J. Phys. Chem. 1995,99, 673-680
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Hydrocarbon Reactivity with Early Transition Metal Atoms and Neutral Diatomic Metal Oxides in the Gas Phase J. Mark P a d s * and Rick D. Laileur Department of Chemistry, Trent University, Peterborough, Ontario, Canada K9J 7B8, and Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6
David M. Rayner Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, Canada KIA OR6 Received: August 3, 1994; In Final Form: October 21, 1994@
Absolute rate coefficients for the reactions of several hydrocarbons with early d-block metal atoms and their corresponding neutral diatomic oxides have been measured in the gas phase at 296 K, using a fast-flow reactor apparatus. Atomic Zr, Nb, and Ta vapors were generated by laser vaporization and entrained in a stream of He buffer gas. Metal atom reactivity with CH4, C3H8, i-C4H10, or C2H4 was monitored after introduction of the reactant gas downstream of the metal vapor source. Diatomic metal oxide reactivity was also investigated, using the same metals and Mo with the previously mentioned gases, by addition of an oxidant gas close to the source region. We report second-order rate coefficients for the reactions of Zr, Nb, Ta, ZrO, NbO, MOO, and TaO with C h , C3H8, i-C4H101 and C2H4. Trends in the rate coefficient data show that the activation energy of a given reaction correlates with the bond dissociation energy of the weakest C-H bond of the hydrocarbon involved. A C-H bond insertion step followed by the elimination of H2 is considered to be the operative mechanism for the reaction of both metal atoms and metal oxides with saturated hydrocarbons.
Introduction Studies of the reactions of organic molecules at small metallic centers such as atoms, atomic ions, and clusters are useful in developing a fundamental understanding of related processes in surface chemistry and catalysis. The well-defined nature of these metallic "reaction sites", as well as the ability to examine their reactions from a theoretical viewpoint without excessive simplification, makes them attractive models for more complex systems. Reactions of hydrocarbons at small metalic centers such as transition metal clusters, atomic and cluster ions, and metal oxide ion species have been investigated1-12 and have revealed complex size and structure-specific trends in the observed chemistry. In particular, the gas-phase chemistry of transition metal ions with alkanes has shown these species to be highly reactive with a varied ~ h e m i s t r y . ' - ~ JThe ~ reactivity of the entire fiist-row transition metal atomic ions with small alkanes has been examined by Weisshaar et al.12 Their observations show that there are strong chemical interactions between the metal ion and various simple alkanes for all metal ions and alkanes studied. Extensive studies by Weisshaar et al.13have been conducted on the first transition metal series to investigate the reactivity of these neutral species with a number of molecules including some small alkanes. Their work has shown that first-row transition metal atoms (M) are unreactive with unstrained alkanes in the gas phase, in contrast to their atomic ion congeners. Weisshaar et al. have also studied the reactivity of the second-row elements Y, Zr, Nb,Mo, and Pd atoms with ethane, propane, n-butane, and c y ~ l o p r o p a n e .Of ~ ~these, only Pd was shown to exhibit any measurable reactivity with an
* To whom all correspondenceshould be addressed at Trent University. e Abstract
published in Advance ACS Abstracts, December 1, 1994.
unstrained alkane.14315 We have recently communicated a portion of the present study in which Zr atoms were shown to be reactive with isobutane.16 Reactions of excited-state transition metal atoms with alkanes in cryogenic matrices are ~ e l l - k n o w n ' ~and J ~ generally lead to the production of a formal C-H bond insertion product. However, only one case of what may have been a ground-state transition metal atom reaction with an unstrained alkane in a cryogenic matrix has been reported. Zr atoms have been shown to react with isobutane and neopentane during co-condensation of Zr atomic vapor with the pure alkanes at 77 K.I9 Both C-H and C-C cleavage by Zr atoms were implicated in the analysis of the co-condensation reaction products. The greater reactivity in gas-phase transition metal ion systems, when compared with that of the corresponding neutral atoms, is generally attributed to (1) the greater strength of the metal-carbon bond in cationic organometallics,20(2) attractive, long-range forces between metal ions and hydrocarbons which are not present with neutral metal atoms,1°J3and (3) stabilization of the dfl-'sl and dn electronic configurations due to the increased effective nuclear charge in metal ions which greatly reduces repulsive energy barriers associated with the dfl-2s2 electronic configuration.l3 The latter configuration is typically the ground state or a low-lying excited state of neutral metal atoms. Gas-phase reactions of alkenes with all first-row and many second-row transition metal atoms have been previously studied, chiefly by Weisshaar et al.13J4and Mitchell et aL21 Some reactions are believed to proceed via association complex formation, since estimated preexponential factors for these reactions are generally close to the gas kinetic limit and exhibit obvious pressure dependence.*' Other metals are believed to react by a bimolecular elimination process, possibly involving initial formation of a long-lived association or C-H bond
0022-3654/95/2099-0673$09.00/0 0 1995 American Chemical Society
674 J. Phys. Chem., Vol. 99, No. 2, 1995
Pamis et al.
