Size-controlled model Ni catalysts on Ga2O3 for CO2 hydrogenation to methanol

Journal of Catalysis 376 (2019) 68–76

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Size-controlled model Ni catalysts on Ga2O3 for CO2 hydrogenation to methanol Hanseul Choi a,c, Sunyoung Oh b,c, Si Bui Trung Tran c, Jeong Young Park a,b,c,⇑ a

Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea c Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 34141, Republic of Korea b

a r t i c l e

i n f o

a b s t r a c t The effect of particle size for Ni nanoparticles supported on b-Ga2O3 was investigated for CO2 hydrogenation to methanol at 0.5 MPa. Model Ni nanoparticles ranging from 3.3 to 10.2 nm were synthesized using the hot injection method by controlling the reaction temperature and time. The smallest Ni nanoparticles (3.3 nm) showed the highest catalytic activity across the entire temperature range and the largest Ni nanoparticles (10.2 nm) showed the highest methanol selectivity. The apparent activation energies for methanol with Ni nanoparticles increased from 6.0 to 18.4 kcal mol
1 as the nanoparticle size increased. Furthermore, it was found that the smallest Ni nanoparticles favor the reverse water gas shift reaction. In situ DRIFT analysis revealed that the gallium oxide itself could produce an intermediate species and the addition of Ni on the oxide support increases the hydrogenation rate. The Ni supported catalysts showed a CO peak, but the smallest Ni nanoparticles showed a larger CO peak than that for the largest Ni nanoparticles, which clearly supports that the smaller nanoparticles favor the reverse water gas shift reaction. Ó 2019 Elsevier Inc. All rights reserved.

Article history: Received 14 May 2019 Revised 27 June 2019 Accepted 28 June 2019

Keywords: Ni nanoparticles CO2 hydrogenation Reverse water gas shift reaction Reaction mechanism Heterogeneous catalyst

CO2 þ H2 $ CO þ H2 O;

1. Introduction Much effort has gone into reducing CO2 levels in the atmosphere caused by the combustion of fossil fuels. A powerful way to reduce the CO2 in the atmosphere is by recycling CO2 into other valuable chemical products. If renewable hydrogen is used when converting CO2 into methanol, this reaction would have a beneficial effect on reducing greenhouse gas emission. Methanol is one of the most common commodity chemicals and it can be produced from the hydrogenation of CO and CO2. This methanol could be used as a fuel or as a raw material to produce other chemicals, such as olefins, gasoline, formaldehyde, dimethyl ether, and so on [1–3]. The methanol production reaction from CO2 requires high pressure (at least 5 MPa) and high temperature (between 200 and 300 °C). To date, copper-based catalysts have been widely used [4–11]. In the reaction, CO becomes another important issue because methanol synthesis (reaction (1)) and the reverse water gas shift (RWGS) reaction (reaction (2)), which produces CO, are competing reactions.

CO2 þ 3H2 $ CH3 OH þ H2 O;


1

DH0 ¼
49:5 kJ mol

ð1Þ

⇑ Corresponding author at: Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. E-mail address: [email protected] (J.Y. Park). https://doi.org/10.1016/j.jcat.2019.06.051 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.


1

DH0 ¼ 41:2 kJ mol

ð2Þ

As shown above, the methanol synthesis reaction is exothermic, meaning it favors low temperature, whereas the RWGS reaction is endothermic and favors higher temperature. However, the use of copper-based catalysts for methanol production also produces significant CO as a by-product during the RWGS reaction. Although the CO by-product is fed back into the reactor, there is still a need to develop new catalysts with low CO selectivity and high methanol activity and selectivity [12,13]. The first use of Ga2O3 for CO2 hydrogenation to methanol was reported in 1995 [14]. Fujitani et al. reported a 20-fold higher catalytic activity with Pd supported on Ga2O3 than Cu/ZnO, and it showed a 120-fold higher activity than Pd supported on SiO2. Since these results were first reported, Ga has been utilized for the CO2 hydrogenation to methanol reaction in the form of gallium oxide and with a promoter [5,8–10]. Ga doped into Pd supported on SiO2 catalysts showed higher catalytic activity from 160 to 250 °C at ambient pressure than that of Cu/ZnO/Al2O3 catalysts [15]. New Ni-Ga intermetallic catalysts supported on SiO2 were reported and the Ni5Ga3 catalyst showed higher activity and stability at ambient pressure than that of conventional Cu/ZnO/Al2O3 [13]. S. E. Collins et al. showed 20- and 4-fold higher methanol TOF and selectivity, respectively, with Ga2O3 than SiO2 as the support [16]. Furthermore, they increased the catalytic activity by adding

H. Choi et al. / Journal of Catalysis 376 (2019) 68–76

gallium into the Pd/SiO2, which showed a 500-fold higher catalytic activity compared with Pd/SiO2 at conventional reaction conditions (250 °C, 3 MPa, H2/CO2 = 3). At the same time, the methanol selectivity with the promoted catalyst increased to 70%, which was 10% higher than for Pd/SiO2. In heterogeneous catalysis, nanoparticle size plays a crucial role in catalytic activity and selectivity because of several molecular factors, including low-coordination sites [17], changes in oxidation states [18,19], and metal–oxide interfaces [20]. For the CO2 hydrogenation reaction, few studies have looked at the size effect of nanoparticles. H. Sakurai et al. used Au for active sites supported on ZnO [21]. In the literature, the particle sizes of the Au nanoparticles were changed from about 5 to 20 nm by controlling the calcination temperature from 400 to 750 °C after deposition using the precipitation method. In the reaction at 5 MPa, the methanol production rate decreased as the size of the Au increased because there are fewer peripheral sites along the metal–oxide interface. Furthermore, Hartadi reported particle size effects on CO2 hydrogenation at relative low pressure (0.5 MPa) with Au nanoparticles [22]. Here, they synthesized Au nanoparticles from 3.2 to 9.6 nm by controlling the calcination temperature from 300 to 600 °C. They reported that the methanol formation rate increased significantly with Au nanoparticles smaller than 4.3 nm, whereas the smaller Au nanoparticles (<7.5 nm) are active producing CO by the RWGS reaction on the support. However, the opposite trend was reported for the CO2 hydrogenation reaction with Co- and Cu-based catalysts. Iablokov et al. synthesized model Co nanoparticles ranging from 3 to 10 nm and the nanoparticles were deposited on a MCF-17 support [23]. In the CO2 hydrogenation reaction at 0.6 MPa, 10 nm nanoparticles showed 2.5 times higher TOF than that of 3 nm nanoparticles at 300 °C. They suggested that the lower activity of the smaller particles originated from a greater susceptibility for oxidation. Ni has not been widely used for methanol synthesis catalysts, but it has been used as a methane synthesis catalyst using CO and CO2 as the reactants [24,25]. It has been reported that the CO2 TOF in the CO2 hydrogenation to methane reaction with 2.5 nm Ni catalyst particles was highest at 400 °C at atmospheric pressure; the TOF decreased when the Ni particles were smaller or larger than 2.5 nm. In this work, we study the size effect of Ni on the CO2 hydrogenation to methanol reaction. It is very important to study with a model nanoparticle as the catalyst so a fundamental mechanistic study could be employed; thus, Ni model nanoparticles ranging from 3.3 to 10.2 nm were synthesized using the hot-injection method and deposited on Ga2O3. The CO2 hydrogenation to methanol reaction was conducted at mild conditions (0.5 MPa). Moreover, the reaction was conducted at similar conditions to allow for in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to determine the reaction mechanism.

