Endothermic dissociative chemisorption of molecular D2 on Ag(111)

8 September 1995

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 243 (1995) 133-139

Endothermic dissociative chemisorption of molecular D 2 on A g ( l l l ) F. Healey, R.N. Carter, G. Worthy, A. Hodgson Surface Science Research Centre, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK

Received 10 May 1995; in final form 7 July 1995

Abstract The dissociative chemisorption of molecular D 2 o n Ag is reported for the first time. Dissociation on the Ag(111) surface is highly activated with an approximately exponential dependence on the energy of the incident molecules, the initial sticking probability increasing from = 1 0 - 7 at 230 meV to 10 -2 at 465 meV, for a beam incident along the surface normal. The sticking probability is sensitive to the internal temperature, or state distribution, of the D 2 beam, dissociative chemisorption being more efficient for higher vibrationally excited states. The threshold for dissociative chemisorption of the ground state lies at too high an energy to be accessed, indicating an activation barrier > 0.8 eV for dissociation of D2(v = 0). Comparison with the activation barrier to D recombinative desorption of 0.28 eV indicates that dissociative chemisorption is endothermic. At large incidence angles and low nozzle temperatures adsorption is dominated by atoms formed in the nozzle, allowing an estimate of 0.6 for the initial sticking probability of D atoms at Ag(lll).

1. Introduction The dissociation of H2 and its isotopes provide a prototype system for the study of the dissociation dynamics of gas-surface reactions since the timescales for motion of the surface atoms and incident gas are very different. This means that for many systems the motion of the substrate can be ignored, as, to a first approximation, can the energy exchange between the gas and the lattice. This allows the dissociation dynamics to be discussed and modelled reasonably realistically on low dimensionality potential energy surfaces which include only a minimum number of surface coordinates and neglect surface motion [1,2]. While there are several notable systems, such as H 2 / S i and CHa/metals, that do not conform to this picture, for the majority of H 2 / m e t a l Elsevier Science B.V. SSDI 0 0 0 9 - 2 6 1 4 ( 9 5 ) 0 0 8 1 5 - 2

systems it provides a reasonable starting point in discussing the dynamics and is a major simplification compared to the dissociation of heavier molecules where energy exchange and motion of the surface are important. The dissociation of molecular H 2 / D 2 on the noble metals is known to be inefficient, with a large barrier to the formation of chemisorbed atoms. For A g both thermal and molecular beam adsorption experiments, at energies up to 135 meV [3], have failed to dissociate molecular H 2, suggesting that the barrier to dissociative chemisorption is even higher than for Cu where Eo(v = 0), the translational energy required to give 50% of the limiting high energy sticking probability for D2(v = 0), is 0.61 eV [4]. The H / D covered A g surface may readily be prepared by exposure of a cold surface to atoms and

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F. Healey et al. / Chemical Physics Letters 243 (1995) 133-139

shows a low coverage desorption feature near 175 K [5-7]. Desorption follows a complicated kinetic scheme, showing zeroth-order kinetics at the lowest coverages and fractional order ( n - - 0 . 5 ) kinetics at coverages corresponding to the (2 × 2) LEED phase [5,7]. The barrier to recombinative desorption was just 26.8 + 0.6 kJ mo1-1 (0.28 eV), and was independent of the kinetic order of desorption at coverages 0 ~<0.5 ML, indicating that the transition state for recombinative desorption was the same, irrespective of the coverage and the kinetic mechanism for desorption. Since D produces a well-ordered overlayer on this surface even at 100 K, the activation energy reflects the energy required to form the D D / A g transition state, rather than a barrier to D diffusion. This barrier is rather lower than for desorption from Cu [8] and reflects the low binding energy of H on Ag where the low lying, filled d bands lead to a weak H-metal bond [9]. There have been a number of scattering studies of He, H 2 and D 2 from A g ( l l l ) and (110) surfaces where diffraction [10] and the selective adsorption resonance [11] structure of the surfaces was investigated. No molecular dissociation was observed, nor has dissociation been reported in high pressure measurements. These observations suggest that the dissociative chemisorption of H 2 on Ag surfaces is probably endothermic, whereas on Cu it is approximately thermoneutral [12], although the energetics are rather poorly characterised. The dissociative chemisorption of D 2 on Cu is well known to show a vibrational enhancement [13,14] and thresholds for dissociation of the different vibrational states have been extracted from the sticking data [4]. Increasing vibrational excitation gives rise to dissociation at reduced translational energies, indicating that the vibrational motion of the molecules is available to promote dissociation and contributes to overcoming the barrier. This implies that the barrier lies at extended H 2 bond separations, the barrier being displaced into the exit channel for dissociation [15,16]. In this respect dissociation can be thought of as being a close parallel to the gas phase reaction of an atom + diatomic, where a late barrier makes vibrational motion of the bond to be broken efficient in promoting reaction [17]. For the gas phase reaction the position of the barrier could be correlated with the energetics of the reaction, an endothermic reaction tending to have a

transition state which is pushed back into the exit channel, with an extended molecular bond. This suggests that, for similar reactions, vibrational promotion will be increasingly important as the reaction becomes more endothermic [17]. H 2 dissociation at Cu shows strong vibrational enhancement, although energy in this coordinate is less efficient at overcoming the barrier than translational motion. If the analogy with the gas phase work holds, vibrational promotion might be expected to be more important for H2 dissociation on Ag where we show the reaction to be endothermic.

2. Experimental Dissociative chemisorption was studied using a molecular beam scattering system that has been described elsewhere [18]. The adsorption probability was determined by monitoring the recombinative desorption of molecular D 2 as a function of exposure to the molecular beam. The relative D 2 exposure was obtained from the background gas pressure in the chamber, measured by a nude quadrupole mass spectrometer. The gas exposure was adjusted to maintain the desorption integral of = 1% to 5% of the saturation coverage (0s~t) for a beam of molecular D 2. The sticking probabilities did not depend on the surface coverage used (0.004 < 0 < 0.15 0sat) and within the precision of the measurements represent the sticking probability for the clean surface. The saturation coverage for surface adsorption of D a t o m s is 1 ML [5] but dissociative chemisorption of a beam of molecular D 2 at 1580 K gave a saturation uptake estimated from the desorption integrals at just 0.6 ML, similar to that reported for D uptake by Lee et al. [6]. Desorption was monitored by a quadrupole mass spectrometer which was enclosed behind a 3 mm aperture and placed immediately in front of the sample during desorption [19]. This enhanced the desorption signal and also discriminated very efficiently against gas desorbing from the heater filament or sample stage. When the crystal was blocked from the direct beam no background desorption was seen at the temperature of the Ag desorption feature, even at the highest exposures used. This allowed very low coverages of D to be measured even after very large gas doses, important when sticking proba-

F. Healey et al. / Chemical Physics Letters 243 (1995) 133-139

bilities less than 10 -7 are to be measured. Relative sticking probabilities were obtained from the coverage as a function of D 2 exposure but direct calibration of these by measuring the D 2 uptake at high energy was not possible for A g ( l l l ) and these data were converted to absolute sticking probabilities in two ways. The absolute calibration of the relative sticking probability for a Cu85Pd15(l10) surface by the King and Wells technique [20] was used directly to calibrate the Ag data. This calibration was checked by using both the direct reflection technique and the recombinative desorption signal for dissociation on an Fe(ll0) surface, which has a relatively high sticking probability [18], immediately after the A g ( l l l ) measurements. The two calibrations were similar ( + 1 5 % ) and the Fe calibration was preferred. The molecular beam formed a 3 mm spot on the sample and the flux was controlled using a chopper with a variable duty cycle. Background effusion rates into the main chamber were measured by blocking the molecular beam in the second stage, prior to the collimating aperture. The nozzle was run with a backing pressure of 5.5 bar of D e and operated at temperatures up to 2100 K. The translational energy of the beam was measured by time of flight using resonance-enhanced multiphoton ionisation and was calibrated against nozzle temperature, giving a translational energy of E T = 5.45/2kT [19]. This implies some cooling of the rotational motion but the vibrational state distributions are unrelaxed under these expansion conditions. The internal state distributions of the beam therefore changes as the translational energy of the beam is varied by heating the nozzle. H 2 / D 2 gas mixtures for seeding experiments were provided by pre-mixing the pure gases in a reservoir. The A g ( l l l ) sample was oriented to < 0.2 ° of the (111) face by Laue and was cleaned by Ar ÷ ion bombardment and annealing to 700 K. The crystal gave sharp (1 × 1) LEED patterns and reproducible D 2 thermal desorption spectra [5].

