The CO content of hydrogen feed to proton exchange membrane fuel cells (PEMFC) must be kept under 1-100 ppm for their proper operation. This can be achieved by using catalysts able to selectively oxidize CO in the presence of excess hydrogen (PROX). The present study reports on the mechanism of PROX reaction on Pt/CeO 2 catalyst, by using catalytic tests, in-situ DRIFTS, high-pressure XPS, HRTEM and TDS techniques. Bulk metallic, pronounced adsorbate-induced surface Pt and a small amount of oxidized Pt sites were detected by in-situ, high pressure XPS under PROX conditions. The pre-oxidized ceria surface was strongly reduced in pure H 2 but significantly re-oxidized under PROX conditions (i.e. O 2 +CO in excess hydrogen) at T=358 K. The remaining small amount of Ce 3+ decreased with increasing temperature. HRTEM found well-crystallized CeO 2 particles (8-10 nm) in the case of activated (pre-oxidized) sample that transformed in a large extent to an oxygen deficient ceria super-cell structure after PROX reaction. Metallic Pt particles (2-3 nm) and small (0.5-0.6 nm) Pt clusters were found by HRTEM. These findings were in accordance with the variations in relative intensity of the corresponding Pt-CO bands (DRIFTS). Different types of carbonate and formate species were detected (XPS and DRIFTS). Their possible role in the reaction mechanism is discussed. Resolved OH bands could not be found by DRIFT in the PROX reaction mixture indicating significant amount of adsorbed water in a hydrogen-bonded structure. Its presence seems to suppress hydrogen oxidation while CO oxidation still takes place, as the metallic particles are covered by CO (DRIFTS). The direct contribution of surface water in a low-temperature water-gas-shift (LTWGS) type reaction in the PROX mixture is proposed.

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Abstract:The presence of natural gas hydrates at all active and passive continental margins has been proven. Their global occurrence as well as the fact that huge amounts of methane and other lighter hydrocarbons are stored in natural gas hydrates has led to the idea of using hydrate bearing sediments as an energy resource. However, natural gas hydrates remain stable as long as they are in mechanical, thermal and chemical equilibrium with their environment. Thus, for the production of gas from hydrate bearing sediments, at least one of these equilibrium states must be disturbed by depressurization, heating or addition of chemicals such as CO 2 . Depressurization, thermal or chemical stimulation may be used alone or in combination, but the idea of producing hydrocarbons from hydrate bearing sediments by CO 2 injection suggests the potential of an almost emission free use of this unconventional natural gas resource. However, up to now there are still open questions regarding all three production principles. Within the framework of the German national research project SUGAR the thermal stimulation method by use of in situ combustion was developed and tested on a pilot plant scale and the CH 4 -CO 2 swapping process in gas hydrates studied on a molecular level. Microscopy, confocal Raman spectroscopy and X-ray diffraction were used for in situ investigations of the CO 2 -hydrocarbon exchange process in gas hydrates and its driving forces. For the thermal stimulation a heat exchange reactor was designed and tested for the exothermal catalytic oxidation of methane. OPEN ACCESSEnergies 2011, 4 152 Furthermore, a large scale reservoir simulator was realized to synthesize hydrates in sediments under conditions similar to nature and to test the efficiency of the reactor. Thermocouples placed in the reservoir simulator with a total volume of 425 L collect data regarding the propagation of the heat front. In addition, CH 4 sensors are placed in the water saturated sediment to detect the distribution of CH 4 in the sample. These data are used for numerical simulations for up-scaling from laboratory to field conditions. This study presents the experimental set up of the large scale reservoir simulator and the reactor design. Preliminary results indicate that the catalytic oxidation of CH 4 operated as a temperature controlled, autothermal reaction in a countercurrent heat exchange reactor is a safe and promising tool for the thermal stimulation of hydrates. In addition, preliminary results from the laboratory studies on the CO 2 -hydrocarbon swapping process in simple and mixed gas hydrates are presented.

