Non-oxidative and selective methanol dehydrogenation to formaldehyde on Cu and Pd/Cu model catalyst surfaces: roles of step sites and deposited Pd.

Selective methanol-to-formaldehyde conversion under oxygen-free, dry conditions remains a key challenge on Cu-based catalysts. Previous surface-science studies have shown that a terrace Cu(111) surface exhibits little reactivity in vacuum. Here we elucidated active-site-controlled reactions on well-defined Cu model surfaces by directly comparing Cu(111), Pd/Cu(111), Cu(977), and Pd/Cu(977) surfaces. After methanol adsorption at low temperature (<100 K) and subsequent heating, the Cu(111) surface exhibits only molecular methanol desorption, while Pd/Cu(111), Cu(977), and Pd/Cu(977) generate formaldehyde accompanied by H2. Methoxy was identified as a stable intermediate by IRAS and XPS under dry conditions. Among these surfaces, the Pd/Cu(977) surface shows the lowest desorption temperature of formaldehyde and the highest formaldehyde yield. Kinetic analysis indicates that the apparent barrier associated with the rate-determining step (CH3O(ad) → CH2O(g) + 1/2H2) decreases systematically from terrace to stepped surfaces and further on the Pd-functionalized stepped surface. On Pd/Cu(977), nearly identical H2 desorption yields in the CH3OH → CH3O + 1/2H2 and CH3O → CH2O + 1/2H2 processes indicate the nearly 100% selective conversion of methoxy to formaldehyde. These results demonstrate that Cu step sites and deposited Pd cooperatively promote methanol-to-formaldehyde conversion under dry conditions.