Stable surface terminations of orthorhombic Mo2C catalysts and their CO activation mechanisms

• Surface terminations of orthorhombic Mo2C catalysts have been computed.
• (1 1 0)-Mo and (1 0 0)-Mo terminations are stable at low carbon chemical potential.
• (1 1 1)-Mo/C and (1 1 0)-Mo/C terminations are stable at high carbon chemical potential.
• CO activation on Mo terminations is dissociation controlled zero-order kinetic.
• CO activation on Mo/C terminations is adsorption controlled first-order kinetic.

The structure and stability of all twenty-two terminations of the seven low-Miller index orthorhombic Mo2C surfaces have been systematically investigated on the basis of the computed surface energies from periodic density functional theory. With the increase of the carbon chemical potential (μC), the most stable surface structure and composition change from the metallic (1 1 0)-Mo and (1 0 0)-Mo terminations to the mixed (1 1 1)-Mo/C and strongly reconstructed (1 1 0)-Mo/C terminations. The calculated stability order and surface area proportions of the (1 0 0), (1 1 0) and (1 1 1) surfaces agree very well with the available X-ray diffraction data. In addition, CO adsorption and dissociation on these surfaces have been computed and micro-kinetic analysis reveals that CO dissociation is rate-determining on the metallic termination, and CO adsorption is rate-determining on the mixed Mo/C-termination. This might explain the observed catalytic differences of orthorhombic Mo2C catalysts prepared from different ways.

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