@article{reifsnyder_lamb_1999, title={Characterization of silica-supported Pd-Au clusters by X-ray absorption spectroscopy}, volume={103}, ISSN={["1089-5647"]}, DOI={10.1021/jp982893b}, abstractNote={Silica-supported Pd−Au clusters were characterized by Pd K and Au L3 extended X-ray absorption fine structure (EXAFS) spectroscopy and Au L2,3 X-ray absorption near-edge structure (XANES) spectroscopy. Pd−Au/SiO2 catalysts with Pd/Au atomic ratios of approximately 1:1 were prepared by coadsorption of [Pd(NH3)4][NO3]2 and [Au(en)2]Cl3 (en = ethylenediamine) onto silica gel from pH 7 and pH 10 slurries. EXAFS spectroscopy of the Pd−Au/SiO2 (1.3:1, pH 7) catalyst after in situ reduction at 300 °C indicates the formation of bimetallic clusters exhibiting Pd surface enrichment. The EXAFS results can be approximated using a cluster “decoration” model in which a Au-rich core is covered by a partial monolayer of Pd. In contrast, EXAFS spectroscopy of the Pd−Au/SiO2 (1:1, pH 10) catalyst after in situ reduction at 350 °C indicates the formation of homogeneous Pd−Au alloy clusters. The homonuclear bond distances in these clusters agree closely with the Vegard's Law prediction for a 1:1 Pd−Au alloy; however, the heteronuclear bond distances are ∼0.02 Å shorter. Au L2,3 XANES spectroscopy of the supported Pd−Au clusters evidences a vanishingly small density of unoccupied Au 5d states. Excellent correspondence is observed between the XANES spectra of supported Pd−Au clusters and those of Pd−Au alloy films having similar compositions.}, number={2}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Reifsnyder, SN and Lamb, HH}, year={1999}, month={Jan}, pages={321–329} }
 @article{reifsnyder_otten_lamb_1998, title={Nucleation and growth of Pd clusters in mordenite}, volume={39}, ISSN={["0920-5861"]}, DOI={10.1016/S0920-5861(97)00121-1}, abstractNote={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 adsorbed 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+, Pdδ+, 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.}, number={4}, journal={CATALYSIS TODAY}, author={Reifsnyder, SN and Otten, MM and Lamb, HH}, year={1998}, month={Mar}, pages={317–328} }
 @article{reifsnyder_otten_sayers_lamb_1997, title={Hydrogen chemisorption on silica-supported Pt clusters: In situ X-ray absorption spectroscopy}, volume={101}, ISSN={["1089-5647"]}, DOI={10.1021/jp970244e}, abstractNote={Hydrogen chemisorption on small silica-supported Pt clusters was investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy. The clusters were found to exhibit a bulklike Pt first nearest neighbor (NN) distance (2.76 Å) and low disorder while covered by chemisorbed hydrogen. In contrast, bare Pt clusters produced by heating in vacuo at 300 °C are characterized by a contracted Pt NN distance (2.66 Å) and greater disorder. These effects are reversed by re-exposure of the bare Pt clusters to H2 at 25 °C. The metal−support interface is characterized by a short Pt−O distance, irrespective of the presence of chemisorbed hydrogen. An apparent L3 edge shift of 0.8 eV relative to bulk Pt is observed for the hydrogen-covered clusters. This shift is attributed to a decrease in the Pt L3 edge resonance (white line) intensity, as no corresponding shift is observed at the L2 edge. A hydrogen-related L2,3 XANES feature at 9 eV appears with nearly equal intensity at each edge. This peak is assigned to electronic transitions from Pt 2p levels to H 1s−Pt 5d antibonding states with mixed d3/2−d5/2 character. From the L2,3 XANES analysis, we find that the number of unoccupied d states in hydrogen-covered Pt clusters is 23% less than in bulk Pt. In contrast, the L2,3 XANES spectra of bare silica-supported Pt clusters are closely similar to those of bulk Pt; quantitative analysis reveals only a slight (4%) decrease in the number of unoccupied d states.}, number={25}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Reifsnyder, SN and Otten, MM and Sayers, DE and Lamb, HH}, year={1997}, month={Jun}, pages={4972–4977} }