Mechanistic elucidation of cascade CO2 hydrogenation enabled by Cu–Fe interfaces and oxygen vacancies

Oxygen vacancies are generally recognized to play significant roles in CO2 adsorption and activation during CO2 hydrogenation. However, by revisiting its structural/electronic affinity for a range of oxygen-containing intermediates in CO2 hydrogenation processes, the additional roles of oxygen vacancies can be long overlooked and underestimated. Herein, using CO2 (photo-)methanation as a model reaction, Co3O4 with abundant oxygen vacancies is employed to investigate the relationship between oxygen vacancies and the formation/conversion of oxygen-containing intermediates. Combined analyses of in situ diffuse reflectance infrared Fourier transform spectroscopy and theoretical calculations reveal that the key intermediate is formate, whose C─O bond cleavage is inferred to be the rate-limiting step during CO2 methanation on Co3O4. Remarkably, leveraging the oxygen vacancy-mediated C─O bond scission to accelerate the conversion of formate, the CH4 production activity (1108.1mmol g-1 h-1) and selectivity (93%) are improved significantly. This comprehensive study provides valuable insights into the multifaceted roles of oxygen vacancies in CO2 hydrogenation reactions, establishing a solid foundation toward the design and development of high-performance oxide-containing/-based catalysts for the conversion of CO2 into various valuable chemicals.

Oxygen vacancies (OVs) on metal oxide surfaces are widelyrecognizedas catalytically active sites; however, the impact of their distributionon the catalytic performance remains underexplored. In this study,we used density functional theory (DFT) calculations combined witha machine learning potential to investigate the distribution of OVson the ZnO­(100) surface and their role in CO2 hydrogenation. We efficiently analyzed over 700,000 potential OVconfigurations by reducing them to unique, irreducible structuresusing the self-developed DefectMaker program. Our results revealedthat higher OV concentrations led to the formation of linear OV structures,which, despite their energetic stability, exhibited lower CO2 hydrogenation efficiency compared to isolated OVs, due to the reducedsurface polarization with linear OVs. Additionally, a comparativeinvestigation on In2O3 surfaces revealed a scattereddistribution of OVs, maintaining the material’s catalytic activityin CO2 hydrogenation. This work provides a deeper understandingof defect engineering in metal oxides for a more efficient CO2 conversion.

Promotion effects of oxygen vacancies on activity of Na-doped CeO2 catalysts for reverse water gas shift reaction

CO2 hydrogenation to ethanol is an attractive strategy for mitigating CO2 emissions. In this work, CoCeOx complex oxide catalysts with different Co/Ce ratios were used for CO2 hydrogenation to ethanol. It was found that a decrease in Co/Ce ratio contributed to oxygen vacancy formation, which facilitated the adsorption and activation of CO2. However, the decrease in Co/Ce ratio hampered H2 adsorption and activation due to the decreased Co sites. Therefore, moderate Co/Ce ratio maintained a balance between H2 and CO2 activation, resulting in the best catalytic activity and STY. CoCeOx catalyst with Co/Ce ratio of 1 exhibited the highest space‐time yield of 1.40 mmolEtOH·gcat−1·h−1 with ethanol selectivity of 12.3% and CO2 conversion of 51.4% under 235 °C and 2 MPa. In situ DRIFTS analysis further proved the synergistic effect between oxygen vacancies and cobalt species. On one hand, oxygen vacancies provide cobalt species with more HCOO* species, promoting the formation of ethanol. One the other hand, cobalt species make the oxygen vacancies from nonrenewable to recyclable by dissociating hydrogen. Our work provides valuable guidance for the design of catalysts and insights into the synergistic effect between oxygen vacancies and cobalt species in CO2 hydrogenation.

