Rh Single Atoms Research Articles

Rh single atoms on TiO2 dynamically respond to reaction conditions by adapting their site

Single-atom catalysts are widely investigated heterogeneous catalysts; however, the identification of the local environment of single atoms under experimental conditions, as well as operando characterization of their structural changes during catalytic reactions are still challenging. Here, the preferred local coordination of Rh single atoms is investigated on TiO2 during calcination in O2, reduction in H2, CO adsorption, and reverse water gas shift (RWGS) reaction conditions. Theoretical and experimental studies clearly demonstrate that Rh single atoms adapt their local coordination and reactivity in response to various redox conditions. Single-atom catalysts hence do not have static local coordinations, but can switch from inactive to active structure under reaction conditions, hence explaining some conflicting literature accounts. The combination of approaches also elucidates the structure of the catalytic active site during reverse water gas shift. This insight on the real nature of the active site is key for the design of high-performance catalysts.

The Key Role of Support Surface Hydrogenation in the CH4 to CH3OH Selective Oxidation by a ZrO2-Supported Single-Atom Catalyst

Direct conversion of methane to methanol has attracted much interest and yet remains a challenge. Here, we investigate the catalytic mechanisms for methane oxidation on the Rh single-atom catalyst ...

Tuning the Dissociative Action of Cationic Rh Clusters Toward NO by Substituting a Single Ta Atom

Modification of Rh chemical activity for decomposition of NO by alloying with Ta was studied by vibrational spectroscopy of gas-phase clusters, where one single Rh atom was substituted by Ta, with NO adsorbed. While NO adsorbs molecularly on pure Rh clusters, it was found to adsorb dissociatively on RhnTa+ (n = 2–8) with the O bound to the Ta on-top site. A reaction path for NO decomposition obtained by density functional theory calculations for octahedral Rh5Ta+ and Rh6+ suggests that the Ta oxygen affinity strongly reduces the energy barrier right before bond cleavage, facilitating NO dissociation. The trend is consistent with the Bell–Evans–Polanyi principle. The addition of other less oxophilic dopant atoms could plausibly enhance catalytic reactivity of Rh.

Catalytic Oxidation of Ethane to Carboxylic Acids in the Liquid Phase at Near Room Temperature at Ambient Pressure

Chemical transformation of shale gas components under mild conditions is ideal for utilizing the shale gas resource to produce high-value intermediates of chemical industries and fuel feedstocks of energy industries. Here we report that single Rh atom sites, Rh1O5 anchored in micropores of HZSM-5, can catalyze oxidation of ethane by hydrogen peroxide in aqueous solution to form acetic acid and formic acid at a temperature ≤ 50 °C at a pressure of ethane at 1.5 bar. Conversion of 1.5 bar of ethane to carboxylic acids at 50 °C in a Parr rector reaches 37% within 2 h. Acetic acid and formic acid are formed through two parallel reaction pathways with apparent activation barriers of 32.5 and 35.5 kJ/mol in the temperature range of 50–72 °C, respectively. Turnover rates for forming carboxylic acids are 0.060 acetic acid molecules on each Rh1O5 site per second (216 mol of acetic acid per mol Rh per hour) and 0.127 formic acid molecules on each Rh1O5 site per second (457 mol of formic acid per mol of Rh per hour)...

CO oxidation on the phosphotungstic acid supported Rh single–atom catalysts via Rh–assisted Mans–van Krevelen mechanism

Recently, there are contradictory picture of the surface processes for polyoxometalates (POMs) supported single–atom catalysts (SACs) in CO oxidation, both Langmuir–Hinshelwood (LH) and Mars–van Krevelen (MvK) mechanisms have been proposed based on experimental and theoretical studies. We here present results of density functional theoretical (DFT) calculations on the neutral phosphotungstic acid (PTA) supported Rh SAC (Rh1/PTA), aiming at the identification of an Rh–assisted MvK mechanism. The main finding of our DFT calculations are: (1) two CO molecules adsorb simultaneously at the Rh site with considerable adsorption energy, which inhibits the adsorptions of O2; (2) anchoring of single Rh atom on the PTA surface weakens W–Ob/Oc bonds and leads to the charge rearrangement of the surface oxygen species; (3) the anchored Rh atom serves as an electron reservoir along the favorable pathway, as supported by the Rh atom acts as electron acceptor in formation of oxygen vacancy and electron donor in the replenishing step; (4) the difference between Ob and Oc atoms of PTA surface is remarkable, the calculated barrier for vacancy formation by reaction with adsorbed CO and Ob atom is lower than that of Oc atom (25 kcal mol–1 vs 40 kcal mol–1, however, previous experimental and theoretical studies neglect this remarkable difference of the Ob and Oc atoms in CO oxidation, which were treated indiscriminately.); (5) although the O2 molecule was activated by the charge transfer from the anchoring Rh atom to π* antibonding orbital of O2, a strong bonding interaction of Rh–O2 leads to a high barrier of 41 kcal mol–1. Therefore, the dissociation of the adsorbed O2 molecule is the rate–determining step for the whole catalytic cycle. Consequences of these findings for the Rh–assisted MvK mechanism on the Rh1/PTA SAC, can be viewed as a model system for POM–supported SACs.

