A mass spectrometry study of the surface chemical activities of thorium oxide and cerium-doped thorium oxide

The capacity for thorium oxide and cerium doped thorium oxide for Fischer-Tropsch catalytic activity in the 0-20 Torr pressure regime was evaluated by Quadrupole Mass Spectrometry. High surface area catalysts were prepared and characterized by adsorption isotherm measurements, scanning electron microscopy, energy dispersive x-ray spectrometry and by gas-solid oxygen isotope exchange kinetics.; Evaluation of catalytic activity was performed in an inert batch reactor system; with automated data acquisition and control using custom software. This system was characterized to allow quantitative acquisition of oxygen exchange kinetics data, and semi-quantitative Fischer-Tropsch kinetics data.; The oxygen exchange mechanisms were found to follow two pathways: single atom exchange between molecular oxygen and lattice oxygen (an R1 process), and a molecular exchange mechanism (an R2 process). It was found that neither process has identical Arrhenius parameters on pure ThO{dollar}sb2{dollar} compared with 10% CeO{dollar}sb2{dollar} doped ThO{dollar}sb2{dollar}. Doping with ceria lowers the activation energy for both R1 and R2 exchanges by 40 KJ, the result being that exchange proceeds faster on the doped thoria than on pure thoria. The literature data for pure CeO{dollar}sb2{dollar} indicates that ceria has a slower overall rate of exchange, in comparison to ThO{dollar}sb2{dollar}. Thoria doped with CeO{dollar}sb2{dollar} has the rates of oxygen exchange altered in a non-additive way, which implies that the doped material must have the Ce completely dispersed at the surface. This has been known to be true in the bulk form from X-ray data.; The product distribution for the Fischer-Tropsch reaction over the two materials indicates that the primary product is propane and methane, with small amounts of 1-butene for pure ThO{dollar}sb2{dollar}. In the case of ThO{dollar}sb{lcub}0.9{rcub}{dollar}Ce{dollar}sb{lcub}0.1{rcub}{dollar}O{dollar}sb2{dollar} catalyst, the product distribution is primarily methane, followed by propane and propene, and 2-butene. This shift in product distribution is accompanied by a 3 fold increase in the rate of consumption of CO with respect to identical conditions over pure thoria catalyst. The shift in product distribution when switching from thoria to Th{dollar}sb{lcub}0.9{rcub}{dollar}Ce{dollar}sb{lcub}0.1{rcub}{dollar}O{dollar}sb2{dollar} is consistent with the redox chemistry of cerium if the synthesis is assumed to proceed by an ene-diolate intermediate.