Hydrogen energy has received widespread attention and has been intensely investigated as an ideal candidate to replace traditional fossil fuels. Among many approaches to hydrogen production, semiconductor photocatalysis is considered to be one of the most ideal methods due to its easy operation and environmental friendliness. However, the absence of catalytic reaction sites and the corresponding rapid quenching of photo-generated carriers lead to a short electron lifetime. In this case, loading a highly dispersed cocatalyst that can effectively separate the photogenerated charge carriers is a feasible approach for enhancing photocatalytic performance.
It is well known that precious metals, especially platinum (Pt), are widely considered as the state-of-art cocatalyst towards photocatalytic H2 generation, but its high prices and scarcity greatly limit its extensive applications. Up to now, various low-cost materials, including metal oxides, carbides, nitrides, sulfides, phosphides, etc., have been developed as promising cocatalysts to enhance the photocatalytic H2-generation performance of semiconductors. However, most catalytic activities of them are still inferior to those of noble metal-based catalysts, which inevitably increases resource consumption and therefore reduce cost performance.
In the past few decades, other less expensive platinum-group alternatives have been applied in hydrogen-evolution reactions given their similar physiochemical nature to Pt. In particular, metallic palladium (Pd) (551 $ per oz) is more economic than Pt (992 $ per oz) and exhibits impressive performance in the dehydrogenation of alkanes, oxygen reduction reaction, hydrogen oxidation/reduction reaction, and CO oxidation owing to its unique electron configuration of 4d105s0. Pd, as an effective hydrogen-evolution cocatalyst has evoked special attention.
However, the present investigations are mainly focused on the nanostructured modulation of metallic palladium, while optimizing its electronic structure to improve the interfacial H2-evolution reaction activity is usually ignored. According to the hydrogen-evolution volcano diagram and related experimental reports, the adsorption energy of a hydrogen atom to palladium is too high and thus restricts the subsequent hydrogen desorption process, which essentially suppresses the overall hydrogen evolution activity.
Therefore, if the electronic properties of palladium metal can be adjusted to reduce the hydrogen adsorption energy, the hydrogen-evolution activity will be greatly improved.
Recently, a research team led by Prof. Huogen Yu from China University of Geosciences designed ultrafine Pd100-xCux alloy nanodot-modified TiO2 photocatalysts via the NaH2PO2-mediated co-deposition route. The obtained Pd100-xCux alloy nanodots (2-5 nm) are uniformly dispersed onto TiO2 and their componential ratios can be well controlled by adjusting the molar ratio of Cu and Pd precursor. Based on in situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) and density functional theory (DFT) results, the as-formed Pd100-xCux alloy nanodots can effectively promote the separation of photo-generated charges and weaken the adsorption strength for hydrogen to optimize the process of hydrogen-desorption process on Pd75Cu25 alloy, thus leading to high photocatalytic H2-evolution activity. Herein, the weakened H adsorption of Pd75Cu25 cocatalyst can be ascribed to the formation of electron-rich Pd after the introduction of Cu with a weak electronegativity. The results were published in Chinese Journal of Catalysis (https://doi.org/10.1016/S1872-2067(21)63830-5).
This work was supported by the National Natural Science Foundation of China (51872221, 21771142, and 22075220), the Fundamental Research Funds for the Central Universities (WUT 2019IB002).
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