3. Results Extended exposure of the 100 K A g ( l l l ) surface to 300 K beam of D E gave no trace of adsorption, allowing a limit (S o < 5 × 10 -8) to be placed on the

135

Translationalenergy / meV 200 400

102

~

~

102

/ / /.¢ / 10"5

m limit

/ /./i,. 500

,

1000 1500 Nozzle temperature

,

10"5 ~"

t o-'

2000

Fig. 1. Initial sticking probability for dissociative chemisorption of D 2 on A g ( l l l ) as a function of the translational energy of the beam (top scale) and the nozzle temperature for a beam incident along the surface normal. The solid lines show vibrational populations as a function of the nozzle temperature while the dashed line shows the fraction of D atoms in the beam, calculated by assuming an equilibrium between D and D 2 at a backing pressure of 5.5 bar.

sticking coefficient at a translational energy of 70 meV with a 300 K internal state distribution. This is consistent with the complete absence of H 2 / D 2 dissociative chemisorption on Ag surfaces for thermal gas adsorption and indicates that dissociation is activated even at steps and defects on the surface. It is also entirely consistent with dissociation being an endothermic process on A g ( l l l ) and the absence of D chemisorption except when dosed directly as atoms [21]. As the nozzle temperature and translational energy of the beam was increased dissociation was observed for translational energies above 220 meV (a nozzle temperature of T, = 930 K), Fig. 1. Under these conditions a sticking probability of 8 × 10 -8 was observed which rose exponentially with increasing translational energy (nozzle temperature). At the highest translational energies which could be achieved with a pure D 2 beam the sticking probability is = 10 -2, approaching values that can be observed by the direct reflection technique. Direct, absolute measurement of the the sticking probability

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F. Healey et al. / Chemical Physics Letters 243 (1995) 133-139

were attempted by the King and Wells technique using the nude QMS in the scattering chamber but any dissociation was below the detection sensitivity, conservatively estimated as S O< 5 × 10 -2. Seeding the D 2 beam in H 2 allows the translational energy of the D 2 to be increased above 0.5 eV without increasing the nozzle temperature further. However, the H 2 / D 2 mixture will isotopically scramble in the hot nozzle creating a statistical mixture of H 2, HD and D 2 with different translational energies, any one of which may stick. This makes thermal desorption measurements unreliable as a monitor of D 2 uptake but the reflection technique can be used since this monitors the loss of D e directly from the scattered beam. King and Wells experiments in which D 2 was seeded in H 2 were carried out for translational energies up to 0.8 eV and nozzle temperatures of 2100 K. Again, no dissociative chemisorption was observed, indicating a D 2 sticking probability of < 5 x 10 -z, whereas sticking could be observed on a Cu85Pd15(l10) surface under similar conditions [19]. It should be emphasised that although the seeding experiment increases the translational energy of the D 2, the vibrational state population distribution remains the same as for the pure beam at that nozzle temperature. The sticking probability would only be expected to follow the curve of Fig. 1 smoothly to higher energies if the internal state distribution was not important in determining the sticking probability. The correlation between the translational energy and the internal state distribution of the beam can be broken either by seeding the beam in an inert carrier gas to reduce the translational energy or by varying the angle of incidence to reduce the component of the translational energy perpendicular to the surface. Fig. 2 shows the variation of the initial sticking probability with nozzle temperature for a fixed translational energy perpendicular to the surface (E l = E T cos20i = 220 meV), obtained by increasing the angle of incidence and the nozzle temperature to keep E l constant. The sticking probability is plotted in an Arrhenius form and shows the effective activation energy for D 2 dissociation with internal energy, for an incidence energy E± = 220 meV. This makes the implicit assumption that the sticking probability for D 2 on A g ( l l l ) is only sensitive to EL and not to the component of the translational energy parallel