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with excess ethanol and collected using centrifugation. The final Ni nanoparticles were re-dispersed in hexane. 2.2. Synthesis of the b-Ga2O3 support and Ni/b-Ga2O3 catalysts Commercial Ga2O3 (Aldrich) was used in this study. To convert it to the beta phase of gallium oxide, the commercial gallium oxide was calcined at 800 °C for 4 h in air. The as-synthesized Ni nanoparticles were deposited on the b-Ga2O3 support via sonication. In brief, 1 g of b-Ga2O3 was placed into a flask and was heated and evacuated to remove any surface moisture. The Ni nanoparticles dispersed in hexane were then injected into the flask targeting a nickel loading of 0.2 wt%. The mixture was sonicated for 30 min and the precipitate was collected using centrifugation. The final precipitate was dried in an oven overnight at 60 °C. 2.3. Catalytic activity measurement The catalytic activity was measured using a packed bed reactor. For catalyst testing, about 200 mg of each sample was placed on quartz wool and positioned inside the reactor [26]. The gas flow was controlled by a mass flow controller (5850E, BROOKS) and the pressure inside the reactor was controlled using a backpressure regulator. The catalysts were pre-treated at 350 °C for 2 h under 10% H2 to remove any remaining surfactant on the Ni surfaces. The reactant gas was composed of 69% H2, 23% CO2, and nitrogen as the balance. All the catalytic activity was measured from 160 to 300 °C at 20 °C increments and a total pressure of 0.5 MPa. For monitoring and controlling the temperature of the catalyst bed, a K-type thermocouple was located right above the catalyst bed. At each temperature, the catalytic activity was measured 5 times. The product was heated to hinder condensation of the gas before entering the gas chromatograph (GC). The final gas composition was monitored every 18 min using the GC (IGC 7200, DS SCIENCE) with a thermal conductivity detector (TCD) for CO and CO2, and a flame ionization detector (FID) for methane and methanol. 2.4. DRIFT analysis The DRIFT analysis was performed using a Fourier-transform infrared spectrometer (Carry 660, Agilent) [27]. A mercury cadmium telluride (MCT) detector was used and the reactant gases were controlled using a mass flow controller (5850E, BROOKS). The analysis was carried out at 0.1 MPa and all spectra were collected with 64 scans at a resolution of 4 cm
1. For the analysis, a self-supported wafer was made and placed in the reactor cell. The sample was pre-treated using 50% H2/He at 350 °C for 2 h, cooled to room temperature under He, and then kept at that condition for an additional hour. 2.5. Characterization

2. Experimental 2.1. Synthesis of Ni nanoparticles Ni nanoparticles were synthesized using the hot-injection method. In a typical synthesis, 0.52 g of Ni acetylacetonate (95%, Aldrich) was dissolved in 8 ml of oleylamine (70%, Aldrich), and the mixture was heated to 100 °C for 30 min under Ar. 5 ml of trioctylphosphine (TOP, 90%, Aldrich) was injected into another flask that was previously evacuated and the temperature was increased to 180–200 °C, and kept at that temperature for 30 min under Ar. The metal precursor mixture was injected into the TOP and aged for 30–60 min to control the size of the Ni nanoparticles. After cooling to room temperature, the black solution was precipitated

The morphologies of the Ni nanoparticles and Ni/b-Ga2O3 catalysts were identified using transmission electron microscopy (TEM, Tecnai F30 ST, FEI). The particle size distributions were obtained by counting 200–250 particles from TEM images. X-ray diffraction (XRD, D-MAX 2500, Rigaku) analysis was used with Ni-filtered Cu Ka radiation at a scanning speed of 4° min
1 from 40° to 80° of 2 theta. The amount of Ni loaded on the support was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, ICP-OES 720, Agilent). The chemical state of the Ni (Ni 2p3/2) was analyzed using X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo VG Scientific) with Al Ka as the X-ray source under ultra-high vacuum conditions of 10
10 Torr. The binding energies calibrated the C 1s peak at 284.8 eV. Metal dispersion of the Ni nanoparticles on the support was measured using H2

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chemisorption (BELCAT-B, BEL JAPAN Inc.). A 50 mg sample was placed in a quartz cell and pretreated at 350 °C for 30 min under H2; the hydrogen gas was then changed to He and held for 30 min. 10% H2/He was introduced several times at 50 °C until the area became stable (<1.2%), as was detected using a thermal conductivity detector. The metal dispersion was calculated assuming a H2/Ni stoichiometry ratio of 2. The surface area and pore diameter were calculated using the Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively, using an ASAP 2020 system (Micromeritics). Before measurement, the samples were degassed at 100 °C overnight.

