Rhodium Clusters Research Articles

Electronic communications between active sites on individual metallic nanoparticles in catalysis

Catalytic activity of metal particles is reported to originate from the appearance of nonmetallic states, but conductive metallic particles, as an electron reservoir, should render electron delivery between reactants more favorably so as to have higher activity. We present that metallic rhodium particle catalysts are highly active in the low-temperature oxidation of carbon monoxide, whereas nonmetallic rhodium clusters or monoatoms on alumina remain catalytically inert. Experimental and theoretical results evidence the presence of electronic communications in between vertex atom active sites of individual metallic particles in the reaction. The electronic communications dramatically lower apparent activation energies via coupling two electrochemical-like half-reactions occurring on different active sites, which enable the metallic particles to show turnover frequencies at least four orders of magnitude higher than the nonmetallic clusters or monoatoms. Similar results are found for other metallic particle catalysts, implying the importance of electronic communications between active sites in heterogeneous catalysis.

Investigating the Electronic Properties and Stability of Rh3 Clusters on Rutile TiO2 for Potential Photocatalytic Applications.

Addressing the pressing needs for alternatives to fossil fuel-based energy sources, this research explores the intricate interplay between Rhodium (Rh3) clusters and titanium dioxide (TiO2) to improve photocatalytic water splitting for the generation of eco-friendly hydrogen. This research applies the density functional theory (DFT) coupled with the Hartree-Fock theory to meticulously examine the structural and electronic structures of Rh3 clusters on TiO2 (110) interfaces. Considering the photocatalytic capabilities of TiO2 and its inherent limitations in harnessing visible light, the potential for metals such as Rh3 clusters to act as co-catalysts is assessed. The results show that triangular Rh3 clusters demonstrate remarkable stability and efficacy in charge transfer when integrated into rutile TiO2 (110), undergoing oxidation in optimal adsorption conditions and altering the electronic structures of TiO2. The subsequent analysis of TiO2 surfaces exhibiting defects indicates that Rh3 clusters elevate the energy necessary for the formation of an oxygen vacancy, thereby enhancing the stability of the metal oxide. Additionally, the combination of Rh3-cluster adsorption and oxygen-vacancy formation generates polaronic and localized states, crucial for enhancing the photocatalytic activity of metal oxide in the visible light range. Through the DFT analysis, this study elucidates the importance of Rh3 clusters as co-catalysts in TiO2-based photocatalytic frameworks, paving the way for empirical testing and the fabrication of effective photocatalysts for hydrogen production. The elucidated impact on oxygen vacancy formation and electronic structures highlights the complex interplay between Rh3 clusters and TiO2 surfaces, providing insightful guidance for subsequent studies aimed at achieving clean and sustainable energy solutions.

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.

Direct reduction of NO into N2 catalyzed by fullerene-supported rhodium clusters.

Catalytic conversion of NO has long been a focus of atmospheric pollution control and diesel vehicle exhaust treatment. Rhodium is one of the most effective metals for catalyzing NO reduction, and understanding the nature of the active sites and underlying mechanisms can help improve the design of Rh-based catalysts towards NO reduction. In this work, we investigated the detailed catalytic mechanisms for the direct reduction of NO to N2 by fullerene-supported rhodium clusters, C60Rh4+, with density functional theory calculations. We found that the presence of C60 facilitates the smooth reduction of NO into N2 and O2, as well as their subsequent desorption, recovering the catalyst C60Rh4+. Such a process fails to be completed by free Rh4+, emphasizing the critical importance of C60 support. We attribute the novel performance of C60Rh4+ to the electron sponge effect of C60, providing useful guidance for designing efficient catalysts for the direct reduction of NO.

Activation of Methane by Rhodium Clusters on a Model Support C20H10.

