(491e) Novel Perovskite Catalysts for the Auto-Reforming of Sulfur Containing Fuels

Fuel cell energy systems have attracted much attention due to their high efficiency and zero- emission. Proton exchange membrane fuel cell (PEMPC) and solid oxide fuel cells (SOFC) in the stationary and auxiliary power units, are the most promising technologies for fuel cell utilization. The ideal fuel for them is pure hydrogen, which is not available as fuel in nature and it needs to be produced from other sources. The technical limitations associated with difficulties of hydrogen transportation, storage, and handling require the use of hydrogen rich gas obtained from liquid organic fuels. It is a special task to convert JP-8 fuel, which is kerosene-based jet fuel. It has been estimated that approximately 60 billion gallons of this fuel are used worldwide each year. However, it is a great challenge to reform such a fuel, because JP-8 involves high hydrocarbon, as well as aromatic compounds. Moreover it also contains a significant amount of sulfur (up to 3000ppm). It is well recognized that during reforming, the fuel with such composition has a vast tendency to a carbon formation on catalyst, which leads to catalyst rapid poisoning and reaction termination. The conversion of hydrocarbon fuels to H2 can be carried out by several catalytic reaction processes, including steam reforming (SR), partial oxidation (PO) and auto-thermal reforming (ATR), which combines SR and PO reactions. The SR has high hydrogen yield, but is endothermic. The PO is highly exothermic, but yields less hydrogen. The ATR process provides higher efficiency and energy density than other conventional processes. Also, a presence of oxygen decreases coke deposition on catalyst. Thus in this work ATR process was used to obtain hydrogen from JP-8 fuel. The catalysts for a steam reforming of sulfur containing higher hydrocarbons typically comprise expensive noble metals, such as Pt, Rh, and Ru. However, high cost and limited yield inhibit their extensive usage, and the high operation temperature (usually above 800 C), makes it difficult to further increase overall efficiencies. Complex mixed metal oxides with perovskite structure attract significant interest in many areas of solid state chemistry including catalysis. The general formula of perovskites is ABO3, where A is a metal with lager ionic radii, typically from rare-earth group and B is a metal with smaller radii, usually from transition metals group. Partial substitution of A, B or both sites allows one to synthesize a wide variety of compositions with different properties. In previous work [1] we have investigated the cerium- and nickel-substituted LaFeO3 perovskites as potential low cost coking resistant catalysts for ATR of a sulfur-free JP-8 fuel surrogate. It was shown that La0.6Ce0.4Fe0.8Ni0.2O3 catalyst exhibits an excellent stability at 775 ºC and 1 atm, with near-equilibrium hydrogen yield at high gas hourly space velocity (GHSV) values (130,000 1/h). However the presence of sulfur in fuel dramatically decreases its activity (conversion drops below 60%). In this work we demonstrate that modified perovskite-based catalyst shows stable performance during reforming of both surrogate and JP-8 fuels, which contain up to 50 ppm of sulfur. Different techniques are used for perovskite preparations, including sol-gel method, solid-state method, mechano-synthesis and combustion synthesis. The latter is an attractive method for production of advanced solid-state materials. The specific feature of this technique is that after its local ignition, a self-sustain propagation of the reaction front occurs throughout the heterogeneous powder mixture of reactants, living behind the desired products. Modification of this technique, so-called solution combustion (SC) synthesis, occurs in a liquid mixture of metal nitrites (oxidizer) and fuel such as glycine, citric acid or urea. Different reaction modes can be used for synthesis of perovskites. They include volume combustion synthesis (VCS), Self-propagating Sol-Gel Combustion (SGC), Impregnated Paper Combustion (IPC) and Impregnated Support Combustion (ISC) [cf. 2, 3]. Using these approaches a variety of perovskite catalyst were synthesized in this work. The corresponding metal nitrates (Alfa Aesar) and glycine were used as the precursors. Obtained products were characterized by BET, SEM, EDS, XRD and chemical analysis to verify products microstructure, as well as phase and chemical compositions. Testing of catalysts in ASR of JP-8 and its surrogate was performed in continuous flow system with fixed bed tubular quartz reactor (7mm ID) at atmospheric pressure. The experiments were performed under following conditions: H2O/C = 3, O2/C = 0.35, temperature: 800°C GHSV = 130000 1/h. Gas stream was analyzed by means of gas chromatograph Agilent Micro GC 3000A, equipped with thermal conductivity detector, using a molecular sieve and Plot Q column with argon and helium as a gas carrier respectively. It was shown that addition of potassium (K) into complex LaFe0.8Ni0.2O3 perovskite significantly improves material resistively for sulfur poisoning. In the range of investigated potassium content (0.3-4wt.%) an average fuel to hydrogen conversion varies from 58 to 75%, reaching maximum at K loading 2 wt%. It is remarkable that the tested catalysts are very resistant against coking. Indeed, while amount of carbon in as-synthesized powders was ~0.15wt.%, the C content on potassium containing catalysts after reaction was below the analysis sensitivity limit. Effects of others additives, mostly from the group of alkali metals were also investigated. It was demonstrated that additions of cesium, sodium, cobalt and combination of cobalt and potassium lead to slightly lower conversions (~ 70%) than for pure potassium. Low activity was observed for catalyst containing lithium (only 63% conversion). All catalysts showed high resistant to carbon deposition. Finally, influence of synthesis methods on the catalysis activity was investigated. For this purpose La0.6Ce0.4Fe0.68Ni0.2K0.12O3 perovskite was synthesized by using four different combustion approaches, i.e. VCS, SGC, IPC and ISC. It was shown that the highest activity (conversion ~75% and H2 yield ~39.3 %) was achieved on catalyst synthesized by IPC method. All obtained effects are discussed in details. Acknowledgement. This work was supported by the U.S. Army CECOM RDEC through Agreement AAB07-03-3-K414. Such support does not constitute endorsement by the U.S. Army of the views expressed in this publication. References: [1] Erri, P., Dinka, P. and Varma, A. ?Novel perovskite-based catalysts for autothermal JP-8 fuel reforming?, Chem. Eng. Sci. (2006, in print). [2] Dinka, P. and Mukasyan, A., ?In Situ Preparation of the Supported Catalysts by Solution Combustion Synthesis?, J. Phys. Chem. (2005), 109(46), 21627-21633. [3] Mukasyan, A., Epstein, P. and Dinka, P. ?Solution Combustion Synthesis of Nanomaterials?, Proceed. Combus. Institute, 31 (2006, in print).