Catalytic Partial Oxidation of Methane: Is a Distinction between Direct and Indirect Pathways Meaningful?

Catalytic partial oxidation of methane (CPOM) at high-temperature, millisecond contact times ( T > 900 °C, τ = 1−50 ms) is a technologically interesting reaction pathway for the production of synthesis gas or hydrogen. While many studies on CPOM over noble metal catalysts have been published in the past 10 years, much debate persists as to whether or not synthesis gas is produced via indirect or direct pathways. Here, we report results from a study into the reaction mechanism of CPOM over Pt catalysts. Catalyst contact times were varied, and product concentrations were correlated with in situ temperature profiles measured throughout the catalyst bed. The results give convincing evidence that the reaction proceeds in two stages: initial direct oxidation to CO and H 2 O at τ < 2 ms, followed by steam reforming of methane. Water gas shift plays only a minor role in all experimental conditions. H 2 is produced predominantly via indirect oxidation, while CO is produced predominantly via direct oxidation. The results indicate that a distinction between “direct” and “indirect” formation of synthesis gas is not useful since the dominant mechanism is different for the carbon and hydrogen pathways.