Enhancing and understanding the stability of Ni catalysts via In-promotion for the steam reforming of oxygenates: An in-depth operando XRD-XAS and modeling investigation

Indium is a well-known stability promoter for high-temperature dry reforming reactions but has not yet been applied to other reforming reactions for hydrogen production, such as those of oxygenated compounds which suffer from rapid deactivation. In addition, little is known about the stabilization mechanism and the limited insights mostly come from ex-situ characterizations. Here, we developed an In-modified Ni-based catalyst by synthesizing a Mg-Al-In mixed oxide support and by depositing Ni via a urea-assisted deposition procedure. The catalyst was evaluated in the steam reforming reaction of acetic acid showing stable activity for more than 24 h at 700 °C and S/C = 3 corresponding to double the hydrogen productivity than the unpromoted catalyst which, on the contrary, was not stable. Superior stability was achieved also with glycerol. We then investigated the catalysts under operando conditions performing a quasi-simultaneous XAS and XRD experiment using synchrotron light, assisted by computational modeling. After reduction, the surface of the Ni nanoparticles is enriched with In, as confirmed by TEM-EDS, in the form of a shell consisting of a Ni3In-like phase. The presence of In in the surface layers competes with C insertion thus protecting the Ni particles from coke deposition. As the reaction proceeds, the intermetallic phase is consumed resulting in Ni nanoparticles that are more stable, although less active, than those of the unpromoted catalyst. This may be due to a different faceting caused by the dealloying of the Ni-In phase, which once again disfavors the coke formation.