Alkane Activation Research Articles

Density Functional Calculations on the Distribution, Acidity, and Catalysis of TiIV and TiIII Ions in MCM‐22 Zeolite

Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions. In this work, density functional calculations were used to explore three current concerns: 1) the distributions of Ti(IV) and Ti(III) ions in the MCM-22 zeolite; 2) the Lewis acidity of the Ti(IV) and Ti(III) sites; and 3) activation of alkane C-H bonds by photocatalysis with Ti-doped zeolites. Neither the Ti(IV) nor Ti(III) ions are randomly distributed in the MCM-22 zeolite. The orders of relative stability are very close for the eight Ti(IV) and Ti(III) sites, and the T3 site is the most probable in both cases. The wavelengths for Ti(IV)-Ti(III) excitations were calculated to lie in the range λ=246.9-290.2 nm. The Ti3(IV) site shows Lewis acidity toward NH(3) in two different modes, and these two modes can coexist with each other. The calculated Ti(IV) coordination numbers, Ti(IV)-O bond elongations, and charge transfers caused by NH(3) adsorption are in good agreement with previous results. Similarly, two different NH(3) adsorption modes exist for the Ti3(III) site; the site that exhibits radical transfer from the lattice O to N atoms is preferred due to the higher adsorption energy. This indicates that the Ti3(III) site does not show Lewis acidity, in contrast to the Ti3(IV) site. At the Ti3(III) site, the energy barrier for activating the methane C-H bond was calculated to be 33.3 kJ mol(-1) and is greatly reduced by replacing the hydrogen atoms with methyl groups. In addition, the reactivity is improved when switching from MCM-22 to TS-1 zeolite. The studies on the various Ti species reveal that lattice O atoms rather than Ti(III) radicals are crucial to the activation of alkane C-H bonds. This work provides new insights into and aids understanding of the catalysis by isolated Ti species in zeolites.

Study of the Catalytic Activity–Semiconductive Properties Relationship For BaTiO3 and PbTiO3 Perovskites, Catalysts for Methane Combustion

The electrical conductivity of barium and lead perovskites used as catalysts for the total oxidation of methane, has been measured under nitrogen, methane–nitrogen mixture, air and methane–air mixture (reaction mixture) at the catalytic reaction temperature. The two compounds appeared to be p-type semiconductors under air with positive holes as the main charge carriers but became n-type when contacted with methane–nitrogen mixture. Their conductivities differ by 1.5 orders of magnitude as n-type semiconductors and by three orders of magnitude when being p-type semiconductors. These results explained the difference in the catalytic activity encountered on the two solids. The alkane activation was proposed to be related in both cases to the p-type semiconducting properties of the solids, likely through hydrogen abstraction by a surface O− species, forming a CH 3 • radical. The overall reaction mechanism on both perovskites can be assimilated to a Mars and van Krevelen mechanism. The electrical conductivity of barium and lead perovskites, catalysts for methane total oxidation, has been studied under different gaseous atmospheres at the reaction temperature and a reaction mechanism has been proposed.

Combined experimental and theoretical investigation into C–H activation of cyclic alkanes by Cp′Rh(CO)2 (Cp′ = η5-C5H5 or η5-C5Me5)

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rate of C-H activation of alkanes by Cp'Rh(CO) {Cp' = η(5)-C(5)H(5) or η(5)-C(5)Me(5); alkane = cyclopentane, cyclohexane and neopentane (Cp only)} in solution at room temperature and allowed the determination of how the change in rate of oxidative cleavage varies between complexes and alkanes. Density functional theory calculations on these complexes, transition states, and intermediates provide insight into the mechanism and barriers observed in the experimental results. Unlike our previous study of the linear alkanes, where activation occurred at the primary C-H bonds with a rate governed by a balance between these activations and hopping along the chain, the rate of C-H activation in cyclic alkanes is controlled mainly by the strength of the alkane binding. Although the reaction of CpRh(CO)(neopentane) to form CpRh(CO)(neopentyl)H clearly occurs at a primary C-H bond, the rate is much slower than the corresponding reactions with cyclic alkanes because of steric factors with this bulky alkane.

