Rh Single Atoms Research Articles

Dry Reforming of Methane on Low‐Loading Rh Catalysts

Dry reforming of methane (DRM) is a promising catalytic process for converting greenhouse gases (CH4 and CO2) into syngas (CO and H2). This study investigates DRM under moderate conditions (550 °C) using supported ultra‐low loading (0.05 wt%) metal (M) catalysts (M = Rh, Ru, Pt, Pd, Ir, Au, and Ni). The reaction was carried out for 6 h with a flow rate of 30 mL/min for both CH4 and CO2, using 0.10 g of catalyst. Among these catalysts, 0.05 wt% Rh/Al2O3 exhibited the highest DRM activity. The effect of catalyst supports revealed that Al2O3 is the most effective support for 0.05 wt% Rh. The DRM activity of Rh species supported on Al2O3 and SiO2 was compared, and the Rh species on Al2O3 outperformed those on SiO2, indicating Al2O3 enhances the DRM activity of Rh. When comparing the DRM activity of Rh nanoparticles and Rh single atoms, it was suggested that Rh nanoparticles might exhibit superior performance for DRM. Coke deposited on 0.05 wt% Rh/Al2O3 is removed by CO2, maintaining stable catalytic activity. These findings provide valuable insights into the design of catalysts that minimize the use of noble metals in DRM reactions.

Engineering single-atom rhodium-C3N sites on covalent organic frameworks for boosting photocatalytic hydrogen evolution

Developing efficient and stable photocatalysts for solar hydrogen (H2) energy conversion is meaningful but challenging. Herein, a novel photocatalyst with Rh single atoms (Rh SAs) anchoring in β-ketoimine-linked covalent organic frameworks (TpPa-1) via RhC3N sites is proposed for achieving highly efficient H2 production in phosphate buffer saline (PBS) solution with sodium ascorbate (SA) as sacrificial agent under visible light. TpPa-1 with abundant N and C-chelate sites provides a reliable basis for anchoring Rh single atoms. The optimized Rh SAs/TpPa-1 exhibits an outstanding hydrogen evolution activity (1836.81μmol h−1 g−1), 9.34 and 2.27 folds enhancement than that of pristine TpPa-1 and Rh NPs/TpPa-1. X-ray absorption fine structure (XAFS) combined with density functional theory (DFT) calculations reveal that the significant improvement in H2 evolution performance on Rh SAs/TpPa-1 originates from the unique RhC3N coordination environment, promoting the charge separation and migration at the atomic interface, and thus decreasing the energy barrier for H* formation. Notably, in situ Raman technique confirmed Rh SAs was the main active sites (RhH) for proton reduction.

A pH-responsive single-atom nanozyme for photothermal-augmented nanocatalytic tumor therapy

Based on the physiological conditions of the tumor microenvironment (TME), many effective therapeutic strategies have been reported by using nanocatalysts. The main challenge is the insufficient catalytic activity of nanocatalysts within the acidic TME, significantly constraining their therapeutic efficacy. Herein, a pH-responsive bifunctional platform is developed with multiple enzyme-like catalytic activities for synergistic tumor therapy by integrating Rh single atoms nanozymes (SA-Rh nanozymes) with photothermal therapy (PTT). The SA-Rh nanozymes display peroxidase-mimicking activities within tumor cells, inducing a synergistic effect of enhanced reactive oxygen species generation for collaborative cancer therapy involving both chemodynamic therapy and PTT. Additionally, SA-Rh nanozymes exhibit catalase mimicking activities, enabling the generation of oxygen, and facilitating efficient nanozyme catalytic therapy in a substrate-cycle manner. Moreover, the catalytic efficiency of SA-Rh nanozymes is found to be higher under weak acidic conditions (pH=6.0) compared to neutral conditions (pH=7.4), thereby enabling the maintenance of catalytic activity in an acidic environment. The incorporation of this feature enhances its catalytic activity within the TME, optimizing the efficacy of nanozyme. The remarkable near-infrared I region absorption capability confers SA-Rh nanozymes with exceptional PTT performance, achieving an impressive efficiency of 34.1 %. Therefore, the SA-Rh nanozymes exhibit a remarkable ability to induce apoptosis in cancer cells through synergistic CDT and PTT modalities, effectively overcoming the shortcomings of current nanozyme catalytic therapy.

