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The reaction of H2S with adsorbed oxygen atoms on platinum (111)
Surface Science 197 (1988) 379-390 North-Holland, Amsterdam
379
THE REACTION OF H2S WITH ADSORBED OXYGEN A T O M S ON PLATINUM (111) * G.E. MITCHELL, M.A. SCHULZ and J.M. W H I T E Department of Chemist.y, University of Texas, Austin, TX 78712, USA Received 11 March 1987; accepted for publication 18 November 1987
The reaction of H2S(g) and adsorbed atomic oxygen on P t ( l l l ) has been studied by SIMS, HREELS, and TPD. H2S(g ) reacts completely at 96 K with 0.22 ML O(a) to give H20(a ) and S(a). The reaction rate shows no measureable temperature dependence between 96 and 135 K and, under the conditions of this study, was equal to the flux of HzS. After a saturation dose of H2S on O ( a ) / P t ( l l l ) , only H2S, H 2 and H~O desorbed in TPD. H R E E L spectra provide evidence of water cluste~ng when H2S exposure continues beyond the amount necessary for complete reaction of the adsorbed O atoms.
1. Introduc~on Knowledge of the interactions of H2S on metal surfaces is helpful in the understanding of hydrodesulfurization catalysts and sulfur poisoning of cracking and automotive exhaust catalysts [1]. The reaction of H2S with adsorbed oxj~en atoms on the P t ( l l l ) surface is also interesting for its potential to provide insight into the mechanism of the reaction of H2(g) + O(a) --, HzO(a). The reduction of oxygen atoms by hydrogen on platinum surfaces has been well-studied [2-8]. The reaction proceeds at measurable rates at temperatures ,'v~ used static secondary ion mass as low as 120 K [4,7]. Ogle and ~,,L.e spectroscopy (SIMS) to measure kinetic parameters of thc reaction at temperatures below the desorption point for H20 [4]. At temperatures between 12~ and 155 K the reaction is activated by about 3 kcai/mol and has a go5 dependence on hydrogen pressure. The reaction profile as measured by following the H30 + secondary ion has a distinctive shape, rising slowly for the second haif. The reaction of water with O ( a ) / P t ( 1 ] ] ) ~o y~eld an intermediate (not OH(a)) has been studied with SIMS av,d TPD by Creighto~ and White [9]. When two molecules of [ q 2 0 a r e added for each O(a) the i~te~mediate reacts with H2(g ) to form water without any induction period [3]. This * Supported in part by the .wo . . . .~. .,., , ~ R.~o~. ........ . . .h. .c~rfi,-,~ . . . . . :rod the Robert A. Welch Foundalic.n.
0039-6028/88/$03.50 ~ Elsevier Science Pubfishers I-;.V. (North-Holland Physics Publishing Division~
380
G.E. Mitcheit et al. / Reaction of H2S with adsorbed 0 on Pt(l l l)
observation was taken as evidence that the reaction path for H2(g ) + O(a)/ Pt(111)-~ H20(a) included the above intermediate [3]. Recently, however, OH(a) (not the product of the reaction of H20(a) + O(a)) has been identified as the key intermediate in the water formation reaction H2(g)+ O(a) on
Pt(111) [8]. There have been many studies of the structure of sulfur adlayers on the surfaces of platinum and other transition metals and of the modification of the catalytic activity of platinum induced by sulfur [10-19]. While the method of choice for introducing sulfur to the surface has been by giecomposition of H2S, only recently has the interaction of H 2S with the Pt(111) surface been detailed [20]. Koestner et al. found that at 110 K, small exposures of HES on Pt(111) completely decompose to S(a) and H(a). At higher exposures SH(a) and H2S(a ) are also detected by high resolution electron energy loss spectroscopy (HREELS). The reaction of H2S with oxygen has been studied on nickel, copper and lead surfaces. Adsorption of H2S on oxygen covered Ni(100) results in H~O desorption at 235 K in TPD [21]. The H20 desorption is reaction limited (probably due to OH(a) + OH(a) -~ HEO(g ) + O(a)) and OH(a) formation may be occurring upon adsorption of the HES. HES is thought to adsorb associatively on oxygen covered Cu(ll0) at 80 K and react to form OH(a) upon heating to 120 K [22]. On Cu(111) X-ray photoemission (XPS) has been used by Roberts' group to demonstrate OH(a) and H20(a ) formation at 105 K after exposing an oxygen covered surface to HES [23]. The authors of the previous study invoked H-bonding as playing a large role in the H atom abstraction reaction. Reaction of H2S with oxidized lead films also produces water at temperatures as low as 85 K [24]. In the study reported b,ere, we have used SIMS, HREELS, temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) to follow the reaction of HzS with atomic oxygen on P t ( l l l ) .
2. Experimental The experiments were performed in an ion pumped UHV chamber described previously [25]. The chamber is equipped with a cylindrical mirror analyzer for AES, a !27 ° cylindrical deflection analyzer for HREELS, a differentially pumped ion gun and a quadrupote mass analyzer for SIMS and TPD. During these experiments the base pressure was generally- 3 × 10 -1° Torr. SIMS was performed using an Ar + current o f - 1 n A / c m 2 at 500 eV, rastered across the sample to minimize local beam damage effects. Previous studies have shown ion curren~ of this magnitude have a negligible effect, on a
G.E. Mitchell et at / Reaction of lieS with adsorbed 0 on P t (l l l )
381
water adlayer for times less than 1000 s [26]. Secondary ions were monitored in the direction parallel to the surface normal. TPD was performed with the same mass spectrometer used for SIMS. The heating rate was 4 K/s. Auger spectroscopy was performed with a beam energy of 3 keV. "D~ftlki~i "~" et al. report a linear relationship between the Auger peak ratios S(150 eV)/Pt(238 eV) and the coverage of sulfur on the P t ( l l l ) surface (determined by radioactive sulfur and low energy electron diffraction (LEED)), at least up to 0.33 ML [10]. Koestner et al. report that a saturation dose of H2S at 110 K, heated to 500 K deposits 0.25 ML S(a) [20]. These facts were used to calibrate the Auger data. The P t ( l l l ) sample was cut and polished by the usual metallurgical techniques. Sulfur was removed by Ar + bombardment or h,'"':ng , . , , , .~ ! e - 1400 K in 2 × 10 -8 Torr H 2. Oxidation was used to remove carbon. Surface cleanliness was confirmed by Auger and oxygen TPD spectra. The criteria considered in the latter involved the area and position of the O(a) recombination TPD peak after a saturation 02 dose at 100 K. The peak position was required to lie within + 5 K of 738 K at a ramp speed of 12.6 K / s and the area was required to lie within _+ 5% of what was found consistently after a series of adsorpdon-desorption cycles. Other work in this laboratory [27] shows that stabilization of the 02 adsorption kinetics involves oxidation of small amounts of impurities in the near surface region. Oxygen (Scientific Gas Products 99.999%), hydrogen (Linde Specialty Gas 99.95%) and H2S (Linde Specialty Gas 99.5%) were used without further purification except for liquid nitrogen cooled traps in the lines for H 2 and 02. Pressures were measured by an ion gauge and corrected for the sensitivity referred to, N2(g ) (2.2 and 0.44 for HaS and Ha) [28]. In all experiments performed with O ( a ) / P t ( l l l ) the atomic oxygen was formed by adsorbing O z at 100 K and heating to 300 K. This procedure gives 0.25 ML of O(a) [29,30]. In our experiments the coverage was about 0.22 + 0.02 ML because of reaction with residual CO(g) to form CO2(g ) during the initial heating.