Effect of 1-butyl-3-methylimidazolium nitrate on separation properties of polymer/AgNO3 membranes for propylene/propane mixtures: Comparison between poly(2-ethyl-2-oxazoline) and poly(ethylene oxide)

A previous study showed that ionic liquids enable AgNO3, which is not readily reduced to silver nanoparticles in the solid state, to be an active olefin carrier in silver polymer electrolyte membranes. We report the effects of an ionic liquid, i.e. 1-butyl-3-methylimidazolium nitrate (BMIM+NO3−), on the separation properties of polymer/AgNO3 complex membranes for propylene/propane mixtures. Interestingly, the change in membrane separation properties with the ionic liquid concentration was completely different between poly(2-ethyl-2-oxazoline) (POZ) and poly(ethylene oxide) (PEO). In the POZ/AgNO3 complex membranes, both the selectivity of propylene/propane and the mixed gas permeance increased up to a 0.1 mole ratio of ionic liquid, after which they decreased. The enhancement of the membrane performance results from an increase in the silver ion activity, whereas the decrease was attributed to the barrier effect of large amounts of IL and less effectiveness of IL in controlling the ionic interactions. On the other hand, in the PEO/AgNO3 membranes, the selectivity increased with increasing ionic liquid, whereas the mixed gas permeance decreased. The increase in selectivity comes from the enhanced silver ion activity, whereas the decrease in permeance was attributed to the decrease in interfacial voids in the membranes.

Activation of Alkanes by Gold‐Modified Lanthanum Oxide

AbstractFunctionalization of alkanes is much sought after for the production of fine and bulk chemicals. In particular, the oxidative activation of alkanes and their conversion to ethene and propene has been studied extensively, owing to the use of these alkenes in polymerization reactions. The greater reactivity of the products in comparison with the reactants has proven a major issue in this reaction as this can result in overoxidation, producing CO and CO2 and, therefore, reducing the alkene yield. Herein, the first application of supported gold catalysts for the direct activation of C2+ aliphatic alkanes with oxygen to form alkenes is demonstrated. This catalyst is particularly notable as it is highly active, selective to propene and ethene, and stable on stream over a 48 h period. Maintaining cationic gold is thought to be critical for the stability and this catalyst design provides the possibility of applying gold‐based catalysts over a much wider temperature range than has been reported.

C−H Bond Activation of Light Alkanes on Pt(111): Dissociative Sticking Coefficients, Evans−Polanyi Relation, and Gas−Surface Energy Transfer

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionC−H Bond Activation of Light Alkanes on Pt(111): Dissociative Sticking Coefficients, Evans−Polanyi Relation, and Gas−Surface Energy TransferG. W. Cushing, J. K. Navin, S. B. Donald, L. Valadez, V. Johánek, and I. Harrison*Cite this: J. Phys. Chem. C 2010, 114, 51, 22790Publication Date (Web):December 8, 2010Publication History Published online8 December 2010Published inissue 30 December 2010https://doi.org/10.1021/jp1112119Copyright © 2010 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views403Altmetric-Citations5LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (37 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Chemical calculations,Desorption,Energy transfer,Hydrocarbons,Kinetic parameters Get e-Alerts

Results of NMR spectroscopic studies of hydrocarbon conversions on solid acid catalysts in the last 25 years

High-resolution solid-state NMR studies of hydrocarbon conversions on the surface of solid acid catalysts are overviewed. The results of identification of hydrocarbons adsorbed on solid acid catalysts are presented. Alkane activation and hydrocarbon conversion mechanisms and the nature of reactive intermediates are discussed. It is demonstrated that NMR spectroscopy can be used not only in the in situ analysis of hydrocarbons on the catalyst surface, but also as a method for seeking new hydrocarbon conversion routes.

