Bidentate Formate Species Research Articles

Exploring the sustained release catalysis of CuAl2O4 spinel for highly effective CO2 conversion to CO

Sustained Release Catalysis concept has been demonstrated over Cu-based spinel catalysts to retard Cu agglomeration via gradually releasing active copper from spinel lattice during catalytic reaction. Herein, the solid-phase synthesized stoichiometric CuAl2O4 spinel is illustrated to be an effective sustained release catalyst for CO2 hydrogenation to CO (RWGS). Characterizations point out a mechanism that active Cu can be liberated from spinel lattice under the RWGS atmosphere, forming Cu-deficiency defect spinel with abundant surface hydroxyl groups on rearranged spinel Al ions. Cu entities and surface hydroxyls can well activate H2 and CO2, producing monodentate and bidentate formate species as crucial intermediates for CO, especially, the monodentate formate located on spinel octahedral AlVI presents superior dissociation performance and contributes more to the CO formation. The successful application of the Sustained Release Catalysis concept in RWGS provides significant insight for understanding the catalytic dynamics of Cu-spinel catalysts in CO2 hydrogenation process.

Unraveling the role of Co3O4 facet for photothermal catalytic oxidation of methanol via operando spectroscopy and theoretical investigation

Tubular, pie- and bread-shaped forms of Co3O4 with exposed {110}, {112} and {111} facets were prepared and compared in their photothermal catalytic performance and reaction pathways during the oxidation of methanol. Among them, the Co3O4 with exposed {110} facet exhibited the best photothermal catalytic performance (95% methanol conversion, 93% CO2 yield) under solar irradiation, while also maintaining good stability and moisture resistance. Reaction mechanism studies showed that the {110} facets had a strong adsorption capacity for formaldehyde, which facilitated its conversion to formate. The transformation of formaldehyde to formate species was the key step. The key step on the {110} facet was conversion of formaldehyde to a mono-dentate formate species, while conversion on the {112} and {111} facets was mainly to bi-dentate formate species. This study demonstrated that the design of preferential exposed crystal facet can regulate the pathway of photothermal catalytic reaction and realize efficient solar energy utilization.

Low temperature CO2 methanation on hydrothermal synthesis of Ni-Ba/Sm2O3 catalysts

A low temperature CO2 methanation is a thermodynamically favorable route to produce highly selective methane while preventing catalyst deactivation. Ni-Ba/Sm2O3 catalysts synthesized using one-pot hydrothermal method exhibited enhanced reducibility with high CO2 adsorption capacity to achieve CO2 conversion at low temperatures. CO2 conversion occurred at 200 °C with 5% conversion, progressively increasing to reach equilibrium at 400 °C with 100% selectivity to methane. BaO promotes surface oxygen vacancy in Sm2O3, which is responsible for forming bidentate formate species during CO2 methanation. Comparative DRIFTS analysis with Ni-Ba/Sm2O3 synthesized using impregnation indicates the catalysts followed different mechanistic pathways depending on the amount of surface oxygen vacancy generated by BaO/Sm2O3 proximity.

Construction of Bi2O3 quantum Dots/SrBi4Ti4O15 S-scheme heterojunction with enhanced photocatalytic CO2 reduction: Role of Bi2O3 quantum dots and mechanism study

The development of layered structure (Bi2O2)2+ semiconductors have received extensive concern in energy and environment research attributable to their distinct layered structure, optical and electronic properties. However, the low charge separation efficiency and the tardy electron transfer process are limited the large-scale studies of bismuth-based materials. Herein, Bi2O3 quantum dots/SrBi4Ti4O15 S-scheme heterojunction (SBTO-1) with atomically ultra-thin structure is successfully synthesized through hydrothermal method by using hexadecyl trimethyl ammonium bromide (CTAB) as assistant. As compared with solely SrBi4Ti4O15, electrochemical tests showed that the as-prepared heterojunction greatly shortened the electron diffusion distance inside the photocatalyst and significantly increased the separation of electron-hole pairs. Meanwhile, the SBTO-1 heterojunction constructed based on Bi2O3 quantum dots can enhance the specific surface area and increase the active sites. Consequently, this S-scheme heterojunction material showed superior photocatalytic performance that CO and CH4 generations reach up to 73.0 and 30.2 μmolg−1, respectively, which are about 3 and 30 times higher than the pure SrBi4Ti4O15. More importantly, even under the air atmosphere, the SBTO-1 composite still presented excellent photocatalytic CO2 reduction ability. Density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) confirmed that electrons can be transferred to SrBi4Ti4O15 by Bi2O3 after constructing heterojunction, which has excellent charge separation ability. In-situ infrared further explained that the bidentate formate species (COO- and -HCOO-) were a crucial intermediate for photocatalytic CO2 reduction. This study provided some fresh insights for the design of bismuth-based photocatalysts that can capture CO2 significantly and effectively transform CO2.

