Rh Clusters Research Articles

Investigation of Thermal Stability and Reactivity of Rh Nanoclusters on an Ultrathin Alumina Film

We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the alumina film at 300 K, had an fcc phase and grew in the (100) orientation; the annealing altered the cluster structure little—the lattice parameter decreased by a factor <2%—but the cluster morphology significantly. With elevated temperature, small clusters (diameter ≤1.5 nm) decreased little in size; in contrast, large clusters (diameter ≥2.0 nm) varied in a complex manner—their mean diameter decreased to about 1.5 nm on annealing to 450 K, despite their similar height, while it increased to above 2.0 nm at temperature ≥570 K. This atypical decrease in size was governed predominantly by energetics. Such a reduced size enhanced the total surface area as well as the reactivity of the clusters toward methanol decomposition, so increased the production of D2 (H2) and CO from decomposed methanol-d4 (or methanol). The result implies a higher temperature tolerance for Rh clusters on the alumina film and a practical approach to prepare small Rh clusters with high reactivity.

Structure and Catalytic Behavior of Alumina Supported Bimetallic Au-Rh Nanoparticles in the Reduction of NO by CO

Alumina-supported bimetallic AuRh catalysts, as well as monometallic reference catalysts, were examined with regard to their structural and catalytic properties in the reduction of NO by CO. Depending on the molar ratio of Au:Rh, the nanoparticles prepared by borohydride co-reduction of corresponding metal salt solutions had a size of 3.5–6.7 nm. The particles consisted of well-dispersed noble metal atoms with some enrichment of Rh in their surface region. NO conversion of AuRh/Al2O3 shifted to lower temperatures with increasing Rh content, reaching highest activity and highest N2 selectivity for the monometallic Rh/Al2O3 catalyst. This behavior is attributed to the enhanced adsorption of CO on the bimetallic catalyst resulting in unfavorable cationic Rh clusters Rh+-(CO)2. Doping with ceria of AuRh/Al2O3 and Rh/Al2O3 catalysts increased the surface population of metallic Rh sites, which are considered most active for the reduction of NO by CO and enhancement of the formation of intermediate isocyanate (-NCO) surface species and their reaction with NO to form N2 and CO2.

Tools for Prescreening the Most Active Sites on Ir and Rh Clusters toward C-H Bond Cleavage of Ethane: NBO Charges and Wiberg Bond Indexes.

B3LYP calculations were carried out to study the insertion of iridium (Ir) and rhodium (Rh) clusters into a C–H bond of ethane, which is often the rate-limiting step of the catalytic cycle of oxidative dehydrogenation of ethane. Our previous research on Ir catalysis correlates the diffusivity of the lowest unoccupied molecular orbital of the Ir clusters and the relative activities of the various catalytic sites. The drawback of this research is that the molecular orbital visualization is qualitative rather than quantitative. Therefore, in this study on C–H bond activation by the Ir and Rh clusters, we conducted analyses of natural bond orbital (NBO) charges and Wiberg bond indexes (WBIs), both of which are not only quantitative but also independent of the basis sets. We found strong correlation between the NBO charges, the WBIs, and the relative activities of the various catalytic sites on the Ir and Rh clusters. Analyses of the NBO charges and the WBIs provide a fast and reliable means of prescreening the most active sites on the Ir and Rh clusters and potentially on other similar transition-metal clusters that activate the C–H bonds of ethane and other light alkanes.

Escaping scaling relationships for water dissociation at interfacial sites of zirconia-supported Rh and Pt clusters.

Water dissociation is an important reaction involved in many industrial processes. In this computational study, the dissociation of water is used as a model reaction for probing the activity of interfacial sites of globally optimized ZrO2 supported Pt and Rh clusters under the framework of density functional theory. Our findings demonstrate that the perimeter sites of these small clusters can activate water, but the dissociation behavior varies considerably between sites. It is shown that the studied clusters break scaling relationships for water dissociation, suggesting that these catalysts may achieve activities beyond the maximum imposed by such relations. Furthermore, we observed large differences in the thermodynamics of the water dissociation reaction between global minimum and near-global minimum isomers of the clusters. Overall, our results highlight the uniqueness of interfacial sites in catalytic reactions and the need for developing new concepts and tools to deal with the associated complexity.

