On the origin of the low-temperature (≈ 150 K) H2 desorption from Pt(111)

Surface Science 171 (1986) L395-L400 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS ON THE ORIGIN OF THE LOW-TEMPERATURE D E S O R P T I O N FROM P t ( l l l )

( = 150 K) H 2

Bene P O E L S E M A , L a w r e n c e S. B R O W N , K l a u s L E N Z , L a u r e n s K. V E R H E I J a n d G e o r g e C O M S A lnstitut fftr Grenzfliichenforschung und Vakuumphysik, Kernforschungsanlage J~lich, Postfach 1913, D-517O Ji~lich, Fed. Rep. of Gerrnany

Received 28 October 1985; accepted for publication 15 January 1986

Chemisorption of hydrogen on Pt(lll) has been reinvestigated using thermal-energy atom scattering (TEAS). A nearly defect-free (~< 0.1% step atoms) Pt(lll) surface was used. In the absence of coadsorbed species, desorption from a saturated H monolayer occurs only at surface temperatures Ts > 180 K. The state frequently observed to desorb around T~= 150 K is atypical of clean H2/Pt(lll ). TEAS measurements imply that this state is related to the incorporation of hydrogen in ice layers and thermal desorption data further support this interpretation. Hydrogen monolayer saturation on the clean, nearly defect-free Pt surface at Ts = 80 K requires exposures in excess of 3 × 104 L, two orders of magnitude larger than observed previously.

T h e i n t e r a c t i o n of h y d r o g e n with p l a t i n u m surfaces has been investigated with a wide variety of techniques over a large n u m b e r of years. A recent h e l i u m d i f f r a c t i o n s t u d y of this p e r e n n i a l a d s o r p t i o n system b y Lee, Cowin a n d W h a r t o n ( L C W ) [1] has triggered the p r e s e n t investigation. A t a d s o r p t i o n t e m p e r a t u r e s a b o v e ---150 K L C W observe a w e l l - o r d e r e d (1 x 1) overlayer structure, with the h y d r o g e n o c c u p y i n g three-fold hollow sites on the P t ( l l l ) surface. A t a d s o r p t i o n t e m p e r a t u r e s b e l o w 150 K, the p o p u l a t i o n of an a d d i t i o n a l state resulting in a d i s o r d e r e d o v e r l a y e r is reported. H e a t i n g this surface to = 160 K is shown to result in an o r d e r e d (1 x 1) layer. L C W relate this o b s e r v a t i o n to d e s o r p t i o n of a l o w - t e m p e r a t u r e fit state of h y d r o g e n d e s o r b i n g at T~---150 K as o b s e r v e d b y M c C a b e a n d S c h m i d t [2] in their t h e r m a l d e s o r p t i o n experiments. A s n o t e d b y W a n g e m a n n et al. [3], this l o w - t e m p e r a t u r e d e s o r p t i o n state showed n o i s o t o p e exchange effects a n d followed first-order d e s o r p t i o n kinetics; so it has been assigned to a m o l e c u larly a d s o r b e d state. Similar m o l e c u l a r states have been r e p o r t e d for h y d r o g e n a d s o r b e d o n nickel [4,5]. In this letter the n a t u r e of the l o w - t e m p e r a t u r e fll d e s o r p t i o n state for the H z / P t ( 1 1 1 ) system is e x a m i n e d . T w o c o m p l e m e n t a r y techniques, thermalenergy a t o m scattering ( T E A S ) a n d t h e r m a l d e s o r p t i o n s p e c t r o s c o p y (TDS), have been e m p l o y e d . In T E A S e x p e r i m e n t s i n f o r m a t i o n on a d s o r b a t e coverage 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

