Trapping-mediated Dissociative Chemisorption Research Articles

Room-temperature dissociative hydrogen chemisorption on boron-doped fullerenes

Using first-principles electronic structure calculations, we show that trapping-mediated dissociative chemisorption of ${\text{H}}_{2}$ molecules may occur on boron-doped fullerenes. Employing the Polanyi--Wigner equation and the van't Hoff--Arrhenius law parameterized by the results of first-principles calculations, we find that a ${\text{H}}_{2}$ molecule adsorbed on boron-doped fullerenes can be dissociated without additional catalysts. The dissociation occurs in $\ensuremath{\sim}0.5\text{ }\text{ps}$ at room temperature, which is also supported by independent molecular dynamics simulations. Our findings indicate that boron-doped fullerenes can be used as an atomic hydrogen storage material, but not a molecular hydrogen one, at ambient conditions.

Multilayer-Induced Reaction of Cyclobutane on Ir(111): Identification of Reaction Products and Quantification of Reaction Kinetics

We have studied the low temperature adsorption of cyclobutane on the hexagonally close-packed (hcp) Ir(111) surface using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Monolayer and submonolayer cyclobutane coverages desorb essentially completely in TPD experiments, whereas in the presence of a condensed cyclobutane multilayer, significant reaction of first-layer cyclobutane is observed. Based on the HREELS and TPD measurements, a C4H8 metallacycle is believed to be the predominate reaction intermediate that is formed in the initial reaction step during multilayer-induced reaction (MIR) of cyclobutane on Ir(111). The TPD spectra suggest that MIR of cyclobutane on Ir(111) results in three different reaction products: (1) surface carbon and hydrogen adatoms formed via complete decomposition of cyclobutane, (2) desorption of n-butane, and (3) desorption of 1-butene. Moreover, the extent of MIR has been quantified and is used in conjunction with a kinetic model in order to quantify the kinetics of the MIR of cyclobutane on this surface. The MIR kinetics are well described by a rate expression which is first order in the coverage of unreacted cyclobutane and the rate coefficient of which can be expressed in Polanyi−Wigner form with a preexponential factor of kr(0) = 6.4 × 1013 s-1 and an activation energy of Er = 8 500 ± 700 cal/mol. This value represents a significant decrease in the activation barrier for reaction of cyclobutane on Ir(111) compared to the value of 10 270 cal/mol recently measured for the trapping-mediated dissociative chemisorption of cyclobutane on this same surface in the submonolayer regime at low temperature (300−400 K). This activation barrier reduction for MIR of cyclobutane on Ir(111) is qualitatively similar to that observed recently for the MIR of cyclobutane on Ru(001). The potential implications of these results toward providing a better understanding of catalytic reactions that occur at the solid−liquid interface are discussed.

Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.

Quantification of multilayer-induced dissociative chemisorption: cyclobutane on Ru(001)

Multilayer-induced dissociative chemisorption of cyclobutane on Ru(001) has been quantified using temperature programmed desorption (TPD). Monolayer and submonolayer coverages of cyclobutane desorb essentially completely, with negligible dissociative chemisorption upon heating. The adsorption of cyclobutane multilayers, however, results in significant dissociative chemisorption. By varying the initial coverage and the TPD ramp rate, we are able to change the maximum temperature at which multiple layers remain on the surface, thus altering the extent of reaction. From these measurements we are able to quantify the kinetics of the reaction. We find that the reaction kinetics are well described by a rate expression which is first order in the coverage of unreacted cyclobutane in the first layer, and whose rate coefficient can be expressed in Polanyi–Wigner form with a pre-exponential factor of 5.0×10 10 s −1 and an activation energy of 7510±500 cal/mol. The latter represents a significant decrease from the barrier of 10 090±180 cal/mol measured previously for the trapping-mediated dissociative chemisorption of cyclobutane in the submonolayer regime. This reduction in activation barrier has potential implications to the study of catalytic reactions at the solid–liquid interface, which are discussed.

