(130e) 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.