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The adsorption of hydrogen and hydrogen sulphide on tungsten sulphide; isotherm and neutron scattering studies
49
Applied Catalysis, l(l981) Elsevier Scientific
49-58 Publishing Company,
Am&dam
- Printed in Belgium
THE ADSORPTION OF HYDROGEN AND HYDROGEN SULPHIDE ON TUNGSTEN SULPHIDE; ISOTHERM AND NEUTRON SCATTERING STUDIES C.J. WRIGHT', D.FRASER*, R.B. MOYES* and P.B. WELLS' 'A.E.R.E. Harwell, Didcot, Oxon. OX11 ORA, United Kingdom. *Department of Chemistry, University of Hull, Hull, Yorkshire, United Kingdom. (Received 7 January 1981, Accepted 27 January 1981) ABSTRACT Adsorption isotherms for hydrogen and hydrogen sulphide on a WS2 surface have been measured at 400°C at pressures up to 1 at. Inelastic neutron scattering spectra have been recorded for the materials produced. Both hydrogen and hydrogen sulphide adsorb dissociatively to produce S-H bonds. Hydrogen sulphide adsorption is smaller than that of hydrogen and it is suggested that this is because dissociation leads also to sulphidation of the surface vacancies at which the adsorption occurs. In both cases the hydrogen atoms produced as a consequence of the dissociation are able to diffuse over the surface to form bonds to available sulphur atoms. INTRODUCTION There has been a recent resurgence of interest in the properties of hydrodesulphurization (H.D.S.) catalysts and much new information about the origins of their activity has been discovered. Few attempts have been made, however, to observe directly the species involved in reactions at their surfaces. We have accordingly investigated the vibration spectra of adsorbates resulting from reactions between unsupported and unpromoted H.D.S. catalysts and hydrogen or hydrogen sulphide. Our first results for molybdenum sulphide (1) and these for tungsten sulphide concern reactions at pressures up to one atmosphere but, following our belief in the need to examine catalyst behaviour under in situ conditions, these papers represent only the beginning of an attempt at full characterizationof the surface at various pressures and temperatures. The published isotherms and isobars for adsorption of hydrogen on tungsten sulphide are intriguing, partly because of the diversity of the observed phenomena and partly because of the incompletenessof some of the work. Donath (2) showed that the extent of isothermal adsorption at 100°C was influenced by the pretreatment and enhanced by
0166~9834/81/0000-0000/$02.50
0 1981 Elsevier Scientific
Publishing Company
50
increasing temperature. However, the work of Friz (3) suggested that Donath's measurements were not made under equilibrium conditions and showed that the volume of hydrogen adsorbed at constant pressure by tungsten sulphide rises progressivelywith temperature up to 25O'C. Friz concluded that hydrogen entered the tungsten sulphide lattice at high temperatureswhereas at low temperatures it was adsorbed at the crystallite surface. Maximum hydrogen uptake corresponded to the stoichiometry Ho oo84WS2. Decrue and Susz (4) observed similar behaviour from samples not at
.
equilibrium and obtained, at lll°C and one atmosphere pressure, a stoichiometry Ho 04WS2. Finally, Gonikberg and Levitskii (5), who measured isotherms between 10 and 97'atmospheres,found the relationship between the uptake, V, and partial pressure, P, to be
v=
I3 +aP$5
Their data more closely fit the relationship V = e t aPH 2 but, nonetheless, they show that uptakes at high pressure do not extrapolate to zero uptake at zero pressure, implying that the heat of adsorption decreases with coverage. At 400°C the stoichiometriesof their products ranged from Ho ogWS2 at 10 at to Ho 16WS2 at 97 at. Since the surface area of their material was not quoted it is not possible to convert these figures to equivalent surface coverages. In sumnary, published evidence has shown that WS2 adsorbs hydrogen in quantities which increase with pressure, but unlike the evidence for MoS2, it does not positively support hydrogen dissolution in tungsten sulphide despite suggestions that this might happen. For the hydrogen MoS2 system, (1,6) work at similar temperatures and pressures showed the existence of dissociative sorption of hydrogen to produce hydrogen-sulphurbonds and uptakes apparently much greater than those which could be accommodated at the surface of the material accessible to nitrogen. It is of interest to compare the behaviour of molybdenum sulphide and tungsten sulphide in the presence of hydrogen and this paper examines the adsorption isotherms and the vibration spectra of hydrogen and hydrogen sulphide on tungsten sulphide at pressures up to one atmosphere. EXPERIMENTAL Tungsten sulphide was made by decomposition of ammonium thiotungstate,a method similar to that described by Decrue and Susz (4). Ammonium thiotungstatewas prepared by dissolving tungsten oxide in the minimum quantity of 0.880 ammonia solution at 80°C and passing hydrogen sulphide through the solution for four hours. The solution was cooled overnight and precipitation of the crystals was aided by the addition of ethanol. The precipitate was dried overnight. The material was transferred to a reaction vessel and treated with hydrogen for 18 h at 400°C to convert the material to tungsten sulphide. After conversion, the reaction vessel was sealed and allowed to cool before connection to a high vacuum apparatus. The sample was pumped to a pressure
51 of less than 10-5 Torr and then heated to 400°C when it was again pumped until the pressure fell below 10m5 Torr. At this temperature hydrogen adsorption was measured using a conventional volumetric technique and a pressure transducer to measure the inlet and equilibrium pressures. At no time was the sample allowed to come into contact with air. Gravimetric analysis by Rooney Laboratories Limited found the composition of this tungsten sulphide to be WS _09. Its crystallite size estimated fran an X-ray diffraction pattern was 40 x 18 a (1 x h). For the neutron experiments the tungsten sulphide was sealed in aluminium containers and then examined on the beryllium filter spectrometer IN18 at the I.L.L. Grenoble (7) and time-of-flight spectrometer 4H5 (8) at A.E.R.E. Harwell. The beryllium filter and time-of-flight spectra were recorded at 80 and 293 K respectively. RESULTS The adsorption isotherm for hydrogen uptake by tungsten sulphide at 400°C is shown Figure 1.
