Thermal Desorption and Reaction of NO Adsorbed on Rhodium Cluster Ions Studied by Thermal Desorption Spectroscopy

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.

Size-dependent reactivity of Rh cationic clusters to reduce NO by CO in the gas phase at high temperatures

The reactivity of the reduction of NO pre-adsorbed on Rh2-9+ clusters by CO was investigated using a combination of an alternate on-off gas injection method and thermal desorption spectrometry. The reduction of RhnNxOy+ clusters by CO was evaluated by varying the CO concentration at T = 903 K. Among the RhnNxOx+ clusters, the Rh3N2O2+ cluster exhibited the highest reduction activity, whereas the other clusters, Rh2,4-9NxOx+, showed lower reactivity. Density functional theory (DFT) calculations for Rh3+ and Rh6+ revealed that the rate-determining step for NO reduction in the presence of CO was NO bond dissociation through the kinetics analysis using the RRKM theory. The reduction of Rh3N2O2+ is kinetically preferable to that of Rh6N2O2+. The DFT results were in qualitative agreement with the experimental results.

CO Oxidation Promoted by NO Adsorption on RhMn2O3- Cluster Anions

CO oxidation represents an important model reaction in the gas phase to provide a clear structure-reactivity relationship in related heterogeneous catalysis. Herein, in combination with mass spectrometry experiments and quantum-chemical calculations, we identified that the RhMn2O3- cluster cannot oxidize CO into gas-phase CO2 at room temperature, while the NO preadsorbed products RhMn2O3-[(NO)1,2] are highly reactive in CO oxidation. This discovery is helpful to get a fundamental understanding on the reaction behavior in real-world three-way catalytic conditions where different kinds of reactants coexist. Theoretical calculations were performed to rationalize the crucial roles of preadsorbed NO where the strongly attached NO on the Rh atom can greatly stabilize the products RhMn2O2-[(NO)1,2] during CO oxidation and at the same time works together with the Rh atom to store electrons that stay originally in the attached CO2- unit. The leading result is that the desorption of CO2, which is the rate-determining step of CO oxidation by RhMn2O3-, can be greatly facilitated on the reactions of RhMn2O3-[(NO)1,2] with CO.

Hydrogen Storage Mechanism by Gas-Phase Vanadium Cluster Cations Revealed by Thermal Desorption Spectrometry

The adsorption of hydrogen on gas-phase vanadium cluster cations, Vn+ (n = 3-14), at 300 K and desorption of hydrogen from hydride clusters, VnHm+, upon heating were observed experimentally by combined thermal desorption spectrometry and mass spectrometry analyses. The ratio m/n was approximately 1.3 for all n values at 300 K, which was reduced to approximately zero at 1000 K. For n = 4, stable cluster geometries of V4Hm+ (m = 0, 2, 4, and 6) were investigated by DFT calculations, revealing that V4 adopted a trigonal pyramidal structure and the H atoms adsorbed mainly on the μ2 bridge sites. The adsorption reaction pathway of one H2 molecule on V4+ was also investigated. The experimentally estimated desorption energies of the H2 molecules were consistent with their calculated binding energies. Among the observed hydride clusters, V6H8+ was found to be significantly thermally durable, probably because of its close-packed octahedral V6 core structure, with H atoms occupying all hollow sites.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

NO Bond Cleavage on Gas-Phase Irn+ Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

The adsorption forms of NO on Ir_n⁺ (n = 3-6) clusters were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. Spectral features indicative both for molecular NO adsorption (the NO stretching vibration in the 1800-1900 cm^-1 range) and for dissociative NO adsorption (the terminal Ir-O vibration around 940 cm^-1) were observed, elucidating the co-existence of molecular and dissociative adsorption of NO. In all calculated structures for molecular adsorption, NO is adsorbed via the N atom on on-top sites. For dissociative adsorption, the O atom adsorbs exclusively on on-top sites (μ₁) of the clusters, whereas the N atom is found on either a bridge (μ₂) or a hollow (μ₃) site. For Ir₅⁺ and Ir₆⁺, the N atom is also found on the on-top sites. The observed propensity for NO dissociation on Ir_n⁺ (n = 3-6) is higher than that for Rh₆⁺, which can be explained by the higher metal-oxygen bond strengths for iridium.

What Determines the Drastic Reactivity of Nbn+ Clusters with Nitric Oxide under Thermalized Conditions?

We report an in-depth study of the adsorption and reaction of NO with cationic Nb_n⁺ (n = 1-20) clusters under thermalized conditions in a laminar flow tube reactor in tandem with a customized triple quadrupole mass spectrometer (FT-TQMS). It is found that the small-sized Nb_n⁺ clusters (2 ≤ n ≤ 7) readily react with NO giving rise to dominant fragmentation products pertaining to the loss of a stable diatomic molecule NbO or NbN. In contrast, the reaction products of larger-sized clusters (n ≥ 10) proceed through diverse channels, including NO adsorption, N₂/N₂O release, and even NO₂ formation. These experimental observations provided the incentive for us to dig deep into the reaction mechanism with the help of DFT calculations. In contrast to the NO-donation coordination in transition metal complexes, here the cationic Nb_n⁺ clusters exhibit dominant electronic donation in initiating the reactions with NO molecules. We fully demonstrated the reaction rate constants, compared the reaction energy diagram of typical Nb_n⁺ clusters, and unveiled the distinct interaction mechanism of niobium clusters available for NO activation and conversion.

