A Highly Active Au / In 2 O 3 -ZrO 2 Catalyst for Selective Hydrogenation of CO 2 to Methanol

: A novel gold catalyst supported by In 2 O 3 -ZrO 2 with a solid solution structure shows a methanol selectivity of 70.1% and a methanol space–time yield (STY) of 0.59 g MeOH h − 1 g cat − 1 for CO 2 hydrogenation to methanol at 573 K and 5 MPa. The ZrO 2 stabilizes the structure of In 2 O 3 , increases oxygen vacancies, and enhances CO 2 adsorption, causing the improved activity. enables the high selectivity for methanol synthesis from CO 2 hydrogenation. The Au / In 2 O 3 -ZrO 2 catalyst shows a CO 2 conversion of 14.8% with a methanol STY of 0.59 g MeOH h − 1 g cat − 1 at 573 K and 5 MPa. Such an outstanding activity indicates that the addition of Zr remarkably improves the activity of the Au / In 2 O 3 catalyst. The catalyst characterizations demonstrate that Au NPs are well dispersed on the surface of the In 2 O 3 -ZrO 2 solid solution. Further investigation through Raman spectra, H 2 -TPR, and CO 2 -TPD indicates that the enhanced H 2 activation is achieved by the Au NPs strongly interacting with catalysts surface due to the SMSI e ﬀ ect. The presence of Zr mainly exhibits four e ﬀ ects: (1) reducing the apparent activation energy, (2) stabilizing the structure of In 2 O 3 , (3) increasing the amount of oxygen vacancies, and (4) enhancing the CO 2 adsorption. This work not only extends the utilization of gold catalysts for CO 2 hydrogenation but also promotes the fundamental studies for CO 2 conversion.


Introduction
With the development of renewable hydrogen, hydrogenation of CO 2 to methanol (Equation (1)) has received increasing attention worldwide. Significant efforts have been made towards the preparation of catalysts with high CO 2 conversion and high methanol selectivity. The most investigated catalysts are promoted copper catalysts based on the industrial Cu/ZnO catalysts for methanol synthesis from syngas [1][2][3]. Recently, ZnO-ZrO 2 [4], GaNi alloy [5], bimetallic PdZn [6], MOF [7,8], and others [1] were reported to have nice activity for CO 2 hydrogenation to methanol. CO 2 + 3 H 2 → CH 3 OH + H 2 O ∆H = −49.7 kJ mol −1 (1) In 2012, our group predicted via density functional theoretical (DFT) studies that In 2 O 3 possesses high activity for CO 2 activation [9]. The further DFT [10] and experimental [11] studies confirmed that In 2 O 3 with oxygen vacancies exhibited high activity for CO 2 hydrogenation to methanol, with unusually high methanol selectivity. Since then, In 2 O 3 -based catalysts have received significant attention, with increasing publications. The reported works include preparation and characterization of In 2 O 3 [12]; combination of In 2 O 3 with other oxides [13,14], especially ZrO 2 that forms the solid solution with In 2 O 3 and improves CO 2 adsorption with optimum oxygen vacancy [15][16][17] and generates strong electronic interaction between oxides [18]; and the addition of metals, including gold [19,20], copper [21,22], cobalt [23], palladium [24][25][26][27], rhodium [28], nickel [29], and platinum [30,31]. The metal catalysts applied significantly improve the hydrogenation ability of In 2 O 3 . It has been found that In 2 O 3 has a strong interaction with metals. This strong interaction causes a total change in catalytic performance. It makes the Ni, Au, Rh, and Pt catalysts become highly selective towards CO 2 hydrogenation to methanol with In 2 O 3 support. Especially for the Au catalyst, most of the reported Au catalysts are highly active for oxidation, with almost a thousand papers per year recently. However, only a few papers were published for hydrogenation. The In 2 O 3 -supported Au catalyst shows the highest selectivity and best activity ever reported for CO 2 hydrogenation to methanol. The methanol selectivity reaches 100% at temperatures below 498 K and is more than 70% at 548 K [19]. It is even 67.8% at 573 K, with a space-time yield (STY) of methanol of 0.47 g MeOH h −1 g cat −1 at 573 K, 5 MPa, and 21,000 cm 3 h −1 g cat −1 .

