Electrochemical Promotion and Evaluation of Nanostructured Palladium-Based Catalysts for Methane Oxidation

Abstract

Methane (CH₄), a fuel known for its clean-burning properties, has attracted significant attention in recent years. This is mainly owing to its remarkably high hydrogen-to-carbon ratio compared to other hydrocarbons. However, methane is a harmful greenhouse gas with a greenhouse effect up to 25 times more potent than that of CO₂. Therefore, the complete combustion of methane is crucial to prevent its release into the atmosphere. Palladium (Pd) has been demonstrated to be the most effective catalyst for methane oxidation among the currently active catalysts. Multiple research studies have examined the enhancement of the catalytic characteristics of the Pd catalyst by including an additional noble metal. Due to their higher cost and limited availability, non-noble metal catalysts have received significant interest as an alternative to noble metals. In the present study, first, the Pd nanoparticles were developed using a new synthesis method that provides a highly porous structure and a low ignition temperature. The resulting nanoparticles were deposited on a solid electrolyte disc and utilized to improve the reactivity of catalysts, considering an in-situ approach, referred to as Non-Faradaic Modification of Catalytic Activity (NEMCA), also known as Electrochemical Promotion of Catalysis (EPOC). The electrochemical promotion of Pd nanoparticles was evaluated in oxidizing reaction conditions, and the Electrochemical Impedance Spectroscopy (EIS) under polarization provided more insight into the origin of electrochemical promotion. In the next step, a second non-noble metal was added to enhance the stability and reduce the cost of the catalyst. Various bimetallic catalysts, including Pd₈Co₂, Pd-FeOₓ, Pd-SnO₂, and Pd-ZnO, were synthesized. These catalysts possess a nanostructure that offers a larger and more reactive surface area for the reaction. Electrochemical promotion was observed for the Pd₈Co₂ catalyst, and the effect of gas mixture composition was evaluated. The catalytic activity of the other bimetallic catalysts and the enhancement of the methane oxidation were investigated using electrochemical techniques, and a preliminary relationship was established between the catalytic rate and electrochemical response for the free-standing Pd and bimetallic catalytic systems, according to the contribution of the lattice O²⁻ species in the overall catalytic activity. To summarize the achievements of this project, studying the monometallic Pd nanoparticles focusing on electrochemical promotion under oxidizing conditions and low-temperature conditions confirmed the electrophobic behaviour of the reaction and validated PdOₓ formation during anodic polarization. The research also reported the electrochemical promotion of methane oxidation over a Pd-Co bimetallic nanoparticle catalyst. A distinct electrochemical response emerged when the gaseous mixture had a low partial oxygen pressure resembling pseudo-capacitance reactions. Upon examination of the Pd-metal oxide catalysts, it was observed that incorporating the metal oxides significantly enhanced the mass catalytic activity of monometallic Pd, with a 12-fold increase in reaction rate. In addition, a preliminary correlation was established between the catalytic rate and electrochemical response, providing insights into the role of lattice O²⁻ species in the overall catalytic activity.