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2009, 113, 8000–8001 Published on Web 04/16/2009
The Kinetics of Ethylidyne Formation from Ethylene on Pd(111) D. Stacchiola† and W. T. Tysoe*,‡ Department of Chemistry and Laboratory for Surface Studies, UniVersity of WisconsinsMilwaukee, Milwaukee, Wisconsin 53211, and Department of Chemistry, Michigan Technological UniVersity, Houghton, Michigan 49931 ReceiVed: March 10, 2009; ReVised Manuscript ReceiVed: April 09, 2009
Recent density functional theory (DFT) calculations of the activation energy for the conversion of ethylene to ethylidyne on Pd(111) (Moskaleva, L. V.; Chen, Z.-X.; Aleksandrov, H. A.; Mohammed, A. B.; Sun, Q.; Ro¨sch, N. J. Phys. Chem. C 2009, 113, 2515) predicted an activation barrier with respect to gas-phase ethylene of ∼46 kJ/mol, substantially lower than the results of previous DFT calculations. Thus, the kinetics of formation of ethylidyne from ethylene on Pd(111) are measured as a function of sample temperature using reflection-absorption infrared spectroscopy to monitor the ethylidyne coverage as a function of time. The results yield an experimental value of the height of the activation barrier with respect to gas-phase ethylene of 49 ( 5 kJ/mol, in good agreement with the results of the DFT calculations. The agreement between experiment and theory indicates that the proposed ethylidyne formation pathway involving an initial, ratelimiting ethylene dehydrogenation step to form vinyl species that finally form ethylidyne via an ethylidene intermediate is correct. Introduction The formation of ethylidyne species from ethylene on noble transition-metal surfaces has been known for many years.1-4 While adsorbed ethylene and not the ethylidyne species is the major contributor to the formation of ethane during palladiumcatalyzed ethylene hydrogenation,5 the formation and removal of ethylidyne species does affect the sites available for ethylene adsorption and hydrogenation6 and therefore the overall hydrogen- and ethylene-pressure dependences of the reaction. A recent paper revisited the pathway and energetics of ethylidyne formation from adsorbed ethylene calculated using density functional theory (DFT) and suggested that the overall reaction proceeds via an initial, rate-limiting dehydrogenation step to form vinyl species that ultimately form thermodynamically stable ethylidyne via an intermediate ethylidene moiety.7 These general conclusions regarding the reaction pathway are in accord with those found previously,8 also using DFT, but where the activation energy for the formation of vinyl species from adsorbed ethylene was found to be substantially larger than in the earlier calculations. Specifically, the activation barrier for vinyl formation referenced to the energy of gas-phase ethylene was calculated to be ∼46 kJ/mol,7 substantially lower than the value of ∼89 kJ/mol found previously.8 While both calculations arrived at the same mechanistic conclusions, the disparity between the energy barriers for the two calculations warrants an experimental measurement of the temperature dependence of the ethylidyne formation kinetics to resolve this issue. The results of these experiments are presented in the following by * To whom correspondence should be addressed. Phone:+(414)229-5222. Fax: +(414)229-5036. E-mail:
[email protected]. † University of WisconsinsMilwaukee. ‡ Michigan Technological University.
10.1021/jp902158s CCC: $40.75
measuring the ethylidyne coverage as a function of exposure at various sample temperatures using infrared spectroscopy. Experimental Section The apparatus that was used to collect the infrared spectra has been described in detail elsewhere.9 Briefly, the sample cell used for these experiments is constructed from a 2 3/4 in. flange six-way cross which was modified by moving one flange by ∼20° to allow infrared radiation to impinge on the sample with the optimal 80° infrared incidence angle. The cell is attached to the main chamber via a gate valve, which when closed completely isolates the infrared cell from the ultrahigh vacuum chamber and when open allows sample transfer into it. Spectra were collected with a Bruker Equinox spectrometer. The Pd(111) single crystal was cleaned using a standard protocol, and its cleanliness was monitored using Auger spectroscopy and temperature-programmed desorption collected following oxygen adsorption. The ethylene (Matheson, researchgrade) was transferred to a glass bottle, which was attached to the gas-handling line for introduction into the vacuum chamber. Results and Discussion The coverage of ethylidyne, measured from the integrated area under the infrared feature at 1330 cm-1,5 plotted as a function of ethylene exposure in Langmuir (L, l L ) 1 × 10-6 Torr s), for various sample temperatures, is displayed in Figure 1. In this case, the measured ethylene pressures are not corrected for ionization gauge sensitivity. This reveals that ethylidyne species are formed relatively slowly and that the uptake kinetics depend on temperature. It has been shown previously that the ethylidyne overlayer saturates at a coverage of 0.25 (where coverages are referenced to the palladium atom density on the (111) surface),5 so that the scale of the plot is adjusted to yield 2009 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8001
Figure 1. Plot of ethylidyne coverage on Pd(111) as a function of exposure for adsorption at various temperatures. The adsorption temperature is marked adjacent to the corresponding spectrum. Shown as an inset is an Arrhenius plot of ln(Akc/kd), where kc is the rate constant for the formation of ethylidyne from ethylene and kd is the ethylene desorption rate constant, versus 1/T, the adsorption temperature.
