The dynamics and energetics of the dissociative chemisorption of gas-phase molecules at soil surfaces are even more complicated. Experimenta) results of the dissociative adsorption as a function of incident kinetic energy, incident ang)e and surface temperature are reported.
EXPER!MENTAL
2 ABSTRACT
Evidence is provided to support a trap-mediated dissociative chemisorption mechanism for the interaction of ethane with a ιr(110)-(1x2) surface. Thus, for a value of about 13 kcat/mo, the probability of dissociative chemisorption decreases rapidly with increasing Tg. 13.4 kcat∕moι, consistent with a trap-mediated chemisorption mechanism; indeed the data supports a quantitative kinetic mode. ) consistent with a trap-mediated chemisorption mechanism. The current understanding of dissociative chemisorption on a bare metal surface involves two different mechanisms: a direct mechanism and a mechanism mediated by trapping the incident particle in a molecularly adsorbed state.
Caption-mediated chemisorption is thought to be an important mechanism in industrial catalytic processes, which are often activated, due to the relatively low kinetic energies of the gaseous reactants. These studies showed a) so that the probability of direct dissociative chemisorption is typically)) a weak function of surface temperature. In addition, there is a strong dependence on the surface temperature Ts in the trapping-mediated chemisorption due to the kinetic competition between desorption and dissociative chemisorption from the trapped state.
In a recent molecular beam study of ethane activation at lr(110)-(1x2) using a beam reflectivity technique, Steinruck et al.34 concluded that there was no evidence for trapping-mediated dissociative chemisorption for surface temperatures between 300 and 1400 K. In the work that we report here we find strong evidence that a trap-mediated mechanism dominates the dissociative chemisorption of ethane on lr(110)-(1x2) for surface temperatures between 154 and 250K and for Ej between 1.2 and 13.4 kcal/mol. As expected for a trap-mediated reaction, So decreases with increasing Ej (up to about 13 kcal/mol).
8 II. EXPERIMENTAL METHODS
As mentioned above, the data reported here were for the initial probability of adsorption determined by a beam reflectivity method similar to that of King and Wells. With this method, the partial pressure of ethane in the diffusion chamber is used as a measure of the flux of C2H6 molecules that do not chemisorb. There is no significant steady-state accumulation of physically adsorbed ethane for any of the experimental conditions reported here.
Initial probabilities of chemisorption are determined by a comparison of the initial ethane partial pressure, after opening the high-speed shutter, with the ethane partial pressure from the jet scattered from the saturated surface. The kinetic energy of the jet is varied by a combination of seeding and variation of nozzle temperature, and is measured by time-of-flight techniques.
RESULTS AND DISCUSSION
The manipulator is cooled with liquid nitrogen, which quickly cools the sample below 80K. The sample temperature is determined by a W∕5%Re of 0.003 inch. 1, the value of So is a function of Tg at each barrel of impact energy. The functional dependence is stronger at lower kinetic energies, as this is the regime in which the trap-mediated chain of chemisorption is dominant.
The probability of trapping-mediated dissociative chemisorption can either be independent of temperature, increase with Tg, or decrease with Tg, depending on the details of the potential energy surface. Simpie's model serves to explain the temperature dependence of trapping-mediated chemisorption of ethane on tr(110)-(1x2). In this case, the dissociative chemisorption probability for the trapping-mediated component in the limit of zero coverage can be written as
Equation (1) can be rewritten as where k⅛ and k°è are the pre-exponential factors of the two rate coefficients, and E^ and Ec correspond to the activation energy. In such a case, the gear plug would be equal to (Ec-Ed)/kg, the difference in activation energies for dissociative chemisorption and desorption from physisorbed wet), divided by the Bottzma constant∩n. Figure 2 is such a collection of data for different values of E⅛ and θj, for which the measured values of ζo^ are used in the evatuati∩g ordinate, since it is expensive that straight teeth provide a good fit att of data over an extremely wide range of measurements (the ordinate spans a factor of more an angle of two magnitudes, and the abscissa is ∆Tg 350K), providing strong support for the proposed trapping-mediated mode of chemisorption). Since the wett physical adsorption depth for ethane is about ^8.0 to 9.0 kcat/mol, the activation barrier for the reaction from the physically adsorbed state is about ^5.8 to 6.8 kcat/mol, which is in perfect agreement with the previous study Wittrig et al.33 The value of the ratio of pre-exponential factors is equal to ½d∕½°c. A ratio of ½d∕½*⅛ greater than unity is expected because entropikatty desorption is favored over dissociation due to the time phase space in which dissociation can occur compared to desorption.