Supported metals represent an important family of catalysts for the transformation of the most stable alkane, methane, under mild conditions. Here, using state-of-the-art mass spectrometry coupled with a newly designed double ion trap reactor that can run at high temperatures, we successfully immobilize a series of Rhn- (n = 4-8) cluster anions on a model support C20H10. Reactivity measurements at room temperature identify a significantly enhanced performance of large-sized Rh7,8C20H10- toward methane activation compared to that of free Rh7,8-. The "support" acting as an "electron sponge" is emphasized as the key factor to improve the reactivity of large-sized clusters, for which the high electron-withdrawing capability of C20H10 dramatically shifts the active Rh atom from the apex position in free Rh7- to the edge position in "supported" Rh7- to enhance CH4 adsorption, while the flexibility of C20H10 to release electrons further promotes subsequent C-H activation. The Rh atoms in direct contact with the support serve as electron-relay stations for electron transfer between C20H10 and the active Rh atom. This work not only establishes a novel approach to prepare and measure the reactivity of "supported" metal clusters in isolated gas phase but provides useful atomic-scale insights for understanding the chemical behavior of carbon (e.g., graphene)-supported metals in heterogeneous catalysis.

Size-Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at LowTemperature and the Consequences for CO Oxidation Activity.

Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2O3using in situ/operando spectroscopy and ex situ aberration-corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperatureand the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Size‐Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

AbstractUnderstanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2O3 using in situ/operando spectroscopy and ex situ aberration‐corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Dramatic Size‐dependence of Rhn+ Clusters in Reacting with Small Hydrocarbons: Rh3+ Cluster Catalysis for Dehydrogenation

AbstractWe report a joint experimental and theoretical study of the gas‐phase reactions between rhodium clusters Rhn+ (n=1–24) and small hydrocarbons. It is found that most Rhn+ clusters react with CH4 to form absorption products Rhn(CH4)1,2+ but Rh3+ reacts to form carbide indicative of complete dehydrogenation; Rh2+ reacts with CH4 to form partial dehydrogenation products Rh2CH2+ and Rh2C2H6+. The distinctive reactivity of Rh3+ is also found for the reactions with C2H2, C2H4 and C2H6, where the complete dehydrogenation product Rh3C2+ is also formed. Analysis of reaction kinetics demonstrates that the cooperation of three Rh atoms plays a key role in the dehydrogenation reaction. Notably, Rh3+ bears a large HOMO‐LUMO gap and reasonable stablity, and the roughly equal contribution of electrostatic and orbital interactions provide advantages to form two symmetric Rh−H−C hydrogen bridge bonds which promote the synergic cluster catalysis for hydrogen atom transfer and C−H bond dissociation. The efficient dehydrogenation mediated by an Rh3+ cluster opens a window to design new catalysts for C−H activation.

Stabilization of Dinuclear Rhodium and Iridium Clusters on Layered Titanate and Niobate Supports.

Atomically dispersed organometallic clusters can provide well-defined nuclearity of active sites for both fundamental studies as well as new regimes of activity and selectivity in chemical transformations. More recently, dinuclear clusters adsorbed onto solid surfaces have shown novel catalytic properties resulting from the synergistic effect of two metal centers to anchor different reactant species. Difficulty in synthesizing, stabilizing, and characterizing isolated atoms and clusters without agglomeration challenges allocating catalytic performance to atomic structure. Here, we explore the stability of dinuclear rhodium and iridium clusters adsorbed onto layered titanate and niobate supports using molecular precursors. Both systems maintain their nuclearity when characterized using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Statistical analysis of HAADF-STEM images revealed that rhodium and iridium dimers had mean cluster-to-cluster distances very similar to what is expected from a random distribution of atoms over a large area, indicating that they are dispersed without aggregation. The stability of dinuclear rhodium clusters supported on titanate nanosheets was also investigated by X-ray absorption fine structure (EXAFS), DRIFTS, and first-principles calculations. Both X-ray absorption spectroscopy and HAADF-STEM simulations, guided by density functional theory (DFT)-optimized structure models, suggested that rhodium dimers adsorb onto the nanosheets in an end-on binding mode that is stable up to 100 °C under reducing conditions. This study highlights that crystalline nanosheets derived from layered metal oxides can be used as model supports to selectively stabilize dinuclear clusters, which could have implications for heterogeneous catalysis.

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.

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.

