Temperature programmed desorption studies of hydrogen on Zn(0001) surfaces

Surface Science 145 (1984) 185-196 North-Holland, Amsterdam

TEMPERATURE PROGRAMMED DESORPTION HYDROGEN ON Zn(OOO1)SURFACES

185

STUDIES OF

L. CHAN and G.L. GRIFFIN Departmenr of ChemicalEngineeringund MaterialsScience, uniuersifyof Minnesota, ~inffe~~oiis, Minnesota5.5455, USA Received 13 January 1984; accepted for publication 4 April 1984

We report for the first time temperature programmed desorption (TPD) spectra of H, from Zn(OOOt) surfaces. Hydrogen is adsorbed in its atomic state, as evidenced by the fact that adsorption must be performed using pre-dissociated hydrogen. However, the observed TPD spectra do not show second-order kinetics, as would be expected for recombinative desorption.

Instead, the peak shapes and the coverage dependence of peak temperatures both appear to obey half-order kinetics, with a desorption energy of 11.1 kO.3 kcal/moI. The kinetic order strongly suggests that island formation occurs, due to attractive interactions between neighboring adsorbed H atoms. An explanation for the attractive interactions is proposed, based on the bonding mechanism in diatomic ZnH and an elementary description of the band structure of Zn metal.

1. Introduction In comparison redox

reactions

with most other metals, Zn has a remarkably low activity involving hydrogen. This makes Zn useful for a number

for of

applications, such as’galvanized coatings and storage battery electrodes. To date there have been no experimental studies of H, adsorption on clean Zn surfaces under controlled conditions. Such studies would be relevant to the technolo~cal appfications noted above, and would also provide a useful comparison with the H, adsorption behavior of the catalytically important Group VIII and IB metals. In this paper we report temperature programmed desorption (TPD) spectra of H, from Zn(OOO1) surfaces. The most novel result is the fact that even though adsorbed hydrogen almost certainly exists in its atomic state, the TPD spectra do not show second-order desorption kinetics. Exhaustive analysis of the spectra shows that half-order kinetics are obeyed, indicating significant attractive interactions and strongly suggesting island formation in the adsorbed H(,) layer. To our knowledge this is only the second reported example of attractive interactions between I-f(,) atoms on a metal surface [l], and the first time that evidence for island formation in an H(,) adlayer has been reported. electrochemical

0039~6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

186

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G. L. Griffin /

TPD of hvdrogen

on Zn(0001)

2. Experimental Each sample was prepared by cleaving a Zn single crystal along the (0001) basal plane. Fracture was performed in liquid nitrogen to improve the quality of the cleaved surfaces. The crystal was spot-welded onto a Ta wire holder and an iron-constantan thermocouple was attached to the edge of the crystal. A thin Au film was evaporated onto the back of the crystal to minimize desorption signals from this surface. The experimental apparatus is shown in fig. 1. It consists of an ultra-high vacuum system specifically configured for TPD experiments using gases with low sticking coefficients. One side of the system contains the sample crystal, a sputtering gun for cleaning the front surface of the crystal, and an ionization gauge for monitoring the dosing pressure during adsorption. The ionization gauge filament also serves to produce dissociated H atoms for adsorption. A turbopump is used to evacuate the relatively high gas load that remains after dosing. The other side of the system contains a quadrupole mass spectrometer for monitoring desorption flux and a 20 l/s ion pump for differential pumping. The two sides of the vacuum system are separated by an isolation valve. A collimating tube is mounted in front of the sample, so that the desorption flux

TURBO PUMP

ISOLATION VALVE ION PUMP

GETTER PUMP

REACTION CHAMEER

Fig. 1. Apparatus for temperature low adsorption probabilities.

