The coadsorption of carbon monoxide and deuterium on potassium predosed Ni(100)

Surface Science 140 (1984) L259-L263 North-Holland, Amsterdam

L259

SURFACE SCIENCE LETTERS THE COADSORPTION OF CARBON MONOXIDE O N P O T A S S I U M P R E D O S E D Ni(100) *

AND DEUTERIUM

H.S. L U F T M A N , Y.-M. S U N a n d J.M. W H I T E Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, USA

Received 17 January 1984; accepted for publication 2 March 1984

The effect of potassium on the coadsorption of carbon monoxide and deuterium on Ni(100), with particular attention given to the low temperature X-CO and . ~ - D 2 desorption states, has been studied by thermal desorption spectroscopy. Potassium shifts the ,~ state peak temperatures to higher values and attenuates their intensities. The effects of potassium on the higher temperature carbon monoxide and deuterium desorption states are similar to the effects observed when one or the other is absorbed alone. A model is presented for the conversion of carbide to graphite on transition metal surfaces and for a possible role which potassium may play. T h e effect of alkali additives on the catalytic p r o p e r t i e s of t r a n s i t i o n metals has been a topic s t i m u l a t i n g active surface science research [1] in recent years. F o r example, in the w o r k of C a m p b e l l a n d G o o d m a n [la] c o n c e r n i n g the influence of p o t a s s i u m on the reactions b e t w e e n C O a n d H E on Ni(100), they n o t e d that increasing the K p r e c o v e r a g e d e c r e a s e d the rate of m e t h a n e f o r m a tion, but increased the rates of C O d i s s o c i a t i o n a n d of the f o r m a t i o n of l o n g e r - c h a i n e d h y d r o c a r b o n s . W e have r e p o r t e d the effect of K c o a d s o r p t i o n with C O [2,3] a n d D 2 [4] i n d i v i d u a l l y on Ni(100) as o b s e r v e d with the U H V techniques of 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), X - r a y a n d ultraviolet p h o t o e l e c t r o n spectroscopies (XPS a n d UPS), change of work function m e a s u r e m e n t s (Aq~) a n d electron energy loss s p e c t r o s c o p y (ELS). H e r e we r e p o r t o u r o b s e r v a t i o n s o n the coadsorption of C O a n d D E on K p r e d o s e d Ni(100) a n d the i m p l i c a t i o n s for the catalytic processes n o t e d b y C a m p b e l l a n d G o o d m a n [11. T h e e x p e r i m e n t a l p r o c e d u r e for cleaning Ni(100), d o s i n g K, C O a n d H2, m e a s u r i n g coverages a n d acquiring T D S have been d e s c r i b e d in earlier p u b l i c a t i o n s [2-5]. In the e x p e r i m e n t s r e p o r t e d here, K was d o s e d f r o m an S A E S getter o n t o clean ( i n d i c a t e d b y A E S ) Ni(100). W i t h this s u b s t r a t e at 120 K, 34 L D 2 followed b y 34 L C O were d o s e d sequentially. T h e resulting T D S data, c o r r e c t e d for d e s o r p t i o n from the b a c k of the crystal [4], are s u m m a r i z e d in figs. 1 a n d 2 for OK = 0.0, 0.1 a n d 0.2 M L (8 K = 0.38 M L is s a t u r a t i o n of the * Supported in part by the National Science Foundation, Grant CHE 80-05107. 0 0 3 9 - 6 0 2 8 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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H.S. Luftman et al. / Coadsorption of CO and D 2 on Ni(lO0) i

I

I

I

I

CO

c

cO

oO

I 200

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i

t 400

I 600

Temperature,

I

i

800 K

Fig. 1. CO TDS from Ni(100) with various OK after sequentially dosing 34 L of D 2 and 34 L of CO at 120 K. The values of OK are: (a) 0.0 ML0 (b) 0.1 ML and (c) 0.2 ML. ",,ote that the first overlayer of K saturates at 0.38 ML.

first K overlayer and 1 ML is defined as one adsorbed species per surface Ni atom). In the absence of K, figs. l a and 2a show the nearly coincident ,~-CO and ~'-D 2 states (210-220 K) that have been discussed earlier in terms of a coadsorbed structure in which carbon monoxide and deuterium are intimately mixed (as opposed to segregated into separate islands) [5-7]. Upon addition of 0.1 ML of K, the resulting desorption spectra, figs. l b and 2b, are attenuated and the 2; states are shifted to higher temperatures (260 K). The other peaks are typical of CO and H 2 adsorbed alone on K-covered Ni [2-4]. For OK = 0.2 ML, the 2; states are no longer detectable. [

i

r

i

i

i

i

D2

'

260

'

a60

'

460

Fig, 2. D 2 TDS from Ni(100) with various #K after sequential dosing 34 L of D 2 and 34 L of CO at 120 K. The potassium coverages are identical to those used in fig. 1 and the spectra in figs. 1 and 2 were taken simultaneously. The signals in curve c have been multiplied by a factor of two.