Highly Active Catalysts Based on the Rh4(CO)12 Cluster Supported on Ce0.5Zr0.5 and Zr Oxides for Low-Temperature Methane Steam Reforming

Syngas and Hydrogen productions from methane are industrially carried out at high temperatures (900 °C). Nevertheless, low-temperature steam reforming can be an alternative for small-scale plants. In these conditions, the process can also be coupled with systems that increase the overall efficiency such as hydrogen purification with membranes, microreactors or enhanced reforming with CO2 capture. However, at low temperature, in order to get conversion values close to the equilibrium ones, very active catalysts are needed. For this purpose, the Rh4(CO)12 cluster was synthetized and deposited over Ce0.5Zr0.5O2 and ZrO2 supports, prepared by microemulsion, and tested in low-temperature steam methane reforming reactions under different conditions. The catalysts were active at 750 °C at low Rh loadings (0.05%) and outperformed an analogous Rh-impregnated catalyst. At higher Rh concentrations (0.6%), the Rh cluster deposited on Ce0.5Zr0.5 oxide reached conversions close to the equilibrium values and good stability over long reaction time, demonstrating that active phases derived from Rh carbonyl clusters can be used to catalyze steam reforming reactions. Conversely, the same catalyst suffered from a fast deactivation at 500 °C, likely related to the oxidation of the Rh phase due to the oxygen-mobility properties of Ce. Indeed, at 500 °C the Rh-based ZrO2-supported catalyst was able to provide stable results with higher conversions. The effects of different pretreatments were also investigated: at 500 °C, the catalysts subjected to thermal treatment, both under N2 and H2, proved to be more active than those without the H2 treatment. In general, this work highlights the possibility of using Rh carbonyl-cluster-derived supported catalysts in methane reforming reactions and, at low temperature, it showed deactivation phenomena related to the presence of reducible supports.

Reduction Reaction of Nitric Oxide on the Rh5V+ Cluster: A Density Functional Theory Mechanistic Study

Recent experiments on the reaction of metal-doped Rh clusters with NO indicate that V-doping is the most effective for the NO decomposition. The reaction mechanism of NO molecules on Rh5V+ cations ...

Stability and electronic properties of Rh-doped ruthenium clusters and their interaction with NH3 molecule

The stability and electronic properties of the Rh-doped ruthenium clusters and their reactivity towards NH3 molecule have been studied using DFT calculations with the BLYP-D3/SDD level of theory. The results show that the doping of Ru clusters with Rh atom improves the catalytic performances of pure Ru clusters, and the Ru5Rh and Ru7Rh clusters are assumed to be less reactive than their neighbours. The interaction of NH3 with clusters exhibits that the Ru atoms are preferred adsorption sites for the NH3 molecule, and the adsorption takes place between the Ru atom of clusters and the N atom of NH3 molecule. The adsorption energies of NH3 on RunRh clusters are in the range of −101.5 to −218.4 kJ mol−1, suggesting a strong adsorption between both species. Upon adsorption process, the electronic properties of the Ru n Rh clusters were substantially changed. The variation of E g (ΔE g) for the Ru n Rh (n ≥ 7) clusters is very important (ΔE g ≥ 55%), suggesting that these clusters are very sensitive to the NH3 molecule. Hence, these clusters can be employed as nanosensors for the detection of the NH3 gas.

Reaction Behavior of the NO Molecule on the Surface of an Mn Particle (M = Ru, Rh, Pd, and Ag; n = 13 and 55): Theoretical Study of Its Dependence on Transition-Metal Element.