Interaction of cyclobutane with the Ru(001) surface: Low-temperature molecular adsorption and dissociative chemisorption at elevated surface temperature

We have studied the interaction of cyclobutane with the hexagonally close-packed Ru(001) surface. High-resolution electron energy loss spectroscopy (HREELS) has been used to identify the vibrational modes of both c-C4H8 and c-C4D8 adsorbed at 90 K as a function of cyclobutane exposure. We have observed a vibrational mode not observed in the gas phase at 2600 cm−1 (2140 cm−1) which is attributed to the strong interaction of the cyclobutane C–H (C–D) bonds with the ruthenium surface. Two different adsorption geometries for cyclobutane on Ru(001) have been proposed based on the dipolar activity of this softened C–H mode. We have also measured the trapping-mediated dissociative chemisorption of both c-C4H8 and c-C4D8 at surface temperatures between 190 and 1200 K. The measured activation energies with respect to the bottom of the physically adsorbed well for c-C4H8 and c-C4D8 are 10 090±180 and 10 180±190 cal/mol, respectively. The trapping-mediated chemisorption of cyclobutane is believed to occur via C–C bond cleavage, as judged by the absence of a kinetic isotope effect. The measured ratios of the preexponential factors for desorption relative to reaction of 21±2 and 47±4 for c-C4H8 and c-C4D8 respectively, are in the expected range considering the greater entropy gain associated with the transition state for desorption relative to the transition state for C–C bond cleavage.

Direct and trapping-mediated dissociative chemisorption of methane on Ir(111)

The initial probabilities of dissociative chemisorption 13CH 4 and CD 4 have been measured on the close-packed Ir(111) surface at both low and high total pressures. At pressures below ∼ 10 −3 Torr, trapping-mediated dissociative chemisorption is the dominant reaction mechanism since the gas temperature is equal to the wall temperature of 300 K, with activation energies for C H (C D) bond cleavage of 12.6 kcal/mol for 13CH 4 and 13.7 kcal/mol for CD 4 with respect to the gas phase energy zero of the methane infinitely far from the surface and at rest. For both methane isotopomers the ratio of the pre-exponential factor for desorption relative to that of reaction is ∼ 180 due to the larger phase space available to the molecule for desorption relative to reaction. The measured differences in C H versus C D bond activation for methane are attributed to zero-point energy differences between each isotopomer and point to classical over-the-barrier reaction dynamics as the reaction pathway for trapping-mediated chemisorption. The incident kinetic energy of the impinging methane was increased by diluting the methane in argon at a total pressure of 1 Torr which raised the gas temperature of the methane to the surface temperature. In this case, methane chemisorbs dissociatively via both trapping-mediated and direct C H bond activation. Direct activation of 13CH 4 is characterized by an activation energy of 17.4 kcal/mol and a pre-exponential factor of ∼ 0.61, while direct activation of CD 4 occurs with an activation energy of 18.1 kcal/mol and the same pre-exponential factor. These activation energies are averages of a distribution of activation barriers which occur because of the nature of the multidimensional potential energy surface describing the interaction of the methane with the surface.

Direct collisionally activated and trapping-mediated dissociative chemisorption of neopentane on clean Pt(111): the activity of surface defect sites

Molecular beam results indicate that the dissociative chemisorption of neopentane on Pt(111) occurs by both direct collisionally activated and trapping-mediated mechanisms. Direct dissociation dominates at translational energies greater than about 110 kJ mol −1, exhibits normal energy scaling and is nearly independent of the surface temperature. The trapping-mediated pathway, which displays a significant temperature dependence, is the controlling dissociative adsorption mechanism at gas-molecule translational energies less than 110 kJ mol −1. Experiments performed on Pt(111) surfaces with differing defect-site densities indicate that trapping-mediated dissociation is facilitated by surface defects. The measured surface-temperature dependence of the trapping-mediated dissociation probability was accurately reproduced by a model which assumes that the rate of migration to defects is rapid compared to the dissociation and desorption rates.