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In this work, we confirm that the In 2 O 3 -ZrO 2 supported Au catalyst presents even higher activity for CO 2 hydrogenation to methanol.

Results
In 2 O 3 -ZrO 2 oxides were prepared via a co-precipitation method, and then Au nanoparticles (NPs) were loaded on the carrier through the deposition-precipitation method. The loading amount of Au was 1%. The samples after reaction at 573 K and 5 MPa were named with "-AR" at the end. As shown in Supplementary Materials Figure S1, the sample with In/Zr molar ratio of 4:1 shows the best catalytic performance. The sample with this In/Zr ratio (labeled as Au/In 2 O 3 -ZrO 2 ) was thus selected for the activity test and further characterization. Based on the N 2 adsorption-desorption test on 77 K, the In 2 O 3 -ZrO 2 and Au/In 2 O 3 -ZrO 2 samples exhibit a similar total specific surface area of 111.5 and 121.6 m 2 g −1 , respectively. As shown in Figure 1a, the results of X-ray diffraction (XRD) show that the characterized peaks for all samples containing indium belong to cubic-In 2 O 3 (PDF file number 06-0416). Meanwhile, the peaks corresponding to Au cannot be observed, which results from the extremely low loading and ultra-high dispersion of Au NPs. This also means that Au NPs remain stable, preventing deactivation by the agglomeration. The absence of metallic indium after reaction also shows that the structure of the Au/In 2 O 3 -ZrO 2 catalyst is stable during the reaction. Moreover, the diffraction peak of the In 2 O 3 -ZrO 2 sample largely differentiates from that of ZrO 2 , which can be deduced that Zr may be penetrated into In 2 O 3 [32]. In Figure 1b, the high-resolution transmission electron microscopy (HRTEM) image of the Au/In 2 O 3 -ZrO 2 sample shows the crystalline lattice distances of 0.180, 0.398, 0.275, and 0.291 nm, which are assigned to cubic-In 2 O 3 (440), (211), (321), and (222) facets, respectively. No lattice fringes of ZrO 2 or Au can be observed. As shown in Figure 1c, the result of element distribution analysis indicates that the supported Au NPs and Zr are highly dispersed before and after reaction (Supplementary Materials Figure S2). These results further confirm the excellent dispersion of Au NPs and the conclusion that In 2 O 3 -ZrO 2 is in a solid solution state.
Catalysts 2020, 10, x FOR PEER REVIEW  2 of 10 MPa, and 21,000 cm 3 h −1 gcat −1 . In this work, we confirm that the In2O3-ZrO2 supported Au catalyst presents even higher activity for CO2 hydrogenation to methanol.

Results
In2O3-ZrO2 oxides were prepared via a co-precipitation method, and then Au nanoparticles (NPs) were loaded on the carrier through the deposition-precipitation method. The loading amount of Au was 1%. The samples after reaction at 573 K and 5 MPa were named with "-AR" at the end. As shown in Supplementary Materials Figure S1, the sample with In/Zr molar ratio of 4:1 shows the best catalytic performance. The sample with this In/Zr ratio (labeled as Au/In2O3-ZrO2) was thus selected for the activity test and further characterization. Based on the N2 adsorption-desorption test on 77 K, the In2O3-ZrO2 and Au/In2O3-ZrO2 samples exhibit a similar total specific surface area of 111.5 and 121.6 m 2 g −1 , respectively. As shown in Figure 1a, the results of X-ray diffraction (XRD) show that the characterized peaks for all samples containing indium belong to cubic-In2O3 (PDF file number 06-0416). Meanwhile, the peaks corresponding to Au cannot be observed, which results from the extremely low loading and ultra-high dispersion of Au NPs. This also means that Au NPs remain stable, preventing deactivation by the agglomeration. The absence of metallic indium after reaction also shows that the structure of the Au/In2O3-ZrO2 catalyst is stable during the reaction. Moreover, the diffraction peak of the In2O3-ZrO2 sample largely differentiates from that of ZrO2, which can be deduced that Zr may be