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

Abstract

Selective hydrogenation of CO2 into methanol is a key sustainable technology, where Cu/Al2O3 prepared by surface organometallic chemistry displays high activity towards CO2 hydrogenation compared to Cu/SiO2, yielding CH3OH, dimethyl ether (DME), and CO. CH3OH formation rate increases due to the metal–oxide interface and involves formate intermediates according to advanced spectroscopy and DFT calculations. Al2O3 promotes the subsequent conversion of CH3OH to DME, showing bifunctional catalysis, but also increases the rate of CO formation. The latter takes place 1) directly by activation of CO2 at the metal–oxide interface, and 2) indirectly by the conversion of formate surface species and CH3OH to methyl formate, which is further decomposed into CH3OH and CO. This study shows how Al2O3, a Lewis acidic and non‐reducible support, can promote CO2 hydrogenation by enabling multiple competitive reaction pathways on the oxide and metal–oxide interface.

Exploiting two-dimensional morphology of molybdenum oxycarbide to enable efficient catalytic dry reforming of methane

“…In terms of required electronic energy, CO 2 cleavage of the adsorbed, bent CO 2 intermediate has a very low energy barrier of +31 kJ mol −1 . This bent CO 2 adsorbate was evaluated previously on several metal carbides 51 and was suggested to enhance the reactivity of CO 2 on interfaces between a metal and a metal carbide 52 or a metal oxide 53 – 55 . DFT calculations on the clean, Mo-terminated surface of δ-Mo 2 C(001) showed that CO 2 activation on molybdenum carbides is also associated with low activation barriers 56 .…”

Enhanced CH3OH selectivity in CO2 hydrogenation using Cu-based catalysts generated via SOMC from GaIII single-sites

“…S23 †), consistent with the observed formation of dimethyl ether during catalysis. Notably, no formate species on Lewis acidic gallium sites (present in the case of Cu/ZrO 2 , Cu-Zr/ SiO 2 , Cu-Ti/SiO 2 or Cu/Al 2 O 3 systems) 6,17,34,35 are observed, indicating that Ga III Lewis acidic sites possibly favor the subsequent hydrogenation of formate into methoxy species and/or increase the thermodynamic stability of methoxy in comparison to formate species. This is also conrmed by the IR spectra of the ex situ Cu-Ga/SiO 2 sample aer reacting with 1 H 2 / 13 CO 2 (Fig.…”

“…While formate species are also likely formed as reaction intermediates on Cu-Ga/SiO 2 under reaction condition, the lower stability of formate compared to methoxy would be consistent with (and explains) the higher CH 3 OH selectivity of this material in contrasts to other systems. Indeed, stable formate species have been shown to be able to generate methyl formate that readily decomposes into CO. 6 Further work is needed to investigate the formation (or not) of formate species as key intermediate in this Cu-Ga/SiO 2 system.…”

“…Regarding selectivity, the hydrogenation of CO 2 can also yield CO via, for instance, the reverse water gas shi (RWGS) reaction or the decomposition of methyl formate, that can be formed in situ via secondary reactions. 6,7 Heterogeneous catalysts that are able to form CH 3 OH with good selectivity and activity mainly consist of copper-based catalysts together with zinc oxide (e.g. Cu/ZnO or Cu/ZnO/Al 2 O 3 ), gallium oxide (e.g.…”

CO2 Hydrogenation to Methanol over La2O3-Promoted CuO/ZnO/Al2O3 Catalysts: A Kinetic and Mechanistic Study

“…As far as catalysts are concerned, efforts have been made to improve the performance of CZA by either replacing/doping Cu with noble metals (e.g., Pt, Pd) [12][13][14][15] or by adding oxides in order to enhance the adsorption properties of the support and to increase the dispersion of the Cu phases [16][17][18][19][20][21]. This is the reason why the majority of studies pinpoint some sort of coupling in the mechanism between the copper phase and the support oxide, with the latter being the anchor point for one of the oxygen atoms of CO 2 [22,23]. In this respect, several promoters have been studied, mainly oxides of metals such as Mn, La, Zr, Cr, and B [21,[24][25][26][27][28].…”