Size-dependent effects of Ge addition on the coking and sintering tendency of PtnGem/alumina (n = 4,7,11) model catalysts

Abstract

The reactivity and adsorbate binding properties of alumina-supported Pt_n and Pt_nGe_m (n/m = 4/1, 7/2, 11/4) model catalysts were studied using a combination of ethylene and CO temperature-programmed desorption (TPD), He⁺ ion scattering (ISS), and detailed density functional theory (DFT). The Pt_n/alumina catalysts gradually deactivated in repeated CO TPD runs and deactivated more rapidly in repeated ethylene TPD runs due to the effects of sintering and carbon deposition (coking). Pt_nGe_m/alumina catalysts were dramatically more stable against deactivation by both sintering and coking. Carbon addition to Pt_n/alumina also increased stability against sintering, but not coking. Ion scattering was used to probe the nature of adsorbate binding, including the effects of both carbon and Ge addition on the accessibility of strong Pt-associated binding sites for ethylene and CO. DFT was used to examine the electronic and geometric structures and ethylene and CO binding properties of Pt_n (n = 4, 7) with added Ge, carbon, or both. Consistent with the ISS results, DFT shows that loss of strong adsorbate binding sites mostly resulted from changes to the electronic properties of the clusters, rather than simple blocking of binding sites by Ge or carbon.