Methanation reactions convert COx (x = 1 or 2) and H2 to CH4. At high temperatures (e.g., &gt;800 K), the exothermicity of these reactions limits the conversion, and the Boudouard reaction, which forms C deposits and inhibits methanation rates, prevails. Here, we report the COx methanation rates on earth-abundant metal (Ni, Co) and alloy (Ni-Co) clusters, which exhibit different affinities towards carbon and oxygen containing species. During these reactions, the parallel water-gas-shift (WGS) reaction attains chemical equilibrium and controls the [CO2]-to-[CO] and the associated [H2O]-to-[H2] pressure ratios. These [CO2]-to-[CO] and the related [CO]2-to-[CO2] ratios are the surrogates of the oxygen and carbon chemical potentials, respectively, at cluster surfaces. The chemical potentials and the metal identity dictate the O* and C* coverages on the clusters during steady-state catalysis. Kinetic measurements in the kinetically controlled regime, chemical titrations of surface C* and O*, and kinetic isotopic measurements during COx methanation show that the identities and coverages of the surface species determine the kinetically-relevant step and the resulting rate expression. Ni clusters prefer to bind to C* rather than O*, and their surfaces become partially covered with C* species during COx-H2 reactions, and consequently the reaction occurs via kinetically relevant addition of H* into CH3*. In contrast, Co clusters prefer to bind to O*, and their surfaces are free of C* and partially covered with O*, on which methanation occurs via kinetically relevant addition of the H from a hydroxyl (OH*) onto CH3*. Ni-Co alloy clusters have an intermediate affinity to C* and O*. These surfaces change from C* to O* covered when the [CO2]-to-[CO] ratio increases and [CO]2-to-[CO2] ratio concomitantly decreases, leading to a transition in the identity of the H donor from H* to OH*. Generally, as the metal identity varies from Ni, Ni-Co, to Co and the difference between the binding energies of C* and O* decreases, the number of active sites blocked by C* decreases, and the methanation rate coefficients increase. We compare the rate coefficients, kinetic isotope effects, and temperature dependencies extracted from rate measurements during COx methanation and the reverse CH4 reforming reactions at similar conditions and demonstrate their thermodynamic consistencies with the computed equilibrium coefficients, equilibrium isotope effects, and free energies of reaction.

Kinetic consequences of carbon and oxygen coverages during COx methanation and CH4 dry reforming on Ni, Co, and Ni-Co clusters

255th National Meeting (2018)

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