penetrated into In2O3 [32]. In Figure 1b, the high-resolution transmission electron microscopy (HRTEM) image of the Au/In2O3-ZrO2 sample shows the crystalline lattice distances of 0.180, 0.398, 0.275, and 0.291 nm, which are assigned to cubic-In2O3 (440), (211), (321), and (222) facets, respectively. No lattice fringes of ZrO2 or Au can be observed. As shown in Figure 1c, the result of element distribution analysis indicates that the supported Au NPs and Zr are highly dispersed before and after reaction (Supplementary Materials Figure S2). These results further confirm the excellent dispersion of Au NPs and the conclusion that In2O3-ZrO2 is in a solid solution state.   The activity test of CO 2 hydrogenation to methanol was conducted in a vertical fixed-bed reactor. The products were analyzed with an online gas chromatograph (Agilent 7890A). For all catalysts, methanol and CO are the main products. Trace methane can be detected at reaction temperatures over 548 K. The carbon balance is better than 99%. As shown in Figure 2a, the Au/In 2 O 3 -ZrO 2 catalyst exhibits the highest catalytic performance among these three catalysts. The CO 2 conversion of the Au/In 2 O 3 -ZrO 2 catalyst is 14.8%, with a methanol selectivity of 70.1% at 573 K and 5 MPa. The methanol STY reaches 0.59 g MeOH h −1 g cat −1 , which is much higher than that over the Au/In 2 O 3 catalyst (0.47 g MeOH h −1 g cat −1 ) [19] and other Au-based catalysts (Supplementary Materials Table S1).
For all catalysts, CO 2 conversion increases with the temperature increases, while the methanol selectivity decreases due to the competitive reverse water-gas shift (RWGS) reaction (Equation (2)). Although CO can be detected at 498 K for the Au/In 2 O 3 -ZrO 2 catalyst, the higher methanol selectivity at 523 K and 548 K is obvious, as compared to the Au/In 2 O 3 catalyst. This result reveals that the Au/In 2 O 3 -ZrO 2 catalyst maintains high selectivity for methanol synthesis from CO 2 hydrogenation. On the other hand, the presence of Zr enhances the CO 2 conversion of the Au/In 2 O 3 catalysts. As shown in Figure 2c, the methanol STY on the Au/In 2 O 3 -ZrO 2 catalyst only decreases slightly and is stabilized at 92% of the initial value after a 6 h reaction at 573 K and 5 MPa. Furthermore, the Au/In 2 O 3 -ZrO 2 catalyst shows the lowest apparent activation energy (Ea) of 74.1 kJ mol −1 (Figure 2d). These results prove the excellent performance for CO 2 hydrogenation to methanol over the Au/In 2 O 3 -ZrO 2 catalyst.
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 10 The activity test of CO2 hydrogenation to methanol was conducted in a vertical fixed-bed reactor. The products were analyzed with an online gas chromatograph (Agilent 7890A). For all catalysts, methanol and CO are the main products. Trace methane can be detected at reaction temperatures over 548 K. The carbon balance is better than 99%. As shown in Figure 2a, the Au/In2O3-ZrO2 catalyst exhibits the highest catalytic performance among these three catalysts. The CO2 conversion of the Au/In2O3-ZrO2 catalyst is 14.8%, with a methanol selectivity of 70.1% at 573 K and 5 MPa. The methanol STY reaches 0.59 gMeOH h −1 gcat −1 , which is much higher than that over the Au/In2O3 catalyst (0.47 gMeOH h −1 gcat −1 ) [19] and other Au-based catalysts (Supplementary Materials Table S1). For all catalysts, CO2 conversion increases with the temperature increases, while the methanol selectivity decreases due to the competitive reverse water-gas shift (RWGS) reaction (Equation (2)). Although CO can be detected at 498 K for the Au/In2O3-ZrO2 catalyst, the higher methanol selectivity at 523 K and 548 K is obvious, as compared to the Au/In2O3 catalyst. This result reveals that the Au/In2O3-ZrO2 catalyst maintains high selectivity for methanol synthesis from CO2 hydrogenation. On the other hand, the presence of Zr enhances the CO2 conversion of the Au/In2O3 catalysts. As shown in Figure  