Hydrogen on Pd(100)-S: the effect of sulfur on precursor mediated adsorption and desorption

Surface Science 237 (1990) 1-19 North-Holland

Hydrogen on Pd( 100) -S: the effect of sulfur on precursor mediated adsorption M.L.

and desorption

Burke

Departmentof Chemistry, Stanford University, Stanford, CA 94305 , US.4 and R.J. Madix

*

Departments of Chemistry and Chemical Engineering, Stanford University, Stanford, CA 94305, IJSA Received

10 January

1990; accepted

for publication

29 May 1990

The influence of adsorbed sulfur on the adsorption and desorption of Ha on Pd(100) was studied using temperature programmed desorption (TPD) for sulfur coverages (0s) from 0.00 to 0.35 ML. The saturation coverage (&_, ) drops linearly with sulfur coverage, and above 0.28 ML of sulfur no hydrogen adsorbs. Direct site blocking is implied by the linear fall of errsat with 6’,, with each sulfur atom effectively blocking 3.7 + 0.5 sites for hydrogen adsorption. The adsorption of H, on the sulfur free Pd(100) surface is best described by a second-order precursor model. When sulfur is added to the surface, however, the effect of the precursor state is diminished, and for f3,> 0.15 hydrogen uptake is adequately modeled by second-order direct adsorption. For low hydrogen coverages values for the activation energy and the preexponential factor for hydrogen adatom recombination drop in compensatory fashion from 85 kJ/mol and 1O-2.5 cm*/s on the sulfur free surface to 49 kJ/mol and 10-6s cm*/s at 6, = 0.15, respectively. The effect of sulfur on the desorption kinetics of hydrogen suggests that the influence of adsorbed sulfur is more complex than simple site blocking. The compensation effect between the preexponential factor and the activation energy from surfaces with sulfur adlayers may arise from a more constrained transition state for desorption on sulfur covered surfaces or from a distribution of activation energies for desorption.

1. Introduction The mechanism by which sulfur adatoms poison surface reactions has been a subject of active debate. Contrasting studies have led to interpretations suggesting both long range, i.e. over several lattice constants, and short range, or steric, interactions to explain experimental results. This controversy exists even for the simple question of the reduction of the saturation coverage of an adsorbate in the presence of a poison such as adsorbed sulfur. In this work we show that both the saturation coverage of adsorbed hydrogen atoms

* To whom correspondence 0039-6028/90/$03.50

should be addressed.

0 1990 - Elsevier Science Publishers

and the probability of dissociative adsorption of H, fall nearly linearly with sulfur adatom coverage, implying that sulfur adatoms act locally as site blockers. Ko and Madix [l] and Benziger and Madix [2] have reported previously that hydrogen uptake can be completely suppressed under ultrahigh vacuum conditions at sulfur saturation, which occurs on W(100) and Fe(lOO) surfaces at 1.0 and 0.5 ML, respectively. Farias et al. have determined that a given coverage of sulfur on Mo(100) reduces the saturation coverage of hydrogen to a greater extent if the sulfur adlayer is ordered, rather than disordered [3]. Regardless of ordering, at a sulfur coverage of one monolayer no hydrogen will adsorb on the Mo(100) surface. In ad-

B.V. (North-Holland)

2

M.L. Burke, R.J. Madrx / Hydrogen on Pd(lOO)-S

dition hydrogen desorption kinetics from the disordered sulfur adlayers does not change as sulfur coverage increases. On clean Pt(ll1) as well as (2 x 2)-S and (6 X fi)R30”-S surfaces, Abbas and Madix found that hydrogen adsorption was reduced by sulfur most rapidly for the high temperature desorption states [4]. Similar to the behavior observed on Mo(lOO), the shape of each hydrogen desorption state was not significantly altered by sulfur. On clean and sulfided Ru(001) Schwarz reported that adsorption of H, occurs via a direct mechanism on the clean surface and that the activation energy and preexponential for hydrogen desorption both decrease as hydrogen coverage increases, indicating a compensation effect [5]. This compensatory decrease is paralleled by the kinetic parameters measured for a fixed hydrogen and an increasing sulfur coverage. coverage Schwarz interpreted this as a site blocking effect of sulfur with desorption kinetics dependent on the local hydrogen coverage rather than the global coverage. Most of the research on the effect of sulfur on hydrogen adsorption and desorption from transition metal single crystal surfaces has been conducted on nickel. Kiskinova and Goodman have studied hydrogen on sulfided Ni(lOO), and report a near linear decrease in the hydrogen saturation coverage up to 0.25 ML of sulfur, with a more gradual decline at -higher sulfur coverages [6]. The initial sticking probability of H, on this surface closely followed a direct model with four sites blocked by each sulfur adatom at lower sulfur coverage, again with a more gradual decline for sulfur coverages greater than 0.15 ML. Goodman and Kiskinova have interpreted the infuence of sulfur as an electronic effect caused by sulfur interaction with metal 3d states. Johnson and Madix found that the addition of sulfur to Ni(lOO) decreases both the activation energy, E,, and preexponential factor, A, for hydrogen desorption between sulfur coverages of 0.00 and 0.28 ML [7-lo]. Hardegree has found that for deuterium adsorption on Ni(lOO) the saturation coverage falls sharply with the addition of small amounts of sulfur [ll]. The initial decrease of OD,sat with sulfur coverage implies that

each sulfur atom blocks sixteen D adsorption sites. and at 0.11 ML of S, the amount of deuterium adsorbed is only 4% of the clean surface value, in contrast to the earlier studies of Johnson and Madix. This large effect of sulfur also differs appreciably from the effect of sulfur on CO molecular adsorption and desorption on Ni(lOO) [3,4,12,13], and Ni(ll1) [14], where the main effect of sulfur appears to be short range site blocking. Palladium is a member of the same group as nickel in the periodic table, so we have chosen Pd(lOO) as a reasonable surface with which to extend the study of the mechanism by which sulfur alters the kinetics of hydrogen adsorption and desorption. Sulfur adsorbed on Pd(lOO) forms a p(2 X 2) pattern at 0.25 ML, and a c(2 X 2) structure at the saturation coverage of 0.50 ML [15]. The dependence of the LEED spot intensity on beam voltage for the c(2 x 2) sulfur layer indicates that sulfur resides in the four-fold hollow [15]. As in the case of nickel, H, dissociatively adsorbs on clean Pd(lOO), as indicated by the second-order desorption kinetics found by Behm et al. [16] and the vibrational studies of Nyberg and TengstAl [17]. Behm et al. also found that hydrogen adsorbs via a mobile precursor and desorbs in two states at 260 and 360 K, respectively; the low temperature state appears as a shoulder for B, 2 0.5. The value of the activation energy (E,) for desorption from the main state was estimated to be 92 kJ/mol, and a saturation coverage of 1.35 ML of H(a) was determined on the basis of LEED and temperature programmed desorption (TPD). Calibrations by Nyberg and Tengst%l, however, revealed an absolute saturation coverage of 1.0 by using LEED intensities and TPD with adsorbed hydrogen and D [18]. He diffraction studies by Rieder and Stocker indicate that the surface corregation at H(a) saturation coverage is extremely small, indicating that any hydrogen present above 1 ML must reside in the subsurface region [19]. Furthermore, Besenbacher et al. found no evidence for subsurface D in their He ion transmission channeling experiments [20] and support the conclusion of Nyberg and Tengst%l that 8,, = 1.0 at saturation. The binding site of adsorbed hydrogen and D on Pd(lOO) has been found to be the four-fold hollow by electron energy loss (EELS)