Reaction of NO molecule on M13 and M55 clusters (M = Ru, Rh, Pd, and Ag) was theoretically investigated to elucidate why its reaction behavior depends on the position of metal element in the periodic table. DFT computations show that NO dissociative adsorption occurs on M = Ru and Rh, NO molecular adsorption occurs on M = Pd, and NO dimerization occurs on M = Ag, which agree with experimental findings. The d-band center and d-band top become lower in energy following the order Ru > Rh > Pd > Ag; this is one of the characteristic features of the periodic table. In the Ag cluster, the valence band-top consists of Ag 5s orbital and its energy is higher than the d-band top of Pd. For NO dissociative adsorption, the M-N and M-O bond strengths are crucially important at the transition state and the product, to which the metal d orbital contributes very much. Ru and Rh clusters have a high energy d-band center and d-valence band top, leading to the formation of strong M-N and M-O bonds. Pd and Ag clusters have a low energy d-band center and d-band top, leading to the formation of weak M-N and M-O bonds. Because the Ag cluster has a high energy 5s valence band that can overlap well with the π* + π* MO of ONNO (NO dimer) moiety due to the same symmetry, charge transfer (CT) occurs from the Ag cluster to the π* + π* MO, which is indispensable for NO dimerization. The 4d-valence band top of Ru and Rh clusters does not fit to the π* + π* MO because of the different symmetry. Though the d-valence band top of the Pd cluster can overlap with the π* + π* MO, its energy is low, which is not good for the CT. Thus, the reactivity of metal cluster for NO is determined by the energy and type (4d or 5s) of the valence band top, which both depend on the position of element in the periodic table; accordingly, Ru and Rh clusters are reactive for NO dissociative adsorption, the Ag cluster is reactive for NO dimerization, but the Pd cluster is not reactive for both and only NO molecular adsorption is possible.

Low Temperature Infrared Study of Carbon Monoxide Adsorption on Rh/CeO2

Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO2 is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas shift reaction (WGSR), and ethanol steam reforming. In this work, the adsorption of CO is studied by infrared (IR) spectroscopy, over CeO2 and 0.6 at. % Rh/CeO2 at a temperature range of 90 to 300 K. CeO2 is in the form of nanoparticles with sizes between 5 and 10 nm and exposing predominantly {111} surface termination in addition to non-negligible fraction of the {100} termination, determined from high resolution transmission electron microscopy (HRTEM). The as prepared Rh/CeO2 contained metallic Rh as well Rh cations in higher oxidation states. At 90 K two IR bands were observed at 2183–2186 and 2161–2163 cm−1, with the former saturating first. The 2163 cm−1 peak was more sensitive to CO pressure than the 2186 cm−1. Heating resulted in the depopulation of the 2163 cm−1 before the 2186 cm−1 peak. The desorption energy computed, assuming a first-order desorption kinetic, was found to be 0.35 eV for the 2186 cm−1 and 0.30 for the 2163 cm−1 IR peak (+/−0.05 eV). The equilibrium constant at 90 K was computed equal to 1.83 and 1.33 Torr−1 for the 2183 and 2161 cm−1, respectively. CO adsorption at 90 K on Rh/CeO2 resulted (in addition to the bands on CeO2) in the appearance of a broad band in the 2110–2130 cm-1 region that contained two components at 2116 and 2126 cm−1. The high frequency of this species is most likely due to adsorption on Rh clusters with very small sizes. The desorption energy of this species was found to be equal to 0.55 eV (+/−0.05 eV). Heating the CO covered Rh/CeO2 surface accelerated the disappearance of CO species over CeO2 and resulted in the appearance of CO2 bands (at about 150 K) followed by carbonate species. At 300 K, the surface was mainly composed of carbonates.

Atomically Dispersed Rh Active Sites on Oxide Supports with Controlled Acidity for Gas-Phase Halide-Free Methanol Carbonylation to Acetic Acid

Acetic acid (AA) is an important bulk commodity chemical produced primarily via methanol carbonylation using homogeneous organometallic catalysts in liquid phase reactors with halides that participate in each catalytic turnover. Here we report a heterogeneous, halide-free, gas-phase process for methanol carbonylation to AA using a catalyst consisting of atomically dispersed rhodium (Rh) active sites on an acidic support. It is demonstrated that active site pairs consisting of atomically dispersed Rh and support acid sites enable highly selective AA production, whereas Rh clusters drive methanol decomposition to CO and CO2. Through a comparison of methanol carbonylation over atomically dispersed Rh species on γ-Al2O3, ZrO2, and Na-modified ZrO2, it was identified that by decreasing the concentration of acidic sites, the production of byproduct dimethyl ether could be minimized. After the acidity of the support was tuned by depositing 5 wt % Na on ZrO2 that contained atomically dispersed Rh, AA selectivity ...

