First-Principles and Microkinetic Simulation Studies of the Structure Sensitivity of Cu Catalyst for Methanol Steam Reforming

High CO₂ and H₂ selectivity are important issues for methanol steam reforming (MSR) to provide H₂ as a clean energy carrier. The structure sensitivity and the factors governing the activity and selectivity for MSR on Cu catalysts are systematically investigated using density functional theory calculations and microkinetic simulation. Potential energy surfaces including water dissociation, methanol dehydrogenation, formaldehyde desorption and coupling with oxygen-containing intermediates, and formation of hydrogen and carbon dioxide are calculated over Cu(111), (100), (221), (211), and (110) surfaces, respectively. It is found that Cu(110) facet is the most active and selective toward carbon dioxide; Cu(221) is also active but highly selective toward formaldehyde, whereas Cu(111) is nearly inactive. Degree of rate control analysis shows that the activity is controlled mainly by methanol dehydrogenation to formaldehyde, whereas the degree of selectivity control shows that the selectivity toward formaldehyde or carbon dioxide depends sensitively on competition between formaldehyde desorption and coupling with surface oxygen. For Cu(110), abundance of both methoxy and oxygen as well as available vacant sites key for its high activity and selectivity toward carbon dioxide, whereas lack of oxygen on Cu(221) makes the corresponding surface highly selective for formaldehyde. The present work highlights the great influence of Cu surface orientations on the activity and selectivity for MSR