Atomic-scale hydrogen dynamics on (sub)nano-sized Ir/γ-Al2O3: chemisorption, spillover, and reverse spillover via combined theory-experiment

Atomic-scale insights into hydrogen chemisorption, spillover, and reverse spillover on metal-oxide-supported metal (sub)nanoparticles are crucial for advancing hydrogen-related technologies. Using a combined calculations-experiments approach, we systematically studied these processes on hydrated γ-Al₂O₃(110) supported Ir_n subnanoparticles (n = 1, 2, 4, 6, 22) and Ir₉₁ nanoparticle. A density-functional theory (DFT) investigation into the influence of Ir (sub)nanoparticles size on the processes of hydrogen chemisorption, spillover, and reverse spillover was initially performed. The maximum H/Ir(surf) adsorption ratio decreases with increasing cluster size (saturation ratios: 3 for Ir₁, 2.11 for Ir₂₂, and <2 for Ir₉₁), accompanied by significant (sub)nano-sized metal deformation under high H-loading. Most hydrogen spillover is thermodynamically unfavorable. Reverse spillover is favored for many supported Ir_n clusters (n = 1, 2, 4, 6). Thermodynamic analysis indicates partial hydrogen retention on small Ir_n clusters under moderate H₂ pressure (0.01 atm) and temperatures at 925 K, due to strong binding at undercoordinated Ir sites. H₂ temperature-programmed reduction/desorption (H₂-TPR/TPD) experiments confirm computational insights. Peaks at ∼518, 709, and 809 K in H₂-TPR curves are assigned to the reduction of surface IrO₂, interfacial Ir-O-Al species, and Ir-induced oxygen-deficient Al₂O₃, respectively. This work elucidates the fundamental atomic-scale nature of hydrogen interactions on supported (sub)nanometer metal clusters.