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.

The kinetic effect of H2O pressure on CO hydrogenation over different Rh cluster sizes

The effect of H2O pressure (PH2O) on CH4 formation turnover rate (TOFCH4) was evaluated as a function of Rh cluster size. In-situ DRIFTS and kinetic measurements were used to study the CO hydrogenation at methanation conditions on two Rh/Al2O3 catalysts with different cluster sizes (Rh-1nm and Rh-3nm). It was observed a significant effect of PH2O on TOFCH4 over Rh-1nm catalysts, while the rate on Rh-3nm resulted insensible to the H2O pressure. The mean Rh cluster sizes were estimated by H2 chemisorption, TEM and XPS analyses. It is confirmed the structural sensitivity of the CO methanation reaction on supported Rh catalysts as the TOFCH4 resulted significantly higher in the larger clusters. Nonetheless, lower apparent activation energy was measured in smaller Rh clusters, which is successfully explained by the energies involved into the parameter of the proposed L-H kinetic model. CO adsorption isotherms were obtained from in-situ DRIFTS experiments at 280–340 °C. The enthalpy and entropy values for CO adsorption indicate that the CO binds more strongly on the low-coordinated surface of Rh atoms. In-situ DRIFTS results demonstrate that the CO coverage is unaffected by PH2 and PH2O, regardless of Rh dispersion, thus excluding H* and H2O* as most abundant surface intermediates of the Kinetics. The infrared and kinetic measurements on both catalysts are consistent with a mechanism of CO bond dissociation assisted by H*. Data are well represented by a single-site-L-H model that successfully captures the effect of PH2, PCO and PH2O on TOFCH4. This model contains a parameter (k’) that represents the ratio between the rates of C* removal by O* and H* at Rh surface and explains the stronger kinetic effect of PH2O on TOFCH4 observed over smaller Rh clusters.

From Mononuclear Complexes to Molecular Nanoparticles: The Buildup of Atomically Precise Heterometallic Rhodium Carbonyl Nanoclusters

Chemical research in synthesizing metal nanoparticles has been a major topic in the last two decades, as nanoparticles can be of great interest in many fields such as biology, catalysis, and nanotechnology. However, as their chemical and physical properties are size-dependent, the reliable preparation of nanoparticles at a molecular level is highly desirable. Despite the remarkable advances in recent years in the preparation of thiolate- or p-MBA or PA-protected gold and silver nanoclusters ( p-MBA = p-mercaptobenzoic acid; PA = phenylalkynyl), as well as the large palladium clusters protected by carbonyl and phosphine ligands that initially dominated the field, the synthesis of monodispersed and atomically precise nanoparticles still represents a great challenge for chemists. Carbonyl cluster compounds of high nuclearity have become more and more part of a niche chemistry, probably owing to their handling issues and expensive synthesis. However, even in large size, they are known at a molecular level and therefore can play a relevant role in understanding the structures of nanoparticles in general. For instance the icosahedral pattern, proper of large gold nanoparticles, is also found in some Au-Fe carbonyl cluster compounds. Rh clusters in general can also be employed as precursors in homo- and heterogeneous catalysis, and the possibility of doping them with other elements at the molecular level is an important additional feature. The fact that they can be obtained as large crystalline species, with dimensions of about 2 nm, allows one to place them not only in the nanometric regime, but also in the ultrafine-metal-nanoparticle category, which lately has been attracting growing attention. In fact, such small nanoparticles possess an even higher density of active catalytic sites than their larger (up to 100 nm) equivalents, hence enhancing atom efficiency and reducing the cost of precious-metal catalysts. Finally, the clusters' well-defined morphology could, in principle, contribute to expand the studies on the shape effects of nanocatalysts. In this Account, we want to provide the scientific community with some insights on the preparation of rhodium-containing carbonyl compounds of increasing nuclearity. Among them, we present the synthesis and molecular structures of two new heterometallic nanoclusters, namely, [Rh23Ge3(CO)41]5- and [Rh16Au6(CO)36]6-, which have been obtained by reacting a rhodium-cluster precursor with Ge(II) and Au(III) salts. The growth of such clusters is induced by redox mechanisms, which allow going from mononuclear complexes up to clusters with over 20 metal atoms, thus entering the nanosized regime.

