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Gold Catalysts Containing Interstitial Carbon Atoms Boost Hydrogenation Activity

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Gold Catalysts Containing Interstitial Carbon Atoms Boost Hydrogenation Activity ARTICLE https://doi.org/10.1038/s41467-020-18322-x OPEN Gold catalysts containing interstitial carbon atoms boost hydrogenation activity Yafei Sun1,6, Yueqiang Cao 2,6, Lili Wang1,6, Xiaotong Mu1, Qingfei Zhao1, Rui Si3, Xiaojuan Zhu1, ✉ Shangjun Chen1, Bingsen Zhang 4, De Chen5 & Ying Wan 1 Supported gold nanoparticles are emerging catalysts for heterogeneous catalytic reactions, including selective hydrogenation. The traditionally used supports such as silica do not favor 1234567890():,; the heterolytic dissociation of hydrogen on the surface of gold, thus limiting its hydrogenation activity. Here we use gold catalyst particles partially embedded in the pore walls of meso- porous carbon with carbon atoms occupying interstitial sites in the gold lattice. This catalyst allows improved electron transfer from carbon to gold and, when used for the chemoselective hydrogenation of 3-nitrostyrene, gives a three times higher turn-over frequency (TOF) than that for the well-established Au/TiO2 system. The d electron gain of Au is linearly related to the activation entropy and TOF. The catalyst is stable, and can be recycled ten times with negligible loss of both reaction rate and overall conversion. This strategy paves the way for optimizing noble metal catalysts to give an enhanced hydrogenation catalytic performance. 1 Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials, and Department of Chemistry, Shanghai Normal University, 200234 Shanghai, China. 2 State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 200237 Shanghai, China. 3 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 201204 Shanghai, China. 4 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 110016 Shenyang, China. 5 Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. 6These authors contributed equally: ✉ Yafei Sun, Yueqiang Cao, Lili Wang. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:4600 | https://doi.org/10.1038/s41467-020-18322-x | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-18322-x old is among the least active metals toward molecules at a reported that spectroscopic characterization shows that cationic solid–liquid or solid–gas interface. For example, it has the Au species such as Au+ and Au3+ are present at the perimeter G 18 highest energy barrier for the dissociation of H2 and the interface , while negatively charged Au clusters at metal/oxide least stable chemisorption state1–3. This inertness has been interfaces have also been confirmed19. This complexity limits the attributed to the full filling of the antibonding d state on correlation of the electronic structure with the catalytic proper- adsorption and the small orbital overlap with the adsorbate ties, and industrial applications. It has been shown that there is no according to the d-band model1,4. Therefore, when gold is loaded direct correlation between the number of cationic or anionic Au on a less-active support, such as silica and activated carbon, the species and catalytic activity20. catalyst shows a very weak catalytic hydrogenation activity due to The motivation for the use of ordered mesoporous carbon as the fact that the net atomic charge transfer to the substrate or host the support is because it is a kind of activated carbon, and porous atoms is very small5. Taking into account the fact that activated carbon-supported noble metal catalysts play an important role in carbon is the most frequently used carrier for metal catalysts in the fine chemicals industry, and also have an indispensable role in industry, the activation of, or the charge transfer into gold atoms, new energy technologies, such as biomass conversion and fuel which are supported on activated carbon, and its relationship cells21,22. Their synthesis involves hydrothermal synthesis, poly- with the catalytic hydrogenation activity are of significant merization for phenolic resins, and carbonization, all of which importance but have been seldom reported. have been realized on an industrial scale23. The carbonization It has been reported that the gold catalysts can sometimes show step with metal species is analogous to the growth of carbon hydrogenation activity when gold nanoparticles or atoms are nanotubes by catalytic chemical vapor deposition24. Carbon supported on reducible oxides6,7. In particular, when supported atoms deposit on the transition metal surface such as iron and 25 on TiO2,Fe2O3, etc., gold nanoparticles exhibit high chemos- palladium, and diffuse into the lattice . Schlögl and coworkers electivity in reducing a nitro group, whereas other reducing reported the formation of interstitial C atoms in a Pd catalyst functions are seen in nitroaromatics8–11. In these cases, the during the hydrogenation of alkyne, which showed significant structure properties of the