3. Results 3.1. T P D
After a saturatiGn H2S are observed in maximum at 168 K. desorb from Pt(lll) hydrogen is adsorbed
dose of H2S on 0.22 ML O ( a ) / P t ( l l l ) , H 2, H20 and TPD (fig. 1). The water desorbs in a single peak with When molecular water is dosed, low coverages of it with a maximum rate at about 170 K [31]. When with atomic oxygen at 100 K on P t ( l l | ) , with heating
G.E. Mitchell et aL / React~on of H2S with adso, bed 0 o~s Pt(l l l)
382
Sat'n H2S + O(a)
/ Pt(lil)
tn .e'4
i72
C
' JO t-
.r4 tO
220
IBB
AJ C F-4 L
H20
E 0 (_
C1 tO
~27 !
U~ 69 ~0 ~E
@
t00
200 Temoerature
300
400
500
/ K
Fig. 1. Temperature programmed desorpdon spectra after exposure of 0.22 ML of O(a) on P t ( l l l ) at 96 K to 14 k of H.:S.
rates >_ i K / s water desorbs in 3 peak:; at about 170, 210, and 300 K [4,7]. Thus, in the H~S + O(a) reaction:, the desorbing water is ~u]ly formed below 170 K. Hydrogen desorbs in a broad peak witl a maximum at 170 K and two shoulders at 185 and - 220 K. By comparison with the work of Koestner et al. [20], the latter two desorption states can be assigned to decomposition of H2S(a ) and SH(a) to S(a)+ H:(g) ',reaction limited desorption) and to the recombinador~ oi H(a) '~o H;~'g) (desorption limited) respectively. The peak
G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt(l l I)
383
aear 170 K is desorbed in unison with H20. Recently, Poelsma et al. [32] have shown that hydrogen can be incorporated into a layer of adsorbed water and be released to the gas phase concurrent with H 2 0 desorption. We ascribe the 170 K H a desorption (fig. 1) to a similar mechanism. The area under the hydrogen peak can be determined by comparison with the desorption of H 2 from an H2S saturated surface. (Tke final sulfur coverage is known for the latter.) The amount of H2(g ) desorbin~ is 0.14 + 0.02 ML. This agrees with Auger ratios which indicate about 0.33 +_ 0.03 ML S(a) left on the surface after TPD. This compares to 0.25 ML S(a) left on the surface after thermal decomposition of a saturation dose of HES on clean Pt(111) [20]. Excess H2S desorbs at 127 K with a shoulder near 150 K (fig. 1). These desorption peaks correspond to H2S desorption from the clean surface [20]. No oxygenated sulfur species such as SO or SO 2 were detected. TPD after H2S exposures below those required to convert all the O(a) to H20 revealed: (1) desorption of ueither H 2 nor H2S occurs, and (2) H20 desorption occurs in two peaks near 170 and 200 K as for the partial reaction of H2(g ) + O ( a ) / P t ( l l l ) [4], but the 300 K H 2 0 peak is not detected for reduction with H2S. 3.2. H R E E L S
Fig. 2 shows HREELS spectra (a) for 0.75 L exposure of H2O u~ clean P t ( l l l ) and (b) after ~,.~,,.,n':~" ,~¢.~HzS + O(a) at 100 K. tt is evident from t~:~e similarity of these spectra that reaction between 1-i25 and ,.,~=,r'~"~,~.~ fc~rm H~O occurs even at 100 K. The vibrational spectra for H20 on Pt(111) prepared by adsorption frGm the gas phase and by reaction of H2(g ) + O(a) have been assigned [33,8]. The losses a t - 570, 1620 and 3400 cm -1 present in both spectra of fig. 2 are assigned to the Pt-O stretch (v(PtO)), H - O - H scissors (8(HOH)) and O - H stretching (v(OH))modes, respectively. Water formed by reaction of H2(g ) + O(a) on P t ( l l l ) exhibits a binding geometry ( - C2v) different from that of water adsorbed from the gas phase [8] (arising from a reluctance of the former to break oxygen-metal interactions in order to form H-bonds). Because of the difference in structure, losses apparent in the spectrum of H 2 0 ( a ) / H 2 0 ( g ) near 260 (hindered translation parallel to the surface; L,.y) 685 and 945 cm -1 (hindered rotations; Rx.y...) (fig. 2a) are absent in the spectra of H 2 0 fcrmed from reaction of H2(g)+ O(a~ [g]. The spectrum of H20(a ) from the reaction of H2S~o ) + O(a) (fig. 2b) Is ~ntermediate to the two cases above. ]In particular, wc assign the shoulders at 260 and - 660 cm-1 to the tx, y and R~. ~.: modes of clustered water respectively, but these are not as intense as those in fig. 2a. The amount of H2S adsorbed for fig. 2b was slightly in excess of the stoichiometfic amount required fo~ H20 (0.05 ML H 2 desorbed in TPD). After larger H2S exposures, ~he librafion near 660 cm-1 becomes mor,~ intense and the HOH scissors mode broadens
384
GE. Mitchell et al. / Reaction of H:S with adsorbed 0 !
I
I
lfA
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I
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I
a4oo
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Pt(l l l)
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on
ii-2.o
!
H2S + 0 (a)/Pt (1tt)
c .IJ c
(b) tt80
I
\I
i620
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L Energy Loss / cm-t Fig. 2. High resolution electron energy loss spectra of: (a) 0.75 L of H20 and ( b ) - 1 L of H2S+0.18 ML of O(a) on Pt(lll).
and moves to somewhat higher frequency. The latter are attributed to increased H 2 0 - H 2 0 interactions (H-bonding), and are consistent with three-dimensional islanding of the adsorbed water formed in the presence of molecular H. S. Because of the interference from the relatively intense losses due to H20(a ) and the !ow coverages of iudrogenated sulfur species which may be present, no losses due to SH(a) or H~S(a) were observed in these experiments, although SIMS results (see below) suggest teat adso,'~tio~ of H2S beyond the stoichio-
G.E. Mitchell et aL / Reaction of H2S with adsorbed 0 on Pt( l ! 1)
385
metric amount needed for reaction with O(a) is associative at 100 K. The Pt-S stretch for S(a) is found at 375 cm-1 [20] and is not resolved from the losses due to H EO(a ) in fig. 2b.
