TEM evidences for factors affecting the genesis of carbon species on bare and K-promoted Nu/MgO catalysts during dry reforming of methane

The structure and morphology of carbon species generated under dry reforming of methane (DRM) at 650 and 800°C on `bare' and `K-doped' Ni/MgO catalysts have been comparatively investigated by Transmission Electron Microscopy (TEM) analyses of `used' samples. K-addition (Kat/Niat, 0.125) strongly improves the resistance of the Ni/MgO catalyst to coking and sintering phenomena at any temperature. At 650°C, an extensive formation of filamentous (whisker carbon) carbon species on bare Ni/MgO catalyst causes the detachment of a large number of Ni particles from the support with a consequent destruction of the structure and remarkable sintering phenomena of the active phase. Considerably lower amounts of carbon deposits with a shell-like (encapsulating carbon) morphology, forming at 800°C on both catalysts, point to the Bouduard reaction as the main route of carbon deposition on Ni-based catalysts during DRM. The electronic effect induced by potassium on the active phase of the Ni/MgO system, timely monitored by a rise in Eapp of DRM from 50 to 70 kJ/mol, markedly hinders the rate of coking also affecting the morphology of carbon whiskers, by inhibiting the processes of C diffusion and nucleation across Ni particles under steady-state conditions.