programmed

desorption

DETECTION CHAMBER

experiments

to study adsorbates

with

L. Ghan, G. L. Griffin / TPD of hydrogen on Zn(OOO1)

187

from the crystal is focussed into the detection chamber, while stray desorption flux from sample leads and the back side of the crystal is primarily removed via the turbopump. The typical experimental procedure is as follows: the surface is cleaned by argon ion sputtering for 30 min ( PAr = 5 X lop5 Torr, beam voltage = 2 kV, ion beam current = 6 PA/cm*). Separate experiments in a system equipped with AES and XPS probes confirm that this is sufficient to produce a clean Zn surface. Hydrogen atoms are then adsorbed onto the surface by admitting H, to the sample chamber while the ionization gauge filament is kept on. The sample is held at - 210 K to minimize adsorption of any H,O produced by the filament. Following the desired exposure (reported in terms of molecular H, exposure), the reaction chamber is evacuated and the TPD spectrum is obtained by resistively heating the crystal (warm-up rate = /I = 2.5 K/s). Control experiments show that no hydrogen adsorption occurs if the filament of the ion gauge is turned off during H, exposure. This confirms that primarily H atoms absorb on the Zn(OOO1) surface at these dosing conditions.

3. Results In fig. 2 we show the TPD spectra for various initial coverages of H atoms on a sputter-cleaned Zn(OOO1) surface. The exposures listed for each curve represent the gas phase molecular H, exposure. We have no method for

319K I

12.5 x IO-%mp

EXPOSURE

=

1 x 105L Ii2 5x104 2x104 1 x104 7.5 x 103 5 x103

I

1 270

, 4sec I 300 330

I 360

TEMPERATURE

(IO

Fig. 2. TPD spectra of H, desorption

from clean Zn(OOO1) (adsorbed as atomic H).

188

L. Ghan, G. L. Griffin /

TPD of hvdrogen

on Zn(0001)

measuring the H atom flux at the sample directly, but assume that it is proportional to the pressure of H, in the sample chamber during exposure. Two features of these spectra are noted: First, a saturation limit exists for H(,, coverage on the Zn(OOO1) surface. No changes are seen in the TPD spectra for H, exposures greater than lo5 L. In addition, the saturation coverage appears to be restricted to the surface monolayer and does not include significant dissolved H beneath the surface. This is indicated by the absence of a long “tail” on the high-temperature side of the TPD peak that would be suggestive of a diffusion limited desorption process. The second, more striking feature of the TPD spectra is their triangular “ramp” shape, with a shallow leading slope and a steep trailing slope. In addition, the peak temperature shifts upward with increasing coverage. Both of these aspects of the desorption behavior are inconsistent with ideal second order kinetics, and instead suggest fractional order kinetics [2]. To examine this result more carefully, in fig. 3 we show isothermal reaction order plots of the desorption rate as a function of adsorbate coverage. These are obtained from the TPD spectra in fig. 2 by comparing the desorption rate at a given temperature versus the amount of adsorbate remaining at that temperature. The instantaneous adsorbate coverage is determined by integrating the area remaining under the TPD spectrum. A logarithmic plot of desorption flux, df?/dt, against amount remaining, 8, yields the reaction order,

5.Or

FRACTIONAL Fig. 3. Reaction remaining.

order

plots

COVERAGE for results

(Q) in fig. 2: desorption

rate

as a function

of adsorbate

L. Ghan, G.L. Griffin / TPD of hydrogen on Zn(OOOI)

189

Wl:

) = ln( k) + m ln( e),

ln(df?/dt

(I)

where k is the desorption rate constant. The data in fig. 3 are correlated by straight lines with an average slope m = 0.47 + 0.05. Thus the desorption spectra all obey half-order kinetics. Once the reaction order is known, we can determine the desorption energy, Ed. If we assume that the desorption rate is described by - de/dt = k,,O”* exp( - EJRT),

(2)

then two methods can be used to find Ed. The simpler approach is to analyze the shift in peak temperature, T,, as a function of initial coverage, 0,. For half-order kinetics this shift is described by [2,3]: ln( 0:/‘/T,‘)

= ln( k,R/PE,)

- (E,/R)(l/T,).

(3)

In fig. 4 we show a plot of ln(r3,“*/T~) versus l/T, for the TPD spectra in fig. 2. The activation energy obtained in this way is Ed = 11.1 + 0.3 kcal/mol. The second method for determining Ed is to analyze a direct Arrhenius plot of the desorption rate. This requires that the observed desorption flux, F, must be divided by coverage factor for the remaining adsorbate at each temperature. In fig. 5 we have plotted F/d’/* versus 1LT for points taken at various temperatures from fig. 2. The desorption energy obtained in this way is Ed = 11.1 + 0.3 kcal/mol, in agreement with the value obtained using the first method.