Fully Dispersed Rh Ensemble Catalyst To Enhance Low-Temperature Activity.

Minimizing the use of precious metal catalysts is important in many applications. Single-atom catalysts (SACs) have received much attention because all of the metal atoms can be used for surface reactions. However, SACs cannot catalyze some important reactions that require ensemble sites. Here, Rh catalysts were prepared by treating 2 wt % Rh/CeO2 hydrothermally at 750 °C for 25 h. Nearly 100% dispersion was obtained, but the surface Rh atoms were not isolated (denoted as ENS). They catalyzed the oxidation of C3H6 or C3H8 at low temperatures, but these oxidations did not occur on the Rh SAC. When the simultaneous oxidation of CO, C3H6, and C3H8 was performed, the T20 (temperature at conversion 20%) for CO oxidation increased significantly from 40 °C for sole CO oxidation to 180 °C on SAC due to the competitive adsorption of hydrocarbons. However, T20 increased much less on ENS, from 60 to 100 °C. ENS exhibited superior activity for low-temperature oxidation. During hydrothermal treatment for 25 h, the Rh size initially increased from 2.3 to 6.7 nm then decreased to 0.9 nm. The surface hydroxyl groups formed on the catalyst surface help detach Rh atoms from Rh clusters, while preventing the reaggregation of dispersed Rh atoms into Rh clusters. This fully dispersed catalyst would have maximum atom-efficiency while catalyzing various surface reactions.

Highly active subnanometer Rh clusters derived from Rh-doped SrTiO3 for CO2 reduction

Sub-nanometer Rh clusters derived from Rh-doped SrTiO3, demonstrated by in-situ X-ray Diffraction (XRD) and X-ray Absorption Fine Structure (XAFS) measurements, are applied as highly active catalysts for CO2 reduction. Compared to the supported Rh/SrTiO3, the catalyst synthesized by a doping-segregation method exhibits a higher space-time yield (STY) to CO with a selectivity of 95% for CO2 reduction by hydrogen; it also shows a higher activity with a larger turnover frequency (TOF) for CO2 reduction by ethane. According to the in-situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) experiments, the higher CO selectivity for CO2 hydrogenation is attributed to the lower CO binding strength resulted by the strong interactions (e.g., charge transfer) between Rh atoms and the oxide support with surface defects. The superior activity is suggested to be originated from the cooperative effect between the highly dispersed sub-nanometer Rh clusters for efficient dissociation of H2/C2H6 and the reconstructed SrTiO3 with oxygen vacancies for preferential adsorption/activation of CO2. The doping-segregation method provides a unique opportunity to tune the size of active metal clusters and the physicochemical properties of the oxide support, offering the potential for applications in a variety of chemical reactions.

Icosahedral Ir, Rh, Pt, and Cu nanoclusters into gold vapor environment: Thermodynamic and structural analysis of the formed core@shell nanoclusters using MD simulations

In this study, metal icosahedral clusters of Ir, Rh, Cu, or Pt have been positioned into gold vapor environment and the condensation process of gold vapor atoms on the metal nanoclusters has been investigated using molecular dynamics (MD) simulations. In this process, we have investigated some thermodynamic and structural properties of the formed core@shell structures at 300 K by increasing the simulation time. In accordance with previous experimental studies, our thermodynamic investigations showed the positioning of Cu, Ir, Rh, and Pt atoms in the gold clusters core is favorable. The positioning of Pt atoms in the gold clusters core is also more favorable than other atoms. We have also discussed the trend of stability of the formed clusters using the different factors such as the cohesive energy, core structure, and surface energy. Our structural examinations also showed that the initial icosahedral structure of the metal clusters has been disappeared during the condensation process; however, the produced core@shell clusters had hcp-like structures.

