Rh Clusters Research Articles

Rh4 cluster supported on the In2O3(111) surface for enhancing the turnover frequency of CO2 hydrogenation to methanol: The application of energetic span model

The turnover frequency of CO2 hydrogenation to methanol on the surface of Rh4 cluster supported In2O3(111) is investigated using the first-principles calculation. The Rh4 cluster and H2 promote oxygen vacancy formation, which facilitates the interaction between the Rh4 cluster and the In2O3(111) carrier and improves the stability of catalyst. The adsorption behavior of CO2 and H2 on catalyst is predicted by electrostatic potential and Fukui(−) index. The loading of Rh4 cluster promotes the adsorption of CO2 and allows easier dissociation of H2 compared to the In2O3(111)_D surface. Afterwards, Gibbs free energy diagrams of three hydrogenation pathways are established and the turnover frequencies of all possible routes are calculated based on the energetic span model. The results reveal that the HCOO pathway is regarded as the optimum hydrogenation mechanism due to its greatest turnover frequency (3.02 × 10−5 s−1), higher than that of In2O3(111)_D and other reported catalysts. The specific process is CO2(g) + 6H → HCOO* + 5H → HCOOH* + 4H → H2COOH* + 3H → H2CO* + 2H + H2O(g) → H2COH* + H + H2O(g) → CH3OH(g) + H2O(g), while the RWGS pathway and by-products are effectively inhibited, thus enhancing the selectivity of methanol. The improvement in catalytic activity may result from the loaded Rh4 cluster facilitating the adsorption of CO2 and the dissociation of H2.

Effect of Surface Temperature on the Distribution and Reactivity of Rh Active Sites for CO Oxidation

AbstractSingle‐atom catalysts are a fast‐emerging area in which late‐transition metal atoms are supported on oxides, metals, and carbonaceous supports. They show great promise for selective chemical reactions due to their well‐defined active sites, and significantly reduce precious metal loading. However, oxide‐supported single‐atom catalysts have drawbacks such as deactivation due to sintering, and there are on‐going debates about the exact nature of the active sites. Herein we report a model system where the surface temperature during Rh deposition on a copper oxide support dictates the Rh size distribution. Our temperature‐programmed desorption experiments reveal that Rh clusters perform CO oxidation at lower temperature than single atoms, and isotopic labelling demonstrates differences in the mechanism in that the C−O bond is broken and reformed on clusters, as opposed to a pure Mars van Krevelen mechanism for CO oxidation at the single‐atom Rh sites. We further examine the cluster size distributions with scanning tunneling microscopy and X‐ray photoelectron spectroscopy which confirm the presence of both clusters and single atoms for the lower temperature deposition and only single atoms for the higher temperature deposition. Together, these results provide a fundamental understanding of the differences in the CO oxidation mechanism on Rh atoms and clusters.

Surface Rh-Boosted Photoelectrochemical Water Oxidation of α-Fe2O3 by Reduced Overpotential in the Rate-Determining Step.

The photoelectrochemical behavior of Rh cluster-deposited hematite (α-Fe2O3) photoanodes (α-Fe2O3@Rh) was investigated. The interactions between Rh clusters and α-Fe2O3 nanorods were elucidated both experimentally and computationally. A facile UV-assisted solution casting deposition method allowed the deposition of 2 nm Rh clusters on α-Fe2O3. The deposited Rh clusters effectively enhanced the photoelectrochemical performance of the α-Fe2O3 photoanode, and electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis were applied to understand the working mechanism for the α-Fe2O3@Rh photoanodes. The results revealed a distinctive carrier transport mechanism for α-Fe2O3@Rh and increased carrier density, while the absorbance spectra remained unchanged. Furthermore, density functional theory (DFT) calculations of the oxygen evolution reaction (OER) mechanism corresponded well with the experimental results, indicating a reduced overpotential of the rate-determining step. In addition, DFT calculation models based on the X-ray diffraction (XRD) measurements and X-ray photoelectron spectroscopy (XPS) results provided precise water-splitting mechanisms for the fabricated α-Fe2O3 and α-Fe2O3@Rh nanorods. Owing to enhanced carrier generation and hole transfer, the optimum α-Fe2O3@Rh3 sample showed 78% increased photocurrent density, reaching 1.12 mA/cm-2 at 1.23 VRHE compared to that of the pristine α-Fe2O3 nanorods electrode.