oxides, including size, phase, exposed promotion in the selectivity for olefins26. DFT calculations con- crystal plane, oxidation state, reducibility, perimeter sites, etc., firmed the presence of surface and subsurface carbon atoms in have dominant effects on the electronic structure of the nano- Pd27,28. Our group found that carburizing a Pd catalyst with particles12. These factors result in the complexity of solid cata- interstitial C atoms produces strong inhibition of the adsorption lysts, and even contradictory results. For example, density of thiourea, a strong adsorbate29. We speculate that the formation functional theory (DFT) calculations show that gold atoms that of interstitial C in the Au lattice will modify its electronic prop- 30 are neutral or with a net charge close to zero can be active for H2 erties although this has not been reported in experiment . dissociation, and are usually located at low-coordinated corner or Here, uniform gold nanoparticles with carbon atoms occupy- edge positions that do not directly bond to the support. However, ing interstitial sites in the lattice (C–Au), supported on ordered the dissociation of molecular hydrogen at the perimeter sites of mesoporous carbon have been synthesized. Combining X-ray 13,14 Au/TiO2 , and a pronounced charge transfer from defect sites absorption spectroscopy (XAS) with X-ray photoelectron spec- on the reduced TiO2(110) surface to Au clusters have also been troscopy (XPS) analysis, we are able to measure the d-electron reported6,9,15–17. The charge transfer calculations, however, are gain of Au. There is a linear relationship between the d-electron not in good agreement with each other. Some researchers have gain of gold and its activation entropy (ΔS0*), and catalytic Fig. 1 Electron micrographs. a Representative HRSEM image, b TEM image of an ultrathin section, c, d HAADF-AC-STEM images for fresh C–Au-2.4/ OMC; AC-STEM images for fresh e C–Au-1.6/OMC, f C–Au-2.4/OMC, and g C–Au-3.9/OMC. The scale bar is 100 nm for (a, b), 5 nm for (c), 2 nm for (d, g), and 1 nm for (e, f). 2 NATURE COMMUNICATIONS | (2020) 11:4600 | https://doi.org/10.1038/s41467-020-18322-x | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-18322-x ARTICLE activity in the chemoselective hydrogenation of the nitro group in Therefore, the catalytic activity is exclusively due to the nano- substituted nitroaromatics. The electron gain of 0.192 of Au for particles. The diffractogram of the Au particles, shows an the C–Au-1.6/OMC catalyst is much higher than that of the well- increasing (111) spacing from 2.35 Å in a typical pure Au foil, to known Au/TiO2 catalyst, showing an almost three times higher 2.36, 2.38, and 2.39 Å in the studied catalysts with Au particle turn-over frequency (TOF) value than the latter. This strategy sizes of 3.9, 2.4, and 1.6 nm, respectively, as measured from the paves the way for optimizing noble metal catalysts to give them AC-STEM images (Fig. 1e–g). It is well-known that lattice con- enhanced hydrogenation catalytic performance. traction occurs in small nanoparticles or clusters, and is related to a higher proportion of outer shell undercoordinated atoms with Results respect to fully coordinated inner shell atoms and is proportional Structure of supported gold interstitial nanocatalysts. A series to the reciprocal of the particle size (2.75% contraction for 2.0 nm of gold interstitial nanocatalysts supported on ordered meso- nanoparticles)33. Therefore, increased lattice fringe separation for porous carbonaceous materials (C–Au/OMC) with different gold the C-modified gold nanoparticles might be related to the dis- nanoparticle sizes was produced by hydrothermal synthesis, solution of C atoms in Au, and their occupation of interstitial polymerization of the phenolic resins, and carbonization. sites in the Au lattice. This effect increases with smaller particle Wide-angle X-ray diffraction (WAXRD) patterns show broad size. diffraction peaks at ~23°, which is attributed to the amorphous Although carbon solubility in Au is low, sp2-bonded structures carbonaceous framework, and at ~38.02°, which is attributed to may nucleate on the Au surface for the growth of low- the growth of very small Au nanoparticles or the presence of dimensional carbon materials including carbon nanotubes, amorphous Au species31 (Supplementary Fig. 1). High-resolution graphene, etc.34,35. In fact, gold has a eutectic alloy phase scanning electron microscope (HRSEM) images show that all the diagram with carbon, and the carbon concentration in bulk Au at studied C–Au/OMC catalysts have a spherical morphology with the eutectic point is ~0.08 at%36. Specifically, the C adatom an average diameter of 90 nm (Fig. 1a, Supplementary Fig. 2). diffusion barrier is low for Au, and a significant acceleration of Ordered mesopores can be clearly seen on the exposed hemi- the diffusion step can be achieved when a monoatomic carbon 37 spheres, reflecting an open pore structure on the surface.
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