hi. L. Burke, R. J. Madix / Hydrogen on Pd(lOO)-S

vibrational studies [17], by He diffraction [19], and by transmission channeling [20]. On Pd(lll), however, Gdowski et al. have found evidence for several forms of subsurface hydrogen [21]. For adsorption at 80 K only a single secondorder desorption feature is present above 310 K, but for exposures greater that 100 langmuir (1L = lop6 Torr . s) and temperatures between 90 and 140 K a zero-order state at 170 K which does not saturate is also present, This 170 K state has been attributed to the decomposition of a near-surface palladium hydride phase with H/Pd 2 0.6. For adsorption temperatures of 224-298 K a considerable amount of hydrogen desorbs near 800 K and has been attributed to hydrogen in solid solution. In the work reported here hydrogen was adsorbed on and desorbed from Pd(lOO) for sulfur precoverages ranging from 0 to 0.35 ML. TPD was used to determine relative hydrogen coverages as well as the kinetic parameters for hydrogen desorption.

2. Experimental All experiments were performed in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure of 1 x 10 torr. LEED was used to monitor surface order, and the elemental composition of the surface was determined using Auger electron spectroscopy (AES). A collimated quadrupole mass spectrometer (QMS) was interfaced with a computer for the TPD experiments. The orifice of the collimator was 0.5 cm in diameter and was slightly smaller than the Pd(lOO) crystal used. This configuration resulted in a pronounced increase in sensitivity and peak resolution in the TPD experiments as compared to an uncollimated QMS used previously. Two capillary array dosers of diameter 0.7 cm were used: one for H,S only and the other for all other gases. The Pd(lOO) sample was cleaned by argon ion bombardment followed by annealing at 1200 K. Surface carbon which segregated from the bulk while annealing was removed by heating the crystal in 0,. Subsequent exposure to CO was used to remove residual oxygen. This procedure resulted in a surface which was clean according to AES

3

and EELS [22]; thus no oxide formation was observed. Sulfided surfaces were prepared by dosing H,S at room temperature followed by flashing the crystal to 550 K to desorb the hydrogen and to order the sulfur. The sulfur coverage was determined by AES by referring the sulfur (152 eV) to palladium (330 eV) peak-to-peak height ratio to the value for a sulfur-saturated surface. A c(2 X 2) LEED pattern was seen at sulfur-saturation, corresponding to 0, = 0.50 [15]. This coverage was obtained by numerous extensive doses of H,S between which AES spectra were recorded. A saturation coverage of sulfur was assumed only after several consecutive H,S doses and subsequent anneals gave no rise in the S(152)/Pd(330) AES ratio. The total H,S exposure required to reach this state was in excess of 2 x 1016 cm-*. A c(2 X 2) LEED pattern was visible at sulfur coverages as low as 0.42 ML, and a p(2 X 2) pattern was seen for t& ranging from 0.25 to 0.37. No sulfur-induced LEED patterns were seen at or below 0.22 ML of sulfur. H, desorption from the back of the crystal was intentionally minimized in these experiments, since extraneous sources of hydrogen alter the apparent desorption kinetics and give erroneous values for hydrogen saturation coverages. This objective was achieved partially by keeping the back of the crystal sulfided to a greater degree than the face. First the entire sample was sulfided with small background doses of H,S (5 X 10-i’ Torr for 500 s) to obtain a surface coverage of sulfur of 0.1 ML. Then the sulfur was removed from the front of the crystal by placing the crystal directly in front of the doser and exposing it to a beam of 0, which gave a background pressure rise of 2 x 10-i’ Torr in the chamber; the crystal was heated to 715 K before placing it in front of the doser, and the 0, exposure was conducted for four minutes while the crystal temperature was lowered stepwise to 545 K. Surface oxygen was then removed via reaction with CO. It was verified by AES that sulfur was removed from the crystal face with this procedure. Since the pressure of 0, at the crystal face for direct dosing in this apparatus has been found to be more than 80 times the background pressure, we assume that the back of the crystal remained sulfided; the total O2 exposure at the

4

M. L. Burke, R.J. Madix / Hydrogen on Pd(lOO)-S

back of the crystal was only enough to result in a surface coverage of approximately 0.02 ML of 0 (a) [23]. At most one-half that amount of sulfur would have been removed as SO,, which is the oxidation product evolved from the surface [24]. This sulfiding/cleaning procedure was repeated twice. Data for the lowest sulfur coverage reported here were obtained for this procedure. Higher sulfur coverages were obtained by first cleaning the crystal face with 0, and CO, again leaving the back of the crystal sulfided. Then repeated cycles of H,S background dosing and annealing were employed until the desired sulfur coverage was obtained. The back of the crystal thus accumulated sulfur throughout the set of experiments, while the front face was adjusted to the sulfur coverage. TPD experiments were performed by dosing H, at 110 K and by desorbing H, through the collimator directly into the mass spectrometer. Hydrogen was dosed directly, by placing the crystal approximately 3 mm in front of the doser. A few background doses were also performed for which the exposures were calculated using the ion gauge readings corrected for the sensitivity of H, relative to N,. The exposures for the direct doses were calculated by comparing the areas under TPD curves for direct and background doses using the same ion gauge. The direct doser enhanced the exposure for H, by a factor of approximately 20. Exposures for H, at each sulfur coverage were varied from zero to 25 ML, where 1 ML of H, is equal to the Pd surface atom density (1.32 X 10” atoms/cm2). The enhancement of the dose onto the front face by the doser, the sulfiding of the rear and edges of the crystal, and the use of the collimator in TPD all ensure that desorption of H, was measured without interference from the back or edges of the crystal.

3. Theory of the effect of site blocking on precursor kinetics Since Behm et al. have inferred that a precursor is involved in the adsorption of H, on Pd(lOO), we chose to consider the effect of a precursor in the analysis of our data. The infuence of a mobile

precursor on adsorption (e.g. refs. [25,26]) and desorption kinetics [27,28] have been previously derived. However, the effect of site blocking by adatoms has not been previously incorporated into the precursor models.

3.1. Elementary

reaction steps

The mechanism for precursor adsorption and desorption was modeled by the following familiar scheme: H,(g)

2 HT 2 2H(a),

k*

where the asterisk denotes the precursor and the k values are the rate constants for each step. The blocking of sites for both the precursor and hydrogen adatoms by sulfur was considered in the derivation of rate equations for each elementary step as follows.

Step I: Trapping of the mobile precursor The rate at which H,(g) is trapped into a precursor state is taken to be the gas-surface collision rate, ri = k, [HZ], multiplied by the trapping probability for the sulfur-free surface, (Y,and by y, the probability that the site onto which the gas molecule impinges is available for precursor binding, i.e. not blocked by sulfur. Assuming hydrogen adatoms have no effect on the trapping probability of H,, after setting k, = ak,, the rate of trapping, r,, is r, =

yk,[H,l.

(2)

Step 2: Desorption of the precursor Although sulfur may reduce the total number of sites and/or the number of adjacent sites available on the surface, the site availability will not effect the rate of a first-order desorption at a given coverage. Any effect of sulfur on the unimolecular reaction step of precursor desorption must lie in the rate constant, k *. The rate of precursor desorption, r *, is then r* = k*[H,*].