Theoretical Insight into Core–Shell Preference for Bimetallic Pt-M (M = Ru, Rh, Os, and Ir) Cluster and Its Electronic Structure

PtmMn (M = Ru, Rh, Os, and Ir; m+n = 38 and 55) clusters are systematically investigated using the DFT method. In an octahedral 38-atom cluster, core–shell structure M6@- Pt32 with M6 core and Pt32 shell is stable for Pt–Rh and Pt–Ir combinations but is not for Pt–Ru and Pt–Os combinations. In a 55-atom cluster, icosahedral M13@Pt42 structure is stable for all Pt-M combinations, indicating that a large cluster is more preferable to stabilizing the core–shell structure than a small cluster. The difference in cohesive energy (Ecoh) between M13 and Pt13 and the distortion energy {Edis(M13)} of M13 are parallel to the segregation energy (Eseg), indicating that these are important factors for stabilizing M13@Pt42. One more crucially important factor is the interaction energy (Eint) between M13 core and Pt42 shell, because Eint is parallel to Eseg and its absolute value is much larger than those of Edis(M13) and Edis(Pt42). The Eint depends on the energy gap between LUMO of M13 core and HOMO of Pt42 shell, indi...

Stability, electronic and magnetic properties of small M-doped rhodium clusters

The geometries, relative stabilities, electronic and magnetic properties of small RhnM (M = Rh, Al, Co, V) (n ≤ 8) clusters were investigated by employing the density functional theory (DFT). CALYPSO method (structure analysis by a particle swarm optimization algorithm) was used to search the structures. The doping of V atom in Rh clusters can enhance the stability of host clusters due to the strong Rh-V interaction. The magnetic moment of Rh clusters can be enhanced by doping Co atom because of the large local magnetic moment of Co atom and the ferromagnetic coupling of Co and Rh atoms. Magnetic moments of the studied clusters are dominantly contributed by d electrons of metal atoms.

Structure, stability, and electronic and magnetic properties of small Rh n Mn (n = 1–12) clusters

The structure, stability, and magnetic properties of Rhn+1 and Rh n Mn clusters (n = 1–12) are systematically investigated within the framework of the generalized gradient approximation density-functional theory (DFT-GGA). The overall structural evolutionary trend shows that the ground state structures of the Rh n Mn are similar to that of the corresponding pure rhodium clusters except for n = 7, 9, 12, while the Rh7Mn, Rh9Mn and Rh12Mn clusters occur substantially geometry reconstruction. The binding energy of Rh n Mn is decreased with the substitution of one Mn atom, thus indicating that Mn doping can weaken the stability of the Rh clusters. The fragmentation energy and the second-order difference energy of the ground-state Rh n Mn clusters imply that the Rh3Mn, Rh5Mn, Rh8Mn and Rh11Mn clusters are more stable than their neighbors. Compared with corresponding pure Rh n clusters, the Mn atom doping increases the total magnetic moment of the Rh n Mn clusters in various degrees, and the physics origin of such a phenomenon is analyzed in detail based on the average bond length, magnetic coupling, and density of state.

Adsorption Forms of NO on Rhn+ (n = 6–16) Revealed by Infrared Multiple Photon Dissociation Spectroscopy

The adsorption of NO on cationic Rh clusters, Rhn+ (n = 6−16), was studied by IRMPD spectroscopy using FELIX in combination with DFT calculations. The IRMPD spectra show that NO adsorbs molecularly on an on-top site of Rhn+ for all n studied, while for n = 7, 12, 13 and 14 evidence is found for a second, bridging, adsorption site. Indeed, the DFT calculations suggest that molecular NO adsorption on a bridge site is more stable than on an on-top site for n = 7. Part of the NO adsorbs dissociatively on Rhn+, and the ratio of dissociative adsorption depends on the size, n. For Rhn+ (n = 6, 8, 9), the dissociative form of NO is predicted more favorable than the molecular form by the DFT calculations, but experimentally observed ratios of dissociative adsorption are less than predicted. The activation barrier existing between the molecular and dissociative adsorption is considered to hinder the NO dissociation.