Role of cluster size in catalysis: spectroscopic investigation of gamma-Al2O3-supported Ir4 and Ir6 during ethene hydrogenation

gamma-Al(2)O(3)-supported Ir(4) and Ir(6) were prepared by decarbonylation of tetra- and hexanuclear iridium carbonyls, respectively, and compared as catalysts for ethene hydrogenation at atmospheric pressure and temperatures in the range 273-300 K. Rates of the reaction were determined along with extended X-ray absorption fine structure (EXAFS) and IR spectra characterizing the clusters in the working catalysts. EXAFS data show that the Ir(4) and Ir(6) cluster frames remained intact during catalysis. Di-sigma-bonded ethene and pi-bonded ethene on the clusters were identified by IR spectroscopy and found to compete as the principal reaction intermediates, with the former predominating at ethene partial pressures less than about 200 Torr and the latter at higher ethene partial pressures. Hydrogen on the clusters is inferred to form by dissociative adsorption of H(2); alternatively, it is provided by OH groups of the support. The rate of ethene hydrogenation on Ir(4) is typically several times greater than that on Ir(6).     

Effects of adsorbates on supported platinum and iridium clusters: Characterization in reactive atmospheres and during alkene hydrogenation catalysis by X-ray absorption spectroscopy

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.     

金属Ir4 Cluster催化丙烯加氢反应势能面的密度泛函理论研究

用密度泛函理论(DFT)对金属Ir4 cluster催化丙烯Propene加氢反应的反应机理进行了理论研究. 在B3LYP理论水平下优化了反应通道上反应物、中间体、过渡态和产物各驻点物种的几何构型, 构建了该反应的基态势能面. 计算结果表明, Ir4 cluster催化丙烯加氢反应, 主要通过3条反应通道(c,d和e)进行. 主反应通道c 是H1原子先经过中间体1加成到丙烯的边端C上形成中间体3, 然后H2原子经过渡态TS3—5, 中间体5和过渡态TS5-P加成到中间C上生成产物P. c通道无论从动力学角度还是热力学角度都是最有利的; 反应通道d和e中的最高势垒和通道c上的相比差别不大, 具有一定的竞争性, 是次通道.     

Reactivity of site-isolated metal clusters: propylidyne on gamma-Al2O3-supported Ir4.

To contrast the reactivity of supported metal clusters with that of extended metal surfaces, we investigated the reactions of tetrairidium clusters supported on porous gamma-Al2O3 (Ir4/gamma-Al2O3) with propene and with H2. Infrared, 13C NMR, and extended X-ray absorption fine-structure spectroscopy were used to characterize the ligands formed on the clusters. Propene adsorption onto Ir4/gamma-Al2O3 at 298 K gave stable, cluster-bound mu3-propylidyne. Propene adsorbed onto Ir4/gamma-Al2O3 at 138 K reacted at approximately 219 K to form a stable, highly dehydrogenated, cluster-bound hydrocarbon species approximated as CxHy (such as, for example, C3H2 or C2H). H2 reacted with Ir4/gamma-Al2O3 at 298 K, forming ligands (likely hydrides), which prevented subsequent reaction of the clusters with propene to form propylidyne. Propylidyne on Ir4 was stable in helium or H2 as the sample was heated to 523 K, whereupon it reacted with oxygen of the support to give CO. Propylidyne on Ir4 did not undergo isotopic exchange in the presence of D2 at 298 K. In contrast, the literature shows that propylidyne chemisorbed on extended metal surfaces is hydrogenated in the presence of H2 (or D2) and exchanges hydrogen with gaseous D2 at room temperature; in the absence of H2, it decomposes thermally to give hydrocarbon fragments at temperatures much less than 523 K. The striking difference in reactivities of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of more than the three metal surface atoms bonded to propylidyne in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters.     

