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• Two routes to prepare Ta-Ir/SiO 2 catalysts via surface organometallic chemistry. • In-depth structural characterization of Ta, Ta-Ir and Ta-Ir 2 surface sites. • Improved catalytic performances in H/D isotope exchange and propane metathesis. A stepwise surface organometallic chemistry (SOMC) methodology consisting in using a silica-supported tantalum species, [(≡SiO)Ta(CH t Bu)(CH 2 t Bu) 2 ], as a reactive center to coordinate an iridium site, was developed to construct tantalum/iridium heterobimetallic edifices. The resulting material, MAT-2 , exhibits enhanced catalytic performances - both in H/D isotopic exchange and alkane metathesis reactions - in comparison to MAT-1 , which was prepared from the direct grafting on silica of a well-defined heterobimetallic Ta/Ir complex. We projected that the difference in catalytic activity was due to the presence of distinct active sites and we used a combination of advanced spectroscopic methods (IR, solid-state NMR and XAS spectroscopies) as well as modeling (computational studies and molecular models) to identify the structure of these surface species. These investigations point towards the presence of three types of active sites at the surface of MAT-2 : the heterobimetallic surface species, [≡SiOTa(CH 2 t Bu) 2 {IrH 2 (Cp*)}] 2-s , which is also found in MAT-1 , as well as an unanticipated heterotrimetallic species, [≡SiOTa(CH 2 t Bu) 2 {IrH 2 (Cp*)}], 3-s along with some unreacted monometallic Ta sites [(≡SiO)Ta(CH t Bu)(CH 2 t Bu) 2 ], 1-s. The well-defined trimetallic surface species 3-s was independently prepared and characterized to support this hypothesis. This study highlights the importance of the synthetic methodology used for the preparation of heterobimetallic species through SOMC, and the difficulty to obtain true single-sites surface species. [ABSTRACT FROM AUTHOR]

[Display omitted] • CNT supported Ru single atoms have been associated to Ru, Pd and Ni particles. • Hydrogen spillover induced a cooperative catalysis between supported Pd and Ni particles and Ru single atoms. • The RuNi/CNT catalyst contains a cocktail of catalytic species including isolated Ru single atoms and Ru single atoms on Ni particles. • The RuNi/CNT catalyst shows much better performances in terms of activity and selectivity compared to the monometallic systems. • DFT calculations revealed that the active species is a Ru SA @Ni NP single atom alloy. Reaching high selectivity at high conversion and high rate in the industrially important selective hydrogenation of α,β-unsaturated aldehydes is a challenging task. Among current approaches reported in the literature, high selectivity towards the unsaturated alcohol or saturated aldehyde is generally achieved at the expense of activity. The use of bimetallic systems can lead either to selectivity or to activity enhancement, but generally not to both. Herein, we show that, for cinnamaldehyde hydrogenation, combining Ni nanoparticles (Ni NP) and Ru single atoms (Ru SA) on CNTs allows improvement of the hydrocinnamaldehyde selectivity, while obtaining a remarkable activity. The STY of Ru SA -Ni NP /CNT is approximately 60 times higher than that of the Ru SA /CNT and 10 times higher than that of the Ni NP /CNT. Importantly, the resulting catalyst shows 94% selectivity at 99% conversion and a good stability in flow. Insights into the cooperative "cocktail"-type catalysis between Ni NP and Ru SA were revealed from density functional theory calculations. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrogen spillover induced a cooperative catalysis between Pd particles and Pd single atoms supported on carbon supports. • Few-layer graphene (FLG) support allows more extended H-spillover than multi-walled carbon nanotubes (CNT) • Pd/FLG is more active for solvent-free total hydrogenation of squalene than Pd/CNT but deactivated. • A new mechanism of thermal catalyst deactivation induced by H-spillover is proposed. General interest in solvent-free catalytic processes is driven by the development of environmentally and economically acceptable procedures. Such processes require the use of highly active and stable catalysts. This study reports on the synthesis, characterization, and kinetic evaluation of two highly active Pd catalysts supported on different carbons support, i.e., multi-walled carbon nanotubes (CNT) and few-layer graphene (FLG). These catalysts, with similar Pd loading (∼2 wt%) and containing both Pd single atoms (Pd SA) and nanoparticles (Pd NP), were systematically tested for solvent-free total hydrogenation of squalene (SQE) to obtain high-purity squalane (SQA). Thanks to the unique cooperative catalysis between Pd SA and Pd NP involving H-spillover, the activities of both catalysts were high compared to catalysts described in the literature. Solvent-free total hydrogenation of SQE could be performed under mild conditions (120 °C, 20 bar H 2 , 4 h) using ultralow Pd loading (60 ppm). However, since this reduction reaction is highly exothermic (ΔH R = − 765 kJ.mol−1), heat management during the reactor operation with these highly active catalysts is crucial to avoid deactivation and thermal runaway. Under these conditions, Pd/FLG was highly active but deactivated, unlike Pd/CNT, which still has high activity, but does not deactivate. Characterizations (ICP, XPS, Raman, TPD-MS, HAADF-STEM) of the Pd/FLG catalyst before and after reaction, supported by calculations based on density functional theory, allowed us to propose a new mechanism of catalyst deactivation involving the chemical reduction of specific surface oxygen groups of the carbon support induced by H-spillover. [ABSTRACT FROM AUTHOR]