Prominent hydrogenation catalysis of a PVP-stabilized Au34 superatom provided by doping a single Rh atom.

A single rhodium atom was precisely doped into a gold cluster Au34 stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) as revealed by mass spectrometry. The Rh-atom-doped Au:PVP exhibited remarkable catalytic activity for hydrogenation reactions of olefins, which was much higher than that of recently reported Pd-atom-doped Au:PVP.

Rh single atom catalyst for direct conversion of methane to oxygenates

Rh single atom catalyst for direct conversion of methane to oxygenates

Rhodium clustering process on defective (8,0) SWCNT: Analysis of chemical and physical properties using density functional theory

The Spin-polarized density functional theory is used to study the effect of a single vacancy in a (8,0) single-walled carbon nanotube (SWCNT) on the Rh clustering process. The vacancy is considered oxygenated and non-oxygenated and, in each case, different Rhn cluster sizes (n=1–4) are taken into account. For the analysis of these systems some physical and chemical properties are calculated, such as binding energy (Eb), work function (WF), magnetic moment, charge transfer, bond length, band gap (Eg), and density of state (DOS). From this analysis it can be concluded that: a single Rh atom and Rh2 dimer are adsorbed on vacancy without oxygen, whereas Rh3 and Rh4 clusters prefer to be adsorbed on oxygenated vacancy. In all cases, Rh adsorption induces a magnetic moment. When the Rh atom and Rh2 dimer are bonded to the defective SWCNT, it has been found that they show a semiconductor behavior that could be interesting to use in the spintronic area. In the case of Rh3 and Rh4 clusters our results show a metallic behavior suggesting that these systems are good candidates for nanotube contact.

Supported Rhodium Catalysts for Ammonia–Borane Hydrolysis: Dependence of the Catalytic Activity on the Highest Occupied State of the Single Rhodium Atoms

AbstractSupported metal nanocrystals have exhibited remarkable catalytic performance in hydrogen generation reactions, which is influenced and even determined by their supports. Accordingly, it is of fundamental importance to determine the direct relationship between catalytic performance and metal–support interactions. Herein, we provide a quantitative profile for exploring metal–support interactions by considering the highest occupied state in single‐atom catalysts. The catalyst studied consisted of isolated Rh atoms dispersed on the surface of VO2 nanorods. It was observed that the activation energy of ammonia–borane hydrolysis changed when the substrate underwent a phase transition. Mechanistic studies indicate that the catalytic performance depended directly on the highest occupied state of the single Rh atoms, which was determined by the band structure of the substrates. Other metal catalysts, even with non‐noble metals, that exhibited significant catalytic activity towards NH3BH3 hydrolysis were rationally designed by adjusting their highest occupied states.

Supported Rhodium Catalysts for Ammonia-Borane Hydrolysis: Dependence of the Catalytic Activity on the Highest Occupied State of the Single Rhodium Atoms.

Supported metal nanocrystals have exhibited remarkable catalytic performance in hydrogen generation reactions, which is influenced and even determined by their supports. Accordingly, it is of fundamental importance to determine the direct relationship between catalytic performance and metal-support interactions. Herein, we provide a quantitative profile for exploring metal-support interactions by considering the highest occupied state in single-atom catalysts. The catalyst studied consisted of isolated Rh atoms dispersed on the surface of VO2 nanorods. It was observed that the activation energy of ammonia-borane hydrolysis changed when the substrate underwent a phase transition. Mechanistic studies indicate that the catalytic performance depended directly on the highest occupied state of the single Rh atoms, which was determined by the band structure of the substrates. Other metal catalysts, even with non-noble metals, that exhibited significant catalytic activity towards NH3 BH3 hydrolysis were rationally designed by adjusting their highest occupied states.