3. Results and discussion 3.1. Synthesis of the Ni nanoparticles and Ni/b-Ga2O3 catalysts In this study, the Ni nanoparticle sizes were tuned by adjusting the injection temperature and reaction time. Fig. 1 shows TEM images of the four sizes of Ni nanoparticles ranging from 3.3 to 10.2 nm. It is obvious that all the nanoparticles have a spherical morphology with uniform size. Nanoparticles with mean sizes of 3.3 ± 0.5, 5.9 ± 0.5, and 7.9 ± 0.7 nm were synthesized by controlling the injection temperature at 180, 190, and 200 °C, respectively, and the reaction time was fixed at 30 min after injection. The

10.2 ± 0.9 nm Ni nanoparticles were synthesized at 200 °C for a prolonged reaction time (60 min). As shown in Fig. S1, the nanoparticles showed a narrow distribution. The size differences could be easily distinguished from Fig. 1 (all scale bars in Fig. 1 are exactly the same). The selected area electron diffraction pattern (SAED) was obtained using 7.9 nm Ni nanoparticles (Fig. 1(c) inset) and indicates that the main facet of the particles was (1 1 1), which is the main facet of the face-centered cubic (FCC) structure. Fig. 2 shows the XRD results for the Ni nanoparticles ranging from 3.3 to 10.2 nm. All the nanoparticles showed three peaks at 44.5, 51.9, and 76.5° of 2 theta and the results are well matched with the simulated pattern for the FCC structure of Ni (JCPDS No. 04– 0850). The main phase of the nanoparticles was (1 1 1), which matches with the SAED pattern, as shown in the inset in Fig. 1(c). The b-Ga2O3 after calcination at 800 °C for 4 h was analyzed using XRD (Fig. S2). After calcination, the beta phase of the gallium oxide was synthesized having (0 0 2) as the main facet; the XRD result is well matched with the simulated pattern. We measured the BET surface area and pore diameter (Fig S2(e) and (f)). The beta phase of the gallium oxide showed a low surface area because of the heat treatment at high temperature and showed a pore diameter of 15.7 nm. TEM images of the Ni/b-Ga2O3 deposited using the sonication method (Fig. S3) clearly show that the Ni nanoparticles were deposited on the support and that the particles were well

Fig. 1. TEM images of the four different sizes of Ni nanoparticles: (a) 3.3 ± 0.5 nm, (b) 5.9 ± 0.5 nm, (c) 7.9 ± 0.7 nm, and (d) 10.2 ± 0.9 nm. All the scale bars are exactly same.

H. Choi et al. / Journal of Catalysis 376 (2019) 68–76

Fig. 2. XRD patterns of the different sizes of Ni nanoparticles.

dispersed. In addition, the particle size of the nickel nanoparticles deposited on the support was similar to that of nanoparticles synthesized using the hot-injection method; the results are shown in Fig. S3. The metal loading was analyzed using ICP-OES and the results were shown in Table 1. A similar metal loading of about 0.2–0.3 wt% was measured for all four samples. Fig. S4 shows the Ni 2p3/2 XPS spectrum of the Ni nanoparticles (without support) after hydrogen treatment at 350 °C for 2 h under 5% hydrogen. To measure the XPS of the nanoparticles, we dropped 100 ll of hexane several times where the nickel nanoparticles were dispersed on the silicon wafer and then dried the sample at 70 °C for 10 min to remove any remaining hexane. The wafer was then transferred into a tubular furnace and reduced using hydrogen. Small peaks appeared at 852.6 and 854.6 eV for both the 3.3 and 5.9 nm Ni nanoparticles. However, the peak at 852.6 eV increased with increasing nanoparticle size, while the peak at 854.6 eV decreased with increasing nanoparticle size. The literature supports the assignment of the binding energies at 852.6 and 854.6 eV to Ni0 and Ni2+ in oxide, respectively [28]. It is obvious that the smaller nanoparticles are easily oxidized. 3.2. Catalytic activity We investigated the catalytic activity for CO2 hydrogenation to methanol at 0.5 MPa with different sizes of Ni nanoparticles on a support while increasing the temperature from 160 to 300 °C. To identify the thermal effect in the reaction temperature region at reaction conditions, we measured the activity with only the reactant gases at the same reaction conditions (not shown). Across the entire temperature region (160–300 °C), no products were detected, meaning there was no thermal effect at reaction conditions. Furthermore, the support effect was studied and the results are shown in Fig. S5. Fig. S5(a) shows the methanol production rate with four different supports. With the exception of b-Ga2O3, all the Table 1 Summary of metal loading, CO2 conversion, and methanol selectivity for the CO2 hydrogenation reaction at 280 °C with the Ni/b-Ga2O3 catalysts.

[a]

Catalysts

Metal loading[a] (%)

CO2 conversion (%)

MeOH selectivity (%)

3.3 nm Ni/b-Ga2O3 5.9 nm Ni/b-Ga2O3 7.9 nm Ni/b-Ga2O3 10.2 nm Ni/b-Ga2O3

0.19 0.33 0.16 0.24

0.94 0.80 0.54 0.45

14.7 21.3 25.0 32.6

Measured by ICP-OES.