H.S. Luftrnan et al. / Coadsorption of CO and 02 oll Ni(lO0)

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As expected, high coverages of K (fig. lc) lead to the appearance of a high temperature CO desorption state (690 K) which is shown elsewhere [2,3] to be the result of recombining C and O atoms. At the same potassium coverage, the high temperature D 2 desorption (B) is attenuated and shifted to higher temperatures,.just as in the absence of coadsorbed CO [4]. In a separate series of experiments Ni(100) was treated with 0.1 ML of K, 34 L D 2 and varying final exposures of CO. CO TDS showed full development of the higher temperature peaks prior to the buildup of the E-CO state, which began to appear at about 2.0 L CO while the ~ - D 2 peak emerged at about 1.2 L CO. These results contrast to observations on K-free Ni(100) (fig. 2 of ref. [5] corrected for a systematic error) where significant E - C O and 2:-H 2 appeared by 1.2 and 0.6 L CO respectively. The coverage of D(a), 0 D, in fig. 2b is no greater than 0.5 ML (the saturation coverage on clean Ni(100) is 1.0 ML [5]. In this case the 2:-D 2 state is readily observed whereas when 0 K = 0.0, 0 D > 0.6 M L is required for the appearance of any desorbing intensity in the 2:-D 2 state. Thus, in the presence of 0.1 ML of K, the 2~ states appear at lower coverages of D(a) but require a higher exposure of CO. The decreased intensity of the 2: peaks with increasing OK and the greater CO exposures required for 2: state desorption are consistant with the decreased sticking coefficient of D 2 (lower stability of D(a)) on the K predosed surface. It has been suggested [5,7,8] that the ~ desorption states arise from a loosely bound surface complex involving 2 H(a) and 1 CO(a) which, upon dissociation leads to both 2: desorption and absorbed surface states. The results reported here indicate that many of the fl-D 2 sites are blocked by K and, thus, an enhancement of the 2: desorption for a given OD with K predosing is not surprising. The increased desorption temperature of the 2: state indicates of the dissociation energy of this complex. A decrease of the C O / H 2 methanation rate has been observed under reaction conditions in the presence of adsorbed K [1]. Our results suggest that the reduction of the sticking coefficient for dissociative H 2 adsorption contributes in a major way to this observation. Under steady state reaction conditions there is a delicate balance between the CO dissociation rate, the H 2 dissociation rate and the methanation rate. These rates control the steady state levels of carbon, hydrogen and methanation intermediates on the surface. When the H 2 dissociation rate drops, the steady state surface hydrogen atom concentration decreases, the surface carbon level increases, and the methanation rate decreases. This in no way discounts other changes in the kinetics of the hydrogen addition steps in methane formation that may be induced by the presence of K. From our results it is clear that the strong interaction to give the ~7 state is not completely inhibited in the presence of 0.1 ML of K. Rather, there appears to be a coupling of the potassium into the desorption of these states and we

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H.S. Luftman et al. / Coadsorption of CO and D e on Ni(lO0)

take this as evidence for a long range interaction of the potassium on the adsorbate dynamics. In the reactive study [1] it was also noted that compared to the reactions on clean Ni(100), in the presence of K: (1) the steady-state concentration of carbidic (reactive) carbon increased, (2) the temperature at which carbidic carbon converted to graphitic (nonreactive) carbon decreased, and (3) there was an increase in the rate of formation of longer-chained hydrocarbons. It has also been noted that the methanation rates are dramatically reduced when graphitic forms of carbon appear on the surface (instead of carbide) [9,10]. In the presence of potassium, we expect the hydrogen atom coverage to decrease and the carbon atom coverage to increase. With higher carbon coverages, the tendency to form graphite will increase. We present here a model for the conversion from carbide to graphite which is based on qualitative thermodynamics and which keeps in mind the arguments presented above concerning the relation between the steady state hydrogen and carbon coverages. The change in entropy (AS) of the conversion of carbidic to graphitic carbon is sensitively dependent upon the surface carbon concentration. At partial surface coverages AS is significantly less than zero, as the carbidic "phase" can be well dispersed over the surface, whereas the graphitic carbon would have these adatoms in large islands. However, this distinction diminishes as the concentration approaches monolayer saturation where AS becomes negligibly small. If one assumes that the transition from carbidic to graphitic carbon is exothermic then the observed phase change occurring in the methanation reaction over Ni(100) [9,10] can be viewed as resulting from the increased rate o f CO dissociation (thereby increasing the surface carbon concentration) with temperature. In the absence of H 2 (i.e. no methanation), the carbon coverage increases with temperature until the relative enthalpies of the graphitic and carbidic forms is the dominant term in the difference between their free energies. The presence of H 2 and the methanation channel provide a means of keeping the C concentration low and thus the carbidic phase is retained to higher temperatures and CO pressures. Potassium, which blocks H 2 adsorption and increases the rate of CO dissociation, permits conversion to graphitic carbon at lower temperatures and thereby lowers the methanation rate. In addition according to this model, the presence of potassium will lead to greater carbon clustering which intuitively would increase the relative rate of multi-carbon molecule formation. This would be due to an increase of the surface carbon concentration at a given temperature as opposed to an increase of the carbon mobility (or a decrease of the activation energy for the transition) induced by the potassium, which we cannot rationalize. This thermodynamic model also suggests that on H- and K-free surfaces that there will be a stable graphitic phase over some range of low surface temperatures and low surface carbon coverages. Such a reversible transition has been observed on R h ( l l l ) [11] and N i ( l l l ) [12]. In conclusion, we have reported here TDS of CO and D 2 coadsorbed on

H.S. Luftman et al. / Coadsorption of CO and De on Ni(lO0)

L263

K - p r e d o s e d N i ( 1 0 0 ) a n d c o m p a r e d these results to t h e T D S of these species a l o n e a n d in pairs. W i t h i n c r e a s i n g p o t a s s i u m c o v e r a g e , the Z states o f C O a n d D 2 shift to h i g h e r t e m p e r a t u r e s i n d i c a t i n g t h a t p o t a s s i u m is s t r o n g l y c o u p l e d i n t o the d e s o r p t i o n of t h e s e species. T h e i n t e n s i t y o f these states at v a r i o u s e x p o s u r e s also r e f l e c t s the e f f e c t of K s i t e - b l o c k i n g o n D 2. A t h e r m o d y n a m i c m o d e l is p r e s e n t e d w h i c h d e s c r i b e s the e f f e c t of K o n C O h y d r o g e n a t i o n processes under reactive conditions.

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