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.