ABSTRACT
TOF-SIMS analyses of state-of-the-art high temperature solid oxide electrolysis cells before and after testing under different operating conditions were performed. The investigated cells consist of an yttria stabilized zirconia (YSZ) electrolyte, a La1-xSrxMnO3-δ composite anode and a Ni-YSZ cermet cathode. The surfaces and cross-sections of the cells were analyzed, and several elemental impurities like Si, Ca and Na were identified and spatially mapped and their enrichment and migration during operation is reported. With advancing operation time, the concentration of these elements, especially Na and Ca, increases. For Si, a concentration gradient is found from the gas inlet to the gas outlet. Additionally, a loss of Ni percolation in the active cathode is observed in the same area where the Si enrichment is found. Based on the obtained TOF-SIMS results, the influence of the operating conditions on degradation is discussed.
ABSTRACT
Adsorption and oxidation of carbon monoxide over a Pd/Al2O3 catalyst layer was investigated both in gas phase and water. Both adsorption and oxidation of CO are significantly affected by the presence of liquid water. Water influences the potential of the metal particles as well as the dipole moment of the adsorbed CO molecule directly, which is reflected both in large red shifts and a higher infrared intensity when experiments are carried out in water. Furthermore, the rate of CO oxidation increases significantly by both the presence of water and by increasing the pH. Enhancement of the oxidation rate is attributed to a weakening of the CO bond by increasing potential of the metal particle, similar to CO oxidation over Pt/Al2O3 as recently published [S. D. Ebbesen et al., J. Catal., 2007, 246, 66]. However, on Pd/Al2O3 the oxidation of palladium is clearly promoted at increasing pH, further enhancing the oxidation of CO over Pd/Al2O3.
ABSTRACT
In relation to the heterogeneous hydrogenation of nitrite, adsorption of NO2-, NH4+, and NH2OH from the aqueous phase was examined on Pt/Al2O3, Pd/Al2O3, and Al2O3. None of the investigated inorganic nitrogen compounds adsorb on alumina at conditions presented in this study. NO2-(aq) and NH4+(aq) on the other hand show similar adsorption characteristics on both Pd/Al2O3 and Pt/Al2O3. The vibrational spectrum of the NO2- ion changed substantially upon adsorption, clearly indicating that NO2- chemisorbs onto the supported metal catalysts. On the contrary, adsorption of NH4+ does not lead to significant change in the vibrational spectrum of the ion, indicating that the NH4+ ion does not chemisorb on the noble metal but is stabilized via an electrostatic interaction. When comparing the adsorption of hydroxylamine (NH2OH(aq)) on Pd/Al2O3 and Pt/Al2O3, significant differences were observed. On Pd/Al2O3, hydroxylamine is converted into a stable NH2(ads) fragment, whereas on Pt/Al2O3 hydroxylamine is converted into NO, possibly via HNO(ads) as an intermediate.
ABSTRACT
Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases.