H-1 NMR studies on the dynamic property of protons in Pd-0-H3PW12O40 systems in the presence of dihydrogen

The dynamic property of protons in the catalysts consisting of Pd-0 metal and H3PW12O40 in the presence of dihydrogen was investigated with H-1 MAS and broad-line NMR techniques. The protons in H3PW12O40 were observed at 9.1 ppm in H-1 MAS NMR. When a composite catalyst consisting of Pd(II) and H3PW12O40 was reduced with dihydrogen, the intensity of the 9.1 ppm signal increased. In addition, hydrogen atoms adsorbed on metallic palladium were observed at 19.8 ppm. The temperature dependence of H-1 MAS NMR reveals the dynamic nature of protons in the catalyst consisting of Pd-0 metal and H3PW12O40. When the composite catalyst was heated to 333 K in the presence of dihydrogen, the line widths of the two peaks were broadened and the peak of hydrogen atoms shifted toward that of acidic protons, indicating that the interconversion occurs between the hydrogen atoms on Pd-0 metal and protons in H3PW12O40 and that hydrogen atoms formed on Pd-0 metal spill-over to the surrounding heteropolyanions. The line width of the peak at 9.1 ppm of the composite catalyst in the presence of dihydrogen depended on the spinning rate in H-1 MAS NMR at 373 K, indicating that the protons in the catalyst are mobile. On the other hand, the line width was independent of spinning rate in the case of H3PW12O40 at this temperature. This clearly shows that the presence of dihydrogen and Pd-0 metal modify the dynamic nature of protons in H2PW12O40.