(402a) Autothermal Reforming of Diesel Fuels on Structured Cordierite Monoliths Coated with Oxide Supports and Rh As Active Phase

The scientific work in the fuel processing and systems group at Forschungszentrum Juelich has the strategic aim of developing high-temperature polymer electrolyte fuel cell (HT-PEFC) systems based on autothermal reforming of diesel fuel and kerosene in the power class of 5 kW_e to 10 kW_e. The main component of the system besides the HT-PEFC stack is the autothermal reformer (ATR).

Several publications were released in the recent years by the fuel processing and systems group at Juelich dealing with the autothermal reformer, the catalytic burner and the complete fuel cell system [1-6] with special focus on reactor development, system development and experimental evaluation thereof. Complementary to these papers, this contribution is about the preparation, experimental evaluation and characterization of six different catalysts for the autothermal reforming.

In general, the catalysts were deposited on monolithic cordierite substrates and consisted of an oxide support and an active Rh phase. The oxide supports were either purchased or synthesized by the Leibniz Institute for Catalysis (LIKAT) in Rostock, Germany. LIKAT also coated the supports on the cordierite substrates followed by decoration with active Rh particles. For simplicity reasons, in this abstract the combination of active phase, support and substrate will only be denoted as “catalyst”, although, from the chemical and catalysis point of view, this label is only valid for the active phase itself.

The following six different catalysts were synthesized: Rh/γ-Al₂O₃, Rh/La-Al₂O₃, Rh/CeO₂, Rh/Gd-CeO₂, Rh/ZrO₂ and Rh/Y-ZrO₂ with theoretical mass fractions of the support and the active phase of approx. 10 ma% and 1 ma%, respectively. Thereby, each value referred to the total mass of the catalyst. The obtained H₂ concentration after the catalytic autothermal reforming was chosen for the evaluation of the specific catalyst activity and stability. Any difference in the activity of the different catalysts or any loss of stability caused by any possible reason (sintering, deposits on the catalyst surface, etc.) directly influences the concentration of H₂according to the general equations (1)-(4).

At first, for each catalyst the H₂O/C molar ratio was varied between 2.5 and 1.3 at a mean gas hourly space velocity (GHSV) of approx. 59,000 h^-1, while the O₂/C molar ratio was kept constant at 0.47. This sequence was repeated at mean GHSVs of approx. 44,000 h^-1 and 29,000 h^-1, respectively. The catalyst was not replaced between the sequences with different GHSVs. Secondly, in a very similar manner and again for each possibly pre-aged catalyst from the first three sequences, the O₂/C molar ratio was varied between 0.49 and 0.31 at a mean GHSV of 54,000 h^-1, while the H₂O/C molar ratio was kept constant at 1.9. Again, this sequence was repeated at mean GHSVs of 41,000 h^-1 and 27,000 h^-1, respectively, without changing the catalyst. These six sequences were first tested with NExBTL diesel fuel and afterwards with Ultimate diesel fuel (cf. Table 1) to test the performance for low and high aromatic diesel fuels, respectively. Again, the catalyst was not replaced, when the fuel was changed. So, a total of twelve consecutive sequences form the evaluation pattern for each catalyst. On the one hand, the goal of this pattern was to find out, which of the six catalysts was the most active one with respect to H₂production. On the other hand, catalyst deactivation should have been provoked by applying the above mentioned rough reaction conditions. Following the experimental evaluation in the test reactor, several methods for catalyst characterization (temperature-programmed desorption, oxidation and reduction as well as thermogravimetry) were applied. The characterization aimed at identifying the redox properties of the fresh and aged catalysts and the extent to which their surfaces were possibly covered by carbonaceous deposits during the experimental sequences.

This contribution describes, explains and discusses the results of the experimental evaluation of the six different catalysts with respect to their catalytic activity and stability for autothermal reforming and finds correlations with the results from the catalyst characterization experiments.

Table 1     Properties of diesel fuels used for the experimental evaluation

[1]          Meißner J, Pasel J, Samsun RC, Peters R, Stolten D. Start-Up and Load-Change Behavior of a Catalytic Burner for a Fuel-Cell-Based APU for Diesel Fuel. Fuel Cells 2014:n/a-n/a. DOI: 10.1002/fuce.201400016

[2]          Meißner J, Pasel J, Samsun RC, Scharf F, Wiethege C, Peters R. Catalytic burner with internal steam generation for a fuel-cell-based auxiliary power unit for middle distillates. Int. J. Hydrogen Energy 2014;39:4131-42. DOI: 10.1016/j.ijhydene.2013.05.167

[3]          Pasel J, Samsun RC, Peters R, Stolten D. Fuel Processing of Diesel and Kerosene for Auxiliary Power Unit Applications. Energy Fuels 2013;27:4386-94. DOI: 10.1021/ef301976f

[4]          Pasel J, Samsun RC, Peters R, Thiele B, Stolten D. Long-term stability at fuel processing of diesel and kerosene. Int. J. Hydrogen Energy 2014;39:18027-36. DOI: 10.1016/j.ijhydene.2014.03.148

[5]          Samsun RC, Wiethege C, Pasel J, Janßen H, Lehnert W, Peters R. HT-PEFC Systems Operating with Diesel and Kerosene for APU Application. Energy Procedia 2012;29:541-51. DOI: 10.1016/j.egypro.2012.09.063

[6]          Samsun RC, Pasel J, Janßen H, Lehnert W, Peters R, Stolten D. Design and test of a 5 kWe high-temperature polymer electrolyte fuel cell system operated with diesel and kerosene. Applied Energy 2014;114:238-49. DOI: 10.1016/j.apenergy.2013.09.054