-10

-12

z -14

-I6

016

018

110, '

xl0 -3

1/T Fig. 2. Arrhenius plot for the dissociative chemisorption probability of D 2 as a function of the nozzle temperature for a fixed translational energy perpendicular to the surface, E± = 220 meV, obtained by varying the angle of incidence of the beam. The sticking probabilities have been corrected for the sticking of atoms from D 2 dissociated in the nozzle, this reduces the slope of the plot by 8% compared to the uncorrected data. The straight line shows a fit to the data giving an activation energy of 126 kJ mol-1 for the variation of S O with nozzle temperature for E± = 220 meV.

to the surface. Although this is a good approximation for H 2 / D z dissociation on C u ( l l l ) [4], and for activated dissociation on many other metals [22], it is a poor assumption for some systems, such as Fe(110) [18,23] and cannot always be expected to hold [24,25]. Separation of the internal state dependence and energy scaling requires a detailed analysis of the seeded beam and angular sticking data, with careful subtraction of the atomic adsorption, and will be described in detail elsewhere [26]. If, however, we assume that, as for Cu, parallel momentum may be ignored to a first approximation, then the slope of Fig. 2 gives an effective Arrhenius activation energy of 126 kJ mo1-1 for the promotion of dissociative chemisorption by the internal energy of the molecule. Since vibrational energy is known to be important in promoting dissociation on Cu, this activation energy [4,13,14] may be compared to the energy of the v = 4 level of D 2 which lies 134.8 kJ mol-1 above the ground state. The low sticking probability for D 2 o n Ag(111) potentially makes this dissociation process very sensitive to the fraction of D atoms in the beam. Berger and Rendulic [27] showed that dissociation on Al was dominated by atoms from the source and this was also important for high nozzle temperatures on

F. Healey et al. / Chemical Physics Letters 243 (1995) 133-139 i "~

•

i .

i ~

290 meV,

.~

- •

307 m e V

.~

o

279 m e V

N o z z l e backi ng p r e s s u r e / b a r

Fig. 3. Variation of SOwith backingpressure for normal incidence at 290 meV energy(O), 307 meV at 60° (rn) and 279 meV at 40° (O). At high incidence angles the stickingprobability increases as the backingpressure is reduced and the fraction of D atoms in the beam increases. The curves show fits to a p~/2 variation which allow the sticking probability of the D atoms to be estimated.

Cu [4]. Fig. 1 shows the sticking data plotted along with the population of the different vibrational states and the fraction of D atoms in the beam. The fraction of D atoms was obtained from a statistical calculation, assuming an equilibrium between D and D 2 at the nozzle pressure used (5.5 bar). Since the sticking probability of D must be less than unity the contribution of the D atoms to the sticking must lie below this curve, indicating that the dissociation of atoms cannot significantly perturb the sticking measurements for data taken at normal incidence. Since for a given nozzle temperature the fraction of D atoms in the beam will vary with the square root of the backing pressure, it is possible to estimate the fraction of sticking due to the D atoms. At low energy varying the backing pressure from 5.5 bar down to 1 bar, a change of a more than a factor of 2 in the D atom concentration, gave no change in the measured sticking probability for a beam incident along the surface normal (Fig. 3). This confirms that under these conditions sticking due to atoms was negligible compared to molecular dissociative chemisorption. When the beam was allowed to strike the crystal at a glancing angle the sticking probability was reduced and became sensitive to the backing pressure. From the pressure dependence under these conditions the contribution of D atoms to the sticking probability can be obtained and compared to the statistical calcu-