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other supports produced no methanol across the entire temperature range (see Table S1); only b-Ga2O3 rapidly produced methanol starting at 260 °C. b-Ga2O3 can simultaneously absorb CO2 and dissociate hydrogen on its surface at temperatures above 200 °C. These absorbed CO2 and dissociated hydrogen atoms can react to produce methanol on the b-Ga2O3 surface [29–31]. Collins et al. proposed that intermediate species, such as bidentate and monodentate formate, were formed on the b-Ga2O3 surface above 200 °C at reaction conditions without metal. One of the big differences between the b-Ga2O3 support and the Pd/b-Ga2O3 catalyst in the reaction was the hydrogenation rate of the intermediates species bound on the b-Ga2O3 surface because of hydrogen spillover from the metallic Pd. Fig. S5(b) shows the production rate of all products from the b-Ga2O3 support. As mentioned, methanol was rapidly produced starting at 260 °C; more importantly, no CO and very little CH4 were produced below 300 °C. These results show that there was no RWGS reaction with the b-Ga2O3 at temperatures up to 300 °C and that the methanol synthesis reaction was the overwhelmingly predominant reaction. Fig. 3 shows the catalytic activity of the CO2 hydrogenation reaction with different sizes of Ni on b-Ga2O3. As mentioned in the experimental section, the catalysts were reduced in hydrogen for 2 h and we confirmed that there was no size change, even for the smallest 3.3 nm Ni during the pre-treatment (Fig. S6). The CO2 conversion was less than 1%, and the smallest 3.3 nm Ni showed the highest CO2 conversion at 280 °C. As shown in Fig. 3 (a), no methanol was produced below 200 °C; starting at 220 °C, methanol was produced regardless of the size of the Ni. The highest methanol activity was 0.0117 s
1 using the 3.3 nm Ni at 280 °C. The methanol TOF increased linearly up to 280 °C for all sizes of Ni, however, the TOF began to decrease at 300 °C. This decrease in the methanol TOF was accompanied by an increase in the CO TOF, which suggests a competitive process between the methanol synthesis and RWGS reactions. The CO TOF clearly supports this (Fig. 3(b)). The activity of CO with all sizes of Ni/b-Ga2O3 catalysts is shown in Fig. 3(b). Methanol was produced starting at a relatively low temperature (220 °C), but CO was first generated starting at relatively high temperatures (260 and 280 °C). The Ni particles (regardless of size) showed the highest CO TOF at 300 °C. The lowest temperature to produce CO was 20 °C lower with Ni particles smaller than 5.9 nm, which indicates that the smaller Ni nanoparticles accelerate the RWGS reaction. Moreover, the CO TOF increased more rapidly with the 3.3 and 5.9 nm Ni than with the 7.9 and 10.2 nm Ni. The smallest Ni showed the highest CO TOF across the entire temperature range, which also supports that the RWGS reaction was accelerated. A similar trend was seen with Au nanoparticles supported on ZnO. Y. Hartadi et al. reported the size effects of Au/ZnO on the CO2 hydrogenation reaction [22]. They controlled the Au nanoparticle size (ranging from 3.3 to 9.6 nm) by controlling the calcination temperature; these catalysts were used in the CO2 hydrogenation to methanol reaction at mild conditions (0.5 MPa). Thus, both the methanol and CO TOF increased as the Au nanoparticle size decreased; the TOF changed abruptly when the Au nanoparticles were less than 5 nm. They concluded that the CO was produced from the RWGS reaction. Ni catalysts supported on Ga2O3 showed a higher methanol TOF than that reported for Au- and Cu-based catalysts supported on ZnO at similar conditions. They controlled the size of the Au nanoparticles from 3.3 to 9.6 nm, which was very similar to the size of the Ni nanoparticles in our current work. They also carried out the CO2 hydrogenation reaction at 0.5 MPa and a H2/CO2 ratio of 3, which are also the same conditions as for our experiments. Therefore, it is reasonable to compare the results of the two sets of experiments conducted at very similar conditions. Hartadi reported methanol TOFs of 0.0013 and 0.0008 s
1 for the 3.2 and 9.6 nm Au nanoparticles at 240 °C, respectively [22]. In comparison, methanol TOFs

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Fig. 3. Catalytic activity of the different sizes of Ni/b-Ga2O3 catalysts during the CO2 hydrogenation reaction (69% H2/23% CO2/N2 balance) at 0.5 MPa. (a) Methanol TOF and (b) CO TOF as a function of temperature.

for the 3.3 and 10.2 nm Ni catalysts of 0.0087 and 0.0031 s
1, respectively, were reported; these TOF are about 6.7 and 3.9 times higher, respectively. In addition, they also measured the methanol TOF for two Cu-based catalysts (purchased from Alfa Aesar and Catalysis & Chemicals Specialties) in another study [32]. The methanol TOF was about 0.0013 s
1 at 240 °C, which is also lower than the TOF from the Ni-based catalysts. Generally, there are many factors that influence the catalytic activity: oxygen vacancies, interface sites, strong metal–support interactions (SMSI), etc. Among them, interface sites are known as active sites for methanol production from CO2. The Haruta group reported the size effect of Au catalysts supported on metal oxides on the CO2 hydrogenation to methanol reaction [21]. The methanol formation rate increased at both 250 and 300 °C as the diameter of the gold decreased. They suggest that the interface sites on the Au nanoparticles are active sites for methanol production. Another study suggests that the perimeter sites are the active sites for methanol production in the CO2 hydrogenation reaction. Strunk et al. proposed that oxygen vacancies on the perimeter of the Au-ZnO interfaces are the active sites for methanol production [33]. Therefore, it could be proposed that the interface between Ni and the support might also be an active site for Ni-based methanol production catalysts.

Fig. 4. Methanol selectivity of the Ni/b-Ga2O3 catalysts for the CO2 hydrogenation reaction as a function of temperature. The methanol selectivity was calculated from all the products: methanol, carbon monoxide, and methane.

Fig. 4 shows the methanol selectivity for the Ni/b-Ga2O3 catalysts from 220 to 300 °C. We calculated methanol selectivity from all the products during the reaction, including methanol, carbon monoxide, and methane. At 200 °C, only the 3.3 nm Ni catalysts showed 93% methanol selectivity; all the other sizes of Ni catalysts showed 100% methanol selectivity. From 220 to 240 °C, the 10.2 nm Ni catalysts showed 100% methanol selectivity and the 3.3 nm Ni catalysts had the lowest methanol selectivity compared with the other catalysts. Furthermore, the selectivity dropped sharply at 260 °C for the 3.3 and 5.9 nm Ni; however, this drop was observed at 280 °C for the 7.9 and 10.2 nm Ni. This rapid decline in methanol selectivity is mainly caused by the CO from the RWGS reaction. The temperature difference of 20 °C (260 °C and 280 °C) at which the methanol selectivity clearly dropped clearly shows that the RWGS reaction occurred more easily with the Ni nanoparticles smaller than 5.9 nm. Fig. S7 shows the TOF for all the products from the 3.3 nm Ni/b-Ga2O3 during the reaction, including CO, MeOH, and CH4. As shown, lots of CO was produced starting at 260 °C; methanol and a very small amount of CH4 were produced across the entire temperature range. The methanol selectivities in this study with a Ni catalyst, regardless of the Ni nanoparticle size, were higher than that for the Au- and Cubased catalysts, which have been used widely to produce methanol from CO2 [22,32,34]. Y. Hatardi et al. studied the particle size effects on CO2 hydrogenation reaction selectivity. Methanol selectivity with a 3.2 nm Au/ZnO catalyst was 72.8% and 85% with 9.6 nm Au on ZnO at 240 °C. In the case of the Cu-based catalyst, the Behm group reported that the methanol selectivity at 240 °C and 0.5 MPa was lower than 20% (purchased from Alfa Aesar, and Catalysts & Chemicals Specialties). Compared with the above results, the Ni/b-Ga2O3 catalyst with 3.3 nm Ni showed a much higher methanol selectivity at 240 °C (89.1%) and even higher methanol selectivities (95.6%, and 100%) were achieved with the 7.9 and 10.2 nm Ni on b-Ga2O3. The differences in selectivity with different sizes of Ni are caused by surface oxidation. It is well known that smaller nanoparticles are more easily oxidized, whereas larger nanoparticles are not. As shown in Fig. S4, the 3.3 nm Ni nanoparticles have more Ni2+ and lower metallic Ni. The 10.2 nm Ni nanoparticles, on the other hand, were more metallic. When we compare reactions (1) and (2), a higher stoichiometric ratio of hydrogen per molecule of CO2 could make methanol, whereas a lower ratio of hydrogen favors the production of CO via the RWGS reaction. Therefore, we could expect that the largest Ni nanoparticles (10.2 nm) that are more metallic can dissociate more hydrogen molecules, which