-11.5-

NE I\ c -m"

-Ed = 11.1kcol/mol

5

I

I

I

3.15

3.20

3.25

lOOO/T,,,(/K) Fig. 4. Analysis of peak temperature shift as a function of initial coverage, for results in fig. 2. (See text, eq. (2))

190

L. Chan,

I 3.25

G.L. Griffin

I

/

TPD

of hydrogen

on Zn(OOO1)

I

3.75

3.50

1000/T(K) Fig. 5. Arrhenius plot of observed for results in fig. 2.

desorption

flux (normalized

by 8’/2 for remaining

adsorbate)

4. Discussion

In the remaining pages we shall comment on the physical significance of the above results. In particular, we wish to distinguish the similarities and differences of H adsorption on Zn versus other metals. 4.1, Adwrption

energy

A fundamental parameter of interest to surface chemists is the bonding energy between a hydrogen atom and a metal surface, E,_,. As noted in recent reviews 14-71, this quantity is almost independent of the metal, at least for low H(,, coverages on Group VIII and IB metals. For example, E,-u_H = 56 kcal/mol [8], E,,,_ ,, = 56 kcal/mol 191, EcO_,., = 60 kcal/mol [lo], ENi_u = 61 kcal/mol (l), E,,_,, = 61 kcal/mol [ll], and EFe_H = 62 kcal/mol [12]. Direct comparison of these numbers with a value for Zn is hampered by the fact that EM_., is related to the equilibrium energy of adsorption (EM_, = i UadS+ 51.6 kcal/mol, the latter term accounting for the Hz dissociation energy), while the TPD technique actually measures the desorption energy, Ed. As evidenced by the need to use pre-dissociated H, for the adsorption step, there is a significant (but undetermined) activation barrier for H, adsorption on Zn(OOO1). In the absence of quantitative knowledge of the barrier height, we can conclude only that the measured value of Ed = 11.1 kcal/mol provides an

L. Ghan, G.L. Griffin / TPD of hydrogen on Zn(fXIOl)

191

upper limit for the Zn-H bond energy, EZn_” I 57 kcal/mol. If the activation barrier is less than 11 kcal/mol, then the H, adsorption reaction is still thermochemically exothermic. This would imply a Zn-H bond energy in the range 51.6 -z E,,_ H < 57 kcal/mol. Such a value would be near the range of Group VIII and IB metals, and would support recent theoretical arguments that H adsorption on these metals occurs primarily through interaction of the H 1s orbital with the metal sp band, rather than the metal d band [13-151. We note that an activation barrier of 3-5 kcal/mol has been reported for H, adsorption on Cu [16]; this number would be relevant for Zn if the role of d orbitals is small in both the equilibrium adsorbate configuration and the transition state for adsorption. If the activation barrier is greater than 11 kcal/mol, this would imply that H, adsorption is thermochemically endothermic. Intuitively this seems unlikely, but it cannot be rigorously ruled out at this point. Comparison with Cu (small activation barrier) and Group VIII metals (no measurable barrier) would then imply that d electrons play the distinguishing (although not the major) role in both the equilibrium and transition state bonding configurations. Clearly, future work should focus on measuring this barrier height for adsorption, in order to resolve the extent of d-electron participation more clearly. One other point worth noting is that unless the activation barrier is extraordinarily large (ca. 70 kcal/mol), the bond energy of a hydrogen atom on Zn metal, EZn_“, is significantly higher than the bond energy for diatomic ZnH (reported as 19.6 kcal/mol [17] and 20.5 kcal/mol [IS]). This is in marked contrast to the Group VIII and IB metals, where the two energies are comparable [5,7]. The unusual case for Zn arises from the abnormally low bond energy of diatomic ZnH. Because the isolated Zn atom has a filled 4s shell, these electrons must be promoted into sp-hybrid orbitals before the H-Zn bonding overlap can develop [13]. In contrast, the electrons near the top of the valence band in Zn metal already have significant sp character [20]. 4.2. Half order desorption kinetics The conclusion that H, desorption obeys half-order kinetics is based on the coverage dependence of the TPD temperature peak and on the shape of the TPD spectra. Both features are essential for this interpretation. An increase in peak temperature with increasing initial coverage by itself might be caused solely by the effect of attractive interactions superimposed on ideal second order kinetics. To illustrate that this is not the case for the present results, we have calculated model TPD spectra based on second order kinetics with a linearly dependent desorption energy (i.e., using mean field theory to account for

