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Heterogeneous Catalysis Of CO₂ Conversion To Methanol On Copper Surfaces.
Published 2014 · Medicine, Chemistry
Among the various valorization reactions of CO2, the Cucatalyzed hydrogenation to methanol belongs to the most promising conversions. Unlike many other approaches, a wellestablished large-scale process that converts synthesis gas (CO, CO2, H2) into methanol is already in operation (approx. 75 Mt/year, currently from fossil sources). In addition to its current role as mainly a base chemical, methanol is also a potential fuel, and a promising storage molecule for the energy sector. Provided a sustainable source of hydrogen becomes available at reasonable costs, this type of chemistry seems feasible to give CO2 molecules from flue gases “a second life” as a synthetic fuel and, thus, to substantially mitigate greenhouse gas emissions already in a shortto medium-term scenario. Given that there are still many open questions even with regard to the conventional syngas process, which has been studied for decades, it is no wonder that this field is currently experiencing a renaissance with many new insights into the structural and functional properties of Cu-based CO2 hydrogenation catalysts and into their mode of operation. In their recent report, Graciani et al. now present copper/ceria as a new promising catalyst system for the methanol synthesis reaction and propose a reaction mechanism based on surface scientific model studies and DFT calculations that deviates from the one that was thought to operate on conventional (Al2O3-promoted) copper/zinc oxide catalysts. Copper/ceria is known to be a powerful catalyst for CO oxidation, water– gas shift and methanol steam reforming, but until now only little attention has been paid to ceria as a promoter in methanol synthesis catalysts. This is astonishing, because there are interesting parallels between the modern view on copper/zinc oxide catalysts and the model that the authors elaborate for copper/ceria. In both systems, there is ample evidence that copper alone, that is, without contact to the support, shows a lower performance than the oxide-containing systems (although there certainly is a distinct activity also of clean copper). In their work, Graciani et al. elegantly show this by comparing the activity of a clean Cu(111) single crystal surface with one on which small ceria islands have been deposited (Figure 1a). The CeOx/Cu(111) was found to be substantially more active— a result that also has been obtained by Fujitani et al. in the copper/zinc oxide system for a ZnOx/Cu(111) model catalyst in a similar experiment. In both cases, the “reducible” nature of the oxide is thought to be important for the synergistic effect in CO2 hydrogenation. This synergistic effect of the oxide is in both Figure 1. Images of advanced surface science models of metal/oxide arrangements that can occur in heterogeneous catalysts. a) STM micrograph of the CeOx/Cu(111) inverse model catalysts studied by Graciani et al. Reprinted with permission from Ref. ; b) DFToptimized model of a ZnO bilayer surface phase supported on Cu(111) studied by Schott et al.; c) HRTEM micrograph of an ultrathin FeO surface film on Pt nanoparticles supported on Fe3O4 by Willinger et al.