Synergy between Ceria Oxygen Vacancies and Cu Nanoparticles Facilitates the Catalytic Conversion of CO₂ to CO under Mild Conditions

Abstract

The performance of supported metal catalysts can depend on many factors, including metal particle size and dispersion and metal− support interactions, and differentiation of these effects is challenging because of their interwoven relationship. Copper/ceria catalysts are wellknown redox catalysts studied in the conversion of CO and CO 2 via oxidation and/or reduction pathways. The redox behaviors of each species, Cu-CuO and CeO x -CeO 2 , are often suggested to be interlinked, allowing ceria-supported copper domains to outperform copper species on other, nonredox active supports. In this work, the catalytic activity of nanosized Cu supported on either cerium oxide or mesoporous silica is explored using samples where the Cu weight loading, particle size, and dispersion of Cu are held constant to highlight the impact of the two supports on catalytic performance without additional influencing factors. The Cu/CeO 2 catalysts are synthesized via a spaceconfined method to limit the growth of CeO 2 particles and to achieve a high dispersion of Cu. Through in situ XRD and XAS, it is shown that the presence of Cu nanoparticles on the CeO 2 support lowers the reduction temperature of CeO 2 , allowing formation of oxygen vacancies at low temperatures <300 °C. The Cu/CeO x catalyst demonstrates 100% CO selectivity in the low temperature (300 °C) and ambient pressure conversion of CO 2 to CO, even when approaching equilibrium conversion. Moreover, this catalyst is approximately 4 times more active than the corresponding Cu/SiO 2 catalyst with otherwise similar structural attributes. The potential reaction pathways are probed by in situ FTIR and in situ XAS at various temperatures, identifying Cu + -CO species and oxygen vacancies forming under some conditions. The collected experimental evidence also suggests a reaction sequence for CO 2 hydrogenation over Cu/CeO x catalysts, consistent with DFT reports in the literature.

Recent Advances on the Rational Design of Non-Precious Metal Oxide Catalysts Exemplified by CuOx/CeO2 Binary System: Implications of Size, Shape and Electronic Effects on Intrinsic Reactivity and Metal-Support Interactions

“…The copper content can significantly affect the mole fraction of Ce 3+ and Cu + species (Figure 22), and in turn, the methanol yield [306]. In addition, a space-confined synthetic approach was applied for the synthesis of highly dispersed copper-ceria catalysts for RWGS, offering 100% CO selectivity at 300 °C and ambient pressure [307]. The enhanced catalytic performance was ascribed to the abundance in interfaces formed among the highly dispersed copper nanoparticles and the Ce 3+ species, thus, favoring H2 spillover [307].…”

Catalytic Conversion of Carbon Oxides in Confined Spaces: Status and Prospects

“…Reaction Promoted evidence and understanding Ref SCuCe CO 2 -to-syngas by CO 2 hydrogenation Space confined method to obtain abundant CuÀ CeO 2 interfaces by high Cu dispersion on CeO 2 , lowers CeO 2 reduction temperature and forms O vacancies, the activity even reaches equilibrium conversion [24] NiMgAl@m-SiO 2 CO 2 -to-syngas by dry reforming of CH 4…”

“…[8,[14][15][16][17][18][19][20][21][22][23] Currently, confinement catalysis has been extensively applied to the catalytic conversion of CO x regarding of improvement in catalytic activity, selectivity, and stability (Scheme 1). [9][10][11][12][24][25][26] Especially, the confined spaces of catalysts efficiently improve the catalytic selectivity for CO x conversion into fuels and high-value chemicals through the modulation of active sites and also the products self-regulation by the confined shell, which represents a promising direction for the selective conversion and utilization of carbon oxides.…”

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

“…Porosoff and Chen [76] compared the RWGS reaction on 1.7% Pt supported on reducible CeO 2 and irreducible Al 2 O 3 at 573 K (H 2 /CO 2 = 3/1, total pressure 30.0 Torr) and found that the forward reaction rate constant based on the mass of a catalyst and the amount of CO uptake (a measure of Pt metal dispersion) on Pt/CeO 2 are ~ 1.7 and ~ 5.7 times higher than those on Pt/Al 2 O 3 [76]. Yang et al [63] prepared ~ 2 nm Cu supported on CeO 2 and mesoporous SiO 2 to exclude the effect of the particle size of Cu. They observed that Cu/CeO x is about four times more active than Cu/SiO 2 at 573 K (Table 1; 0.2 g of catalyst, H 2 /CO 2 = 3/1, 1 bar, weight hourly space velocity of 1.08 h −1 ) and assigned this improvement to the formation of oxygen vacancies on the Cu/CeO x catalyst.…”

“…In summary, the activity of metal catalysts on reducible supports (CeO 2 and TiO 2 ) in a RWGS reaction is higher than that on irreducible supports (SiO 2 and Al 2 O 3 ; Table 1), but they may form more CH 4 . No CH 4 forms on Au-and Cu-based catalysts, whereas CH 4 may form on Pt and Pd catalysts [62,63]. For Pt catalysts, CH 4 selectivity is higher on reducible supports (TiO 2 and CeO 2 ) than on irreducible supports (SiO 2 and Al 2 O 3 ) possibly because of the removal of both O atoms from CO 2 by oxygen vacancies and the hydrogenation of the formed C species to CH 4 [76].…”