B. Poelsema et al. / l~)w-temperature H 2 desorption from Pt(l 11)

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is obtained from changes in the diffracted intensities (here the specular intensity) as a result of the diffuse scattering from adsorbates [6,7]. TEAS probes here the presence of hydrogen o n the surface. Fig. 1 shows a typical adsorption curve as measured by TEAS, i.e. relative height of the specular helium peak as a function of hydrogen exposure. (The adsorption was done from ambient hydrogen after adjusting the partial pressure of H 2 in the U H V chamber to a suitable level.) As the exposure (or coverage) increases, the height of the specular peak decreases, passes through a deep minimum, and ultimately returns to a considerable level. The large specular intensity at the two ends of the curve is due to the good reflectivity of the clean, defect-free P t ( l l l ) surface [8] and of the only slightly corrugated [1] and also defect-free H-full monolayer, respectively. (The stiffness of the H 2 layer is evidenced by its " D e b y e - W a l l e r temperature" of --~ 190 K [1] not very much lower than that of Pt(111), -- 230 K [8].) With increasing coverage, in the low-exposure region, the specular intensity decreases due to diffuse scattering from H adatoms (see e.g. ref. [6]). At coverages below H saturation, in the high-exposure region, the H vacancies in the H adlayer lead likewise to a diffuse scattering of the He beam. This latter effect is similar to the thermal He scattering from ion-bombardment-induced vacancies on clean metal surfaces [9]. The general behavior in fig. 1 measured at 80 K is similar to that of the curves we obtained at higher ~ ( < 180 K) on the same surface and to that previously observed for hydrogen adsorption on other metal surfaces [7,10]; it will be discussed in more detail elsewhere [11]. The comparison of the adsorption curve in fig. 1 with the data of LCW [1] reveals similarities and discrepancies. The curves for ~ >/160 K presented by LCW in fig. 9 have a similar shape to our curves: after passing through a

B. Poelsema et al. / Low-temperature H 2 desorption from Pt(l 11)

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Fig. 2. Desorption curve; relative peak height of the He specular beam during linear heating (0.3 K s 1) of an initially (TS= 80 K) fully hydrogen-covered Pt(lll) surface ( x ); between 80 < 7~s< 180 K the curve is perfectly reversible (no H desorption). The arrow pointing downward indicates the decrease of the He specular beam peak height during H20 adsorption on the fully H-covered surface at Ts = 80 K. By subsequent heating H20 desorbs around 150 K ((i)) and the He peak height recovers the value corresponding to the surface covered with H alone.

m i n i m u m the specular H e b e a m intensity increases m o n o t o n i c a l l y with the H 2 e x p o s u r e t o w a r d s saturation. T h e difference in this h i g h - t e m p e r a t u r e range is that while in the L C W plot s a t u r a t i o n a p p e a r s to be reached after exposures of the o r d e r o f 102 L H 2, in our e x p e r i m e n t s roughly two orders of m a g n i t u d e larger exposures are needed. (The need for high exposures is related to very low defect densities [11].) In the l o w - t e m p e r a t u r e range ~ < 150 K the shape of the curves p r e s e n t e d b y L C W in fig. 9 changes d r a m a t i c a l l y : after passing t h r o u g h the m i n i m u m the specular H e b e a m intensity reaches a m a x i m u m a n d then decreases to zero. However, as a l r e a d y stated o u r curves m a i n t a i n their s h a p e in the whole range of t e m p e r a t u r e s 80 < ~ < 180 K a n d thus, as obvious in fig. 1, even at T~ = 80 K does the specular intensity, i.e. the surface reflectivity, increase m o n o t o n i c a l l y up to very high exposures. This o b s e r v a tion is p a r t i c u l a r l y i m p o r t a n t in the p r e s e n t context, as it implies that in our e x p e r i m e n t s n o d i s o r d e r i n g occurs u p o n s a t u r a t i o n of the h y d r o g e n a d l a y e r even well b e l o w ~ = 150 K. This, in turn, indicates that p o p u l a t i o n of the low-temperature/31 state suggested b y L C W does not occur. F u r t h e r evidence for the a b s e n c e of this state is s u p p l i e d b y the " d e s o r p t i o n " e x p e r i m e n t in fig. 2. The height of the specular h e l i u m p e a k is p l o t t e d as a function of surface t e m p e r a t u r e in the absence of H 2 in the gas p h a s e a n d s t a r t i n g with the s a t u r a t e d H m o n o l a y e r at ~ = 80 K. A c o n s t a n t h e a t i n g rate of 0.3 K s - t was used. First, the surface t e m p e r a t u r e has been r e p e a t e d l y v a r i e d b a c k a n d forth between 80 a n d 180 K. T h e height of the s p e c u l a r p e a k

