Coking Can Enhance Product Yields in the Dry Reforming of Methane

Unlike the more common steam reforming of methane, dry (or CO₂) reforming of methane (DRM) can directly consume CO₂, the most abundant greenhouse gas, to produce useful syngas. However, catalyst deactivation during DRM, which is generally attributed to carbon deposition (coking), has traditionally impeded its industrialization. In this study, we focus on the coking process, the oxidation process taking place during DRM, and the nature of carbon deposits. We provide evidence that the carbon deposits on a standard Ni catalyst surface can be a reaction intermediate which is produced during the DRM reaction and removed by reaction with CO₂. We demonstrate that coke is present in two principal forms: (1) two-dimensional graphite and (2) one-dimensional carbon nanotubes. Our data indicate that the two-dimensional carbon, which can cover and completely deactivate the catalyst, only accumulates significantly in a CO₂-deficient environment. After 30 min under the DRM reaction conditions employed, the main form of coke present is one-dimensional carbon, which covers ~50% of the catalyst surface, as estimated by the decrease in CH₄conversion. The DRM activity does not further change significantly over this 30 min time period, which implies that there is a steady state involving carbon deposition and carbon consumption. The carbon deposits which are oxidized to CO contribute to the DRM yield. This study therefore redefines the role of coke during the DRM reaction and has significant implications for reaction conditions that optimize DRM reaction yields and hence has potential economic and environmental benefits.