Exploring Pd adsorption, diffusion, permeation, and nucleation on bilayer SiO2/Ru as a function of hydroxylation and precursor environment: From UHV to catalyst preparation

Abstract

The hydroxylation-dependent permeability of bilayer SiO 2 supported on Ru(0001) was investigated by XPS and TDS studies in a temperature range of 100 K to 600 K. For this, the thermal behavior of Pd evaporated at 100 K, which results in surface and sub-surface (Rusupported) binding arrangements, was examined relative to the extent of pre-hydroxylation. Samples containing only defect-mediated hydroxyls showed no effect on Pd diffusion through the film at low temperature. If, instead, the concentration of strongly bound hydroxyl groups and associated weakly bound water molecules was enriched by an electron-assisted hydroxylation procedure, the probability for Pd diffusion through the film is decreased via a pore-blocking mechanism. Above room temperature, all samples showed similar behavior, reflective of particle nucleation above the film and eventual agglomeration with any metal-atoms initially binding beneath the film. When depositing Pd onto the same SiO 2 /Ru model-support via adsorption of [Pd(NH 3 ) 4 ]Cl 2 from alkaline (pH 12) precursor solution, we observe notably different adsorption and nucleation mechanisms. The resultant Pd adsorption complexes follow established decomposition pathways to produce model-catalyst systems compatible with those created exclusively within UHV despite lacking the ability to penetrate the film due to the increased size of the initial Pd precursor groups.

Insights into Silica Bilayer Hydroxylation and Dissolution

“…We recall that hydroxylated SiO 2 /Ru surfaces can be prepared by adsorption of water at 100 K followed by heating to room temperature (thermal route) [32], or, in order to enhance the hydroxyl concentration, with an additional electron irradiation step prior to heating (electron-assisted route) [34,35]. Desorption of multilayer ice is detected from both samples at around 160 K. Note the slightly smaller ice desorption signal from the electron-bombarded film (red trace in Fig.…”

“…Following up on the previously reported enhancement of silica bilayer hydroxylation by electron irradiation of adsorbed ice layers [34,35], we present herein a detailed investigation of oxygen isotopic exchange between water and silica and the dependence of electron irradiation parameters on hydroxylation activity. In addition, we extend our investigations to silica/solution interfaces and study the dissolution of the films at various solution pHs and temperatures.…”

“…Calculations using density functional theory (DFT) provided structural models of hydroxylated surfaces possessing terminal silanol groups with vibrational properties consistent to those detected experimentally [35]. While this hydroxylated silica model system represents a starting point for studying the interaction of adsorbed water and silanol groups with supported metals [34], or to further explore possibilities for anchoring functional groups, the mechanism by which electron irradiation enhances hydroxylation of the silica thin-film has not yet been clarified.…”

“…This is most probably the reason for the small surface coverage of Pd obtained in the study of [Pd(NH 3 ) 4 ] 2? adsorption onto bilayer silica from solution, which was conducted to mimic a catalyst preparation process that requires surface silanol sites to bind the Pdcomplexes [34]. More recently, it has been shown that the abundance of both molecularly adsorbed water and silanol groups may be enhanced via an electron-assisted hydroxylation route, which consists of electron irradiation of an ice layer adsorbed on the silica bilayer at low temperature [34,35].…”

“…adsorption onto bilayer silica from solution, which was conducted to mimic a catalyst preparation process that requires surface silanol sites to bind the Pdcomplexes [34]. More recently, it has been shown that the abundance of both molecularly adsorbed water and silanol groups may be enhanced via an electron-assisted hydroxylation route, which consists of electron irradiation of an ice layer adsorbed on the silica bilayer at low temperature [34,35]. Calculations using density functional theory (DFT) provided structural models of hydroxylated surfaces possessing terminal silanol groups with vibrational properties consistent to those detected experimentally [35].…”