Structure and Reactivity of Nickel and Iron Modified Palladium(111) Surfaces

Abstract

Palladium based bimetallic catalysts have been identified as promising candidates for hydrodeoxygenation reactions for reforming heavy oxygen-containing bio-oil components. Investigating Pd(111) single crystal surface and well-defined M-Pd(111) (M = Ni, Fe, Co) model surfaces offers fundamental insights and knowledge of how the surface structure and alloying affect their catalytic properties at a molecular level.

Ultrathin films of Ni, Fe and Co (up to 2 monolayers in thickness) were deposited on Pd(111) surfaces at 300 K under ultrahigh vacuum (UHV) conditions. The film growth, surface structure and morphology, surface composition, and thermal stability were examined by using Auger electron spectroscopy (AES), low energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM).

The growth of Ni, Fe and Co ultrathin films on Pd(111) surfaces at 300 K generally followed a layer-by-layer mechanism, with non-ideal layering before the first monolayer (ML) fully covered the substrate. After heating a 1 ML-Ni/Pd(111) surface to 500 K, a “sandwich-like” structure with Ni concentrated in the subsurface layer and a Pd-dominated topmost surface was observed. After heating 1 ML-Fe/Pd(111) and 1 ML-Co/Pd(111) surfaces to 600 -700 K, the surfaces exhibited metastable alloy phases with about 10% Fe or Co residing in the topmost surface layer, respectively. Specifically, a (2×2) Fe/Pd(111) alloy surface was formed after heating a 2 ML-Fe/Pd(111) surface to 600 – 650 K. After heating to 800 K, these 3d metals further diffused into deeper Pd subsurface regions, leaving pure Pd atoms on the topmost surface in all three cases.

The surface chemical properties and reactivity of the (2×2) Fe/Pd(111) alloy surface was then evaluated by using temperature programmed desorption (TPD). Compared to the clean Pd(111) surface, a strong electronic effect and site blocking effect was observed for CO adsorption on the (2×2)Fe/Pd(111) surface and ethanol decomposition pathways were inhibited.

Ordered iron oxide films were generated after reacting Fe/Pd(111) surfaces with oxygen. A monolayer-thick FeO(111) film with a characteristic Moiré structure was formed on Pd(111), which was further studied as a template for supporting Au and Pt nanoparticles.