Methane steam reforming
(MSR) plays a key role in the production
of syngas and hydrogen from natural gas. The increasing interest in
the use of hydrogen for fuel cell applications demands development
of catalysts with high activity at reduced operating temperatures.
Ni-based catalysts are promising systems because of their high activity
and low cost, but coke formation generally poses a severe problem.
Studies of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)
indicate that CH
4
/H
2
O gas mixtures react with
Ni/CeO
2
(111) surfaces to form OH, CH
x
, and CH
x
O at 300 K. All of these
species are easy to form and desorb at temperatures below 700 K when
the rate of the MSR process is accelerated. Density functional theory
(DFT) modeling of the reaction over ceria-supported small Ni nanoparticles
predicts relatively low activation barriers between 0.3 and 0.7 eV
for complete dehydrogenation of methane to carbon and the barrierless
activation of water at interfacial Ni sites. Hydroxyls resulting from
water activation allow for CO formation via a COH intermediate with
a barrier of about 0.9 eV, which is much lower than that through a
pathway involving lattice oxygen from ceria. Neither methane nor water
activation is a rate-determining step, and the OH-assisted CO formation
through the COH intermediate constitutes a low-barrier pathway that
prevents carbon accumulation. The interactions between Ni and the
ceria support and the low metal loading are crucial for the reaction
to proceed in a coke-free and efficient way. These results pave the
way for further advances in the design of stable and highly active
Ni-based catalysts for hydrogen production.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.