(3)

hf. L. Burke, R.J. Madix / Hydrogen on Pd(lOO)-S

Step 3: Dissociative adsorption of the precursor Sulfur can block sites for both the precursor and hydrogen adatom binding. The number of sites blocked by sulfur for precursor binding was not assumed equal to those blocked for H(a) binding in order to allow the degree of site blocking for each species to be treated independently. For example, adsorbed sulfur could block only neighboring sites for the precursor and both nearest and next nearest neighboring sites for hydrogen adatoms. However, there are several implicit assumptions in the treatment. First of all we assume that the precursor coverage is low, so precursorprecursor interactions can be neglected. Also, as in previous treatments of precursor adsorption we treat the precursor as having a preferred binding site [29-311, and to simplify matters we assume that the H, precursor and hydrogen adatoms occupy the same type of site. In addition the precursor is assumed to bind above H(a) as well as above vacant sites, and all sites blocked by sulfur are assumed to be randomly distributed. The rate of dissociative adsorption, r,, equals k, [H;] x B (the probability that H; occupies a site available for hydrogen adatom adsorption) X C (the probability that an adjacent site is available for hydrogen adatom adsorption). For random site occupation the term C is equal to the fraction of the surface sites available to H(a), or ( errsat - S,), where 8u sat is the fractional surface coverage of hydrogen’adatoms at saturation. The total number of surface sites available to the precursor is N - Q, where N is the total number of surface sites and Q is the number of precursor sites blocked by sulfur. The term B then equals [(N-

In one case Q I M, where A4 is the number of H(a) binding sites blocked by sulfur. Then the sites blocked for binding of the precursor by sulfur are also blocked for hydrogen adatoms, and B becomes (N-Q>-(M-Q>-(H)

or N-M-H N-Q

N-Q

where hydrogen is the number adatoms. This in turn simplifies to

B=

+‘,,,I-&I),

of

’ hydrogen

N-Q with y = N

Since Q 5 M, in this case it follows that y 2 Orisa,, so that Bsl. However, if Q L M and the sites blocked by sulfur for hydrogen adatoms are also blocked for the precursor, B becomes (N -

orl-

Q) - (0) - [(NN-Q

Q>/(N-

M)l H

H N_M.

The term in brackets takes into account that H(a) is distributed among N - M sites with only N - Q of them available to the precursor. This may be rearranged to give a value of B of

The result for the rate of precursor thus

dissociation

is

(4)

Q>

- (sites accessible to H; but unavailable H(a) product

of dissociation)]

/(N-Q), which equals [(N-

5

Q>

- (sites blocked for H(a), but not for Hz) - (sites unblocked by H(a))]/(N

for HT, but occupied -

Q>.

for the

T=y

r = 4Lsat

Q/M 5 1 (Y 2 b.sa, >J for Q/M 2 1 ( Y 5 b,sat >.

for

Note that when Q equals M, and the number of sites for H adatoms and the precursor blocked by sulfur is identical, r = y = errsat. Both of the above equations for r are thus correct at Q = M. Step 4: Adatom recombination to form the precursor In a method introduced by Schwarz [5], the effect of site blocking by sulfur may be accounted

M.L. Burke, R.J. Madix

6

for by introducing such that

a local coverage

for H(a), BH.lC,c.

8, 0 H,loc= 8. H.S.M

(5)

This relationship accounts for an increase in the effective coverage of H(a) in the presence of sulfur. In addition, the reduction of the active surface area due to site blocking required to give the proper rate per unit area of surface not blocked by sulfur is accounted for by multiplying by oH,\at. This yields a rate for adatom recombination, r,, of

(6) where OH is the global, not the local hydrogen coverage. Effectively, the rate of associative adsorption of hydrogen at a given hydrogen coverage is increased by the inverse of the saturation hydrogen coverage at that sulfur coverage. 3.2. Adsorption kinetics The rates of the elementary steps may be used to evaluate the effects of a mobile precursor and the site-blocking adatoms on the overall adsorption kinetics. The rate of dissociative adsorption, is then r, rd found from eqs. (4) and (6); r, rads ) may be neglected for low temperature adsorption. The equation for r,& thus contains the concentration of the precursor, which may be eliminated by using the steady state approximation, assuming that the precursor coverage is low. As a result the rate of adsorption is

ayki [H2I rddS= 1 + (Tk*/k,)(O,,,,,

(7)

- f+,P2.

The sticking probability, s, is simply rads/kI [HZ], and the initial sticking probability, s,,, is found by substituting 8, = 0. The sticking probability relative to that for the hydrogen-free surface is thus -I

e2 H.sat @H,sat- 8H)2 -

’

r( k */k, 1 k */k,)

+ O;_,,, .

K gives a measure of the contribution of the precursor to the rate of adsorption since it varies with k */k,. If k * B k,, there is no enhancement of the adsorption rate from the precursor, and this is reflected in a value of K of 1. However, if rek* CC k,, K goes to zero, and the precursor sults in a pronounced increase in the rate of adsorption. The sticking probability equations, (8) and (9) are completely analogous to those of Ehrlich [25] and of King and Wells [26] when orrFat and r are both assigned values of 1; i.e. when sulfur-mediated site blocking of the H; precursor and of hydrogen adatoms are excluded. Our analysis shows that the inclusion of site blocking diminishes the extent to which the precursor state enhances the rate of adsorption; as flHaa, is reduced to zero by adsorbed sulfur, K ‘increases toward 1, and the adsorption model approaches the direct second-order limit. This behavior will occur unless the value of Tk*/k, falls more Such a rapid decrease in rapidly than 0,!,,,, Tk*/k, is not expected since r should fall between BH,aat and 1 (eq. (4)) and sulfur should promote desorption rather than adsorption, causing k*/k, to increase. K cannot equal one unless r( k */k, ) x=- 8&,, . but in that case eq. (8) simplifies to a direct adsorption model with

.s -=

(“H.sat - o~)2 l32 H.M

.SO

If eq. (8) integrated, exposure is found:

OH

OH 1 1 -

(1

‘H/‘H.s;,,

-0~

of

1

=%F.

where E is the H, exposure. If K = 1, the equation simplifies to that for direct adsorption. 3.3. Desorption

’

the dependence

kinetics

(8)

where

K= r(

/ Hydrogen on Pd(lOO)-S

(9)

The overall rate of desorption, rdr,, equals the rate at which the precursor desorbs minus the rate of trapping into the precursor state, or r * - r,. In the desorption regime rt is negligible since there is little H, present in the gas phase, and introducing

M.L. Burke, R.J. Madix / Hydrogen on Pd(lOO)-S

[H;] from the steady-state (3) yields Tk,k* rdes =p 0 Hmka

approximation

Gi I’k */k,

+ ( 6H,sat - 8,)2

into eq.

1 02) .