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results in a higher methanol selectivity; whereas, the smallest Ni nanoparticles (3.3 nm) that are less metallic dissociate relatively fewer hydrogen molecules, which would favor the RWGS reaction. Overall, Ga2O3-based Ni catalysts showed a higher catalytic activity and methanol selectivity than Au nanoparticles with a similar size and even higher activity than Cu-based catalysts, which have been widely used in methanol synthesis. Therefore, we could offer a new alternative for methanol synthesis catalysts. This enhanced catalytic activity might be one solution to lower the pressure of the reaction process. Fig. 5(a) shows an Arrhenius plot of methanol with the different sizes of Ni/b-Ga2O3 catalysts for the CO2 hydrogenation to methanol reaction. The activation energies were determined from the slope of the methanol TOF from 220 °C, where methanol was first produced, to 280 °C because the RWGS reaction becomes dominant at 300 °C with the 3.3 and 5.9 nm Ni. The calculated activation energies varied from 6.0 to 18.4 kcal mol
1. The smallest Ni nanoparticles (3.3 nm) showed the lowest activation energy and the largest Ni nanoparticles (10.2 nm) showed the highest activation energy. The activation energies were similar to those reported in the literature. In the CO2 hydrogenation to methanol reaction, activation energies were reported of 6.2–18 kcal mol
1 [8,12,35–37]. The lowest activation energy presented in this study is similar to that reported for catalysts containing Ga. R. Ladera et al. reported an activation energy of 7.4 kcal mol
1 for Cu/Ga2O3/ZnO/ZrO2 [38]. Medina reported an activation energy of 18.6 kcal mol
1 for methanol formation from CO2 with Cu/SiO2 catalysts. However, the activation energy decreased to 6.2 kcal mol
1 with Ga doping [12]. Fig. 5(b) summarizes the methanol TOF and selectivity at 280 °C depending on the size of the Ni catalysts on b-Ga2O3. As discussed above, the methanol selectivity decreased as the Ni size decreased, whereas the methanol TOF increased. This will be discussed further in the DRIFT results below. The stability of the 10.2 nm Ni on b-Ga2O3 catalyst was also measured at the highest temperature (300 °C). As shown in Fig. S8, the catalysts showed reasonable stability after 24 h. The 10 nm Ni nanoparticles on b-Ga2O3 catalyst were analyzed after the reaction using HRTEM (Fig. S9). Even though the size of the nanoparticles did not change, a thin layer formed on the Ni surfaces. We expect that the thin layer was GaOx because it is well known that a partially reduced state of the support could be formed on supported metal particles after reduction treatment [39–42]. Zhang reported that the partially reduced form of titania, TiOx, migrated under reducing conditions and formed a thin layer

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on Pd nanoparticles. Similar results occurred with iron oxide and tin oxide. Therefore, excess hydrogen during CO2 hydrogenation could reduce the gallium into a partially reduced state, GaOx, which migrated on the Ni surface and formed a thin GaOx layer. However, there is still no proven mechanism that supports the formation of a covered metallic surface during the reaction or reduction pretreatment. Therefore, in our current situation, we expect that the thin layer is GaOx and that the phenomenon will be studied in more depth in the near future. 3.3. In situ DRIFT results and reaction mechanism First, we flowed pure CO2 at room temperature for 10 min with b-Ga2O3 and 3.3 nm Ni/b-Ga2O3; the results are shown in Fig. 6(a). Both showed main three peaks at 1626, 1409, and 1220 cm
1, which are ascribed to the vas(CO3), vs(CO3), and d(OH) modes, respectively, from the bicarbonate species (HCO
3 ) on the gallium oxide. The peaks at 1587, and 1330 cm
1 are assigned to the vas(CO3), and vs(CO3) modes, respectively, from the bidentate carbonate species on gallium oxide. The peaks at 1483 and 1390 cm
1 were ascribed to vas(CO3) and vs(CO3), respectively, from the polydentate carbonate species on the gallium oxide [31]. Fig. 6(b) shows the DRIFT spectra during CO2 hydrogenation with b-Ga2O3 and 3.3 nm Ni/b-Ga2O3. We chose the reaction temperature at 300 °C, because the trend of catalytic activity and selectivity was clearly visible at this temperature. Both catalysts showed several peaks from 1200 to 2200 cm
1. In detail, the bands at 1550, 1380, and 1359 cm
1 were assigned to vas(COO
), d(CH), and vs(COO
) of the formate species [43,44]. The difference between the two bands of vas(COO
) and vs(COO
) (191 cm
1) is similar to the results found in the literature, which means that the formate species were bound on the surface with a bridging configuration [45]. The band at 1694 cm
1 indicates vs(C@O) in the formate species, which are bound on the surface with a monodentate configuration [46]. Both samples showed peaks at 2110 and 2180 cm
1, which correspond to gaseous CO, and a broad band at 3500 cm
1, which is assigned to the hydroxyl group in water. A broader hydroxyl group appeared for the 3.3 nm Ni/b-Ga2O3 catalysts than for the b-Ga2O3, whereas the gaseous CO peaks showed the opposite tendency. As mentioned above, the RWGS reaction occurred with the Ni catalyst, so we suggest that the hydroxyl group and gaseous CO in the DRIFT spectra were from the RWGS reaction. However, the b-Ga2O3 showed larger gaseous CO peaks and a smaller hydroxyl group, which means the adsorbed CO2

Fig. 5. (a) Arrhenius plots of the Ni/b-Ga2O3 catalysts with different Ni nanoparticle sizes and (b) methanol TOF and selectivity at 280 °C versus Ni nanoparticle size.