3.3. SIMS Using the H 3 0 + SIMS intensity as a monitor of the watcr cc_,verage, we followed the reaction of H2S(g) with O ( a ) / P t ( l l l ) in a constant pressure of H2S (fig. 3). As a benchmark, the reaction H2(g ) + O(a) (fig. 3a) was performed at 135 K and a hydrogen pressure of 4.5 x 10 -8 Torr. The rate of the latter reaction is undetectable at 96 K. Fig. 3b shows H 3 0 + (role = 19) and a
I
|
I
0
H2(g) + Olal --> H20(al T-
t35K
PH2 - 4 . 5 x t 0 . 8 t o r t
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~
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.
0
240
.
.
.
.
4aO
tort
&
+
720
%0
1200
Time / Seconds Fig. 3. Time dependence of H3O+ and H3S + SIMS signals during titration of Ofa) for (a) 4 x 1G-'~ Torr H2(g)+0.22+_0.02 ML of O(a)at 135 K. (b) 3.6x 10 . 9 Torr H2S(g)+0.22+0.02 ML of O(a) at 96 and 135 K.
386
G.E. Mitchell et al. / Reaction of HzS with adsorbed 0 on Pt(l l l)
H 3S ÷ ( m / e = 35) versus time curves for the reaction of H 2S with O(a)/Pt(111) at 135 and 96 K at the same oressure of H2S. The ion intensities have been normalized to the measured primary Ar ÷ current. For t < 290 s, the ~hapes of the role = 19 curve are very s~milar to the shape of the H2(g)+ O(a) curve shown in panel a. For the H:~S + O(a) reaction, the slopes at 96 and 135 K are the same in both the induction and rapid regioiis. The slight offset of the two curves (along the time axis) may be due to a small difference in the initial O(a) coverages (.- 0.01 ML) for the two experiments. We interpret these results to indicate there is no significant temperature dependence (no activation barrier) for water formation under these conditions. In order to obtain information regarding the H2S coverages, we also followed the HaS + SIMS ion (the largest + ion for H 2 S / P t ( l l l ) ) during the experiments reported in fig. 3b. By analogy with water [26], it is likely that mass 35 (H3 S÷) contains contributions derived from both adsorbed H2S and SH. Bearing this in mind, two comments are made regarding the m/e = 35 spectra in fig. 3b. First, in both cases the mass 35 intensity begins to increase as the mass 19 signal saturates. Apparently up until this point all the hydrogen is being used to form water and very little is bonded to the sulfur atoms. Second, the mass 35 intensity in the 135 K reaction saturates earlier and at a much lower level than in the 96 K reaction (note the expansion factors in fig. 3b). Since 135 K is above the molecular H2S desorption peak and below the H2 resorption peak, we assign the Ha S+ seen at 135 K to SH(a) and not H2S(a ). The signal-to-noise and interfering bands of H20(a) prevent unambiguous identification of SH(a) by HREELS (see above). After completion of water formation (~ > 290 s) at 135 K the m/e = 19 and (less rapidly) the m/e = 35 signals begin decreasing. That this decrease in secondary ion yield is not due to sputtering was determined by an experiment like fig. 3b in which the ion beam was turned off for a time; the signal continued to decrease while the beam was off. We envision three possible mechanisms which could cause the observed decrease in the H3O+ yield: (1) three-dimensional island formation, such that the two-dimensional area covered by H20(a ) decreases; (2) a decrease in the work function; and (3) displacement of HzO(a ) by H2S(a ) (or a decomposition product of the latter). HREEL ~pectra (reported above) suggest the importance of three-dimensional island,nn v ~,~wever. HREEL spectra taken after annealing to 130 K do not show a significant increase in intensity v~ &c ]:,br~cm~! modes, indicating that further three-dime~, sional islanding is not important. To test for the t ~ r d possibility, temperature programmed S~MS (TPSIMS) was performed after formation of water from HzS + O(a) at 96 K. The H30+ intensity declines by 30% between 120 K (H2S desorption temperature) and 156 K (where water begins to desorb), at which point the signal falls rapidly to zero. The decrease in the H30 + yield between 120 and 156 K in the TPSIMS experiment does not involve desorption of H20(a ) and (assuming the decrease in H30 + yield in the
G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt( l l l ) !
I
!
387
o
H2S + O(a) - - > H20(o} T = 96K
U) -,-I
r-
3.6xt0 -9 t o r t
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¢= 4-= C H 03 =£ H + O I "O3
•
O
I
I
0
240
:~80
720
g60
~.200
Time / saconds Fig. 4. Time dependence of H 3 0 + signals during reaction of H2S with O(a) on P t ( l l l ) at 96 K. Two H2S pressures were used (1.8 and 3.6 × 10 -9 Tort).
TPSIMS and isothermal SIMS experiments have the same cause) we rute out mechanism 3. As discussed bele-w, a work function change provide~ a reasona~ ble explanation of this SIMS data. The influence of the pressure of H2S on water formation was also investigated (fig. 4). The reaction was performed at constant temperature (96 K) at H2S pressures differing by a factor of 2 (3.6 × 10 -9 and 1.8 × 10 -9 Tort). Defining the induction time as the intersection with the baseline of a line drawn through the linear portion of the curve in the rapid reaction region, gives 129 and 346 s for the induction time with high and low H2S fluxes respectively. The final H3O+ signal intensities in fig. 4 indicate a small disparity in the initial O(a) coverage of 0.03 ML. Taking into account the disparity in initial oxygen, the induction time is - 2.3 times ionger for the case w~t~ ha]~ ~Lc H ~ ~"~. However, the reaction rate in the rapid region (slope t0r die linear portion of the curve) for the ia~,~., ~u~ is ,~,~y I ~ times that for the smaller flux. The average overall reaction rate is then proportional to d~ f~ux. At 3°6 )< 10 -9 Torr lhe flux of HzS to the surface is 8.7 × 10 --4 ML/s. A fluence cff H2S equal to the initial O(a) coverage ( - 0 . 2 1 ML) is reached a~ 250 s, and similarly at the lower flux, 0.24 ML of H2S fluence is reached at
388
G.E. Mitchell et at. / Reaction of l-l~S with adsorbed 0 on Pt(l l l)
570 s. As shown in fig. 4, tl~e reactions are nearly completed at these times. The overall rate of the reaction to form water is therefore essentially the same as the flux of H2S to the surface under the conditions of these experiments.