There are two limiting cases in terms of the contribution of the precursor to desorption [27]. First, if rk* s+ k,, eq. (12) reduces to a normal second-order desorption equation modified to account for the effects of site blocking by sulfur:

kd -e;.

rdes= e

H&It

The second limit applies for Tk*/k, 8,)2, and takes the form

0s = 0.00

HJI’d(100)

(b) 0.97

(13) e (r3W,sat-

which is valid for 8, < OH,+, since the rate expression diverges at hydrogen saturation. If the terms arising from site blocking by sulfur are eliminated from eqs. (12)-(14) by setting both ber and ensat equal to 1, these rate equations come identical to those derived by Gorte and Schmidt for second-order desorption mediated by a precursor state [27]. The only effect of site blocking in the limit of direct desorption (eq. (13)) is thus an increase in the effective preexponential, which is manifested by the en,@t term in the denominator. Recall that this arises from a combination of an enhancement of BH,ioc and a reduction of the total active surface area by sulfur (section 3.1, step 4). An identical contribution occurs for precursor-mediated desorption, eq. (14), but is essentially offset by r in the numerator (section 3.1, step 3). Physically, if r-~ 1, then sulfur excludes precursor molecules from sites which are blocked for hydrogen adatom binding, and are thus inactive toward precursor dissociation. This exclusion increases the probability that a given precursor molecule will reside over an unblocked site. Thus for a given errsat the rate of precursor dissociation is enhanced as r decreases (with eH,sat a lower bound on r), so the rate of desorption is decreased. Lastly, site blocking increases the rate of precursor-mediated desorption by reducing the number of adjacent sites available

I/

I

”

200

I

r

8

1

’

300

Temperature

I

400

r

’

”

5 01

(K)

Fig. 1. Thermal desorption of H, from Pd(lOO) with 19s= 0.00. Exposures in ML of H, are (a) 12.5, (b) 3.75, (c) 1.25, (d) 0.37, (e) 0.12, (f) 0.04, (g) 0.01 and (h) 0. Heating rates are 12-14 K/S.

for dissociation of the precursor. For a given hydrogen coverage, sulfur reduces ( eH,sat - r9,)2 (eq. (14)), and thus decreases the rate of readsorption of the precursor with a concomitant increase in the desorption rate.

4. Results 4.1. TPD features For the clean Pd(lOO) surface (es < 0.01 ML) H, desorption curves appear second-order at low coverage since the peak temperature shifts down with increasing coverage (fig. 1). The peaks are nearly symmetrical, but they are slightly skewed to higher temperatures, as expected for second-order desorption [32]. At higher coverages, however, the onset of desorption shifts noticeably to lower tem-

x

M.L. Burke,

R.J. Madrx

perature, with a value near 235 K at saturation. The peak temperature at saturation is 350 K. An exposure of 12.5 ML of H, is sufficient to saturate the clean surface and the surface at all other sulfur coverages; a 25 ML exposure does not result in a H(a) coverage higher than that for the 12.5 ML dose. These desorption spectra are quite different from those of Behm et al. [16]; their data showed a distinct low temperature state near 260 K which grew in at exposures above 0.5 L. Based on our results we believe that the low temperature state found by Behm et al. arises from desorption from the back of the crystal, with impurities causing the downward temperature shift. The results which lead us to this conclusion are as follows. (1) During initial experiments conducted in this work we found that H, desorbing from the back and sides of the crystal contributed to our TPD results. In the initial experiments a significant TPD signal was obtained after dosing H, on a Pd(lOO) surface saturated with sulfur (0.5 ML), and later experiments have shown that no H, will adsorb at this sulfur coverage if care is taken to also ensure that the back of the crystal is sulfided. (2) Hydrogen desorption peak temperatures are shifted down in the presence of sulfur, and presumably other impurities. Thus hydrogen desorption from the crystal back combined with higher impurity levels on the back than on the front of the crystal could yield a distinct desorption feature. In order to address the argument that the low temperature state seen by Behm et al. is characteristic of clean Pd(lOO) but is suppressed in our work by the sulfiding-cleaning procedure. we note that this state was absent even when H,S dosing was not performed. Previous TPD results from this laboratory also show no evidence of a low temperature shoulder for H, desorption from Pd(lOO) [23]. If desorption from the crystal back was a problem. this may explain why Behm et al. found the saturation surface coverage of H(a) on Pd(lOO) to be 1.35 ML. No absolute calibration was attempted here, but a saturation value of 1.0 ML is assumed based on the results of Nyberg and Tengstal [18] and Besenbacher et al. [20]. Hydrogen desorption from a surface precovered with 0.08 ML of sulfur is similar to that

/ H.vdrogen on Pd(lOO)-S

H2/Pd(100) ? y = 0.08 (IlllC 2)

I

100

I

/

,

r

200

-r,-r

---,

300

Temperature

-_,

300

(K )

Fig. 2. Thermal desorption of H, from Pd(100) with 0, = 0.08. Exposures in ML of Hz are (a) 12.5, (b) 3.75, (c) 1.25. (d) 0.37. (e) 0.12. (f) 0.04. (g) 0.01 and (h) 0. Heating rates are 12-14 K/S.

from the clean surface, but there are some notable differences (fig. 2). The peak temperature is downshifted to a value of 331 K at saturation. Also, a low temperature shoulder emerges for Hz coverages above 0.71 ML, which may be due to destabilization of H(a) by sulfur and reduced H-S distances at high hydrogen coverages. For the hydrogen saturated surface the onset of desorption is shifted down to 200 K. For 19,= 0.15, only one desorption state is seen; at hydrogen saturation the TPD curve is quite broad, but it is nearly symmetric with a peak temperature of 299 K (fig. 3); the temperature at which desorption began was again near 200 K for hydrogen saturation. At a sulfur coverage of 0.25 ML there is only a small amount of Hz desorbed, but the TPD curve shapes are similar to that for OS= 0.15 (fig. 4): the main difference is a peak

M.L. Burke, R.J. Madix / Hydrogen on Pd(IOO)-S

H#‘d(lOO)

I>

100

200

9

&=0.15

300

Temperature

400

5 00

200

(IS)

Fig. 3. Thermal desorption of H2 from Pd(100) with 6’s = 0.15. Exposures in ML of H, are (a) 12.5, (b) 3.75, (c) 1.25, (d) 0.37, (e) 0.12, (f) 0.04, (g) 0.01 and (h) 0. Heating rates are 12-13 K/S.

300

400

Temperature

50

(K)

Fig. 4. Thermal desorption of H2 from Pd(100) with 6’, = 0.25. Exposures in ML of H, are (a) 12.5, (b) 3.75, (c) 1.25, (d) 0.37, (e) 0.12, (f) 0.04, (g) 0.01 and (h) 0. Heating rates are 11-12 K/s.

temperature at saturation of 309 K. Hydrogen TPD for a sulfur coverage of 0.35 ML shows no H, desorption for H, exposures up to 12.5 ML. H Saturation

Coverage

vs S Coverage

Pd( 100)

4.2. Effect of a&orbed sulfur on hydrogen saturation coverage

The saturation coverage of hydrogen drops off linearly with sulfur coverage. Extrapolation of the data indicates that no hydrogen will adsorb for a sulfur coverage of 0.28 ML (fig. 5). As will be discussed in section 5.1, this behavior implies that sulfur poisoning of hydrogen adsorption is a short-range effect, with one sulfur atom blocking approximately four (3.7 f 0.5) H(a) binding sites. Also supporting the site-blocking model for the effect of sulfur on BH,sat is the similarity of our results with calculations by Campbell et al. for

o.o?,

LIZ

0.0

I,

0.1

r

I

I,

I

m\, t 3

0.2

Sulfur Coverage (ML) Fig. 5. Saturation

H(a) coverage on Pd(100) sulfur coverage.