CO2 hydrogenation over rhodium cluster catalyst nucleated within a manganese oxide framework

Rhodium-based manganese oxide frameworks were explored as a prototype for carbon dioxide reactive capture and conversion. Three-dimensional frameworks of MnOx were utilized as support structures to isolate Rh metal centers. V, Na, and Zn were introduced as counterions to stabilize the structure and for their beneficial effect as promoters. With this multicomponent catalyst, Rh active centers with MnOxs and varied counterions, we were able to selectively tune the catalytic performance of the material via the choice of counterion and structure of the host material. With cryptomelane-type tunnel manganese oxides octahedral molecular sieve (OMS2), we found that Rh-V-OMS2 was highly stable even after 48 hours on stream with a reaction rate of around 1.5×10−4 mol CO2/gRh/s, surpassing the net reactivity of other initially more active combinations. Furthermore, during CO2 hydrogenation, in situ XAFS showed that single Rh atoms nucleated into nanoparticles/ sub-nanometer clusters with a coordination number of 5.5 or less. Our finding of the correlation between the reaction rate and particle size offers the potential for enhanced control over the reaction rate by tuning particle size. Our activity study with control experiments demonstrates that the activities of the catalysts are proved due to the unique metal support interaction offered by the Rh-X-MnO.

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.

Adjacent MnOx clusters enhance the hydroformylation activity of rhodium single-atom catalysts

Hydroformylation reactions promoted by Rh single-atom catalysts typically exhibit a negative reaction order for CO and a positive reaction order for H2 and alkenes. To enhance the catalytic activity, efforts have been concentrated on reducing the CO adsorption strength and/or enhancing the hydrogenation capacity at Rh sites. This report introduces an optimized Rh single-atom catalyst, utilizing positively charged Mn species to weaken the CO adsorption strength. Our strategy involves placing MnOx clusters in the proximity to the Rh single atom. This arrangement allows the reduction of Rh3+ to produce electronically deficient Rhδ+ species (1 < δ < 2), rather than the usual formation of the lower valence state Rh+ species. The weakened CO adsorption strength on the more positively charged Rhδ+ results in reduced CO coverage on the Rh active site, and enhanced adsorption of propylene. Our mechanistic study reveals that H2 and CO adsorption on Rh catalyzes the reduction of adjacent Mn(IV) species, inducing the formation of a Rh-MnOx paired active site. The neighboring MnOx clusters hinder the extensive reduction of Rh3+ to Rh+. This study presents MnOx as an inorganic ligand in Rh-MnOx pair site modifies the electronic properties of Rh sites to modulate the hydroformylation activity of Rh single-atom catalysts.

Efficient CO2 capture and in-situ electrocatalytic conversion on MgO surface by metal single-atom engineering: A comprehensive DFT study

CO2 capture and electrochemical CO2 conversion are both extremely energy-intensive processes, and synergistic coupling between the two processes can increase the ecological sustainability of the system and decrease the cost of the generated products. Here, we investigated the enhancement of CO2 capture performance on MgO (100) surface by thirteen types of metal single-atom engineering using density functional theory (DFT) method. The doping of Pt, Ru, or Rh single atom transforms the adsorption of CO2 on the surface of MgO (100) from physical to chemical processes. The MgO surface doped with Ru single atom has the maximum CO2 adsorption energy of −1.46 eV. Notably, this chemical interaction with the surface activates inactive O = C = O bond of CO2 significantly, which is advantageous for the generation of the intermediate *COOH during the electrochemical conversion process. Reaction pathway analysis shows that Ru@MgO surface exhibits high activity and selectivity for the electrochemical conversion of CO2 to CH4. DFT results show that Ru@MgO can be used to separate CO2 from flue gas and electro-catalyze CO2 into CH4 in-situ efficiently, paving the way for the development of a novel CO2 separation technology with low energy consumption and high efficiency.