137

lation of the D atom concentration in the beam, giving an estimate of 0.6 + 0.2 for the sticking probability of D atoms on A g ( l l l ) . (The quoted error represents the scatter on our estimates of S D at nozzle temperatures of 1260-1435 K but does not allow for any systematic errors associated with the absolute calibration of the sticking probability ( = 15%) or in the statistical calculation of the fraction of D in the beam.) This value can be compared with a recent measurement of 0.18 + 0.06 for H sticking on Cu(ll0) [28] and would seem to be consistent with an increased energy exchange for the heavier isotope and the conclusion [28] that trapping of H / D is dominated by phonon excitation. This procedure could not be used to measure S D at higher nozzle temperatures since broadening of the translational energy distribution of the beam gave rise to increased sticking from the high energy tail as the gas pressure was reduced. However, even at the highest nozzle temperatures used here the calculated limit for atomic dissociation is well below the measured sticking probability and should have minimal impact for molecules incident along the surface normal. Sticking of D atoms has more effect on measurements at larger angles of incidence, where molecular dissociation is reduced, but the activation energy obtained from Fig. 2 is not changed substantially by removing the correction for atomic sticking (E a increases slightly to 136 kJ mo1-1) and is not consistent with atomic dissociation (D O= 4.556 eV, 439.6 kJ mol- 1).

4. Discussion D 2 dissociative chemisorption on A g ( l l l ) is highly activated and can be observed only for translational energies above 220 meV and gas temperatures above 940 K. A crude extrapolation of the sticking data of Fig. 1 to thermal energies (64 meV and 273 K) gives a sticking probability of order 10 -15 , consistent with the total absence of dissociation after exposure of Ag surfaces to thermal H 2 / D 2. The absence of any dissociative chemisorption at low energies implies that dissociation is activated for all sites on the surface and is not mediated by defects.

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H 2 / D 2 dissociation on Cu can be modelled [4] in terms of a set of simple ' S ' shaped step functions for each vibrational state of the incoming molecules, where, if the influence of rotational energy is neglected [29], the sticking probability depends on the vibrational state and the normal component of the translational energy. For D 2 dissociation on C u ( l l l ) the translational energy thresholds for sticking decrease steadily from 0.61 eV for the v = 0 state to -- 0.22 eV for D2(v = 2) and reach a limiting sticking probability of order 0.3 to 0.5 at high energy [30]. The strong variation of the sticking probability for A g ( l l l ) with the internal energy of the gas, at a constant normal translational energy, is indicative of a strong internal state dependence for the dissociative chemisorption probability. The apparent activation energy of 126 kJ mol -a at 220 meV normal translational energy is comparable to the energy of the v = 4 level of D 2, which lies 134.8 kJ mo1-1 above the ground state, and the magnitude of the sticking probability is consistent with the thermal population of this level. The absence of a measurable sticking probability for D 2 using the King and Wells technique, indicates that the dissociation probability for the pure D 2 beam at 0.49 eV and a vibrational temperature of 2100 K was less than 0.05. Similarly, seeding D 2 to an energy of 0.8 eV at an internal temperature of 2100 K also showed S O< 0.05, whereas under the same conditions a clear King and Wells adsorption signal could be seen for dissociation on Cu85Pd15{110}(2 × 1), which has a sticking probability similar to that of C u ( l l l ) [19]. The population of both the v = 0 and v = 1 states is significant under these conditions, 87% and 11% respectively, indicating a sticking probability on A g ( l l 1) of less than 0.06 and 0.5 for these two levels respectively. If it is assumed that the sticking probability for D 2 on A g ( l l l ) follows a similar functional form to that of Cu, i.e. the sticking probability for each state shows a threshold behaviour with the sticking probability of order 0.5 at energies above the threshold, this implies a translational activation barrier for the ground vibrational state of at least 0.8 eV on A g ( l l l ) . A comparison of the energetics for D 2 dissociation on Cu and A g ( l l l ) is shown schematicly in Fig. 4, the absence of D2(v = 0) dissociation on Ag being reflected in the larger barrier to dissociative

recombination

2D(ad)

.

