(472a) Hydrodechlorination of 1,2-Dichloroethane over Ag-Pd Catalysts Prepared By Controlled Surface Reactions

Hydrodechlorination is an attractive process for the treatment of toxic chlorinated waste, such as 1,2-dichloroethane, to produce valuable hydrocarbon products^1,2. Previous studies have shown Group VIII metals, such as Pt and Pd to be active for this reaction^1,3–8. Furthermore, the addition of Group IB metals, such as Cu or Ag has been shown to improve selectivity to produce ethylene over ethane^{1,3–11}. Although numerous studies in the literature have examined hydrodechlorination reactions over AgPd catalysts^{1,3–6,10–12}, traditional catalyst synthesis methods produce bimetallic catalysts with a wide range of structures, making it challenging to elucidate structure activity relationships. In this work, AgPd catalysts have been synthesized by controlled surface reactions to form well defined active site structures.

We present our results on the hydrodechlorination of 1,2-dichloroethane to form ethane, ethylene, and chloroethane over Pd and AgPd catalysts. These Pd and AgPd catalysts are synthesized on carbon, SiO₂, and TiO₂ supports, and we show that the formation of uniform bimetallic structures is most effective on the TiO₂ support. The bimetallic catalysts are synthesized by depositing Pd by controlled surface reactions onto a supported Ag catalyst, as has previously been demonstrated for other metal systems^13–19. UV visible spectroscopy is used to quantify the extent of interaction between the Pd precursor (cyclopentadienyl Pd allyl) and the support. We observe no interaction between the Pd precursor and the titania support, indicating that selective deposition of Pd onto Ag/TiO₂ structures occurs during the synthesis process. On carbon and silica supports, some interaction between Pd precursor and support is observed, and Pd deposition onto Ag is not entirely selective. It is determined that formation of bimetallic AgPd structures is most effective for AgPd/TiO₂, followed by AgPd/C and then AgPd/SiO_{2 ­}catalysts.

FTIR spectra of adsorbed CO on the AgPd catalysts provide additional evidence for variations in the active site structure depending on the degree of interaction between Ag and Pd. On the TiO₂-supported AgPd catalyst, CO adsorbs to Pd primarily in a linear configuration, while CO adsorbs in both linear and multifold configurations on the SiO₂-supported AgPd catalyst. This difference in CO adsorption indicates that Pd is well dispersed on the Ag nanoparticles for the TiO₂-supported catalyst, while a higher fraction of contiguous Pd sites is present on the SiO₂-supported catalyst.

The ethylene selectivity decreases from 97% to 1% on AgPd/TiO₂ catalysts as the Ag:Pd atomic ratio increases from 1:2 to 1:0.35. Similarly, an ethylene selectivity of 82% is achieved on the AgPd/C catalyst. On these catalysts, the selectivity to chloroethane is less than 1% and the balance of the selectivity is to ethane. The AgPd/SiO₂ catalyst, however, is 91% selective to ethane and 9% selective to chloroethane. These results indicate that the well dispersed Pd achieved at low Pd loadings on TiO₂ supported catalysts and carbon supported catalysts are selective to ethylene, while the contiguous Pd species formed at higher Pd loadings on TiO₂ and on SiO₂ are selective to ethane.

We have synthesized AgPd catalysts on TiO₂, carbon, and SiO₂ supports with various surface structures by using controlled surface reactions. The range of surface structures formed on these supports at various Pd loadings has allowed us to develop structure-selectivity relationships for the hydrodechlorination of 1,2-dichloroethane, observing high ethylene selectivity when Pd is well isolated because of intimate contact with Ag.

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