as a function

of

10

M.L. Burke, R.J. Madix / Hydrogen on Pd(lOO)-S

adsorption onto surfaces in the presence of randomly distributed site blocking agents [33]. For dissociative adsorption of H, on Pt(ll1) they have shown that the uptake of hydrogen should fall linearly with the coverage of blocking agent. No H, adsorption was found to occur at 0.25 ML of blocking agent in the case where four H(a) sites are blocked by each site-blocker. Although two adjacent sites may be required for H, dissociation. the saturation coverage drops linearly since diffusion in the adlayer subsequent to dissociation permits sites which are blocked to dissociation of H, (two sites required) but not to hydrogen adatom binding (one site required since OH.uat= 1) to be occupied. Our experimental result for Pd(lOO) is completely analogous. Note that sulfur adsorption up to 0.22 ML appears to be random, as no LEED patterns form below this coverage. 4.3. Adsorption kinetics The dependence of the H(a) coverage on H, exposure indicates that a mobile precursor is involved in the adsorption process for sulfur coverages up to at least 0.08 ML. The direct and precursor adsorption models for a second-order process were employed to model the data. A firstorder direct model, in which the exponential in eq. (10) is changed from 2 to 1, was also used to model the adsorption data, but it was disregarded because it overestimated the H(a) coverage over most of the exposure range (fig. 6, dotted line). A first-order precursor mechanism was not considered since it would result in an enhancement of the adsorption rate over the first-order direct model, which already overestimates the adsorption rate. In the case of the sulfur free surface and for 0.08 ML of sulfur, the second-order direct adsorption model (fig. 6, dashed line) does not fit our data nearly as well as the precursor model (solid line). but as sulfur coverage increases to 0.15 and 0.25 ML the direct model fits well. This trend is expected from the definition of K, which goes to 1 as tirtsa,approaches zero (eq. (9)). At 19~= 0.00 the precursor model describes the dependence of coverage on exposure quite well for a K value of 0.20; when 0.08 ML of sulfur is preadsorbed a

0.0 -L

Fig. 6. Experimental hydrogen adatom coverage as a function of Hz exposure at 110 K on Pd(lOO) with 0s values of 0.00 (X). 0.08 (0). 0.15 (0) and 0.25 (i). The solid lines represent a second order-precursor model fit to the data using K values in table 1. The dashed lines result from a second-order direct adsorption model. The dotted lines are derived from a firstorder direct model.

value of K of 0.30 yields the best fit to the data. It thus appears that the precursor plays a role in H, adsorption at sulfur coverages I 0.08 ML. For sulfur coverages of 0.15 and 0.25 ML, K values of 0.75 and 1.00, respectively, yield best fits. The K value of 1 for 0.25 ML of sulfur implies a direct adsorption model, and the fit of the precursor model at 0.15 ML of sulfur is nearly indistinguishable from the direct model. Table 1 summarizes these results where values of s,, relative to so for the sulfur-free surface, sO/[so( 8, = O)], are listed along with the background exposures, chg and K values used in the fits. Also given is the standard deviation in the calculated coverages, a,, which was minimized in the determination of adsorption parameters. Table 1 Parameters

0.00 0.08 0.15 0.25

for hydrogen

adsorption

.%I/S”


(0s=O,

(MLofHL)

1.00 0.69 0.31 0.05

0.081 0.081 0.081 0.081

K

0.20 0.30 0.75 1.00

models a,

(ML of H)

Direct

Precursor

0.077 0.040 0.017 0.005

0.012 0.025 0.016 0.005

M. L. Burke, R.J. Madix

g:~,lool \

Relative Sticking Probability

,

x

Y

\

/ Hydrogen on Pd(lOO)-S

11 Relative Sticking Probability for H, on Pd(lOO)-S

(a)0.00

x x x 2nd order direct

\

(b) 0.08

- - 1st order direct x

24

2 0.4 *

‘b

x

0.0

0.2

0.4 &I/

0.6

I

0.8

1.0

&at

Fig. 7. Relative sticking probabilities versus relative hydrogen coverage for a second-order precursor model with K = 0.2 (solid line), a second-order direct model (crosses) and a firstorder direct model (dashed line).

Even on the clean surface the precursor’s contribution is only modest. The relative sticking probability is plotted in fig. 7 versus BH/BH,sat for the precursor model as well as for second- and first-order direct models. The enhancement over the direct second-order model is considerable in the coverage range near 0.5 ML, but the role of the precursor is not so pronounced as to yield a nearly constant sticking probability over a wide coverage range. The increase in the value of K as sulfur coverage increases indicates that the contribution from the precursor diminishes; this is reflected by the dependence of s/s0 on BH/t&tsat shown in fig. 8. Values of so/[sO( 0s = 0)] monotonically decrease as sulfur coverage increases (fig. 9) [34]. This indicates that, in the least, sulfur does block sites for precuror adsorption, and sulfur may entirely disrupt the precursor mechanism. Experimental data (filled squares) is plotted along with predicted curves for direct adsorption (dashed line) and precursor models which result from assuming y=0 H,sat (solid line), and y = 1 (dotted line). The predicted curves for s,/[s,(Bs = O)] versus Bs were calculated using eq. (7) by evaluating r.,J[r,,,(& = 0)] at 0u = 0. For the direct model k*/k, a 1, for the precursor models the limiting case of k*/k, -x 1 was used, and for all models site

hd a-I,sat Fig. 8. Relative sticking probabilities coverage for second-order precursor data for (a) 19,= 0.00 with K = 0.20, 0.30, (c) t& = 0.15 with K = 0.75 and 1.00.

versus relative hydrogen model fits to adsorption (b) 6’, = 0.08 with K = (d) f?s = 0.25 with K =

blockage by sulfur was assumed to be linear with all H(a) sites blocked by 0.28 ML of sulfur. A precursor model where sulfur does not block precursor adsorption (dotted line) clearly over-predicts the sticking probability; the value relative to the clean surface would remain near unity until sulfur coverages become high enough to limit the availability of sites for H, dissociation.

Initial Sticking Probability

of Hs on Pd(lOO)-S 1 . Data 2nd Order Precursor Y=l -2nd Order Precunor

- - 2nd Order Direct

0.1

0.2

Sulfur Coverage (ML) Fig. 9. Ratio of the initial sticking probability of H, on sulfur covered Pd(lOO) at 110 K to that of the sulfur-free surface as a function of sulfur coverage. Data points (m) are shown along with values predicted by a direct adsorption model (dashed line) and by precursor models with k*/k, -s 1 and with y values of eH,sa, (solid line) and 1 (dotted line).

