Plasma catalysis: separating plasma and surface contributions for an Ar/N2/O2 atmospheric discharge interacting with a Pt catalyst

Abstract

Atmospheric pressure non-equilibrium plasmas can form nitrogen oxide (NO _x ) compounds directly from nitrogen and oxygen without a catalyst, and at lower catalyst temperatures than would be possible without plasma. In this work, the oxidation of plasma-produced NO from an Ar/N₂/O₂ non-equilibrium atmospheric-pressure plasma-jet (APPJ) over a platinum-on-alumina powder catalyst was investigated with in-situ infrared spectroscopy. Products downstream of the catalyst bed were analyzed along with catalyst surface species. The catalyst was exposed to plasma at both constant temperature and a cyclic temperature ramp in order to study long-lasting and transient surface changes. Primary incident reactive species to the catalyst were assessed to be NO and O₃. Pt-Al₂O₃ at 350 °C increased oxidation of NO relative to Al₂O₃ or an empty chamber. The surface state of Pt-Al₂O₃ evolves during plasma-effluent exposure and requires upwards of 20 min exposure for stabilization compared to Al₂O₃. Once stable surface conditions are achieved, thermal cycling reveals a repeatable hysteresis pattern in downstream products. At low temperature, oxygen and NO _x accumulate on the catalyst surface and react at elevated temperatures to form NO₂. Increasing plasma power and O₂:N₂ ratio increases the hysteresis of the heating relative to the cooling curves in the pattern of NO₂ formation. The limitation on NO oxidation at high temperatures was assessed to be Pt-O which is depleted as the catalyst is heated. Once stored species have been depleted, NO oxidation rates are determined by incoming reactants. Two overlapping NO oxidation patterns are identified, one determined by surface reactants formed at low temperature, and the other by reactants arriving at the surface at high temperature. The plasma is responsible for providing the reactants to the catalyst surface, while the catalyst enables reaction at high temperature or storage at low temperature for subsequent reaction.