First-principles insights into step-site catalysis on Pt–Pd alloys for passive autocatalytic recombiners under oxygen-rich conditions

Passive autocatalytic recombiners are essential safety devices for mitigating hydrogen hazards in nuclear power plant containments. Their performance is determined by the catalytic activity of the Pt–Pd alloy surfaces employed. In practice, these surfaces are highly irregular and rich in step defects. The atomic-scale mechanisms of hydrogen recombination at these step sites under oxygen-rich conditions remain poorly understood. In this work, we used first-principles calculations to systematically investigate the step-site reactivity of Pt 3 Pd (111) surfaces for catalytic hydrogen recombination. We reveal a coverage-dependent cooperative oxygen binding mechanism between up-step and down-step sites. Pre-adsorbed up-step oxygen atoms stabilize the otherwise metastable down-step site by nearly 1 eV through electronic effects, reducing the OH* formation barrier from 1.167 eV to 0.695 eV. This translates into a rate enhancement of approximately four orders of magnitude at 500 K. Electronic structure analyses, including Bader charge and projected density of states, demonstrate that this cooperative effect originates from cumulative depletion of d-band electrons from step-edge Pd and Pt atoms by pre-adsorbed oxygen, which weakens the vertical metal–oxygen bond. Kinetic analysis reveals three competing OH* consumption pathways with distinct site-dependent rates. At 500 K, the ordering follows up-step > free > up-step, with rate ratios of approximately 2:1:0.002. These results establish a mechanistic framework for understanding step-mediated catalysis on alloy surfaces under oxygen-rich conditions and provide atomic-scale guidance for the design of high-performance PAR catalysts for nuclear safety applications.