268459 Density Functional Theory Study of High Sulfur Tolerant Rh-Ni Bimetallic Catalysts in Steam Reforming Reactions
Hydrogen (H2) production has become an important research area due to the possibility of efficient and environment-friendly energy conversion of hydrogen gas. A representative application of hydrogen gas is as an anode fuel for fuel cells such as proton exchange fuel cells (PEMFC) and solid oxide fuel cells (SOFC). Fuel cell systems can be combined with a reforming process for on-board and on-site fuel cell applications where hydrogen is supplied by reforming natural gas, liquid hydrocarbons or alcohols. Especially, liquid hydrocarbon fuels such as jet fuels and diesel fuels are appropriate for on-board and on-site hydrogen production systems due to their higher energy density with their advantages of safety, handling, and well-established infrastructures. The major challenge of steam reforming using liquid hydrocarbons is sulfur poisoning on catalyst surfaces as the liquid fuels inherently contain a certain amount of sulfur compounds. The accumulated sulfur species make the reforming catalysts lose their activity for the reforming reaction, which finally results in carbon deposit formation on the surface to block catalytic sites. The sulfur poisoning problem can be improved by developing sulfur tolerant catalysts.
Recently, as a sulfur tolerant catalyst, a Rh-Ni bimetallic catalyst was suggested by Strohm et al. for liquid hydrocarbon steam reforming. Nickel is a major catalyst used in industrial steam reforming processes due to its low-cost and high activity. The main disadvantage of Ni catalysts is their weak resistance to solid carbon formation which becomes more prominent and severe for reforming liquid hydrocarbons containing aromatics. Due to such a limitation of Ni catalysts, noble metals including Rh and Ru minimizing carbon formation on the catalytic surface can be used for liquid hydrocarbon reforming, however noble metals are easily deactivated in the presence of the sulfur compounds in fuels. Strohm et al. showed that the weak sulfur resistance of the Rh catalyst can be improved by forming Rh-Ni bimetallic catalysts through the addition of Ni to Rh. We will detail the use density functional theory (DFT) methods to investigate the sulfur tolerance mechanism of Rh-Ni bimetallic catalysts. Sulfur poisoning thermodynamically occurs through S adsorption formed by H2S dissociation. The Rh1Ni2 model shows both a lower binding energy of sulfur and weaker interaction between S and co-adsorbed reactants compared to pure a Rh surface, suggesting higher sulfur tolerance of Rh-Ni binary catalysts. For an energetic analysis of actual reforming reaction, the full reaction energy path of propane reforming was successfully calculated over single and binary metal surfaces using BEP and scaling relationships. In addition to providing mechanistic information, these results combined with microkinetic modeling can be applied to find other metal combinations constituting a new bimetallic catalyst with high sulfur resistance.
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