Carbide and nitride supported methanol steam reforming catalysts.

Abstract

Transition metal carbides and nitrides have many interesting properties. These include high surface areas, good thermal stability, and catalytic properties similar to noble metals. This research investigates the use of carbides and nitrides as methanol steam reforming (MSR) catalysts. The results show that carbide and nitride supported catalysts were more promising than their bimetallic or monometallic constituents. This study was the first to design and construct a combinatorial synthesis system for the addition of transition metals onto carbide and nitride supports. Over 500 catalyst formulations, with varying supports (Mo₂C, Mo₂N) and metal combinations (Co, Cu, Fe, Ni, Sn, Pd, Pt, Ru) were synthesized and screened. High H₂ production rates and low CO₂ selectivity was observed on the noble metal catalysts. An opposite trend was observed for the base metals. The low activity over base metals was due to their poor adsorption capacity onto the support. Supported noble-base metal bimetallic catalysts displayed a trend with medium activity and selectivity. This was due to a lower loading of noble metals as a result of competition between noble and base metals for adsorption sites. On the bimetallic systems, each supported metal acted as a separate entity instead of bimetallic clusters. Several catalysts were identified for further study. They include Mo₂N, Mo₂C, Pt/Mo₂N, Pt/Mo₂C, Pd-Cu/Mo₂N, Pt-Fe/Mo₂C, and Cu-Fe/Mo₂C. Reaction pathway studies were performed to elucidate the CO₂ selectivity relationship of the noble and base metal catalysts. Nine reactions involving reaction intermediates (HCOOH, HCHO, HCOOCH₃) were examined. Two mechanisms were proposed: (1) a Formate Route yielding CO₂ and H₂ over base metals and (2) Decomposition & WGS Route yielding CO and H₂ over noble metals. Bimetallic noble and base metal catalysts contained characteristics of both pathways. The initial step of CH₃OH dehydrogenation to HCHO was common to both pathways. Dehydrogenation was observed during CH₃OH chemisorption. With an understanding of reaction pathway, the MSR reaction conditions could be modified to improve the CO₂ selectivity over supported noble metal catalysts. With further work in optimizing both the catalyst formulations and activation procedure, it is believed that carbide and nitride supported catalysts has the potential of surpassing the activity of a commercial Cu/Zn/Al₂O₃ catalyst.