Thermodynamic properties of gaseous cerium molybdates and tungstates studied by Knudsen effusion mass spectrometry.

Abstract

CeO2 -WO3 and CeO2 -MoO3 catalysts have shown excellent performance in the selective reduction of NOx by ammonia (NH3 -selective catalytic reduction) over a wide temperature range. Strong interaction between CeO2 and WO3 or MoO3 might be the dominant reason for the high activity of these mixed oxides. Studies of ceria-containing gaseous salts involve considerable experimental difficulties, since the transition of such salts to vapor requires high temperatures. To predict the possibility of the existence of gaseous associates formed by cerium and molybdenum (tungsten) oxides it is important to know their thermodynamic characteristics. Until the present investigation, gaseous cerium oxyacid salts were unknown. Knudsen effusion mass spectrometry was used to determine the partial pressures of vapor species and the equilibrium constants of gas-phase reactions, as well as the formation and atomization enthalpies of gaseous cerium molybdates and tungstates. CeO2 was evaporated from molybdenum and tungsten effusion cells containing gold metal as a pressure standard. A theoretical study of gaseous cerium gaseous molybdates and tungstates was performed by several quantum chemical methods. In the temperature range 2050-2400 K, CeO, CeO2 , XO2 , XO3 , CeWO3 , CeXO4 , CeXO5 (X = Mo, W) and CeMo2 O7 were found to be the main vapor species over the CeO2 - Mo (W) systems. On the basis of the equilibrium constants of the gaseous reactions, the standard formation enthalpies of gaseous CeWO3 , CeXO4 , CeXO5 (X = Mo, W) and CeMo2 O7 at 298 K were determined. Energetically favorable structures of gaseous cerium salts were found and vibrational frequencies were evaluated in the harmonic approximation. The thermal stability of gaseous cerium oxyacid salts was confirmed by high-temperature mass spectrometry. Reaction enthalpies of the gaseous cerium molybdates and tungstates from gaseous cerium, molybdenum and tungsten oxides were evaluated theoretically and the obtained values are in reasonable agreement with the experimental one.