The AC monitoring of a mixed conducting Li-Ni-Co-O catalyst during temperature programmed reactions

The temporal analysis of effluent species with temperature linearly increasing with time for two reactions, carbon dioxide hydrogenation (250° to 600°C) and the oxidative coupling of methane (400° to 800°C) have been studied. A non-stoichiometric, mixed-oxide catalyst, Li_0.9Ni_0.5Co_0.5O_2-δ, containing both mobile ions and electrons at elevated temperatures was investigated. The exit gas stream was monitored by using the multiple-ion monitoring facility of a quadrupole mass spectrometer. The AC electrical characteristics of the catalyst pellet between 100 Hz and 1 MHz were simultaneously monitored in-situ. A clear correlation between catalytic behaviour and electrical properties was obtained. Frequency-dependent AC losses due to surface dipole effects were dominant during phases of strong chemisorption and reaction, whilst reversion to thermally activated behaviour occurred during times of reduced chemical activity. In the case of carbon dioxide hydrogenation, the catalyst becomes partially reduced as a result of the chemical reaction, as shown by the changes in conductance-temperature-frequency characteristics. The original oxidised state could be recovered by re-oxidation at 700°C for 1 hour in air. For the partial oxidation of methane, mobile lattice oxygen participates in the reaction. The temporary changes in the state of oxidation of the catalyst during this reaction are correlated with changes in the electrical characteristics of the material. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy