Active sites and effects of co-adsorbed H2O on isolated methanol dehydrogenation over Pt/γ-Al2O3

•Infrared spectra of CO adsorbed on Pt particles with varying size distributions revealed several sites with distinct reactivity.•Larger Pt particles dehydrogenate methanol faster and at lower temperatures than smaller Pt particles with or without co-adsorbed H2O.•Even on smaller Pt particles, methanol appears to dehydrogenate on the metal sites of highest coordination first.•Co-adsorbed H2O promotes the formation of bridging CO but hinders dehydrogenation on small Pt particles.