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Fig. 6. DRIFT spectrum (a) after flowing CO2 for 10 min at 25 °C and (b) during the CO2 hydrogenation reaction at 300 °C.

was reduced to make CO on the surface. One of the big differences in the DRIFT results is the peak from 3050 to 3300 cm
1 that is assigned to the hydroxyl group in alcohol (methanol in this work) [47]. Although we observed methanol from the CO2 hydrogenation reaction with b-Ga2O3 (shown in Fig. S5), the DRIFT spectra did not show a methanol peak, mainly because of the difference in pressure between the reaction conditions (0.5 MPa) and the DRIFT conditions (0.1 MPa). Therefore, we could conclude that deposition of Ni on a gallium oxide support can accelerate methanol production. S.E. Collins et al. proposed that the hydrogenation rate of carboncontaining species on gallium oxide increased from spillover of dissociated hydrogen atoms from the Pd metal as active sites [31]. Fig. 7(a) shows the DRIFT spectra with 3.3 and 10.2 nm Ni nanoparticles supported on b-Ga2O3 during the CO2 hydrogenation reaction at 100 °C. Both catalysts showed formate peaks as intermediates from 1200 to 1800 cm
1. However, only the 3.3 nm Ni catalysts showed a broad peak at 3280 cm
1 from the hydroxyl group in the methanol, meaning that the smaller Ni nanoparticles could produce methanol more easily than the larger Ni nanoparticles. It is well known that smaller nanoparticles have a higher

activity in catalysis. M. Cargnello et al. studied the effect of changing VIII metal catalyst size on catalytic activity. Regardless of the metal component, all the catalysts showed higher activity with smaller nanoparticles. In the case of Pt, the rate of CO2 produced with the smaller nanoparticles was more than 10 times higher than that for the larger nanoparticles. They suggest that the perimeter atoms were the active sites for the CO oxidation reaction [48]. Lee et al. also studied the effect of nanoparticle size on catalytic activity. They used 1.7 and 4.5 nm Pt nanoparticles on Au/TuO2. In the hydrogen oxidation reaction, the smaller nanoparticles showed about a 1.6-fold higher TOF than that for the larger nanoparticles at 373 K [20]. The Haruta group also reported a decreased methanol production rate in the CO2 hydrogenation reaction per exposed surface area of Au as the Au diameter increased. They suggested that this trend resulted from a decrease in the peripheral sites between the gold and oxide interface [21]. Therefore, we could suggest that a plausible active site is the interface between the Ni nanoparticles and the gallium oxide along the perimeter of the nanoparticles. The large peak of CO at 2075 cm
1 with 3.3 nm Ni means that the RWGS reaction was favorable with

Fig. 7. (a) DRIFT spectra from the 3.3 and 10.2 nm Ni/b-Ga2O3 catalysts during the CO2 hydrogenation reaction at T = 100 °C and P = 0.1 MPa and (b) corresponding magnified DRIFT spectra from 2025 to 2075 cm
1.

H. Choi et al. / Journal of Catalysis 376 (2019) 68–76

the smaller Ni nanoparticles; methanol was also produced more easily with the smaller Ni nanoparticles than with the larger Ni nanoparticles (Fig. 7(b)). The hydroxyl group from the methanol peak showed a marked increase starting at 200 °C as the temperature increased, as did the alkyl CAH stretch peak from methane starting at the same temperature (shown in Fig. S10). Fig. S11 shows the DRIFT spectra with 10.2 nm Ni nanoparticles supported on SiO2 during the CO2 hydrogenation reaction to show the gallium oxide effect. Here, we chose the 10.2 nm Ni nanoparticles as model catalysts because the larger Ni nanoparticle size had fewer side reactions at the reaction conditions at low temperature. At 100 °C, the peak from 1200 to 1800 cm
1 shows the intermediate species. However, large peaks at 1850 and 2050 cm
1, which are assigned to CO, and at 2850 and 2985 cm
1, which corresponded to methane, appeared even at 100 °C. These results suggest that if Ni nanoparticles exist, then the adsorbed CO2 on the surface would be reduced to CO and methane. Therefore, we could conclude that the use of gallium oxide as a support has the advantage in the CO2 hydrogenation reaction to produce methanol.

4. Conclusions In summary, we synthesized Ni model nanoparticles ranging from 3.3 to 10.2 nm using the hot injection method by controlling the reaction time; these Ni model nanoparticles were then deposited on a b-Ga2O3 support. The Ni/b-Ga2O3 was used as catalysts for the CO2 hydrogenation to methanol reaction at 0.5 MPa. The smallest Ni nanoparticles showed the highest methanol TOF at 280 °C, whereas the largest Ni nanoparticles showed the lowest methanol TOF. We associate the high catalytic activity of the smaller Ni nanoparticles with the large perimeter with the support. Furthermore, we show that surface oxidation influences methanol selectivity. Larger Ni particles were not easily oxidized, which facilitated dissociation of more hydrogen, and thus promoted methanol production by showing higher methanol selectivity. Compared with a Cu-based catalyst, the 3.3 nm Ni catalysts showed about a 7-fold higher methanol TOF at 240 °C. In addition, regardless of the Ni nanoparticle size, higher methanol selectivity was observed than that of the Cu-based catalyst. However, the methanol TOF decreased at 300 °C because of CO formation, which indicates that the RWGS reaction occurred easily with the smaller Ni nanoparticles. In situ DRIFT results revealed that the hydrogenation rate increased in the presence of Ni, and the catalysts followed the formate route in the reaction. Also, a larger CO peak was observed with the 3.3 nm Ni than that with the 10.2 nm Ni, meaning that the RWGS reaction was favorable with the smaller Ni nanoparticles in the CO2 hydrogenation to methanol reaction.

Declaration of Competing Interest The authors declare no competing financial interest.

Acknowledgment The work was supported by the Institute for Basic Science (IBS) [IBS-R004].

Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.06.051.

