Migrating Al species hindering NiO reduction on Al containing catalyst carriers

Abstract

Reduction behavior of NiO supported on industrial like steam reforming carriers is studied under wet reduction conditions using TPR, in situ XRD, and STEM-EDS mapping. It is found that the interaction with the support greatly influences the reduction. When free MgO is in the carrier, it will react with Ni during impregnation and calcination to form a NixMg1−xO solid solution, which results in increasingly high reduction temperatures with increasing Mg content. Increased reduction temperatures are also observed in a catalyst based on a MgAl2O4 carrier, but without excess MgO. In this case, the higher reduction temperatures are caused by migration of Al-species from the support to the surface of the NiO particles during calcination. Calcination at 850°C results in NiO particles completely covered by Al-species, forming a thin protective surface spinel, resulting in reduction temperatures above 700°C. Based on the observed data, it is proposed that Al-species are mobile during calcination already at 450°C resulting in a partial coverage of NiO particles and higher reduction temperatures compared to pure nanocrystalline NiO. Formation of the protective surface spinel seems to be a general feature for Al-containing carriers as similar reduction behavior is observed on MgAl2O4, calcium aluminate, and α-Al2O3 carriers.