Experimental and Theoretical Characterization of Rh Single Atoms Supported on γ-Al2O3 with Varying Hydroxyl Contents during NO Reduction by CO

Suppressing Metal Leaching and Sintering in Hydroformylation Reaction by Modulating the Coordination of Rh Single Atoms with Reactants

Hydroformylation reaction is one of the largest homogeneously catalyzed industrial processes yet suffers from difficulty and high cost in catalyst separation and recovery. Heterogeneous single-atom catalysts (SACs), on the other hand, have emerged as a promising alternative due to their high initial activity and reasonable regioselectivity. Nevertheless, the stability of SACs against metal aggregation and leaching during the reaction has rarely been addressed. Herein, we elucidate the mechanism of Rh aggregation and leaching by investigating the structural evolution of Rh1@silicalite-1 SAC in response to different adsorbates (CO, H2, alkene, and aldehydes) by using diffuse reflectance infrared Fourier transform spectroscopy, X-ray adsorption fine structure, and scanning transmission electron microscopy techniques and kinetic studies. It is discovered that the aggregation and leaching of Rh are induced by the strong adsorption of CO and aldehydes on Rh, as well as the reduction of Rh3+ by CO/H2 which weakens the binding of Rh with support. In contrast, alkene effectively counteracts this effect by the competitive adsorption on Rh atoms with CO/aldehyde, and the disintegration of Rh clusters. Based on these results, we propose a strategy to conduct the reaction under conditions of high alkene concentration, which proves to be able to stabilize Rh single atom against aggregation and/or leaching for more than 100 h time-on-stream.

Catalytic removal of gaseous pollutant NO using CO: Catalyst structure and reaction mechanism

Carbon monoxide (CO) has recently been considered an ideal reducing agent to replace NH3 in selective catalytic reduction of NOx (NH3-SCR). This shift is particularly relevant in diesel engines, coal-fired industry, the iron and steel industry, of which generate substantial amounts of CO due to incomplete combustion. Developing high-performance catalysts remain a critical challenge for commercializing this technology. The active sites on catalyst surface play a crucial role in the various microscopic reaction steps of this reaction. This work provides a comprehensive overview and insights into the reaction mechanism of active sites on transition metal- and noble metal-based catalysts, including the types of intermediates and active sites, as well as the conversion mechanism of active molecules or atoms. In addition, the effects of factors such as O2, SO2, and alkali metals, on NO reduction by CO were discussed, and the prospects for catalyst design are proposed. It is hoped to provide theoretical guidance for the rational design of efficient CO selective catalytic denitration materials based on the structure-activity relations.

Approaching Molecular Definition on Oxide-Supported Single-Atom Catalysts

ConspectusSingle-atom catalysts (SACs) offer unique advantages such as high (noble) metal utilization through maximum possible dispersion, large metal-support contact areas, and oxidation states usually unattainable in classic nanoparticle catalysis. In addition, SACs can serve as models for determining active sites, a simultaneously desired as well as elusive target in the field of heterogeneous catalysis. Due to the complexity of heterogeneous catalysts bearing a variety of different sites on metal particles and the respective support as well as at their interface, studies of intrinsic activities and selectivities remain largely inconclusive. While SACs could close this gap, many supported SACs remain intrinsically ill-defined due to complexities arising from the variety of different adsorption sites for atomically dispersed metals, hampering the establishment of meaningful structure-activity correlations. In addition to overcoming this limitation, well-defined SACs could even be utilized to shed light on fundamental phenomena in catalysis that remain ambiguous when studies are obscured by the complexity of heterogeneous catalysts.In this Account, we describe approaches to break down the complexity of supported single-atom catalysts through the careful choice of oxide supports with specific binding motives as well as the adsorption of well-defined ligands such as ionic liquids on single metal sites. An example of molecularly defined oxide supports is polyoxometalates (POMs), which are metal oxo clusters with precisely known composition and structure. POMs exhibit a limited number of sites to anchor atomically dispersed metals such as Pt, Pd, and Rh. Polyoxometalate-supported single-atom catalysts (POM-SACs) thus represent ideal systems for the in situ spectroscopic study of single atom sites during reactions as, in principle, all sites are identical and thus equally active in catalytic reactions. We have utilized this benefit in studies of the mechanism of CO and alcohol oxidation reactions as well as the hydro(deoxy)genation of various biomass-derived compounds. More so, the redox properties of polyoxometalates can be finely tuned by changing the composition of the support while keeping the geometry of the single-atom active site largely constant. We further developed soluble analogues of heterogeneous POM-SACs, opening the door to advanced liquid-phase nuclear magnetic resonance (NMR) and UV-vis techniques but, in particular, to electrospray ionization mass spectrometry (ESI-MS) which proves powerful in determining catalytic intermediates as well as their gas-phase reactivity. Employing this technique, we were able to resolve some of the long-standing questions about hydrogen spillover, demonstrating the broad utility of studies on defined model catalysts.