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A series of catalysts composed of Pt-Fe nanoparticles supported on mesoporous silica SBA-15 (ca. 7 nm pore diameter) have been prepared by ultrasound-assisted coimpregnation of the metal precursors and evaluated in the nonoxidative dehydrogenation of isobutane. Prereduced catalytic systems were characterized by STEM-HAADF + EDS mapping and XAS to determine the chemical environment of the highly dispersed platinum active sites on the iron host matrix. While the monometallic platinum (nanoparticles) supported on SBA-15 material presented a rapid catalyst deactivation under reaction conditions, coordinatively unsaturated Pt-Fe (Pt ≪ Fe) sites located in the mesopores of SBA-15 showed a high steady-state activity (43% conversion) and selectivity (96% to isobutylene) in the dehydrogenation of isobutane at 550 °C for several hours. Temperature-programmed reduction profiles determined not only the substantially higher reducibility of FeO species with doping amounts of platinum in the as-prepared (calcined) catalysts but also the detrimental structural changes undergone after consecutive reaction-regeneration cycles. Finally, reactivation under controlled conditions allows to minimize irreversible catalyst deactivation after successive cycles.

A frontier challenge in single-atom (SA) catalysis is the design of fully inorganic sites capable of emulating the high reaction selectivity traditionally exclusive of organometallic counterparts in homogeneous catalysis. Modulating the direct coordination environment in SA sites, via the exploitation of the oxide support's surface chemistry, stands as a powerful albeit underexplored strategy. We report that isolated Rh atoms stabilized on oxygen-defective SnO2 uniquely unite excellent TOF with essentially full selectivity in the gas-phase hydroformylation of ethylene, inhibiting the thermodynamically favored olefin hydrogenation. Density Functional Theory calculations and surface characterization suggest that substantial depletion of the catalyst surface in lattice oxygen, energetically facile on SnO2, is key to unlock a high coordination pliability at the mononuclear Rh centers, leading to an exceptional performance which is on par with that of molecular catalysts in liquid media.

A detailed understanding of the interactions among the active components in gallium promoted Cu/ZnO catalysts, depending on the speciation of the gallium, are reported using in situ/operando spectroscopic studies, and their effect in the CO hydrogenation to methanol unraveled. In this contribution, the promoting effect of Ga-doped in the wurtzite ZnO lattice of a Cu/ZnO/GaO catalyst is compared to that of a zinc gallate (ZnGaO) phase. Remarkably, a strong inhibition of CO formation, together with an enhanced methanol formation, are observed in the Ga-doped ZnO sample, specifically at conditions where the competitive reverse water gas shift reaction predominates. The catalytic performance has been correlated with the microstructure of the catalyst where a surface enrichment with reduced ZnO species, together with the stabilization of positive charged copper species and an increase in the amount of surface basic sites for CO adsorption are observed on the most selective sample.

The precise synthesis and stabilization of oxide-supported bimetallic clusters in the low-to-sub nanometer size regime is highly relevant in various fields, from optics and sensing to electrochemistry and catalysis. In surface-driven phenomena such as catalysis, avoiding metal segregation and agglomeration is essential for performance and stability under relevant operation conditions. Here we show how high-temperature oxidative crystal redispersion and oxide atom-trapping phenomena provide a widely valid route towards atomically dispersed bimetallic precursors which, upon reductive metal agglomeration, result in very small, uniformly sized and remarkably stable bimetallic clusters. For a PdPt/MgO system, oxidative redispersion leads to isolated Pd and Pt atoms stabilized by the MgO support up to overall surface metal contents ofca.1.0 Mnm, beyond which loading, atom-trapping oxide sites become exhausted and metal aggregation sets in. On the contrary, more conventional, milder-temperature activation protocols lead to significant metal segregation and markedly bimodal particle size populations already from comparatively lower metal contents. As proven byin situX-ray absorption spectroscopy and atomic-resolution STEM microscopy, the stabilization of isolated Pd and Pt cations within nanometer distances on the common MgO support is essential for the synthesis ofca.1 nm bimetallic PdPt clusters by reductive agglomeration. The uniformly sized PdPt aggregates developed on MgO from single-atom precursors display a notably higher activity for the oxidative activation of methane with carbon dioxide (dry reforming) at 923 K compared to analogue materials synthesizedviamilder calcination/reduction protocols. Moreover, despite their high surface-to-volume ratio, the small bimetallic clusters in the former display an outstanding stability against metal agglomeration under the demanding reaction conditions, likely as a result of a lower driving force for Ostwald ripening growth pr

