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Interactions of sulfur with nickel surfaces: Adsorption, diffusion and desorption
433
Surface Science 13 1 ( 1983) 433-447 North-Holland Publishing Company
I~~~~IONS ADSORPTION,
OF SULFUR WITH NICKEL SURFACES: DIFFUSION AND DESORF’TION
M, BLASZCZYSZYN, Institute of Experimental
R. BLASZCZYSZYN
and R. Ml$LEWSKI
Physics, University of Wrociaw, 50205 Wrocfaw, Poland
and A.J. MELMED
and T.E. MADEY
Surface Science Division, National Bureau of Standards,
Washington, DC 20234, USA
Received 27 December 1982; accepted for publication 10 May 1983
The kinetics of adsorption, surface diffusion and thermal desorption of sulfur on Ni surfaces have been studied using field electron emission microscopy methods. The sticking probability for elemental sulfur sublimed onto a Ni specimen is approximately unity for Ni substrate temperatures from 77 to 530 K. The maximum average work function for - monolayer coverage of S ranged from 4.7 to 5.05 eV in different experiments; for fractional monolayer doses of S, surface diffusion was rapid at T > 400 K, with activation energies ranging from IS- to 28 kcal/mol. For multilayer adsorption of sulfur, diffusion occurs without motion of a sharp bounda~, and there is evidence of extensive surface reaction between S and Ni (emission from smalt “crystalhtes” is evident in the field emission patterns). Sulfur desorbs from Ni at temperatures above 1500 K.
1. Intruduction
Sulfur is an impurity which is known to affect the lifetime of nickel catalysts; for example, S is a common poison which causes catalyst deactivation [l-4] in an energy-related catalytic process, the catalytic methanation reaction (CO + 3 H, + CH, + H,O). However, sulfur is also known to be a promoter for another reaction, the formation of nickel carbonyl [S] (4 CO + Ni -+ Ni(CO),). For a detailed understanding of the roles of S in catalytic reactions over Ni surfaces, it is useful to characterize the kinetics of adsorption, diffusion and desorption of S on Ni surfaces. There is an abundance of literature concerned with various aspects of the sulfur-nickel interactions. The structures of S on Ni single crystals have been studied by LEED [6,7], and surface segregation has been characterized by AES (81. The influence of S on the kinetics of CO adsorption and desorption [9- 1 l] and on the rates of catalytic reactions [l-4] is well known. However, there is 0039-6028/83/0000-0000/$03.00
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little or no information available concerning the kinetics of adsorption and diffusion of elemental S over Ni surfaces, nor has the thermal desorption of S from Ni been reported. In the present work, we apply the techniques of field emission microscopy to the S/Ni system. We report measurements of electron emission properties, sticking probabilities, activation energies for surface diffusion. and surface structures which accompany the thermal desorption of S.
2. Experimental In general, conventional experimental methods [ 121 were used in the present work. Field electron microscope (FEM) observations and measurements were made in sealed-off glass tubes immersed in liquid nitrogen, providing an ambient pressure < 1.3 X 10d8 Pa (1 X lo-” Torr). The Ni field emitters were prepared by spotweiding electrolytically etched 99.995% pure Ni wire to W support loops, and the sulfur sources were six-times vacuum-distilled and sealed in glass ampoules. Cleaning the field emitter at the start of an experiment was achieved by lengthy periods of heating to temperatures > 1.500 K in vacuum. However, the
Fig. 1. Field electron shadow images of annealed Ni field emitters (T- 400-1000 nm. (a)-(f) Discussed in text.
1600 K). Tip diameters
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success rate was less than 508, due to catastrophic ruptures of the tips. This rather low success rate can be understood from the results of some earlier (unpublished) studies by one of the authors (A.J.M.) in which Ni field emitters were thermally cleaned and observed by field-electron shadow microscopy [ 131 at various stages during the cleaning process and after further heat treatments in vacuum. Some typical emitter profiles are shown in fig. 1. The tips having shapes shown in figs. lb, Id, le, and If are extremely weak and specimens with such shapes undoubtedly would catastrophically fail if subjected to the FEM field [ 14]. The shape shown in fig. la is the sturdiest of those shown, and was achieved by starting with a much wider cone-angle tip. Further heating caused this shape to change to shapes similar to those of figs. lb- 1f. Thus, we conclude that the success rate for achieving clean Ni field emitters is increased by (1) starting with the highest purity Ni, so that the total heating time for removing impurities is not excessive, and (2) polishing tips with cone angles as Large as possible, consistent with adequately sharp tips 1141. After bakeout, a high purity sulfur ampoule was broken allowing sulfur to be sublimed into a small electrically heatable glass cylinder. This source was located in direct line-of-sight of, and perpendicular to, the Ni tips and was operated in a continuously heated, constant temperature mode with reproducible dosing accomplished by magnetically opening and closing a shutter. Calibration of the qu~titative deposition rate was only approximate, however, principally due to uncertainties in the geometry of the effusion source and the source temperature. Thus, we assumed that monolayer coverage prevailed just prior to the appearance of discrete nuclei (see, for example, fig. 2 and the accompanying discussion). Although the dosing rate was not known absolutely, it was very constant and reproducible, and at a source temperature of 300 K, the composition of the incident S beam is known to be - 80% S, and - 20% s, [15]. Tip temperature was controlled by an electronic temperature controller with temperature rise times typically about one second from 77 K to the diffusion temperatures. Always, sulfur deposition was done with no field applied to the Ni tip, which was kept either at 77 K or at some elevated temperature. Likewise, no field was applied during the thermal spreading of deposited sulfur; all FEM observations were made with the emitter cooled to 77 K.
