Influence of the preparation procedure in the activity of Pd supported catalysts used in CO2 reforming of CH4

IGNACIO O. COSTILLA; CARLOS E. GIGOLA; MIGUEL D. SÁNCHEZ

Karlsruhe

Conferencia; 1st International Conference on Materials for Energy; 2010

In spite of the high reaction temperature (> 650oC) required to obtain a reasonable level of conversion, CO2 reforming of CH4 is a promising reaction to be used in the production of synthesis gas from CH4/CO2 mixtures coming directly from natural gas wells. The literature shows that Pt/ZrO2 and Rh supported on Al2O3 and MgO are highly active and stable catalysts for the reforming reaction. Catalytic activity and selectivity of Pd catalysts for dry reforming have not been extensively studied despite the lower cost of Pd as compared with Pt or Rh. In this work we have studied the effect of the preparation method on the catalytic activity of Pd catalysts with low metallic loading (< 0.5%) supported on a commercial a-Al2O3. The catalysts were prepared by a recharging method consisting of several impregnation steps with a solution of Pd(AcAc)2 in benzene (2 x 10-3 g Pd(AcAc)2 /ml) followed by washing and calcination at 500oC. Catalysts with different metal loading and dispersion were prepared by adjusting the number of washings steps. The samples were characterized by H2 chemisorption, FTIR and XPS. For H2 chemisorption, sorption and back sorption isotherms were measured in a volumetric glass adsorption apparatus. FTIR studies of adsorbed CO (1 Torr) were carried out at room temperature using a NICOLET 20 DXB instrument at 4 cm-1 resolution. XPS measurements were made in a PHI 548 spectrometer equipped with a preparation chamber for heat treatment in hydrogen up to 700oC. The catalytic activity was determined in a flow reactor operated at 650 °C temperature using a feed mixture of CH4/CO2/Ar (25/25/50). The characterization by FTIR and H2 chemisorption demonstrates that catalysts with a metallic dispersion above 30% and a low metal content (0.37%) are obtained when several washing steps after impregnation are used. The XPS spectra indicate the presence of a small fraction of Pd(4+) after an hour reduction treatment at 700°C, due to the presence of small particles with high substrate interaction. The catalytic test showed that the catalyst with high metal dispersion undergoes a slow activation during 24 hours of reaction time that correlates with a particle-size increase and a diminution of the fraction of Pd(4+) species. On the other hand, samples with low metal dispersion (< 0.25%) exhibit a steady deactivation due to carbon deposition and sintering . The formation of palladium particles with a (100) surface structure that favours the adsorption and dissociation of CH4, seems to be a necessary condition to obtain a high activity for the reforming reaction.