Ladder Oxygenation of Group VIII Metal Clusters and the Formation of Metalloxocubes M13O8+

The diversity of valence and bonding of transition metals makes their oxidation processes perplexing at reduced sizes. Here we report a comprehensive study on the oxidation reactions of rhodium clusters Rhn± (n = 3-30) and find that Rh3,4O4+, Rh5-7O6+, and Rh8-13O8+ always dominate the mass distributions showing size-dependent ladder oxygenation which is closely associated with the O-binding modes. While the Rh8-13O8+ clusters display a μ3-O binding mode (hollow site adsorption), Rh3-4O4+ and Rh5-7O6+ favor the μ2-O binding mode (edge-site adsorption) or a mixture of the two modes. The μ3-O binding mode is inclined to yield a cubic Rh13O8, while the μ2-O binding mode gives rise to oxygen-bridge protection for the metal clusters. Such ladder oxidation was also observed for Ptn+, Fen+, Con+, and Nin+ clusters. We propose a three-dimensional diagram for the oxidation states and O-binding modes of metals, and highlight the metalloxocubes M13O8+ for cluster-genetic materials.

Cryo infrared spectroscopy of N2 adsorption onto bimetallic rhodium-iron clusters in isolation.

We investigated the N2 adsorption behavior of bimetallic rhodium-iron cluster cations [RhiFej(N2)m]+ by means of InfraRed MultiplePhotoDissociation (IR-MPD) spectroscopy in comparison with density functional theory (DFT) modeling. This approach allows us to refine our kinetic results [Ehrhard et al., J. Chem. Phys. (in press)] to enhance our conclusions. We focus on a selection of cluster adsorbate complexes within the ranges of i = j = 3-8 and m = 1-10. For i = j = 3, 4, DFT suggests alloy structures in the case of i = j = 4 of high (D2d) symmetry: Rh-Fe bonds are preferred instead of Fe-Fe bonds or Rh-Rh bonds. N2 adsorption and IR-MPD studies reveal strong evidence for preferential adsorption to Rh sites and mere secondary adsorption to Fe. In some cases, we observe adsorption isomers. With the help of modeling the cluster adsorbate complex [Rh3Fe3(N2)7]+, we find clear evidence that the position of IR bands allows for an element specific assignment of an adsorption site. We transfer these findings to the [Rh4Fe4(N2)m]+ cluster adsorbate complex where the first four N2 molecules are exclusively adsorbed to the Rh atoms. The spectra of the larger adsorbates reveal N2 adsorption onto the Fe atoms. Thus, the spectroscopic findings are well interpreted for the smaller clusters in terms of computed structures, and both compare well to those of our accompanying kinetic study [Ehrhard et al., J. Chem. Phys. (in press)]. In contrast to our previous studies of bare rhodium clusters, the present investigations do not provide any indication for a spin quench in [RhiFej(N2)m]+ upon stepwise N2 adsorption.

Adsorption Forms of NO on Iridium-Doped Rhodium Clusters in the Gas Phase Revealed by Infrared Multiple Photon Dissociation Spectroscopy.

The adsorption of an NO molecule on a cationic iridium-doped rhodium cluster, Rh5Ir+, was investigated by infrared multiple photon dissociation spectroscopy (IRMPD) of Rh5IrNO+·Arp complexes in the 300-2000 cm-1 spectral range, where the Ar atoms acted as a messenger signaling IR absorption. Complementary density functional theory (DFT) calculations predicted two near-isoenergetic structures as the putative global minimum: one with NO adsorbed in molecular form in the on-top configuration on the Ir atom in Rh5Ir+, and one where NO is dissociated with the O atom bound to the Ir atom in the on-top configuration and the N atom on a hollow site formed by three Rh atoms. A comparison between the experimental IRMPD spectrum of Rh5IrNO+ and calculated spectra indicated that NO mainly adsorbs molecularly on Rh5Ir+, but evidence was also found for structures with dissociatively adsorbed NO. The estimated fraction of Rh5IrNO+ structures with dissociatively adsorbed NO is approximately 10%, which was higher than that found for Rh6+, but lower than that for Ir6+. The DFT calculations indicated the existence of an energy barrier in the NO dissociation pathway that is exothermic with respect to the reactants, which was considered to prevent NO from dissociating readily on Rh5Ir+. The height of the barrier is lower than that for NO dissociation over Rh6+, which is attributed to the higher binding energy of atomic O to the Ir atom in Rh5Ir+ than to a Rh atom in Rh6+.

