Understanding Methanol Synthesis on Inverse ZnO/CuO_x/Cu Catalysts: Stability of CH₃O Species and Dynamic Nature of the Surface

Abstract

Inverse ZnO/Cu catalysts are key systems in the conversion of CO 2 , a common atmospheric pollutant, into methanol, a high-value chemical and fuel. The chemistry of methanol and methoxy groups over inverse ZnO/Cu 2 O/Cu(111) catalysts was investigated employing ambient pressure X-ray photoelectron spectroscopy (AP-XPS), scanning tunneling microscopy (STM), and calculations based on density functional theory (DFT). The results of AP-XPS show that the adsorption of methanol on the binary oxide substrate at 300 K leads to formation of *CH 3 O and *HCOO species with a minor amount of *CH x . Most of the methoxy groups disappeared from the surface after heating to 450 K, the onset temperature for the formation of methanol during the hydrogenation of CO 2 . The results of AP-XPS, STM, and DFT point to preferential adsorption of methoxy on the ZnO regions of the binary oxide. On the supported ZnO or on a ZnO−Cu 2 O interface, the breaking of the O−H bond in methanol is an exothermic process with a negligible (1−2 kcal/mol) or non-existent energy barrier depending on the size and shape of the ZnO islands. STM shows large changes in the morphology of ZnO/Cu 2 O/Cu(111) surfaces upon reaction with methanol. The produced *CH 3 O, *HCOO, and *CH x species are localized in groups of active sites that have a dynamic nature and their structure changes during the adsorption/desorption processes.

The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst

“…There is not a straightforward interpretation for the peaks in the carbon region, but given that multiple species can coexist at various sites, we can rule out graphitic carbon as an intermediate, as it gives rise to a much narrower peak at a lower binding energy of 285.4 eV ( 36 ). We stress that the interpretation of formate and methoxy on ZnO and CuZn from CO 2 and methoxy on CuZn from CO is basically consistent with the proposed mechanisms of methanol synthesis ( 4 , 6 , 14 ) and has been observed in low-pressure or vacuum experiments ( 7 , 24 , 30 , 37 – 40 ).…”

Selective Methane Oxidation to Methanol on ZnO/Cu2O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors

“…Beginning at 300 K and under an atmosphere of 10 mTorr O 2 and 20 mTorr CH 4 , that is, a stoichiometric ratio for the CH 4 + 1/2O 2 → CH 3 OH reaction, the associated surface chemistry is distinct from that of pure CH 4 in that there are now oxidation products on the surface (Figure b). Upon deconvolution, a feature is seen near 285.4 eV which likely comes from CH 3 O bound to copper oxide . CH 3 O groups bound to ZnO are expected at 286.3 eV, , and the feature will overlap with the peak for CH 4 gas.…”

“…Upon deconvolution, a feature is seen near 285.4 eV which likely comes from CH 3 O bound to copper oxide . CH 3 O groups bound to ZnO are expected at 286.3 eV, , and the feature will overlap with the peak for CH 4 gas. The existence of CH 3 O in Figure b is consistent with the production of CH 3 OH on ZnO/Cu 2 O/Cu­(111) after exposing the surface to a CH 4 /O 2 mixture (Figure ).…”

Recent Progress in Direct DME Synthesis and Potential of Bifunctional Catalysts

“…Recently, inverse oxide/metal catalysts with metastable ''graphite-like'' ZnO layers in Cu/ZnO/Al 2 O 3 catalysts have also attracted considerable interest due to the metal-support interaction, [39,40,42,43] with ZnO sites being considered as hydrogen reservoirs to favor methanol formation.…”