Studies of Supported Catalysts for Environmental Applications: - Flow Reactor Measurements, Monte Carlo Simulations and High Throughput Screening

Doktorsavhandling, 2001

The use of heterogeneous catalysts for environmental applications, primarily for reduction of harmful emissions from vehicles and stationary sources, has successively increased during the last three decades. Driven by emission regulations, research and development within this area attract large interest and resources, especially for automotive applications. One of the problems still to be solved is the poor catalytic efficiency at low temperatures, e.g., reduction of cold start emissions from cars. The objectives of this thesis work were to develop and use a broad range of methods and tools for studies of environmental catalysis, focusing on catalytic total oxidation reactions on supported catalysts at low temperatures. Supported catalysts were studied in detail by different characterisation and temperature programmed methods, Monte Carlo simulations were utilised to study the reaction kinetics on a single supported catalyst particle, and a new catalyst high throughput screening (HTS) system was developed. The influence of pre-treatment, gas composition and metal (Ce or Co) oxide promotion on the low-temperature oxidation activity of supported Pt and Pd catalysts were studied. Pre-oxidised cobalt oxide catalysts were found to be highly active for CO oxidation at temperatures as low as 200 K. At these low temperatures the activity is independent of the presence of platinum. When tested with other compounds typically present in cold start exhausts from Otto engines, the low temperature CO oxidation activity on cobalt oxide was inhibited to various degrees. The Monte Carlo simulations were utilised to study the steady-state kinetics of the 2A + B2 ==> 2AB model reaction on a nm-sized catalyst particle. The attention was focused on the adsorbate-diffusion-mediated interplay of reaction kinetics occurring on different facets, in the more general case with finite magnitudes of the rate constants. Different rate constants for adsorption, desorption, reaction, and diffusion were investigated. In addition, the contribution of the facet edges to the reaction rate was investigated. The newly developed HTS system is primarily designed for evaluation of heterogeneous catalysts for environmental applications (although the approach is generic), and with a specific aim to establish a platform for screening of catalysts prepared by a wide range of methods. The construction of the HTS system and the first measurements, demonstrating the system performance, are presented. A positional gas-sampling probe, connected to a mass-spectrometer, is used to measure the catalytic activity of individual samples in a flat array of supported catalysts.