Facilitating CO2 methanation over oxygen vacancy-rich Ni/CeO2: Insights into the synergistic effect between oxygen vacancy and metal-support interaction

Effect of reduction pretreatment on the structure and catalytic performance of Ir-In2O3 catalysts for CO2 hydrogenation to methanol

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts: Effects of ZnO morphology and oxygen vacancy

The valorization of carbon oxides on metal/metal oxide catalysts has been extensively investigated because of its ecological and economical relevance. However, the ambiguity surrounding the active sites in such catalysts hampers their rational development. Here, in situ infrared spectroscopy in combination with isotope labeling revealed that CO molecules adsorbed on Ti3+ and Cu+ interfacial sites in Cu/TiO2 gave two disparate carbonyl peaks. Monitoring each of these peaks under various conditions enabled tracking the adsorption of CO, CO2, H2, and H2O molecules on the surface. At room temperature, CO was initially adsorbed on the oxygen vacancies to produce a high frequency CO peak, Ti3+−CO. Competitive adsorption of water molecules on the oxygen vacancies eventually promoted CO migration to copper sites to produce a low-frequency CO peak. In comparison, the presence of gaseous CO2 inhibits such migration by competitive adsorption on the copper sites. At temperatures necessary to drive CO2 and CO hydrogenation reactions, oxygen vacancies can still bind CO molecules, and H2 spilled-over from copper also competed for adsorption on such sites. Our spectroscopic observations demonstrate the existence of bifunctional active sites in which the metal sites catalyze CO2 dissociation whereas oxygen vacancies bind and activate CO molecules.

Influence of oxygen vacancies of CeO2 on reverse water gas shift reaction

Strong metal-support interactions (SMSIs) are essential for optimizing the performance of supported metal catalysts by tuning the metal-oxide interface structures. This study explores the hydrogenation of CO2 to methanol over Cu-supported catalysts, focusing on the synergistic effects of strong metal-support interaction (SMSI) and oxygen vacancies introduced by the CO2 treatment to the catalysts on the catalytic performance. Cu nanoparticles were immobilized on Mg-Al layered double oxide (LDO) supports and modified with nitrate ions to promote oxygen vacancy generation. Further calcination in a 15% CO2/85% N2 atmosphere at various temperatures not only resulted in the formation of SMSI and electronic metal-support interaction (EMSI) between Cu and MgO, but also generated abundant oxygen vacancies on MgO. The optimized 7.5%Cu/MA-C700 catalyst (Cu supported on MgAl-LDO treated in CO2 at 700 °C) exhibited significantly higher methanol production and turnover frequency compared to the air-calcined counterparts. In situ FTIR studies further revealed that oxygen vacancies led to the formation of more monodentate formate species, thus enhancing methanol production. This research provides a novel approach to engineering the catalyst interface structure and the interaction between the active metal and the support, particularly for the irreducible metal oxide support, for efficient hydrogenation of CO2 to methanol.

Modulation of inter-elemental synergy and oxygen vacancy content of CdZrOx solid solution catalysts by Ga for effective CO2 hydrogenation to methanol

Promotion of surface oxygen vacancies on the light olefins synthesis from catalytic CO2 hydrogenation over Fe[sbnd]K/ZrO2 catalysts

Reconstruction of interface oxygen vacancy for boosting CO2 hydrogenation by Cu/CeO2 catalysts with thermal treatment

CO2 hydrogenation to light olefins with high-performance Fe0.30Co0.15Zr0.45K0.10O1.63

Methanol synthesis from CO2 hydrogenation on the defective In2O3(110) surface with surface oxygen vacancies has been investigated using periodic density functional theory calculations. The relative stabilities of six possible surface oxygen vacancies numbered from Ov1 to Ov6 on the perfect In2O3(110) surface were examined. The calculated oxygen vacancy formation energies show that the D1 surface with the Ov1 defective site is the most thermodynamically favorable while the D4 surface with the Ov4 defective site is the least stable. Two different methanol synthesis routes from CO2 hydrogenation over both D1 and D4 surfaces were studied, and the D4 surface was found to be more favorable for CO2 activation and hydrogenation. On the D4 surface, one of the O atoms of the CO2 molecule fills in the Ov4 site upon adsorption. Hydrogenation of CO2 to HCOO on the D4 surface is both thermodynamically and kinetically favorable. Further hydrogenation of HCOO involves both forming the C–H bond and breaking the C–O bond, resulting in H2CO and hydroxyl. The HCOO hydrogenation is slightly endothermic with an activation barrier of 0.57 eV. A high barrier of 1.14 eV for the hydrogenation of H2CO to H3CO indicates that this step is the rate-limiting step in the methanol synthesis on the defective In2O3(110) surface.