4. Discussion
Investigations of the reaction of H2(g ) and O(a) between 120 and 150 K have determined that adsorbed hydrogen atoms are an intermediate and that isotope effects are consistent with H(a) diffusion being the rate limiting step of the reaction [34]. This step must then involve the measured 3 kcal/mol activation barrier. Since the reaction of H2S(g ) + O(a) does not show an activation barrier, it must either cause a decrease in the activation barrier for H(a) diffusion or not involve H(a) diffusion as a step in the reaction. The latter, suggesting direct transfer of the H from H2S(a) to O(a), is preferred for the following reasons. (1) Surface diffusion of H2S(a ) would be facile at the temperatures employed in these studies (according to the rule of thumb that the onset temperature is 1/3 to 1/2 the desorption temperature), and (2) since H2S readily dehydrogenates on the clean surface, hydrogen atom transfer to O(a) or OH(a) should also be easy. The shape of the SIMS reaction profile is very similar to the curves for the reduction of O(a) with H2(g ). In the latter case the induction period has been attributed to the formation of an intermediate (OH(a)) [8]. The rapid increase in ~,lle H3O+ SIMS intensity at about 150 s in fig. 3a is attributed to the reaction of OH(a) with H(a) 0,o form H20(a). Apparently the same intermediate is also involved for H~,~;(g) + O(a) --, H20(a ). Since the length af the inductic'," region fo~ H2S(g ) + O(a) is temperature independent (fig. 3b), the reaction to form the intermediate must also be temperature independent in this case. The lack of a thermal barrier is additional evidence against assigning the product of H20(a ) + O(a) as the key intermediate because the latter reaction has an appreciable activation barrier [35]. We believe the decrease in the H3 O+ intensity after completion of the water formation reaction at 135 K (and observed in TPSIMS between 120 and 156 K) is caused by a change in work function. On Ru(ll0) dissociative adsorption of H2S to produce S(a) + 2 H(a) at low exposures increases @, while SH(a) formation at higher exposures is accompanied by a decrease in @, and finally near saturation coverage, adso~tion of H2S(a ) leaven ~ unchanged [36]. Adsorption of H2S on this surface is very simiiar to the case of P t ( l l l ) [20] and therefore we expect a similar inflaence on ~ for Pt. Indeed, work function measurements made by the secondary onset method (300 eV electron beam) during H2S ex!,:,osure at 100 K show an initial increase (0.15 V) followed by a decrease to - 0.52 V (SH(a) formation). Koestner et al. have shown that H2S decomposition is dependent on the availability of unoccupied surface sites. At
G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt(l i l)
389
completion of the water formation reaction, there i s - 0 . 2 5 ML H20(a ) and - 0.25 ML S(a) on the surface and at 100 K there is, according t o H3 s + SIMS yields (also note the H2S TPD peak at 120 K), at least 10 times more H2S(a ) than at 135 K (fig. 3b). The H25 dccumposition channel is apparentiy not open on the crowded surface at 96 K, but at 135 K the short residence time for H2S(a ) frees up sites for SH(a) formation. The decrease in • caused by SH(a) would then decrease the yield of H30 +. The amount of S(a) left on the surface after heating an O(a) covered surface is larger (0.33 ML) than found for the clean surface (0.25 ML) [20]. On the ckan surface, decomposkion of ;!3 :~ :~,.h~b/icd by biu¢ldng of sites for H(a) by previously produced H(a), S(a) a~:d SH(a). On the oxygen covered surface the first 0.5 ML of hydrogen atoms from H2S is used to make water and assuming one site per adsorbed species, fewer sites will be blocked, thus H2S decomposition during TPD can proceed much further. Three-dimensional islanding of the water caused by H2S adsorption as indicated by HREELS will serve to further increase the number of sites available for H2S decomposition during TPD.
5. Summary In summ_ary, H 2 S is an extremely efficient reducing agent on Pt(lll), completely reducing •"~"~ .,~,, to water with no apparent activation barrier at 96 K. This is in contrast to H2(g) which exhibits an activation barrier of 3 kcat./mol for reduction of O ( a ) / P t ( l l l ) and does not re,~.t with the latter b e l o w - !2.~ l( [d]_ . The facility of !he reaction at 96 K is strong evidencc f.,3r an intermediiate differing from that f,~rmed in the reaction of H2©(a ) + O(a).
References [1] L.L. Hegedus and R.W. McCabe, Catalyst Poisoning (Dekker, New York~ 1984). [2] P.R. Norton, in: The Chemical Physics of Solid Surface's and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 27. [3] G E . Mitchell, S. Akhter and J.M. Whi~e, Surface Sci. 166 (1986) 283. [4] K.M. Ogle and LM. White, Surface Sck 139 (1984) 43. [5] J.L. Gland, G.B. Fisher and E.B. Kollin, J. Catalysis 77 (1982) 263. [6] G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 (1980) 683. IO. [7] G.B. Fisher, J.L. Gland and S.J. Schmieg, .L Vacuum Sci. Technol. 20 (1982) J~1o [8] G.E. Mitchell and J.M. Whhe, Chem. Phys. Lollers 135 (1987) 84. [9] J.R. Creighton and 3.M. White, Surface %i. 122 (1982) L648. [10] Y. Berthier, M. Perdeau and J. Oudar, Surface Sci. 36 (1973) 225. [11] C . M . r . , d i e r , Y. Berthier, E. Margot and J. Oudar, J. Microsc. Spectrosc. Electrons 8 (1983) 269. [12] C.~A. Pradier, Y. Berthier and ]. Oudar. Surface'3ci. 130 (19:,3) 229.
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[13] T.E. Fischer and 3.R. Kelemen, J. Catalysis 53 (1978) 24. [14] T.E. Fischer and S.R. Kelemen, Surface Sci. 69 (1977) 1. [15] J. Billy and M. Abon, Surface Sci. 146 (1984) L525. [16] W. Heegemann, K.H. Meister, E. Bechtold and K. Hayek, Surface Sci. 49 (1975) 161. [17] N.M. Abbas and R.J. Madix, Appl. Surface Sci. 7 (1981) 241. [18] H.P. Bonzel and R. Ku, J. Chem. Phys. 58 (1973) 4617. [19] H.P. Bonzel and R. Ku, J. Chem, Phys. 59 (1973) 1641. [20] R.J. Koestner, M. Salmeron, E.B. Kollin and J.L. Gland, Surface Sci. 172 (1986) 668. [21] Y. ghou and J.M. White, Surface Sci. 194 (1988) 438. [22] K. Prabhakaran, P. Sen and C.N.R. Rao, Surface Sci. 169 (1986) L301. [23] L. Moroney, S. Rassias and M.W. Roberts, Surface Sci. 105 (1981) L249. [2a] K. Kishi and M.W. Roberts, Trans. Faraday Soc. I, 71 (1975) 1721. [25] G.E. Mitchell, M.A. Henderson and .LM. White, J. Phys. Chem., submitted. [26] J.IL Creighton and J.M. White, Chem. Phys. Letters 92 (1982) 435. [27] S. Akhter, C.M. Greenlief, H.W. Chen and J.M. White. Appl. Surface Sci. 25 (1986) 154. [28] ILL. Summers, NASA Technical Note TND-5285 (NASA, Washington, DC, June, 1969). [29] J.L. Gland, Surface Sci. 93 (1980) 487. [30] P.R. Norton, J.A. Davies and T.E. Jackman, Surface Sci. 122 (1982) L593. [31] G.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446. [32] B. Poelsma, L.S. Brown, K. Lenz, L.K. Verheij and G. Comsa, Surface Sc/. 171 (1986) L395. [33] B.A Sexton, Surface Sci. 94 (1980) 435. [34] K.M. Ogle and J.M. White, Surface Sci. 169 (1986) 425. [35] J.R. Creighton and J.M. White, Surface Sci. 136 (1984) 449. [36] G.B. Fisher, Surface Sci. 87 (1979) 215.
