The effect of tungsten-oxide supports on Pt, Pd, and Ru catalysts

• The catalytic effects of a WO x overlayer depend on the metal identity. • WO x forms overlayer on metal nanoparticles and suppresses gas adsorption. • Pt and Pd on WO x /Al 2 O 3 show less activity loss than adsorption experiments suggest. • Competitive reactant adsorption could displace WO x , reducing Pt and Pd activity. • Strong WO x binding deactivates Ru, preventing reactants from accessing Ru sites. The catalytic properties of Pt, Pd, and Ru supported on thin WO x films were compared to their γ-Al 2 O 3 -supported counterparts in order to understand the role of WO x as a support. The catalysts were prepared by vapor deposition of Pt, Pd, or Ru onto either γ-Al 2 O 3 or 0.5-nm-thick WO x films deposited onto the γ-Al 2 O 3 by Atomic Layer Deposition (ALD). Following reduction at 573 K, CO adsorption was completely suppressed on each of the WO x /γ-Al 2 O 3 -supported metals and could not be restored by high temperature oxidation. For Pd and Pt, rate constants for cyclohexane dehydrogenation and toluene hydrogenation were modestly lower on WO x /γ-Al 2 O 3 than on γ-Al 2 O 3 , even though CO adsorption was suppressed. It is hypothesized that the high activity of the WO x /γ-Al 2 O 3 -supported metals is due to reactant molecules competing with mobile WO x overlayers to access metal sites. For Ru, cyclohexane dehydrogenation, toluene hydrogenation, and n-hexane hydrogenolysis were completely poisoned when WO x /γ-Al 2 O 3 was used as the support. DFT calculations indicate that WO x binds much more strongly to Ru compared to Pt and Pd, explaining why reactants cannot access Ru sites on Ru/WO x /γ-Al 2 O 3 .