Unravelling the Role of Oxygen Vacancies in the Mechanism of the Reverse Water–Gas Shift Reaction by Operando DRIFTS and Ultraviolet–Visible Spectroscopy

Abstract

The reaction mechanism of reverse water-gas shift (RWGS) reaction was investigated using two commercial gold-based catalysts supported on Al2O3 and TiO2. The surface species formed during the reaction and reaction mechanisms were elucidated by transient and steady-state operando DRIFTS studies. It was revealed that RWGS reaction over Au/Al2O3 proceeds through the formation of formate intermediates that are reduced to CO. In the case of Au/TiO2 catalyst, the reaction goes through a redox mechanism with the suggested formation of hydroxycarbonyl intermediates, which further decompose to CO and water. The Ti 3+ species, the surface hydroxyls, 2 and oxygen vacancies jointly participate. The absence of carbonyl species adsorbed on gold particles during the reaction for both catalysts indicates that the reaction pathway involving dissociative adsorption of CO2 on Au particles can be discarded. To complete the study Operando UV-Vis spectroscopy was successfully applied to confirm the presence of Ti 3+ and to understand the role of the oxygen vacancies of TiO2 support in activating CO2 and thus the subsequent RWGS reaction.

CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

“…[13,14] Regarding CO,i th as mainly been proposed to be formed on copper-based catalysts via two different mechanisms:T he so-called redox and formate pathways.T he former consists of the direct conversion of CO 2 to CO (that may involve M À COOH intermediates) and the subsequent reduction of Cu 2 Osurface species to Cu 0 metal forming H 2 O, while the latter involves the decomposition of formate into CO. [9,[15][16][17][18][19][20][21][22][23][24][25][26][27] Additionally,the formation of not only CO but also CH 3 OH is often proposed to be related to the presence of oxygen defect sites originating from the oxide support. [28][29][30][31][32][33][34][35][36] ForC u/ZnO/Al 2 O 3 ,t he origin of the promotional effect towards selective CH 3 OH formation is still amatter of debate. Fori nstance,C uZn alloy or the Cu/ZnO interface has been proposed to generate the active site responsible for the high CH 3 OH selectivity.…”

CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

“…The former consists of the direct conversion of CO 2 to CO (that may involve M−COOH intermediates) and the subsequent reduction of Cu 2 O surface species to Cu 0 metal forming H 2 O, while the latter involves the decomposition of formate into CO . Additionally, the formation of not only CO but also CH 3 OH is often proposed to be related to the presence of oxygen defect sites originating from the oxide support …”

Catalytic Reduction of CO2 to CO via Reverse Water Gas Shift Reaction: Recent Advances in the Design of Active and Selective Supported Metal Catalysts

“…Their experimental work has further confirmed that the stronger CO 2 adsorption on Pt/TiO 2 than that on Pt/SiO 2 improves the TOF of CO 2 conversion in the RWGS reaction by 4-5 times (Table 1) [58]. Bobadilla et al [62] reported that the activation energy of a RWGS reaction on Au/TiO 2 is 30 kJ/mol, which is much lower than that of 79 kJ/mol on Au/Al 2 O 3 ; as a result, the RWGS activity on Au/TiO 2 is higher than that on Au/Al 2 O 3 . As shown in Fig.…”

“…4, under identical reaction conditions (0.2 g of catalyst, H 2 /CO 2 = 4/1, gas hourly space velocity [GHSV] is 12,000 h −1 ), the conversion of CO 2 for Au/TiO 2 approaches the thermodynamic equilibrium of CO 2 conversion, while it is much lower on Au/Al 2 O 3 ( Table 1) [62]. Bobadilla et al [62] further performed DRIFTS and UV-vis spectroscopy studies and proposed that the presence of oxygen vacancies (or Ti 3+ cations) during the reaction facilitates the formation of a surface hydroxycarbonyl (OCOH) intermediate, which decomposes to form CO and H 2 O (Fig. 3b).…”

“…They also compared the OSC properties of Au/CeO 2 and Au/TiO 2 and suggested that oxygen vacancies away from an Au particle may be involved in the RWGS reaction on Au/CeO 2 , because of the higher oxygen mobility in CeO 2 than in TiO 2 . However, Chen et al [38] combined Fourier transform infrared (FTIR) experiments and a temperature-programmed surface reaction and proposed that redox and associate mechanisms exist on a Pt/ CeO 2 catalyst via a related mechanism that involves a surface formate species as the major path under their reaction [62] condition. These works have indicated that oxygen vacancies play an important role in the RWGS reaction, even though its reaction mechanism may vary depending on reaction conditions.…”