(90f) Microwave Active Conductive Metal Oxides for CO2 Dry Reforming of Methane

The dry reforming of methane reaction (DRM) produces H₂ and CO (syngas) from CO₂ and CH₄. In this regard, DRM enables “carbon neutral” syngas production because it consumes CO₂ as a reactant, whereas conventional methane steam reforming generates over nine tons of CO₂ for each ton of H₂. Using CO₂ to produce commodity chemicals, such as H₂ and CO, can generate revenue to offset carbon capture costs, reduce the carbon footprint of fossil-fuel powered processes, and allow sustainable use of our country’s fossil fuel resources. Unfortunately, the high operating temperatures of DRM (>860°C) introduce challenges compared with the lower-temperature, carbon-positive, methane steam reforming.

Recently, microwave-assisted catalysis has been proposed as an enabling technology for high temperature chemical processes. Unlike traditional thermal heating, microwaves can rapidly heat catalysts to extremely high temperatures, which allows reactors to utilize excess renewable energy on an intermittent basis (load follow) to promote traditionally challenging, thermally-driven reactions. Moreover, microwaves directly heat the catalyst material, and not the entire reactor body, which reduces many of the traditional heat management challenges associated with high temperature DRM. Microwave absorption is a function of the electronic and magnetic properties of a material, and a properly designed catalyst can function as a both a microwave heater and a reactive surface for driving the desired reaction. However, microwave absorption is extremely sensitive to the catalyst’s chemical state and electronic structure, and effective catalysts must maintain microwave activity across a wide range of temperatures and in both oxidative and reductive environments.

We have constructed a single-mode microwave reactor and investigated microwave assisted DRM using conductive metal oxides. Conductive perovskites (ex: La_0.8Sr_0.2CoO₃) are attractive because they readily convert microwave energy into heat while also potentially providing active metal and oxide surfaces for catalyzing the reaction. Additionally, perovskites have good thermal stability at temperatures in excess of 1000^oC, unlike conventional carbon-based microwave absorbers which must be kept below 900^oC to avoid pyrolytic carbon sublimation. The conductivity of these oxides allows them to inductively heat in the microwave like metals or carbon-based absorbers, but with the advantage of maintaining conductivity even in oxidative environments. We have demonstrated the rapid heating of the microwave DRM reactor and were able to ramp from a ground state to reaction temperature (>700^oC) in less than 3 minutes with consistent on-off cyclability. By modifying the transition metal dopants used at the perovskite B site, we were able to tune the catalyst performance such that stable conversions of 90% of CO₂ and CH₄ could be achieved using less than 100W of applied microwave power, demonstrating the suitability of microwave catalysts for rapid, efficient, and carbon neutral syngas production.