25 Years Full of Chemical Discovery

25 Years Full of Chemical Discovery

Understanding the factors affecting the activation of alkane by Cp ′ Rh(CO) 2 (Cp ′ = Cp or Cp*)

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rates of activation of alkanes by Cp'Rh(CO) (Cp(')=η(5)-C(5)H(5) or η(5)-C(5)Me(5)). We have monitored the kinetics of C─H activation in solution at room temperature and determined how the change in rate of oxidative cleavage varies from methane to decane. The lifetime of CpRh(CO)(alkane) shows a nearly linear behavior with respect to the length of the alkane chain, whereas the related Cp*Rh(CO)(alkane) has clear oscillatory behavior upon changing the alkane. Coupled cluster and density functional theory calculations on these complexes, transition states, and intermediates provide the insight into the mechanism and barriers in order to develop a kinetic simulation of the experimental results. The observed behavior is a subtle interplay between the rates of activation and migration. Unexpectedly, the calculations predict that the most rapid process in these Cp'Rh(CO)(alkane) systems is the 1,3-migration along the alkane chain. The linear behavior in the observed lifetime of CpRh(CO)(alkane) results from a mechanism in which the next most rapid process is the activation of primary C─H bonds (─CH(3) groups), while the third key step in this system is 1,2-migration with a slightly slower rate. The oscillatory behavior in the lifetime of Cp*Rh(CO)(alkane) with respect to the alkane's chain length follows from subtle interplay between more rapid migrations and less rapid primary C─H activation, with respect to CpRh(CO)(alkane), especially when the CH(3) group is near a gauche turn. This interplay results in the activation being controlled by the percentage of alkane conformers.

Propane transformations on aluminum chloride—cobalt chloride clusters: a quantum chemical study

Density functional PBE/TZ2p quantum chemical calculations of activated complexes and pathways of model catalytic transformations of propane under the action of aluminum chloride-cobalt chloride ionic bimetallic complexes were carried out. The formation of an intermediate with a broken C-C bond can occur on the cationic cluster CoAlCl4 + characterized by the strongest coordination of propane molecule. The activation barrier to the reaction is ΔG = 25.0 kcal mol−1. Activation of alkane C-H bonds follows the alkyl pathway involving the formation of bimetallic alkyl complexes. The interaction of activated hydrocarbon fragments bound to transition metal atoms in cobalt-chloroaluminate clusters can result in alkane metathesis products (in this case, ethane and a polymetallic cluster containing an extendedchain alkyl radical).

ChemInform Abstract: Carbene‐Transition Metal Complexes Formed by Double C—H Bond Activation

AbstractReview: [65 refs.]

Harnessing Biology to Produce Inorganic Materials

Siluria's first application of its phage/catalysis approach is in the production of ethylene, the world's largest commodity chemical. The company uses a process called oxidative coupling of methane (OCM) to produce ethylene. The chemistry of OCM has been known for about 30 years, with no shortage of effort or scientific rigor to develop it selectively and efficiently for commercialization. “Nobody has been able to use OCM selectively for ethylene because they have not been able to get a catalyst that would convert the majority of methane to ethylene,” says Scher who explains that Siluria's catalysts can do this easily.Hot natural gas, and air are the inputs into a gas phase reactor. At temperatures several hundred degrees below what was previously achieved with conventional catalysts, Siluria's phage-designed catalyst transforms them into ethylene. The biological scaffold will have been burned off, leaving behind an inorganic material—in this case, ethylene oxide.Should this technology work at scale for ethylene production, it could replace a 70 year, energy-intensive process called steam cracking. Cracking consists of breaking long carbon chains, specifically naphtha, which is oil-derived, with very hot steam at temperatures near 900°C to produce short carbon chains in the process. “This is an endothermic process, requiring a lot of energy,” explains Scher. You have to keep putting heat into the reaction, usually in the form of burning natural gas, in order to maintain the reaction at 900°C or higher.”Methane contains very strong carbon/hydrogen bonds. “Siluria is pursuing an approach to activating methane at low temperatures to produce ethylene, a problem heavily invested in over the better part of refining chemistry, which is traditionally not able to be accomplished without radicals generated at high temperature,” says Turner. “Because this is an area in traditional chemistry where a lot of smart people have worked for a long time, if one is going to get beyond the state of the art as it is today in alkane activation, it is going to take a fresh approach like the one Siluria is pursuing.”Siluria, founded in 2008, is the second commercial venture to get its start from Belcher's work in this area. Cambrios Technologies Corp. was founded in 2002 on the work of Belcher and Hu to produce electronic materials. Belcher remains a member of Siluria's board of directors and a scientific advisor.“The path to improved economics in the chemical industry lies in innovation in catalysis,” says Tkachenko. Adds Turner, “If Siluria's technology truly does provide for carbon/hydrogen activation at very low temperatures on solid inorganic catalysts, it would represent a significant breakthrough and open a wide variety of potential catalytic transformations.”Going forward, Siluria hopes to discover catalysts for other high-value target reactions, including those for acetic and acrylic acids, phenols, and to increase yields from existing processes such as those used for maleic anhydride and phthalic anhydride.

