(600bg) Enzyme-Inspired Sulfur Tolerant Iron Nanocatalysts

Sulfur poisoning of iron catalysts is a major problem in industrial catalysis that results in low yields and low conversion.  A very promising technique to efficiently control sulfur poisoning of catalysts is to mimic how naturally occurring metalloenzymes, like Ni-Fe hydrogenase and Fe nitrogenase, do it.  These enzymes have Fe, Ni and S atoms in their inherent chemical structure and yet, behave as extremely energy-efficient catalysts.  We use these Fe-S structures as a baseline to synthesize sulfur tolerant heterogeneous catalysts.  Fe8, Fe20 and Fe40 clusters and their sulfided counterparts have been synthesized via the dendrimer template coordination technique in aqueous solution. The advantage of using dendrimers for templating is that the average number of metal atoms in a metallic cluster can be controlled by the ratio of metal ions in solution to the tertiary amine chelation sites when the metal ion amount is less than the number of amine groups. Consequently, the resulting cluster is designated by the metal and the average number of metal atoms on a cluster.  UV-vis absorption spectroscopy was used to monitor the comnplexation process between the metal ions and the internal amine groups of the dendrimer solution.  The inclusion of the sulfur atoms in the Fe clusters was achieved by the controlled addition of thiophene gas to the supported sample. Extended X-ray Absorption Fine Structure (EXAFS) and X-ray crystallography techniques were used to determine the possible structures of the resulting bimetallic [Fe-S] clusters. We have used density functional theory to predict the stability and feasibility of such chemical structures.  The effect of sulfur poisoning of the metal catalysts on a chemical reaction was studied for the ethylene hydrogenation reaction.  When Pt particles (3 nm or larger) were exposed to thiophene (S precursor), an order of magnitude decrease in the reaction rate was observed because of sulfur poisoning at the Pt surface. This result is in agreement with sulfur poisoning precious and base metal catalysts. However, the small bio-inspired Fe8-S and Fe20-S clusters show relatively little deactivation due to sulfur.  We also observed that the sulfur-tolerance of the Fe-S clusters increased with decreasing cluster size, i.e. [Fe8-S] showed much higher chemical activity than the [Fe40-S] cluster. We believe that with decreasing cluster size, the Fe-S clusters form similar shapes as the metal co-ordination spheres in the enzymes leading to very low sulfur poisoning. Using DFT calculations, we verified this hypothesis by studying the ethylene hydrogenation reaction on two different model catalyst surfaces, viz. a 0.5 monolayer Sulfur coated on a bulk Fe slab that represented Sulfur poisoning in large nanoparticles and, a cubical [Fe4S4] cluster resembling the enzyme structures.  Comparison between the activation energies of the ethylene hydrogenation reaction between the two model catalyst surfaces indeed support our experimental observations of very low sulfur poisoning on small [Fe-S] clusters.