CO oxidation over platinum-ceria catalysts -- Structural dynamics and reaction mechanisms

Licentiate thesis, 2021

Catalytic oxidation of carbon monoxide (CO) continues to be important due to its wide applicability in emission control, in-door air cleaning, fuel cell efficiency enhancement, chemical feedstocks purification etc. The transportation sector emits most of the CO emissions. This is because modern powertrains and driving patterns result in exhausts with low temperatures at which the catalyst cannot operate efficiently. Thus, catalysts for CO oxidation needs to be developed as to be more active at low temperatures, ideally, even at ambient conditions. The metal oxide supported platinum catalyst is a practical choice thanks to its high robustness, i.e., resistance to sintering and deactivation by water, carbon dioxide and sulfur species. Although it has been extensively studied, the influence of chemical and structural dynamics on the catalytic activity under reaction conditions is still debated, especially for industrial catalysts.

This work aims at understanding the catalytic function of platinum highly dispersed onto ceria, which is a reducible support. The kinetic behaviours during catalytic extinction and reaction orders have been experimentally determined using a fixed-bed flow reactor. To explore the structure-function relationships, operando infrared and X-ray absorption spectroscopy have been used. Also, detailed reaction pathways have been simulated using kinetic Monte Carlo with kinetic parameters determined from ab initio calculations.

The CO oxidation kinetics for Pt/ceria is qualitatively different from that of reference Pt/alumina. The extinction profile for Pt/ceria catalyst exhibits a smooth decay in CO conversion rather than a stepwise drop as for the Pt/alumina catalyst. This is due to the two supports modifying the Pt particles differently as well as complementary reaction paths towards CO2 facilitated by boundary sites only for the Pt/ceria catalyst. Furthermore, operando spectroscopy reveals that the Pt particles bind strongly with ceria showing an unaltered Pt-O bond distance of 2 Å during catalytic extinction. Although difficult to experimentally determine, charge transfer from Pt particles to ceria supplemented with reverse spillover of ceria lattice oxygen to the vicinity of Pt particles likely occur.