3. Results 3.1. Field emission patterns The field electron emission patterns obtained and recorded in this work served to provide a qualitative microscopic description of the uniformity or anisotropy of S adsorption, surface diffusion and desorption, as well as data
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Fig. 2. Field electron images depicting the equilibrated distribution of accumulated small doses of sulfur on Ni. (a) Initial clean (111) oriented Ni field emitter. Prior to each of the succeeding micrographs the Ni tip, at 77 K, was exposed to a sulfur dose for some time and then heated to 530 K to spread the additional sulfur. The tip was then cooled again for photography. The cunudative dose time (minutes) and the applied voltage (volts) for constant emission current for each micrograph is: (a) 0.6282; (b) 2,6404; (c) 8,6717; (d) 22,7214; (3) 30,7358; (f) 38,7512.
for the quantitative determination of surface diffusion activation energy. In addition, inspection of the FEM patterns provided evidence interpreted [ 161 to mean that the Ni surfaces were clean after high temperature heat treatment. The surface coverage resuling from the adsorption and spreading of low doses of sulfur showed no great anisotropy as a function of the nickel substrate crystallography. However, when the coverage reached a critical value, discussed below, heterogeneous nucleation occurred. The nucleation sites did not appear to be reproducibly related to special c~stallograp~c features of the substrate. A sequence of FEM patterns resulting from the deposition of a series of “small” (i.e., fractional monolayer) doses of S onto initially clean Ni at 77K, followed by spreading at 530 K of each dose is shown in fig. 2. Gradual changes in the patterns are apparent in figs. Za-2e, indicating approximately uniform, increasing coverage. Fig. 2f shows the onset of nucleation; discrete nuclei, seen as bright spots, are evident at the edges of the leftmost (111) plane, which was the (111) plane directly in view of the S source. Much larger doses of S could be spread only by heating the Ni tip to higher temperatures, as seen in the sequence of FEM patterns shown in figs. 3 and 4. At the lower spreading temperatures represented in fig. 3, the large accumu-
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Fig. 3. Field electron images depicting sulfur on Ni. Electron emission current to each succeeding micrograph sulfur some time. The tip temperature, sulfur 320 K. 2400 s, 6107 V; (c) 320 K, 5580 V; (f) 460 K, 60 s, 6219 V.
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the spreading (during specimen heating) of large doses of was 1 x IO-’ A for each image. (a) Clean Ni, 5759 V. Prior was impinged upon the heated specimen from below for deposition time and tip voltage (for photograph) are: (b) s, 6252 V; (d) 290 K, 7080 s, 6198 V; (e) 400 K, 120 s, 6188
lations of sulfur formed many clusters, rather than diffused to a significant extent, while at the higher spreading temperatures of figs. 4e and 4f large doses of sulfur indeed diffused over the surface, but the complicated appearance of the patterns indicates that extensive rearrangement of the original Ni surface occurred. 3.2. Work function of S on Ni Fig. 5 is a plot of the work function as a function of increasing S coverage during the adsorption of S on a (110) oriented Ni tip. The S was deposited onto one side of the tip while heating to 530 K. Following deposition of S, the S was spread by continuing to heat at 530 K to insure that the deposit was uniform; a sequence of FEM patterns similar to those of fig. 2 above accompanied the work function measurements of fig. 5. The work function I$ following each dose was determined using the Fowler-Nordheim equation, based on current (i) versus voltage (V) measurements at 77 K, and assuming
438
Fig. 4. Field from below for spreading 1 x IO-’ A, (e) 5957 V,
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electron micrographs depicting the spreading of a large dose of sulfur on Ni deposited the Ni tip. For each photograph, the voltage, emission current, tip temperature used and the spreading time are given: (a) 5342 V, 1 x 10e7 A, 1610 K, 10 s; (b) 5562 V, 77 K, 1200 s; (c) 7701 V, 1 x IO-’ A, 460 K, 15 s; (d) 5834 V, 1 x IO-’ A, 460 K, 60 s; 1 x IO-’ A, 570 K, 675 s; (f) 6063 V, 5 X 10-s A, 685 K, 15 s.
(pa (clean Ni) = 4.50 eV. Also shown in fig. 5 is the Fowler-Nordheim preexponential factor versus dose. Data similar to fig. 5 were measured for 3 different tips, two with (111) orientation and one with (110) orientation. In all cases, the work function was observed to rise monotonically with increasing S dose for low S coverages. However, there was some difficulty in reproducing the maximum value of work function (the saturation value of + ranged from 4.7 to 5.05 eV in different experiments). The lack of repr~ucibility in work function measurements at higher S doses was clearly related to the lack of uniformity in emission due to the formation of small crystallites which were visible in the FEM patterns (see, for example, fig. 2f). Thus, for low sulfur coverages, the sulfur diffuses in a uniform fashion over the entire tip, producing a symmetrical pattern of electron emission. For high sulfur coverages, a surface reaction on the “dosed” side of the tip leads to the formation of “crystallites”, perhaps due to a surface reaction to form Ni sulfide. For the subsequent discussion, we shall assume somewhat arbitrarily that
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. 5
_* .
. . ..* .
InA
. 4
.
.O.O
8
.
0 0 0
4.9 0 +
600K,
0 cl
0
(eV)
530K
o
00 0
4.8 0
0 0
4.6 0
4.5
I
I
I
20
30
40
I
IO
EVAPORATION TIME t (min) Fig. 5. Pre-exponent (In A) and work function ( $I), determined using Fowler-Nordheim equation, for sulfur evaporated onto a Ni field emitter. Following each deposition of S and spreading at 530 K, the tip was cooled to 77 K for the measurements.
the sulfur coverage immediately below which the crystallites were observed to form corresponds to “monolayer” coverage of S, 0, = 1. We note that for different S sources, the deposition time necessary to reach 6s = 1 varied from - 10 to - 40 min. For each source and Ni tip, however, 8, = 1 occurred at a reproducible deposition time. 3.3. Sticking probability of S / Ni Fig. 6 contains plots of 9 versus S dose for a (111) oriented Ni tip. The three sets of data correspond to deposition of S at 77, 380 and 530 K respectively; in each case, the S was spread over the tip by heating to 530 K before measuring the i versus Vcharacteristic at 77 K. The data at 530 K correspond to the FEM
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CRYSTqLLITES
4.60
4.50 EVAPORATIONTIME
t(min)
Fig. 6. Approximate work function (determined using + = 4,,( V/Vo)2/3) versus sulfur deposition time for Ni tips held at three different temperatures, 77 K (0), 380 K (+) and 530 K (A), indicating that the sticking probability of sulfur is independent of temperature.