Pt3Ru6 clusters supported on gamma-Al2O3: synthesis from Pt3Ru6(CO)21(mu3-H)(mu-H)3, structural characterization, and catalysis of ethylene hydrogenation and n-butane hydrogenolysis

The supported clusters Pt-Ru/gamma-Al2O3 were prepared by adsorption of the bimetallic precursor Pt3Ru6(CO)21(mu3-H)(mu-H)3 from CH2Cl2 solution onto gamma-Al2O3 followed by decarbonylation in He at 300 degrees C. The resultant supported clusters were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and as catalysts for ethylene hydrogenation and n-butane hydrogenolysis. After adsorption, the nu(CO) peaks characterizing the precursor shifted to lower wavenumbers, and some of the hydroxyl bands of the support disappeared or changed, indicating that the CO ligands of the precursor interacted with support hydroxyl groups. The EXAFS results show that the metal core of the precursor remained essentially unchanged upon adsorption, but there were distortions of the metal core indicated by changes in the metal-metal distances. After decarbonylation of the supported clusters, the EXAFS data indicated that Pt and Ru atoms interacted with support oxygen atoms and that about half of the Pt-Ru bonds were maintained, with the composition of the metal frame remaining almost unchanged. The decarbonylated supported bimetallic clusters reported here are the first having essentially the same metal core composition as that of a precursor metal carbonyl, and they appear to be the best-defined supported bimetallic clusters. The material was found to be an active catalyst for ethylene hydrogenation and n-butane hydrogenolysis under conditions mild enough to prevent substantial cluster disruption.     

FTIR matrix-isolation study of the reaction products of vanadium atoms with propene: observation of allylvanadium hydride as a precursor to sacrificial hydrogenation of propene

Vanadium atoms have been reacted with different partial pressures of propene in Ar under matrix-isolation conditions, and the products have been observed using Fourier transform infrared (FTIR) spectroscopy. Under dilute propene in Ar conditions, new features are observed in the IR spectra corresponding to a C-H insertion product, identified here as H-V-(η(3)-allyl). Use of d(3)-propene (CD(3)-CH═CH(2)) demonstrates that the initial V-atom insertion occurs at the methyl group of the propene molecule, and DFT calculations have been used to support the identity of the initial product. Upon increasing the partial pressure of propene, additional features corresponding to propane (C(3)H(8)) are observed, with the hydrogen-atom source for the observed hydrogenation demonstrated to be additional propene units. Analysis of a systematic increase in the partial pressure of propene in the system demonstrates that the yield of propane correlates with the decrease of the allyl product, demonstrating the H-V(allyl) species as a reactive intermediate in the overall hydrogenation process. An overall mechanism is proposed to rationalize the formation of the insertion product and ultimately the products of hydrogenation, which agrees with previous gas-phase and matrix-isolation work involving propene and the related system, ethene.     

EXAFS/FT-IR characterization of tetra-iridium carbonyl clusters bound to tris-(hydroxymethyl)phosphine grafted silica surface catalytically active for propene oxidation to acetone

Structure and catalytic activity of Ir₄(CO)₁₂ bound to tris-(hydroxymethyl)phosphine (THP) grafted silica (THP/SiO₂) was investigated by means of EXAFS, FT-IR and kinetic studies. It was found that Ir₄(CO)₁₂ was uniformly attached on THP/SiO₂ by substitution of CO by THP (Ir₄/THP/SiO₂). The tetra-iridium carbonyl cluster framework was remained during the substitution of THP ligands and two THP ligands coordinated to the iridium carbonyl clusters to form Ir₄(CO)₁₀(THP/SiO₂)₂. species. When Ir₄/THP/SiO₂ was evacuated at 373 K, bridge CO was desorbed and Debye-Waller factor of Ir---Ir contribution derived from EXAFS analysis was increased which suggested that the cluster framework was distorted by the evacuation at 373 K. The resulting sample evacuated at 373 K was an active catalyst for hydroformylation of ethene and partial oxidation of propene, while the Ir₄/THP/SiO₂ without evacuation exhibited poor catalytic activities. The propene oxidation reaction proceed on the Ir₄/THP/SiO₂ evacuated at 323–353 K under subatmospheric pressures to give acetone as a product in high selectivity. The ethene hydroformylation proceed on the evacuated Ir₄/THP/SiO₂ at lower temperatures compared with other conventional iridium catalysts. EXAFS characterization and kinetic studies suggested that the catalytic activities were associated with the structural distortion of the iridium cluster framework due to surface attachment by the bidentate phosphine substitution.     