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B. Poelsema et al. / Low-temperature H 2 desorption from Pttl l 1)

behaves in a perfectly reversible manner in this temperature range. (The finite slope is due to the Debye-Waller effects.) This fact excludes the desorption of hydrogen from a saturated layer at temperatures below 180 K [12]. Any desorption would lead to irreversible behavior in the temperature cycles in the range 80 < 7~ < 180 K depicted in fig. 2. Therefore the existence of a 150 K fll state for hydrogen on P t ( l l l ) is in obvious contradiction with the present TEAS results. Above ~ = 180 K desorption sets in and, of course, the curve ceases to be reversible. By continuing the heating, the hydrogen is gradually desorbed and a curve shape similar to that of the "adsorption curve" in fig. 1, but run in inverse direction, is obtained. The present results are thus at variance with those of LCW, also obtained using helium scattering. In an effort to resolve this issue, we have considered the possibility of water coadsorption in the LCW experiments. This seems plausible. Water desorbs from P t ( l l l ) at ~ - - 1 5 0 K [13], and is commonly present as an impurity in vacuum systems and also in molecular beams. Because the sticking probability for water is much larger than that for hydrogen at higher coverages, a relatively low water-impurity level would suffice to result in substantial water adsorption at surface temperatures T~ < 150 K. Indeed, during the course of our work we have found that special precautions must be taken to minimize the water-impurity level in the probing helium beam, in the H 2 supply manifold and in the UHV residual gas. Deliberate coadsorption of water below ~ = 150 K leads to a disordering of the surface as probed by TEAS. This is also shown in fig. 2. Following H-adlayer completion, a H 2 0 exposure brings the specular peak height essentially to zero (arrow pointing down). As seen further in fig. 2 (open circles), a subsequent heating of the surface leads to a complete recovery of the original reflectivity after desorption of the water at T~ ~ 150 K. If the disordering at low temperatures observed by LCW with helium diffraction is attributed to the influence of coadsorbed water, the question arises as to whether the H 2 desorption peak at T~ ~ 150 K, i.e. the/~1 peak in the thermal desorption spectra of McCabe and Schmidt [2], is somehow related to coadsorbed water as well. Although the data do suggest this, TEAS is not capable of addressing this issue directly. While this technique provides an extremely sensitive probe of surface coverages, it has the limitation of not being mass specific, so that in general it is not possible to distinguish between different adsorbed species on the basis of TEAS alone. In the case of coadsorption, TEAS has to be backed up by mass-specific data obtained e.g. from TDS measurements performed with a mass analyzer. Whereas TEAS probes the coverage on the surface, TDS actually probes the rate of coverage reduction by measuring a partial pressure rise due to desorption. The present experimental arrangement was not designed for TDS, and unfortunately the quadrupole mass analyzer sees not only the sample surface, but also parts of

