CO Oxidation over Oxide Supported Platinum Catalysts
This work scrutinizes the kinetics of CO oxidation over Pt/alumina and Pt/ceria catalysts through analysis of reaction orders obtained experimentally from flow-reactor measurements and theoretically by kinetic Monte Carlo simulations and connects this to kinetic model formulation. Further, the catalytic structure-function relationship is explored by operando infrared and X-ray absorption spectroscopy. The influence of water and nitrogen oxide on the CO oxidation kinetics is investigated with in situ infrared spectroscopy. Finally, iron oxide is explored as an active support for platinum with a focus on the structural dynamics of Pt/FeOx under reaction conditions.
The results show that reaction orders depend on reaction conditions and operating mechanism, and the adsorbate-adsorbate interactions play a crucial role. Pt/ceria is active at lower temperatures than Pt/alumina thanks to lattice oxygen in the ceria support that participates according to a Mars-van Krevelen mechanism. This mechanism is promoted by water but inhibited by nitrogen oxide through nitrate formation. On Pt/alumina, the reaction proceeds via the Langmuir-Hinshelwod mechanism, which is also promoted by water and inhibited by nitrates. Finally, using iron oxide as support for Pt opens for a catalyst design with a support even more interacting with Pt than ceria in terms of redox properties at low temperatures.
active catalyst support
nitrogen oxide inhibition
Chalmers, Kemi och kemiteknik, Tillämpad kemi
Carbon monoxide is a primary air pollutant that indirectly contributes to climate change by forming ground-level ozone. Unlike the ozone layer in the stratosphere, which absorbs dangerous UV light, ozone near the Earth’s surface is a harmful air pollutant. The annual release of carbon monoxide amounts 20,000,000 tons and comes mainly from transportation, i.e., from car, truck, and ship engines. The challenge today is to construct a catalyst that can prevent carbon monoxide emissions already from the start of the engines when the exhausts are cold. Further, modern engines and electric hybrids produce exhaust with few combustible components that can lower the catalyst temperature. Also, many driving patterns and stop-and-go functions lead to cold exhausts.
Platinum catalysts are robust and efficient for converting carbon monoxide to carbon dioxide. Platinum is incredibly valuable, which is why platinum catalysts are expensive. We thus want to maximize the use and efficiency of platinum in the catalysts. The way to go is to make nanosized platinum, and even single platinum atoms, and put them on inexpensive support materials like alumina. Carbon monoxide, however, is not only bad for us but also for the platinum catalysts at low temperatures. When the exhaust is cold, carbon monoxide poisons the platinum catalysts by blocking oxygen adsorption, which is needed to convert carbon monoxide to carbon dioxide. We are solving this problem by sneaking in the oxygen that already exists in the support material underneath the platinum. However, not all support materials could provide extra oxygen to convert carbon monoxide. Only materials that could switch oxidation states can help, like ceria. Can we use all the oxygen from ceria? Unfortunately, not. Only the oxygen in the vicinity of platinum can be used to convert carbon monoxide to carbon dioxide.
Finding an inexpensive substrate material with as much oxygen as possible available at low temperatures for platinum catalysts to convert carbon monoxide to carbon dioxide will create a better and safer environment!
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5270
PJ-salen, Fysikgården 2B
Opponent: Professor Susanne Mossin, from Technical University of Denmark (DTU), Copenhagen, Denmark