Catalytic hydrogenation of CO2 to light olefins by using K-doped FeCx catalysts derived from the Fe-chitosan complex

K-doped FeCx catalysts derived from the carbothermal reduction of the Fe-chitosan complex were investigated and tested in the hydrogenation of CO2 to light olefins (C2-6=). Catalyst characterization, including bulk composition, porosity, crystallinity, Fe coordination, morphology, surface property, and basicity, was conducted. The physicochemical properties of tested catalysts were correlated with their performances. The mechanistic study discovered that bidentate carbonates, monodentate carbonate, bicarbonate, and formate species were intermediates involved in C2-6= synthesis. The yield of C2-6= was found to be dependent on the composition of the carbide phases of K-doped FeCx catalysts. The most active catalyst, i.e., FeK@CS-(0.5)-py, showed the most promising space-time yield of C2-6= (13.7 μmolC2-6=/gFe/s).

Effective CO2 methanation over site-specified ruthenium nanoparticles loaded on TiO2/palygorskite nanocomposite

Here a site-specifically assembled Ru nanoparticles (2–8 nm) on palygorskite (Pal) loaded TiO2 catalyst (TiO2/Pal) is reported to trigger the CO-radical mediated CO2 methanation reactions. The catalyst was prepared by the strong electrostatic adsorption (SEA) method based on the interactions between the metal source and the TiO2 support, which have successfully prevented the Ru nanoparticles (NPs) from aggregation. The optimized Ru(4%)-TiO2/Pal sample showed fairly good catalytic activities (μmax = 88.7%) to CO2 and selectivity of CH4 (Cmax ∼ 100%). The catalyst also demonstrated excellent stability: no obvious decedent in CO2 conversion and no carbon deposition were detected after 60 h methanation test. In situ FTIR measurement showed the CO species were an intermediate state, which reacted with the adsorbed hydrogen to produce hydro carbonate, bidentate or monodentate formate species, and finally reached the methane molecular. The SEA method has proven to be an effective way to produce site-specified nano-composites under delicately controlled conditions. Nanofibrous Pal acted as TiO2 carrier and separator, surface hydroxyls supplier, and origin of repulsive force to the Ru species, which finally resulted in outstanding catalytic performance.

Structural evolution and catalytic performance in CO2 hydrogenation reaction of ZnO-ZrO2 composite oxides

ZnO-ZrO2 composite oxides are used as catalysts for CO2 hydrogenation reaction. Herein, we prepare a series of ZnO-ZrO2 composite oxides via co-precipitation method and comprehensively study their structural evolution and catalytic performance in CO2 hydrogenation reaction. Via combined spectroscopic and microscopic characterization techniques, we found that, as the Zr content increased, phase structures of ZnO-ZrO2 composite oxides evolved from the mixture of hexagonal ZnO phase and Zn-doped ZrO2 solid solution phases to the pure Zn-doped ZrO2 solid solution phase. Zn-doped ZrO2 solid solution nanoparticles of mixed-phased ZnO-ZrO2 composite oxides were observed to preferentially nucleate and grow within the bulk of ZnO nanoparticles. Surfaces of mixed-phased ZnO-ZrO2 composite oxides are enriched with Zn, while the pure Zn-doped ZrO2 solid solutions exhibit uniform bulk and surface compositions. ZnO-ZrO2 composite oxides are much more catalytic active than pure ZnO, t-ZrO2 and m-ZrO2 in catalyzing CO2 hydrogenation reactions. The pure Zn-doped ZrO2 solid solution phase exhibit high selectivity toward CH3OH production, while pure ZnO, t-ZrO2, m-ZrO2 and mixed-phased ZnO-ZrO2 composite oxides exhibit high selectivity toward CO production. Bidentate formate species and carbonates/bicarbonates species are the key intermediates of the CO2 hydrogenation to methanol and CO, respectively. These results deepen the fundamental understanding of structure–activity relation and reaction mechanisms for ZnO-ZrO2 composite oxides-catalyzed CO2 hydrogenation reaction.