B. Poelsema et al. / Low-temperature H 2 desorption from Pt(l I 1)

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the s a m p l e holder. A c c o r d i n g l y , we only discuss qualitatively our T D S experiments, which have been p e r f o r m e d to clarify the H 2 - H 2 0 c o a d s o r p t i o n issue a d d r e s s e d above. First, we have m e a s u r e d a h y d r o g e n d e s o r p t i o n s p e c t r u m o b t a i n e d after sequential ~,xposures of 35 L H 2 0 followed b y 3 × 104 L H 2 at T, = 80 K. W e observe an i m p o r t a n t h y d r o g e n d e s o r p t i o n p e a k at the t e m p e r a t u r e at which the ice layer desorbs. W e ascribe this l o w - t e m p e r a t u r e d e s o r p t i o n to h y d r o g e n i n c o r p o r a t e d in the ice multilayers; i.e. h y d r o g e n is released d u r i n g e v a p o r a tion of the ice layers. S u p p o r t for this i n t e r p r e t a t i o n is that the area of this l o w - t e m p e r a t u r e h y d r o g e n p e a k is s u b s t a n t i a l l y larger than the area below the curve m e a s u r e d when d e s o r b i n g a full h y d r o g e n m o n o l a y e r . F u r t h e r it a p p e a r s that the m a g n i t u d e of the l o w - t e m p e r a t u r e H 2 d e s o r p t i o n signal is p r o p o r tional to the a m o u n t of p r e a d s o r b e d water. Indeed, after sequential exposures of twice as m u c h water (70 L H 2 0 ) a n d the same a m o u n t of H 2 (3 × 104 L H 2), the l o w - t e m p e r a t u r e H 2 d e s o r p t i o n signal was d o u b l e d . A n i n d e p e n d e n t check run, m a d e after exposing 70 L H 2 0 (and no H2), shows that no a p p r e c i a b l e f r a g m e n t a t i o n of H 2 0 into H2~ occurs. In a d d i t i o n to the l o w - t e m p e r a t u r e peak, d e s o r p t i o n a r o u n d 300 K is o b s e r v e d similar to the cases when h y d r o g e n is directly a d s o r b e d on clean Pt. This is a p p a r e n t l y due to h y d r o g e n diffusing through the ice layer. W e c o n c l u d e that a s u b s t a n t i a l a m o u n t of h y d r o g e n can be i n c o r p o r a t e d in an ice adlayer. These H 2 molecules d e s o r b o b v i o u s l y together with this a d l a y e r at T~ = 150 K. T h e "/31 state", o b s e r v e d in several T D S studies, m a y therefore be a t t r i b u t e d to h y d r o g e n a d s o r b e d in an ice a d l a y e r p r e s e n t as an i m p u r i t y at the surface. In s u m m a r y , we have shown that: - T h e r e is no ( d i s o r d e r e d ) /11 state of h y d r o g e n a d s o r b e d on clean P t ( l l l ) d e s o r b i n g at T, = 150 K. - H y d r o g e n can be i n c o r p o r a t e d in an ice adlayer. This h y d r o g e n is released d u r i n g e v a p o r a t i o n of the ice layer.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

,L Lee, J.P. Cowin and L. Wharton, Surface Sci. 130 (1983) 1. R.W. McCabe and L.D. Schmidt, Surface Sci. 65 (1977) 189. K. Wangemann, J. Ri~stig and K. Christmann, Deut. Physik. Ges. Verhandl. 20 (1985) 935. A. Benninghoven, P. Beckmann, D. Greifendorf, K.-H. M't~llerand M. Schlemmer, Surface Sci. 107 (1981) 148. R.J. Behm, Diplomarbeit, Munich (1976). B. Poelsema, G. Mechtersheimer and G. Comsa, Surface Sci. 111 (1981) 519. T.Engel and H. Kuipers, Surface Sci. 90 (1979) 162. B. Poelsema, R.L, Palmer, G. Mechtersheimer and G. Comsa, Surface Sci. 117 (1982) 60. B. Poelsema, L.K. Verheij and G. Comsa, Phys. Rev. Letters 53 (1984) 2500.

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[10] H. Wilsch and K.H. Rieder, J. Chem. Phys. 78 (1983) 7491. [11] B. Poelsema, K. Lenz, L.S. Brown, L.K. Verheij and G. Comsa, in progress. [12] This observation agrees with very careful hydrogen TDS experiments performed by Steiniger (Ji~lich, 1763 (1982)) showing, indeed, that no desorption of H 2 from a nearly defect-free Pt(111) surface takes place at temperatures below 180 K. [13] G.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446.