Patricio Reyes, Alejandra Figueroa and Gina Pecchi

Faculty of Chemistry, Universidad of Concepción, Casilla 160-C, Concepción, Chile.

Fax 56-41-245974, Email: preyes@udec.cl

The catalytic combustion is one of the several industrial processes for pollution abatement. Catalysts able to perform combustion are divided in two groups: noble metals for which reactions may start at temperatures as low as room temperature and transition metal oxides which are less efficient, but also more resistant towards high temperatures. With regard to the noble metal supported catalysts, most of the studies have been focused on platinum and palladium ^1-2. The nature of the support has also been investigated, being Al₂O₃, SiO₂-Al₂O₃, SiO₂, ThO₂, ZrO₂, and TiO₂, among others extensively studied ³. The sinterization and the poisoning of the metal component is an important problem in treatments of industrial emissions. This is caused by the presence in the feed of water vapour and compounds containing some heteroatoms. The migration of the metal crystallites in the presence of water, may be minimized using the sol gel method in the preparation of the metal supported catalysts. In fact, one of the advantages reported for the sol-gel method in the preparation of metallic catalysts, is that it may produce catalysts in which the sintering is highly restricted due to the metal component is strongly interacting with the support and in some cases remains buried into the support⁴.

In the present work two different serie of Pd/SiO₂ and Pd-Cu/SiO₂ catalysts containing 0.5wt.% Pd and different Cu loading (0.3 up to 1.2 wt%) have been prepared. One prepared by the sol gel method using tetraethoxysilane, acetylacetonate of palladium and copper nitrate, as precursors of SiO₂, Pd and Cu respectively. According to previous studies, a pH of gelation of 5 using acetic acid as hydrolysis catalyst was chosen. The other series was prepared by impregnation or coimpregnation of the same precursors on a commercial silica.

Surface area and porosity, temperature programmed reduction, hydrogen and carbon monoxide chemisorption and TEM have been used to characterize the catalyst.

The catalytic activity in the combustion of methane was studied under stoichiometric an oxidant conditions to evaluate the catalytic behaviour of the different series. The reaction was studied at different temperatures from 473 up to the temperature required for a total conversion.

Nitrogen adsorption isotherm at 77K indicates great differences in both series. Surface area for the catalysts prepared by impregnation is close to the support one (130 m²/g), whereas in those obtained by cogelation, the values are much higher and changes significantly with the addition of Cu. In the former, the average pore diameter is 4.5 nm and in the later is arround 1.0 nm, indicating an important proportion of micropores. TPR experiments show significant differences in the profiles of both serie. In the bimetallic Pd-Cu prepared by impregnation, a clear effect on the copper reduction catalyzed by the presence of Pd was observed, being this effect much smaller in the sol-gel series.

Table I summarizes some characterization and catalytic activity results of the studied catalysts. Pd dispersion in all catalysts is high, specially in those obtained by the sol-gel procedure. Only slight changes were detected in H/Pd ratios as Cu is added to the catalyst, being higher in the bimetallic Pd-Cu compared with the monometallic palladium, which may suggest Pd-Cu interactions. This finding is in line with XPS results. In fact, Pd/Si surface ratio is higher in the bimetallic catalysts. Additionally, the B.E values of the Pd 3d_5/2 for Pd/SiO₂ is 335.2 eV correspond to Pd° species. In Pd-Cu catalysts a slight shift to lower values is detected suggesting the possibility of alloy formation in a small extent. The Cu/Si surface ratio is almost 4 times lower than the nominal ratio, indicating poor Cu dispersion, in agreement with TEM results.

Table I. H/Pd ratios, metal particle size, atomic surface ratios and ignition temperature in methane combustion for 0.5wt.% Pd catalysts

The ignition temperature is defined as the temperature required to obtain 50% of conversion. In both serie the ignition temperature is lower for the monometallic Pd catalysts, whereas in the bimetallic Pd-Cu system this value is almost constant for the catalysts prepared by impregnation but an increase in the ignition temperature with Cu loading is observed. This may be related to differences in Pd-Cu interactions, being higher in the impregnated ones. As expected, the ignition temperature is lower in excess of oxygen in the feed for Pd and Pd-Cu catalysts. Comparing both serie of catalysts, the activity is higher in all the catalysts prepared by impregnation compared with the sol-gel. This can be attributed to differences in accessibility of the reactants to the active site, due to the differences in porosity of the supports. However, after experiments carried out under drastic conditions, new combustion cycles were carried out, showing a decrease in the activity in the catalysts prepared by impregnation whereas in those obtained by the sol-gel method, the activity does not change.

References:

1.- R. Prassad, L. Kennedy, E. Ruckenstein, Catal. Rev. Sci. Eng. 26 (1984) 1.

2.- G. Pecchi, P. Reyes, I. Concha and J.L.G. Fierro, J. Catal., 179 (1998) 309.

Our main research field is Catalysis by Metals. We have been involved in several research projects with researchers from Spain, France, England, Mexico, Argentine and Brazil.

Research Projects in Progress

1. Selective hydrogenation of a ,b insaturated aldehydes ( Both in gas phase and in liquid medium)
2. Combustion of Volatile organic compounds on metal supported catalysts