CO2 hydrogenation to methanol promoted by Cu and metastable tetragonal CexZryOz interface

Designing effective catalyst to improve the activity of CO2 hydrogenation to methanol is a potential avenue to realize the utilization of CO2 resources. Herein we construct three kinds of Cu/CexZryOz (CCZ) catalysts with different crystal phases of CexZryOz solid solutions, which demonstrate distinct activity and methanol selectivity in the order of metastable tetragonal-CCZ (CCZ-t″, parts of oxygen in CexZryOz were replaced by tetragonal phase from cubic fluorite phase) > tetragonal-CCZ (CCZ-t) > cubic-CCZ (CCZ-c) for CO2 hydrogenation to methanol. Structural analysis reveals that oxygen vacancies, surface hydroxyls and unsaturated Cu species of CCZ all follow the same sequence as that of activity and methanol selectivity, indicating that the above features are beneficial to improve the catalytic reaction performance. Temperature programmed experiments and mechanism studies show that the interface between Cu and tetragonal (t and t″) CexZryOz can promote CO2 adsorption, and the adsorbed CO2 is more reactive and can generate active bidentate carbonate species, which can be hydrogenated to form active monodentate and bidentate formate species under CO2 and H2 atmosphere. These intermediates should be crucial to the formation of methanol product. CCZ-t″ has stronger H2 activation ability than CCZ-t, which makes the former catalyst have more intermediates and higher methanol selectivity. In contrast, CO2 mainly adsorbs on cubic CexZryOz support of CCZ-c, but its H2 spillover ability is low, which hinders the reaction process. In addition, the strong adsorption of surface intermediates on CCZ-c is also not conducive to methanol formation. Results here demonstrate that constructing active Cu-support interfaces may be an important approach to design effective catalyst for CO2 hydrogenation.

CO2 Hydrogenation to Methanol over Partially Reduced Cu-SiO2P Catalysts: The Crucial Role of Hydroxyls for Methanol Selectivity

The involvement of surface hydroxyl species in controlling methanol selectivity for a CO2 hydrogenation reaction was investigated over Cu-phyllosilicate (Cu-SiO2P) catalysts prepared by a urea-assisted hydrothermal synthesis method. The role of hydroxyls involvement was investigated by treating the Cu-SiO2P catalyst between 225 and 350 °C in H2 gas. The presence of Cu-containing phyllosilicate structures in Cu-SiO2P catalysts was confirmed through TEM and XPS analyses. As evidenced by in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy, Cu+ centers are dominant surface species for Cu in Cu-SiO2P catalysts during the hydrogenation reaction. Also, bidentate formate species are the prominent intermediates to direct methanol formation via methoxy intermediate species. The hydroxyl intervention in CO2 hydrogenation for a hydroxyl-abundant Cu-SiO2P catalyst reduced at 225 °C is confirmed by the appearance of a new band at 2944 cm–1 (−C–H) by high-pressure in situ DRIFTS CO2 hydrogenation experiments. H2/CO-TPR revealed the uniformity and presence of surface hydroxyls in Cu-SiO2P catalysts. The best catalyst Cu-SiO2P reduced at 225 °C gave a stable CO2 conversion of 3.5% and a methanol yield of 140 mg MeOH/g-cat.h (a methanol selectivity of 77%) at 225 °C and 3 MPa pressure for a 24 h reaction time. The presence of hydroxyls generated due to partial reduction of the Cu-SiO2P catalyst and surface enriched with Cu+ species could be the reasons for its superior catalytic performance.

Highly efficient Cu/CeO2-hollow nanospheres catalyst for the reverse water-gas shift reaction: Investigation on the role of oxygen vacancies through in situ UV-Raman and DRIFTS

Self-assembled CeO2 with 3D hollow nanosphere, nanoparticle and nanocube morphologies were prepared and used to support Cu particles. The obtained samples (denoted as Cu/CeO2-hs, Cu/CeO2-np and Cu/CeO2-nc, respectively) were tested for the reverse water-gas shift (RWGS) reaction. Under the reaction conditions, V(H2):V(CO2) = 3:1 and weight hourly space velocity (WHSV) = 300,000 mL g−1 h−1, the Cu/CeO2-hs sample exhibited the best catalytic performance among the as-prepared catalysts. In order to reveal the key factors affecting the catalytic performance, the physicochemical properties of the catalysts were analyzed by XRD, BET, SEM, TEM, H2-TPR, quasi in-situ XPS and in situ UV-Raman techniques. The results point out that the particle size of Cu is not the rate-determining factor while the amount of surface oxygen vacancies is highly correlated with the catalytic reaction rate. In situ UV-Raman and in situ DRIFTS experiments indicate the formation of bidentate carbonate and formate species on oxygen vacancy sites which is thought to be the key intermediates for the RWGS reaction. These findings unravel the crucial roles of surface oxygen vacancies on CO2 adsorption and activation and also highlight a feasible way for the design of highly efficient catalysts for the important RWGS reaction in chemical industry.