379
THE REACTION OF H2S WITH ADSORBED OXYGEN A T O M S ON PLATINUM (111) * G.E. MITCHELL, M.A. SCHULZ and J.M. W H I T E Department of Chemist.y, University of Texas, Austin, TX 78712, USA Received 11 March 1987; accepted for publication 18 November 1987
The reaction of H2S(g) and adsorbed atomic oxygen on P t ( l l l ) has been studied by SIMS, HREELS, and TPD. H2S(g ) reacts completely at 96 K with 0.22 ML O(a) to give H20(a ) and S(a). The reaction rate shows no measureable temperature dependence between 96 and 135 K and, under the conditions of this study, was equal to the flux of HzS. After a saturation dose of H2S on O ( a ) / P t ( l l l ) , only H2S, H 2 and H~O desorbed in TPD. H R E E L spectra provide evidence of water cluste~ng when H2S exposure continues beyond the amount necessary for complete reaction of the adsorbed O atoms.
1. Introduc~on Knowledge of the interactions of H2S on metal surfaces is helpful in the understanding of hydrodesulfurization catalysts and sulfur poisoning of cracking and automotive exhaust catalysts [1]. The reaction of H2S with adsorbed oxj~en atoms on the P t ( l l l ) surface is also interesting for its potential to provide insight into the mechanism of the reaction of H2(g) + O(a) --, HzO(a). The reduction of oxygen atoms by hydrogen on platinum surfaces has been well-studied [2-8]. The reaction proceeds at measurable rates at temperatures ,'v~ used static secondary ion mass as low as 120 K [4,7]. Ogle and ~,,L.e spectroscopy (SIMS) to measure kinetic parameters of thc reaction at temperatures below the desorption point for H20 [4]. At temperatures between 12~ and 155 K the reaction is activated by about 3 kcai/mol and has a go5 dependence on hydrogen pressure. The reaction profile as measured by following the H30 + secondary ion has a distinctive shape, rising slowly for the second haif. The reaction of water with O ( a ) / P t ( 1 ] ] ) ~o y~eld an intermediate (not OH(a)) has been studied with SIMS av,d TPD by Creighto~ and White [9]. When two molecules of [ q 2 0 a r e added for each O(a) the i~te~mediate reacts with H2(g ) to form water without any induction period [3]. This * Supported in part by the .wo . . . .~. .,., , ~ R.~o~. ........ . . .h. .c~rfi,-,~ . . . . . :rod the Robert A. Welch Foundalic.n.
0039-6028/88/$03.50 ~ Elsevier Science Pubfishers I-;.V. (North-Holland Physics Publishing Division~
380
G.E. Mitcheit et al. / Reaction of H2S with adsorbed 0 on Pt(l l l)
observation was taken as evidence that the reaction path for H2(g ) + O(a)/ Pt(111)-~ H20(a) included the above intermediate [3]. Recently, however, OH(a) (not the product of the reaction of H20(a) + O(a)) has been identified as the key intermediate in the water formation reaction H2(g)+ O(a) on
Pt(111) [8]. There have been many studies of the structure of sulfur adlayers on the surfaces of platinum and other transition metals and of the modification of the catalytic activity of platinum induced by sulfur [10-19]. While the method of choice for introducing sulfur to the surface has been by giecomposition of H2S, only recently has the interaction of H 2S with the Pt(111) surface been detailed [20]. Koestner et al. found that at 110 K, small exposures of HES on Pt(111) completely decompose to S(a) and H(a). At higher exposures SH(a) and H2S(a ) are also detected by high resolution electron energy loss spectroscopy (HREELS). The reaction of H2S with oxygen has been studied on nickel, copper and lead surfaces. Adsorption of H2S on oxygen covered Ni(100) results in H~O desorption at 235 K in TPD [21]. The H20 desorption is reaction limited (probably due to OH(a) + OH(a) -~ HEO(g ) + O(a)) and OH(a) formation may be occurring upon adsorption of the HES. HES is thought to adsorb associatively on oxygen covered Cu(ll0) at 80 K and react to form OH(a) upon heating to 120 K [22]. On Cu(111) X-ray photoemission (XPS) has been used by Roberts' group to demonstrate OH(a) and H20(a ) formation at 105 K after exposing an oxygen covered surface to HES [23]. The authors of the previous study invoked H-bonding as playing a large role in the H atom abstraction reaction. Reaction of H2S with oxidized lead films also produces water at temperatures as low as 85 K [24]. In the study reported b,ere, we have used SIMS, HREELS, temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) to follow the reaction of HzS with atomic oxygen on P t ( l l l ) .