Trapping-mediated dissociative chemisorption of ethane and propane on Ru(001)

The initial probabilities of precursor-mediated dissociative chemisorption of 13C 2H 6, C 2D 6, C 3H 8, CH 3CD 2CH 3 and C 3D 8 have been measured on the hexagonally close packed Ru(001) surface. The activation energies for CH bond cleavage are 9310 cal mol −1 for 13C 2H 6 and 9900 cal mol −1 for C 2D 6 with respect to the bottom of the physically adsorbed well of the ethane molecule. For each isotopomer of propane, these measured activation energies are 10 430, 10 620 and 10 910 kcal mol −1 for C 3H 8, CH 3CD 2CH 3 and C 3D 8, respectively. For each alkane investigated the ratio of the preexponential factor for desorption relative to that of reaction is ∼ 100 due to the larger phase space available to the molecule for desorption relative to reaction. An examination of the selective activation of primary (1°) versus secondary (2°) CH bonds in propane reveals that 2° CH bond cleavage is favored energetically by 240 cal mol −1 and entropically by a factor of 2.1 over 1° bond cleavage. However, the formation of 1° Ru-propyl intermediates is favored over the formation of 2° Ru-propyl intermediates at all investigated temperatures due to the greater number of 1° CH bonds in propane compared to 2° CH bonds. The measured differences in CH versus CD bond activation for both ethane and propane are attributed to zero-point energy differences between each isotopomer and point to classical over the barrier reaction dynamics as the reaction pathway.

Trapping-mediated dissociative chemisorption of cyclopropane on Ru(001) via CC bond cleavage

The initial probabilities of precursor-mediated dissociative chemisorption of c-C 3H 6 and c-C 3D 6 have been measured on the hexagonally close-packed Ru(001) surface at surface temperatures from 400 to 1100 K. The measured activation energies for both c-C 3H 6 and c-C 3D 6 are 9470±120 cal mol −1 with respect to the bottom of the physically adsorbed well. Trapping-mediated dissociative chemisorption of cyclopropane occurs via CC rather than CH bond cleavage at all temperatures studied, as judged by the lack of a kinetic isotope effect. The preferred reaction pathway of CC bond cleavage is a consequence of the ring strain contained in molecular cyclopropane. For both isotopomers of cyclopropane, the ratio of the pre-exponential factor of desorption to that of reaction is ∼100 due to the larger phase space available to the molecule for desorption relative to reaction.

Trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110)

We have employed molecular beam techniques to investigate the molecular trapping and trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110) at low beam translational energies, Ei≤5 kcal/mol, and surface temperatures, Ts, from 85 to 1200 K. For Ts=85 K, C3H8 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, equal to 0.94 at Ei=1.6 kcal/mol and ξ=0.86 at Ei=5 kcal/mol. At Ei=1.9 kcal/mol and Ts=85 K, ξ of C3D8 is equal to 0.93. From 150 K to approximately 700 K, the initial probabilities of dissociative chemisorption of propane decrease with increasing Ts. For Ts from 700 to 1200 K, however, the initial probability of dissociative chemisorption maintains the essentially constant value of 0.16. These observations are explained within the context of a kinetic model which includes both C–H (C–D) and C–C bond cleavage. Below 450 K propane chemisorption on Ir(110) arises essentially solely from C–H (C–D) bond cleavage, an unactivated mechanism (with respect to a gas-phase energy zero) for this system, which accounts for the decrease in initial probabilities of chemisorption with increasing Ts. With increasing Ts, however, C–C bond cleavage, the activation energy of which is greater than the desorption energy of physically adsorbed propane, increasingly contributes to the measured probability of dissociative chemisorption. The activation energies, referenced to the bottom of the physically adsorbed molecular well, for C–H and C–C bond cleavage for C3H8 on Ir(110) are found to be Er,CH=5.3±0.3 kcal/mol and Er,CC=9.9±0.6 kcal/mol, respectively. The activation energies for C–D and C–C bond cleavage for C3D8 on Ir(110) are 6.3±0.3 kcal/mol and 10.5±0.6 kcal/mol, respectively. The desorption activation energy of propane from Ir(110) is approximately 9.5 kcal/mol. These activation energies are compared to activation energies determined recently for ethane and propane adsorption on Ir(111), Ru(001), and Pt(110)–(1×2), and ethane activation on Ir(110).

