Scientists at the National University of Singapore have developed a general wet chemistry approach for the scalable and automated synthesis of a library of single-atom ultra-high density (UHD-SAC) catalysts for 15 common transition metals on chemically distinct supports via two -stategy thermal annealing control.
Catalysts play an important role in a number of industrial chemical processes and there is a growing need for more advanced versions to improve their efficiency. Single-atom heterogeneous catalysts (SACs) are a new class of catalysts that consist of single metal atoms individually dispersed on the surface of supports. Their unique geometric and electronic properties have the potential to dramatically improve the selectivity of targeted catalytic reactions and reduce operating costs. Since the concept of SAC was invented in 2011, interest in this class of SAC materials has increased globally focusing on their use to improve the efficiency of chemical transformations for sustainable industrial processes. A fundamental challenge for the implementation of this pioneering class of catalysts in many technical applications is the lack of synthetic routes to produce them with high surface densities. Achieving the latter is particularly important for maximizing catalyst productivity in large-scale industrial processes.
A NUS research team led by Professor Jiong Lu of the Department of Chemistry and Institute of Functional Intelligent Materials at the National University of Singapore tackled this difficult problem by developing a scalable and versatile two-step annealing method to prepare ultra-high-density SAC libraries. This work is a collaboration between Professor Javier Pérez-Ramírez from ETH Zurich, Professor Jun Li from Tsinghua University and Dr Xiaoxu Zhao from Nanyang Technological University (NTU). The method is based on the control of ligand removal from metal precursors and their associated interactions with the support to saturate the material surface with metal atoms.
A selective anchoring mechanism that maximizes the probability of binding the metal atom to all available coordination sites on the material surface helps maintain a high level of metal cover. The metal atoms which are not attached are then washed away. This prevents potential sintering of the metal in the subsequent high temperature annealing step used to remove residual ligands. The annealing step also allows the stabilization of much higher metal contents compared to conventional impregnation routes (see figure (a)). This evolutionary synthetic pathway for the development of UHD-SAC has been demonstrated for 15 common transition metals using chemically distinct supports of different nature (including nitrogen doped carbon, polymeric carbon nitride, oxide of cerium, alumina and titanium oxide) with a load exceeding 20% by weight. (see figure (b)). In addition, the proposed approach easily lends itself to a standardized and automated approach. protocol (see Figure (c) and Figure (d)) demonstrating its robustness and provides a viable route to explore a large number of libraries of mono- or multimetallic catalysts.
The team showed the potential benefits of a high load of SAC in separate catalyst systems, which range from electrochemical, thermal and organic catalysis, illustrating the need to optimize the surface metal density for a specific catalytic application. Additionally, the charge-dependent site-specific activity observed in separate catalyst systems reflects the well-known complexity of heterogeneous catalyst design. This can now be solved with a SAC library with widely adjustable metal loads.
Prof Lu said, “Our work has solved long-standing problems in single-atom catalysis, including charge density and scalable manufacturing of this pioneering class of UHD-SAC. This is crucial for their industrial implementation in sustainable chemical and energy transformations. ”
Mesoporous structure improves catalytic performance of single atom catalysts
Xiao Hai et al, Two-Step Scalable Annealing Method for the Preparation of Ultra-High Density Single-Atom Catalyst Libraries, Nature Nanotechnology (2021). DOI: 10.1038 / s41565-021-01022-y
Provided by the National University of Singapore
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