One step formation of propene from ethene or ethanol through metathesis on nickel ion-loaded silica

Increased propene production is presently one of the most significant objectives in petroleum chemistry. Especially the one-step conversion of ethene to propene (ETP reaction, 3C?H? →2C?H?) is the most desired process. In our efforts, nickel ion-loaded mesoporous silica could turn a new type of ETP reaction into reality. The one-step conversion of ethene was 68% and the propene selectivity was 48% in a continuous gas-flow system at 673 K and atmospheric pressure. The reactivity of lower olefins and the dependences of the ETP reaction on the contact time and the partial pressure of ethene were consistent with a reaction mechanism involving dimerization of ethene to 1-butene, isomerization of 1-butene to 2-butene, and metathesis of 2-butene and ethene to yield propene. The reaction was then expanded to an ethanol-to-propene reaction on the same catalyst, in which two possible reaction routes are suggested to form ethene from ethanol. The catalysts were characterized mainly by EXAFS and TPR techniques. The local structures of the nickel species active for the ETP reaction were very similar to that of layered nickel silicate, while those on the inert catalysts were the same as that of NiO particles.     

Ethene insertion into vanadium hydride intermediates formed via vanadium atom reaction with water or ethene: a matrix isolation infrared spectroscopic study

The reaction of V atoms with H2O and various concentrations of C2D4 in argon has been investigated by matrix isolation infrared (IR) spectroscopy. Both C2D6 and CD2H-CD2H are observed as the major products of a set of parallel processes involving hydrogenation of ethene where the formal source of hydrogen is either C2D4 or H2O. Portions of the IR spectrum of CD2H-CD2H isolated in an argon matrix are observed for the first time. For experiments involving low concentrations of C2D4, irradiation of the matrix with light of wavelengths >455 nm results in VH2 formation, with limited observation of ethene hydrogenation. The source of H2 is believed to be due to photoelimination of molecular hydrogen from HO-V-H species, during matrix deposition, with OV as an additional product. Recombination of OV with available H2 in the matrix is proposed as the source of OVH2 under low ethene conditions. No evidence for VD2 formation is observed under our conditions. At higher C2D4 concentrations, VH2 formation is suppressed, while products of ethene hydrogenation are maximized. A second process competing with H2 elimination in which HO-V-H reacts with C2D4 is proposed. Parallel reaction schemes involving V atom insertion into the O-H bonds of water or the photoinduced insertion of V atoms into the C-D bonds of C2D4 are proposed to account for the observed hydrogenation products. In each mechanism, insertion of C2D4 into the V-H or V-D bonds of transient intermediates is followed by photoinduced elimination of the associated ethane isotopomer.     

Redox chemistry of tantalum clusters on silica characterized by X-ray absorption spectroscopy

SiO(2)-supported clusters of tantalum were synthesized from adsorbed Ta(CH(2)Ph)(5) by treatment in H(2) at 523 K. The surface species were characterized by X-ray absorption spectroscopy (extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES)) and ultraviolet-visible spectroscopy. The EXAFS data show that SiO(2)-supported tantalum clusters were characterized by a Ta-Ta coordination number of approximately 2, consistent with the presence of tritantalum clusters, on average. When these were reduced in H(2) and reoxidized in O(2), the cluster nuclearity remained essentially unchanged, although reduction and oxidation occurred, respectively, as shown by XANES and UV-vis spectra; in the reoxidation, the tantalum oxidation state change was approximately two electronic charges per tritantalum cluster. The data demonstrate an analogy between the chemistry of group 5 metals on the SiO(2) support and their chemistry in solution, as determined by the group of Cotton.     

Surface modification of mesoporous, macroporous, and amorphous silica with catalytically active polyoxometalate clusters

Transition-metal-substituted polyoxometalates (TMSP) of the type [MII(H2O)PW11O39]5- (M = Co, Zn) and [SiW9O37(CoII(H2O))3]10- have been chemically anchored to modified macroporous (400 nm pores), mesoporous (2.8 nm pores), and amorphous silica surfaces. The materials were characterized by solid-state 31P MAS NMR, UV-vis, FT-IR spectroscopy, and N2 adsorption experiments to verify cluster attachment and the structure of the TMSP on the support. On the basis of the spectroscopic evidence, clusters were attached datively to the surface, and the integrity of the [CoPW11] cluster was maintained for nonaqueous impregnation with TBA5[CoPW11]; partial degradation of the cluster occurred when it was impregnated from aqueous solution using the K5[CoPW11] salt. Both the amine surface groups and the cobalt centers in the clusters were found to be necessary to prevent cluster loss during washing or reaction processes. The catalytic activities of the supported TMSP clusters were tested by the epoxidation of cyclohexene to cyclohexene oxide in the presence of isobutyraldehyde. The percent conversion of the substrate and the amount of product formed per unit time were similar for [CoPW11] clusters on each of the three silica supports, but slightly lower than for purely homogeneous reactions. [SiW9Co3] clusters with three available cobalt centers exhibited higher catalytic activity with nearly identical performance on a silica support or in homogeneous solution.     

