Palladium catalysis for energy applications [report]

L D Pfefferle, Abhaya Datye
2001 unpublished
Palladium (Pd) is an attractive catalyst for a range of new combustion applications comprising primary new technologies for future industrial energy needs, including gas turbine catalytic combustion, auto exhaust catalysts, heating and he1 cells. Most gas turbine companies are now developing some version of catalytic combustion of natural gas using Pd for advanced turbine designs, and multiple design efforts are also underway for ultra-low NO, lean burn catalytic combustors for burning natural
more » ... as, bio-derived fuels, and conventional liquid fuels in microturbines for distributive power generation and hybrid electric vehicle gas turbines. Y conditions as discussed in point #2 below. This is important in explaining reactivity 2. Confirmation of the nucleation mechanism was obtained for Pd( 1 1 1) using scanning 1 J .-DISCLAIMER This report was prepared as an account of work rpoasored by an agency of the United States Government Ndthcr the United States Ooverxlmeat nor any agency thereof* nor m y of tl@ cmploytcs, makes any -ty. utpttss or implied, or tssamb aby kgal liability or retp0nsibilit)r for the accuracy, completeness. or usefulness of my information, apparatus, product, or proctss disclosed or represents that its use wwkl not infringe pmately owned rights. Reference herein to my spe-&IC commerdal prcdnct, proctss, or Service by trade name, trademark manufacturer, or oth& docs not necessarily constitute or imply its endorsement. recornmcnd;rtion. or favoring by the Unittd States Government or any agency thmof. The *icws md opinions of authors txprrsstd herein do not n d y state or tefkct those of the United States Government or any agency &emf. . DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. implications for the long lifetime coexistence of phases as noted in point #2 above and in Appendix I. We obtained similar results for a zirconia supported catalyst, consistent with data reported by other authors [lo], but also found that oxygen exchange with the zirconia support and the PdO catalyst is fast especially at 1 [ , 700K so interpretation of the data is not straightforward. 7. Oxidation of alkanes over PdO is strongly affected by water poisoning (reaction order of Abstract Zirconia-supported Pd160 and bulk Pd160 were used as methane combustion catalysts for a reaction mixture containing 1% methane and 4% l8Oz in helium. The methane oxidation reaction was performed in pulsed experiments and the distribution of oxygen isotopes among the reaction products was monitored. The "0 content of the catalyst following labeled reaction mixture pulses was determined by catalyst reduction with either diluted hydrogen or diluted methane pulses. At low reaction temperature both COz and water contained primarily I6O. As temperature increased, however, the l80 distribution in water and carbon dioxide changed differently. The isotopic composition of water molecules reflected the oxygen isotopic distribution in the bulk of the catalyst particles as determined by post reaction reduction experiments. The larger concentration of l 8 0 in the carbon dioxide is explained by the differences in residence time and mobility of the products water and COz on the catalyst. The hydrogedwater samples the bulk, while the C02 reflects the surface composition. The behavior of the zirconia supported catalyst was similar to the bulk PdO at the lowest temperature; however, as the temperature was increased above 600K, oxygen exchange with the support became important. catalyst surface: a bridge-bound oxygen to two palladium atoms. The surface is involved in the methane reaction mechanism by successive reductiodreoxidation cycles. Reoxidation uses both bulk and gas phase oxygen, and also oxygen from the support in the case of zirconia supported catalyst. Under these conditions the gas phase oxygen exchange with the catalyst is limited by the methane oxidation surface reaction. The catalyst behavior is explained by the presence of a single oxygen species at the Abstract combustion over zirconia supported palladium catalyst was qualitatively studied at different reaction temperatures under both pulsed and continuous flow experiments. Water inhibition of methane oxidation was found to depend strongly on the time constant for equilibration of the adsorptioddesorption of water, which, in turn, is a strong function of temperature. In pulsed experiments, at temperatures below 723K the water inhibition effect was found to be important, in agreement with previously reported results and consistent with results reported by other authors. This also coincides with the temperature at which desorption time gap between the measured COz and water peaks disappears. Response to changes in water concentration was found to be slow, thus not equilibrated at low temperatures. However, under continuous flow water poisoning is important to higher temperatures due to readsorption. The time constant for equilibration of water adsorptioddesorption during methane , Abstract been found to be strongly dependent on the oxygen content of the PdO catalyst particles. Reaction mixture pulses of 1% methane, 4% oxygen and helium balance were injected over partially reduced catalysts obtained by chemical reduction with methane, and over completely reduced catalysts produced by thermal decomposition of the PdO phase. Catalyst activity was observed to initially increase with the degree of reduction, reaching a maximum and then decreasing continuously as the oxygen is depleted. The degree of chemical reduction is temperature dependent and at high temperatures, CO production is associated with extraction of subsurface oxygen. Reoxidation of the completely reduced catalyst is slow and strongly inhibited, while the partially reduced catalysts reoxidize at higher rate. However, if a completely reduced layer forms on top of the oxide core, the reoxidation inhibition phenomenon is present and the reoxidation rate considerably decreases. At low temperature reaction conditions oxygen exchange from the bulk can be faster than reoxidation from the gas phase. The activity of ceria-zirconia supported palladium catalysts for methane combustion has
doi:10.2172/799205 fatcat:zsta6rqavfgltmnxnpbtdqessq