The role of promoters for Ni catalysts in low temperature (membrane) steam methane reforming

In the search for active and stable Ni-based catalysts for steam methane reforming in membrane reactors, the effect of three different promoters La, B and Rh was compared. Promoted and unpromoted Ni catalysts were characterized by TEM, TPR and X-ray absorption spectroscopy. The average Ni particle size is between 4 and 10 nm. Promoters affected both dispersion and reducibility of Ni. Smaller particles were found to be more difficult to reduce than larger ones. The use of B resulted in very small Ni particles. The degree of Ni reduction strongly increased by use of La and Rh promoters, whereas B strongly impeded Ni reduction. The initial intrinsic rate per surface metal atom was found to increase linearly with the Ni metal dispersion, suggesting that the rate is controlled by dissociative methane adsorption over low-coordinated surface atoms. The data indicate that Rh and La act as structural promoters to the activity. Catalysts modified by B show a much higher activity of the Ni surface atoms. Catalyst stability was investigated by using feed compositions representing the inlet of the membrane reactor and the hydrogen lean reformate towards its outlet. Stability increases in the order La < Rh < B. Deactivation of the catalysts is caused by insufficient removal of carbon species from the surface of Ni particles and the formation of stable, graphitic carbon deposits, most likely covering the surface of metal. This is substantially suppressed when the Ni particles are small. B is an excellent structural promoter to obtain small Ni particles, Rh stabilizes metallic Ni and La aids in the removal of some of the carbon deposits more effectively by gasification.

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► B, Rh and La promoters influence particle size and reducibility of Ni.
► Surface atom based activity of Ni correlates to Ni dispersion.
► Boron improves stability of Ni due to formation of nickel boride active phase.