Understanding Methanol Synthesis on Inverse ZnO/CuO<sub><i>x</i></sub>/Cu Catalysts: Stability of CH<sub>3</sub>O Species and Dynamic Nature of the Surface
Ivan Orozco, Erwei Huang, Mausumi Mahapatra, Jindong Kang, Rui Shi, Slavomír Nemšák, Xiao Tong, Sanjaya D. Senanayake, Ping Liu, José A. Rodríguez
Abstract
Inverse ZnO/Cu catalysts are key systems in the conversion of CO2, a common atmospheric pollutant, into methanol, a high-value chemical and fuel. The chemistry of methanol and methoxy groups over inverse ZnO/Cu2O/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 *CH3O and *HCOO species with a minor amount of *CHx. 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 CO2. 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–Cu2O 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/Cu2O/Cu(111) surfaces upon reaction with methanol. The produced *CH3O, *HCOO, and *CHx species are localized in groups of active sites that have a dynamic nature and their structure changes during the adsorption/desorption processes.