The roles of step-site and zinc in surface chemistry of formic acid on clean and Zn-modified Cu(111) and Cu(997) surfaces studied by HR-XPS, TPD, and IRAS

The adsorption, desorption, and decomposition of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resolution x-ray photoelectron spectroscopy, temperature programmed desorption, and infrared reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid molecules adsorbed at 83 K were dissociated to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH molecules were dissociated into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH molecules were already dissociated into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid molecules were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissociation at low temperature. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissociation of formic acid. The desorption peak maxima of the formate decomposition products (CO2 and H2) on Zn-Cu(997) were shifted to higher temperatures than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface.

Reverse water-gas shift in a packed bed DBD reactor: Investigation of metal-support interface towards a better understanding of plasma catalysis

Palladium-loaded ZnO (Pd/ZnO) catalyst was synthesized by co-precipitation method and evaluated for plasma-catalytic reverse water-gas shift (RWGS) reaction. A CO2 conversion of 32.5 % was achieved on “Pd/ZnO + Plasma”, and was around two times higher than that of “ZnO + Plasma” (17.3 %). Catalyst characterization by X-ray diffraction, high-resolution transmission electron microscopy, temperature-programmed reduction and quasi in situ X-ray photoelectron spectroscopy revealed the possible formation of ZnOx@PdZn (ZnOx decorated PdZn alloy) at the interface of Pd active phase and ZnO support. The role of ZnOx@PdZn in plasma-catalytic CO2 conversion was systematically investigated by plasma-excited CO2 temperature-programmed desorption and in situ Fourier transform infrared spectroscopy. It was found that ZnOx@PdZn at Pd-ZnO interface improved significantly plasma-catalytic CO2 conversion by promoting the production of bidentate carbonate and bidentate formate species. In situ Raman spectroscopy studies proved that the ZnOx@PdZn remained largely intact during the RWGS process. Our in situ spectroscopic findings thus unveil that the ZnOx@PdZn at Pd-ZnO interface favors the activation of CO2, and, consequently, leads to a higher plasma-catalytic performance for CO2 conversion.

Pathways of Methane Transformation over Copper‐Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

AbstractThe reaction of methane with copper‐exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic CuII species. Several types of surface‐bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono‐ and bidentate formates, CuI monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper‐exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.

Pathways of Methane Transformation over Copper-Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy.

The reaction of methane with copper-exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic CuII species. Several types of surface-bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono- and bidentate formates, CuI monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper-exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.

In Situ Characterization of Mesoporous Co/CeO2 Catalysts for the High-Temperature Water-Gas Shift

Mesoporous Co/CeO2 catalysts were found to exhibit significant activity for the high-temperature water-gas shift (WGS) reaction with cobalt loadings as low as 1 wt %. The catalysts feature a uniform dispersion of cobalt within the CeO2 fluorite type lattice with no evidence of discrete cobalt phase segregation. In situ XANES and ambient pressure XPS experiments were used to elucidate the active state of the catalysts as partially reduced cerium oxide doped with oxidized cobalt atoms. In situ XRD and DRIFTS experiments suggest facile cerium reduction and oxygen vacancy formation, particularly with lower cobalt loadings. In situ DRIFTS analysis also revealed the presence of surface carbonate and bidentate formate species under reaction conditions, which may be associated with additional mechanistic pathways for the WGS reaction. Deactivation behavior was observed with higher cobalt loadings. XANES data suggest the formation of small metallic cobalt clusters at temperatures above 400 °C may be responsible. N...

Carbon Dioxide Methanation over Nickel-Based Catalysts Supported on Various Mesoporous Material

Diverse supports (ZSM-5, SBA-15, MCM-41, Al2O3 and SiO2) with various mesoporous structures were introduced to fabricate nickel-based catalysts for the CO2 methanation by incipient wetness impregnation method. Ni/ZSM-5 catalyst displayed the most active catalytic properties, followed with Ni/SBA-15, Ni/Al2O3, Ni/SiO2 and Ni/MCM-41 catalysts. The excellent catalytic property of Ni/ZSM-5 catalyst was resulted from the basic property and the synergistic effect between nickel metal and support. The reactivity of the reaction intermediate monodentate formate in Ni/ZSM-5 catalyst was more active than that of bidentate formate species as identified by in situ infrared spectroscopy. The Ni/ZSM-5 catalyst performed with excellent stability and no deactivation up to 100 h. XRD, BET and TGA/DTA characterization further indicated that Ni/ZSM-5 catalyst had excellent resistance to carbon deposition and metal sintering. What is more, the kinetics of CO2 methanation over Ni/ZSM-5 catalyst was also studied.

