Synthesis of a binary alloy nanoparticle catalyst with an immiscible combination of Rh and Cu assisted by hydrogen spillover on a TiO(2) support

This work demonstrated the use of TiO(2) as a promising platform for the synthesis of non-equilibrium RhCu binary alloy nanoparticles (NPs). These metals are regarded as immiscible based on their phase diagram but form NPs with the aid of the significant hydrogen spillover on TiO(2) with concurrent proton–electron transfer. The resulting RhCu/TiO(2) exhibited 2.6 times higher catalytic activity than Rh/TiO(2) during hydrogen production from the hydrolysis of ammonia borane (AB), due to a synergistic effect. Theoretical simulations showed a higher energy value for the adsorption of AB on the RhCu alloy and a lower activation energy for the rate determining N–B bond dissociation by the attack of H(2)O during AB hydrolysis compared to monometallic Rh. High-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the formation of RhCu alloy NPs with a mean diameter of 2.0 nm on the TiO(2). H(2)-temperature programmed reduction and in situ X-ray absorption fine structure analyses at elevated temperature under H(2) demonstrated that Rh(3+) and Cu(2+) precursors were simultaneously reduced only on the TiO(2) support. This effect resulted from the improved and limited reducibility of Cu(2+) and Rh(3+), respectively. The rate of hydrogen spillover of TiO(2) is faster as compared to γ-Al(2)O(3) and MgO as evidenced by sequential H(2)/D(2) exchanges during in situ Fourier transform infrared analyses. Density functional theory calculations also showed that the migration of H atoms on TiO(2) proceeds with a lower energy barrier than that on Al(2)O(3), and the reduction of Cu(2+) species is facilitated by H spillover on the support rather than by direct reduction by H(2). These results confirm the vital role of TiO(2) in the formation of the alloy and may represent a new strategy for the synthesis of different non-equilibrium solid solution alloys.