Nanoalloy Catalysts for Boosting H2 Production from Hcooh Decomposition: A First-Principles Study

The oxidation of small organic molecules has been widely studied owing to fundamental interest and relevance to fuel technology. Among these molecules, formic acid (HCOOH) has received much attention as a source of hydrogen because of low-toxicity, transportability, and easy-storability. HCOOH may decompose via dehydrogenation or dehydration, depending on the reaction conditions. It was found that the activity of the noble metal catalysts for the H₂ production via HCOOH decomposition was of the following order Pd > Rh > Pt > Au. Since the carbon monoxide (CO) generated from HCOOH dehydration can poison the Pt-based electrode in a fuel cell, a highly selective catalyst toward HCOOH dehydrogenation is required and such desired selectivity can be obtained by modifying the chemical and physical properties of surface by adopting the bimetallic Pd-M alloy catalysts, where M is transition metal. In this talk, I will deliver the underlying mechanism for the enhancement of H₂ productivity and selectivity in metal alloy catalysts. The followings are the main presenting topics. (1) We found that the H₂ production rate strongly depends on the Pd shell thickness in Ag-Pd core shell catalyst, in particular, the Pd thinnest shell (Pd monolayer) showed the highest activity. (2) In the bimetallic Pd/M catalyst, we discovered that HCOOH decomposition strongly relies on the variation of surface charge polarization (ligand effect) and lattice distance (strain effect), which are caused by the heteronuclear interactions between surface Pd and core M atoms. Especially, the contraction of bond distance between the surface Pd atoms and the increase of charge density in surface Pd atoms compared to the pure Pd (for example, Pd/Cu, Pd/Rh) are responsible for the enhancement of the selectivity to H₂ formation via HCOOH decomposition. (3) We employed Pd₃M-Pd core−shell models (M = Sc, Ti, V, and Cr) and discovered the Pd/Pd₃Sc and Pd/Pd₃V surfaces exhibit particularly enhanced selective H₂ production capability. Significant charge transfer from the subsurface Sc atoms to the surface Pd atoms and subsequent extremely low level of d band occupancy (<0.1) around the Sc atoms are identified as a key factor in deriving the modification of the Pd surface electronic structure. In contrast, the promotion by V is mainly attributed to the increase of the activation energy barrier of the CO producing reaction path as a result of the severe upshift in the transition state energy along with the relatively negligible modification of the initial state energy for the rate-determining step. Our theoretical calculation offers the fundamental mechanism of HCOOH decomposition on metal alloy catalysts and also provides physical and chemical intuition for the next generation bimetallic catalyst for hydrogen and fuel cell applications.