The development of active and selective non-noble metal-based catalysts for the chemoselective reduction of nitro compounds in aquo media under mild conditions is an attractive research area. Herein, the synthesis of subnanometric and stable cobalt nanoclusters, covered by N-doped carbon layers as core-shell (Co@NC-800), for the chemoselective reduction of nitroarenes is reported. TheCo@NC-800catalyst was prepared by the pyrolysis of the Co(tpy)complex impregnated on Vulcan carbon. In fact, the use of a molecular complex based on six N-Co bonds drives the formation of a well-defined and distributed cobalt core-shell nanocluster covered by N-doped carbon layers. In order to elucidate its nature, it has been fully characterized by using several advanced techniques. In addition, this as-prepared catalyst showed high activity, chemoselectivity and stability toward the reduction of nitro compounds with Hand under mild reaction conditions; water was used as a green solvent, improving the previous results based on cobalt catalysts. Moreover, theCo@NC-800catalyst is also active and selective for the one-pot synthesis of secondary aryl amines and isoindolinones through the reductive amination of nitroarenes. Finally, based on diffraction and spectroscopic studies, metallic cobalt nanoclusters with surface CoNpatches have been proposed as the active phase in theCo@NC-800material.

Supported silver species are among the most promising catalysts for the depletion of ammonia emission by selective catalytic oxidation (NH-SCO). Here, an investigation on the influence of small pore CHA and RHO zeolite structures on the silver species formed and their catalytic activity for the NH-SCO reaction has been conducted. To this end, AgRHO(4) and AgCHA(4) zeolites with similar Si/Al molar ratios (≈ 4) and silver content (∼15 wt%, Ag/Al ≈ 0.6), and AgCHA(2) with Si/Al ≈ 2 and higher silver loading, while maintaining the Ag/Al ratio (∼26 wt%, Ag/Al ≈ 0.6), have been submitted to different treatments and characterized by using a large variety of techniques (XRD, UV-Vis,Ag NMR,in situXAS andoperandoFT-IR). The reduction of the AgCHA and AgRHO zeolites at low temperature (100-200 °C) produces silver clusters, which remain in the AgRHO zeolite when the temperature is increased to 400 °C. However, the silver species in the AgCHA zeolites evolve to nanoparticles (NPs) at 400 °C under H. The catalytic tests show that metal particles are the active sites while silver clusters are inactive for the NH-SCO reaction. Also, there are important differences in the stability of the reduced Ag species under oxidizing or under reaction conditions at 400 °C. Metal NPs are partially redispersed and oxidized to (Ag), while silver clusters are completely oxidized to Ag. Our results indicate that silver clusters are stabilized in the RHO-type and not in the CHA-type zeolite, and thus they display very different catalytic activities for the NH-SCO reaction.

[EN] Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones toward a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and Sd (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O-2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial deconfinement of the monatomic metal centers driven by CO at precatalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96 +/- 19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.

[EN] Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru-1/CeO2 and Rh-1/CeO2 SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov alpha-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (>95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.

