Intrinsic reactivity of Ni, Pd and Pt surfaces in dry reforming and competitive reactions: Insights from first principles calculations and microkinetic modeling simulations

- CH4 activation is similar for Ni, Pd, Pt while CO2 dissociation is kinetically more demanding following the trend Ni < Pd < Pt.
- The Ni (1 1 1) surface is the only active surface in dry reforming, the CH4 activation being the rate-limiting step.
- The reverse water gas shift reaction is hydrogen-assisted for the case of Pt.
- The Boudouard reaction is favored over methane cracking for Ni whereas the latter is less energy demanding for Pd and Pt.
- Coke formation is less favorable when moving down in the metal series.

CO2 reforming of methane (dry reforming) is a challenging reaction due to the high temperatures needed and the undesired side reactions taking place under such conditions. Hence, further fundamental understanding is required not only for dry reforming but also for competitive reactions such as steam reforming, reverse water gas-shift, or coke formation. In this contribution, a comparative mechanistic study of the dry reforming reaction network at 700 °C is provided for Ni, Pd and Pt (1 1 1) surfaces by using density functional theory (DFT).While the CH4 dissociation energy barrier is found to be metal independent, our calculations show that CO2 dissociation is kinetically more demanding following the trend Ni < Pd < Pt, in agreement with the position of the average d band of the metal. As a consequence, the activation of CO2 via C-O bond cleavage is less energy demanding than the CH4 activation in the case of Ni, but not for Pd and Pt surfaces. Additionally, hydrogen-assisted carbon dioxide cleavage routes are more accessible when moving down in the metal series and the energy span of the reverse water gas shift reaction depends on the nature of the metal. Another difference among them concerns the energetics of carbon-carbon bond formation (related to coke formation) on the metal surface from adsorbed carbon species, which becomes less favorable from Ni to Pt. All three metals share the same preferred dry reforming route via HCO∗ intermediate and the overall energy span increases in the order Ni ≈ Pd ≪ Pt. However, from our microkinetic simulations, only Ni (1 1 1) surface is active in the dry reforming reaction, which is related to the balance between energy barriers of CH4 and CO2 activation processes. The CH4 activation is found to be the rate-limiting step for Ni according to our sensitivity analysis. Our results provide further mechanistic understanding on the dry reforming over group 10 metal series surfaces, a key step in order to obtain more active and selective catalysts.

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