,_
H, I WS,
I-
H,SI
$.0375f-
WS,
P
m 8
In
9 m
3 0.02 -
0
I 200
I LOO
o
ADSORPTION
.
DESDRPTION I 600
I 600
BRANCH
I 1000
EOUILIBRIUM
FIGURE 1
g
BRANCH
g J
3 9
PRESSURE
1
0
,
,
200
LOO
q
DE;RPTI;
600
BRA;,
600
1000
ITDRR)
Adsorption isotherms for hydrogen and hydrogen sulphide uptake by WSg.
Equilibrium, as assessed from the cessation of gas uptake, was achieved at all pressures within five minutes. A further demonstration that a good approximation to equilibrium was attained was provided by the similarity of the adsorption and desorption isotherms. The experimental points could fit Langmuir models, assuming either molecular or atomic adsorption. The only point of difference between these two fits was that the dissociative model required a value for the monolayer uptake which was significantly greater than that suggested by the limiting value of the pressure -
52
volume curve. This behaviour has been noticed before for dissociative chemisorption (9). The limiting value of the uptake at 673 K was equivalent to a stoichiometry of HO 04BWS2. Further uptake was recorded on cooling the sample so that the stoichiometry WS at 423 K. Below this temperature uptake was too slow for the approach became H0.056 2 to equilibrium to be measured. The volume of nitrogen adsorbed by this sample at p/p, = 0.3 was determined with a flow technique, using a one point B.E.T. model, to be 20 m* g-' so that the limiting uptake of hydrogen at 673 K was equivalent to about 0.7 of a monolayer coverage. The adsorption isotherm for hydrogen sulphide uptake by tungsten sulphide at 400°C is shown in Figure 1. The rate of attainment of equilibrium was much slower than for the case of hydrogen uptake and the uptake at saturation coverage was considerably less. The maximum uptake was equivalent to WS2(H2S)o.065.Again, the shape of the isotherm did not discriminate between molecular and atomic adsorption. The adsorption was not truly reversible because the desorbed gas was not H2S but hydrogen. Samples prepared by our standard method (i.e. evacuated at 10m5 Torr at 4OO'C) all contained residual hydrogen. A determination of the quantity of this residual hydrogen, by following its exchange with deuterium using a mass spectrometer, showed that the residual quantity was equivalent to Ho oo7WS2.
NEUTRON
FIGURE 2
ENERGY CM“
TRANSFER
Inelastic neutron scattering spectrum of hydrogen adsorbed by WS2.