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References [1] W.C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Furler, S.M. Haile, A. Steinfeld, High-Flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria, Science 330 (2010) 1787. [2] J. Ma, N. Sun, X. Zhang, N. Zhao, F. Xiao, W. Wei, Y. Sun, A short review of catalysis for CO2 conversion, Catal. Today 148 (2009) 221. [3] A. Baiker, Utilization of carbon dioxide in heterogenerous catalytic synthesis, Appl. Organomet. Chem. 14 (2000) 751. [4] J. Słoczyn´ski, R. Grabowski, A. Kozłowska, P. Olszewski, J. Stoch, J. Skrzypek, M. Lachowska, Catalytic activity of the M/(3ZnOZrO2) system (M=Cu, Ag, Au) in the hydrogenation of CO2 to methanol, Appl. Catal. A 278 (2004) 11. [5] J. Toyir, P. Piscina, J. Fierro, N. Homs, Catalytic performance for CO2 conversion to methanol of gallium-promoted copper-based catalysts: influence of metallic precursors, Appl. Catal. A 34 (2001) 255. [6] X. Guo, D. Mao, S. Wang, G. Wu, G. Lu, Combustion synthesis of CuO–ZnO–ZrO2 catalysts for the hydrogenation of carbon dioxide to methanol, Catal. Commun. 10 (2009) 1661. [7] R. Raudaskoski, M.V. Niemelä, R.L. Keiski, The effect of ageing time on coprecipitated Cu/ZnO/ZrO2 catalysts used in methanol synthesis from CO2 and H2, Top Catal. 45 (2007) 57. [8] J. Sloczynski, R. Grabowski, P. Olszewski, A. Kozlowska, J. Stoch, M. Lachowska, J. Skrzypek, Effect of metal oxide additives on the activity and stability of Cu/ ZnO/ZrO2 catalysts in the synthesis of methanol from CO2 and H2, Appl. Catal. A 310 (2006) 127. [9] J. Toyir, P. Piscina, J. Fierro, N. Homs, Highly effective conversion of CO2 to methanol over supported and promoted copper-based catalysts: influence of support and promoter, Appl. Catal. A 29 (2001) 207. [10] X. Liu, G.Q. Lu, Z. Yan, Nanocrystalline zirconia as catalyst support in methanol synthesis, Appl. Catal. A 279 (2005) 241. [11] J. Liu, J. Shi, D. He, Q. Zhang, X. Wu, Y. Liang, Q. Zhu, Surface active structure of ultra-fine Cu/ZrO2 catalysts used for the CO2 + H2 to methanol reaction, Appl. Catal. A 218 (2001) 113. [12] J.C. Medina, M. Figueroa, R. Manrique, J. Rodríguez Pereira, P.D. Srinivasan, J.J. Bravo-Suárez, V.G. Baldovino Medrano, R. Jiménez, A. Karelovic, Catalytic consequences of Ga promotion on Cu for CO2 hydrogenation to methanol, Catal. Sci. Technol. 7 (2017) 3375. [13] F. Studt, I. Sharafutdinov, F. Abild-Pedersen, C.F. Elkjaer, J.S. Hummelshoj, S. Dahl, I. Chorkendorff, J.K. Norskov, Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol, Nat. Chem. 6 (2014) 320. [14] T. Fujitani, M. Saito, Y. Kanai, T. Watanabe, J. Nakamura, T. Uchijima, Development of an active Ga2O3 supported palladium catalyst for the synthesis of methanol from carbon dioxide and hydrogen, Appl. Catal. A 125 (1995) L199. [15] E.M. Fiordaliso, I. Sharafutdinov, H.W.P. Carvalho, J.-D. Grunwaldt, T.W. Hansen, I. Chorkendorff, J.B. Wagner, C.D. Damsgaard, Intermetallic GaPd2 nanoparticles on SiO2 for low-pressure CO2 hydrogenation to methanol: catalytic performance and in situ characterization, ACS Catal. 5 (2015) 5827. [16] S.E. Collins, J.J. Delgado, C. Mira, J.J. Calvino, S. Bernal, D.L. Chiavassa, M.A. Baltanás, A.L. Bonivardi, The role of Pd–Ga bimetallic particles in the bifunctional mechanism of selective methanol synthesis via CO2 hydrogenation on a Pd/Ga2O3 catalyst, J. Catal. 292 (2012) 90. [17] G.A. Somorjai, J.Y. Park, Molecular factors of catalytic selectivity, Angew. Chem. Int. Ed. Engl. 47 (2008) 9212. [18] K. Qadir, S.H. Joo, B.S. Mun, D.R. Butcher, J.R. Renzas, F. Aksoy, Z. Liu, G.A. Somorjai, J.Y. Park, Intrinsic relation between catalytic activity of CO oxidation on Ru nanoparticles and Ru oxides uncovered with ambient pressure XPS, Nano Lett. 12 (2012) 5761. [19] M.E. Grass, Y. Zhang, D.R. Butcher, J.Y. Park, Y. Li, H. Bluhm, K.M. Bratlie, T. Zhang, G.A. Somorjai, A reactive oxide overlayer on rhodium nanoparticles during CO oxidation and its size dependence studied by in situ ambientpressure X-ray photoelectron spectroscopy, Angew. Chem. Int. Ed. Engl. 47 (2008) 8893. [20] H. Lee, I.I. Nedrygailov, C. Lee, G.A. Somorjai, J.Y. Park, Chemical-reactioninduced hot electron flows on platinum colloid nanoparticles under hydrogen oxidation: impact of nanoparticle size, Angew. Chem. Int. Ed. Engl. 54 (2015) 2340. [21] H. Sakurai, M. Haruta, Synergism in methanol synthesis from carbon dioxide over gold catalysts supported on metal oxides, Catal. Today 29 (1996) 361. [22] Y. Hartadi, D. Widmann, R.J. Behm, CO2 hydrogenation to methanol on supported Au catalysts under moderate reaction conditions: support and particle size effects, ChemSusChem 8 (2015) 456. [23] V. Iablokov, S.K. Beaumont, S. Alayoglu, V.V. Pushkarev, C. Specht, J. Gao, A.P. Alivisatos, N. Kruse, G.A. Somorjai, Size-controlled model Co nanoparticle catalysts for CO2 hydrogenation: synthesis, characterization, and catalytic reactions, Nano Lett. 12 (2012) 3091. [24] M. Romero-Saez, A.B. Dongil, N. Benito, R. Espinoza-Gonzales, N. Escalona, F. Gracia, CO2 methanation over nickel-ZrO2 catalyst supported on carbon nanubutes: A comparison between two impregnation strategies, Appl. Catal. A 237 (2018) 817. [25] T.A. Le, M.S. Kim, S.H. Lee, T.W. Kim, E.D. Park, CO and CO2 methanation over supported Ni catalysts, Catal. Today 293–274 (2017) 89. [26] H. Choi, M. Carboni, Y.K. Kim, C.H. Jung, S.Y. Moon, M.M. Koebel, J.Y. Park, Synthesis of high surface area TiO2 aerogel support with Pt nanoparticle catalyst and CO oxidation study, Catal. Lett. 148 (2018) 1504.