Heterometallic rhodium clusters as electron reservoirs: Chemical, electrochemical, and theoretical studies of the centered-icosahedral [Rh12E(CO)27]n- atomically precise carbonyl compounds.

In this paper, we present a comparative study of the redox properties of the icosahedral [Rh12E(CO)27]n- (n = 4 when E = Ge or Sn and n = 3 when E = Sb or Bi) family of clusters through in situ infrared spectroelectrochemistry experiments and density functional theory computational studies. These clusters show shared characteristics in terms of molecular structure, being all E-centered icosahedral species, and electron counting, possessing 170 valence electrons as predicted by the electron-counting rules, based on the cluster-borane analogy, for compounds with such metal geometry. However, in some cases, clusters of similar nuclearity, and beyond, may show multivalence behavior and may be stable with a different electron counting, at least on the time scale of the electrochemical analyses. The experimental results, confirmed by theoretical calculations, showed a remarkable electron-sponge behavior for [Rh12Ge(CO)27]4- (1), [Rh12Sb(CO)27]3- (3), and [Rh12Bi(CO)27]3- (4), with a cluster charge going from -2 to -6 for 1 and 3 and from -2 to -7 for cluster 4, making them examples of molecular electron reservoirs. The [Rh12Sn(CO)27]4- (2) derivative, conversely, presents a limited ability to exist in separable reduced cluster species, at least within the experimental conditions, while in the gas phase it appears to be stable both as a penta- and hexa-anion, therefore showing a similar redox activity as its congeners. As a fallout of those studies, during the preparation of [Rh12Sb(CO)27]3-, we were able to isolate a new species, namely, [Rh11Sb(CO)26]2-, which presents a Sb-centered nido-icosahedral metal structure possessing 158 cluster valence electrons, in perfect agreement with the polyhedral skeletal electron pair theory.

Cryokinetics and spin quenching in the N2 adsorption onto rhodium cluster cations

ABSTRACT The kinetics of N2 adsorption onto gas phase Rh i + clusters (i = 5–15) within 26 K He buffer gas have been investigated by Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry, with particular focus to clusters i = 5,6,7,9. The adsorption limits m = mmax of [Rh i (N2) m ]+ clusters strongly depend on the size of the clusters. Some Rh i + clusters reveal an intermittent adsorption limit mx , which implies a rough cluster surface. Most of the clusters reveal smooth surfaces, which consist of Rh atoms with similar next neighbour coordination. The kinetic fits and relative rate constants reveal the sequential adsorption of N2 as the only significant reaction channel. In some cases, we find N2 desorption steps at high N2 loads indicating adsorption–desorption equilibria. For these steps, we determined Gibbs energies from the relative rate constants. We observed a significant reluctance towards N2 adsorption for the Rh5 + cluster, indicated by significant desorption at low N2 coverage. Our DFT calculations model the energetics of the N2 adsorption onto Rh5 + in terms of coverage dependent ‘spin valley’ curves, and we conclude that an adsorption induced spin quenching takes place upon considerable N2 coverage, m > 4.

Interactions between water and rhodium clusters: molecular adsorption versus cluster adsorption.

Understanding metal-water interactions and hydrogen-bonding in water droplets is important but highly challenging. Various transition metals may serve as effective coordination centers to water; however, not in all cases is water bonded to a metal center as single molecules. We report here the observations of gas-phase rhodium clusters and their interactions with water. A series of rhodium-water clusters, Rhn±,0(H2O)m (n = 3-30, m = 1-5), with isotope labels were detected by mass spectrometry after exposure to different water concentrations, among which Rh8+(H2O)4 and Rh9+(H2O)3 were prominent in the mass distributions, showing a size-dependent preference of water adsorption on rhodium clusters. Comprehensive density functional theory calculations reveal that the lowest energy structure of Rh9+(H2O)3 possesses a hydrogen-bonded cyclic (H2O)3 water trimer on the top of a tri-capped Rh9+ trigonal prism. The tri-capped Rh9+ trigonal prism and the cyclic (H2O)3 water trimer match in sizes, charge distributions, and orbital symmetries to form effective electrostatic cluster-cluster interactions. In contrast, Rh8+(H2O)4 contains four water molecules separately attached to a bi-capped octahedron, Rh8+, at four corners via single-molecule adsorption. The difference between covalent molecular adsorption and electrostatic cluster-cluster interaction in these two proto-typical rhodium hydrates is further demonstrated by detailed natural bonding orbital, electrostatic surface potential, and charge decomposition analyses.