The inelastic neutron scattering spectrum of hydrogen adsor$d by tungsten sulphide (Figure 2) shows excitations at 694, 1380 and 2074 cm
and a broader band
of scattering at higher energies partly resolved into peaks at 2470 and 2679 cm-'. This broad band of scattering, at energies neither equal or close to an integral
53
multiple of 694 cm", must be due to another 2ndamental such as an H-S stretching vibration. Such fundamentals occur at 2500 cm
for thiols (10). A self-consistent
-1 assignment of the spectrum is that it shows hydrogen vibrations. parallel at 694 cm -1 and perpendicular at 2470 or 2679 cm , to the WS2 surface. The scattering at 1380 -1 and 2074 cm-' is due to the first and second harmonics of the scattering at 694 cm . The values of the two highest energy frequencies are similar to those that have been observed for hydrogen-treatedMoS2 by infrared spectroscopy near to 2500 and 2640 cm-' (11). This assignment is confirmed by an analysis of the intensities of the four peaks in the spectrum. Their theoretical intensities can be calculated from the product of two terms, the first representing the intensity of scattering from the hydrogen behaving as a simple harmonic oscillator and the second representing the influence on the intensity of the vibrations of the lattice surrounding the hydrogen I = IS.H.0. x exp g
1
[ The first term, IS.H.O_, can be readily calculated from a knowledge of the vibration frequencies of the hydrogen atoms in the sample. The second term is unknown but, since M and uL are constants for any lattice, the logarithm of the ratio of the observed to the predicted intensities for the four excitations should be directly proportional to
Q2,
the momentum transfer squared, at each excitation, when the correct vibration
frequencies have been chosen to calculate IS. H , o . (12.13). Figure three shows values of this ratio plotted against Q2 for three of the different structural models represented in Figure 4. The first is one in which the hydrogen atom lies above the layer of sulphur atoms to which it is multiply bonded; the second is one in which the hydrogen atom lies in the plane parallel to the basal plane surrounded by sulphur or metal neighbours and the third is one in which the hydrogen atom lies above a single sulphur atom to which it is bonded. The vibration frequencies corresponding to these models, chosen to be consistent both with our experimental observations and the known -1 vibration frequencies of similar compounds (10,12), were for model one uA = 1380 cm , -1 -1 = 694 an-', for model two uA = 694 cm , uE = 1380 cm , and for model three WE q 694 cm", uA = 2500 cm-'. uA and I+ are respectively the singly degenerate WE vibration perpendicular to the basal planes and the doubly degenerate vibration parallel to the basal planes. In model one the stretching frequency for the bridged hydrogen atom has been chosen to be 50 % of the known unbridged S-H stretching frequency, on the ground that a similar reduction in frequency is found for metalhydrogen stretching vibrations (14). These three models are, on chemical grounds, the most likely to describe the hydrogen-tungsten-sulphideinteraction. Model three best fits the data and the error bars of Figure 3 are calculated from the estimated experimental errors in the intensity determination.A fourth model, also worthy of attention, is one in which the adsorbed hydrogen atoms occupy sulphur vacancy sites
54
and so interact, through single bonds, with the tungsten atoms. In this case, metalhydrogen vibrations would be,observed in the neutron spectra which would have energies -1 for those of bending and between 600 and 850 cm-' and between 1800 and 2200 cm stretching character respectively (14). With our present experimental technique we are not able to eliminate such a model on spectroscopic grounds, since it closely resembles model three. Instead we propose that this model can be eliminated because -1 and for the reasons of stoichiometry that we of the scattering near to 2500 cm enlarge upon in the discussion.
7-o -
f IWI
s...,
IWI
5,‘.
‘~.I+ ‘.
S(WI
zao: * :: . ?70-
6S
S____S
7-
I 20
40
60
60
lHOMENTUM
FIGURE 3
100 TRANSFERI’
120
140
160
I 160
A’-’
Ratios of the predicted to the observed scattering intensity for the peaks
in the H/WS2 spectrum, ploted against Q*. Although these results all confirm the presence of single H-S bonds, there is a number of points concerning the high frequency bands in the spectrum which are not completely described by this model. The "observed" intensity, used to calculate the point for the highest energy band in Figure 3. is an integrated value for the whole band which therefore masks the fact that the relative intensities of the scattering
55
at 2470 and 2679 cm"
are not those that would be predicted. Our simple model
predicts intensities of approximately 2:l for excitations corresponding to the fourth harmonic of the parallel excitation and the fundamental of the perpendicular excitation. This compares with the equal intensities observed at these two frequencies and a value for the fourth harmonic energy based on the energies of the lower harmonics -1 of 2765 cm . The spectrum recorded after adsorption of hydrogen sulphide on the tungsten sulphide surface is similar to that recorded for adsorbed hydrogen, except for an -1 additional shoulder in the low frequency region of the spectrum at 613 cm .
spy*
I
s,Q
1
/ Y -
O’
a
0
0
0
\!
....
,....w
“Y...w
FIGURE
coverage
56
occurs at a stoichiometry (H2S)0.0065WS2.If
it is assumed that both hydrogen and
hydrogen sulphide dissociate into their constituent atoms, then the number of surface sites that can be occupied by the hydrogen sulphide fragments is, at most, 40 % of the number of sites occupied by the dissociated hydrogen molecules. In consequence, not only must the sites occupied by the different species be of different types but, if the hydrogen sulphide adsorbs at sulphur vacancies as seems likely from poisoning studies (15), the hydrogen atoms cannot also be adsorbing at those sites. This is the additional reason for rejecting model four, to which reference was made earlier.
z E k
0
500
NEUTRON
FIGURE 5
750
1000
ENERGY TRANSFER cm-l
Inelastic neutron scattering spectrum of hydrogen sulphide adsorbed by WS2.
