B. K. Min

The nucleation, growth, and stability of oxide-supported metal clusters

The optimization of oxide-supported metal clusters as heterogeneous catalysts requires a detailed understanding of the metal cluster–oxide interface. Model catalysts, prepared by deposition of a catalytically active metal onto a thin film oxide support, closely mimic real-world catalysts, yet are amenable to study using surface sensitive techniques. Surface science methods applied to model catalysts, combined with the use of in situ high-pressure reaction studies, have provided a wealth of information about cluster structure and reactivity. STM capabilities for imaging individual particles under reaction temperatures and pressures offer a new approach for studying supported cluster catalysts on a particle-by-particle basis. This article describes recent work in our laboratories using variable temperature STM to investigate the role of the support and its defects in the nucleation and stabilization of metal clusters.

Ammonia decomposition on Ir(100): from ultrahigh vacuum to elevated pressures

Ammonia decomposition on Ir(100) has been studied over the pressure range from ultrahigh vacuum to 1.5 Torr and at temperatures ranging from 200 to 800 K. The kinetics of the ammonia decomposition reaction was monitored by total pressure change. The apparent activation energy obtained in this study (84 kJ/mol) is in excellent agreement with our previous studies using supported Ir catalysts (Ir/Al2O3 82 kJ/mol). Partial pressure dependence studies of the reaction rate yielded a positive order (0.9±0.1) with respect to ammonia and negative order (−0.7 ±0.1) with respect to hydrogen. Temperature-programmed desorption data from clean and hydrogen co-adsorbed Ir(100) surfaces indicate that ammonia undergoes facile decomposition on both these surfaces. Recombinative desorption of N2 is the rate-determining step with a desorption activation energy of ∼63 kJ/mol. Co-adsorption data also indicate that the observed negative order with respect to hydrogen pressure is due to enhancement of the reverse reaction (NHx + H → NHx+1, x=0–2) in the presence of excess H atoms on the surface.

Thermal stability of Pd supported on single crystalline SiO2 thin films

The effect of annealing temperature on a model Pd/SiO2 catalyst has been investigated using Auger electron spectroscopy (AES) and scanning tunneling microscopy. Pd clusters on a single crystalline SiO2 thin film are not altered with respect to size or shape upon heating to 700 K; however, interdiffusion and sintering of the Pd clusters take place between 750 and 1050 K. At 1000 K, AES data imply the formation of Pd–silicide. Above 1050 K, desorption of Pd occurs concomitant with the decomposition of SiO2.

Ag growth on Mo(112)-Oa and MoO2 surfaces

The growth of Ag clusters on preadsorbed oxygen and oxide-covered Mo(112) has been investigated using scanning tunneling microscopy (STM). The objective of these experiments is the synthesis of adjacent areas on a surface with distinctly different metal–support interactions in order to investigate the relationship between the morphology of a supported metal cluster and the strength of the cluster–support interaction. The STM results show that more highly dispersed Ag clusters with a greater number density are obtained on that surface that interacts to a greater extent with the metal. Heating leads to the formation of two-dimensional–Ag nanostructures on oxygen-free Mo(112) (strong metal–support interaction) and three-dimensional cluster growth on oxide-covered Mo(112) (relatively weak metal–support interaction).

Toward an Understanding of Catalysis by Supported Metal Nanoclusters

OAK (B204) The goal of this program is an atomic-level understanding of catalysis by supported metal nanoclusters, especially the surface intermediates in selective oxidation by noble metal nanocatalysts.