2c, the methanol STY on the Au/In2O3-ZrO2 catalyst only decreases slightly and is stabilized at 92% of the initial value after a 6 h reaction at 573 K and 5 MPa. Furthermore, the Au/In2O3-ZrO2 catalyst shows the lowest apparent activation energy (Ea) of 74.1 kJ mol −1 (Figure 2d). These results prove the excellent performance for CO2 hydrogenation to methanol over the Au/In2O3-ZrO2 catalyst. To further understand the factors responsible for the enhanced catalytic performance, Raman analyses were performed. As shown in Figure 3a, the band at 132 cm −1 is assigned to the In-O vibration (VIn-O) [25]. The scattering features at 306 cm −1 (I1) and 493 cm −1 belong to the bending Raman spectra above. The H2-TPR profiles demonstrate the increasing surface oxygen vacancies over the Au/In2O3-ZrO2 catalyst, as a reason for its prominent catalytic activity.  To further investigate the adsorption behavior of CO2 on the catalyst surface, temperatureprogrammed desorption of CO2 (CO2-TPD) was conducted. Figure 4b presents the profiles of CO2-TPD, involving several signals in the temperature ranges of 325-425, 485-570, 570-755, and 730-865 K. The single peak around 360 K for all samples belongs to the desorption of physically absorbed CO2 [35]. Other peaks from low to high temperatures can be attributed to chemically absorbed CO2 on the different sites, ascribed to weak, medium, and strong basic sites, respectively. Comparing the Au/In2O3-ZrO2 sample with other samples, we see that the desorption peaks shift to the much higher temperatures of 707 and 798 K, respectively. This result implies that the addition of Zr enhances the strength of CO2 adsorption on these sites, probably owing to the enhanced basicity [36]. It can be To further understand the factors responsible for the enhanced catalytic performance, Raman analyses were performed. As shown in Figure 3a, the band at 132 cm −1 is assigned to the In-O vibration (V In-O ) [25]. The scattering features at 306 cm −1 (I 1 ) and 493 cm −1 belong to the bending vibration and stretching vibrations of InO 6 octahedrons, respectively [33]. The stretching vibration of In-O-In, occurring at 364 cm −1 (I 2 ), was confirmed to reflect the degree of oxygen defects [19]. For fresh samples, the band of V In-O visibly shifts towards lower frequencies due to the addition of Zr. Moreover, the characteristic peaks of the In 2 O 3 -ZrO 2 and Au/In 2 O 3 -ZrO 2 samples are completely the same as those of In 2 O 3 without the peaks attributed to ZrO 2 (Supplementary Materials Figure S3). These results affirm the penetration of Zr atoms into the indium oxides [32]. The intensities of I 2 increase with the addition of Zr, suggesting that more oxygen vacancies are generated. However, the loading of Au results in lower I 2 intensities, likely owing to the coverage of Au NPs on oxygen vacancies [30]. Subsequently, the variation of related peaks over the samples after H 2 reduction for different times under 473 K (labeled with "-HR-1.5h" or "-HR-1h"at the end) was further investigated. As shown in Figure 3b,c, H 2 reduction results in the increase of oxygen vacancies for all samples, according to the much larger I 2 band [30]. Moreover, prolonging reduction time enables oxygen vacancies to increase sequentially. The area ratio of I 2 and I 1 (labeled as I 2 /I 1 ) was calculated to characterize the amount of oxygen vacancies (Figure 3d) information of the sample can be obtained under the excitation laser with the wavelength of 532 nm [34]. The rapid increment of oxygen vacancies can be attributed to the over-reduction of pure In 2 O 3 [30]. It results in metallic indium without activity for H 2 activation [13]. Thus, it can be deduced that the strong metal-support interaction (SMSI) between Au and carriers could be conducive to maintain the structure of In 2 O 3 [19], which can also be performed through the incorporation of Zr into In 2 O 3 . Furthermore, compared to Au/In 2 O 