Atomic-level insights in optimizing reaction paths for hydroformylation reaction over Rh/CoO single-atom catalyst.

Rh-based heterogeneous catalysts generally have limited selectivity relative to their homogeneous counterparts in hydroformylation reactions despite of the convenience of catalyst separation in heterogeneous catalysis. Here, we develop CoO-supported Rh single-atom catalysts (Rh/CoO) with remarkable activity and selectivity towards propene hydroformylation. By increasing Rh mass loading, isolated Rh atoms switch to aggregated clusters of different atomicity. During the hydroformylation, Rh/CoO achieves the optimal selectivity of 94.4% for butyraldehyde and the highest turnover frequency number of 2,065 h−1 among the obtained atomic-scale Rh-based catalysts. Mechanistic studies reveal that a structural reconstruction of Rh single atoms in Rh/CoO occurs during the catalytic process, facilitating the adsorption and activation of reactants. In kinetic view, linear products are determined as the dominating products by analysing reaction paths deriving from the two most stable co-adsorbed configurations. As a bridge of homogeneous and heterogeneous catalysis, single-atom catalysts can be potentially applied in other industrial reactions.

Enhanced performance of Rh1/TiO2 catalyst without methanation in water‐gas shift reaction

Water‐gas shift (WGS) reaction is an important process for industrial hydrogen production. The side reaction of methanation often causes unavoidable loss of H2 along with this reaction. Here, we report a Rh1/TiO2 single‐atom catalyst (SAC) with appreciable loading of 0.37 wt %, which exhibited an overall CO conversion of ∼95% but without any methanation at 300°C, even under CO2‐ and H2‐rich WGS stream. The specific activity of this SAC was around four times higher than that of cluster catalyst, which meanwhile suffered from unfavorable methanation. It was found that Rh single atoms promoted the formation of more oxygen vacancies on the TiO2 support to activate H2O to generate H2 and prohibited the dissociation of H2 compared with Rh clusters, leading to the enhanced activity and selectivity for WGS. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2081–2088, 2017

The spin significance in the capture and activation of N2O by small Rh nanoparticles

This work presents many possible theoretical reaction pathways of N2O reduction to N2 and O on Rhn nanoparticles (n=1–4) using density functional theory (DFT) method and the zero order regular approximation (ZORA), which explicitly considers the scalar relativistic corrections. The Rh spin multiplicity is an essential condition to dissociate N2O, because can promote or inhibit the electron back donation from the metal. Rh activates N2O by exothermic and spontaneous reactions. For each case presented, the optimized geometry adsorption site, reaction energy, spin multiplicity and Voronoi charges are calculated. On a single Rh atom in the ground and low-lying excited states, the N2O is captured only. On the Rh2 quintet ground state, N2O is also chemisorbed and dissociation occurs for the next two excited states (triplet and septet). In the Rh3 case, there are N2O adsorption and dissociation for Rh3 quartet ground state and for most excited states. Several dissociation cases take place when N2O is parallel to the plane and parallel to a Rh3 bond. On the Rh4, however, there are two optimal geometries: tetrahedral and square; for both cases there are N2O adsorption and dissociation.

Investigation of superhalogen properties of RhFn (n = 1–7) clusters using quantum chemical method

In this paper we have studied the interaction of Rhodium (Rh) atom with Fluorine (F) using density functional theory. Up to seven F atoms are bound to a single Rh atom which results in increase of electron affinities of the given molecule successively, reaching a peak value of 9.05eV for RhF7. By using HOMO–LUMO gap, molecular orbital analysis, binding energy of these clusters, we examined its stability and reactivity. It is found that energy required for dissociation of F2 molecules are higher than energy required for dissociation of F atoms. The unusual properties are brought about by involvement of inner shell 4d-electrons, which not only allow RhFn clusters to belong to the class of superhalogens but also show that its valence can exceed the nominal value of 1.

A Density Functional Theory Investigation of the Bimetallic Clusters Nb2Rh and NbRh2 and the Complexes They Form with CO

The interaction of CO with the bimetallic clusters Nb2Rh and NbRh2 has been theoretically investigated using density functional theory. The lowest energy structure of Nb2Rh is found to be a doublet Cs scalene triangle and the global minimum of Nb2Rh–CO is a dissociative structure with C1 symmetry. The lowest energy minimum of NbRh2 is found to be a doublet C2v isosceles triangle and the global minimum of NbRh2–CO is a dissociative structure with Cs symmetry. In comparison with our previous work on Rh3 + CO (J. Comp. Chem., 2008, 29, 1497), these results show that substitution of a single Rh atom with Nb is sufficient to dissociate CO.