2. Experimental The experiments were performed in an ion pumped UHV chamber described previously [25]. The chamber is equipped with a cylindrical mirror analyzer for AES, a !27 ° cylindrical deflection analyzer for HREELS, a differentially pumped ion gun and a quadrupote mass analyzer for SIMS and TPD. During these experiments the base pressure was generally- 3 × 10 -1° Torr. SIMS was performed using an Ar + current o f - 1 n A / c m 2 at 500 eV, rastered across the sample to minimize local beam damage effects. Previous studies have shown ion curren~ of this magnitude have a negligible effect, on a
G.E. Mitchell et at / Reaction of lieS with adsorbed 0 on P t (l l l )
381
water adlayer for times less than 1000 s [26]. Secondary ions were monitored in the direction parallel to the surface normal. TPD was performed with the same mass spectrometer used for SIMS. The heating rate was 4 K/s. Auger spectroscopy was performed with a beam energy of 3 keV. "D~ftlki~i "~" et al. report a linear relationship between the Auger peak ratios S(150 eV)/Pt(238 eV) and the coverage of sulfur on the P t ( l l l ) surface (determined by radioactive sulfur and low energy electron diffraction (LEED)), at least up to 0.33 ML [10]. Koestner et al. report that a saturation dose of H2S at 110 K, heated to 500 K deposits 0.25 ML S(a) [20]. These facts were used to calibrate the Auger data. The P t ( l l l ) sample was cut and polished by the usual metallurgical techniques. Sulfur was removed by Ar + bombardment or h,'"':ng , . , , , .~ ! e - 1400 K in 2 × 10 -8 Torr H 2. Oxidation was used to remove carbon. Surface cleanliness was confirmed by Auger and oxygen TPD spectra. The criteria considered in the latter involved the area and position of the O(a) recombination TPD peak after a saturation 02 dose at 100 K. The peak position was required to lie within + 5 K of 738 K at a ramp speed of 12.6 K / s and the area was required to lie within _+ 5% of what was found consistently after a series of adsorpdon-desorption cycles. Other work in this laboratory [27] shows that stabilization of the 02 adsorption kinetics involves oxidation of small amounts of impurities in the near surface region. Oxygen (Scientific Gas Products 99.999%), hydrogen (Linde Specialty Gas 99.95%) and H2S (Linde Specialty Gas 99.5%) were used without further purification except for liquid nitrogen cooled traps in the lines for H 2 and 02. Pressures were measured by an ion gauge and corrected for the sensitivity referred to, N2(g ) (2.2 and 0.44 for HaS and Ha) [28]. In all experiments performed with O ( a ) / P t ( l l l ) the atomic oxygen was formed by adsorbing O z at 100 K and heating to 300 K. This procedure gives 0.25 ML of O(a) [29,30]. In our experiments the coverage was about 0.22 + 0.02 ML because of reaction with residual CO(g) to form CO2(g ) during the initial heating.
3. Results 3.1. T P D
After a saturatiGn H2S are observed in maximum at 168 K. desorb from Pt(lll) hydrogen is adsorbed
dose of H2S on 0.22 ML O ( a ) / P t ( l l l ) , H 2, H20 and TPD (fig. 1). The water desorbs in a single peak with When molecular water is dosed, low coverages of it with a maximum rate at about 170 K [31]. When with atomic oxygen at 100 K on P t ( l l | ) , with heating
G.E. Mitchell et aL / React~on of H2S with adso, bed 0 o~s Pt(l l l)
382
Sat'n H2S + O(a)
/ Pt(lil)
tn .e'4
i72
C
' JO t-
.r4 tO
220
IBB
AJ C F-4 L
H20
E 0 (_
C1 tO
~27 !
U~ 69 ~0 ~E
@
t00
200 Temoerature
300
400
500
/ K
Fig. 1. Temperature programmed desorpdon spectra after exposure of 0.22 ML of O(a) on P t ( l l l ) at 96 K to 14 k of H.:S.
rates >_ i K / s water desorbs in 3 peak:; at about 170, 210, and 300 K [4,7]. Thus, in the H~S + O(a) reaction:, the desorbing water is ~u]ly formed below 170 K. Hydrogen desorbs in a broad peak witl a maximum at 170 K and two shoulders at 185 and - 220 K. By comparison with the work of Koestner et al. [20], the latter two desorption states can be assigned to decomposition of H2S(a ) and SH(a) to S(a)+ H:(g) ',reaction limited desorption) and to the recombinador~ oi H(a) '~o H;~'g) (desorption limited) respectively. The peak
G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt(l l I)
383
aear 170 K is desorbed in unison with H20. Recently, Poelsma et al. [32] have shown that hydrogen can be incorporated into a layer of adsorbed water and be released to the gas phase concurrent with H 2 0 desorption. We ascribe the 170 K H a desorption (fig. 1) to a similar mechanism. The area under the hydrogen peak can be determined by comparison with the desorption of H 2 from an H2S saturated surface. (Tke final sulfur coverage is known for the latter.) The amount of H2(g ) desorbin~ is 0.14 + 0.02 ML. This agrees with Auger ratios which indicate about 0.33 +_ 0.03 ML S(a) left on the surface after TPD. This compares to 0.25 ML S(a) left on the surface after thermal decomposition of a saturation dose of HES on clean Pt(111) [20]. Excess H2S desorbs at 127 K with a shoulder near 150 K (fig. 1). These desorption peaks correspond to H2S desorption from the clean surface [20]. No oxygenated sulfur species such as SO or SO 2 were detected. TPD after H2S exposures below those required to convert all the O(a) to H20 revealed: (1) desorption of ueither H 2 nor H2S occurs, and (2) H20 desorption occurs in two peaks near 170 and 200 K as for the partial reaction of H2(g ) + O ( a ) / P t ( l l l ) [4], but the 300 K H 2 0 peak is not detected for reduction with H2S. 3.2. H R E E L S
Fig. 2 shows HREELS spectra (a) for 0.75 L exposure of H2O u~ clean P t ( l l l ) and (b) after ~,.~,,.,n':~" ,~¢.~HzS + O(a) at 100 K. tt is evident from t~:~e similarity of these spectra that reaction between 1-i25 and ,.,~=,r'~"~,~.~ fc~rm H~O occurs even at 100 K. The vibrational spectra for H20 on Pt(111) prepared by adsorption frGm the gas phase and by reaction of H2(g ) + O(a) have been assigned [33,8]. The losses a t - 570, 1620 and 3400 cm -1 present in both spectra of fig. 2 are assigned to the Pt-O stretch (v(PtO)), H - O - H scissors (8(HOH)) and O - H stretching (v(OH))modes, respectively. Water formed by reaction of H2(g ) + O(a) on P t ( l l l ) exhibits a binding geometry ( - C2v) different from that of water adsorbed from the gas phase [8] (arising from a reluctance of the former to break oxygen-metal interactions in order to form H-bonds). Because of the difference in structure, losses apparent in the spectrum of H 2 0 ( a ) / H 2 0 ( g ) near 260 (hindered translation parallel to the surface; L,.y) 685 and 945 cm -1 (hindered rotations; Rx.y...) (fig. 2a) are absent in the spectra of H 2 0 fcrmed from reaction of H2(g)+ O(a~ [g]. The spectrum of H20(a ) from the reaction of H2S~o ) + O(a) (fig. 2b) Is ~ntermediate to the two cases above. ]In particular, wc assign the shoulders at 260 and - 660 cm-1 to the tx, y and R~. ~.: modes of clustered water respectively, but these are not as intense as those in fig. 2a. The amount of H2S adsorbed for fig. 2b was slightly in excess of the stoichiometfic amount required fo~ H20 (0.05 ML H 2 desorbed in TPD). After larger H2S exposures, ~he librafion near 660 cm-1 becomes mor,~ intense and the HOH scissors mode broadens
384
GE. Mitchell et al. / Reaction of H:S with adsorbed 0 !