L. Ghan. G.L. CriJfin / TPD of hydrogen

192

attractive

on Zn(0001)

interactions):

-de/dt

=

k,e2

The simulated

exp[ -(E,

spectra

+

wej/zw].

are shown

(4)

in fig. 6, based

E, = 17.7 kcal/mol, and w = 2.4 kcal/mol. By comparing these spectra with the observed temperature shapes

peak

shift is reasonably

are poorly

reproduced.

narrower

as coverage

accurate

approximations

auto-catalytic observed Ni(ll0)

effect

increases. than

The

calculated

This narrowing mean

described

field

both the temperature

using half-order

kinetics.

spectra

become

[21,22]

well with

reproducing

the observed

the “ramp”

with increasing

using more

and is due to the

desorption

energy.

of formate

species

peak shift and the spectral

in fig. 2, particularly

shape of the spectra and the broadening

initial coverage.

the end of the desorption

The simulated

process,

because

spectra

It is not for H, on on various

shape can be

In fig. 7 we show simulated

results

spectral

significantly

is also predicted

based on eq. (2), using k, = 6 X lo6 s and Ed = 11.1 kcal/mol. quite

s-l,

in fig. 2, we see the

but the overall

spectra

theory

dependent

= 10”

of H, on Zn(OOOl), but has been reported

[l], and for the decomposition

metals [23]. In contrast,

results

well-described,

of a coverage

in our spectra

on k, = v,,n,,,

TPD

with

respect

to

of the spectra

do not behave

of the singularity

spectra

These compare

well at

at 8 = 0 in the

“A TEMPERATURE

(K 1

Fig. 6. Simulated TPD spectra based on second-order (7)); k, = lOI s, Ed = 11 kcal/mol, w = 2.5 kcal/mol,

kinetics with attractive interactions B = 2.5 K/s.

(cf. eq.

193

L. Chon, G.L. Griffin / TPD of hydrogen on Zn(OOOI)

derivative of eq. (2). In the real system, we would expect the adlayer behavior to convert to first or second order kinetics as the adsorbate coverage approaches zero (e.g., as the size of the “evaporating” islands (see below) approaches molecular dimensions). 4.3. Island formation The simplest explanation for half-order kinetics is that desorption occurs primarily from edge atoms in condensed adsorbate “islands” (i.e., segregated domains of high local adsorbate density). These islands would exist as a result of attractive interactions between adsorbed H atoms, and desorption would occur preferentially at island edges because the adsorbate atoms located there have fewer nearest neighbors than do adatoms in the interior of the islands. The number of islands, which is assumed to remain fixed during desorption, would be determined by nucleation kinetics during the adsorption process, and by the concentration of substrate defects that could serve as nucleation centers. Before asserting this conclusion, one must consider whether the existence of islands should be detectable at all in the TPD experiment. More specifically, will ordered domains in an adsorbate layer be stable for the time required for them to desorb completely? An approximate answer can be inferred from critical point arguments. Half order kinetics will only be observed as long as there are domains of high adsorbate density that are distinguishable from regions of low (or vanishing) density. Such a condition can exist, even if there is adsorbate mobility between domains of high and low density, provided the surface is below the critical temperature, T,, for phase separation in the adsorbate layer. Thus, a crude criterion for the detectability of adsorbate islands in a TPD experiment is that

TEMPERATURE (K) Fig. 7. Simulated TPD spectra based on half-order kinetics (cf. eq. (2)); ko = 1.5 kcal/mol, B = 2.5 K/s.