Atomic XAFS as a tool to probe the reactivity of metal oxide catalysts: quantifying metal oxide support effects

The potential of atomic XAFS (AXAFS) to directly probe the catalytic performances of a set of supported metal oxide catalysts has been explored for the first time. For this purpose, a series of 1 wt % supported vanadium oxide catalysts have been prepared differing in their oxidic support material (SiO2, Al2O3, Nb2O5, and ZrO2). Previous characterization results have shown that these catalysts contain the same molecular structure on all supports, i.e., a monomeric VO4 species. It was found that the catalytic activity for the selective oxidation of methanol to formaldehyde and the oxidative dehydrogenation of propane to propene increases in the order SiO2 < Al2O3 < Nb2O5 < ZrO2. The opposite trend was observed for the dehydrogenation of propane to propene in the absence of oxygen. Interestingly, the intensity of the Fourier transform AXAFS peak decreases in the same order. This can be interpreted by an increase in the binding energy of the vanadium valence orbitals when the ionicity of the support (increasing electron charge on the support oxygen atoms) increases. Moreover, detailed EXAFS analysis shows a systematic decrease of the V-Ob(-M(support)) and an increase of a the V-O(H) bond length, when going from SiO2 to ZrO2. This implies a more reactive OH group for ZrO2, in line with the catalytic data. These results show that the electronic structure and consequently the catalytic behavior of the VO4 cluster depend on the ionicity of the support oxide. These results demonstrate that AXAFS spectroscopy can be used to understand and predict the catalytic performances of supported metal oxide catalysts. Furthermore, it enables the user to gather quantitative insight in metal oxide support interactions.     

On the role of thermal activation in selective photochemistry: mechanistic insight into the oxidation of propene on the V4O11- cluster

An experimental methodology for a mechanistic analysis of gas phase chemical reactions is presented in the context of structure-reactivity relationships of metal oxide clusters relevant to photocatalysis. The spectroscopic approach is demonstrated with the investigation of the photoinduced oxygenation of propene on the V(4)O(11)(-) cluster, where the thermal activation and subsequent photoreaction are deduced with the information from (i) the temperature dependency of the aggregation kinetics in the propene-seeded helium atmosphere of an ion-trap reactor; (ii) the fluence dependency in the yield of different product channels of the photoreaction and (iii) the intensity dependency in the fragmentation of neutral reaction products that are probed via in situ multi-photon ionization. For the thermal reaction, selective hydrogen abstraction from the allylic position of propene accompanied by the linkage to the cluster at the dioxo moiety is postulated as the mechanism in the aggregation of propene on the V(4)O(11)(-) cluster. In accordance with an insightful neutralization-reionization study (Schr?der et al., J. Mass. Spectrom., 2010, 301, 84), the subsequent photoinduced reaction is defined by an allylic oxidation in the formation of acrolein from the initial allyloxy radical photoproduct. The relevance of the observed selectivity is discussed in view of the electronic structure and bond motifs offered by high valence oxide systems such as the V(4)O(11)(-) cluster.     

Molecular structure of a supported VO4 cluster and its interfacial geometry as a function of the SiO2, Nb2O5, and ZrO2 support

The influence of the support oxide on the molecular structure of a VO(4) cluster and its interfacial geometry has been determined for SiO(2), Nb(2)O(5), and ZrO(2) as supports. Raman, IR, UV-vis-NIR diffuse reflectance, electron spin resonance, and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported vanadium oxide clusters after dehydration. It has been found that for all supports under investigation the vanadium ion is tetrahedral coordinated and consists of one V=O and three V-O bonds. For a VO(4)/SiO(2) catalyst it has been established that only one O neighbor is shared with the SiO(2) support via a V-O(b)-Si(support) bond with an angle of approximately 101 degrees (+/-0.5 degrees ) and a V...Si distance of 2.61 A. The absence of a second vanadium atom in the vicinity of the vanadium oxide cluster further subverts the classical assignment of the 920 cm(-1) Raman band to a V-O-V related vibration. The EXAFS results combined with structural modeling using Cerius(2) software lead to structural constraints, which imply a similar V-O(b)-M(support) interaction for Nb(2)O(5) and ZrO(2) as well. The V-O(b) and the V...M(support) distances depend on the geometry of each support surface. The results show that the classical model with three V-O(b)-M(support) bonds could not be experimentally observed with EXAFS under the applied measuring conditions. Additional experiments with IR and Raman spectroscopy under experimental conditions mimicking those of the EXAFS measurements reveal the presence of V-OH groups, giving further support for the presence of a O=V(OH)(2)-O(b)-M moiety at the support surface.     