3 , the Au/In 2 O 3 -ZrO 2 sample exhibits the higher I 2 /I 1 values for not only fresh samples but also the samples after reduction, implying the increase of oxygen vacancy density. These results coincide with the better catalytic activity of the Au/In 2 O 3 -ZrO 2 catalyst. H 2 temperature-programmed reduction (H 2 -TPR) was employed to investigate the reduction behaviors of the catalysts. The H 2 -TPR profiles are shown in Figure 4a. For all samples, the reduction peaks located below 500 K can be assigned to the surface reduction [30]. The onset of reduction peak at 530 K belongs to the reduction of bulk In 2 O 3 . Due to the fact that In 2 O 3 can be reduced by hydrogen more easily than ZrO 2 [17], the enhanced stability of Au/In 2 O 3 -ZrO 2 under the hydrogen atmosphere is thereby achieved. The XRD patterns of the spent samples were further investigated. As shown in Supplementary Materials Figure S4, there is a noticeable shift towards a lower angle for the In 2 O 3 -AR sample compared to fresh In 2 O 3 , while the Au/In 2 O 3 and Au/In 2 O 3 -ZrO 2 samples can remain at the same angle after the reaction at 573 K and 5 MPa. Our previous work reported that the lattice expansion of In 2 O 3 caused by thermal effect and oxygen vacancies could be performed on the shift of XRD diffraction peak [19]. Moreover, it is similar for lattice parameters of In 2 O 3 and Au/In 2 O 3 , heated under argon and reactant mixture at 573 K, respectively [19]. As the thermal effect can be excluded, it can be conducted that this shift may be attributed to the increasing oxygen vacancies of bulk In 2 O 3 . Thus, it can be deduced that the addition of Zr and Au could effectively hinder the over-reduction of In 2 O 3 under a reactant mixture including H 2 . In the temperature range from 275 to 530 K, the surface reduction peak shifts towards the lower temperature (below 360 K) with the addition of Au NPs. This result confirms that Au NPs promote the formation of surface oxygen vacancies. In addition, a broader and intense H 2 consumption peak at 350 K for the Au/In 2 O 3 -ZrO 2 sample is obvious. The incorporation of Zr into In 2 O 3 promotes the formation of surface oxygen vacancies on the Au/In 2 O 3 -ZrO 2 sample. The higher onset temperature of surface reduction and the disappeared peak at 456 K indicate the stabilization function of Zr on In 2 O 3 structure. These results are in line with the results of Raman spectra above. The H 2 -TPR profiles demonstrate the increasing surface oxygen vacancies over the Au/In 2 O 3 -ZrO 2 catalyst, as a reason for its prominent catalytic activity.
Catalysts 2020, 10, x FOR PEER REVIEW 5 of 10 Raman spectra above. The H2-TPR profiles demonstrate the increasing surface oxygen vacancies over the Au/In2O3-ZrO2 catalyst, as a reason for its prominent catalytic activity.  To further investigate the adsorption behavior of CO2 on the catalyst surface, temperatureprogrammed desorption of CO2 (CO2-TPD) was conducted. Figure 4b presents the profiles of CO2-TPD, involving several signals in the temperature ranges of 325-425, 485-570, 570-755, and 730-865 K. The single peak around 360 K for all samples belongs to the desorption of physically absorbed CO2 [35]. Other peaks from low to high temperatures can be attributed to chemically absorbed CO2 on the different sites, ascribed to weak, medium, and strong basic sites, respectively. Comparing the Au/In2O3-ZrO2 sample with other samples, we see that the desorption peaks shift to the much higher temperatures of 707 and 798 K, respectively. This result implies that the addition of Zr enhances the strength of CO2 adsorption on these sites, probably owing to the enhanced basicity [36]. It can be To further investigate the adsorption behavior of CO 2 on the catalyst surface, temperature-programmed desorption of CO 2 (CO 2 -TPD) was conducted. Figure 4b presents the profiles of CO 2 -TPD, involving several signals in the temperature ranges