patterns shown in fig. 2. For technical reasons, the work function fig. 6 was determined from the voltage necessary to maintain emission current via the relation
C#Ishown in a constant
(1) The fact that the three sets of data are completely superimposable for tip temperatures ranging from 77 to 530 K indicates that the sticking probability S for S on Ni is independent of temperature over the entire range of S coverage up to 8, = 1, where crystallites are observed in the FEM patterns (cf. fig. 2). Although we cannot determine the value of sticking probability in an unequivocal fashion from the data of fig. 6, there is good reason to believe that S = 1 over the entire range of T and 8. As mentioned previously, the composition of the incident S beam is - 80% S, and - 20% S, for a source temperature of 300 K [ 151. Since the sticking probability for these species is almost certainly
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unity at 77 K, the constancy of the + versus dose data at higher T implies the same value of sticking probability in the other experiments shown in fig. 6. 3.4. Diffusion of S over Ni To study the diffusion of S over Ni, three types of experiments were performed: (1) diffusion of small doses (6, < 1) deposited at 77 K, (2) diffusion of large doses (0, > 1) deposited at 77 K, and (3) diffusion during deposition at diffusion temperatures. The resulting behavior of the deposited S was complex, and in some respects unusual, compared to other adsorption systems previously studied. The results of these different experiments will be discussed separately. 3.4. I. Small doses, 8, < I
Following the deposition of a small dose of S (13, < 1) onto one side of a Ni field emission tip, subsequent heating to T z 400 K resulted in diffusion of S into the initially-clean regions of the tip. Diffusion in this fractional monolayer
4.66 436K 416 K
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4.64
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+(eV)
4.62 4.61
4.60
4.56
4.57 0
20
40
60
60
100
120
140
160
160
200
t (?.I
Fig. 7. Approximate work function, determined using $J = c#z~( V/Vo)2/3, for Ni tips at several temperatures as a function of sulfur spreading time. The dashed line marks the diffusion endpoint which was used to construct the Arrhenius plot Q4 of fig. 8. The initial point in each case was at
t = 0.
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Fig. 8. Arrhenius plots for diffusion of S over four different Ni tips. The reciprocal diffusion times (I/t) were determined as shown in fig. 7. The field emission patterns corresponding to the four data sets are shown in fig. 9.
coverage regime proceeded without the formation of a sharp boundary. Thus, it was not possible to determine the activation energies for surface diffusion by measuring the rate of advance of a boundary between the S-covered and the clean surface. Instead, the average work function C$of the tip, calculated using eq. (1) and corresponding to different stages of diffusion, was measured after cooling to 77 K following each diffusion interval. Fig. 7 shows the values of 9 as a function of diffusion time t, for different diffusion temperatures. Note that the values of $I do not correspond to true average work functions, since the adlayers were not equilibrated. However, they mark successive stages of diffusion. The data of fig. 7, as well as similar data for other Ni tips measured in the range 400 to 500 K, were used to determine activation energies as follows. In fig. 7, a diffusion endpoint was chosen as indicated by the horizontal dashed line. The times t necessary to reach the endpoint were measured, and plotted in Arrhenius form in fig. 8 [In l/t versus l/T]. The line labelled Q4 corresponds to the data of fig. 7. The activation energies for diffusion of S over Ni were measured on three additional Ni tips, and the data are also shown in fig. 8. The activation energies Q determined from these four plots range from 15 + 2 to 27.7 & 2 kcal/mol; differences in Q may be explained by differences in the initial S coverage as well as in tip geometry. Fig. 9 shows field emission patterns which correspond to the Arrhenius plots of fig. 8. The left column shows patterns from the clean tip before sulfur
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a
b
d Fig. 9. Field emission patterns corresponding to the data of fig. 8. For each corresponds to the clean tip before deposition of S, and the right pattern following deposition and diffusion of S. In all cases, deposition of S was (bottom of pattern). The activation energies for S diffusion for each endpoint kcal/mol; (b) Q2 = IS.2 kcal/mol; (c) Q, = 20.0 kcal/mol; (d) Q4 = 27.7
row, the left pattern shows the endpoint from below the tip were: (a) Q, = 16.7 kcal/mol.
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deposition from below, and the right column shows patterns of the diffusion endpoints which were used to construct the Arrhenius plots. Note that different tip orientations were obtained in each case. 3.4.2. Large doses, 6, > I In most cases of multilayer adsorption, surface diffusion occurs via migration of the topmost layer over the chemisorbed first layer, with precipitation at the edge of the deposit and subsequent advance of the boundary. This process has been likened to an “unrolling of a carpet”, and the boundary of the deposit remains sharp as it advances [16]. This does not appear to occur for large doses of S on Ni. Upon heating after deposition (see sequence of fig. 3), a sharp boundary appears near the deposit edge, but does not advance with further heating or following further deposition of additional S. As the temperature is increased to T z 320 K, a boundary-free diffusion into the initially-clean regions occurs, and nucleation of brightly-emitting protuberances (clusters of Ni sulfide?) occurs in the originally-dosed region. For large doses, the tip must be heated to T > 650 K to achieve equilibration of the S coverage. 3.4.3. Large doses while heating The deposit behaved in a manner somewhat similar to the behavior in the second type experiments above, with two notable differences. With the tip kept at 320 K during large dosing, a very slow surface diffusion process was observed to occur in the (111) vicinals. The effect was entirely absent unless dosing was done during tip heating. Also, a sharp moving boundary was seen to very slowly advance during dosing at 320 K. 3.5. Thermal desorption of sulfur At the conclusion of an adsorption or surface diffusion experiment, it was often possible to successfully reclean the Ni field emitter by heating to temperatures > 1500 K in vacuum. A typical sequence of field emission patterns recorded at various stages of the recleaning process is shown in fig. 10. Note that initially the pattern orientation is such that a (111) plane is near the projection center. However, the final clean surface has an orientation with a (001) plane near the center. This is clear evidence that the removal of sulfur from the Ni by heating is accompanied by extensive removal of Ni as well. Thus, in the sequence documented in fig. IO, the Ni crystallite which originally comprised the tip apex was entirely evaporated. Also, we conclude from the occurrence of different field emission patterns during the sequence that extensive reconstruction of the Ni surface is caused by high temperature interaction with sulfur. After achieving a field emission pattern which appeared clean, such as shown in fig. IOf, the Ni tips were heated for various lengths of time, up to five
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Fig. 10. Field electron micrographs characterizing the thermal desorption of S/Ni at temperatures z 1500 K. A large deposition of sulfur was progressively removed prior to each of the micrographs from (a) to (f).