TABLE 1: Electronic Transitions Used To Excite LIF for insertion ~ o m p l e x . Of ~ ~the . ~ first-row ~ transition metals studied the Detection of Metal Atoms and Diatomic Metal Oxides to date, only Ni and Cu have been found to react with ethene, while Sc, Ti, V, and Ni have been shown to react with larger atom/ diatomic oxide transition wavelength/nm description alkenes.13,21 Significantly faster reactions with alkenes, including ethene, are observed with second-row elements Y,Zr, Nb, 593.69 atomic transition42 zr Z3p3-a3F2 and Pd. This enhancement is thought to be due to rather subtle 473.35 atomic transition42 Nb Z4D07n-a6D5,2 428.15 atomic transition4* Ta Z4Pslz-a4Fsn effects such as favorable long-range overlap of the larger, more 586.0 R band head43 zro C’Z+-XCz+ diffuse 4d orbitals during the initial stages of bond formation 469.0 R band head@ Nbo c4x--x4z+ and stronger bonds in the initially produced metal-alkene TaO Ln~n-XtAsn 428.28 R band head (0,0)45 complex. R, band head& MOO B5n-1-X511-1 482.69 The activity of pure and alkali metal promoted bulk metal oxides in oxidative coupling of methane is w e l l - d o c ~ m e n t e d . ~ ~ * ~oxides ~ with three alkanes and ethene. The results show these The first step in this type of process is believed to be a C-H species to be moderately reactive with isobutane and highly bond abstraction at an 0- center on the surface of the solid, reactive with ethene. Consideration of trends in the reactivity occurring as a natural or an alkali-metal induced defect. Despite observed suggest that a single mechanism involving C-H bond the fundamental nature of neutral diatomic transition metal cleavage can account for most of the observed changes in the oxides (MO) as models for 0- centers, virtually nothing is rate coefficients when the reactant alkane is varied and that known about their reactivity with alkanes, although the kinetics electronic structural effects play a less important role in of the reactions of T i 0 and ScO with oxidant molecules have determining the reactivity of the metal system in question. A been reported by Ritter and W e i ~ s h a a r . ~The ~ , ~H~ atom preliminary report has been made on the reactivity of one abstraction reactions of the diatomic oxides LiO, MgO, and A10 second-row transition metal, Zr,and a neutral diatomic transition with methane have been considered in a theoretical investigation metal oxide, Zro, with isobutane at room temperature.16 by Berve and Pettersson?6 They have predicted that significant activation energy barriers of 25-67 kT mol-’ are involved. The Experimental Approach contribution of an abstraction process to the reaction of A10 The fast-flow reactor employed in the present study has been with methane has recently been examined by Belyung et ~ 1 . ~ ’ described p r e v i o ~ s l y . ~ Briefly, ~ ~ ~ *metal ~ ~ atoms were generated They have concluded that one or more channels, such as C-H by focusing the full output of a pulsed XeCl excimer laser bond insertion, are also involved in the overall mechanism at (Lumonics 860-4) on a rotating and translating metal target. lower temperatures. Vapor containing metal atoms was entrained in a continuous The reactions of diatomic transition metal oxide ions with stream of He buffer gas (High Purity, 99.995%, further purified alkanes and other molecules are reasonably well documented. by passing it through a liquid nitrogen-cooled molecular sieve The reactions of VO+ and TiO+ with CHq or H2 have been trap) at ~ 5 Torr 0 in a 2 mm diameter channel. The He gas examined by Kappes and Staley? who found that the former flow rate was maintained at 15 000 sccm using a mass flow oxide ion reacted much more quickly than the latter, with a controller (MKS, Canada). The buffer gas containing the metal rate coefficient of 1.5 x lo-” cm3 molecule-’ s-l for reaction vapor expanded into the flow reactor. The reactor tube was with both CHq and H2. Jackson et al. have shown that FeO+ maintained at pressures between 0.45 and 9.0 Torr, by adjusting reacts readily with saturated hydrocarbons, mainly by a mechaa gate valve downstream of the detection zone and upstream of nism involving C-H insertion followed by H2O elimination.6 an Edwards EH2600 580 L s-l mechanical booster pump backed Alkane and alkene reactions with gas-phase CrO+ have also by an Edwards ElM275 rotary pump. Methane (Research been reported by Kang and B e a ~ c h a m p . ~Several ,~ processes Grade, 99.9995%), propane (Instrument Grade, 99.5%), isobuincluding C-C cleavage, dehydrogenation, and oxidation of tane (Research Grade, 99.995%), ethene (CP, 99.5%), n-butane alkanes were observed, showing this species to be highly (CP, 99.0%), and SF6 (CP, 99.8% liquid phase) were introduced reactive. through a