The inelastic scattering spectrum for adsorbed hydrogen sulphide is similar to that for adsorbed hydrogen, showing that the hydrogen sulphide must dissociate at the WS2 surface. For each original H2S molecule two H-S bonds are formed to different sulphur atoms. This finding is of considerable interest since it has been known for some time that hydrogen sulphide poisons the activity of sulphide catalysts for hydrodesulphurization,hydrogenolysis,hydrogen-deuteriumexchange and hydrogenation reactions. Voorhoeve and Stuiver (15) showed that in benzene and cyclohexane hydrogenationsan increase in the hydrogen sulphide partial pressure influenced the
57
reaction kinetics through the pre-exponentialfactor rather than the activation energy and, from this, they concluded that hydrogen sulphide removes active sites which must be sulphur deficient metal centres. The reaction proceeded, so they speculated, to produce sulphur and hydrogen gas. In the light of our spectroscopic data this mechanism can now be modified so that it reads H2S + S-- +oS + 2SHThe hydrogen sulphide adsorbs at a vacancy site, where it dissociates to form an HS group and a hydrogen atom. The HS group fills the vacancy and deactivates it, whilst the hydrogen atom diffuses over the surface to form an H-S bond with another available sulphur atom. Hydrogen molecules could also dissociate at these vacancies, or perhaps at the coordinatively unsaturated edge sulphur atoms, but in this case the hydrogen atoms diffuse away from their dissociation centre leaving the vacancies unaltered. This mechanism explains differences in the limiting uptakes of hydrogen sulphide and hydrogen adsorption. In the former adsorption terminates when all the vacancies have been occupied whereas, in the latter adsorption could continue until all the sulphur atoms have been converted to SH. It can be shown that this mechanism for H2S adsorption is quite consistent with the surface area of the WS2 sample. Assuming particles of hexagonal colunnar form, with each edge of the particle having length L. the number of sulphur vacancies in the surfaces perpendicular to the basal planes will be equal to the number of H2S molecules adsorbed, i.e. 0.0065 per tungsten atom, if 20 % of the sulphur atoms in the edge planes were absent. Although calculations for other particle shapes will require different fractions of the sulphur sites to be vacant to produce the same number of vacancies per tungsten atom, it is clear that the ratio of vacancies to occupied sites at the surface, required to explain the uptake is reasonable.
It
is interesting to contrast this value for the hydrogen sulphide uptake, 0.0065
mol/mol, with the uptakes of oxygen by molybdenum sulphide recently determined by Tauster and coworkers (16). Their samples of molybdenum sulphide, numbers thirteen and fifteen, prepared by a similar method to our preparation of tungsten sulphide, had oxygen uptakes of 0.005 and 0.006 mol/mol. Despite the different surface areas of these samples (MoS2 2.3~10~. 3.7x103. WS2 7.4x103 m2 mol")
it is tempting to suggest
that oxygen and hydrogen sulphide chemisorption is occuring at the same sites and therefore the magnitude of the oxygen chemisorption on these materials is a measure of the number of the sulphur vacancies, rather than the total area of the edge planes. Since Tauster et al (16) find a clear correlation between oxygen chemisorption and H.D.S. activity (for dibenzothiophene),these results are consistent with the suggestion that hydrodesulphurizationoccurs at sulphur vacancies in an analogous way to the mechanism of hydrogenation over sulphide catalysts proposed by Voorhoeve and Stuiver (15).
