Operando Spectroscopic Investigation of Supported Pd and Low-surface Area Mixed Metal Oxide Catalysts for Automotive Emissions After-treatment

The catalytic oxidation of carbon monoxide remains a topic of active research in the field of heterogeneous catalysis, despite nearly a century of exhaustive characterization. Platinum group metals (PGMs) are most often used industrially for this reaction, and their fully reduced state has long been considered to be the most active. The development of in-situ and operando characterization techniques has led to new insights into the state of the PGM surface during reaction. With these new findings has come contention regarding the most active state of the catalyst during the oxidation of CO to CO2.;In this thesis, operando IR and X-ray absorption spectroscopies were used to study CO oxidation in a net oxidizing atmosphere over two initially different forms of a highly dispersed Pd/γ-Al2O3 catalyst: fully oxidized, and fully reduced. Fully oxidized PdO/Al2O 3 was found to undergo partial reduction prior to the onset of activity, while at light-off, its surface was reoxidized. In the case of fully reduced Pd/Al2O3, the formation of a surface oxide did not occur until above the light-off temperature. Interestingly, at high temperatures and conversions, both catalysts were extensively reoxidized to a disordered, PdO-like phase. When the temperature ramp was reversed, extinction was observed at the same temperature for both catalysts. Both showed hysteresis between their light-off and extinction temperatures, but in opposite directions. The extinction temperature of the catalyst that was initially PdO/Al2O3 was higher than its light-off temperature, i.e., extensive high temperature oxidation had served to decrease activity. However, the same oxide phase showed improved activity relative to Pd/Al 2O3, compared to light-off, suggesting that the disordered oxide is more active than either reduced Pd nanoparticles or ordered PdO nanoparticles.;Low surface area mixed metal oxide catalysts are also of interest in the catalytic oxidation of CO, because of their greater resistance to changes in oxidation state and sintering relative to highly dispersed metal nanoparticles. Many types of PGM-doped and un-doped mixed metal oxides have shown high activity for this reaction, including perovskites, pyrochlores, and spinels. In this thesis, the activity of the delafossite CuScO2 in CO oxidation was investigated, using both standard ex-situ techniques and in-situ X-ray absorption spectroscopy. CuScO2 has the ability to intercalate additional oxygen into its lattice, accompanied by oxidation of Cu(I) to Cu(II). After exposure to CO/O2 above the light-off temperature, the catalyst had been partially oxidized. In subsequent light-off cycles, activity improved with each cycle. In-situ X-ray absorption spectroscopy confirmed that, under reaction conditions, oxygen is incorporated into the lattice. Reactor studies of CuScO2 with intercalated with 18O showed that lattice oxygen is available for reaction and is likely the cause of decreased light-off temperature.