, a larger number of *COOH and *CO intermediates are formed as H 2 is more readily activated over Pd-O(H)-In sites, leading to CH 3 OH synthesis through the reverse water-gas pathway [45].After the formation of Pd 2 In 3 sites over 1.2Pd-InO x , H 2 becomes more accessible to be activated compared to when only Pd-O(H)-In sites are present [42].It could be further observed that the content of *HCO 3 − intermediates shows a trend of initially increasing and then decreasing, which aids in the continued conversion of *HCO 3 − to *COOH and *CO, further hydrogenating to produce CH 3 OH.

Figure 6 .
Figure 6.Possible reaction pathways corresponding to different active sites.

Figure 5 .
Figure 5. (a) Percentage of different intermediates over the xPd/InO x catalysts calculated by in situ DRIFT.(b) The trend of in situ DRIFT peak area changes for *HCO 3 − .

Figure 5 .
Figure 5. (a) Percentage of different intermediates over the xPd/InOx catalysts calculated by in situ DRIFT.(b) The trend of in situ DRIFT peak area changes for *HCO3 − .

Figure 6 .
Figure 6.Possible reaction pathways corresponding to different active sites.

Figure 6 .
Figure 6.Possible reaction pathways corresponding to different active sites.