Nature of the Active Sites and Reaction Mechanism during Methanol Steam Reforming over Cu/ZnO: An Isotopic Modulated Excitation Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study.

Hydrogen production with high efficiency and low CO selectivity is the objective of developing methanol steam reforming (MSR) catalysts. The nature of the active sites and reaction mechanisms, however, has not been fully resolved, hampering the rational design of improved catalysts. Herein, we apply the isotope modulation excitation-phase sensitive detection-diffuse reflectance infrared Fourier transform spectroscopy (ME-PSD-DRIFTS) method to identify active surface species and determine the reaction pathway of the dynamic processes. The findings show that methoxy, hydroxyl, and formate species are the key reactive intermediates on the Cu/ZnO catalyst, while carbonate species formed at higher temperatures are spectators that do not participate in the reaction. In addition, the MSR reaction on Cu/ZnO undergoes a "formate" route. Typically, methanol absorbs and dissociates to surface methoxy, which then reacts with hydroxyl from the dissociation of water to form surface formate species and finally decomposes to form CO2 and H2. The interfacial sites play key roles in methoxy dehydrogenation and water dissociation. The results highlight the potential of ME-PSD-DRIFTS to provide a detailed molecular level understanding of the reaction mechanism(s) in the MSR reaction and the dynamics of surface catalytic processes for heterogeneous catalysts in general.