Study of the state of Ag and Pd in silver-palladium catalysts on a titanium dioxide substrate by IR spectroscopy of adsorbed CO

Abstract

We have used IR spectroscopy of adsorbed CO to study the effect of the method for preparation of silver-palladium catalysts on a TiO 2 substrate and exposure to an oxidizing or reducing gaseous atmosphere on the state of Ag and Pd centers. We have shown that as a result of the metal-support interaction, the reducibility of the Ag and Pd particles is increased, which leads to a change in their adsorption and catalytic properties.Introduction. Supported bimetallic catalysts containing Pd and Ag have high activity and selectivity in various exhaustive and partial oxidation processes [1,2]. We know [3-6] that Ag has a considerable effect on the sorption and catalytic properties of supported palladium catalysts. Thus in [1,2] it is shown that modification of aluminum-palladium catalysts by silver leads to an increase in their activity in oxidation of carbon monoxide, methanol, and ethylene.According to [2][3][4], the change in the catalytic activity of bimetallic systems is connected with interaction of Pd and Ag on the surface of the supports, which leads to formation of new active centers involving them. Thus in the opinion of the authors of [4,5], synergism in the catalytic effect of Ag-Pd/Al 2 O 3 in oxidation of CO is due to formation of Ag-Pd alloys. In [6], in-situ IR spectroscopy of adsorbed CO was used to establish the substantial effect of Ag additives on the sorption characteristics of Pd/SiO 2 .In the spectra of CO adsorbed on the Ag-Pd/SiO 2 surface, the ratio of linear and bridge complexes of palladium with CO changes appreciably compared with unmodified samples. Analogous data were obtained in [5], suggesting an increase in the fraction of the linear forms of adsorbed CO when Ag is introduced.Interaction of Pd and Ag on the surface of the support and the composition of the surface particles are mainly determined by the conditions under which the catalysts are prepared and activated. Thus in [3], the presence of monometallic and bimetallic particles was established on the surface of a Pd-Ag/Al 2 O 3 catalyst obtained by deposition of both components from one impregnating solution. Preliminary modification of the Al 2 O 3 by palladium leads to formation of interstitial structures, while modification of the support by silver does not yield such structures.In [6], it is shown that oxidizing treatment of Ag-Pd/SiO 2 leads to segregation of the metals, while high-temperature reduction by hydrogen induces surface migration of the Ag.The aim of this work was to study the effect of the preparation conditions for silver-palladium catalysts on a titanium dioxide substrate and their treatment in oxidizing or reducing gaseous atmospheres on the formation of Ag and Pd centers, and also their activity in oxidation reactions.The experiment. Titanium dioxide, obtained by precipitation of TiO(OH) 2 by ammonia from a TiOSO 4 solution, was used as the support to prepare the catalysts [7]. Depending on the method for deposition of the metals, the following catalyst samples were obtained.Sample No. 1 (Pd/TiO 2 ). The TiO 2 s...