Electrocatalytic nitrite reduction to ammonia on an Rh single-atom catalyst

Electrocatalytic NO2− reduction to NH3 (NO2RR) holds great promise as a green method for high-efficiency NH3 production. Herein, an Rh single-atom catalyst where isolated Rh supported on defective BN nanosheets (Rh1/BN) is reported to exhibit the exceptional NO2RR activity and selectivity. Extensive experimental and theoretical studies unveil that the high NO2RR performance of Rh1/BN arises from the single-atom Rh sites, which not only promote the activation and hydrogenation of NO2−-to-NH3 process, but also hamper the undesired hydrogen evolution. Consequently, Rh1/BN assembled in a flow cell exhibits the highest NH3 yield rate of 2165.4 μmol h−1 cm−2 and FENH3 of 97.83 % at a high current density of 355.7 mA cm−2, ranking it the most efficient catalysts for NO2−-to-NH3 conversion.

Understanding the role of supported Rh atoms and clusters during hydroformylation and CO hydrogenation reactions with in situ/operando XAS and DRIFT spectroscopy.

Supported Rh single-atoms and clusters on CeO2, MgO, and ZrO2 were investigated as catalysts for hydroformylation of ethylene to propionaldehyde and CO hydrogenation to methanol/ethanol with in situ/operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS). Under hydroformylation reaction conditions, operando spectroscopic investigations unravel the presence of both single atoms and clusters and detected at first propanal and then methanol. We find that the formation of methanol is associated with CO hydrogenation over Rh clusters which was further confirmed under CO hydrogenation conditions at elevated pressure. The activity of catalysts synthesized via a precipitation (PP) method over various supports towards the hydroformylation reaction follows the order: Rh/ZrO2 > Rh/CeO2 > Rh/MgO. Comparing Rh/CeO2 catalysts synthesized via different methods, catalysts prepared by flame spray pyrolysis (FSP) showed catalytic activity for the hydroformylation reaction at lower temperatures (413 K), whereas catalysts prepared by wet impregnation (WI) showed the highest stability. These results not only provide fundamental insights into the atomistic level of industrially relevant reactions but also pave the way for a rational design of new catalysts in the future.

Tailoring the Near-Surface Environment of Rh Single-Atom Catalysts for Selective CO2 Hydrogenation

Tailoring the Near-Surface Environment of Rh Single-Atom Catalysts for Selective CO₂ Hydrogenation

Sulfur Poisoning and Self-Recovery of Single-Site Rh1 /Porous Organic Polymer Catalysts for Olefin Hydroformylation.

Sulfur poisoning and regeneration are global challenges for metal catalysts even at the ppm level. The sulfur poisoning of single-metal-site catalysts and their regeneration is worthy of further study. Herein, sulfur poisoning and self-recovery are first presented on an industrialized single-Rh-site catalyst (Rh1 /POPs). A decreased turnover frequency of Rh1 /POPs from 4317 h-1 to 318 h-1 was observed in a 1000 ppm H2 S co-feed for ethylene hydroformylation, but it self-recovered to 4527 h-1 after withdrawal of H2 S, whereas the rhodium nanoparticles demonstrated poor activity and self-recovery ability. H2 S reduced the charge density of the single Rh atom and lowered its Gibbs free energy with the formation of inactive (SH)Rh(CO)(PPh3 -frame)2 , which could be regenerated to active HRh(CO)(PPh3 -frame)2 after withdrawing H2 S. The mechanism and the sulfur-related structure-activity relationship were highlighted. This work provides an understanding of heterogeneous ethylene hydroformylation and sulfur-poisoned regeneration in the science of single-atom catalysts.

Sulfur Poisoning and Self‐Recovery of Single‐Site Rh1/Porous Organic Polymer Catalysts for Olefin Hydroformylation

AbstractSulfur poisoning and regeneration are global challenges for metal catalysts even at the ppm level. The sulfur poisoning of single‐metal‐site catalysts and their regeneration is worthy of further study. Herein, sulfur poisoning and self‐recovery are first presented on an industrialized single‐Rh‐site catalyst (Rh1/POPs). A decreased turnover frequency of Rh1/POPs from 4317 h−1 to 318 h−1 was observed in a 1000 ppm H2S co‐feed for ethylene hydroformylation, but it self‐recovered to 4527 h−1 after withdrawal of H2S, whereas the rhodium nanoparticles demonstrated poor activity and self‐recovery ability. H2S reduced the charge density of the single Rh atom and lowered its Gibbs free energy with the formation of inactive (SH)Rh(CO)(PPh3‐frame)2, which could be regenerated to active HRh(CO)(PPh3‐frame)2 after withdrawing H2S. The mechanism and the sulfur‐related structure–activity relationship were highlighted. This work provides an understanding of heterogeneous ethylene hydroformylation and sulfur‐poisoned regeneration in the science of single‐atom catalysts.

