Active Probing of a RuO2/CZ Catalyst Surface as a Tool for Bridging the Gap Between CO Oxidation Catalytic Tests in a Model and Realistic Exhaust Gas Stream.

Abstract

Herein, we present a paper that attempts to bridge the gap between CO oxidation catalytic tests performed in a model stream and a more realistic exhaust gas stream by incorporating characterization methods that allow for active probing of the catalyst surface. The results have shown that it is not just the abundance of a given type of species on the surface that impacts the activity of a system but also the ease of extraction of ions from their surface (time-of-flight secondary ion mass spectrometry) and the response of the support to change in the feed composition (dynamic in situ X-ray diffraction (XRD) with variable atmosphere). The study utilizes the method of doping a catalyst (RuO₂/CZ) with a small amount of alkali-metal (K⁺ or Na⁺) carbonates in order to slightly modify its surface to gain insight into parameters that may cause discrepancies between model stream activity and complex stream activity. The most pronounced difference is that in the model stream, which contains only CO and O₂ in helium, both alkali ions improve the activity of the system at temperatures around 175 °C, whereas in the complex stream, which mimics the exhaust stream from a diesel engine under oxygen lean conditions, the K⁺-doped catalyst is slightly worse than RuO₂ /CZ and RuO₂ + Na⁺/CZ and much worse in propane combustion. The total hydrogen consumption values (temperature-programmed reduction) and the O_ads/O_latt ratios (X-ray photoelectron spectroscopy) both place the RuO₂ + K⁺/CZ system between the other two and hence provided no reason for the unusual behavior of the K⁺-doped catalyst. In contrast, both in situ XRD measurement tests and ToF SIMS results show a pronounced difference between the RuO₂ + K⁺/CZ catalyst and the other two systems, which indicates that the interaction of the surfaces with the reagents might be the cause of the discrepancy. The CO₂-TPD results show that this system retains more CO₂, i.e., the product, at adsorption sites, which might reduce the adsorption of other reagents, i.e., oxygen ions, CO, and propane, hence lowering the overall activity of the system.