(112k) Dual Pathway Mechanism for Preferential CO Oxidation Over Pt-CeO2 Catalysts

A Pt-CeO₂ catalyst was studied and its reaction products examined to study the kinetics of a Preferential Oxidation of CO in the presence of H₂ (PROX). Two parallel reaction mechanisms have been found to occur. One reaction mechanism is a Langmuir-Hinshelwood (L-H) mechanism. This mechanism requires a CO molecule and an unpaired oxygen atom to be attached to adjacent catalyst active sites. Once these two species are bound to the platinum surface, a reaction occurs when the carbon monoxide molecule combines with the free oxygen to form carbon dioxide, which is then released to the atmosphere as a gas. Additional oxygen and carbon monoxide from the atmosphere surrounding the catalyst will fill in the voids where the former reactants were located. The second mechanism for CO oxidation on the Pt-CeO₂ catalyst is a Mars and van Krevelen mechanism. This reaction pathway occurs when all of the CO adsorption sites on the platinum surface are saturated with carbon monoxide. Under CO saturation conditions the oxygen used to oxidize the CO comes from the ceria support. We see that the Mars and van Krevelen mechanism is the most dominant reaction mechanism in the regime where a high CO/low O₂ environment exists. At high CO concentrations, the platinum surface is completely covered with a layer of CO, thus inhibiting the attachment of O₂, H₂, or any other competitive species. Thus, in accordance with the workings of the Langmuir-Hinshelwood (L-H) mechanism, it becomes very unlikely that a CO molecule and an O₂ molecule will bind to two adjacent active platinum catalyst sites, and the more favorable Mars and van Krevelen reaction pathway occurs to a greater extent between the ceria support and the platinum metal site. The experimental regime where both the CO and O₂ concentrations are low mirrors the above mentioned scenario of a high CO and a low O₂ concentration because the CO gas has a greater affinity for the active platinum catalyst sites than oxygen does. Therefore, once the catalyst system reaches steady state, CO will once again blanket all of the active platinum catalyst sites and prevent the little amount of O₂ present from adhering to the platinum surface. Thus, the bulk of the catalyst activity occurs at the platinum-ceria interface, and the Mars and van Krevelen mechanism dominates again. Finally, when a low CO/high O₂ environment exists, the amount of CO in the atmosphere is insufficient to blanket the platinum surface, and thus all of the bulk species can interact with the platinum surface. Thus, the CO selectivity of the reaction will decrease significantly as more H₂ adsorbs on the surface and is oxidized to H₂O. Also, more CO₂ will be produced since the CO present can undergo oxidation to CO₂ by either the Mars and van Krevelen pathway (Mv-K) or via the Langmuir-Hinshelwood (L-H) pathway.