minutes, at various temperatures below the cleaning temperature as a test for the occurrence of sulfur out-diffusion from the specimen bulk or some other surface rearrangements which would indicate the presence of residual sulfur. Such tests always gave negative results, demonstrating that all detectable sulfur was indeed removed by the thermal cleaning process.
4. Discussion The surface diffusion of S over Ni is notable for the absence of extensive sharp boundary diffusion [16] for sulfur doses in excess of monolayer (“large doses”), as well as for fractional monolayer doses. These results may be interpreted as follows. Sulfur sublimed from the doser is predominantly in the S, polymeric form. When S, (and other polymeric forms of S) adsorb on clean Ni, these species dissociate and diffuse as atoms upon heating to T z 400 K. However. for large doses, S, adsorbed in the outer layers above the Ni is in contact with additional S, or a sulfur/nickel complex. Under these conditions dissociation is less probable, and it appears likely that the S, remains intact, or forms another allotrope of molecular sulfur [17]. As the surface is heated to
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T-e 350 K, the absence of sharp boundary motion over the surface suggests that the diffusion energy for a sulfur polymer is 2 the desorption energy for that polymer, i.e., species such as S, desorb at a faster rate than they are able to diffuse. Another possibility is that the bulk reaction between S and Ni to form Ni,S, may compete with surface diffusion. During dosing with the Ni emitter at 320 K, a sharp boundary was observed to advance very slowly (cf. section 3.4.3). At this temperature, the surface lifetime of the steadily-arriving S, is apparently long enough for a small fraction of these species to diffuse over the edge of the S/Ni layer and dissociate on the clean Ni, thus extending the boundary at a slow rate. Finally, we note that surface diffusion is rapid (>, 100 A/s) on a microscopic scale in the temperature range > 500 K which is the typical temperature range for hydrogenation of CO to form CH, over Ni catalysts [l-4]. Sulfur poisoning can occur due to impurity S deposited onto high area Ni catalysts from the carrier gas (e.g., from H,S impurities) or by diffusion of S from the bulk. Under reaction conditions (> 500 K), S atoms can readily diffuse to equilibrium sites, so that surface diffusion will not be rate limiting bn the poisoning reaction of typical high area catalysts (particle sizes - 60 A in radius). In contrast, surface diffusion is slow on a macroscopic scale, so that long range diffusion through a catalyst bed of impurity S deposited at the entrance will not be a practical problem.
5. Summary The main observations of this study are summarized as follows: (1) The sticking probability for elemental sulfur on Ni is - 1 over a wide range of substrate temperature (77-530 K). (2) The adsorption of sulfur causes an increase in work function; the maximum work function for a sulfur-dosed Ni field emission tip is measured to be - 4.7 to 5.05 eV in different experiments. (3) For fractional monolayers of S, diffusion proceeded rapidly above - 400 K with average activation energies ranging from 15 to 28 kcal/mol in different experiments. (4) For multilayers of sulfur, diffusion with significant motion of a sharp boundary does not occur. Upon heating, extensive bulk reaction occurs between S and Ni. (5) Sulfur desorbs from Ni above 1500 K. Acknowledgement This work was supported in part by the Maria Sklodowska-Curie tion.
Founda-
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References [l] [2] [3] [4]
R.A. Dalla Betta, A.G. Piken and M. Shelef, J. Catalysis 40 (1975) 173. C.H. Bartholomew, G.D. Weatherbee and G.A. Jarvi, J. Catalysis 60 (1979) 257. M.A. Vannice, Catalysis Rev. Sci. Eng. 14 (1976) 153. R.D. Kelley and D.W. Goodman, in: Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 427. [5] G. Greiner and D. Menzel, Surface Sci. 109 (1981) L510. [6] J. Benard, Catalysis Rev. 3 (1969) 93. [7] J.E. Demuth and T.N. Rhodin, Surface Sci. 45 (1974) 249. [S] P.H. Holloway and J.B. Hudson, Surface Sci. 33 (1972) 56. [9] W. Erley and H. Wagner, J. Catalysis 53 (1978) 287. [lo] M. Kiskinova and D.W. Goodman, Surface Sci. 108 (1981) 64. [ll] S. Johnson and R.J. Madix, Surface Sci. 108 (1981) 77. [ 121 R. Blaszczyszyn, M. Blaszczyszyn and R. Meclewski, Surface Sci. 5 1 (1975) 396. [13] A.J. Melmed, Appl. Phys. Letters 12 (1968) 100. [14] A.J. Melmed, J. Appl. Phys. 38 (1967) 1885. [ 151 See, for example, R.E. Honig, RCA Rev. 23 (1962) 567. [ 161 R. Gomer, Field Emission and Field Ionization (Cambridge, MA, 1961). [ 171 B. Meyer, in: Inorganic Sulfur Chemistry, Ed. G. Nickless (Elsevier, Amsterdam, 1968) p. 241.
