Electrical Conductivity and Defect Equilibria of MoO_3-d, a Catalyst for Biofuel Upgrading

The transportation sector currently relies heavily on non-renewable fossil-based fuels to meet its energy requirements. Alternative energy sources need to be developed to meet the projected energy demand in the future in a sustainable and environmentally conscious manner. Bio-oils, generated via biomass fast pyrolysis, represent an attractive avenue for the production of renewable fuels and chemicals. We are investigating MoO3-δ, a promising catalyst for bio-oil upgrading, that selectively transforms various phenolic compounds into aromatic hydrocarbons with high yields using low H2 pressures by hydrodeoxygenation (HDO). While it is widely suggested that both ionic (oxygen vacancies) and electronic defects (e.g. Mo5+/3+), in both the oxide catalyst and oxide support (e.g. TiO2, ZrO2 and Al2O3), play controlling roles in the catalytic reactions, surprising little is known about how operating conditions (temperature, dopants/impurities, and atmosphere (pO2, pH2O, pH2, carbon activity, etc.)) impact the concentration or the diffusivities of these key defects, nor how they influence the reaction kinetics. In this presentation, we discuss our efforts to combine catalysis with defect chemical and impedance spectroscopic approaches to achieving a deeper understanding of the role that defects in metal oxide catalysts and supports play in influencing catalytic selectivity, efficiency, stability and reaction rates. The relationship between the defect equilibria and electrical conductivity is reported for a range of oxygen partial pressures (10-5 - 1 atm O2) and temperatures (300 – 600 oC). Relevant energetics of defect formation and transport are extracted, and their relationships to relevant catalysis processes are discussed.