Systematic Investigation of Earth-Abundant Transition Metal Promoters for Pd-Catalyzed Methane Complete Combustion

Systematic Investigation of Earth-abundant Transition Metal Promoters for Pd-Catalyzed Methane Complete Combustion

Joshua J. Willis, Ian Andrew Naccarella, Helen Yan, Joseph Maalouf, Kester Wade, Matteo Cargnello

Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, CA 94305, USA

 

There is a critical need for reducing emissions of methane, the second most prevalent greenhouse gas. Palladium is widely accepted the best metal catalyst. Still rates are unsatisfactory below 400˚C where new technologies for methane utilization (e.g. low-temperature combustion engines) are currently operating. Also, steam dramatically deactivates Pd-based catalysts through the formation of less active hydroxide phases. Thus, there is a critical need to utilize Pd to the best possible extent and improve its activity and stability. Earth-abundant transition metals (Mn, Fe, Co, Ni and Cu) as promoters are a promising route to optimizing Pd utilization and destabilizing the formation of Pd hydroxide.

In order to perform systematic studies of promoting effects in Pd-based catalysts, here we present the results of using highly monodisperse palladium nanocrystals as the active phase. In this way, any variation in catalytic activity can be directly attributed to the promoter structure and properties. By deposition of the nanocrystals onto promoted γ-alumina (M-Al₂O₃) and activation using a fast thermal annealing process, a library of Pd/M-Al₂O₃ is obtained. By controlling the Pd nanocrystal structure and size, promoter-activity relationships can be drawn. Kinetic characterization demonstrates a unique promoting effect for Pd-Mn systems for methane complete combustion, with decrease activation energy and a three-fold increase in rates compared to the corresponding Pd/M-Al₂O₃. Experiments in the presence of steam also show pronounced effects of the promoters on the activity of the Pd-only catalyst. Our work provides clear elements for improving the activity of Pd-based combustion catalysts and in general a framework for understanding promoter-activity relationships using highly uniform catalysts under realistic reaction conditions.