Surface activation of platinum group metal clusters for efficient and durable oxygen reduction in proton exchange membrane fuel cells

To develop efficient and durable acidic oxygen–reduction–reaction (ORR) catalysts based on all platinum group metals (PGMs) is crucial for large-scale application of proton-exchange membrane fuel cells (PEMFCs) but challenging. Here, we report a nitrogen coordination-induced strong metal–support interaction that can tune the surface atoms of ORR-inactive PGM clusters into efficient and durable active sites. Taking Rh as an example, the carbonization of Rh-overdoped zeolitic imidazolate framework-8 results in a large number of Rh clusters (with a little atomic Rh) in porous nitrogen-doped carbon. The cluster surface atoms coordinate with the nitrogen of the carbon support, forming much stronger metal–support interactions than that of common N-doped carbon-supported metal nanoparticles. The activity of surface-activated Rh clusters is close to that of Pt/C. The regulation rules for the surface active sites inherit most of the characteristics of the corresponding single-atom catalysts, but without their severe instability problem. This surface activation strategy has also shown applicable to other PGMs, thereby it is a promising way to alleviate the reliance of PEMFCs on platinum.

Theoretical insights into the support effect on the NO activation over platinum-group metal catalysts.

We systematically explored NO activation at metal/oxide interfaces by the combination of Sr3Ti2O7, Sr3Fe2O7, CeO2, anatase-TiO2, ZrO2, and γ-Al2O3 supports and the platinum-group metal cluster (Pd4, Pt4, and Rh4) using slab-model density functional theory calculations. These metal clusters can be strongly adsorbed at these metal oxide surfaces. The Pt4 and Rh4 clusters show larger adsorption energies than the Pd4 cluster, yet the γ-Al2O3(100) surface shows smaller adsorption energies than other metal oxide surfaces. One oxygen vacancy close to the metal cluster was constructed to evaluate the NO activation at those metal/oxide interfaces. The O atom of NO refills the oxygen vacancy after NO dissociation, while the N adatom is left on the metal cluster. The exothermic process was identified for the NO activation except for the Sr3Fe2O7 case, indicating the significant role of the interplay between the metal cluster and oxygen vacancy.

Structure and magnetism of rhodium particles as a size effect

The magnetic properties of Rh clusters with face-centered-cubic (FCC) and hexagonal-closed-packed (HCP) structures were studied by first-principles calculations. The results show that both the structure and magnetic properties have size effects. Bulk Rh exists as a FCC structure. However, as the size of Rh particles shrinks, the FCC and HCP structures have comparable energies, consistent with the coexistence of the two structures experimentally observed previously. Although the bulk Rh is nonmagnetic, the nanoparticles or clusters of Rh in both the FCC and HCP structures are magnetic. The average magnetic moment gradually increases as the clusters shrink. The origin of the magnetism of Rh clusters can be attributed to the reduction of coordination number, other than the lattice expansion. The findings provide insights for understanding the magnetic properties of noble metal clusters.

