Nucleation and growth of Pd clusters in mordenite

The nucleation and growth of Pd clusters in mordenite were investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and Fourier transform infrared (FTIR) spectroscopy of absorbed CO. Calcination of [Pd(NH₃)₄]²⁺-exchanged mordenite at 350°C in O₂ results in decomposition of the amine complex and formation of square-planar Pd²⁺ oxo species within the mordenite pores. Reduction of these species at 150°C in H₂ yields Pd clusters with an average nuclearity of 3. On an average two 2.22 Å Pd-O bonds are associated with each Pd₃ cluster; we infer that this interaction serves to anchor the clusters within the pores. After reduction at 150°C, the FTIR spectrum of irreversibly adsorbed CO is indicative of a mixture of Pd⁺, Pd^δ+, and Pd⁰ carbonyl species. Reduction at 350°C produces larger intrazeolitic Pd clusters (average nuclearity of 6) that exhibit only a weak interaction with the mordenite, as evidenced by their facile aggregation in the presence of CO at 30°C. Reduction at 450°C yields large 20 Å Pd clusters that we infer are located on external mordenite surfaces or locally disrupt the intracrystalline structure.