M. L. Burke. R.J. Madix

12

The values observed for the initial sticking probabilities fall between values predicted by the model for direct adsorption, with s,/[s,,(~, = O)] = B,&t, and the precursor model with sO/[sO( 0, = O)] = en,._,. Care must be used in interpreting this comparison since our models invoke random site blocking. Clearly each sulfur atom blocks more than one H(a) site. Blocked sites are distributed on the surface as groups of sites surrounding each sulfur adatom. Monte Carlo calculations by Campbell et al. for the initial sticking probability of adsorbates on surfaces in the presence of blocking agents show that the functional form (1 - OH)“, where 13, is the fractional coverages of sites blocked (= 3.76, in our case) and n is the site requirement of the adsorbate, is strictly valid only if one site is blocked per blocking agent, and the blocked sites are randomly distributed. If each blocking agent blocks more than one site the initial sticking probability will be higher than that predicted by (1 - 19,)” [33]. Our result that initial sticking probabilities are higher than those predicted for a direct adsorption process may thus arise from a non-random distribution of blocked sites, rather than enhanced adsorption arising from a precursor; we cannot discern between the two using our analytical treatment where random site blocking must be assumed. However, negative deviations from the precursor model do not contradict a precursor adsorption mechanism for low sulfur coverages since the contribution to adsorption by the precursor may be diminished at high 0% 4.4. Desorption

kinetics

Desorption of hydrogen at high coverages does not conform to simple kinetics, and peak shapes indicate that the process may be precursor-mediated. This is illustrated in fig. 10, where the experimental TPD for a hydrogen saturation coverage on the sulfur-free surface is shown with computer simulations for direct desorption (open squares) and for precursor-mediated desorption for a firstorder precursor (diamonds) and for a second-order precursor (crosses). The curves were fit at three points only: the temperature where the rate is a maximum and the two temperatures where the

/ Hvdrogen on Pd(lOO)-S

L

!

‘,i.

1

IO0

200

300

Temperature

400 (K)

Fig. 10. TPD data and computer simulations for 0 H,I = 1.00: ( x ) second-order precursor model with E, = 112 kJ/mol and A = 8 X 10’ cm2/s; (0) first-order precursor model using E, = 80 kJ/mol and A = 2XlO-4 cm2/s; (0) direct desorption model using E, = 45 kJ/mol and A = 2~ lo-’ cm?/s.

rate is one-half of the maximum. Models for direct desorption and for second-order precursor-mediated desorption utilized eqs. (13) and (14), respectively, with parameters grouped to give overall values for both the preexponential and the activation energy. In the case of a first-order precursor, eq. (14) was modified to contain a first-order dependence on (Briaat - 8,, ), instead of the second-order dependence. The low temperature tail on the desorption trace is inadequately reproduced by the direct desorption model, but is fit well by the precursor models. The first-order precursor model gives the best fit over the full TPD trace; this probably arises because the kinetic equations used are valid only in the limiting case of a large contribution

M. L. Burke, R.J. Madix

from the precursor. The precursor only plays a modest role in the adsorption of H, (section 4.3) and the same should follow for the desorption process. A first-order precursor model simulates this intermediate behavior even though the readsorption of the precursor actually requires two sites. This analysis suggests that a precursor intermediate is involved in the desorption process, but is not proof since complex functionalities of the activation energy and preexponential could be constructed to model the desorption also. We have limited our quantitative kinetic analysis of TPD results to low initial hydrogen coverages, B,,i for several reasons. First, in the limit of low 6, the kinetic models for precursor-mediated desorption simplify to give the same form as that for direct desorption (eqs. (13) and (14)). This is

I

H2/Pd(100)

0s = 0.00

(m/e 2)

-Data

x 2nd Order Precursor 0 1st Order Precursor 0 D~rcct &sorption

10(I

200

Fig. 11. TPD

400

300

Temperature

500

(K)

data and computer simulations for 6’s = 0.00, second-order precursor model with E, = 92 kJ/mol and A = 2X lo-* cm*,& (0) first-order precursor model using E, = 88 kJ/mol and A = 8 X 10K3 cm’/s; (0) direct desorption model using E, = 84 kJ/mol ,and A = 2 X 10m3 cm’/s.

e H,, = 0.14: (x)

/ Hydrogen on Pd(lOO)-S

13

illustrated by simulations of H, desorption from the clean surface for B,,i = 0.14 (fig. ll), where equally good fits are obtained with each of the three kinetic models mentioned above. Also, kinetic parameters obtained from each model are virtually identical at low Brri; activation energies for desorption calculated for the desorption trace in fig. 11 vary by only 4 kJ/mol from the mean value of 88. Thus regardless of the model chosen, kinetic parameters at low B,,i should be accurate. Most importantly this method allows us to determine the effects of S-H interactions independently of H-H interactions, since the latter are minimized at low hydrogen coverages. Kinetic parameters were calculated from TPD results based on peak temperatures (T,) and the peak width at one half the maximum rate (AT1,2) using the method of Chan et al. [32]. This is the simplest method which results in accurate kinetic parameters. To minimize errors resulting from a single peak analysis, all TPD traces with an initial focd hydrogen coverage (eH,i_loc = eH,i/BH.sat) < 0.20 were analyzed, and mean values of E and log,4 are reported for each sulfur coverage. This also results in an equivalent basis of comparison for all sulfur coverages. Only two TPD traces had low enough initial coverages for 0, values of both 0.00 and 0.08, but three were used for 0, = 0.15. The signal-to-noise was prohibitively poor for TPD traces with 8, i_,Oczz 0.20 on the surface with 0.25 ML of sulfur: thus no kinetic parameters were calculated at this sulfur coverage. Both A and E, are reduced by sulfur, from 10-2.5 cm*/s and 85 kJ/mol, respectively, on the sulfur-free surface to 10-3.5 cm’/s and 75 kJ/mol at Bs = 0.08, and to 10-6.8 cm*/s and 49 kJ/mol for 6, = 0.15 (fig. 12). Table 2 lists the kinetic parameters obtained from each TPD trace as well as the mean values. Inherent values of A have been obtained by multiplying the effective A values by &r,sat to account for the effects of site blocking on desorption kinetics (eq. (13)). This adjustment is negligible for t9, values of 0.00 and 0.08, and the correction to log A is only 0.4 even at 0.15 ML of sulfur. Although the desorption rate from the surface with 0.25 ML of sulfur was too low to be accurately analyzed for BH,i_locI 0.2, note that Tp and AT,,, at saturation are nearly

M. L. Burke, R.J. Mudix

/ Hydrogen on Pd(lOO)-S

5. Discussion

Kinetic Parameters for Desorptwn H,/‘Pd(lOO)-S

5.1. Nature of the influence

.

.

(b)

/ Sulfur

Coverage

(ML)

Fig. 12. Variation of (a) the activation energy and (b) the log of the preexponential factor for H, desorption from Pd(lOO) as a function of sulfur coverage. Initial local hydrogen coverages are 5 0.20 ML.

identical to the same values for desorption of a hydrogen saturation coverage at 0, = 0.15. The kinetics of desorption on the two surfaces are thus indistinguishable.

