Consequences of Propane Dehydrogenation and Oxidative Regeneration on Ni-Phosphide Phase Stability

The effective removal of carbon deposits formed during nonoxidative alkane dehydrogenation can improve the overall lifetime of catalysts. Here, the effects of oxidative regeneration and re-reduction on the performance of unsupported Ni2P for propane dehydrogenation were investigated. The mass-normalized propylene production rate increased over the first three regeneration cycles, accompanied by a gradual transition to the Ni12P5 phase. Pure Ni12P5 showed similar mass-normalized rates to the in situ formed Ni2P/Ni12P5 mixed phase. Experiments with fresh Ni12P5 resulted in a lower propylene selectivity than fresh Ni2P, even after partial, in situ conversion to Ni12P5, indicating that a mixed phase has the best combination of high propylene production and propylene selectivity. A mixed phase, Ni12P5/Ni2P, was synthesized and showed intermediate propylene production and selectivity between the two pure phases. The mixed phase exhibited higher per-site rates via the formation of new active sites. Simple oxidation and reduction cycling experiments with Ni2P indicated that the dehydrogenation reaction step, and not the regeneration, is the primary cause of the phase transition. Ni2P was tested for 10 cycles of reaction, resulting in extensive Ni12P5 formation, which was fully converted to the Ni2P phase through P addition