Jentoft, R.

Lerch, M.

Ressler, T.

Rödel, C.

Schlögl, R.

Schomäcker, R.

Soerijanto, H.

Wang, D.

Weinberg, G.

Wild, U.

English

MPG.PuRe

Hydrogen Production from Ammonia Decomposition over Zirconium OxynitrideH. Soerijanto1,2, C. Rödel2, U. Wild1, M. Lerch2, R. Schomäcker2, R. Schlögl1, R. Jentoft1, G. Weinberg1, D. Wang1, T. Ressler21Fritz-Haber-Institut der MPG, Abt. Anorg. Chemie, Faradayweg 4-6, 14195 Berlin2Institut für Chemie, TU Berlin, Straße des 17. Juni 135, 10623 BerlinHydrogen production from hydrocarbons has the disadvantage of CO impurities which can poison the Pt-electrodes of fuel cells. The production of hydrogen from ammonia is absolutely CO-free and so can be used for fuel cell applications without costly CO removal procedures. One of fuel cell types: AFC (Alkaline Fuel Cell) can be operated at temperatures between 60°C and 200°C. This low operating temperature and the possibility for miniaturization [1] opens up a broad application for small portable systems. Additionally, it is still interesting for the chemical industry to find a new catalyst generation which works under milder reaction conditions than energy intensive conditions of Haber Bosch Process (Fe3O4, 500°C, 200 bar). Driven by these two goals a zirconia oxynitride catalyst was prepared, characterized, and tested for ammonia decomposition. Ammonia decompossition: NH3 0,5  N2 + 1,5 H2 ∆H0 = +46,2 kJ/molIntroductionCatalyst preparationExperimental resultsFinancial support from the Deutsche Forschungsgemeinschaft, DFG (LE 781/9-1, SCHL 332/6-1), is gratefully acknowledged.[1] R.Z. Sorensen, .J.E. Nielsen, S. Jensen, O. Hansen, T. Johannessen, U. Quaade, C.H. Christensen, Cat. Comm. 6 (2005) 229-232[2] M. Lerch, J. Am. Ceram. Soc., 79 (10), (1996), 2641-2644[3] M. Lerch, O. Rahäuser, J. Mat. Sci. 32 (1997), 1357-1363 Literature AcknowledgementSummary:1. Zirconium oxynitride is a promising material for the new generation of ammonia catalysts. It showed an ignition activation due the phase transition.2. The dramatic change in activity is also correlated to a rapid change in the electronic structure of the surface that accompanies the formation of the more active β´´ ZrON phase.3. The results presented show for the first time a direct correlation between the onset of ion conductivity as indicated by the phase change to the β´´-type, a modified electronic structure of the surface, and the catalytic performance of a heterogeneous catalyst.4. The simulation of conversion evolution as a function of temperatures gave an effective activation energy of 106 kJ/mol. The simulation of the kinetic measurements using a mechanistic approach gave an effective activation energy of 117 kJ/mol. It coincides well with the activation energy of ion conductivity in zirconia based materials.5. Hydrogen adsorbs strongly at the surface and inhibits the reaction. Kinetic measurements were modelled with an Eley-Rideal  rate law and gave a dissociative adsorption energy of 41 kJ/mol for nitrogen over ZrON. 6. The ZrON is also an excellent candidate for ammonia cracker in AFC. Figure 1: Evolution of XRD patterns measured during thermal treatment of a mixture of ZrON (•) and ZrO2 (♦) in the temperature range from 100°C to 750°C. The phase change from the β´ phase to the β´´ phase of ZrON is indicated (bar).Zirconium oxynitride (ZrON) pellets were prepared from isostatically pressed commercial zirconia powder nitrided at 1900°C in nitrogen atmosphere for 2 h (graphite heated resistance furnace), resulting in a mixture of nitrogen-free monoclinic ZrO2 and the ß”-type of zirconium oxynitride phases (~Zr7O9,5N3) [2]. Subsequently, the material was quenched from a vertical tube furnace from 1300°C (nitrogen atmosphere) in water to form the ß’-type. An image of a quenched sample is displayed.X-ray analysis after quenching shows a reduced amount of monoclinic zirconia and the metastable ß’-phase of zirconium oxynitride (~Zr7O11N2) [3]. For the catalyst testing described here a sample consisting of 51 wt-% ZrO2 and 49 % β´ phase was used. Hot gas extraction of the as-prepared material yielded a total amount of nitrogen of 1.63 wt-% (theoretical 1.62 wt-%). After three month time on stream during ammonia decomposition the material exhibited only a minor decrease in the amount of nitrogen (-0.30 wt-%). Catalytysis investigationA blank reactor filled with glass beads and third reactor with 1 g magnetite as catalyst were also measured for control and comparison. Catalysis measurements were carried out in a fixed bed reactor at atmosphere pressure with various temperatures and feed concentrations by applying a mixture of ammonia and helium. Effluent gases were analyzed by ammonia sensor (Binos IR detector, Rosemount) and subsequently washed by sulfuric acid in a gas flask. The experimental results were fitted to the reaction material balance to determine the activation energy.Figure 3: Evolution of width (left) and position (rights) of XPS Zr 3d3/2 peak of β´ ZrON during heating and subsequent cooling. The dashed line indicates theonset of the phase change from β´ ZrON to β´´ ZrON and catalytic activity.Scheme of catalysis testing device 26 28 30 32 34 36 38 Intensity Diffraction angle 2θ (Cu Kα) / ° 100 °C 200 °C 300 °C 400 °C 450 °C 500 °C 550 °C 600 °C 650 °C 700 °C 750 °C ZrON / ZrO2  ♦ ♦ • • ♦  2.1 2.2 2.3 300 400 500 600 Zr 3d3/2 (FWHM) / eV Temperature / °C Heating Cooling 184.0 184.2 184.4 184.6 184.8 300 400 500 600 Binding energy Zr 3d3/2 / eV Temperature / °C Heating Cooling PMF HePIR SensorMF NH3NH3HeGFHE = -10°CMetal fritGlas beadsCatalyst bedFigure 4: Bright-field TEM images show the coexistence of the monocline ZrO2 phase and the modulated β” phase. Figure 6: Van‘t Hoff plot for determination of adsioption constants of the gases over ZrON00.20.40.60.81150 250 350 450 550 650T / °CConversion Xblank reactor, glas beadsß' ZrON, 4 g, heating ß' ZrON, 4 g, coolingoptimized Fe3O4, 1g00,30,60,90 7 14TOS / minConversion XFigure 2: Evolution of NH3 conversion as a function of temperature during NH3 decomposition (feed 50 ml/min He with 4100 ppm NH3) on Fe3O4, ZrON(heating and cooling), and blank reactor. The activity at the conversion jump appeared 4 minutes after switching from 540 °C to 560 °C 00,0010,0020,0030,0040,0050,0060,0070,0080,0090 0,002 0,004 0,006 0,008 0,01pNH3 / barr = - ∆∆ ∆∆pNH3/ ∆∆ ∆∆t4600 ppm7400 ppm9900 ppmln KH2 = 1332,6/T + 5,9151ln KN2 = 4962,6/T - 6,9839ln KNH3 = 3140,9/T - 4,9988-3-1135790,00125 0,0013 0,00135 0,0014 0,00145 0,00151/T  / K-1ln KNH3 H2N2Figure 5: The result of kinetic measurements at different feed concentrations.shows an inhibition phanomena at NH3 decomposition on  ZrON

Hydrogen Production from Ammonia Decomposition over Zirconium Oxynitride

Hydrogen Production from Ammonia Decomposition over Zirconium Oxynitride

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