In situ diagnostics of the decomposition of silacyclobutane on a hot filament by vacuum ultraviolet laser ionization mass spectrometry

The gas-phase reaction products of silacyclobutane (SCB) and 1, 1-dideuterio-silacyclobutane (SCB-d(2)) from a hot-wire chemical vapor deposition (HWCVD) chamber were diagnosed in situ using vacuum ultraviolet (VUV) laser single-photon ionization (SPI) coupled with time-of-flight (TOF) mass spectrometry. The SCB molecule was found to decompose at a filament temperature as low as 900 degrees C. Both Si- (silylene, methylsilylene, and silene) and C-containing (ethene and propene) species were produced from the SCB decomposition on the filament. Ethene and propene were detected by the mass spectrometer. It is demonstrated that the formation of ethene is favored over that of propene. The experimental study of hot-wire decomposition of SCB-d(2) shows that propene is most likely produced by a process that is initiated by a 1,2-H(D) migration to form n-propylsilylene, followed by an equilibration with silacyclopropane, which then decomposes to propene. The detection of ethene in our experiment indicates that a competitive route of fragmentation exists for SCB decomposition on the filament. It has been shown that this competitive route occurs without H/D scrambling. The highly reactive silylene, silene, and methylsilylene species produced from SCB decomposition underwent either insertion reactions into the Si-H bonds of the parent molecule or pi-type addition reaction across the double and triple CC bonds. The dimerization product of silene, 1,3-disilacyclobutane, at m/z = 88 was also observed.     

Activation of hydrocarbons on acidic zeolites: superior selectivity of methylation of ethene with methanol to propene on weakly acidic catalysts

The study of methylation of ethene with methanol to propene over MFI zeolites with different heteroatoms has found that an efficient catalyst with weak acidities prevented the side reactions related with the formation of ethene oligomers from occurring, as evidenced by in situ IR spectroscopy, leading to superior propene selectivity in the product distribution.     

Rhodium complex with ethylene ligands supported on highly dehydroxylated MgO: synthesis, characterization, and reactivity

Mononuclear rhodium complexes with reactive olefin ligands, supported on MgO powder, were synthesized by chemisorption of Rh(C(2)H(4))(2)(C(5)H(7)O(2)) and characterized by infrared (IR), (13)C MAS NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. IR spectra show that the precursor adsorbed on MgO with dissociation of acetylacetonate ligand from rhodium, with the ethylene ligands remaining bound to the rhodium, as confirmed by the NMR spectra. EXAFS spectra give no evidence of Rh-Rh contributions, indicating that site-isolated mononuclear rhodium species formed on the support. The EXAFS data also show that the mononuclear complex was bonded to the support by two Rh-O bonds, at a distance of 2.18 A, which is typical of group 8 metals bonded to oxide supports. This is the first simple and nearly uniform supported mononuclear rhodium-olefin complex, and it appears to be a close analogue of molecular catalysts for olefin hydrogenation in solution. Correspondingly, the ethylene ligands bonded to rhodium in the supported complex were observed to react with H(2) to form ethane, and the supported complex was catalytically active for the ethylene hydrogenation at 298 K. The ethylene ligands also underwent facile exchange with C(2)D(4), and exposure of the sample to carbon monoxide led to the formation of rhodium gem dicarbonyls.     

Organometallic complexes in supported ionic-liquid phase (SILP) catalysts: a PHIP NMR spectroscopy study

para-Hydrogen induced polarization (PHIP) NMR spectroscopy emerges as an efficient and robust method for on-line monitoring of gas-phase hydrogenation reactions. Here we report detailed investigations of supported ionic liquid phase (SILP) catalysts in a continuous gas-phase hydrogenation of propene with PHIP NMR spectroscopy. A relocation of the rhodium complex in the thin layer of ionic liquid in the SILP catalyst at the initial stage of the propene hydrogenation is demonstrated. PHIP NMR spectroscopy can provide profound insight into the evolution of SILP catalysts during hydrogenation reactions.     