Electronic and magnetic properties of embedded Rh clusters in Ni matrix

The electronic structure and magnetic properties of rhodium clusters with sizes of one to 43 atoms embedded in a nickel host are studied by first-principles spin-polarized calculations within the local density functional formalism. A single Rh atom in Ni matrix is found to have a magnetic moment of 0.45 mu B. Rh13 and Rh19 clusters in Ni matrix have lower magnetic moments compared with the free ones. The most interesting finding is that Rh43 cluster, which is bulk-like and non-magnetic in vacuum, becomes ferromagnetic when embedded in the nickel host.

Magnetic Properties of Embedded Rh Clusters in Ni Matrix

ABSTRACTThe electronic structure and magnetic properties of rhodium clusters with sizes of 1 - 43 atoms embedded in the nickel host are studied by the first-principles spin-polarized calculations within the local density functional formalism. Single Rh atom in Ni matrix is found to have magnetic moment of 0.45μB. Rh13 and Rhl 9 clusters in Ni matrix have lower magnetic moments compared with the free ones. The most interesting finding is tha.t Rh43 cluster, which is bulk-like nonmagnetic in vacuum, becomes ferromagnetic when embedded in the nickel host.

Role of silver promoter in carbon monoxide hydrogenation and ethylene hydroformylation over Rh/SiO 2 catalysts

The effect of silver promotion on CO hydrogenation and ethylene hydroformylation over Rh/SiO 2 has been studied. Catalyst characterization before reaction reveals that Ag and Rh form separate crystallites; part of the Ag is atomically spread on the surface of Rh crystallite and blocks the Rh ensembles required for the bridge CO. Although both reactions restructure the catalyst surface and increase the ratio of bridge to linear CO sites on AgRh/SiO 2 catalysts, the ratio of the bridge to the linear CO sites remains lower on Ag-Rh/SiO 2 than on Rh/SiO 2 under reaction conditions. Ag is found to increase the selectivity and the rate of formation for acetaldehyde (C 2 oxygenate) during CO hydrogenation and for propionaldehyde (C 3 oxygenate) during ethylene hydroformylation on Rh/SiO 2. The increased activity and selectivity for C 2 and C 3 oxygenates is attributed to a high ratio of the linear CO to bridge CO sites and a possible presence of isolated Rh + sites under reaction conditions. The results are consistent with a previous report that the single Rh atom site that chemisorbs linear CO is active for CO insertion.

Catalytic site requirements for elementary steps in syngas conversion to oxygenates over promoted rhodium

Following P.H. Emmett's pioneering work, much research was focused on the mechanisms of Fischer-Tropsch catalysis, and it has been discovered that certain base metal ions promote the activity and selectivity of Rh metal to catalyze the conversions of CO + H/sub 2/ to aldehydes and alcohols. Recent IR, Moessbauer, and EXAFS evidence in conjunction with catalytic tests suggests that the highly oxophilic ions Mn, Ti, Zr, and Nb, when located at the Rh surface, enhance CO dissociation, possibly through direct interaction with the oxygen atom of tilted adsorbed CO. A different mechanism emerges for Fe and Zn ions. These seem to block sites, thus inhibiting the formation of bridging CO and its dissociation to C + O. These steps are characterized by large ensemble requirements. Remarkably, the rate of CO insertion on Rh appears enhanced but the rate of hydrogenation is markedly suppressed by Zn. Consequently, the hydroformylation of olefins and the synthesis of methanol are efficiently catalyzed. This result suggests that the latter reactions require only very small Rh ensembles; possibly they are catalyzed by single Rh atoms.

Vibrational spectroscopy using HREELS of benzene adsorbed on the Rh(111) crystal surface

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) have been used to study the structure of adsorbed benzene (C 6H 6 and C 6D 6) monolayers on the Rh(111) surface at 300K. A surface bonding geometry is proposed for benzene adsorbed to give a c(2✓3×4) rectangular structure, which involves very little perturbation of the molecular structure with the ring plane parallel to the surface. Only one chemical environment for adsorbed benzene is indicated by a single frequency shift of the symmetric CH out-of-plane bending mode. The adsorption site is tentatively assigned to benzene centered over a single Rh atom.