ChemInform Abstract: Transformation of Butanes Over ZSM-5 Zeolites. Part 2. Formation of Aromatic Hydrocarbons Over Zn-ZSM-5 and Ga-ZSM-5.

Selectivities for aromatic hydrocarbons in the transformation of butanes are greatly enhanced by incorporating zinc or gallium cations into H-ZSM-5. This work is concerned with the activation of butanes over Zn-ZSM-5 and Ga-ZSM-5 at 773 K. The zinc and gallium cations serve as catalysts for dehydrogenation of the starting alkanes. In the case of the transformation of butane, 80–85% of butane molecules undergo dehydrogenation on the cationic centres, while the remainder (15–20%) undergo cracking on the acidic sites. In the case of isobutane, ca. 70% of isobutane molecules undergo dehydrogenation and ca. 30% undergo cracking. The mode of activation of alkanes depends on the ease of carbenium-ion formation and the acid strength of the zeolite. The metal cations act also as very efficient catalytic centres for dehydrogenating the intermediate alkenes into aromatic hydrocarbons.Because of the higher acidity of Ga-ZSM-5, the intermediate alkenes undergo hydrogen-transfer reactions to form alkanes as well as aromatics. This leads to a lower yield of aromatics over Ga-ZSM-5 in comparison with Zn-ZSM-5.

ChemInform Abstract: Activation of Alkanes by Cr+: Unique Reactivity of Ground-State Cr+(6S) and Thermochemistry of Neutral and Ionic Chromium-Carbon Bonds.

Guided ion beam mass spectrometry is used to study the reactions of ground-state Cr{sup +}({sup 6}S) with propane, butane, methylpropane, dimethylpropane, and selectivity deuteriated propane and methylpropane. Thermal energy reactions of Cr{sup +}({sup 6}S) with 4-octyne are also investigated. Ground-state Cr{sup +} ions undergo no bimolecular reactions at thermal energies with any of the alkanes, but do react at elevated kinetic energies. The only products formed at thermal energy in the alkane systems are the collisionally stabilized adduct complexes. Approximate lifetimes for these adducts are determined. Analyses of the endothermic processes in the alkane systems yields 298 K bond energies for several chromium-ligand species. These include the neutral and ionic chromium methyl species [D{degrees}(Cr-CH{sub 3}) = 37.9 {+-} 2.0 kcal/mol and D{degrees}(Cr{sup +}-CH{sub 3}) = 30.3 {+-} 1.7 kcal/mol], several other chromium ion-alkyl species [D{degrees}(Cr{sup +}-C{sub 2}H{sub 5}) = 35.0 {+-} 2.1 kcal/mol, D{degrees}(Cr{sup +}-1-C{sub 3}H{sub 7}) = 32.1 {+-} 1.4 kcal/mol, D{degrees} (Cr{sup +}-2-C{sub 3}H{sub 7}) = 28.5 {+-} 1.3 kcal/mol], and chromium ion-vinyl and -various alkylidenes [D{degrees}(Cr{sup +}-C{sub 2}H{sub 3}) = 59.0 {+-} 2.3 kcal/mol, D{degrees}[Cr{sup +}=CHCH{sub 3}] = 52 {+-} 3 kcal/mol, D{degrees}[Cr{sup +}=CHCH{sub 2}CH{sub 3}] = 36 {+-} 3 kcal/mol, D{degrees}[Cr{sup +}=C(CH{sub 3}){sub 2}] = 39more » {+-} 3 kcal/mol]. The observed reactivity requires unusual reaction mechanisms that are discussed in detail. 60 refs., 9 figs., 4 tabs.« less

