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In situ ATR-IR study of the adsorption of cinchonidine on Pd/Al2O3. Differences and similarities with adsorption on Pt/Al2O3
|Published in||Journal of Catalysis. 2002, vol. 210, no. 1, p. 160-170|
|Abstract||Pd/Al2O3 model catalysts have been prepared by physical vapour deposition and characterised by means of XPS, STM, and in situ ATR–IR spectroscopy. Morphological changes in the Pd film induced by dissolved hydrogen leads to enhanced infrared absorption and could be followed with both STM measurements and IR spectroscopy. Adsorption of CO, pyridine, quinoline, 2-methylquinoline, and the chiral auxiliary cinchonidine has been studied in situ at 283 K in CH2Cl2 solvent. Two different species have been observed for cinchonidine on Pd. One is oriented with the quinoline moiety nearly parallel to the Pd surface, likely through the Π-system, whereas in the second the σ-bonding through the N lone pair prevails and induces a tilting of the ring with respect to Pd. No indication of the presence of α-quinolyl species has been found, in contrast to adsorption on Pt/Al2O3 catalysts. Compared to adsorption on Pt, cinchonidine is more weakly bound on Pd under hydrogenation conditions. Also, the relative stability of the Π- and N lone pair-bonded species is different for the two metals, with the Π-bonded species being relatively more stable on Pt. Similarities and differences found in the adsorption of the chiral modifier on the two metals are discussed and traced mainly to the different d-orbital diffuseness of Pd and Pt.|