Being one of the cleaner fossil fuels, natural gas has garnered a lot of attention in recent times as alternative to conventional automotive fuel, but its main component, methane (CH4), is a greenhouse gas 28 times more potent than carbon dioxide. Methane is thermodynamically stable with strong C-H bonds and exhibits low reactivity. Thus, one of the critical challenges in the natural gas-based transportation industry lies in the efficient low-temperature combustion of methane over automotive catalysts. Increasingly stringent emission limits continue to justify the use of precious metals, such as platinum and palladium for tailpipe emission catalysis. Yet, typical three-way catalytic converters remain ineffective for methane combustion below ca. 400 °C. Recently, iridium oxide has shown exceptional performance toward methane activation with C-H bond cleavage activity at temperatures as low as –123 °C. To further improve the low temperature methane oxidation activity of platinum group metals, we therefore considered iridium as promoter. To this end, periodic, van der Waals corrected density functional theory (DFT) calculations were used to probe methane activation on Ir single atom doped platinum Pt(211) and palladium Pd(211) surfaces. After determining the preferred location of Ir atoms among terrace, edge or subsurface sites under oxidizing and reducing conditions, the effect of Ir promotion on methane activation was assessed. Favorable methane conversion is related to the ease of C-H bond dissociation, which can be related through Brønsted-Evans-Polanyi relationships to the binding strength of CH3* and H*. The dissociative chemisorption energy of methane is, thus, a good reactivity descriptor for methane activation. Under anaerobic conditions, the presence of Ir at edge site improved the thermodynamics of dissociative chemisorption of CH4 by 0.5 and 0.2 eV for Pd and Pt respectively. Furthermore, the activation of CH4 in the presence of pre-adsorbed O*, at the atop Ir-doped edge site unveiled an improvement in the dissociative chemisorption energy, denoting a positive effect of oxygen on methane activation. The fundamental insight from this study provides guidance for the continuous improvement of automotive emission catalysts with focus on methane abatement, enabling sustainable large-scale implementation of natural gas-powered vehicles and paving the path for a greener environment.

Enhanced methane activation over Ir single atom doped platinum group metals under aerobic and anaerobic conditions

ACS Spring Meeting 2021

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