Precursor-mediated dissociation of n-butane on a PdO(1 0 1) thin film

We investigated the molecular adsorption and dissociation of n-butane on a PdO(1 0 1) thin film using temperature-programmed reaction spectroscopy (TPRS) experiments and density functional theory (DFT) calculations. At low coverage, n-butane adsorbs on PdO(1 0 1) in a molecular state that is more strongly bound than n-butane physisorbed on Pd(1 1 1). This molecularly adsorbed state of n-butane on PdO(1 0 1) corresponds to a σ-complex that forms on the rows of coordinatively unsaturated (cus) Pd atoms of the oxide surface. TPRS results show that a fraction of the n-butane layer undergoes C–H bond cleavage below ∼215 K and that the resulting fragments are completely oxidized by the surface upon continued heating. The evolution of product yields with the n-butane coverage as well as site blocking experiments provide strong evidence that the n-butane σ-complex serves as the precursor to initial C–H bond cleavage of n-butane on PdO(1 0 1). DFT calculations confirm the formation of an n-butane σ-complex on PdO(1 0 1). In the preferred bonding geometry, the n-butane molecule aligns parallel to a cus-Pd row and adopts a so-called η 1(2H) configuration with two coordinate H–Pd bonds per molecule. Our DFT calculations also show that σ-complex formation weakens C–H bonds, causing bond elongation and vibrational mode softening. For methane, we predict that coordination with a cus-Pd atom lowers the barrier for C–H bond cleavage on PdO(1 0 1) by more than 100 kJ/mol. These results demonstrate that dative bonding between alkane molecules and cus-Pd atoms serves to electronically activate C–H bonds on PdO(1 0 1) and suggest that adsorbed σ-complexes play a general role as precursors in alkane activation on transition metal oxide surfaces.

Carbene–Transition Metal Complexes Formed by Double CH Bond Activation

The activation of a single sp(3) C-H bond of alkanes and their derivatives by electron-rich transition metal complexes has been a topic of interest since the landmark work by Bergman and Graham in 1982. Ten years later, it was shown that compounds of 5d elements, such as osmium and iridium, even enable a double alpha-C-H bond activation of alkane or cycloalkane derivatives containing an OR or NR(2) functional group, thus opening up a new route to obtain Fischer-type transition metal carbene complexes. Subsequent work focused in particular on the conversion of methyl alkyl and methyl aryl ethers into bound oxocarbenes and also of dimethyl amines to bound aminocarbenes. In the context of this work, it was recently shown that square-planar oxocarbene-iridium(I) complexes prepared in this way exhibit an unusual mode of reactivity: They react with CO(2), CS(2), COS, PhNCO, and PhNCS by an atom- or group-transfer metathesis, which has no precedent. Organic azides RN(3) and N(2)O behave similarly. Recent results confirm that this novel type of metathesis can be made catalytic, thus offering a novel possibility for C-H bond functionalization.

ChemInform Abstract: Recent Applications in Catalysis of Surface Organometallic Chemistry