Decomposition of nitric oxide by rhodium cluster cations at high temperatures.

Decomposition reactions of NO molecules on gas-phase Rhn+ (n = 6-9) clusters were investigated by gas-phase thermal desorption spectrometry and density functional theory calculations. We found that NO adsorbs on the clusters, forming RhnNxOx+ at room temperature. Upon heating, NO desorption was observed below 800 K. Above 800 K, while for n = 7 and 8, each of Rh7N3O3+, Rh7N4O4+, and Rh8N3O3+ was found to release an N2 molecule, no N2 formation was clearly observed for Rh6,9NxOy+. We considered that both Rh7N3O3+ and Rh8N3O3+ have at least two dissociated NO molecules, while Rh6NxOx+ (x = 1-3) has one or less. Our computational results for Rh8N3O3+ suggested that the formation of an N-N bond in the Rh8N3O3+ structure must overcome an energy barrier of ∼2 eV, which is the highest among the suggested possible reaction pathways. These findings suggested that the size-dependent activity of NO decomposition is governed primarily by how NO molecules are adsorbed on Rhn+ clusters, i.e. whether two or more N atoms from dissociated NO molecules exist in the NO adsorbed clusters, and secondly, by the readiness of the N-N bond formation.

Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation.

Dinitrogen activation is known as one of the most challenging subjects in chemistry. A number of well-defined metal complexes, nitrides, and clusters have been studied that show catalysis for dinitrogen activation. However, direct evidence of a complete cleavage of the N≡N triple bond at mild conditions is rather limited to date. Herein, we report a study on the dissociation of N2 on small rhodium clusters assisted by surface plasma radiation. From mass spectrometry observation, a few rhodium nitride clusters with an odd number of nitrogen atoms are produced, such as the Rh3N2m-1+ (m = 1-5) series, indicative of N≡N bond dissociation in the mild plasma atmosphere. Interestingly, Rh3N7+ is identified with outstanding mass abundance among the RhnN2m-1+ products, and its ground-state structure is in the form of Rh3N(N2)3+ by capping a nitrogen atom on the top of Rh3+ plane and hanging three N2 molecules beneath the three Rh atoms respectively, giving rise to a C3v symmetry and excellent stability. We demonstrate the catalysis of such a three-atom rhodium cluster and reveal a dinitrogen activation strategy by thermodynamics- and dynamics- favorable chain reactions of multiple N2 molecules with two rhodium clusters under plasma atmosphere.

Catalytic activity of Rhn/MegO-nanotube for NO adsorption and dissociation: A van der waals density functional study

We have studied the adsorption and dissociation of nitric oxide (NO) on a series of interfaces consisting of a rhodium cluster (Rhn, n = 1–5) and an MgO nanotube, using van der Waals density functional theory calculations and have found that the Rhn clusters can tightly adhere to an MgO (100) nanotube. The change in the configuration of the nanotube substrate induces differences in the energetically preferred structures for some Rhn clusters (n > 2). With an MgO-nanotube substrate, the low dimensional Rhn clusters tend to be stable, and provide additional active sites to the NO. Hence they have an excellent ability to trap and dissociate NO. We see strong hybridization between N/O 2p and Rh 4d. Moreover, NO dissociation is activated, with the transition state below the reference state, indicating the possibility of the NO dissociating. Binding to NO also yields a change in the total magnetic moment of the Rhn/MgO-NT system, and this affects the adsorption energy of NO. Finally, we propose that the morphology of the metal clusters could be directly regulated by the configuration of the nanotube substrate, which encourages us to explore and design various types of nanotubes for different catalytic purposes.