Surface Science 13 1 ( 1983) 433-447 North-Holland Publishing Company
I~~~~IONS ADSORPTION,
OF SULFUR WITH NICKEL SURFACES: DIFFUSION AND DESORF’TION
M, BLASZCZYSZYN, Institute of Experimental
R. BLASZCZYSZYN
and R. Ml$LEWSKI
Physics, University of Wrociaw, 50205 Wrocfaw, Poland
and A.J. MELMED
and T.E. MADEY
Surface Science Division, National Bureau of Standards,
Washington, DC 20234, USA
Received 27 December 1982; accepted for publication 10 May 1983
The kinetics of adsorption, surface diffusion and thermal desorption of sulfur on Ni surfaces have been studied using field electron emission microscopy methods. The sticking probability for elemental sulfur sublimed onto a Ni specimen is approximately unity for Ni substrate temperatures from 77 to 530 K. The maximum average work function for - monolayer coverage of S ranged from 4.7 to 5.05 eV in different experiments; for fractional monolayer doses of S, surface diffusion was rapid at T > 400 K, with activation energies ranging from IS- to 28 kcal/mol. For multilayer adsorption of sulfur, diffusion occurs without motion of a sharp bounda~, and there is evidence of extensive surface reaction between S and Ni (emission from smalt “crystalhtes” is evident in the field emission patterns). Sulfur desorbs from Ni at temperatures above 1500 K.
1. Intruduction
Sulfur is an impurity which is known to affect the lifetime of nickel catalysts; for example, S is a common poison which causes catalyst deactivation [l-4] in an energy-related catalytic process, the catalytic methanation reaction (CO + 3 H, + CH, + H,O). However, sulfur is also known to be a promoter for another reaction, the formation of nickel carbonyl [S] (4 CO + Ni -+ Ni(CO),). For a detailed understanding of the roles of S in catalytic reactions over Ni surfaces, it is useful to characterize the kinetics of adsorption, diffusion and desorption of S on Ni surfaces. There is an abundance of literature concerned with various aspects of the sulfur-nickel interactions. The structures of S on Ni single crystals have been studied by LEED [6,7], and surface segregation has been characterized by AES (81. The influence of S on the kinetics of CO adsorption and desorption [9- 1 l] and on the rates of catalytic reactions [l-4] is well known. However, there is 0039-6028/83/0000-0000/$03.00
Q 1983 North-Holland
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of sulfur with nickel surfmes
little or no information available concerning the kinetics of adsorption and diffusion of elemental S over Ni surfaces, nor has the thermal desorption of S from Ni been reported. In the present work, we apply the techniques of field emission microscopy to the S/Ni system. We report measurements of electron emission properties, sticking probabilities, activation energies for surface diffusion. and surface structures which accompany the thermal desorption of S.
2. Experimental In general, conventional experimental methods [ 121 were used in the present work. Field electron microscope (FEM) observations and measurements were made in sealed-off glass tubes immersed in liquid nitrogen, providing an ambient pressure < 1.3 X 10d8 Pa (1 X lo-” Torr). The Ni field emitters were prepared by spotweiding electrolytically etched 99.995% pure Ni wire to W support loops, and the sulfur sources were six-times vacuum-distilled and sealed in glass ampoules. Cleaning the field emitter at the start of an experiment was achieved by lengthy periods of heating to temperatures > 1.500 K in vacuum. However, the
Fig. 1. Field electron shadow images of annealed Ni field emitters (T- 400-1000 nm. (a)-(f) Discussed in text.
1600 K). Tip diameters
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success rate was less than 508, due to catastrophic ruptures of the tips. This rather low success rate can be understood from the results of some earlier (unpublished) studies by one of the authors (A.J.M.) in which Ni field emitters were thermally cleaned and observed by field-electron shadow microscopy [ 131 at various stages during the cleaning process and after further heat treatments in vacuum. Some typical emitter profiles are shown in fig. 1. The tips having shapes shown in figs. lb, Id, le, and If are extremely weak and specimens with such shapes undoubtedly would catastrophically fail if subjected to the FEM field [ 14]. The shape shown in fig. la is the sturdiest of those shown, and was achieved by starting with a much wider cone-angle tip. Further heating caused this shape to change to shapes similar to those of figs. lb- 1f. Thus, we conclude that the success rate for achieving clean Ni field emitters is increased by (1) starting with the highest purity Ni, so that the total heating time for removing impurities is not excessive, and (2) polishing tips with cone angles as Large as possible, consistent with adequately sharp tips 1141. After bakeout, a high purity sulfur ampoule was broken allowing sulfur to be sublimed into a small electrically heatable glass cylinder. This source was located in direct line-of-sight of, and perpendicular to, the Ni tips and was operated in a continuously heated, constant temperature mode with reproducible dosing accomplished by magnetically opening and closing a shutter. Calibration of the qu~titative deposition rate was only approximate, however, principally due to uncertainties in the geometry of the effusion source and the source temperature. Thus, we assumed that monolayer coverage prevailed just prior to the appearance of discrete nuclei (see, for example, fig. 2 and the accompanying discussion). Although the dosing rate was not known absolutely, it was very constant and reproducible, and at a source temperature of 300 K, the composition of the incident S beam is known to be - 80% S, and - 20% s, [15]. Tip temperature was controlled by an electronic temperature controller with temperature rise times typically about one second from 77 K to the diffusion temperatures. Always, sulfur deposition was done with no field applied to the Ni tip, which was kept either at 77 K or at some elevated temperature. Likewise, no field was applied during the thermal spreading of deposited sulfur; all FEM observations were made with the emitter cooled to 77 K.