Trapping-mediated dissociative chemisorption of propane on Ir(110)

We have employed molecular beam techniques to investigate the molecular trapping and trapping-mediated dissociative chemisorption of propane on Ir(110) at low beam translational energies, Ei≤5 kcal/mol, and a wide range of surface temperatures, Ts, from 85 to 1200 K. At Ei=1.6 kcal/mol and Ts=85 K, molecular propane is physically adsorbed on Ir(110) with a trapping probability of 0.94. At Ei=5 kcal/mol and Ts=85 K the trapping probability drops to 0.86. For Ts≳150 K propane is dissociatively chemisorbed, with initial probabilities of dissociative chemisorption decreasing with increasing surface temperature from 150 to approximately 700 K. At surface temperatures from 700 to 1200 K, the initial probability of dissociative chemisorption maintains the essentially constant value of 0.16. These observations are explained within the context of a kinetic model which includes both C–H and C–C bond cleavage. Below 500 K propane chemisorption on Ir(110) arises essentially solely from C–H bond cleavage, an unactivated mechanism (with respect to a gas-phase energy zero) for this adsorption system, which accounts for the decrease in initial probabilities of chemisorption with Ts. With increasing Ts, however, C–C bond cleavage, the activation energy of which is greater than the desorption energy of physically adsorbed propane, increasingly contributes to the measured probability of dissociative chemisorption. The activation energies, referenced to the bottom of the physically adsorbed molecular well, for C–H and C–C bond cleavage for C3H8 on Ir(110) are found to be Er,CH=5.3±0.3 kcal/mol and Er,CC=9.9±0.6 kcal/mol, respectively, while the desorption activation energy of propane from Ir(110) is approximately 9.5 kcal/mol. These activation energies are compared to activation energies determined recently for ethane and propane adsorption on Ir(111), and ethane activation on Ir(110).

Kinetics and dynamics of the trapping-mediated dissociative chemisorption of oxygen on Ru(001)

Supersonic molecular beam techniques were used to determine the initial dissociative chemisorption probability, S0, of O2 on the Ru(001) surface as a function of incident kinetic energy, Ei, angle of incidence, θi, and surface temperature, Ts. For Ts between 77 and 900 K, two kinetic energy regimes are observed as Ei increases from 30 to 700 meV. In this article we discuss the mechanism for chemisorption in the low kinetic energy regime. S0 decreases with Ei for incident energies less than ∼70 meV, and there is little dependence of S0 on θi in this low energy range. Additionally, S0 decreases rapidly as Ts is increased in this regime, and we believe that a simple trapping-mediated mechanism dominates adsorption. The low Ei data are fit to a simple one-dimensional potential energy surface model that includes the molecule initially trapping into a physical adsorption well and then surmounting a barrier into the dissociatively chemisorbed state. Analysis of an Arrhenius construction yields a difference in activation energies for desorption and dissociation from the physically adsorbed state that is approximately 28 meV below the vacuum zero, for both Ei =30 and 53 meV.

Dynamics of trapping-mediated dissociative chemisorption of molecular oxygen on reconstructed Ir(110)

We have measured the initial probability of dissociative chemisorption of molecular oxygen on the reconstructed Ir(110) surface as a function of incident energy, E i, and polar angle, θ i, for surface temperatures of 88, 400, and 1000 K using a supersonic molecular beam. Under these conditions, when the incident energy is less than 4 kcal/mol, the probability of chemisorption decreases with increasing incident energy, indicative of a trapping-mediated mechanism. This probability is insensitive to polar angle for all conditions reported here, indicating that the total incident energy, rather than the component of energy perpendicular to the surface (the normal energy), dictates the trapping.

Quantification of the Influence of Surface Structure on C—H Bond Activation by Iridium and Platinum

The trapping-mediated dissociative chemisorption of ethane on the closest packed Ir(111) surface has been investigated, and the activation energy and preexponential factor of the surface reaction rate coefficient have been measured. These results are compared to those of ethane activation on Pt(111) and on the missing row reconstructed Ir(110)-(1x2) and Pt(110)-(1x2) surfaces, allowing a quantitative determination of the effect surface structure has on the catalytic activation of C-H bonds. In the order Pt(111), Pt(110)-(1x2), Ir(111), and Ir(110)-(1x2), the activation energies for the dissociative chemisorption of ethane are 16.6, 10.5, 10.3, and 5.5 kilocalories per mole, demonstrating that the electronic and geometric effects are of approximately equal importance for ethane activation on these catalysts.

