In situ Studies of Platinum Catalyst Sintering

Doctoral thesis, 2016

The damaging effect of pollutants from combustion engine exhaust on human health and environment is well recognized. Since the 70s, the three-way catalytic converter (TWC) has substantially improved the urban air quality by simultaneous conversion of CO, NOx and unburned hydrocarbons to O2, N2 and H2O. The active phase in the TWC is small precious metal particles (e.g. platinum, palladium, and rhodium) that are dispersed on high surface area oxide support such as gamma-alumina. The TWC, however, has a limited durability due to catalyst deactivation where sintering is one major process at high temperatures. Presently, metal overloading is required for the TWC to meet legislations, and detailed understanding of sintering mechanisms and kinetics is crucial to design catalyst formulations with extended durability. This thesis addresses fundamental issues connected to supported platinum sintering in oxidizing environments by two different approaches; (i) indirect nanoplasmonic sensing (INPS), as operando tool to probe the changes in an ensemble of Pt catalyst nanoparticles in real time, and (ii) transmission electron microscopy (TEM), by which Pt nanoparticles can be visualized after exposure to sintering conditions. By employing these tools, the sintering kinetics and evolution of Pt particle size distributions on alumina and silica supports are studied under oxidizing conditions. In the first part of the thesis, the possibility to apply the INPS platform to study sintering on different flat supports in different gas environments is demonstrated. Time-resolved sintering kinetic data are deduced by correlating the INPS data with ex situ TEM analysis for Pt particles supported on silica and alumina in O2 and NO2 environment. The applicability of INPS is furthermore extended to monitor metal sintering in porous supports. This is achieved by successful deposition and metal impregnation of porous gamma-alumina on the INPS sensor chip. The second part of the thesis is dedicated to detailed TEM studies of Pt model catalyst sintering. Intermittent particle size distributions are analyzed as a function of time and temperature during oxygen-induced sintering of Pt on flat alumina and silica supports. Transient bimodal size distributions are clearly observed, and support heterogeneity is discussed as a possible mechanism. Moreover, controlled support heterogeneity was introduced by fabrication of an alumina support with arrays of nanocone structures. Significant differences in the evolution of Pt particle size and coverage on the cone and flat alumina surfaces are observed, with clear redispersion of Pt on the cones.