Table 2 Kinetic parameters from Tp and A~,,,

for H, desorption

from Pd(lOO)-S derived

0s

%,

k-,oc

Tp (K)

AT,/,

E (kJ/moU

log[ A

0.00

0.14 0.18 mean

0.14 0.18

316 315

46.5 44.5

83 87 85

-2.8 - 2.3 - 2.5

0.08

0.14 0.16 mean

0.17 0.20

361 359

41 47

16 75 75

- 3.5 -3.6 - 3.5

0.15

0.03 0.04 0.06 mean

0.07 0.09 0.14

341 339 331

61 63 62

51 48 47 49

- 6.4 - 6.9 -7.1 -6.8

(cm’/s)l

of sulfur

The major effect of sulfur on hydrogen adsorption is the linear drop of the saturation hydrogen coverage and the nearly linear decrease in the initial sticking probability of H, with increasing sulfur coverage. Both of these effects are indicative of site blocking by sulfur, whereby each sulfur atom blocks an average of approximately four (3.7 + 0.5) hydrogen adsorption $tes. The van der Waals diameter of sulfur of 3.9 A and the Pd(lOO) interatomic spacing of 2.75 A implies that each sulfur atom fills not only the four-fold site which it occupies, but also extends its steric influence into the four nearest neighbor hollow sites. An isolated sulfur atom would thus block up to 5 sites; the reduction of the average number of sites blocked per sulfur atom is attributed to intersecting ranges of site blocking for contiguously adsorbed sulfur adatoms. The result that H, adsorption is completely blocked at 0.28 ML of sulfur, along with the presence of a p(2 x 2) LEED pattern at this coverage, indicates that the four-fold hollow in the center of the p(2 X 2)-S unit cell is not available for hydrogen adsorption at 110 K and the UHV conditions of the experiments. Although this site is beyond the steric influence of the adsorbed sulfur atoms, it is an isolated binding site. If a minimum of two adjacent hollow sites are required for H, dissociative adsorption and each sulfur adatom blocks its four nearest neighbor hollow sites, then these isolated sites are inaccessible. Similarly, H, adsorption beyond 8, = 0.25 up to 0.28 ML of sulfur may result from binding of sulfur at defect sites. It is possible that the central hollow of the p(2 X 2)-S adlayer is energetically unfavorable to hydrogen atom population, rather than just being sterically inaccessible. The destabilization of H(a) by sulfur may be large enough at this site to reduce the H(a) binding energy below the level necessary for exothermic adsorption and, therefore finite population at the low pressures used in UHV studies is not observed. The binding energy for hydrogen adatoms on the clean surface is 260

ML.

Burke, R.J. Madix

kJ/mol, calculated by assuming non-activated adsorption. The H(a) binding energy at the central hollow of the p(2 X 2)-S unit cell must be reduced by only 1520% below the clean surface value to make dissociative adsorption of H, energetically unfavorable. Based on the E, for desorption at low coverages of H, there is a 7% reduction in the binding energy of hydrogen adatoms upon the addition of 0.15 ML of sulfur to the clean surface; this destabilization should be even more pronounced at 0.25 ML of sulfur, due to the additive destabilization of the four sulfur adatoms surrounding the “isolated” four-fold hollow. The details of the effects of adsorbed sulfur on the desorption kinetics is difficult to deconvolute since a multiplicity of reaction channels may exist with different activation energies (see section 5.2 for further discussion). In addition, sulfur is certain to hinder the diffusion of hydrogen adatoms on the surface, further complicating the observed kinetics (e.g. see ref. [35]). Whether S-H interactions are involved in desorption which are more long-ranged than the sum of van der Waals radii is currently unknown. If the observed reduction of E, for hydrogen desorption by sulfur is, however, attributed to hydrogen adatom destabilization, only a minor perturbation of hydrogen adatoms in the vicinity of sulfur adatoms needs to be invoked to explain the 2% and 7% reductions in the H-Pd binding energy observed at sulfur coverages of 0.08 and 0.15 ML, respectively. Possible causes include long range influences of sulfur, such as electrostatic interactions and modification of the Pd electronic wavefunctions by the additive interactions of the sulfur atoms evaluated at a given point on the surface. Both hydrogen and sulfur atoms acquire a partial negative charge when adsorbed on Pd [l&36]. As a result the two adspecies should repel each other, with the magnitude of the effect dependent upon the charges on H(a) and S(a). Alternatively, calculations by Feibelman and Hamann for ordered l/4 ML overlayers of S, P and Cl adsorbed on a two layer thick Rh(100) film show that these adatoms may cause a reduction in the local density of states at the Fermi level of the metal (E,-LDOS) [37]. The lowering of E,-LDOS extends to Rh atoms which are next nearest

/ Hydrogen on Pd(IOO)-S

neighbors coverages

15

to the adatoms, but neither low adatom nor longer distances were probed.

5.2. Compensation effect induced by surfur The tandem decrease of both A and E, is a compensation effect. A plot of In A versus E, (fig. 13) shows that the three experimental points are colinear and yield an isokinetic temperature of 443 K (slope = l/Rqs,), i.e. the temperature at which the rate constants at all sulfur coverages are equal. The compensation effect is an often encountered, but little understood phenomenon, which has been reviewed in a theoretical paper by Peacock-Lopez and Suhl [38]. They argue that the coupling of reactant energy levels to states of the bath, in this case the Pd(lOO) surface, inherently increases the entropy of activation when the activation energy is increased. A more direct physical interpretation is that raising the energy of the reactants (i.e. lowering the activation energy) inherently increases their entropy and correspondingly decreases the entropy of activation. This entropy rise may take the form of lower adsorbate-metal vibrational frequencies or increased translational freedom, for example. If the entropy of activation is attributed to translational degrees of freedom, the four order of

‘1 In A versus

-16-18 40

E for HJPd(

100)-S

,/

I

I

I

I

50

60

70

80

Activation

’

Energy (kJ/mol)

Fig. 13. In A versus I$ obtained for initial local hydrogen coverages s 0.20 ML and sulfur coverages of 0.00-0.15 ML. The slope (l/RT,,,) is indicative of an isokinetic temperature of 443 K.

16

M.L. Burke, R.J. Madix

magnitude decrease in the preexponential factor for desorption corresponds to a reduction of approximately l/2 degree of freedom in the entropy of activation for the surface with 0.15 ML of sulfur, relative to the clean surface [39]. Considering the specifics of the H/Pd/S system suggests that changes in the transition state for desorption may be an important factor in the origin of the compensation effect. Adsorbed sulfur will certainly restrict H(a) motion on the surface, but this would result in an increase in the preexponential, everything else remaining equal. However. in one limit the rate constant for desorption via a precursor is k,k*/k, (eq. (14)) thus the transition state for desorption is that of the precursor desorbing into the gas phase times the equilibrium constant (K * = k,/k,). Then AS,&, = AS! + AS** - AS:, or AS&, = S** - S(2H(a)). In this case the transition state to desorption from the clean surface is highly mobile, Further, a shift from precursor-mediated to direct desorption as OS increases is implied by the reduced contributions from the precursor in hydrogen adsorption (section 4.3). In a direct desorption mechanism the rate constant is simply k, (eq. (13)) so the transition state lies between hydrogen adatoms and gas phase H,. If there is a preferred trajectory for recombination of the hydrogen atoms, as the work of Lee and DePristo indicates for H, on Ni(lOO) [40], the transition state should be much more constrained than for the desorbing precursor. Both the restriction of precursor mobility and the movement toward a direct mechanism with a more constrained transition state would lower the entropy of activation and thus the preexponential factor, as sulfur coverage increases. In effect, the sulfur may change the nature of the transition state. The multiplicity of activated pathways for hydrogen desorption which will naturally result from the destabilization of some H(a) sites relative to others due to the presence of sulfur may also lead to an apparent compensation effect. This effect may be enhanced if the surface coverage of the sulfur is nonuniform, since there will be an even greater range of activation energies. The resulting TPD can be considered as a sum of desorption from individual states with different activation