58
REFERENCES C.J. Wright, C. Sampson, D. Fraser, R.8. Moyes, P.8. Wells and C. Riekel, J. Chem. Sot. Faraday 1, 76 (1980) 1588. E.E. Donath, Advances in Catalysis Vol. 8, p 245 (1956), Academic Press, New York. H. Friz, Zeit. Fur. Electrochimie, 54 (1950) 538. J. Decrue and 8. Susz, Helv. Chim. Acta., 39 (1956) 619. M.G. Gonikberg and 1.1. Levitskii, Izvest. Akad. Nauk. S.S.S.R., Otdel. Khim. Nauk.. (19601 1170. 6 E.H.M: Badger, R.H. Griffith and W.8.S. Newling, Proc. Roy. Sot., (1949) 184. 7 Neutron Beam Facilities at the High Flux Reactor, Institut Laue-Langevin,Grenoble, France (1974). 8 A.H. Baston and D.H.C. Harris, Neutron Beam Instruments at Harwell, A.E.R.E. R9278. H.M.S.O. (1978). A. Hansen and H.L. Gruber, J. Catal., 20 (1971) 97. 1: L.J. Bellamy, The infrared spectra of complex molecules, Methuen and Co. Ltd., London, p 351, (1958). P. Ratnasamy and J.J. Fripiat, Trans. Faraday Sot.. 66 (1970) 2897. ;: CO;iekel, H.G. Reznik, R. Schollhorn and C.J. Wright, J. Chem. Phys., 70 (1979) C.J.'Wright, J. Chem. Sot. Faraday Trans II, 73 (1977) 1497. U.A. Jayasooriya,M.A. Chesters, M.W. Howard, S.F.A. Kettle, D.B. Powell and N. Sheppard, Surface Sci., 93 (1980) 526. 15 R.J.H. Voorhoeve and J.C.M. Stuiver, J. Catal., 23 (1971) 243. 16 S.J. Tauster, T.A. Pecoraro and R.R. Chianelli, J. Catal., 63 (1980) 515. ;:
Applied Catalysis, l(l981) Elsevier Scientific
49-58 Publishing Company,
Am&dam
- Printed in Belgium
THE ADSORPTION OF HYDROGEN AND HYDROGEN SULPHIDE ON TUNGSTEN SULPHIDE; ISOTHERM AND NEUTRON SCATTERING STUDIES C.J. WRIGHT', D.FRASER*, R.B. MOYES* and P.B. WELLS' 'A.E.R.E. Harwell, Didcot, Oxon. OX11 ORA, United Kingdom. *Department of Chemistry, University of Hull, Hull, Yorkshire, United Kingdom. (Received 7 January 1981, Accepted 27 January 1981) ABSTRACT Adsorption isotherms for hydrogen and hydrogen sulphide on a WS2 surface have been measured at 400°C at pressures up to 1 at. Inelastic neutron scattering spectra have been recorded for the materials produced. Both hydrogen and hydrogen sulphide adsorb dissociatively to produce S-H bonds. Hydrogen sulphide adsorption is smaller than that of hydrogen and it is suggested that this is because dissociation leads also to sulphidation of the surface vacancies at which the adsorption occurs. In both cases the hydrogen atoms produced as a consequence of the dissociation are able to diffuse over the surface to form bonds to available sulphur atoms. INTRODUCTION There has been a recent resurgence of interest in the properties of hydrodesulphurization (H.D.S.) catalysts and much new information about the origins of their activity has been discovered. Few attempts have been made, however, to observe directly the species involved in reactions at their surfaces. We have accordingly investigated the vibration spectra of adsorbates resulting from reactions between unsupported and unpromoted H.D.S. catalysts and hydrogen or hydrogen sulphide. Our first results for molybdenum sulphide (1) and these for tungsten sulphide concern reactions at pressures up to one atmosphere but, following our belief in the need to examine catalyst behaviour under in situ conditions, these papers represent only the beginning of an attempt at full characterizationof the surface at various pressures and temperatures. The published isotherms and isobars for adsorption of hydrogen on tungsten sulphide are intriguing, partly because of the diversity of the observed phenomena and partly because of the incompletenessof some of the work. Donath (2) showed that the extent of isothermal adsorption at 100°C was influenced by the pretreatment and enhanced by
0166~9834/81/0000-0000/$02.50
0 1981 Elsevier Scientific
Publishing Company
50
increasing temperature. However, the work of Friz (3) suggested that Donath's measurements were not made under equilibrium conditions and showed that the volume of hydrogen adsorbed at constant pressure by tungsten sulphide rises progressivelywith temperature up to 25O'C. Friz concluded that hydrogen entered the tungsten sulphide lattice at high temperatureswhereas at low temperatures it was adsorbed at the crystallite surface. Maximum hydrogen uptake corresponded to the stoichiometry Ho oo84WS2. Decrue and Susz (4) observed similar behaviour from samples not at
.