Formic acid oxidation at palladium electrode in acidic media containing chloride anions: An in situ ATR-SEIRAS investigation

The effect of chloride anions in electrolyte on formic acid oxidation (FAO) at Pd electrode is investigated by cyclic voltammetry and in situ surface-enhanced infrared absorption spectroscopy with attenuated total reflection (ATR-SEIRAS). Electrochemical measurement indicates that 1–2×10−5M Cl− in 0.1M HClO4+0.1M HCOOH solution leads to a significant decrease of formic acid oxidation current. Molecular level results from the ATR-SEIRAS indicate that such a small amount of chloride suppresses the adsorption of ClO4− and HCO3−, increased the CO coverage and somehow stabilized the bidentate formate (HCOOB) species over the main oxidation potentials. The blocking of active Pd surface sites by Cl− and COad is mainly responsible for the oxidation current density decay, and the inconformity of υ(HCOOB) band intensity and oxidation current implies that HCOOB is probably not the active intermediate in the direct pathway of formic acid oxidation.

A novel W-doped Ni-Mg mixed oxide catalyst for CO2 methanation

Novel Ni-W-Mg mixed oxide catalysts (NiWMgOx) were prepared by homogeneous precipitation and attempted for the methanation of CO2. Adding W remarkably promoted the activity with improved stability, anti-CO-poisoning ability and resistance against coke formation compared to the undoped NiMgOx catalyst. The superior reactivity of monodentate formate towards hydrogenation than that of bidentate formate species was identified by DRIFTS analysis and the formation of more active monodentate formate species was indisputably facilitated by W additives, leading to the greatly enhanced catalytic activity. H2-TPR and CO2-TPD characterization showed that doping W increased the number of stable CO2 adsorption sites and helped in anchoring the Ni sites as a result of strengthened Ni-Mg interaction, both of which were responsible for the enhanced CO2 methanation activity and the improved resistance against sintering.

Adsorption of Formic Acid on the Fe3O4(001) Surface

The adsorption of formic acid (HCOOH) on the Fe3O4(001) surface was studied using X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy (IRRAS), low-energy electron diffraction, and scanning tunneling microscopy (STM). At room temperature, HCOOH dissociates to form formate (HCOO–) and hydroxyl groups, facilitated by the close proximity of undercoordinated Fe3+/O2– cation/anion pairs at the Fe3O4(001) surface. Bidentate formate species are observed in IRRAS, and their position on Fe–Fe bridge sites can be inferred from STM data. At 70 K, HCOOH is adsorbed both dissociatively and molecularly, suggesting two active sites for dissociation. Our study also demonstrates that IRRAS is possible on Fe3O4 single crystals with good sensitivity but that unusual peak shapes occur because the substrate is midway between a perfect conductor and a perfect dielectric.

The loading effect of silver nanoparticles prepared by impregnation and solution plasma methods on the photocatalysis of Ga2O3

Ag loaded Ga2O3 (Ag/Ga2O3) photocatalysts for the reduction of CO2 with water have been prepared by impregnation (IMP) and two types of solution plasma methods (SPM1 and SPM2). Using X-ray absorption near edge structure (XANES) and Fourier transform infrared (FT-IR) spectroscopies, we have investigated the local electronic structures of Ag/Ga2O3 photocatalysts as well as the adsorption behaviors of CO2 during the reaction. Both Ag L3-edge and O K-edge XANES analyses reveal the Ag–Ga2O3 interaction, i.e., the charge-transfer from O atoms to Ag atoms, by demonstrating the decrease in the unoccupied Ag 4d-state density and increase in the unoccupied O 2p-state density. The strength of the interaction depends on the preparation method, and increases in the order of Ag/Ga2O3 (SPM2), Ag/Ga2O3 (SPM1) and Ag/Ga2O3 (IMP). In addition, FT-IR measurements have disclosed that Ag/Ga2O3 (IMP) obtains a larger amount of strongly basic sites as a result of the strongest interaction between Ag and Ga2O3. Although the amount of the adsorbed CO2 is different in each Ag/Ga2O3 sample, in the following formation process of bidentate formate species, no remarkable difference is detected among all samples. The bidentate formate species are likely to interact with H2O molecules to produce CO under photoirradiation, and this process would be affected by the strength of the Ag–Ga2O3 interaction, because this reaction hardly proceeds over Ag/Ga2O3 (SPM2) having the weakest Ag–Ga2O3 interaction.