Open Access

Show Abstract ▼

Abstract: Metal chloride complexes react with tris(trimethylsilyl)phosphine under mild condition to produce metal phosphide nanoparticles, and chlorotrimethylsilane as a byproduct. The formation of Si-Cl bonds that are stronger than the starting M-Cl bonds acts as a driving force for the reaction. The potential of this strategy is illustrated through the preparation of ruthenium phosphide nanoparticles using [RuCl2(cymene)] and tris(trimethylsilyl)phosphine at 35°C. Characterization with a combination of techniques including electron microscopy, X-ray absorption spectroscopy, and solid-state NMR spectroscopy, evidences the formation of small (diameter of 1.3 nm) and amorphous NPs with an overall Ru50P50 composition. Interestingly, these NPs can be easily immobilized on functional support materials, which is of great interest for potential applications in catalysis and electrocatalysis. Mo50P50 and Co50P50 NPs could also be synthesized following the same strategy. This approach is simple and versatile and paves the way toward the preparation of a wide range of transition metal phosphide nanoparticles under mild reaction conditions.

Open Access

Show Abstract ▼

Abstract: Electrocatalytic oxygen reduction reaction (ORR) has been intensively studied for efficient and environmentally benign energy conversion processes. However, insufficient understanding of ORR 2e--pathway mechanism at the atomic level inhibits rational design of electrocatalysts with both high activity and selectivity, causing concerns including catalyst degradation due to Fenton reaction or poor efficiency of H2O2 electrosynthesis. Herein we show that the generally accepted ORR electrocatalyst design based on a Sabatier volcano plot argument optimises activity but is unable to account for the 2e--pathway selectivity; an extended “dynamic active site saturation” model that examines in addition the hydrogenation kinetics linked to the OOH* adsorption energy enables us to resolve the activity-selectivity compromise. Through electrochemical and operando spectroscopic studies on the ORR process governed by a series of Co-N x /carbon nanotube hybrids, a construction-driven approach that aims to create the maximum number of 2e- ORR sites by directing the secondary ORR electron transfer step towards the 2e- intermediate is proven to be attainable by manipulating O2 hydrogenation kinetics. Control experiments reveal the O2 hydrogenation chemistry is related to a catalyst reconstruction with lower symmetry around the Co active centre induced by the application of a cathodic potential. The optimised catalyst exhibits a ~100% H2O2 selectivity and an outstanding activity with an ORR potential of 0.82 V versus the reversible hydrogen electrode to reach the ring current density of 1 mA cm-2 by using rotating ring-disk electrode measurement, which is the best-performing 2e- ORR electrocatalyst reported to date, and approaches the thermodynamic limit.

Open Access

Show Abstract ▼

Abstract: Electrochemical urea splitting provides a sustainable and environmentally benign route for facilitating energy conversion. Nonetheless, the sustained efficiency of urea splitting is impeded by a scarcity of active sites during extended operational periods. Herein, an atomic heterostructure engineering strategy is proposed to promote the generation of active species via synthesizing unique Ru–O4 coordinated single atom catalysts anchored on Ni hydroxide (Ru1–Ni(OH)2), with ultralow Ru loading mass of 40.6 μg cm−2 on the nickel foam for commercial feasibility. Leveraging in situ spectroscopic characterizations, the structure-performance relationship in low and high urea concentrations was investigated and exhibited extensive universality. The boosted generation of dynamic Ni3+ active sites ensures outstanding activity and prominent long-term durability tests in various practical scenarios, including 100 h Zn–urea–air battery operation, 100 h alkaline urine electrolysis, and over 400 h stable hydrogen production in membrane electrode assembly (MEA) system under industrial-level current density.

