(574b) Nickel-Based Autothermal Reforming Catalysts: Effect of Particle Size On Sulfur Tolerance

Autothermal reforming (ATR) of liquid hydrocarbon
fuels over nickel-based catalysts produces an equilibrium-limited mixture of hydrogen-rich
syngas that is suitable as feed for solid oxide fuel cells. Autothermal
reforming involves exothermic partial oxidation as well as endothermic steam
reforming pathways that lead to non-isothermal temperature profiles in the
catalyst bed. The widespread adoption
of Ni-based ATR catalysts is hindered by the vulnerability of the catalysts to
deactivate when low concentrations of sulfur compounds are present in the fuel.

In a systematic study of
isooctane ATR in presence of small concentrations of thiophene, we noticed that
catalyst sites responsible for steam reforming were more vulnerable to
deactivation than the sites responsible for partial oxidation. The presence of
thiophene led to an increase in the catalyst bed temperature profile and a
higher yield of intermediate products, such as isobutylene, propylene and
ethylene. Long-term stable performance of the catalyst could be achieved under
reaction conditions that favored nearly complete conversion of thiophene to hydrogen
sulfide.

However,
it was not clear to what extent the nickel particle size affected the
susceptibility of the catalysts to deactivation.  Therefore, we conducted a unique
experimental study where we attempted to isolate the effect of nickel particle
size from other contributing factors. 
The total number of active nickel surface sites was removed as a
variable in the exploration of particle size effects by selectively
pre-treating catalysts to create a series of samples with different Ni loadings
and particle sizes but identical nickel surface areas.  The activities of these catalysts were
measured for autothermal reforming of sulfur-free and thiophene-containing
isooctane.  Under sulfur-free
conditions, the isooctane conversion and yield of synthesis gas increased with
increasing Ni particle size. However, in presence of thiophene, catalysts with
larger nickel particles were more susceptible to deactivation. Catalysts with small Ni particle sizes exhibited poor
steam reforming activity, but were less vulnerable to deactivation by
thiophene.

There are two possible explanations for the role of Ni
particle size. First, it is conceivable that the Ni surface ensembles in the
smallest Ni particles are not large enough for adsorption and carbon-carbon
bond breaking of large hydrocarbon molecules. A second possible explanation is that
the smaller Ni particles are more prone to oxidation under ATR reaction
conditions, with the resulting NiO surfaces having diminished hydrocarbon-reforming
activity.

From the standpoint
of catalyst design, our results clearly show that larger nickel particle sizes
are beneficial for sulfur tolerance, with the caveat that sulfur will mainly
deactivate the sites responsible for steam reforming while leaving the partial
oxidation sites intact.