this saturation coverage. It has been demonstrated that the integrated area under the 1330 cm-1 feature scales reasonably well with the ethylidyne coverage.10 The ethylidyne formation kinetics are modeled by assuming that ethylene adsorbs reversibly on the Pd(111) surface and converts via a first-order process to ethylidyne species, in accord with the reaction pathways calculated using DFT.7,8 Thus, S is the sticking probability of ethylene on Pd(111), kd the desorption rate constant of ethylene, and kc the rate constant for ethylidyne formation from adsorbed ethylene. Assuming that ethylene adsorbs via Langmuir kinetics, an adsorption isotherm for ethylene can be written as
Θethylene )
APethylene APethylene kd + 0.33
(1)
where the saturation coverage of ethylene is taken to be 0.3311,12 and A ) SN/(2πmkTg)1/2, where N is the number of palladium atoms per unit area on the (111) face, Tg the gas temperature, m the mass of the molecule (ethylene), and k the Boltzmann constant. In the case of the adsorption experiments shown in Figure 1, Pethylene < 10-6 Torr, so that AP , kd, and eq 1 can be simplified to
Θethylene )
(2)
The rate of ethylidyne formation is taken to be first order in ethylene coverage and is given by
(
Θethylidyne dΘethylidyne ) kcΘethylene 1 dt 0.25
)
(3)
where the saturation coverage of ethylidyne is taken to be 0.25. Combining eqs 2 and 3 and integrating yields
(
(
Conclusions The formation kinetics of ethylidyne from ethylene are measured on Pd(111) as a function of sample temperature and yield an experimental value of the height of the activation barrier with respect to gas-phase ethylene of 49 ( 5 kJ/mol, in good agreement with the results of recent DFT calculations that predicted a value of ∼46 kJ/mol. The agreement between experiment and theory suggests that the proposed ethylidyne formation pathway involving an initial, rate-limiting ethylene dehydrogenation step to form vinyl species that finally form ethylidyne via an ethylidene intermediate is correct. Acknowledgment. We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Grant Number DE-FG02-92ER14289. References and Notes
APethylene kd
Θethylidyne ) 0.25 1 - exp -
good, thus suggesting that the assumptions made in deriving eq 4 are valid. The logarithms of the fitting parameters are plotted in Arrhenius form versus 1/T, and the results are displayed as an inset to Figure 1. This results in a good straight line, where the slope corresponds to an energy of 49 ( 5 kJ/mol. From above, the constant A depends only weakly on temperature and is taken to be a constant in comparison with the strong temperature dependences of kc and kd. These are written explicitly in their Arrhenius forms act as kc ) Ac exp(-Eact c /RT) and kd ) Ad exp(-Ed /RT), so that the parameter can then be written as (Akc/0.25kd) ) (AAc/0.25Ad) act exp[(-Eact c - Ed )/RT]. Thus, the measured value of energy from the Arrhenius plot in Figure 1 (49 ( 5 kJ/mol) corresponds to act (Eact c - Ed ) and is the energy difference between gas-phase ethylene and the rate-limiting activation step for the formation of vinyl species from adsorbed ethylene. This value is in excellent agreement with the more recently calculated value of ∼46 kJ/mol7 but is less than the ∼89 kJ/mol from the earlier calculation.8 Finally, the kinetic parameters for ethylene desorption have been measured previously and yield Edact ) 54 ( 5 kJ/mol.11,12 The calculated values are 62 kJ/mol from the earlier work8 and 71 kJ/mol for the more recent DFT calculation.7 These experimental results are closer to the earlier DFT calculation. Nevertheless, the good overall agreement between the results of the DFT calculations obtained in ref 7 and the experimental data, particularly for the formation of ethylidyne from ethylene, suggests that the mechanistic conclusions arrived at in this (and the previous) paper are correct and suggest that ethylidyne forms on Pd(111) by an initial, rate-limiting dehydrogenation of ethylene to form a vinyl species and a subsequent reaction to form ethylidyne via an ethylidene intermediate.
AkcE 0.25kd
))
(4)
where the exposure E is calculated from Pethylenet. The function in eq 4 is fit to the experimental data in Figure 1 using a single parameter Akc/0.25kd, and the fits are shown as dashed lines through the data, where the agreement between experiment and theory is
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