The Interplay between Hydroxyl Coverage and Reaction Selectivity of CO Conversion over the MnOH_x/Pt Catalyst

Hydroxyl groups (OH) and oxygen vacancies (V-O) have proven to be vital in numerous oxide catalytic systems for catalytic conversion of C1 molecules. In the present work, combining surface science experiments and density functional theory calculations, we construct a submonolayer MnOHx film on a Pt(111) substrate with different OH coverages (denoted as theta(OH)) and reveal its impact on reaction pathways under ultrahigh vacuum (UHV) and a CO atmosphere. OH directly desorbs in the form of H2O under UHV, while its evolution under a CO atmosphere can be divided into three stages with decreasing theta(OH): (i) CO + 2OH -> CO2 + H2O (85% <= theta(OH) <= 94%); (ii) CO + 2OH -> CO2 + H-2 + O-L (O-L, lattice O; 50% <= theta(OH) <= 85%); and (iii) 2CO + 2OH -> 2CO(2) + H-2 (26% <= theta(OH) <= 50%). In nature, surface OH and CO can form carboxyl intermediates, which promote the H2O generation and the O-H to Mn-H diffusion compared to the processes without CO assistance. Furthermore, as theta(OH) decreases and V-O increases, H binding on MnOHx/Pt(111) is weakened while OH binding is strengthened, which favors the generation of H-2 instead of H2O. These results provide inspiration to understand the mechanism for H- or OH-involved CO conversion reactions and to modulate a reaction selectivity via tailoring OH and V-O densities.