of 325-425, 485-570, 570-755, and 730-865 K. The single peak around 360 K for all samples belongs to the desorption of physically absorbed CO 2 [35]. Other peaks from low to high temperatures can be attributed to chemically absorbed CO 2 on the different sites, ascribed to weak, medium, and strong basic sites, respectively. Comparing the Au/In 2 O 3 -ZrO 2 sample with other samples, we see that the desorption peaks shift to the much higher temperatures of 707 and 798 K, respectively. This result implies that the addition of Zr enhances the strength of CO 2 adsorption on these sites, probably owing to the enhanced basicity [36]. It can be proposed that the enhanced CO 2 adsorption on the Au/In 2 O 3 -ZrO 2 sample benefits methanol synthesis. Time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were also conducted to gain the insights into CO 2 adsorption. As shown in Supplementary Materials Figure S5, the CO 2 interaction with Au/In 2 O 3 -ZrO 2 prefers to produce carbonate species rather than bicarbonate species due to the change of intensities [37]. The bicarbonate species disappears much faster after the Ar desorption, especially over the Au/In 2 O 3 sample. Moreover, compared to the Au/In 2 O 3 sample, the blue shift over the Au/In 2 O 3 -ZrO 2 sample for the bands at 900-1100 cm −1 , attributed to CO 2 absorbed on vacancies, suggests the stronger adsorption of CO 2 [17,38]. These results reveal that the addition of Zr strengthens the interaction between CO 2 and the catalyst surface. Based on the discussion above, the SMSI between Au and In 2 O 3 -ZrO 2 solid solution facilitates H 2 dissociation. Meanwhile, the incorporation of Zr into In 2 O 3 leads to the increasing oxygen vacancies, as well as the enhanced CO 2 adsorption. The H adatoms, dissociated by the Au NPs strongly interacting with the carrier, can spill over to the oxygen vacancy sites and react with the CO 2 molecule activated on the oxygen vacancy sites. This significantly promotes the CO 2 conversion into methanol.
In conclusion, we have shown that the integration of Au, In 2 O 3 , and ZrO 2 enables the high selectivity for methanol synthesis from CO 2

Catalyst Preparation
Indium-zirconium oxide carriers were prepared by a co-precipitation approach. Appropriate amounts of In(NO 3 ) 3  Thereafter, the precipitate was filtered and then washed with deionized water several times. Prior to the calcination at 723 K (10 K min −1 , 3 h), the sample was dried at 353 K overnight. The obtained carrier was labeled as In 2 O 3 -ZrO 2 . The In 2 O 3 and ZrO 2 samples were prepared with the same protocol. The Au/In 2 O 3 -ZrO 2 sample was prepared via a deposition-precipitation method, which is described in detail elsewhere [19]. The only difference was that the sample after freeze-drying overnight was directly used for the catalytic activity test, without a further reduction step. The samples with other molar ratios of In/Zr were denoted as Au/In 2 O 3 -ZrO 2 (m:n), where m:n is the molar ratio of In/Zr.

Characterization
The phase compositions of the samples were characterized by a Bruker D8 Focus diffractometer equipped with Cu Kα radiation source (λ = 1.54056 Å). Diffraction data were collected at a scanning speed of 8 • min −1 over the 2θ range of 10 • -70 • . The N 2 adsorption/desorption isotherms, measuring on an AUTOSORB-1-C instrument (Quantachrome) at 77 K, were used to determine the total specific surface area (S BET ).
The Raman spectra were recorded by an INVIA Raman analyzer (Renishaw PLC. Ltd., Gloucestershire, UK) with laser irradiation at 532 nm. The SEM images and EDX spectra were used to characterize microstructure and element distribution. All samples were measured on a Hitachi S-4800 SEM at an operating voltage of 25 kV without any conductive coating process. The transmission electron microscopy (TEM) and HRTEM images were obtained using a JEOL JEM-F200 microscope at the accelerating voltage of 200 kV.
