Synthesis of a binary alloy nanoparticle catalyst with an immiscible combination of Rh and Cu assisted by hydrogen spillover on a TiO2 support

Abstract

This work demonstrated the use of TiO₂ 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 TiO₂ with concurrent proton–electron transfer. The resulting RhCu/TiO₂ exhibited 2.6 times higher catalytic activity than Rh/TiO₂ 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 H₂O 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 TiO₂. H₂-temperature programmed reduction and in situ X-ray absorption fine structure analyses at elevated temperature under H₂ demonstrated that Rh³⁺ and Cu²⁺ precursors were simultaneously reduced only on the TiO₂ support. This effect resulted from the improved and limited reducibility of Cu²⁺ and Rh³⁺, respectively. The rate of hydrogen spillover of TiO₂ is faster as compared to γ-Al₂O₃ and MgO as evidenced by sequential H₂/D₂ exchanges during in situ Fourier transform infrared analyses. Density functional theory calculations also showed that the migration of H atoms on TiO₂ proceeds with a lower energy barrier than that on Al₂O₃, and the reduction of Cu²⁺ species is facilitated by H spillover on the support rather than by direct reduction by H₂. These results confirm the vital role of TiO₂ in the formation of the alloy and may represent a new strategy for the synthesis of different non-equilibrium solid solution alloys.