I
I
lfA
X75
260685
I
"
I
a4oo
!
|
x200
Pt(l l l)
0.75L H20/Pt(ttt)
H v ~
4J °P,I
on
ii-2.o
!
H2S + 0 (a)/Pt (1tt)
c .IJ c
(b) tt80
I
\I
i620
I co
a4oo
L Energy Loss / cm-t Fig. 2. High resolution electron energy loss spectra of: (a) 0.75 L of H20 and ( b ) - 1 L of H2S+0.18 ML of O(a) on Pt(lll).
and moves to somewhat higher frequency. The latter are attributed to increased H 2 0 - H 2 0 interactions (H-bonding), and are consistent with three-dimensional islanding of the adsorbed water formed in the presence of molecular H. S. Because of the interference from the relatively intense losses due to H20(a ) and the !ow coverages of iudrogenated sulfur species which may be present, no losses due to SH(a) or H~S(a) were observed in these experiments, although SIMS results (see below) suggest teat adso,'~tio~ of H2S beyond the stoichio-
G.E. Mitchell et aL / Reaction of H2S with adsorbed 0 on Pt( l ! 1)
385
metric amount needed for reaction with O(a) is associative at 100 K. The Pt-S stretch for S(a) is found at 375 cm-1 [20] and is not resolved from the losses due to H EO(a ) in fig. 2b.
3.3. SIMS Using the H 3 0 + SIMS intensity as a monitor of the watcr cc_,verage, we followed the reaction of H2S(g) with O ( a ) / P t ( l l l ) in a constant pressure of H2S (fig. 3). As a benchmark, the reaction H2(g ) + O(a) (fig. 3a) was performed at 135 K and a hydrogen pressure of 4.5 x 10 -8 Torr. The rate of the latter reaction is undetectable at 96 K. Fig. 3b shows H 3 0 + (role = 19) and a
I
|
I
0
H2(g) + Olal --> H20(al T-
t35K
PH2 - 4 . 5 x t 0 . 8 t o r t
m
~
m/e
19
C
r...
H2Slg) + Ole) --> H2OIa} E {IJ
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.
0
240
.
.
.
.
4aO
tort
&
+
720
%0
1200
Time / Seconds Fig. 3. Time dependence of H3O+ and H3S + SIMS signals during titration of Ofa) for (a) 4 x 1G-'~ Torr H2(g)+0.22+_0.02 ML of O(a)at 135 K. (b) 3.6x 10 . 9 Torr H2S(g)+0.22+0.02 ML of O(a) at 96 and 135 K.
386
G.E. Mitchell et al. / Reaction of HzS with adsorbed 0 on Pt(l l l)
H 3S ÷ ( m / e = 35) versus time curves for the reaction of H 2S with O(a)/Pt(111) at 135 and 96 K at the same oressure of H2S. The ion intensities have been normalized to the measured primary Ar ÷ current. For t < 290 s, the ~hapes of the role = 19 curve are very s~milar to the shape of the H2(g)+ O(a) curve shown in panel a. For the H:~S + O(a) reaction, the slopes at 96 and 135 K are the same in both the induction and rapid regioiis. The slight offset of the two curves (along the time axis) may be due to a small difference in the initial O(a) coverages (.- 0.01 ML) for the two experiments. We interpret these results to indicate there is no significant temperature dependence (no activation barrier) for water formation under these conditions. In order to obtain information regarding the H2S coverages, we also followed the HaS + SIMS ion (the largest + ion for H 2 S / P t ( l l l ) ) during the experiments reported in fig. 3b. By analogy with water [26], it is likely that mass 35 (H3 S÷) contains contributions derived from both adsorbed H2S and SH. Bearing this in mind, two comments are made regarding the m/e = 35 spectra in fig. 3b. First, in both cases the mass 35 intensity begins to increase as the mass 19 signal saturates. Apparently up until this point all the hydrogen is being used to form water and very little is bonded to the sulfur atoms. Second, the mass 35 intensity in the 135 K reaction saturates earlier and at a much lower level than in the 96 K reaction (note the expansion factors in fig. 3b). Since 135 K is above the molecular H2S desorption peak and below the H2 resorption peak, we assign the Ha S+ seen at 135 K to SH(a) and not H2S(a ). The signal-to-noise and interfering bands of H20(a) prevent unambiguous identification of SH(a) by HREELS (see above). After completion of water formation (~ > 290 s) at 135 K the m/e = 19 and (less rapidly) the m/e = 35 signals begin decreasing. That this decrease in secondary ion yield is not due to sputtering was determined by an experiment like fig. 3b in which the ion beam was turned off for a time; the signal continued to decrease while the beam was off. We envision three possible mechanisms which could cause the observed decrease in the H3O+ yield: (1) three-dimensional island formation, such that the two-dimensional area covered by H20(a ) decreases; (2) a decrease in the work function; and (3) displacement of HzO(a ) by H2S(a ) (or a decomposition product of the latter). HREEL ~pectra (reported above) suggest the importance of three-dimensional island,nn v ~,~wever. HREEL spectra taken after annealing to 130 K do not show a significant increase in intensity v~ &c ]:,br~cm~! modes, indicating that further three-dime~, sional islanding is not important. To test for the t ~ r d possibility, temperature programmed S~MS (TPSIMS) was performed after formation of water from HzS + O(a) at 96 K. The H30+ intensity declines by 30% between 120 K (H2S desorption temperature) and 156 K (where water begins to desorb), at which point the signal falls rapidly to zero. The decrease in the H30 + yield between 120 and 156 K in the TPSIMS experiment does not involve desorption of H20(a ) and (assuming the decrease in H30 + yield in the
G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt( l l l ) !
I
!
387
o
H2S + O(a) - - > H20(o} T = 96K
U) -,-I
r-
3.6xt0 -9 t o r t
f,. ,,,¢
t . 8 x t 0 -9 torv ,el
¢= 4-= C H 03 =£ H + O I "O3
•
O
I
I
0
240
:~80
720
g60
~.200
Time / saconds Fig. 4. Time dependence of H 3 0 + signals during reaction of H2S with O(a) on P t ( l l l ) at 96 K. Two H2S pressures were used (1.8 and 3.6 × 10 -9 Tort).
