Isotope effect in hydrogen/deuterium-absorbing Pd nanoparticles revealed by X-ray powder diffraction and by a multi-component MO method

The isotope effect in Pd nanoparticles that absorb hydrogen or deuterium (i.e., H/D isotope effect) was studied experimentally and theoretically. First, the geometries (i.e., lattice parameters) of such Pd nanoparticles exposed to hydrogen or deuterium gas were measured by using X-ray powder diffraction to determine the lattice parameters. Then, the geometrical and electronic relaxations of Pd_nH^- and Pd_nD^- (n = 1-6) clusters, which modeled Pd nanoparticles exposed to hydrogen or deuterium gas, were calculated by using a multi-component molecular orbital (MC_MO) method, which uses first principles of quantum mechanics to account for the quantum effect of proton/deuteron. Experimental results from the diffraction patterns show that the bond distances of Pd nanoparticles exposed to hydrogen gas (and thus had absorbed hydrogen) were about 0.005 Å longer than those of exposed to deuterium gas. These results were confirmed by analytical results from the MC_MO calculation for Pd_nH^- and Pd _nD^- clusters. Therefore, the local geometrical changes due to the H/D isotope effect control the geometrical changes of the entire nanoparticle. Both the experimental and analytical results also show that the nanoparticle (cluster) size influences the extent of the H/D isotope effect on the geometrical changes. Based on the analytical results, the electronic charge densities are only slightly influenced by the H/D isotope effect.