Cluster size selectivity in the product distribution of ethene dehydrogenation on niobium clusters

Ethene reactions with niobium atoms and clusters containing up to 25 constituent atoms have been studied in a fast-flow metal cluster reactor. The clusters react with ethene at about the gas-kinetic collision rate, indicating a barrierless association process as the cluster removal step. Exceptions are Nb8 and Nb10, for which a significantly diminished rate is observed, reflecting some cluster size selectivity. Analysis of the experimental primary product masses indicates dehydrogenation of ethene for all clusters save Nb10, yielding either Nb(n)C2H2 or Nb(n)C2. Over the range Nb-Nb6, the extent of dehydrogenation increases with cluster size, then decreases for larger clusters. For many clusters, secondary and tertiary product masses are also observed, showing varying degrees of dehydrogenation corresponding to net addition of C2H4, C2H2, or C2. With Nb atoms and several small clusters, formal addition of at least six ethene molecules is observed, suggesting a polymerization process may be active. Kinetic analysis of the Nb atom and several Nb(n) cluster reactions with ethene shows that the process is consistent with sequential addition of ethene units at rates corresponding approximately to the gas-kinetic collision frequency for several consecutive reacting ethene molecules. Some variation in the rate of ethene pick up is found, which likely reflects small energy barriers or steric constraints associated with individual mechanistic steps. Density functional calculations of structures of Nb clusters up to Nb(6), and the reaction products Nb(n)C2H2 and Nb(n)C2 (n = 1...6) are presented. Investigation of the thermochemistry for the dehydrogenation of ethene to form molecular hydrogen, for the Nb atom and clusters up to Nb6, demonstrates that the exergonicity of the formation of Nb(n)C2 species increases with cluster size over this range, which supports the proposal that the extent of dehydrogenation is determined primarily by thermodynamic constraints. Analysis of the structural variations present in the cluster species studied shows an increase in C-H bond lengths with cluster size that closely correlates with the increased thermodynamic drive to full dehydrogenation. This correlation strongly suggests that all steps in the reaction are barrierless, and that weakening of the C-H bonds is directly reflected in the thermodynamics of the overall dehydrogenation process. It is also demonstrated that reaction exergonicity in the initial partial dehydrogenation step must be carried through as excess internal energy into the second dehydrogenation step.     

Spectroscopic IR, EPR, and operando DRIFT insights into surface reaction pathways of selective reduction of NO by propene over the Co-BEA zeolite

Interaction of a Co-BEA catalyst with individual components (NO, C(3)H(6), CO, O(2)) and mixtures simulating the real feed of the selective catalytic reduction (SCR) of nitric oxide in static and pulse experiments at variable temperatures was investigated by means of IR, EPR, and operando DRIFT spectroscopy coupled with QMS/GC analysis of the products. Speciation of cobalt active sites into Co(II), mono- and polynuclear oxo-cobalt species as well as CoO clusters was quantified by IR using CO and NO as probe molecules. The key intermediates, by-products, and final products of the SCR reaction were identified and their spectroscopic signatures ascertained. Based on the spectroscopic operando results, a concise mechanistic scheme of the selective catalytic reduction of nitric oxide by propene, triggered by a two-electron Co(II)/Co(0) redox couple, was developed. It consists of a complex network of the sequential/parallel selective reduction steps that are interlocked by the rival nonselective oxidation of the intermediates and their thermal decomposition. It has been shown that the SCR process is initiated by the chemoselective capture of NO from the reaction mixture by the cobalt active sites leading to the cobalt(II) dinitrosyls, which in the excess of oxygen are partially oxidized to surface nitrates and nitrites. N(2)O is produced by semi-decomposition of the dinitrosyl intermediates on the mononuclear centers, whereas NO(2)via NO oxidation on the polynuclear oxo-cobalt sites. Cyanide and isocyanate species, formed together with propene oxygenates in the course of the C=C bond scission, are the mechanistically pivotal reaction intermediates of C(3)H(6) interaction with the dinitrosyles and NO(3)(-)/NO(2)(-) surface species. Dinitrogen is produced by three main reaction routes involving oxidation of cyanides by NO/NO(2), reduction of dinitrosyls, nitrates, and nitrites by propene oxygenates (medium temperature range) or their reduction by carbon monoxide (high temperature range).