(242e) Catalytic Steam Gasification of Carbon Derived From Isooctane Decomposition Over Ni Supported On Ceria-Zirconia Catalyst

For solid-oxide fuel cell based
auxiliary power units on board of vehicles, it is important to develop
catalytic reforming technologies enabling the use of liquid transportation
fuels. One of the many challenges is the conversion of heavy hydrocarbons into
hydrogen-rich reformate without the deposition of deleterious carbon species.
Extensive studies of carbon deposition have been carried out in the past
decades for the reforming of methane. It was found that carbon deposition can
be minimized by modifying catalyst formulations and optimizing reaction
conditions, like temperature and the ratios of steam to carbon (S/C) and oxygen
to carbon (O/C). Nevertheless, the situation during reforming of liquid
feedstocks is more complex than methane reforming as liquid fuel reforming
involves the breaking of not only C-H, but also C-C bonds. We postulated in
previous studies that thermal and oxidative cracking of liquid hydrocarbons
produces light hydrocarbons (C₁-C₄) under reforming
conditions, and that these molecules are not only the primary routes to
reformate gases but also contribute to carbon deposition.

However, carbon accumulation on
a catalyst is determined by the dynamic balance between carbon deposition and
carbon gasification, especially in the steam environment under SR and ATR
conditions. It has been also reported in the literature that the pronounced
resistance of CeO₂ to carbon deposition is attributed to improved
carbon gasification rates. Therefore, one of the strategies available for
eliminating or retarding the carbon deactivation of reforming catalysts is to
accelerate gasification rate. The in-situ carbon characterization during
reforming reactions is unavailable. 
The final form of carbon species left on the catalyst is obtained from
the balance between carbon deposition and carbon gasification, and it may not
directly reflect the nature of the initially deposited carbon. Therefore, in
this work we attempted to reconstruct the carbon species that resemble the
structure of the originally deposited carbon species during reforming. Based on
the characterizations by temperature-programmed oxidation (TPO) and scanning
electron microscopy (SEM), the typical carbon-deposited catalysts, which were
obtained by performing catalytic decomposition of isooctane over Ni/Ce-Zr-O,
were chosen for steam gasification. Different temperature programmed procedures
were compared and the steam gasification was measured by on-line
thermogravimetric analysis. The gasification rate was determined from the
weight loss of the sample, while the gasification products CO, CO₂,
and H₂ were analyzed by on-line by IR and MS, respectively. Combined
with TPO and SEM characterization of carbon types, the results of gasification
were interpreted in light of carbon deposition-related reforming reactions.