From single-site tantalum complexes to nanoparticles of Ta x N y and TaO x N y supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy.

Open Access Article. Published on 08 June 2017. Downloaded on 29/09/2017 10:48:15. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Chemical Science View Article Online EDGE ARTICLE Cite this: Chem. Sci., 2017, 8, 5650 View Journal | View Issue From single-site tantalum complexes to nanoparticles of TaxNy and TaOxNy supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy† Janet C. Mohandas, a Edy Abou-Hamad,‡a Emmanuel Callens, ‡a Manoja K. Samantaray, a David Gajan,b Andrei Gurinov, a Tao Ma, c Samy OuldChikh, a Adam S. Hoffman,c Bruce C. Gates *c and Jean-Marie Basset *a Air-stable catalysts consisting of tantalum nitride nanoparticles represented as a mixture of TaxNy and TaOxNy with diameters in the range of 0.5 to 3 nm supported on highly dehydroxylated silica were synthesized from TaMe5 (Me ¼ methyl) and dimeric Ta2(OMe)10 with guidance by the principles of surface organometallic chemistry (SOMC). Characterization of the supported precursors and the supported nanoparticles formed from them was carried out by IR, NMR, UV-Vis, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopies complemented with XRD and high-resolution TEM, with dynamic nuclear polarization surface enhanced NMR spectroscopy being especially helpful by providing enhanced intensities of the signals of 1H, 13C, 29Si, and 15N at their natural abundances. The characterization data Received 27th March 2017 Accepted 8th June 2017 provide details of the synthesis chemistry, including evidence of (a) O2 insertion into Ta–CH3 species on the support and (b) a binuclear to mononuclear transformation of species formed from Ta2(OMe)10 on the support. A catalytic test reaction, cyclooctene epoxidation, was used to probe the supported nanoparticles, DOI: 10.1039/c7sc01365e with 30% H2O2 serving as the oxidant. The catalysts gave selectivities up to 98% for the epoxide at rsc.li/chemical-science conversions as high as 99% with a 3.4 wt% loading of Ta present as TaxNy/TaOxNy. Introduction Dispersed species ranging from single-metal-atom complexes to clusters and nanoparticles (NPs) of metals and metal oxides are materials that nd many applications, especially in catalysis.1–3 Numerous methods have been used to synthesize such NPs, oen involving colloidal, solution-based, or chemical or physical vapour-deposition techniques.4,5 Solution-based methods oen suffer from the need for capping agents or surfactants to stabilize the clusters or NPs, and deposition methods lack ne control and typically lead to highly nonhomogeneous a King Abdullah University of Science & Technology, KAUST Catalysis Center (KCC), 23955-6900 Thuwal, Saudi Arabia. E-mail: b Institut de Sciences Analytiques (CNRS/ENS-Lyon/UCB-Lyon 1), Université de Lyon, Centre de RMN à Très Hauts Champs, 69100, Villeurbanne, France c Department of Chemical Engineering, University of California, Davis, California 95616, USA. E-mail: † Electronic supplementary information (ESI) available: Complete experimental procedures, supporting characterization techniques, data and the details for the prepared compounds are provided. See DOI: 10.1039/c7sc01365e ‡ These authors contributed equally. 5650 | Chem. Sci., 2017, 8, 5650–5661 materials.6,7 In typical catalytic applications, NPs are dispersed on high-area porous supports such as silica, alumina, and zeolites.8,9 The supported NPs are oen unstable, undergoing agglomeration or sintering during operation.10 A foundation for the synthesis of NPs from molecular building blocks on supports is provided by surface organometallic chemistry. Thus, the supported species can be made without the need for templates, capping agents, or surfactants, and the syntheses thereby offer good prospects for control of the compositions and sizes of the dispersed species. Essential reactant species in such syntheses are the support surface functional groups, exemplied by the –OH groups on silica. Control of the sizes of NPs synthesized from single-metal-atom precursors on supports may be facilitated by the initial bonding of the precursor species to the support to minimize agglomeration. Thus, for instance, surface organometallic chemistry has guided the synthesis of monometallic (Pt) and bimetallic (Pt– Sn) clusters on silica, giving catalysts with high activities and selectivities for catalytic hydrogenolysis, isomerization, and dehydrogenation reactions.11–13 However, only little work has been done to extend the concepts to non-metal NPs including This journal is © The Royal Society of Chemistry 2017 View Article Online Open Access Article. Published on 08 June 2017. Downloaded on 29/09/2017 10:48:15. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Edge Article semiconductors, and herein we demonstrate their use to guide the synthesis of silica-supported metal nitride semiconductor NPs.14 Such materials offer prospects as new catalysts, and they could also offer unique optoelectronic properties of potential value in displays, quantum computing, or photovoltaic devices.8,15–22 Unsupported nanocrystalline samples consisting of TaON and Ta3N5 were used by Gao et al.17 to catalyze cyclooctene epoxidation, an industrially important reaction. Epoxidation reactions are sensitive to catalyst surface acidity or basicity, and by incorporating nitrogen in place of oxygen in the framework of Ta2O5, these authors tuned the catalyst basicity and improved its properties. Syntheses of metal nitrides oen lead to mixed phases or non-stoichiometric compositions. Strategies to control metal nitride synthesis have been developed, some involving the application of various nitriding agents such as gaseous or liquid NH3, urea, cyanamide, etc.20,22 A traditional method involving gaseous NH3 is convenient for the synthesis of impurity-free metal nitrides, because the high treatment temperatures lead to the release of N2 and H2, which react with oxides and form water,21,23–25 which can be removed as a gas. Hence, we reasoned that it would be of interest to attempt analogous syntheses to make dispersed metal nitrides on a support, being guided by surface organometallic chemistry and starting with molecular metal complexes anchored to the support. We chose silica as the support because its surface chemistry is well understood and the principles of surface of organometallic chemistry are a powerful guide to manipulating syntheses on it. We chose tantalum as a precursor metal to allow a comparison of our results with those of Gao et al. for the performance of tantalum nitride catalysts for cyclooctene epoxidation. Understanding the chemistry of species dispersed on solid surfaces emerges best when a battery of complementary characterization techniques is applied to allow elucidation of elementary reactions. Dynamic (...truncated)