Quantification of primary and tertiary CH bond cleavage in alkane activation: isobutane dissociation on Pt(110)-(1 × 2)

The trapping-mediated dissociative chemisorption of three isotopes of isobutane ((CH 3) 3CH, (CH 3) 3CD, and (CD 3) 3CH) has been investigated on the Pt(110)-(1 × 2) surface, and both the apparent activation energies and the apparent pre-exponential factors of dissociative chemisorption have been measured. Of more significance, the probabilities of primary and tertiary CH bond cleavage for alkane activation on this surface were evaluated. The activation energy of primary CH bond cleavage is 137 cal/mol greater than that of tertiary CH bond cleavage for isobutane, and the two true activation energies that embody the single measured activation energy were determined for each of the three isotopes. The true pre-exponential factors were evaluated, and tertiary CH bond cleavage was also found to be preferred on entropie grounds. The magnitude of the effect is a factor of 3.2.

Quantification of primary and tertiary C bond cleavage in alkaline activation: isobutane dissociation on Pt(110)-(1 sx 2)

The trapping-mediated dissociative chemisorption of three isotopes of isobutane ((CH 3) 3CH, (CH 3) 3CD, and (CD 3) 3CH) has been investigated on the Pt(110)-(1 × 2) surface, and both the apparent activation energies and the apparent pre-exponential factors of dissociative chemisorption have been measured. Of more significance, the probabilities of primary and tertiary C bond cleavage for alkane activation on this surface were evaluated. The activation energy of primary C bond cleavage is 137 cal/mol greater than that of tertiary C bond cleavage for isobutane, and the two true activation energies that embody the single measured activation energy were determined for each of the three isotopes. The true pre-exponential factors were evaluated, and tertiary C bond cleavage was also found to be preferred on entropic grounds. The magnitude of the effect is a factor of 3.2.

Quantification of Primary Versus Secondary C-H Bond Cleavage in Alkane Activation: Propane on Pt

The trapping-mediated dissociative chemisorption of three isotopes of propane (C(3)H(8), CH(3),CD(2)CH(3), and C(3)D(8)) has been investigated on the Pt(110)-(1 x 2) surface, and both the apparent activation energies and the preexponential factors of the surface reaction rate coefficients have been measured. In addition, the probabilities of primary and secondary C-H bond cleavage for alkane activation on a surface were evaluated. The activation energy for primary C-H bond cleavage was 425 calories per mole greater than that of secondary C-H bond cleavage, and the two true activation energies that embody the single measured activation energy were determined for each of the three isotopes. Secondary C-H bond cleavage is also preferred on entropic grounds, and the magnitude of the effect was quantified.

Trapping-mediated dissociative chemisorption of ethane on Ir(110)-(1×2)

Evidence is presented to support a trapping-mediated dissociative chemisorption mechanism for ethane interacting with an Ir(110)-(1×2) surface. The data were obtained from supersonic molecular-beam measurements with an incident kinetic energy Ei ranging between 1.2 and 24.1 kcal/mol, a surface temperature Ts between 154 and 500 K, and an incident angle θi between 0° and 45°. For Ei less than approximately 13 kcal/mol, the probability of dissociative chemisorption S0 decreases rapidly with increasing Ts. For a surface temperature of 154 K, S0 decreases with increasing Ei for 1.2≤Ei ≤13.4 kcal/mol, consistent with a trapping-mediated chemisorption mechanism. Indeed, the data also support quantitatively a kinetic model consistent with a trapping-mediated chemisorption mechanism. The difference in the activation energies for desorption and chemisorption from the physically adsorbed, trapped state Ed −Ec is 2.2±0.2 kcal/mol. In the trapping-mediated regime, S0 is found to be rather insensitive to incident angle, scaling with Ei cos0.5 θi .