equilibrium and obtained, at lll°C and one atmosphere pressure, a stoichiometry Ho 04WS2. Finally, Gonikberg and Levitskii (5), who measured isotherms between 10 and 97'atmospheres,found the relationship between the uptake, V, and partial pressure, P, to be
v=
I3 +aP$5
Their data more closely fit the relationship V = e t aPH 2 but, nonetheless, they show that uptakes at high pressure do not extrapolate to zero uptake at zero pressure, implying that the heat of adsorption decreases with coverage. At 400°C the stoichiometriesof their products ranged from Ho ogWS2 at 10 at to Ho 16WS2 at 97 at. Since the surface area of their material was not quoted it is not possible to convert these figures to equivalent surface coverages. In sumnary, published evidence has shown that WS2 adsorbs hydrogen in quantities which increase with pressure, but unlike the evidence for MoS2, it does not positively support hydrogen dissolution in tungsten sulphide despite suggestions that this might happen. For the hydrogen MoS2 system, (1,6) work at similar temperatures and pressures showed the existence of dissociative sorption of hydrogen to produce hydrogen-sulphurbonds and uptakes apparently much greater than those which could be accommodated at the surface of the material accessible to nitrogen. It is of interest to compare the behaviour of molybdenum sulphide and tungsten sulphide in the presence of hydrogen and this paper examines the adsorption isotherms and the vibration spectra of hydrogen and hydrogen sulphide on tungsten sulphide at pressures up to one atmosphere. EXPERIMENTAL Tungsten sulphide was made by decomposition of ammonium thiotungstate,a method similar to that described by Decrue and Susz (4). Ammonium thiotungstatewas prepared by dissolving tungsten oxide in the minimum quantity of 0.880 ammonia solution at 80°C and passing hydrogen sulphide through the solution for four hours. The solution was cooled overnight and precipitation of the crystals was aided by the addition of ethanol. The precipitate was dried overnight. The material was transferred to a reaction vessel and treated with hydrogen for 18 h at 400°C to convert the material to tungsten sulphide. After conversion, the reaction vessel was sealed and allowed to cool before connection to a high vacuum apparatus. The sample was pumped to a pressure
51 of less than 10-5 Torr and then heated to 400°C when it was again pumped until the pressure fell below 10m5 Torr. At this temperature hydrogen adsorption was measured using a conventional volumetric technique and a pressure transducer to measure the inlet and equilibrium pressures. At no time was the sample allowed to come into contact with air. Gravimetric analysis by Rooney Laboratories Limited found the composition of this tungsten sulphide to be WS _09. Its crystallite size estimated fran an X-ray diffraction pattern was 40 x 18 a (1 x h). For the neutron experiments the tungsten sulphide was sealed in aluminium containers and then examined on the beryllium filter spectrometer IN18 at the I.L.L. Grenoble (7) and time-of-flight spectrometer 4H5 (8) at A.E.R.E. Harwell. The beryllium filter and time-of-flight spectra were recorded at 80 and 293 K respectively. RESULTS The adsorption isotherm for hydrogen uptake by tungsten sulphide at 400°C is shown Figure 1.
,_
H, I WS,
I-
H,SI
$.0375f-
WS,
P
m 8
In
9 m
3 0.02 -
0
I 200
I LOO
o
ADSORPTION
.
DESDRPTION I 600
I 600
BRANCH
I 1000
EOUILIBRIUM
FIGURE 1
g
BRANCH
g J
3 9
PRESSURE
1
0
,
,
200
LOO
q
DE;RPTI;
600
BRA;,
600
1000
ITDRR)
Adsorption isotherms for hydrogen and hydrogen sulphide uptake by WSg.
Equilibrium, as assessed from the cessation of gas uptake, was achieved at all pressures within five minutes. A further demonstration that a good approximation to equilibrium was attained was provided by the similarity of the adsorption and desorption isotherms. The experimental points could fit Langmuir models, assuming either molecular or atomic adsorption. The only point of difference between these two fits was that the dissociative model required a value for the monolayer uptake which was significantly greater than that suggested by the limiting value of the pressure -
52
volume curve. This behaviour has been noticed before for dissociative chemisorption (9). The limiting value of the uptake at 673 K was equivalent to a stoichiometry of HO 04BWS2. Further uptake was recorded on cooling the sample so that the stoichiometry WS at 423 K. Below this temperature uptake was too slow for the approach became H0.056 2 to equilibrium to be measured. The volume of nitrogen adsorbed by this sample at p/p, = 0.3 was determined with a flow technique, using a one point B.E.T. model, to be 20 m* g-' so that the limiting uptake of hydrogen at 673 K was equivalent to about 0.7 of a monolayer coverage. The adsorption isotherm for hydrogen sulphide uptake by tungsten sulphide at 400°C is shown in Figure 1. The rate of attainment of equilibrium was much slower than for the case of hydrogen uptake and the uptake at saturation coverage was considerably less. The maximum uptake was equivalent to WS2(H2S)o.065.Again, the shape of the isotherm did not discriminate between molecular and atomic adsorption. The adsorption was not truly reversible because the desorbed gas was not H2S but hydrogen. Samples prepared by our standard method (i.e. evacuated at 10m5 Torr at 4OO'C) all contained residual hydrogen. A determination of the quantity of this residual hydrogen, by following its exchange with deuterium using a mass spectrometer, showed that the residual quantity was equivalent to Ho oo7WS2.
NEUTRON
FIGURE 2
ENERGY CM“
TRANSFER
Inelastic neutron scattering spectrum of hydrogen adsorbed by WS2.