H 2 -TPR was carried out using the Micromeritics Autochem II 2920 chemisorption analyzer. About 100 mg of the as-prepared sample was pretreated under flowing helium for 1 h at 473 K to remove the absorbed impurities. Thereafter, the sample was cooled down to 273 K by liquid nitrogen under flowing helium. Then the sample was heated to 973 K (10 K min −1 ) after switching to a mixture gas of 10% H 2 in N 2 . The effluent gas was analyzed using a thermal conductivity detector (TCD). The signals were normalized to the weight of the sample for the following analysis. CO 2 -TPD was conducted on the same apparatus as H 2 -TPR. After the same pretreating process as H 2 -TPR, the sample (about 100 mg) was cooled down to 323 K under flowing helium, followed by CO 2 adsorption for 1 h. After being purged with flowing helium for 1 h to remove physically absorbed CO 2 at the same temperature, the sample was heated to 1073 K under flowing helium at a rate of 10 K min −1 . The effluent gas was analyzed by the TCD. The signals were normalized to the weight of the sample for the following analysis.
Time-resolved in situ DRIFTS was performed using a Tensor 27 spectrometer (Bruker) equipped with a MCT detector by recording 32 scans at a resolution of 2 cm −1 . After the pretreatment under flowing argon for 2 h at 323 K, the background spectra were firstly collected and subtracted from the sample spectra. For CO 2 adsorption, a 1.0% CO 2 in Ar gas with the flow rate of 20 mL min −1 was fed into the system at 323 K. After the collecting of time-resolved spectra for 30 min, the gas was switched to pure Ar. The desorption time-resolved spectra were collected after flushing for 10 min.

Catalyst Activity Test
The activity test of CO 2 hydrogenation to methanol was conducted in a vertical fixed-bed reactor setup. The reactor was loaded with 0.2 g of the catalysts and 1.0 g of SiC, which was purged with N 2 , at room temperature. After 0.5 h, the gas flow was switched to the reactant mixture (H 2 /CO 2 /N 2 = 76/19/5, molar ratio) until the pressure reached 5 MPa. The reaction was conducted with a temperature range from 473 to 573 K. The gaseous hourly space velocity (GHSV) was 21,000 cm 3 h −1 g cat −1 . The reaction conditions were determined by the reported studies [17,39] and our previous study on Au/In 2 O 3 [19] in order to get better catalytic performance. Products were analyzed with an online gas chromatograph (Agilent 7890A) equipped with a flame ionized detector (FID) and a TCD. All the valves and lines between the reactor outlet and GC inlet were heated to 413 K to prevent the condensation of methanol. CO 2 conversion (X CO2 ) and selectivity (S i ) and space-time yield (STY, g i g cat where F and M are the molar flow rate and the molar mass of product i, respectively, and m cat is the catalyst sample weight. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/11/1360/s1. Table S1: Comparison of Au catalysts for CO 2 hydrogenation to methanol. Figure S1: (a) CO 2 conversion and methanol selectivity, and (b) methanol space-time yield (STY) respect to the molar In content, which is equal to m/(m + n) for the Au/In 2 O 3 -ZrO 2 (m:n) sample. Reaction conditions: 5 MPa, H 2 /CO 2 /N 2 = 76/19/5, GHSV = 21,000 cm 3 h −1 g cat −1 . Figure S2: TEM images of Au/In 2 O 3 -ZrO 2 (a), Au/In 2 O 3 -ZrO 2 -AR (b) and Au/In 2 O 3 -ZrO 2 -AR (c); SEM image (d) and the corresponding EDX mapping of Au/In 2 O 3 -ZrO 2 -AR. Figure S3: Visible Raman spectra of fresh ZrO 2 , Au/In 2 O 3 -ZrO 2 , In 2 O 3 -ZrO 2 , and In 2 O 3 . Figure S4: Enlarged XRD patterns of peaks around 32 • for fresh samples and the samples after reaction. Figure S5: Time-resolved in situ DRIFTS as a function of CO 2 adsorption time and Ar desorption time at 323 K after pretreatment in Ar over Au/In 2 O 3 (a) and Au/In 2 O 3 -ZrO 2 (b). Symbol definition: bicarbonate (white frame), carbonate (red frame), CO 2 absorbed on the vacancies featuring the higher CO 2 adsorption energy (yellow frame).
Author Contributions: C.L., conceptualization, supervision, and review and revision of the final version of the paper; Z.L., investigation, methodology, and visualization and writing-original draft preparation; K.S., methodology, and review and editing; J.W., review and editing; Z.Z., visualization. All authors have read and agreed to the published version of the manuscript.