Direct method to etch-embed Ce onto Cu-based Oxide

In a paper to be published in the forthcoming issue in NANO, a team of researchers from Nanchang University have attempted to directly engineer the surface structure of Cu-based substrate to get a series of Ce-O-Cu catalysts for NH₃-SCR of NO. The obtained catalysts were structured as CuO matrix with interactive surface composed by Cu(I)-Cu(II) and Ce(III)-Ce(IV) co-present species, exhibiting the distinct synergistic effect and leading to attractive catalytic performance even with SO₂ present in reactant mixture.

What is a feasible preparation strategy for engineering oxide composites with interactive structures for advanced heterogeneous catalysis? Here, an attempt to directly construct and manipulate Ce-O-Cu composited surface structures from the design of Ce(IV) to chemically embed Ce into the surface of Cu₂O substrate was made, and the thermally-derived samples were investigated as catalysts for NH₃ selective catalytic reduction (NH₃-SCR) of NO.

The feature of surface structure of solid catalyst such as oxide composites was the decisive factor to govern the performance for heterogeneous catalysis reactions. However, the traditional preparation methods to construct oxide composites such as co-precipitation, sole-gel or solvothermal synthesis were not qualified to manipulate the surface structure of solid materials.

Unlike these body-mingling preparations from traditional methods, in this work, an attempt to directly construct and manipulate composited oxide surface upon a redox replacement preparation was made by employing Ce(IV) precursor to etch Cu₂O in order to embed Ce species onto Cu-based oxide surface, and the catalysts derived from thermal stabilization treatments were investigated for NH₃-SCR of NO. In principle, the standard potentials for two redox pairs [Ce(IV)/Ce(III)] and [Cu₂O/Cu²⁺] were 1.72 V and 0.20 V, respectively, the lower potential of [Cu₂O/Cu²⁺] pair means Cu₂O would act as the reductant when it encountered the Ce(IV) species, i.e., Ce(IV) species could etch Cu₂O to produce Cu²⁺ and Ce(III) species. Since Cu²⁺ would remove into aqueous solution, the left space on Cu-based solid substrate could facilitate the surface embedding of Ce species. Although Cu(II) contained compounds such as CuO that were always used as precursors or substrates, such Cu(II) compounds would not react with Ce ion species no matter whether Ce(IV) or Ce(III) was employed due to the thermodynamic limitation from the above redox potential disparity. Therefore, the design and preparation of Ce-O-Cu composited samples initiated from employing Ce(IV) contained Ce(NO₃)₆^2- ion to etch-embed Ce onto the surface of Cu₂O substrate, followed by a thermal-stabilization processing to get catalysts.

With comprehensive characterizations, the design was demonstrated to be quite efficient in creating Cu(I)-Cu(II) and Ce(III)-Ce(IV) co-existed interactive surface on CuO matrix with ultra-low dosage of Ce (0.83-2.3 wt.%) in comparison with literature works. More interestingly, the surface metal species distributions, redox properties, active site distributions and strength, and surface reactivity could be sensitively and conveniently manipulated on such structures, which offered a chance to get superior catalytic performance such as higher NO conversion (≥90% with ≥95% selectivity to N₂) below 300°C, fold-increased TOF, good catalytic durability even under the presence of SO₂ on well-manipulated sample compared to Ce-Cu reference catalysts prepared from traditional method. All of the above catalytic results were obtained with gas hourly space velocity (GHSV) at 100,000 h^-1, and more practical reaction conditions such as (SO₂+H₂O) co-presence were also imposed on our samples. The superior performance under these conditions together with novel structure and attractive physical chemistry properties of our samples suggest that chemically embedding preparation and derived structures could be a competitive solution to obtain efficient oxide composite materials for practical and advanced applications.

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The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (NSFC, No.21003071, No.21563018 and No.21663016) and Doctoral Fund of Ministry of Education of China (No.20093601120007).

Additional authors are Qian Sun, Chun Zeng, Meng-Meng Xing, Bo Chen, San-Guo Hong (Professor) and Ning Zhang (Professor), Qian Sun and Chun Zeng contributed equally to the paper; corresponding author of the paper is Dan Zhao (Associate Professor), [email protected].

For more insight into the research described, readers are invited to access the paper on NANO.

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Caption: Chemically weaving the surface of Cu-based oxide with Ce initiated from employing Ce(IV) precursor to etch-embed Ce into Cu₂O surface was demonstrated to be quite efficient for construction of synergistic catalysis interface in nano or sub-nanoscale, leading to the dramatic activity enhancements for NH₃-SCR of NO, especially when SO₂ is present.

NANO is an international peer-reviewed monthly journal for nanoscience and nanotechnology that presents forefront fundamental research and new emerging topics. It features timely scientific reports of new results and technical breakthroughs and publishes interesting review articles about recent hot issues.

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For more information, contact Tay Yu Shan at [email protected].