[EN] Catalytic hydrogenation of bromate using Pd catalysts supported on activated carbon fibers is a smart solution to treat bromate polluted water. These catalysts have been analyzed by different techniques for an in-deep characterization of the active sites. The in situ X-ray absorption spectroscopy and the CO chemisorption studies showed that Pd-0 nanoparticles with different crystal sizes were generated on the support during hydrogen activation at 200 degrees C and that the PdHx-phase was formed during the cooling to room temperature. As PdHx species formed on Pd-0 nanoparticles are responsible for bromate reduction, the most active catalysts are those having Pd-0 nanoparticles with large crystal sizes, where PdHx species are easily formed. The catalysts are fully stable in succesive reaction runs. It has been also shown that bromate reduction rate depends on the bromate concentration and on the hydrogen partial pressure, with a pseudo-first reaction order towards both reactants.

Catalytic hydrogenation of bromate using Pd catalysts supported on activated carbon fibers is a smart solution to treat bromate polluted water. These catalysts have been analyzed by different techniques for an in-deep characterization of the active sites. Thein situX-ray absorption spectroscopy and the CO chemisorption studies showed that Pdnanoparticles with different crystal sizes were generated on the support during hydrogen activation at 200 °C and that the PdH-phase was formed during the cooling to room temperature. As PdHspecies formed on Pdnanoparticles are responsible for bromate reduction, the most active catalysts are those having Pdnanoparticles with large crystal sizes, where PdHspecies are easily formed. The catalysts are fully stable in succesive reaction runs. It has been also shown that bromate reduction rate depends on the bromate concentration and on the hydrogen partial pressure, with a pseudo-first reaction order towards both reactants.

Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones toward a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and 5d (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial deconfinement of the monatomic metal centers driven by CO at precatalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96 ± 19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.

The potential of oxide-supported rhodium single atom catalysts (SACs) for heterogeneous hydroformylation was investigated both theoretically and experimentally. Using high-level domain-based local-pair natural orbital coupled cluster singles doubles with perturbative triples contribution (DLPNO-CCSD(T)) calculations, both stability and catalytic activity were investigated for Rh single atoms on different oxide surfaces. Atomically dispersed, supported Rh catalysts were synthesized on MgO and CeO. While the CeO-supported rhodium catalyst is found to be highly active, this is not the case for MgO, most likely due to increased confinement, as determined by extended X-ray absorption fine structure spectroscopy (EXAFS), that diminishes the reactivity of Rh complexes on MgO. This agrees well with our computational investigation, where we find that rhodium carbonyl hydride complexes on flat oxide surfaces such as CeO(111) have catalytic activities comparable to those of molecular complexes. For a step edge on a MgO(301) surface, however, calculations show a significantly reduced catalytic activity. At the same time, calculations predict that stronger adsorption at the higher coordinated adsorption site leads to a more stable catalyst. Keeping the balance between stability and activity appears to be the main challenge for oxide supported Rh hydroformylation catalysts. In addition to the chemical bonding between rhodium complex and support, the confinement experienced by the active site plays an important role for the catalytic activity.

Although ruthenium is a common promoter for cobalt-based Fischer-Tropsch synthesis (FTS) catalysts, the origin of Ru promotion and promoter concentration effects remains controversial. To gain a fundamental understanding of the Ru promotion effects, we herein performed an in situ spectroscopic study by X-ray absorption spectroscopy (XAS), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS)-Auger of working Co/TiO2 catalysts (ca. 12 wt % Co, pure anatase TiO2 phase) promoted with different amounts of Ru (0.1-1.2 wt %). At typical FTS conditions (220 °C, 20 bar, H2/CO = 2), the activity in terms of both metal-time-yield (MTY) and initial turnover frequency (TOF) was maximized for the catalysts promoted with 0.1-0.2 wt % Ru, for which most of the Ru was alloyed with Co in bimetallic nanoparticles. The analysis of Auger electrons evidenced that the lower (apparent) TOF of accessible Co0 sites in the absence of Ru and at Ru concentrations beyond 0.2 wt % is bridged to the development, under reaction conditions, of FTS-inactive cobalt carbide (CoCx) and cobalt oxide (CoOx) patches, respectively, at the outermost surface of cobalt crystallites.