The hydrogenation of acrolein over pure and supported silver has been investigated with a focus on the influence of catalyst structure and reaction pressure (mbar to 20 bar range) on activity and selectivity. An onset of formation of allyl alcohol beyond 100 mbar reaction pressure (at 250 degrees C) is ascribed to a change in adsorption geometry upon increasing coverage. Smaller silver particles (in the nanometer range), the proximity of a reducible oxide component as well as high pressure lead to enhanced allyl alcohol formation; the selectivity to the other main product propionaldehyde is reduced. The silver dispersion changed depending on the reaction pressure. Moreover, the presence of oxygen, most likely as subsurface oxygen, and the presence of defects are of paramount importance for the catalytic behaviour. The considerable changes of the silver catalysts under reaction conditions and the pressure dependence call for in situ measurements to establish true structure-activity/selectivity relationships for this system.

The aim of the PROX reaction is to reduce the CO content of hydrogen feed to proton exchange membrane fuel cells (PEMFC) by selective oxidation of CO in the presence of excess hydrogen. Both Pt and Pd on ceria are active in CO oxidation (without hydrogen) while Pd is poorly active in the presence of hydrogen. In this paper we aimed at finding the reasons of such behavior, using the same techniques for Pd/CeO 2 as for Pt/CeO 2 in Part I: catalytic tests, in-situ DRIFTS, high-pressure XPS, HRTEM and TDS. The reaction mechanism of CO oxidation (without hydrogen) was also examined. It does not occur via the exactly same mechanism on Pt and Pd/CeO 2 catalyst. In the presence of hydrogen (PROX) at low temperature (T=350-380 K), the formation of Pd β-hydride was confirmed by high-pressure in-situ XPS. Its formation greatly suppressed the possibility of CO oxidation, because oxygen both from gas phase and support sites reacted fast with hydride H to form water, and this water desorbed from Pd easily. Nevertheless, CO adsorption was not hampered here. These entities transformed mainly to surface formate and formyl (-CHO) species instead of oxidation as observed by DRIFTS. The participation of a low-temperature water-gas-shift type reaction proposed for the platinum system [Part I] was hindered. Increasing temperature led to decomposition of the hydride phase and a parallel increase in the selectivity towards CO oxidation was observed. However, it remained still lower on Pd/CeO 2 than on Pt/CeO 2 .

The gas phase hydrogenation of acrolein over silver has been studied in a broad pressure range from ~2 mbar to 20 bar and with various silver materials (single crystals, sputtered silver, silica supported Ag nanoparticles) in an attempt to examine the question of "pressure and materials gap" in catalysis. High pressures as well as nanoparticles favour the formation of allyl alcohol (selectivities up to 42 %), whereas with the opposite conditions propionaldehyde is by far the main product. A critical minimum reaction pressure was identified: below ca. 100 mbar no allyl alcohol was formed. In situ-XAS measurements have been performed at 7.5 mbar in order to gain insight into the interaction of acrolein with silver samples. Despite the fact that beam-induced processes have been observed, it is concluded that at low pressures, acrolein orientates parallel to the surface on Ag (111) and is present at the surface in the form of hydrogenated propionaldehyde-like species. The influence of catalyst structure and pressure on the adsorption geometry of acrolein as well as the possible rate-determining step in acrolein hydrogenation are discussed.

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The gas phase hydrogenation of acrolein over 7.5% Ag/SiO 2 has been studied in a broad pressure range from 7.5 mbar to 20 bar. Higher pressures favour the formation of allyl alcohol (selectivities up to 42 %), whereas at low pressures propionaldehyde is by far the main product. In situ-XAS has been performed at 7.5 mbar in order to gain insight into the interaction of acrolein with Ag(111). Hydrogenated propionaldehyde-like surface species could be detected which orientated parallel to the surface. The observed intermediate correlates perfectly with the online catalytic data.