3. Results 3.1. Field emission patterns The field electron emission patterns obtained and recorded in this work served to provide a qualitative microscopic description of the uniformity or anisotropy of S adsorption, surface diffusion and desorption, as well as data
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Fig. 2. Field electron images depicting the equilibrated distribution of accumulated small doses of sulfur on Ni. (a) Initial clean (111) oriented Ni field emitter. Prior to each of the succeeding micrographs the Ni tip, at 77 K, was exposed to a sulfur dose for some time and then heated to 530 K to spread the additional sulfur. The tip was then cooled again for photography. The cunudative dose time (minutes) and the applied voltage (volts) for constant emission current for each micrograph is: (a) 0.6282; (b) 2,6404; (c) 8,6717; (d) 22,7214; (3) 30,7358; (f) 38,7512.
for the quantitative determination of surface diffusion activation energy. In addition, inspection of the FEM patterns provided evidence interpreted [ 161 to mean that the Ni surfaces were clean after high temperature heat treatment. The surface coverage resuling from the adsorption and spreading of low doses of sulfur showed no great anisotropy as a function of the nickel substrate crystallography. However, when the coverage reached a critical value, discussed below, heterogeneous nucleation occurred. The nucleation sites did not appear to be reproducibly related to special c~stallograp~c features of the substrate. A sequence of FEM patterns resulting from the deposition of a series of “small” (i.e., fractional monolayer) doses of S onto initially clean Ni at 77K, followed by spreading at 530 K of each dose is shown in fig. 2. Gradual changes in the patterns are apparent in figs. Za-2e, indicating approximately uniform, increasing coverage. Fig. 2f shows the onset of nucleation; discrete nuclei, seen as bright spots, are evident at the edges of the leftmost (111) plane, which was the (111) plane directly in view of the S source. Much larger doses of S could be spread only by heating the Ni tip to higher temperatures, as seen in the sequence of FEM patterns shown in figs. 3 and 4. At the lower spreading temperatures represented in fig. 3, the large accumu-
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Fig. 3. Field electron images depicting sulfur on Ni. Electron emission current to each succeeding micrograph sulfur some time. The tip temperature, sulfur 320 K. 2400 s, 6107 V; (c) 320 K, 5580 V; (f) 460 K, 60 s, 6219 V.
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the spreading (during specimen heating) of large doses of was 1 x IO-’ A for each image. (a) Clean Ni, 5759 V. Prior was impinged upon the heated specimen from below for deposition time and tip voltage (for photograph) are: (b) s, 6252 V; (d) 290 K, 7080 s, 6198 V; (e) 400 K, 120 s, 6188
lations of sulfur formed many clusters, rather than diffused to a significant extent, while at the higher spreading temperatures of figs. 4e and 4f large doses of sulfur indeed diffused over the surface, but the complicated appearance of the patterns indicates that extensive rearrangement of the original Ni surface occurred. 3.2. Work function of S on Ni Fig. 5 is a plot of the work function as a function of increasing S coverage during the adsorption of S on a (110) oriented Ni tip. The S was deposited onto one side of the tip while heating to 530 K. Following deposition of S, the S was spread by continuing to heat at 530 K to insure that the deposit was uniform; a sequence of FEM patterns similar to those of fig. 2 above accompanied the work function measurements of fig. 5. The work function I$ following each dose was determined using the Fowler-Nordheim equation, based on current (i) versus voltage (V) measurements at 77 K, and assuming
438
Fig. 4. Field from below for spreading 1 x IO-’ A, (e) 5957 V,
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electron micrographs depicting the spreading of a large dose of sulfur on Ni deposited the Ni tip. For each photograph, the voltage, emission current, tip temperature used and the spreading time are given: (a) 5342 V, 1 x 10e7 A, 1610 K, 10 s; (b) 5562 V, 77 K, 1200 s; (c) 7701 V, 1 x IO-’ A, 460 K, 15 s; (d) 5834 V, 1 x IO-’ A, 460 K, 60 s; 1 x IO-’ A, 570 K, 675 s; (f) 6063 V, 5 X 10-s A, 685 K, 15 s.
(pa (clean Ni) = 4.50 eV. Also shown in fig. 5 is the Fowler-Nordheim preexponential factor versus dose. Data similar to fig. 5 were measured for 3 different tips, two with (111) orientation and one with (110) orientation. In all cases, the work function was observed to rise monotonically with increasing S dose for low S coverages. However, there was some difficulty in reproducing the maximum value of work function (the saturation value of + ranged from 4.7 to 5.05 eV in different experiments). The lack of repr~ucibility in work function measurements at higher S doses was clearly related to the lack of uniformity in emission due to the formation of small crystallites which were visible in the FEM patterns (see, for example, fig. 2f). Thus, for low sulfur coverages, the sulfur diffuses in a uniform fashion over the entire tip, producing a symmetrical pattern of electron emission. For high sulfur coverages, a surface reaction on the “dosed” side of the tip leads to the formation of “crystallites”, perhaps due to a surface reaction to form Ni sulfide. For the subsequent discussion, we shall assume somewhat arbitrarily that
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. 5
_* .
. . ..* .
InA
. 4
.
.O.O
8
.
0 0 0
4.9 0 +
600K,
0 cl
0
(eV)
530K
o
00 0
4.8 0
0 0
4.6 0
4.5
I
I
I
20
30
40
I
IO
EVAPORATION TIME t (min) Fig. 5. Pre-exponent (In A) and work function ( $I), determined using Fowler-Nordheim equation, for sulfur evaporated onto a Ni field emitter. Following each deposition of S and spreading at 530 K, the tip was cooled to 77 K for the measurements.
the sulfur coverage immediately below which the crystallites were observed to form corresponds to “monolayer” coverage of S, 0, = 1. We note that for different S sources, the deposition time necessary to reach 6s = 1 varied from - 10 to - 40 min. For each source and Ni tip, however, 8, = 1 occurred at a reproducible deposition time. 3.3. Sticking probability of S / Ni Fig. 6 contains plots of 9 versus S dose for a (111) oriented Ni tip. The three sets of data correspond to deposition of S at 77, 380 and 530 K respectively; in each case, the S was spread over the tip by heating to 530 K before measuring the i versus Vcharacteristic at 77 K. The data at 530 K correspond to the FEM
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CRYSTqLLITES
4.60
4.50 EVAPORATIONTIME
t(min)
Fig. 6. Approximate work function (determined using + = 4,,( V/Vo)2/3) versus sulfur deposition time for Ni tips held at three different temperatures, 77 K (0), 380 K (+) and 530 K (A), indicating that the sticking probability of sulfur is independent of temperature.
