Acetylene Oligomerization over Pd Nanoparticles with Controlled Shape: A Parahydrogen-Induced Polarization Study
Substantial signal enhancements achieved by using parahydrogen in catalytic hydrogenations are powerful tools for mechanistic studies of chemical reactions involving molecular H-2. Potentially, this technique can be extended to other reaction classes, providing new information about reaction mechanisms. Moreover, this can lead to new substances with highly polarized spins. Herein, we report strong signal enhancements of oligomerization reaction products observed during the selective hydrogenation of acetylene over Pd nanoparticles of different shapes and sizes supported on SiO2. C4 oligomeric products (1,3-butadiene, 1-butene, 2-butene) demonstrated a high degree of nuclear spin polarization, with the highest degree observed for 1-butene [more than 1.7% vs (2.4 X 10(-3))% at thermal equilibrium], which was an order of magnitude larger than that of the triple C-C bond hydrogenation products. No dependence of polarization on the metal surface statistics Or, generally, on the nanoparticle morphology (cubic, octahedral, cuboctahedral) could be observed. In contrast, the particle size effect was such that larger particles provided higher signal enhancements. This observation is in line with the increased activity over larger Pd nanoparticles observed during acetylene hydrogenation over the same catalysts.