76

H. Choi et al. / Journal of Catalysis 376 (2019) 68–76

[27] S. Oh, S. Back, W.H. Doh, S.Y. Moon, J. Kim, Y. Jung, J.Y. Park, Probing surface oxide formations on SiO2-supported platinum nanocatalysts under CO oxidation, RSC Adv. 7 (2017) 45003. [28] A.P. Grosvenor, M.C. Biesinger, R.S.C. Smart, N.S. McIntyre, New interpretations of XPS spectra of Ni metal and oxides, Surf. Sci. 600 (2006) 1771. [29] S.E. Collins, M.A. Baltanas, A.L. Bonivardi, Hydrogen chemisorption on gallium oxide polymorphs, Langmuir 21 (2005) 962. [30] S.E. Collins, M.A. Baltanas, A.L. Bonivardi, Infrared spectroscopic study of the carbon dioxide adsorption on the surface of Ga2O3 polymorphs, J Phys Chem C 110 (2006) 5498. [31] S. Collins, M. Baltanas, A. Bonivardi, An infrared study of the intermediates of methanol synthesis from carbon dioxide over Pd/b-Ga2O3, J. Catal. 226 (2004) 410. [32] Y. Hartadi, D. Widmann, R.J. Behm, Methanol formation by CO2 hydrogenation on Au/ZnO catalysts – effect of total pressure and influence of CO on the reaction characteristics, J. Catal. 333 (2016) 238. [33] J. Strunk, K. Kähler, X. Xia, M. Comotti, F. Schüth, T. Reinecke, M. Muhler, Au/ ZnO as catalyst for methanol synthesis: the role of oxygen vacancies, Appl. Catal. A 359 (2009) 121. [34] Y. Hartadi, D. Widmann, R.J. Behm, Methanol synthesis via CO2 hydrogenation over a Au/ZnO catalyst: an isotope labelling study on the role of CO in the reaction process, Phys. Chem. Chem. Phys. 18 (2016) 10781. [35] J. Yoshihara, S.C. Parker, A. Schafer, C.T. Campbell, Methanol synthesis and reverse-water gas shift kinetics over clean polycrystalline copper, Catal. Lett. 31 (1995) 313. [36] A. Karelovic, P. Ruiz, The role of copper particle size in low pressure methanol synthesis via CO2 hydrogenation over Cu/ZnO catalysts, Catal. Sci. Technol. 5 (2015) 869. [37] Y. Choi, K. Futagami, T. Fujitani, J. Nakamura, The role of ZnO in Cu-ZnO methanol synthesis catalysts-morphology effect or active site model, Appl. Catal. A. 208 (2001) 163. [38] R. Ladera, F.J. Pérez-Alonso, J.M. González-Carballo, M. Ojeda, S. Rojas, J.L.G. Fierro, Catalytic valorization of CO2 via methanol synthesis with Ga-promoted Cu–ZnO–ZrO2 catalysts, Appl. Catal. B. 142–143 (2013) 241.

[39] T. Okanishi, T. Matsui, T. Takeguchi, R. Kikuchi, K. Eguchi, Chemical interaction between Pt and SnO and influence on adsorptive properties of carbon monoxide, Appl. Catal. A 298 (2006) 181. [40] S. Zhang, P.N. Plessow, J.J. Willis, S. Dai, M. Xu, G.W. Graham, M. Cargnello, F. Abild-Pedersen, X. Pan, Dynamical observation and detailed description of catalysts under strong metal-support interaction, Nano Lett. 16 (2016) 4528. [41] H. Tang, Y. Su, B. Zhang, A.F. Lee, M.A. Isaacs, K. Wilson, L. Li, Y. Ren, J. Huang, M. Haruta, B. Qiao, X. Liu, C. Jin, D. Su, J. Wang, T. Zhang, Classical strong metalsupport interactions between gold nanoparticles and titanium dioxide, Sci. Adv. 3 (2017) e1700231. [42] R.N. Alnoncourt, M. Friedrich, E. Kunkes, D. Rosenthal, F. Girgsdies, B. Zhang, L. Shao, M. Schuster, M. Behrens, R. Schlogl, Strong metal-support interactions between palladium and iron oxide and their effect on CO oxidation, J. Catal. 317 (2017) 220. [43] G. Pang, D. Wang, Y. Zhang, C. Ma, Z. Hao, Catalytic activities and mechanism of formaldehyde oxidation over gold supported on MnO2 microsphere catalysts at room temperature, Front. Environ. Sci. Eng. 10 (2015) 447. [44] G. Busca, J. Lamotte, J. Lavalley, V. Lorenzelli, FT-IR study of the adsorption and transformation of formaldehyde on oxide surfaces, J. Am. Chem. Soc. 109 (1987) 5197. [45] J.P. Durand, S.D. Senanayake, S.L. Suib, D.R. Mullins, Reaction of formic acid over amorphous manganese oxide catalytic systems. An in situ study, J. Phys. Chem. C 114 (2010) 20000. [46] G.Y. Popova, A.A. Budneva, T.V. Andrushkevich, Identification of adsorption forms by IR spectroscopy for formaldehyde and formic acid on K3PMo12O40, React. Kinet Catal. 61 (1997) 353. [47] S.E. Collins, L.E. Briand, L.A. Gambaro, M.A. Baltanas, A.L. Bonivardi, Adsorption and decomposition of methanol on gallium oxide polymorphs, J. Phys. Chem. C 112 (2008) 14988. [48] V.V.T. Cargnello, T.R. Doan-Nguyen, R.E. Gordon, E.A. Diaz, R.J. Stach, C.B. Gorte Murray, Control of metal nanocrystal size reveals metal-support interface role for ceria catalysts, Science 341 (2013) 771.