/ Hydrogen on Pd(lOO)-S

broadenergies, giving rise to “ inhomogeneous the peak ening” of the TPD curves. Certainly, temperature may be shifted down and the peak width increased relative to the clean surface TPD due to lower energy pathways. This effect on the desorption curves would be interpreted as a decrease in both the activation energy and preexponential. 5.3. Comparison

with previous

work

The linear decrease we have found for the saturation coverage of hydrogen on Pd(100) as a function of increasing sulfur coverage is similar to results for Ru(001) [5] and Ni(lOO) [6]. Schwarz has reported that the saturation coverage of hydrogen falls linearly with f& on Ru(001) and that no hydrogen adsorbs at sulfur coverages < 0.25 ML [5]. The origin of this effect was attributed to sulfur merely blocking sites for hydrogen adsorption. Similarly, on Ni(lOO) Kiskinova and Goodman found that ortaat drops linearly up to a sulfur coverage of 0.25 ML. but then decreases more gradually [6]. Our kinetic parameters for H2 desorption from clean Pd(lOO), with E, = 85 kJ/mol and A = 10-‘.scm2/ s , a g ree well with those of Behm et al. who estimated an activation energy of 92 kJ/mol by assuming A = lo-* cm2/s [16]. It is most instructive, however. to compare our results with those obtained for other sulfur-covered surfaces. In previous work E, and A for H, desorption from clean Ni(lOO) were reported to be independent of hydrogen coverage for the highest temperature. or cr state [7]: the values of 89 kJ/mol and lo-‘-’ cm2/s obtained for E, and A, respectively, are quite similar to the values found here for sulfur-free Pd(lOO). At the highest sulfur coverages studied, E;, was estimated to be 63 kJ/mol at low hydrogen coverages (f?H.,oc = 0.2). Using the reported peak temperature, E, and or,,, we estimated A to be 10e5 cm2/s. Comparing our result of 49 kJ/mol and 10e6.’ cm*/s on Pd(lOO) indicates that the degree of destabilization of H(a) by adsorbed sulfur is greater on Pd(lOO) than on Ni(lOO), although the effect of sulfur on hydrogen desorption kinetics is qualitatively the same on both surfaces.

M. L. Burke, R.J. Madix / Hydrogen on Pd(lOO)-S

Similarly, Kiskinova and Goodman reported that peak temperatures for H, desorption from Ni(lOO) decreased as sulfur coverage was increased, but they performed no kinetic analysis [6]. On Ru(OOl), Schwarz also found that the addition of sulfur results in a reduction in T, as well as an increase in AT,,, for desorption of a fixed coverage of hydrogen [5]. This effect, however, was attributed to increased local hydrogen coverages in the presence of sulfur, since a similar decrease in Tp and increase in AT,,, occur on the sulfur-free surface as hydrogen coverage is increased. The same interpretation is obviously not valid for our Pd(lOO) results since Tp for H, desorption in the presence of sulfur is shifted down by as much as 50 K from that for a saturation hydrogen coverage on the sulfur-free surface. Studies of CO adsorption and desorption on sulfided Ni(lOO) by Hardegree et al. [12], Madix et al. [8,9] and Gland et al. [13] all give a similar picture of the effect of sulfur as that found here. Although small amounts of sulfur cause a sharp decrease in the saturation coverage of CO due to the disruption of high coverage compression structures, the main effect of sulfur on CO saturation coverage is due to site blocking. On Ni(lOO) sulfur causes a slight decreases in the activation energy for CO desorption. Hardegree et al. showed that this decrease may be caused by electrostatic interactions, based on the range of the interaction and the charges on adsorbed sulfur and CO obtained from work function results. On the Pd(lOO) surface Jorgensen and Madix found that the influence of sulfur on adsorbed CO is only slightly more extensive than pure site blocking in that CO desorption temperatures were slightly lowered by even small sulfur coverages [41]. In addition, their EELS data for Pd(lOO) show that the v(C0) frequency at low CO coverages gradually increases with sulfur coverage with a 30 cm -’ increase at sulfur saturation. That the increase in the Y(CO) frequency was observed for low CO coverages is significant in that this allows the frequency shifts to be attributed to sulfur-CO interactions, since CO-CO interactions should be negligible at low coverage. The increase in Y(CO) is characteristic of a decrease in the degree of charge donation from the metal into the CO 7~*

17

orbital and possibly a weakening of the metal-CO bond. Similarly, for CO on Ni(ll1) Trenary et al. have concluded from their infrared spectroscopy studies that the effects of sulfur on coadsorbed CO are short-ranged [14]. CO adsorbed at 300 K at an ambient pressure of 0.1-1.0 Torr displays changes in the number of IR bands as sulfur coverage is increased, but the frequency of each band is independent of sulfur coverage. Analogous to the results of Gland et al. for Ni(lOO), Trenary et al. have concluded that this is indicative of site blocking induced changes in the CO binding site, but does not reflect long range interactions. Another system displaying only small perturbations of adsorbate-metal bonding by sulfur is SO, on Pd(lOO) [42]. EELS spectra at low SO, coverages were virtually identical on the sulfur-free and p(2 X 2)-S surfaces. The binding configuration as assigned from intensity ratios of SO, symmetric and asymmetric S-O stretches remains the same for both cases, but slight increases in the frequency of both of these vibrational modes indicate that donation from the metal into the SO, 7~* orbital was reduced when 0.25 ML of sulfur was present.

6. Conclusions Adsorbed sulfur lowers the saturation coverage of hydrogen on Pd(lOO) through a site-blocking mechanism, with hydrogen adsorption completely blocked at a sulfur coverage of 0.28 ML. The adsorption kinetics of H, on the sulfur-free surface reflect the influence of a precursor, and the contribution from the precursor is gradually diminished as sulfur coverage is increased. The initial sticking probability is near unity on the clean surface, and decreases with the addition of sulfur in a manner consistent with sulfur blocking about the same number of sites for the precursor state as for H(a) binding. Hydrogen desorption kinetics indicate that S-H interactions are more complex than pure site blocking. Both activation energies and preexponential factors for desorption are decreased by sulfur from 85 kJ/mol and 10-2.5 cm2/s, respectively, on the sulfur-free surface to 49 kJ/mol and

18

M.L. Burke, R.J. Madix / Hydrogen on Pd(lOO)-S

10e6.s cm2/s at a sulfur coverage of 0.15 ML. TPD lineshapes are consistent with a precursor influence on desorption of saturation hydrogen coverages from the sulfur-free surface; precursormediated and direct desorption models are indistinguishable at low hydrogen coverages, however. A compensation effect is displayed by the decrease in both the activation energy and preexponential factor for hydrogen desorption as sulfur coverage is increased. A possible explanation of this effect is that on the sulfur-free surface the transition state is precursor-like, with a high degree of mobility, and as sulfur is added this translational freedom is decreased due to movement toward a direct desorption process with a more constrained transition state nearer to the onset of adatom recombination. However, the compensation effect may also be explained by a distribution of activation barriers caused by the adsorbed sulfur. Even with no change in the preexponential for desorption a multiplicity of activation energies will lead to peak temperatures shifted down from that which is characteristic of the highest binding energy of H(a) on the surface and broadened desorption peaks, thus producing a “macroscopic” compensation effect.

Acknowledgements We gratefully acknowledge the support of this work by the National Science Foundation (NSFCHE86-15910). M.L.B. would like to thank the National Science Foundation for support through a graduate fellowship. We also thank C.T. Campbell for helpful discussions.

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