The inelastic neutron scattering spectrum of hydrogen adsor$d by tungsten sulphide (Figure 2) shows excitations at 694, 1380 and 2074 cm
and a broader band
of scattering at higher energies partly resolved into peaks at 2470 and 2679 cm-'. This broad band of scattering, at energies neither equal or close to an integral
53
multiple of 694 cm", must be due to another 2ndamental such as an H-S stretching vibration. Such fundamentals occur at 2500 cm
for thiols (10). A self-consistent
-1 assignment of the spectrum is that it shows hydrogen vibrations. parallel at 694 cm -1 and perpendicular at 2470 or 2679 cm , to the WS2 surface. The scattering at 1380 -1 and 2074 cm-' is due to the first and second harmonics of the scattering at 694 cm . The values of the two highest energy frequencies are similar to those that have been observed for hydrogen-treatedMoS2 by infrared spectroscopy near to 2500 and 2640 cm-' (11). This assignment is confirmed by an analysis of the intensities of the four peaks in the spectrum. Their theoretical intensities can be calculated from the product of two terms, the first representing the intensity of scattering from the hydrogen behaving as a simple harmonic oscillator and the second representing the influence on the intensity of the vibrations of the lattice surrounding the hydrogen I = IS.H.0. x exp g
1
[ The first term, IS.H.O_, can be readily calculated from a knowledge of the vibration frequencies of the hydrogen atoms in the sample. The second term is unknown but, since M and uL are constants for any lattice, the logarithm of the ratio of the observed to the predicted intensities for the four excitations should be directly proportional to
Q2,
the momentum transfer squared, at each excitation, when the correct vibration
frequencies have been chosen to calculate IS. H , o . (12.13). Figure three shows values of this ratio plotted against Q2 for three of the different structural models represented in Figure 4. The first is one in which the hydrogen atom lies above the layer of sulphur atoms to which it is multiply bonded; the second is one in which the hydrogen atom lies in the plane parallel to the basal plane surrounded by sulphur or metal neighbours and the third is one in which the hydrogen atom lies above a single sulphur atom to which it is bonded. The vibration frequencies corresponding to these models, chosen to be consistent both with our experimental observations and the known -1 vibration frequencies of similar compounds (10,12), were for model one uA = 1380 cm , -1 -1 = 694 an-', for model two uA = 694 cm , uE = 1380 cm , and for model three WE q 694 cm", uA = 2500 cm-'. uA and I+ are respectively the singly degenerate WE vibration perpendicular to the basal planes and the doubly degenerate vibration parallel to the basal planes. In model one the stretching frequency for the bridged hydrogen atom has been chosen to be 50 % of the known unbridged S-H stretching frequency, on the ground that a similar reduction in frequency is found for metalhydrogen stretching vibrations (14). These three models are, on chemical grounds, the most likely to describe the hydrogen-tungsten-sulphideinteraction. Model three best fits the data and the error bars of Figure 3 are calculated from the estimated experimental errors in the intensity determination.A fourth model, also worthy of attention, is one in which the adsorbed hydrogen atoms occupy sulphur vacancy sites
54
and so interact, through single bonds, with the tungsten atoms. In this case, metalhydrogen vibrations would be,observed in the neutron spectra which would have energies -1 for those of bending and between 600 and 850 cm-' and between 1800 and 2200 cm stretching character respectively (14). With our present experimental technique we are not able to eliminate such a model on spectroscopic grounds, since it closely resembles model three. Instead we propose that this model can be eliminated because -1 and for the reasons of stoichiometry that we of the scattering near to 2500 cm enlarge upon in the discussion.
7-o -
f IWI
s...,
IWI
5,‘.
‘~.I+ ‘.
S(WI
zao: * :: . ?70-
6S
S____S
7-
I 20
40
60
60
lHOMENTUM
FIGURE 3
100 TRANSFERI’
120
140
160
I 160
A’-’
Ratios of the predicted to the observed scattering intensity for the peaks
in the H/WS2 spectrum, ploted against Q*. Although these results all confirm the presence of single H-S bonds, there is a number of points concerning the high frequency bands in the spectrum which are not completely described by this model. The "observed" intensity, used to calculate the point for the highest energy band in Figure 3. is an integrated value for the whole band which therefore masks the fact that the relative intensities of the scattering
55
at 2470 and 2679 cm"
are not those that would be predicted. Our simple model
predicts intensities of approximately 2:l for excitations corresponding to the fourth harmonic of the parallel excitation and the fundamental of the perpendicular excitation. This compares with the equal intensities observed at these two frequencies and a value for the fourth harmonic energy based on the energies of the lower harmonics -1 of 2765 cm . The spectrum recorded after adsorption of hydrogen sulphide on the tungsten sulphide surface is similar to that recorded for adsorbed hydrogen, except for an -1 additional shoulder in the low frequency region of the spectrum at 613 cm .
spy*
I
s,Q
1
/ Y -
O’
a
0
0
0
\!