Zooming in on the initial steps of catalytic NO reduction using metal clusters

The study of reactions relevant to heterogeneous catalysis on the surface of well-defined metal clusters with full control over the number of consituent atoms and elemental composition can lead to a detailed insight into the interactions between metal and reactants. We here review experimental and theoretical studies involving the adsorption of NO molecules on mostly rhodium-based clusters under near-thermal conditions in a molecular beam. We show how IR spectrosopic characterization can give information on the binding nature of NO to the clusters for at least the first three NO molecules. The complementary technique of thermal desorption spectrometry reveals at what temperatures multiple NO molecules on the cluster surface desorb or combine to form rhodium oxides followed by N₂ elimination. Variation of the cluster elemental composition can be a powerful method to identify how the propensity of the critical first step of NO dissociation can be increased. The testing of such concepts with atomic detail can be of great help in guiding the choices in rational catalyst design.

The study of reactions relevant to heterogeneous catalysis on metal clusters with full control over the number of constituent atoms and elemental composition can lead to a detailed insight into the interactions governing catalytic functionality.

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⁺·Ar_p 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⁺.

Decomposition of nitric oxide by rhodium cluster cations at high temperatures

Decomposition reactions of NO molecules on gas-phase Rh_n⁺ (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 Rh_nN_xO_x⁺ 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 Rh_6,9N_xO_y⁺. We considered that both Rh₇N₃O₃⁺ and Rh₈N₃O₃⁺ have at least two dissociated NO molecules, while Rh₆N_xO_x⁺ (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 Rh_n⁺ 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.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

Theoretical Study of NO Dissociative Adsorption onto 3d Metal Particles M55 (M = Fe, Co, Ni, and Cu): Relation between the Reactivity and Position of the Metal Element in the Periodic Table

NO dissociative adsorption onto 3d metal particles M55 (M = Fe, Co, Ni, and Cu) was investigated theoretically using density functional theory computations. A transition state exists at higher energy in the Cu case but at lower energy in the Fe, Co, and Ni cases than the reactant (sum of M55 and NO), indicating that Cu55 is not reactive for NO dissociative adsorption because NO desorption occurs more easily than the N-O bond cleavage in this case, but Fe55, Co55, and Ni55 are reactive because NO desorption needs a larger destabilization energy than the N-O bond cleavage. This result agrees with the experimental findings. The energy of transition state E(TS) becomes higher in the order of Fe < Co < Ni ≪ Cu. Exothermicity E exo (relative energy to the reactant) decreases in the order of Fe > Co > Ni ≫ Cu. These results indicate that the reactivity for NO dissociative adsorption decreases kinetically and thermodynamically in this order. In addition, the E(TS) and E exo values show that 3d metal particles are more reactive than 4d metal particles when a comparison is made in the same group of the periodic table. Charge transfer (CT) from the metal particle to NO increases as the reaction proceeds. The CT quantity to NO at the TS increases in the order of Cu < Ni < Co < Fe, identical to the increasing order of reactivity. The negative charges of the N and O atoms of the product (N and O adsorbed M55) increase in the order of Ni < Co < Cu < Fe, identical to the increasing order of E exo except for the Cu case; in the Cu case, the discrepancy between the order of E exo and those of the N and O negative charges arises from the presence of valence 4s electron of Cu because it suppresses the CT from N and O to Cu55. From these results, one can infer that the d-valence band-top energy of M55 plays an important role in determining the reactivity for NO dissociative adsorption. Truly, the d valence orbital energy decreases in the order of Fe > Co > Ni ≫ Cu and the 3d metal > 4d metal in the same group of the periodic table, which reflects the dependence of reactivity on the metal element position in the periodic table.

Oxophilicity as a Descriptor for NO Cleavage Efficiency over Group IX Metal Clusters

Iridium and rhodium are group IX elements that can both catalytically reduce NO. To understand the difference in their reactivity toward NO, the adsorption forms of NO onto clusters of Ir and Rh are compared using vibrational spectra, recorded via infrared multiple-photon dissociation spectroscopy. The spectra give evidence for the existence of at least two specific adsorption forms. The main Ir₆⁺NO isomer is one in which NO is dissociated, whereas one other is a local minimum structure in the reaction pathway leading to dissociative adsorption. In contrast to adsorption onto Rh₆⁺, where less than 10% of the isomeric population was found in the global minimum associated with dissociative adsorption, a substantial fraction (about 50%) of NO dissociates on Ir₆⁺. This higher efficiency is attributed to a considerably reduced activation barrier for dissociation on Ir₆⁺. The key chemical property identified for dissociation efficiency is the cluster's affinity to atomic oxygen.

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.

Effects of Second-Metal (Al, V, Co) Doping on the NO Reactivity of Small Rhodium Cluster Cations

Reactions of pure and doped rhodium cluster cations, Rh_nX⁺ (n = 2-6; X = Al, V, Co, Rh), with NO molecules were investigated at near-thermal energy using a guided ion beam tandem mass spectrometer. We found that the doping with Al and V increases the total reaction cross section mostly. Under single-collision conditions, Rh₂X⁺ reacts with NO to produce Rh₂N⁺ with release of metal monoxide, XO, whereas Rh_nX⁺ (n = 3-6) adsorb NO. For the specific clusters Rh_nAl⁺ (n = 3 and 4) and Rh_nV⁺ (n = 4-6), the NO adsorption is often accompanied by the release of one Rh atom. In addition, we examined the reactions of Rh₅X⁺ (X = Al, V, Co, Rh) with NO under multiple-collision conditions and observed the cluster dioxide formation and the N₂ release, i.e., NO decomposition. Particularly, the V-doping is most effective for the NO decomposition. One possible explanation for the present results is that the formation of a stable dopant metal-oxygen bond directly leads to the increase of NO dissociative adsorption energy and the reduction of the energy barrier between the molecular and dissociative adsorption, thereby encouraging the NO decomposition on the small Rh_nX⁺ clusters studied.