Single‐Atom Rh on High‐Index CeO2 Facet for Highly Enhanced Catalytic CO Oxidation

AbstractReducible oxide‐supported noble metal nanoparticles exhibit high activity in catalyzing many important oxidation reactions. However, atom migration under harsh reaction conditions leads to deactivation of the catalyst. Meanwhile, single‐atom catalysts demonstrate enhanced stability, but often suffer from poor catalytic activity owing to the ionized surface states. In this work, we simultaneously address the poor activity and stability issues by synthesizing highly active and durable rhodium (Rh) single‐atom catalysts through a “wrap‐bake‐peel” process. The pre‐coated SiO2 layer during synthesis of catalyst plays a crucial role in not only protecting CeO2 support against sintering, but also donating electron to weaken the Ce−O bond, producing highly loaded Rh single atoms on the CeO2 support exposed with high‐index {210} facets. Benefiting from the unique electronic structure of CeO2 {210} facets, more oxygen vacancies are generated along with the deposition of more electropositive Rh single atoms, leading to remarkably improved catalytic performance in CO oxidation.

Single-Atom Rh on High-Index CeO2 Facet for Highly Enhanced Catalytic CO Oxidation.

Reducible oxide-supported noble metal nanoparticles exhibit high activity in catalyzing many important oxidation reactions. However, atom migration under harsh reaction conditions leads to deactivation of the catalyst. Meanwhile, single-atom catalysts demonstrate enhanced stability, but often suffer from poor catalytic activity owing to the ionized surface state. In this work, we address the poor activity and stability issues simultaneously by synthesizing highly active and durable rhodium (Rh) single-atom catalysts through a "wrap-bake-peel" process. The pre-coated SiO2 layer during synthesis of catalyst plays a crucial role in not only protecting CeO2 support against sintering, but also donating electron to weaken the Ce-O bond, producing highly loaded Rh single atoms on the CeO2 support exposed with high-index{420}. Benefiting from the unique electronic structure of CeO2facets, more oxygen vacancies are generated along with the deposition of more electropositive Rh single atoms, leading to remarkably improved catalytic performance in CO oxidation.

Methane Carboxylation Using Electrochemically Activated Carbon Dioxide

AbstractDirect synthesis of CH3COOH from CH4 and CO2 is an appealing approach for the utilization of two potent greenhouse gases that are notoriously difficult to activate. In this Communication, we report an integrated route to enable this reaction. Recognizing the thermodynamic stability of CO2, our strategy sought to first activate CO2 to produce CO (through electrochemical CO2 reduction) and O2 (through water oxidation), followed by oxidative CH4 carbonylation catalyzed by Rh single atom catalysts supported on zeolite. The net result was CH4 carboxylation with 100 % atom economy. CH3COOH was obtained at a high selectivity (&gt;80 %) and good yield (ca. 3.2 mmol g−1cat in 3 h). Isotope labelling experiments confirmed that CH3COOH is produced through the coupling of CH4 and CO2. This work represents the first successful integration of CO/O2 production with oxidative carbonylation reaction. The result is expected to inspire more carboxylation reactions utilizing preactivated CO2 that take advantage of both products from the reduction and oxidation processes, thus achieving high atom efficiency in the synthesis.

Methane Carboxylation Using Electrochemically Activated Carbon Dioxide.