Hydrogen adsorption study on nanostructured Ag–Rh films grown by supersonic cluster beam deposition

Following recent literature work, an interesting result on hydrogen sorption by Ag–Rh clusters was reported which do not follow the common understanding of hydrogen absorption in bulk alloys. Thus, nanostructured Ag–Rh films with a composition of 75% Ag–25% Rh were grown by supersonic cluster beam deposition onto microbalance quartz crystals in order to measure hydrogen absorption. The films were exposed to H2 gas to study hydrogen absorption at different partial pressure. The Ag–Rh films were then characterized by XPS and AFM. The Ag/Rh atomic ratio was found to be 75/25 by XPS. Surface roughness was determined by AFM to be 3 nm RMS in all the films. In disagreement with the quoted literature report, no hydrogen absorption was observed in our Ag–Rh films up to 5.3×103Pa of hydrogen pressure. The response of the balance was checked and validated by measuring hydrogen absorption on 10 and 100 nm Pd films. The hydrogen absorption by Pd yielded an atomic H/Pd ratio of ∼0.65 in agreement with our prior measurements.

Controlled-Release Mechanism Regulates Rhodium Migration and Size Redistribution Boosting Catalytic Methane Conversion

The migration of Rh atoms under a gas/reactive environment has a great impact on the dynamic restructuring and size redistribution of Rh catalysts in a variety of structure-sensitive catalytic reactions. To date, regulating the size distribution of active Rh species via controlled atomic migration remains challenging. Here, we show a controlled-release mechanism to regulate Rh atom migration through two-dimensional (2D) zeolite nanosheets, enabling quasi-continuous size redistribution of active Rh species from a single atom to nanoparticles with reversibility. Utilizing state-of-the-art in situ characterizations, a reversible aggregation/redispersion of Rh catalysts in/out of the 2D zeolite was directly observed under H2 or CO environment. The interplay between gas environment and support confinement was disclosed that substantially restrained the dynamic migration of Rh species and enabled a quasi-continuous control of Rh size distribution over a wide temperature window and size range. The catalytic testing for mild oxidation of methane demonstrated a volcano correlation of methanol activity with increasing particle size. The sub-nanometer Rh clusters with an average diameter of 0.9 nm exhibited the highest methanol activity of 39.7 molCH3OOH·molRh–1·h–1 with remarkable selectivity as high as 73.2%, far beyond that of single atom species and larger particles. Density functional theory calculations and electron paramagnetic resonance spectroscopy further revealed that this size dependency is related to the preferential formation of •OH radicals on Rh clusters with different sizes, which acts as a key initiator for methane activation. Our results thus provide a practical approach to synthesize size-specific metal catalysts via controlled atomic migration.

Encapsulating Metal Nanoparticles into a Layered Zeolite Precursor with Surface Silanol Nests Enhances Sintering Resistance**

AbstractSupported metal nanoparticles are used as heterogeneous catalysts but often deactivated due to sintering at high temperatures. Confining metal species into a porous matrix reduces sintering, yet supports rarely provide additional stabilization. Here, we used the silanol‐rich layered zeolite IPC‐1P to stabilize ultra‐small Rh nanoparticles. By adjusting the IPC‐1P interlayer space through swelling, we prepared various architectures, including microporous and disordered mesoporous. In situ scanning transmission electron microscopy confirmed that Rh nanoparticles are resistant to sintering at high temperature (750 °C, 6 hrs). Rh clusters strongly bind to surface silanol quadruplets at IPC‐1P layers by hydrogen transfer to clusters, while high silanol density hinders their migration based on density functional theory calculations. Ultimately, combining swelling with long‐chain surfactant and utilizing metal‐silanol interactions resulted in a novel, catalytically active material—Rh@IPC_C22.

Encapsulating Metal Nanoparticles into a Layered Zeolite Precursor with Surface Silanol Nests Enhances Sintering Resistance.

Supported metal nanoparticles are used as heterogeneous catalysts but often deactivated due to sintering at high temperatures. Confining metal species into a porous matrix reduces sintering, yet supports rarely provide additional stabilization. Here, we used the silanol-rich layered zeolite IPC-1P to stabilize ultra-small Rh nanoparticles. By adjusting the IPC-1P interlayer space through swelling, we prepared various architectures, including microporous and disordered mesoporous. In situ scanning transmission electron microscopy confirmed that Rh nanoparticles are resistant to sintering at high temperature (750 °C, 6 hrs). Rh clusters strongly bind to surface silanol quadruplets at IPC-1P layers by hydrogen transfer to clusters, while high silanol density hinders their migration based on density functional theory calculations. Ultimately, combining swelling with long-chain surfactant and utilizing metal-silanol interactions resulted in a novel, catalytically active material-Rh@IPC_C22.