This paper reviews recent applications of well-defined silica-supported hydrides of the group 4 and 5 transition metals in the field of carbon-carbon and carbon-hydrogen bonds activation of alkanes. The synthesis and characterization of the zirconium hydride is presented. The monohydride (≡SiO) 3 Zr-H is obtained by hydrogen treatment at ca. 150°C of the well-defined surface species ≡Si-O-ZrNp 3 (Np = CH 2 C(CH 3 ) 3 ). This surface complex is formally an 8 electron species and is consequently very electrophilic. Similarly, hafnium and titanium hydrides are obtained by treatment under hydrogen of ≡Si-O-MNp 3 (M = Hf,Ti). In the case of titanium the reaction is not quantitative in the sense that a non-negligible amount of titanium(III) is formed. The tantalum hydride (≡SiO) 2 Ta-H is obtained by hydrogen treatment at ca. 150°C of (≡Si-O) x Ta(=CHC(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) 3-x (x = 1,2), prepared by reaction of Ta(=CHC(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) 3 with the hydroxyl groups of silica. Examples of applications of these hydrides in the field of the activation of alkanes at moderate temperatures are then given. All these surface hydrides can achieve the hydrogenolysis of alkanes at low temperature. When the titanium hydride is used, a simultaneous reaction of skeletal isomerization occurs. In all cases, the mechanism of C-C bond cleavage passes through an elementary step of β-alkyl transfer. The mechanism of hydroisomerization observed with the titanium hydride passes also by an elementary step of β-alkyl transfer but, in this case, the β-H elimination-olefin reinsertion occurs quite rapidly so that a skeletal isomerization also occurs. The zirconium hydride can also catalyze under olefin pressure the olefin polymerization and under hydrogen pressure the polyolefin hydrogenolysis. Here the equilibrium between the olefin insertion into a metal alkyl and the β-alkyl transfer is shown to occur with the same catalyst in agreement with the concept of microreversibility. A new catalytic reaction called 'alkane metathesis' has been discovered with the tantalum hydride. By this reaction, alkanes are catalytically transformed into higher and lower alkanes. The mechanism by which this reaction occurs is not fully understood. The products distribution, especially with labeled alkanes, is explained by a concerted mechanism by which a Ta-C bond and a C-C bond of the alkane can be cleaved and reformed simultaneously via a kind of four centered σ-bond metathesis which has no precedent in classical organometallic chemistry.

ChemInform Abstract: Catalytic Asymmetric C-H Activation of Alkanes and Tetrahydrofuran.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reversible Heterolytic Methane Activation of Metal‐Free Closed‐Shell Molecules: A Computational Proof‐of‐Principle Study

AbstractUtilization of the acid/base effects simultaneously is one of the basic principles used by transition‐metal (TM) complexes to activate H–H/C–H σ bonds. In principle, the high reactivity of metal‐free FLPs (frustrated Lewis pairs) towards H2 can also be attributed to these effects. On the basis of our proposed integrated FLPs, we pushed the effects to a higher (if not a limit) level at which the extremely unreactive methane C–H σ bond can be activated by our designed metal‐free closed‐shell molecules. Three molecules (M3c, M4b, and M4c) among the reported have activation free energies (22.4, 20.0, and 20.2 kcal mol–1, respectively) comparable with (or less than) the 22.3 kcal mol–1 of a TM model complex that features a Ti=N double bond. The derivative of the TM model has been experimentally shown to be capable of activating methane. Moreover, some of the activation reactions are (or nearly) thermoneutral. For example, the methane activations of M3c, M4b, and M4c are exothermic by –1.9, –4.5, and –5.2 kcal mol–1 of free energies, respectively. The kinetics and thermodynamics imply that the molecules could be further developed to realize catalytic methane activation. The electronic structure analyses reveal that, although our metal‐free molecules and the TM model complex share the same principle in activating the C–H bond, there are differences as to how they go about maintaining the effective active sites. The reported molecules could be the targets for experimental realizations. The strategy could be applied to the design of similar molecules to realize more general C–H bond activation of alkanes.

Alkane Oxidation on Rh(111) Single-Crystal Surfaces under High-Temperature, Short-Contact-Time Conditions: A Molecular Beam Kinetic Study

The kinetics of the partial oxidation of alkanes with molecular oxygen on Rh(111) single-crystal surfaces were studied by using a collimated effusive molecular beam under ultrahigh vacuum conditions. These experiments were conceived to probe the details of the mechanism of these processes under high-temperature, short-contact-time conditions. It was determined that the primary products in these reactions are H2 and CO, the components of syngas, as it has been suggested by the so-called direct mechanism. The production of water is also detected, but at a slower rate because of the slow formation of the surface hydroxo intermediate involved. Carbon dioxide, on the other hand, is never produced, because carbon monoxide desorbs before having the opportunity to react further with adsorbed oxygen atoms. Both surface and subsurface atomic oxygen species form during the reaction, but only the more labile surface intermediate is relevant, blocking Rh sites for alkane activation and reacting to form CO and H2O.