TPSIMS and isothermal SIMS experiments have the same cause) we rute out mechanism 3. As discussed bele-w, a work function change provide~ a reasona~ ble explanation of this SIMS data. The influence of the pressure of H2S on water formation was also investigated (fig. 4). The reaction was performed at constant temperature (96 K) at H2S pressures differing by a factor of 2 (3.6 × 10 -9 and 1.8 × 10 -9 Tort). Defining the induction time as the intersection with the baseline of a line drawn through the linear portion of the curve in the rapid reaction region, gives 129 and 346 s for the induction time with high and low H2S fluxes respectively. The final H3O+ signal intensities in fig. 4 indicate a small disparity in the initial O(a) coverage of 0.03 ML. Taking into account the disparity in initial oxygen, the induction time is - 2.3 times ionger for the case w~t~ ha]~ ~Lc H ~ ~"~. However, the reaction rate in the rapid region (slope t0r die linear portion of the curve) for the ia~,~., ~u~ is ,~,~y I ~ times that for the smaller flux. The average overall reaction rate is then proportional to d~ f~ux. At 3°6 )< 10 -9 Torr lhe flux of HzS to the surface is 8.7 × 10 --4 ML/s. A fluence cff H2S equal to the initial O(a) coverage ( - 0 . 2 1 ML) is reached a~ 250 s, and similarly at the lower flux, 0.24 ML of H2S fluence is reached at
388
G.E. Mitchell et at. / Reaction of l-l~S with adsorbed 0 on Pt(l l l)
570 s. As shown in fig. 4, tl~e reactions are nearly completed at these times. The overall rate of the reaction to form water is therefore essentially the same as the flux of H2S to the surface under the conditions of these experiments.
4. Discussion
Investigations of the reaction of H2(g ) and O(a) between 120 and 150 K have determined that adsorbed hydrogen atoms are an intermediate and that isotope effects are consistent with H(a) diffusion being the rate limiting step of the reaction [34]. This step must then involve the measured 3 kcal/mol activation barrier. Since the reaction of H2S(g ) + O(a) does not show an activation barrier, it must either cause a decrease in the activation barrier for H(a) diffusion or not involve H(a) diffusion as a step in the reaction. The latter, suggesting direct transfer of the H from H2S(a) to O(a), is preferred for the following reasons. (1) Surface diffusion of H2S(a ) would be facile at the temperatures employed in these studies (according to the rule of thumb that the onset temperature is 1/3 to 1/2 the desorption temperature), and (2) since H2S readily dehydrogenates on the clean surface, hydrogen atom transfer to O(a) or OH(a) should also be easy. The shape of the SIMS reaction profile is very similar to the curves for the reduction of O(a) with H2(g ). In the latter case the induction period has been attributed to the formation of an intermediate (OH(a)) [8]. The rapid increase in ~,lle H3O+ SIMS intensity at about 150 s in fig. 3a is attributed to the reaction of OH(a) with H(a) 0,o form H20(a). Apparently the same intermediate is also involved for H~,~;(g) + O(a) --, H20(a ). Since the length af the inductic'," region fo~ H2S(g ) + O(a) is temperature independent (fig. 3b), the reaction to form the intermediate must also be temperature independent in this case. The lack of a thermal barrier is additional evidence against assigning the product of H20(a ) + O(a) as the key intermediate because the latter reaction has an appreciable activation barrier [35]. We believe the decrease in the H3 O+ intensity after completion of the water formation reaction at 135 K (and observed in TPSIMS between 120 and 156 K) is caused by a change in work function. On Ru(ll0) dissociative adsorption of H2S to produce S(a) + 2 H(a) at low exposures increases @, while SH(a) formation at higher exposures is accompanied by a decrease in @, and finally near saturation coverage, adso~tion of H2S(a ) leaven ~ unchanged [36]. Adsorption of H2S on this surface is very simiiar to the case of P t ( l l l ) [20] and therefore we expect a similar inflaence on ~ for Pt. Indeed, work function measurements made by the secondary onset method (300 eV electron beam) during H2S ex!,:,osure at 100 K show an initial increase (0.15 V) followed by a decrease to - 0.52 V (SH(a) formation). Koestner et al. have shown that H2S decomposition is dependent on the availability of unoccupied surface sites. At
G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt(l i l)
389
completion of the water formation reaction, there i s - 0 . 2 5 ML H20(a ) and - 0.25 ML S(a) on the surface and at 100 K there is, according t o H3 s + SIMS yields (also note the H2S TPD peak at 120 K), at least 10 times more H2S(a ) than at 135 K (fig. 3b). The H25 dccumposition channel is apparentiy not open on the crowded surface at 96 K, but at 135 K the short residence time for H2S(a ) frees up sites for SH(a) formation. The decrease in • caused by SH(a) would then decrease the yield of H30 +. The amount of S(a) left on the surface after heating an O(a) covered surface is larger (0.33 ML) than found for the clean surface (0.25 ML) [20]. On the ckan surface, decomposkion of ;!3 :~ :~,.h~b/icd by biu¢ldng of sites for H(a) by previously produced H(a), S(a) a~:d SH(a). On the oxygen covered surface the first 0.5 ML of hydrogen atoms from H2S is used to make water and assuming one site per adsorbed species, fewer sites will be blocked, thus H2S decomposition during TPD can proceed much further. Three-dimensional islanding of the water caused by H2S adsorption as indicated by HREELS will serve to further increase the number of sites available for H2S decomposition during TPD.
5. Summary In summ_ary, H 2 S is an extremely efficient reducing agent on Pt(lll), completely reducing •"~"~ .,~,, to water with no apparent activation barrier at 96 K. This is in contrast to H2(g) which exhibits an activation barrier of 3 kcat./mol for reduction of O ( a ) / P t ( l l l ) and does not re,~.t with the latter b e l o w - !2.~ l( [d]_ . The facility of !he reaction at 96 K is strong evidencc f.,3r an intermediiate differing from that f,~rmed in the reaction of H2©(a ) + O(a).
References [1] L.L. Hegedus and R.W. McCabe, Catalyst Poisoning (Dekker, New York~ 1984). [2] P.R. Norton, in: The Chemical Physics of Solid Surface's and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 27. [3] G E . Mitchell, S. Akhter and J.M. Whi~e, Surface Sci. 166 (1986) 283. [4] K.M. Ogle and LM. White, Surface Sck 139 (1984) 43. [5] J.L. Gland, G.B. Fisher and E.B. Kollin, J. Catalysis 77 (1982) 263. [6] G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 (1980) 683. IO. [7] G.B. Fisher, J.L. Gland and S.J. Schmieg, .L Vacuum Sci. Technol. 20 (1982) J~1o [8] G.E. Mitchell and J.M. Whhe, Chem. Phys. Lollers 135 (1987) 84. [9] J.R. Creighton and 3.M. White, Surface %i. 122 (1982) L648. [10] Y. Berthier, M. Perdeau and J. Oudar, Surface Sci. 36 (1973) 225. [11] C . M . r . , d i e r , Y. Berthier, E. Margot and J. Oudar, J. Microsc. Spectrosc. Electrons 8 (1983) 269. [12] C.~A. Pradier, Y. Berthier and ]. Oudar. Surface'3ci. 130 (19:,3) 229.
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G.E. Mitchell et al. / Reaction of H2S with adsorbed 0 on Pt(l l l)
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