(243f) Catalyst Design Strategies for Sulfur-Tolerant Tar Reforming Catalysts

Steam reforming of the tars produced during biomass gasification requires the use of a catalyst. Supported nickel has been proven to be an excellent catalyst for this application [1]. However, a major issue in the long term stability and activity of the catalyst is its tolerance towards small (ppm) levels of sulfur-containing compounds, including H₂S, present in the process feed stream [2]. Sulfur is known to bind strongly on the active Ni surface, blocking sites required for the reforming reaction [3]. In some systems, sulfur has also been associated with significant metal restructuring and bulk sulfide formation, which lowers the long term catalyst stability and regenerability [4].

We have used density functional theory calculations (DFT) to identify multimetallic surfaces that may exhibit improved sulfur tolerance with comparable tar reforming activity. DFT calculations have focused on H₂S adsorption and dissociation on strained Ni (111) surfaces (Ni_x), Ni₃M/Ni (111) surface alloys, Ni₃M (111) homogenous alloys (M = Sn, Pd, Pt, Ru), and surfaces promoted with alkaline modifiers. These studies targeted the identification of surfaces that showed reduced affinity for H₂S and its surface decomposition products while maintaining strong adsorption energies for ethylene, a model tar compound. Based on the identification of various sulfur-tolerant catalysts by DFT, a number of Ni-based catalysts were prepared and then tested for their sulfur tolerance and activity in a packed bed reactor. The results of these experimental studies correlated strongly with trends predicted from the computations. In particular, experimental studies confirmed predictions that (i) NiSn bimetallics show improved sulfur tolerance at a cost of reduced activity, (ii) Mg-promoted Ni catalysts exhibit improved rates but with poor sulfur tolerance, and (iii) NiRu bimetallics show improved rates and improved sulfur tolerance. The likely mechanisms for the effects of the various metals will be discussed.

References

1. Benardo, C.A., Alstrup, I., and Rostrup Nielsen, J.R. Journal of Catalysis 96,517 (1985).

2. Sargent, G.A., and Lih-Ren Chao, J. Applications of Surface Science 7, 104 (1981).

3. Owens, W.T., Rodriguez, N.M., and Baker R.T.K. Catalysis Today 21, 3 (1994).

4. Baker, R., and Horz, G. Vacuum 46, 1101 (1995).