• D2(g) dissociation

Ag ,,=,

e, q

Fig. 4. Schematic diagram showing the potential energy for adsorption of D2 molecules from the gas phase to form atoms chemisorbed on Ag or Cu. The formationof chemisorbedatoms is slightly exothermicon Cu but endothermicon Ag.

chemisorption. Recombinative desorption from A g ( l l l ) occurs at low temperature, reflecting the weak binding of D on Ag, with an Arrhenius activation energy of just 0.28 eV [5,7]. For C u ( l l l ) Anger et al. [8] report an activation energy for recombinative desorption of -~ 0.8 eV, dissociation being nearly thermoneutral [12] with recent calculations indicating it to be slightly exothermic [9]. The experimental measurements of the activation energy for recombinative desorption and the threshold for dissociation cannot be directly compared, since the transition state is not necessarily the same in both cases. Recombinative desorption occurs by thermal excitation from near the minimum of the barrier, whereas the dissociation threshold E0(v = 0) obtained from sticking measurements is the translational translational energy required to obtain 50% of the limiting sticking probability. Rettner et al. [30] have shown how the thermal activation energy for sticking depends on both the threshold energies and widths of the sticking functions, E a being = 0.5 eV even though E0(c = 0) is 0.61 eV. For A g ( l l l ) the large difference between the activation energy for desorption and the threshold for dissociation of D2(v = 0) indicates that dissociative chemisorption is endothermic, by = 0.5 eV or more, assuming the activation

F. Healey et al. / Chemical Physics Letters 243 (1995) 133-139

energy for dissociation scales with E 0 in the same way as for Cu [30]. The increase in the barrier to dissociation is consistent with the weak binding between H / D and Ag which has been ascribed to the very low lying, filled d bands [31]. Generalized gradient calculations of the dissociation of H 2 at Cu [32,33] have shown that the barrier height across the unit cell is closely correlated with the chemisorption energy of the H atoms. On A u ( l l l ) Hammer and Ncrskov [9] find dissociation to be endothermic and preliminary calculations indicate a similar result on Ag [34]. The large barrier to dissociation found here and the weak chemisorption bond on Ag are consistent with endothermic dissociative chemisorption and the complete absence of dissociative chemisorption at thermal energies. 4. Conclusions

We have shown that at high energies D 2 dissociates on A g ( l l l ) with a sticking probability that depends on both the translational energy and the internal energy of the molecules. Assuming that the sticking probability of individual vibrational levels saturates with S(v) > 0.05 at high energies, the barrier for D2(v = 0) dissociation is > 0.8 eV. Comparison with the activation energy for recombinative desorption indicates that D 2 dissociation is endothermic on A g ( l l l ) . References [1] S. Holloway, Surface Sci. 300 (1994) 656. [2] J. Harris, Faraday Discussions Chem. Soc. 96 (1993) 1. [3] L. Mattera, R. Musenich, C. Salvo and S. Terreni, Faraday Discussions Chem. Soc. 80 (1985) 115. [4] C.T. Rettner, D.J. Auerbach and H.A. Michelsen, Phys. Rev. Letters 68 (1992) 1164. [5] F. Healey, R.N. Carter and A. Hodgson, Surface Sci. 328 (1995) 67. [6] G. Lee, P.T. Sprunger, M. Okada, D.B. Poker, D.M. Zehner and E.W. Plummer, J. Vacuum Sci. Technol. A 12 (1994) 2119.

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