Open Access

Show Abstract ▼

Abstract: The research for surface active sites with outstanding catalytic activity and selectivity is continuing apace. The rationally designed catalyst with optimised energy level and geometric configuration is key to achieving novel reaction pathways with superior performance. Compared to the conventional supported nanomaterials as catalysts, single atomic site catalysts (SASCs) not only inherit the excellent recyclability but are also featured with ultimate atom efficiency, high structural uniformity and tunable coordination environment. These great advantages of SASCs are due to their unique atomic dispersion nature that preserves features of both heterogeneous and homogeneous catalysts. Especially the homogeneity of SASC enables convincing identification and characterisation of real active sites, making it an ideal platform to establish the definitive structure-activity relationship and to validate reaction mechanisms. Therefore, rational designed SASC has become the most prominent material to fabricate desired active sites with outstanding catalytic activity and selectivity. In this thesis, chemical synthesis strategies and characterisation techniques for SASCs are carefully reviewed. The limitations and future perspectives from a subjective view of the current methodology are discussed in detail as well. As inspired by pioneers’ work, rational designed Ru and Cu SASCs are prepared to investigate their distinct relationships between catalyst structures and reaction behaviours during CO oxidation reaction. With the help of combined in situ characterisation techniques, the structural evolution of active sites for both Ru and Cu catalysts were carefully studied. In the first project, the ultimate rational design of Ru active centre is realised by building surface single-sites to mimic molecular Ru catalysts. Inspired by a homogeneous Ru(II) complex, an air-stable surface -[bipy-Ru(II)(CO)2Cl2] single-site is designed through precise engineering of geometric and electronic structures from -[bipy-Ru(III)Cl4]- site. Such Ru(II) single-site enable oxidation of CO while the Ru(III) site is completely inert, providing an excellent prototype of the synthetic strategy which is generally applicable to transition metals. The second project focuses on the electronic metal-support interactions (EMSI) which describe electron flow between metal sites and a metal oxide support. For CuO-CeO2 catalysts, the electron withdrawing effect on Cu species introduced by electrophilic Ce4+ is maximised for atomically dispersed Cu sites over CeO2 surface. Experiment evidence shows the energy levels of 3d orbitals of isolated Cu(I)/(II) sites are decreased by Ce4+ cations in the support framework. It is demonstrated by in situ study that a [Cu(I)O2]3- site on CeO2 could selectively adsorb molecular O2 and form a rarely reported electrophilic 2-O2 species, leading to ten times higher activity than CuO clusters in CO oxidation. The third project is derived from the previous study of CuO-CeO2 catalysts, in which an unbalanced electron transfer between Cu and Ce is observed for CuO clusters dominant samples. To explain the reaction pathway of CeO2 supported CuO clusters in CO oxidation, an electronic metal-support-carbon interaction (EMSCI) based on EMSI is proposed. In the CuO-CeO2 redox, an additional flow of electron from metallic Cu to surface carbon species is observed by combined in situ studies, providing a complete picture of the mass and electron flow in the catalytic redox cycles.

Open Access

Show Abstract ▼

Abstract: Ru(II) compounds are widely used in catalysis, photocatalysis and medical applications. They are usually obtained in reductive environment as molecular O 2 can oxidize Ru(II) to Ru(III) and Ru(IV). Here we report the design, identification and evolution of an air‐stable surface ‐[bipy‐Ru(II)(CO) 2 Cl 2 ] site that is covalently mounted onto a polyphenylene framework. Such Ru(II) site was obtained by reduction of ‐[bipy‐Ru(III)Cl 4 ] ‐ with simultaneous ligand exchange from Cl ‐ to CO. This structural evolution was witnessed by a combination of in situ X‐ray and infrared spectroscopy studies. The ‐[bipy‐Ru(II)(CO) 2 Cl 2 ] site enables oxidation of CO with a turnover frequency of 0.73 × 10 ‐2 s ‐1 at 462 K, while the Ru(III) site is completely inert. This work contributes to the studies of structure‐activity relationship by demonstrating a practical control over both geometric and electronic structures of single‐site catalysts at molecular level, which can be further applied in other single site catalyst researches.