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Crystallographic facet selective HER catalysis: exemplified in FeP and NiP₂ single crystals.

ABSTRACT: How the crystal structures of ordered transition-metal phosphide catalysts affect the hydrogen-evolution reaction (HER) is investigated by measuring the anisotropic catalytic activities of selected crystallographic facets on large (mm-sized) single crystals of iron-phosphide (FeP) and monoclinic nickel-diphosphide (m-NiP₂). We find that different crystallographic facets exhibit distinct HER activities, in contrast to a commonly held assumption of severe surface restructuring during catalytic activity. Moreover, density-functional-theory-based computational studies show that the observed facet activity correlates well with the H-binding energy to P atoms on specific surface terminations. Direction dependent catalytic properties of two different phosphides with different transition metals, crystal structures, and electronic properties (FeP is a metal, while m-NiP₂ is a semiconductor) suggests that the anisotropy of catalytic properties is a common trend for HER phosphide catalysts. This realization opens an additional rational design for highly efficient HER phosphide catalysts, through the growth of nanocrystals with specific exposed facets. Furthermore, the agreement between theory and experimental trends indicates that screening using DFT methods can accelerate the identification of desirable facets, especially for ternary or multinary compounds. The large single-crystal nature of the phosphide electrodes with well-defined surfaces allows for determination of the catalytically important double-layer capacitance of a flat surface, C _dl = 39(2) μF cm^-2 for FeP, useful for an accurate calculation of the turnover frequency (TOF). X-ray photoelectron spectroscopy (XPS) studies of the catalytic crystals that were used show the formation of a thin oxide/phosphate overlayer, presumably ex situ due to air-exposure. This layer is easily removed for FeP, revealing a surface of pristine metal phosphide.

SUBMITTER: Owens-Baird B

PROVIDER: S-EPMC8159208 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:In this work, we have disclosed the facile syntheses of morphologically diverse Cu2O nanoparticles using our laboratory designed modified hydrothermal reactor employing low-cost copper (II) acetate precursor compounds. The reaction conditions dovetail the effect of ethylene glycol (EG) and glucose to exclusively evolve the morphology tuned Cu2O nanomaterial at different pHs. The morphology tuning produces octahedron (Oh), dwarf hexapod (DHP), and elongated hexapod (EHP) Cu2O structures only with the optimized reagent concentrations. Interestingly, all of them were bestowed with a (111) facet, a superlative facet for facile nitroarene reduction. Thus, the morphology reliant catalytic reaction becomes evident. However, when used individually, EG and glucose evolve ill-defined CuO/Cu2O and Cu2O structures, respectively. We have observed that a change in pH of the medium at the onset of the reaction is obligatory for the evolution of tailor-made morphologically diverse Cu2O nanoparticles. However, preformed Cu2O particles do not suffer further structure/morphology changes under deliberate pH (6.0-9.0) change. With the as-obtained Oh, DHP, and EHP Cu2O structures, we further delve into the realm of catalysis to understand the splendor of the nanocatalyst, morphology and surface area dependence, facet selective reactivity, and other factors affecting the catalytic efficiency. The remarkable rate of catalysis of 4-nitrophenol (4-NP), evident from the catalyst activity parameter (k a = 123.6 g-1 s-1), to produce 4-aminophenol in the presence of a reducing agent like sodium borohydride (NaBH4) of the as-prepared catalysts is evidence of the collaborative effects of the effective surface area, surface positive charge, and active (111) facet of the Cu2O nanocatalyst. We have also studied the effect of other common anions, namely, Cl-, NO2 -, NO3 -, CO3 2-, and SO4 2- on the reduction process. To obtain a general consensus about facets, we compared (100) and (111) faceted Cu2O nanocatalysts not only for 4-NP reduction but also for the reduction of toxic chromium Cr(VI) in the presence of formic acid to further emphasize the importance of facet selectivity in catalysis and the versatility of the morphology tuned as-prepared Cu2O.

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Crystallographic facet selective HER catalysis: exemplified in FeP and NiP₂ single crystals.

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