X

10’ s, Ed = 11

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L. Ghan. G. L. Griffin /

TPD

of hydrogen

on Zn(0001)

the critical temperature must be greater than the desorption peak temperature. In the present case, this implies T, > 320 K. A lower limit for the adsorbate interaction energy can then be obtained using statistical mechanical results for lattice gas models. If we assume the six-fold symmetry of the Zn(OOO1) surface is reflected in the adsorbate layer, then the critical temperature is given by [22]: kT, = 1.8205<,

(5)

where c is the pairwise additive interaction energy between two nearest neighbor adsorbate atoms. using T, > 320 K, this yields E > 0.35 kcal/mol. It is interesting to note that this value, when multiplied by six to account for a full complement of nearest neighbors, gives a value of w = 2.1 kcal/mol, which is in reasonable agreement with the value found above while attempting to model the desorption results using the mean-field approximation (cf. fig. 6). Of course, the observation of half-order kinetics by itself does not prove the existence of islands. Further evidence would be desirable, such as EELS measurements that the adsorbate vibrational frequency might be independent of coverage. It would also be useful to have a measurement of island size. 4.4. Origin of attractive

interactions

Several workers have predicted the existence of attractive interactions between adsorbates [24-261. The major cause of these interactions is usually attributed to “through-substrate” effects; i.e., the metal-adsorbate bond of one site is perturbed toward greater stability by the presence of neighboring metal-adsorbate bonds. Calculations for model systems show that the magnitude and even the sign of the interaction energy is a strongly coupled function of the electronic parameters of the system (e.g., band width, band occupancy, energy level spacing between adsorbate and the metal, etc.). Therefore it is difficult to propose a rigorous explanation for the observed interactions in the present system. However, we would like to offer a qualitative explanation based on simple chemical bonding concepts and elementary band theory. As noted above, the bonding in diatomic ZnH involves the H 1s orbital and a hybridized Zn sp orbital. Cluster calculations indicate that p= character is also enhanced when a single H atom is adsorbed on the surface of an sp metal [27]. Next, we note that the lower portion of surface bands derived from p, orbitals are centered around the r point in the Brillouin zone. This means that atomic components of the p,-derived metal wavefunction have similar signs for neighboring Zn atoms (i.e., a net bonding p: overlap occurs because the Fermi energy is near the bottom of the pz-like bands). When we now consider the case of two H atoms adsorbed at neighboring sites, two effects can be expected:

L. Ghan, G.L. Griffin / TPD of hydrogen on Zn(OOO1)

195

(i) The H 1s components of two neighboring H-Zn bonding orbitals will also have the same signs. This will allow bonding overlap between neighboring H atoms, and thus may give rise to “direct” attractive interactions. (ii) The adsorption of a single H atom will disrupt the participation of the underlying Zn atom(s) in the p,-derived bands. Thus p, electrons on neighboring Zn atoms will lose some of their substrate bonding energy, and will become more energetically available to participate in a second H-Zn surface bond. This will give rise to “indirect” or through-substrate attractive interactions. While a well-reasoned choice between (i) and (ii) cannot be made at this level of simplicity, we favor indirect interactions as the more likely cause, for two reasons: Direct interactions resulting from positive H 1s overlap would be expected to also reduce the activation barrier for dissociative adsorption. In the case of Zn, however, the dissociation barrier is higher than on other metals. Secondly, the p, orbital symmetry postulated as necessary to induce indirect interactions is a result of the way in which the 4s-closed shell nature of atomic Zn is reflected in the bulk band structure of Zn metal. This effect would not be expected on Group IB or transition series metals, which would account for the almost total absence of reported attractive interactions for H adsorbed on other metals.

5. Summary

The desorption energy of hydrogen from Zn(OOO1) surfaces is 11.1 * 0.3 kcal/mol. Unless the activation barrier for dissociated’ H, desorption is exceptionally large, this implies that the Zn-H bond energy is similar to that for Group VIII and IB metal surfaces. The desorption spectra for different initial coverages demonstrate the existence of attractive interactions between adsorbed H atoms. The spectra obey half-order desorption kinetics, which suggests quite strongly that island formation takes place. A qualitative explanation based on through-substrate interactions in Group IIB metals is presented to account for these attractive interactions. Further experimental measurements of the island formation process and a more quantitative theoretical description of the bonding process both appear to be worthwhile avenues for future research.

Acknowledgement

This work was supported by the Corrosion Research Center at the University of Minnesota, through Department of Energy grant no. DE-AC0279ER10450.

L. Ghan, G. L. Griffin /

TPD

of hydrogen

on Zn(0001)

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