shower-head type moveable inlet, positioned 70 cm The absence of information on the reactivity of neutral from the detection region and 46 cm downstream from the metal transition metal oxides is primarily due to the difficulty of vapor source. Reactant gas flow rates were maintained using producing these high-temperature species at lower temperatures a mass flow controller (MKS, Canada). All gases were supplied in the gas phase. In an attempt to address this problem, a simple by Matheson, Canada. variant of a fast-flow, laser ablation source reactor, originally Metal atoms and diatomic metal oxides were detected by designed for the investigation of metal atoml3,l4 and metal laser-induced fluorescence (LIF), using a pulsed, excimer~ l u s t e r reactions, ~ ~ , ~ ~ has been used in the present study. pumped dye laser beam (Lumonics HyperDYE 300 and 860 Addition of an oxidant gas, such as oxygen, close to the source XeCl excimer lasers) perpendicularly oriented with respect to region of the original reactor, results in the generation of both the gas flow direction and the photomultiplier tube axis. diatomic metal oxides from their corresponding neutral metal Table 1 lists the electronic transitions probed by LIF in the atoms. The flight time from the source region to the reagent detection of the metals and their corresponding diatomic inlet is made long enough to allow the newly formed metal oxides.30 Total fluorescence was collected with a Hamamatsu oxide diatomics to thermalize vibrationally before contact with IP28 photomultiplier (PMT). The PMT output was conditioned a molecular reactant occurs. Depletion of the oxidant in the with a current amplifier, sampled and held, and passed to a early stages of the flow, coupled with the relatively slow personal computer. oxidation of diatomic metal oxides to higher oxides, results in Given the pulsed nature of the metal atom source and a stable signal from the metal oxide with which reliable reaction detection laser, reagent contact times may be obtained unamkinetics measurements may be made. We have explored this biguously in the following way.28,29The “time of flight” for approach as a general method for the investigation of MO the metal atom or oxide between the source and the detection reactivity and find it promises to be widely applicable. zone is f i s t determined by varying the probe laser delay time to maximize the LIF signal intensity associated with the metallic In this paper we present the full details of a study involving reagent. Contact times are then easily obtained by multiplying the reactivity of several transition metal atoms and diatomic
J. Phys. Chem., Vol. 99, No. 2, 1995 675
Reactivity of Transition Metals and Metal Oxides the time-of-flight values by the fraction of the length between the source and the detection zone in which the reactant gas is in contact with the metal reagent. Typically, the delay between the vaporization and probe lasers varied between 5.5 and 95.0 ms depending on the flow tube pressure. This delay range corresponds to contact times of 3.3-57.3 ms. Diatomic metal oxides were generated by addition of air into the flow system through an inlet near the source. The oxidant gas pressure was critical, as the diatomic metal oxide (MO) oxidized further to produce higher oxides in the presence of excess oxidant. The flow rate of oxidant was optimized to give the most intense LIF signal for the metal oxide in question. Under conditions of high total flow tube pressure, the high reactivity of the atoms with molecular oxygen resulted in there often being sufficient trace oxide impurities in the metal rod to produce the desired diatomic metal oxide. All other aspects of the experiments with the diatomic metal oxides were as reported above for the metal atoms. Under pseudo-first-order removal conditions, the partial pressure of M or MO, p(M), at the detection point is given by eq 1.
where p(M)o is the initial partial pressure of M or MO, k(2) is the second-order rate coefficient for removal of M or MO,p(Q)is the partial pressure of a molecular quencher, and z is the reagent contact time.28,29,31Under conditions of full excitation saturation, the LIF signal (Z) at the detection zone is directly proportional to the number density of the species detected. Thus, a plot of ln(Z/Zo) versus p(Q)z (eq 2) yields a linear curve with slope of -H2) for a simple, second-order removal process, where ZO is the LIF signal intensity in the absence of a molecular quencher .
Experimental measurements of second-order rate coefficients were obtained by measuring LIF intensities at the detection zone for different partial pressures of quencher molecules. For most of the systems examined, a weak chemiluminescent emission background signal (