Dynamic Structural Evolution of [Rh(NO)2]+ Complex/Rh Metal Cluster in Zeolite during de-NOx via in Situ Formed NH3 under Lean/Rich Periodic Conditions

The dynamic structural evolution of Rh species in mordenite (MOR) zeolite was investigated using in situ spectroscopic techniques and density functional theory (DFT) calculations. In situ X-ray absorption spectroscopy and operando infrared (IR) revealed that metallic Rh species were oxidized to afford isolated [Rh(NO)2]+ species under NO flow at 200 °C, whereas small Rh metal clusters are formed under the subsequent H2 flow. Ab initio thermodynamics analysis shows that the plausible structures under NO and H2 at 200 °C are [Rh(NO)2]+ and Rh clusters in MOR, which is consistent with the experimental observations. A comparative study of Rh-loaded Al2O3 suggests that Al sites in MOR increase the thermodynamic stability of isolated Rh+ species and thus prevent their overoxidation to Rh2O3 under NO. NO capture in the form of [Rh(NO)2]+ and its selective reduction toward NH3 under H2 flow were observed by in situ IR measurements. The RhMOR catalyst exhibited ∼60% of NOx conversion above 200 °C under periodic lean/rich conditions. Transition-state calculations showed that the activation barrier for NO reduction to NH3 on [Rh(NO)2]+ (178 kJ/mol) is higher than that for Rh13 (156 kJ/mol), suggesting that Rh metal clusters are preferable NH3 formation sites, where the Rh13-catalyzed NO reduction into N2 and N2O was less preferable than NH3 formation, which is consistent with the experimental results. Combined with operando IR experiments under lean (NO + O2) and rich (NO + H2) conditions, we show that the reversible dynamic structural evolution of Rh species ([Rh(NO)2]+ ↔ Rh metal clusters under lean and rich conditions) is a key mechanistic feature for unsteady-state de-NOx via the capture of NO, its selective reduction to NH3, and the selective reduction of NO with NH3 formed in situ.

Controlling sintering and carbon deposition with post-coated MgO confined group VIII metal-based catalysts towards durable high-temperature steam reforming

Metal-sintering and carbon-deposition are two bottlenecks in hydrocarbon (e.g., n-dodecane) steam reforming for hydrogen production. Effective inhibition of carbon-deposition by MgO-pre-coating support was intensively confirmed. Unfortunately, we found that weak interaction between Rh-NPs and MgO-pre-coating-CZO induced serious metal-sintering (4→47 nm). To materialize bifunctional suppression, MgO-post-coating confined metal-NPs strategy is proposed. Under the conditions of 700 °C, WHSV = 10 h−1, and S/C = 3 for 50 h, negligible metal-sintering (4→7 nm) and carbon-deposition (0.7 mg C/g cat.) occurred for 5%MgO-1%Rh-Ce0.75Zr0.25O2, whereas Rh-NPs grew to 36 nm and carbon-deposition was 25.6 mg C/g cat. for 1%Rh-Ce0.75Zr0.25O2. Owing to enhanced metal-support-interaction, formed MgO/Rh/CZO interfaces, and increased proportion of Rh0, 5%MgO-1%Rh-Ce0.75Zr0.25O2 showed superior performance. DFT-based calculation results unveiled that MgO-post-coating increased the desorption energies of Rh cluster (0.21→3.29 eV) and individual atoms (3.89→4.35 eV). This study provides an efficient route for realizing sintering resistances of supported metal catalysts in high-temperature hydrothermal atmosphere.

Synergistic Effects in the Activity of Nano-Transition-Metal Clusters Pt12 M (M=Ir, Ru or Rh) for NO Dissociation.