patterns shown in fig. 2. For technical reasons, the work function fig. 6 was determined from the voltage necessary to maintain emission current via the relation
C#Ishown in a constant
(1) The fact that the three sets of data are completely superimposable for tip temperatures ranging from 77 to 530 K indicates that the sticking probability S for S on Ni is independent of temperature over the entire range of S coverage up to 8, = 1, where crystallites are observed in the FEM patterns (cf. fig. 2). Although we cannot determine the value of sticking probability in an unequivocal fashion from the data of fig. 6, there is good reason to believe that S = 1 over the entire range of T and 8. As mentioned previously, the composition of the incident S beam is - 80% S, and - 20% S, for a source temperature of 300 K [ 151. Since the sticking probability for these species is almost certainly
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unity at 77 K, the constancy of the + versus dose data at higher T implies the same value of sticking probability in the other experiments shown in fig. 6. 3.4. Diffusion of S over Ni To study the diffusion of S over Ni, three types of experiments were performed: (1) diffusion of small doses (6, < 1) deposited at 77 K, (2) diffusion of large doses (0, > 1) deposited at 77 K, and (3) diffusion during deposition at diffusion temperatures. The resulting behavior of the deposited S was complex, and in some respects unusual, compared to other adsorption systems previously studied. The results of these different experiments will be discussed separately. 3.4. I. Small doses, 8, < I
Following the deposition of a small dose of S (13, < 1) onto one side of a Ni field emission tip, subsequent heating to T z 400 K resulted in diffusion of S into the initially-clean regions of the tip. Diffusion in this fractional monolayer
4.66 436K 416 K
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+(eV)
4.62 4.61
4.60
4.56
4.57 0
20
40
60
60
100
120
140
160
160
200
t (?.I
Fig. 7. Approximate work function, determined using $J = c#z~( V/Vo)2/3, for Ni tips at several temperatures as a function of sulfur spreading time. The dashed line marks the diffusion endpoint which was used to construct the Arrhenius plot Q4 of fig. 8. The initial point in each case was at
t = 0.
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Fig. 8. Arrhenius plots for diffusion of S over four different Ni tips. The reciprocal diffusion times (I/t) were determined as shown in fig. 7. The field emission patterns corresponding to the four data sets are shown in fig. 9.
coverage regime proceeded without the formation of a sharp boundary. Thus, it was not possible to determine the activation energies for surface diffusion by measuring the rate of advance of a boundary between the S-covered and the clean surface. Instead, the average work function C$of the tip, calculated using eq. (1) and corresponding to different stages of diffusion, was measured after cooling to 77 K following each diffusion interval. Fig. 7 shows the values of 9 as a function of diffusion time t, for different diffusion temperatures. Note that the values of $I do not correspond to true average work functions, since the adlayers were not equilibrated. However, they mark successive stages of diffusion. The data of fig. 7, as well as similar data for other Ni tips measured in the range 400 to 500 K, were used to determine activation energies as follows. In fig. 7, a diffusion endpoint was chosen as indicated by the horizontal dashed line. The times t necessary to reach the endpoint were measured, and plotted in Arrhenius form in fig. 8 [In l/t versus l/T]. The line labelled Q4 corresponds to the data of fig. 7. The activation energies for diffusion of S over Ni were measured on three additional Ni tips, and the data are also shown in fig. 8. The activation energies Q determined from these four plots range from 15 + 2 to 27.7 & 2 kcal/mol; differences in Q may be explained by differences in the initial S coverage as well as in tip geometry. Fig. 9 shows field emission patterns which correspond to the Arrhenius plots of fig. 8. The left column shows patterns from the clean tip before sulfur
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a
b
d Fig. 9. Field emission patterns corresponding to the data of fig. 8. For each corresponds to the clean tip before deposition of S, and the right pattern following deposition and diffusion of S. In all cases, deposition of S was (bottom of pattern). The activation energies for S diffusion for each endpoint kcal/mol; (b) Q2 = IS.2 kcal/mol; (c) Q, = 20.0 kcal/mol; (d) Q4 = 27.7
row, the left pattern shows the endpoint from below the tip were: (a) Q, = 16.7 kcal/mol.
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deposition from below, and the right column shows patterns of the diffusion endpoints which were used to construct the Arrhenius plots. Note that different tip orientations were obtained in each case. 3.4.2. Large doses, 6, > I In most cases of multilayer adsorption, surface diffusion occurs via migration of the topmost layer over the chemisorbed first layer, with precipitation at the edge of the deposit and subsequent advance of the boundary. This process has been likened to an “unrolling of a carpet”, and the boundary of the deposit remains sharp as it advances [16]. This does not appear to occur for large doses of S on Ni. Upon heating after deposition (see sequence of fig. 3), a sharp boundary appears near the deposit edge, but does not advance with further heating or following further deposition of additional S. As the temperature is increased to T z 320 K, a boundary-free diffusion into the initially-clean regions occurs, and nucleation of brightly-emitting protuberances (clusters of Ni sulfide?) occurs in the originally-dosed region. For large doses, the tip must be heated to T > 650 K to achieve equilibration of the S coverage. 3.4.3. Large doses while heating The deposit behaved in a manner somewhat similar to the behavior in the second type experiments above, with two notable differences. With the tip kept at 320 K during large dosing, a very slow surface diffusion process was observed to occur in the (111) vicinals. The effect was entirely absent unless dosing was done during tip heating. Also, a sharp moving boundary was seen to very slowly advance during dosing at 320 K. 3.5. Thermal desorption of sulfur At the conclusion of an adsorption or surface diffusion experiment, it was often possible to successfully reclean the Ni field emitter by heating to temperatures > 1500 K in vacuum. A typical sequence of field emission patterns recorded at various stages of the recleaning process is shown in fig. 10. Note that initially the pattern orientation is such that a (111) plane is near the projection center. However, the final clean surface has an orientation with a (001) plane near the center. This is clear evidence that the removal of sulfur from the Ni by heating is accompanied by extensive removal of Ni as well. Thus, in the sequence documented in fig. IO, the Ni crystallite which originally comprised the tip apex was entirely evaporated. Also, we conclude from the occurrence of different field emission patterns during the sequence that extensive reconstruction of the Ni surface is caused by high temperature interaction with sulfur. After achieving a field emission pattern which appeared clean, such as shown in fig. IOf, the Ni tips were heated for various lengths of time, up to five
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Fig. 10. Field electron micrographs characterizing the thermal desorption of S/Ni at temperatures z 1500 K. A large deposition of sulfur was progressively removed prior to each of the micrographs from (a) to (f).
