(699f) The Role of Water in Low-Temperature CO Conversion Using Transition Metal Oxide Supported Nobel Metal Nanoclusters: Structure, Surface Bonding and Energetics

Carbon monoxide, one of the most common and widely distributed air pollutants, has significant negative public health and environmental effects. Catalytic oxidation is considered to be the most effective strategy to minimize CO emission, which requires high performance CO conversion catalysts under ambient conditions. Unfortunately, most of the currently developed low temperature CO oxidation catalysts merely functionalize without the presence of moisture, which is a major component of automobile exhaust. Additionally, moisture introduces great complexity and plays distinct roles in the presence or absence of noble metals. More specifically, water vapor is a devastatingly poisonous specie for transition metal oxides, whereas it acts as a catalytic promoter for noble metals. However, the exact roles of moisture and the water – catalyst interaction mechanisms as a function of water coverage remain unclear on molecular scale. In this study, a series of meso-structured transition metal oxides with fine-tuned oxidation states, such as NiO, Mn₃O₄, Fe₃O₄, were synthesized to support platinum group metal catalytic species, including Pt and Pd. We took advantage of the synergy between noble metal nanoparticles and transition metal oxide supports, which enabled greatly enhanced CO conversion activity. Owing to their tunable oxidation states, transition metal oxides possess superior oxygen storage and release capability. Moreover, the effects of water during the CO conversion process were investigated by a spectrum of structural, surface and calorimetric analyses as the water partial pressure varies. In-situ X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were performed to reveal the structural and bonding evolution for both CO and water molecules. The signature technique of our group, adsorption calorimetry, was employed to investigate the energetics of CO – catalyst interactions, and CO conversion mechanism at different water coverage. We confirm that the noble metal nanoparticles distribute homogeneously resulting in greatly enhanced catalytic activities. We find out that the catalytic CO conversion at low-temperature using transition metal oxide supported noble metal catalysts is tightly correlated to the water coverage. This conclusion is supported by the structural, spectroscopic and calorimetric results. In all these experiments, the oxidation states of transition metal oxide supports were controlled.