The dissociation of environmentally hazardous NO through dissociative adsorption on metallic clusters supported by oxides, is receiving growing attention. Building on previous research on monometallic M13 clusters [The Journal of Physical Chemistry C 2019, 123 (33), 20314-20318], this work considers bimetallic Pt12 M (M=Rh, Ru or Ir) clusters. The adsorption energy and activation energy of NO dissociation on the clusters have been calculated in vacuum using Kohn-Sham DFT, while their trends were rationalized using reactivity indices such as molecular electrostatic potential and global Fermi softness. The results show that doping of the Pt clusters lowered the adsorption energy as well as the activation energy for NO dissociation. Furthermore, reactivity indices were calculated as a first estimate of the performance of the clusters in realistic amorphous silica pores (MCM-41) through ab initio molecular dynamics simulations.

DFT insights into the electronic structure of Rh single-atom catalysts stabilized on the CeO2(1 1 1) surface

The insights into an Rh single-atom catalyst (SAC) stabilized on CeO2(111) surface is studied by employing DFT + U calculations on various structural models and comparing with the experimental spectral data from the literature. The possible models investigated include (Rh)ads, (RhOH)ads, (RhO2H2)ads, (RhO)ads, (RhO2)ads, (Rh)subCe, and (Rh)subO species. For a comparison with single Rh atom species, Rh4 cluster deposited on CeO2(111) surface is also investigated. Based on the calculated results of CO adsorption properties and the comparison with experimental data , we can figure out that single-atom Rh species stably reside on CeO2(111) surface as (Rh)ads, (RhOH)ads, (RhO)ads structures.

Design of hydrodechlorination catalysts on the basis of chloromethanes-metallic active sites interactions

The adsorption of chloromethanes, namely trichloromethane (TCM), dichloromethane (DCM) and monochloromethane (MCM), and hydrogen on metal clusters of Pd, Pt, Rh and Ru was studied by molecular simulation to learn on the design of more efficient catalysts for the hydrodechlorination (HDC) of those species. All the above metals showed high dechlorination ability, following in all cases the order TCM > DCM > MCM. The computational study consigns that dissociative adsorption of chloromethanes is promoted on the zero-valent metal (M0) sites while non-dissociative adsorption, associated to catalyst deactivation, is strongly favored on the electro-deficient ones.The dechlorination ability of M0 species follows the order Ru > Rh > Pt > Pd, with greater differences in the dechlorination of TCM. Conversely, hydrogenation ability remarkably decreases from Pd and Pt to Rh and Ru catalysts. This determines a more favorable balance of reactant adsorption energies in the HDC with Pd, especially for TCM HDC. Based on the theoretical findings, catalysts with high proportion of zero-valent metal species, supported on CeO2-Sm2O3 (CS), were prepared, characterized and tested in the HDC of TCM and DCM in lab-scale continuous fixed bed experiments. The conclusions obtained by simulation were supported by the experimental results. The high proportion of zero-valent species in the Pd/CS catalyst, besides the favorable interactions between active sites and both chloromethanes and H2, allow an outstanding performance of the catalyst in the HDC of those chloromethanes, with a significantly higher dechlorination activity and stability than the other metals for the HDC of TCM.

Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalysts.

The electrochemical nitrate reduction reaction (NITRR) provides a promising solution for restoring the imbalance in the global nitrogen cycle while enabling a sustainable and decentralized route to source ammonia. Here, we demonstrate a novel electrocatalyst for NITRR consisting of Rh clusters and single-atoms dispersed onto Cu nanowires (NWs), which delivers a partial current density of 162 mA cm-2 for NH3 production and a Faradaic efficiency (FE) of 93 % at -0.2 V vs. RHE. The highest ammonia yield rate reached a record value of 1.27 mmol h-1 cm-2 . Detailed investigations by electron paramagnetic resonance, in situ infrared spectroscopy, differential electrochemical mass spectrometry and density functional theory modeling suggest that the high activity originates from the synergistic catalytic cooperation between Rh and Cu sites, whereby adsorbed hydrogen on Rh site transfers to vicinal *NO intermediate species adsorbed on Cu promoting the hydrogenation and ammonia formation.