minutes, at various temperatures below the cleaning temperature as a test for the occurrence of sulfur out-diffusion from the specimen bulk or some other surface rearrangements which would indicate the presence of residual sulfur. Such tests always gave negative results, demonstrating that all detectable sulfur was indeed removed by the thermal cleaning process.
4. Discussion The surface diffusion of S over Ni is notable for the absence of extensive sharp boundary diffusion [16] for sulfur doses in excess of monolayer (“large doses”), as well as for fractional monolayer doses. These results may be interpreted as follows. Sulfur sublimed from the doser is predominantly in the S, polymeric form. When S, (and other polymeric forms of S) adsorb on clean Ni, these species dissociate and diffuse as atoms upon heating to T z 400 K. However. for large doses, S, adsorbed in the outer layers above the Ni is in contact with additional S, or a sulfur/nickel complex. Under these conditions dissociation is less probable, and it appears likely that the S, remains intact, or forms another allotrope of molecular sulfur [17]. As the surface is heated to
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T-e 350 K, the absence of sharp boundary motion over the surface suggests that the diffusion energy for a sulfur polymer is 2 the desorption energy for that polymer, i.e., species such as S, desorb at a faster rate than they are able to diffuse. Another possibility is that the bulk reaction between S and Ni to form Ni,S, may compete with surface diffusion. During dosing with the Ni emitter at 320 K, a sharp boundary was observed to advance very slowly (cf. section 3.4.3). At this temperature, the surface lifetime of the steadily-arriving S, is apparently long enough for a small fraction of these species to diffuse over the edge of the S/Ni layer and dissociate on the clean Ni, thus extending the boundary at a slow rate. Finally, we note that surface diffusion is rapid (>, 100 A/s) on a microscopic scale in the temperature range > 500 K which is the typical temperature range for hydrogenation of CO to form CH, over Ni catalysts [l-4]. Sulfur poisoning can occur due to impurity S deposited onto high area Ni catalysts from the carrier gas (e.g., from H,S impurities) or by diffusion of S from the bulk. Under reaction conditions (> 500 K), S atoms can readily diffuse to equilibrium sites, so that surface diffusion will not be rate limiting bn the poisoning reaction of typical high area catalysts (particle sizes - 60 A in radius). In contrast, surface diffusion is slow on a macroscopic scale, so that long range diffusion through a catalyst bed of impurity S deposited at the entrance will not be a practical problem.
5. Summary The main observations of this study are summarized as follows: (1) The sticking probability for elemental sulfur on Ni is - 1 over a wide range of substrate temperature (77-530 K). (2) The adsorption of sulfur causes an increase in work function; the maximum work function for a sulfur-dosed Ni field emission tip is measured to be - 4.7 to 5.05 eV in different experiments. (3) For fractional monolayers of S, diffusion proceeded rapidly above - 400 K with average activation energies ranging from 15 to 28 kcal/mol in different experiments. (4) For multilayers of sulfur, diffusion with significant motion of a sharp boundary does not occur. Upon heating, extensive bulk reaction occurs between S and Ni. (5) Sulfur desorbs from Ni above 1500 K. Acknowledgement This work was supported in part by the Maria Sklodowska-Curie tion.
Founda-
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References [l] [2] [3] [4]
R.A. Dalla Betta, A.G. Piken and M. Shelef, J. Catalysis 40 (1975) 173. C.H. Bartholomew, G.D. Weatherbee and G.A. Jarvi, J. Catalysis 60 (1979) 257. M.A. Vannice, Catalysis Rev. Sci. Eng. 14 (1976) 153. R.D. Kelley and D.W. Goodman, in: Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 427. [5] G. Greiner and D. Menzel, Surface Sci. 109 (1981) L510. [6] J. Benard, Catalysis Rev. 3 (1969) 93. [7] J.E. Demuth and T.N. Rhodin, Surface Sci. 45 (1974) 249. [S] P.H. Holloway and J.B. Hudson, Surface Sci. 33 (1972) 56. [9] W. Erley and H. Wagner, J. Catalysis 53 (1978) 287. [lo] M. Kiskinova and D.W. Goodman, Surface Sci. 108 (1981) 64. [ll] S. Johnson and R.J. Madix, Surface Sci. 108 (1981) 77. [ 121 R. Blaszczyszyn, M. Blaszczyszyn and R. Meclewski, Surface Sci. 5 1 (1975) 396. [13] A.J. Melmed, Appl. Phys. Letters 12 (1968) 100. [14] A.J. Melmed, J. Appl. Phys. 38 (1967) 1885. [ 151 See, for example, R.E. Honig, RCA Rev. 23 (1962) 567. [ 161 R. Gomer, Field Emission and Field Ionization (Cambridge, MA, 1961). [ 171 B. Meyer, in: Inorganic Sulfur Chemistry, Ed. G. Nickless (Elsevier, Amsterdam, 1968) p. 241.