Direct synthesis of CH3 COOH from CH4 and CO2 is an appealing approach for the utilization of two potent greenhouse gases that are notoriously difficult to activate. In this Communication, we report an integrated route to enable this reaction. Recognizing the thermodynamic stability of CO2 , our strategy sought to first activate CO2 to produce CO (through electrochemical CO2 reduction) and O2 (through water oxidation), followed by oxidative CH4 carbonylation catalyzed by Rh single atom catalysts supported on zeolite. The net result was CH4 carboxylation with 100 % atom economy. CH3 COOH was obtained at a high selectivity (>80 %) and good yield (ca. 3.2 mmol g-1 cat in 3 h). Isotope labelling experiments confirmed that CH3 COOH is produced through the coupling of CH4 and CO2 . This work represents the first successful integration of CO/O2 production with oxidative carbonylation reaction. The result is expected to inspire more carboxylation reactions utilizing preactivated CO2 that take advantage of both products from the reduction and oxidation processes, thus achieving high atom efficiency in the synthesis.

Boosting the Hydroformylation Activity of a Rh/CeO2 Single-Atom Catalyst by Tuning Surface Deficiencies

Controlling the interactions between atomically dispersed metal atoms and the support plays significant roles in determining the activity and selectivity of single-atom catalysts. In this report, we tuned the local coordination environment of Rh single atoms on CeO2 via calcination to construct a highly active hydroformylation catalyst. Single-atom Rh/CeO2 calcined at a high temperature exhibits more oxygen vacancies, which lead to the formation of a large amount of low-coordination Rh active species that are more active for hydroformylation. Under the optimum conditions, the best Rh/CeO2 catalyst achieved a TOF at approximately 5000 h–1 with 100% aldehyde selectivity in propylene hydroformylation to butanal. In situ FTIR spectroscopy and in situ XPS characterizations provide strong evidence that Rh on 800 °C-calcined CeO2 is easily activated to form surface HRh(CO)2 active species, favoring propylene adsorption and CO insertion. This work highlights the significance of engineering metal–support interactions in tuning the hydroformylation performance of single-atom catalysts and contributes to mechanistic insights into single-atom Rh-catalyzed hydroformylation reactions.

In-situ adaptive evolution of rhodium oxide clusters into single atoms via mobile rhodium-adsorbate intermediates

It is a common phenomenon for supported metal catalysts to undergo thermally-induced or adsorbate-induced reconstruction. Great efforts have been devoted to making these reconstruction adaptive to the reaction environment instead of deactivation. Herein, we reported the evolution of initially inactive RhOx clusters on Al2O3 into the formation of catalytically active oxygen vacancies and Rh single atoms via mobile Rh-CO intermediates during hydroformylation of propene. The activated catalyst exhibited a high specific activity of 3.0 × 104 mol molRh–1 h–1 towards hydroformylation reaction. Mechanistic studies revealed the evolution paths. Specially, RhOx clusters were reduced by CO to form oxygen vacancy where the surrounding unsaturated Rh atoms enabled the chemisorption of CO*. Rh atoms that were ejected from RhOx clusters diffused on Al2O3 supports to generate Rh single atom via the formation of carbonyl or geminal dicarbonyl species. Meanwhile, the Rh atoms on clusters were also leached to the solution by the adsorbed CO molecules, followed by partial re-adsorption on the support. This work not only offers an efficient catalyst for propene hydroformylation, but also advances the understandings of dynamic evolution of catalysts.

The theoretical study of Rh single atom catalysts decorated C3N monolayer with N vacancy for CO oxidations

The theoretical study of Rh single atom catalysts decorated C3N monolayer with N vacancy for CO oxidations

Hydroformylation over polyoxometalates supported single-atom Rh catalysts

Atomic dispersion of Rh on phosphotungstic acid (PTA) salts was achieved by a self-assembled precipitation method using alkali metal ions as coprecipitation reagents. During styrene hydroformylation, the supported Rh single-atom catalyst (Rh1/M-PTA, M refers to an alkali metal ion) demonstrated an optimum turnover frequency (TOF) of 1076 h^-1. With increasing ionic radius, the pore size of the catalysts increased in the following order: Rh₁/K-PTA<Rh₁/Rb-PTA<Rh₁/Cs-PTA. The catalytic activity showed the same trend, suggesting a positive correlation between pore size and hydroformylation performance. Further experimental data suggested that temperature is an important factor affecting not only the activity but also the selectivity. This study enriches the understanding of the structure and catalytic properties of PTA-supported single-atom materials. The cation-controlled synthesis of catalysts may also be applied to prepare other single-atom catalysts with tunable pore size distributions.