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Methanol and methane formation over palladium dispersed on the lanthanide rare earth oxides

Journal of Catalysis
Publication Date
DOI: 10.1016/0021-9517(87)90158-8


Abstract To complete our study of the effect of the rare earth oxide (REO) support on the CO hydrogenation reaction over Pd, this metal was dispersed on the oxides of the late lanthanide elements—Tb through Lu. The Pd surface area was measured before and after kinetic runs at 0.1 MPa (1 atm) and 1.5 MPa with these Pd REO catalysts. Methanation activation energies were 30.4 ± 1.1 kcal mole −1', which was virtually the same as that for those determined for the early Pd REO catalysts. The turnover frequency (TOF) at 548 K for methane ranged from 1.2 × 10 −3 to 4.8 × 10 −3 s −1, which is up to 40 times that on Pd powder. At 1.5 MPa and 523 K, stable activity was achieved after 48 h on stream and all catalysts showed a selectivity of 83–90% to oxygenates (excluding CO 2), with over 90% of this fraction composed of CH 3OH. The average activation energy for methanol synthesis was 19.6 ± 0.8 kcal mole −1, again nearly identical to values for Pd on the early REOs, and TOFs varied from 1.6 × 10 −3 to 6.1 × 10 −3 s −1. When these results are combined with those of the previous study (C. Sudhakar and M. A. Vannice, J. Catal., 95, 227 (1985)), the patterns for methanation at 0.1 MPa and CH 3OH synthesis at 1.5 MPa are established as a function of the position of the REO in the periodic table. There is a clear influence of the support on catalytic properties. No trend occurs between methanation activity and the acidity of the support, but a correlation exists between the CH 4 TOF and the activation energy for electrical conductivity, which is consistent with a redox sequence involving oxygen vacancies at the PdREO interface which enhance activity by facilitating bond rupture of the CO molecule. The TOF for CH 3OH synthesis shows a strong correlation with the basicity of the REO, which varies with the lanthanide contraction. This behavior also indicated that the support participates directly in the reaction sequence, and this trend strongly supports a mechanism involving formate species on the REO surface, which are formed most readily on the most basic oxides, such as La 2O 3. The possibility of a heterogeneous analog of the Cannizzaro reaction is pointed out, and this route may also influence methanol formation.

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