SYNOPSIS
3 for both normal energy scaling (tower + symbol) and Ejcos° 5θj scaling (bottom x symbol) at E}=20.9 kcal/mol and Tg=500K are based solely on the data shown in Fig. 3 that normal energy scaling accurately describes the direct chemisorption channel· Normal energy scaling is very common for direct, dissociative chemisorption in activated systems3.4; although direct dissociative chemisorption of N2 on both W(110)2 and W(100)26 scales with total energy. Ec is about 5.8 to 6.8 kcal/mol, while the physical adsorption well depth Ed is about 8.0 to 9.0 kcal/mol.
The difference between these two energies, Ed-Ec, was accurately measured and found to be 2.2+0.2 kcal/mol. The trapping-mediated component of dissociative chemisorption increases with increasing angle of incidence and scales as Ejcosθ.5θj. The increase in So with increasing θj is due to an increase in the trap probability, which also scales as Ejcoso.5θj.i4 The trap-mediated component of So actually scales as Ejcoso.5θj because the trap probability scales this way.
At higher kinetic energies, direct dissociative chemisorption was observed, the probability of which scales with the normal component of the kinetic energy. This work was supported by the Office of Basic Energy Sciences of the Department of Energy under grant number DE-FG03-. Acknowledgment is thus made to the donors of the Petroιeum Research Fund of the American Chemicaι Society for partia) support of this research under grant number PRF 19819-AC5-C.
17 REFERENCES
20 FIGURE CAPTιONS
1 CHAPTER V)
An important aspect of these studies has been the evaluation of the relative motion of the transfer of momentum parallel and perpendicular to the trapping process through measurements of the incidence effect on the rate at which trapping decreases with increasing energy. Most previous studies have been conducted at temperatures at which the residence time of trapped species is sufficient to allow the trap to be extracted by surface coverage or adsorption of incident ftux. A detailed description of the apparatus has been presented previously2.ιι, and so we present only a brief description here.
The insensitivity of the results to the beam conditions suggests that the radiation in these beams is negative. However, if the surface temperature is too high (2i300 K for this study), two effects combine to estimate the separation of the direct-inelastic and capture-desorption components. Here it is necessary to estimate the shape of the non-pia∩e distribution for the direct-ineasticity.
As a result of the uncertainty in the off-ptan angular width, the absolute capture probability values reported here should only be considered accurate to within ±10%. The value of ζo at the two higher surface temperatures atso decreases with increasing Ej. However, c)oser inspection of traction E; data for fig. As the surface temperature increases, the Ctearty parattet momentum becomes increasingly important in the trapping process, indicating a corresponding increase in the effective waviness of the interaction potential. However, the most reliable is also the actual surface roughness due to thermal) dispιacement of surface atoms. therefore, this behavior must be explained by simpte singte-co)ιision modets.
14 REFERENCES
- Conclusions
Measurements of the probability of molecular adsorption of ethane on the Ir(110)-(lx2) surface have been performed using the reflectivity method of King and Wells (12), using a molecular beam device which will be described in detail elsewhere. described (16). . At higher temperatures, the fractional coverage of the second layer and thus the 'tail' in the experi. This is the ratio between the probability of success of desorption and the probability of success of migration.
This, as explained in the text, allows absolute determination of the desorption rate from the second layer. The supersonic Ar beams are directed at the sample placed in the UHV chamber on the manipulator, which provides precise control of the incident angle and surface temperature in the range from —85 to 2500 K. The sample con. Height-time distributions for Ar scattering from 2H-W( 100) in the direction of the surface normal for two different incident beams.
The charge frequency <5,(NH,) is very sensitive and shifts up with increasing charge on the metal atom, having average values of 1158 cm ^ ' in the compound M ( NH, )⅛, 1333 cm*' in M (NH ,)^ compounds, and in mean average values in the case of M(NH,)^. Thus, both on the surface and in coordination compounds, the repeated (non-formal) "oxidation state" (defect) is of great importance in determining the vibrational frequencies of the co. This money- . !e) is the frequency shift of this mode in coordination. thione compounds as the charge on the meta) atom decreases. Tabie Hι) shows that the structure and bond of ethidine in both cases are quite similar.
A comparison of the EEL spectra of C;H< and C2D is essential) to the correct identification of these vibrational modes. Our EEL spectra of thermal evoiution of ethyiidyne and acetylide on Ru(00i) and complementary thermal desorption spectra show that virtuai)y aii of ethyiidyne dehydrogenates to surface carbon below 360 X, ie. emits only H(a) on and H(a) the surface.