Efficient Electrochemical Nitrate Reduction to Ammonia with Copper‐Supported Rhodium Cluster and Single‐Atom Catalysts

AbstractThe electrochemical nitrate reduction reaction (NITRR) provides a promising solution for restoring the imbalance in the global nitrogen cycle while enabling a sustainable and decentralized route to source ammonia. Here, we demonstrate a novel electrocatalyst for NITRR consisting of Rh clusters and single‐atoms dispersed onto Cu nanowires (NWs), which delivers a partial current density of 162 mA cm−2 for NH3 production and a Faradaic efficiency (FE) of 93 % at −0.2 V vs. RHE. The highest ammonia yield rate reached a record value of 1.27 mmol h−1 cm−2. Detailed investigations by electron paramagnetic resonance, in situ infrared spectroscopy, differential electrochemical mass spectrometry and density functional theory modeling suggest that the high activity originates from the synergistic catalytic cooperation between Rh and Cu sites, whereby adsorbed hydrogen on Rh site transfers to vicinal *NO intermediate species adsorbed on Cu promoting the hydrogenation and ammonia formation.

The ethanol–metal interaction in bimetallic clusters of Pt and Rh

The ethanol–metal interaction in bimetallic clusters of Pt and Rh

Importance of Species Heterogeneity in Supported Metal Catalysts.

The structural heterogeneity of surface metal species, which is represented by the distribution in size, morphology, and local coordination environment of the active metal component, is almost inevitable in practical supported metal catalysts. This is often regarded as a major hindrance to the full utilization of metal loading and the high mass-specific catalytic activity. In this work, by quantitative evaluation of the individual reaction steps of a probe reaction, cyclohexanol dehydrogenation (an important reaction for hydrogen storage and transportation as well as high valued chemical production), we demonstrate that the inherent heterogeneity of supported Rhodium catalysts prepared by conventional synthesis has unique advantages in a complex heterogeneous catalytic reaction. The isolated Rh species (Rh1) is extremely active for the first step of dehydrogenation, the transformation of cyclohexanol to cyclohexanone, while the Rh ensemble sites (Rhe, including Rh clusters, Rhn, and Rh nanoparticles, Rhp) are highly efficient for the successive reaction step, cyclohexanone to phenol, for which the Rh1 sites are almost inactive. Only with the coexistence of both active structures could the optimal reaction performance be achieved, which ambiguously demonstrates the importance of species heterogeneity in some multistep catalytic reactions. Our study provides a new view of the benefits from structural heterogeneity in practical catalysts and sheds light on the catalyst design strategy for complex catalytic reactions.

Reaction product-driven restructuring and assisted stabilization of a highly dispersed Rh-on-ceria catalyst

Understanding the structural dynamics of a catalyst under reaction conditions is challenging but crucial regarding catalyst design. Here, by a combination of in situ/operando characterization and first-principles modelling, we show that supported rhodium (Rh) catalysts undergo restructuring at the atomic scale in response to carbon monoxide (CO), a gaseous product formed during steam reforming of methane. Despite transformation of the initially prepared single-Rh-cation catalyst into Rh nanoparticles during hydrogen pretreatment, the formed Rh nanoparticles redispersed to low-nuclearity, CO-liganded Rh clusters (Rhm(CO)n (m = 1–3, n = 2–4)) under catalytic conditions. Theoretical simulations under reaction conditions suggest that the pressure of the CO product stabilizes Rhm(CO)n sites, while in situ/operando spectroscopy revealed a reversible restructuring between Rh3(CO)3 clusters and CO-ligand-free Rh clusters driven by CO pressure. Our findings demonstrate the importance of including product molecules in the atomic-scale understanding of catalytic active sites and mechanisms.