....
,....w
“Y...w
FIGURE
coverage
56
occurs at a stoichiometry (H2S)0.0065WS2.If
it is assumed that both hydrogen and
hydrogen sulphide dissociate into their constituent atoms, then the number of surface sites that can be occupied by the hydrogen sulphide fragments is, at most, 40 % of the number of sites occupied by the dissociated hydrogen molecules. In consequence, not only must the sites occupied by the different species be of different types but, if the hydrogen sulphide adsorbs at sulphur vacancies as seems likely from poisoning studies (15), the hydrogen atoms cannot also be adsorbing at those sites. This is the additional reason for rejecting model four, to which reference was made earlier.
z E k
0
500
NEUTRON
FIGURE 5
750
1000
ENERGY TRANSFER cm-l
Inelastic neutron scattering spectrum of hydrogen sulphide adsorbed by WS2.
The inelastic scattering spectrum for adsorbed hydrogen sulphide is similar to that for adsorbed hydrogen, showing that the hydrogen sulphide must dissociate at the WS2 surface. For each original H2S molecule two H-S bonds are formed to different sulphur atoms. This finding is of considerable interest since it has been known for some time that hydrogen sulphide poisons the activity of sulphide catalysts for hydrodesulphurization,hydrogenolysis,hydrogen-deuteriumexchange and hydrogenation reactions. Voorhoeve and Stuiver (15) showed that in benzene and cyclohexane hydrogenationsan increase in the hydrogen sulphide partial pressure influenced the
57
reaction kinetics through the pre-exponentialfactor rather than the activation energy and, from this, they concluded that hydrogen sulphide removes active sites which must be sulphur deficient metal centres. The reaction proceeded, so they speculated, to produce sulphur and hydrogen gas. In the light of our spectroscopic data this mechanism can now be modified so that it reads H2S + S-- +oS + 2SHThe hydrogen sulphide adsorbs at a vacancy site, where it dissociates to form an HS group and a hydrogen atom. The HS group fills the vacancy and deactivates it, whilst the hydrogen atom diffuses over the surface to form an H-S bond with another available sulphur atom. Hydrogen molecules could also dissociate at these vacancies, or perhaps at the coordinatively unsaturated edge sulphur atoms, but in this case the hydrogen atoms diffuse away from their dissociation centre leaving the vacancies unaltered. This mechanism explains differences in the limiting uptakes of hydrogen sulphide and hydrogen adsorption. In the former adsorption terminates when all the vacancies have been occupied whereas, in the latter adsorption could continue until all the sulphur atoms have been converted to SH. It can be shown that this mechanism for H2S adsorption is quite consistent with the surface area of the WS2 sample. Assuming particles of hexagonal colunnar form, with each edge of the particle having length L. the number of sulphur vacancies in the surfaces perpendicular to the basal planes will be equal to the number of H2S molecules adsorbed, i.e. 0.0065 per tungsten atom, if 20 % of the sulphur atoms in the edge planes were absent. Although calculations for other particle shapes will require different fractions of the sulphur sites to be vacant to produce the same number of vacancies per tungsten atom, it is clear that the ratio of vacancies to occupied sites at the surface, required to explain the uptake is reasonable.
It
is interesting to contrast this value for the hydrogen sulphide uptake, 0.0065
mol/mol, with the uptakes of oxygen by molybdenum sulphide recently determined by Tauster and coworkers (16). Their samples of molybdenum sulphide, numbers thirteen and fifteen, prepared by a similar method to our preparation of tungsten sulphide, had oxygen uptakes of 0.005 and 0.006 mol/mol. Despite the different surface areas of these samples (MoS2 2.3~10~. 3.7x103. WS2 7.4x103 m2 mol")
it is tempting to suggest
that oxygen and hydrogen sulphide chemisorption is occuring at the same sites and therefore the magnitude of the oxygen chemisorption on these materials is a measure of the number of the sulphur vacancies, rather than the total area of the edge planes. Since Tauster et al (16) find a clear correlation between oxygen chemisorption and H.D.S. activity (for dibenzothiophene),these results are consistent with the suggestion that hydrodesulphurizationoccurs at sulphur vacancies in an analogous way to the mechanism of hydrogenation over sulphide catalysts proposed by Voorhoeve and Stuiver (15).
58
REFERENCES C.J. Wright, C. Sampson, D. Fraser, R.8. Moyes, P.8. Wells and C. Riekel, J. Chem. Sot. Faraday 1, 76 (1980) 1588. E.E. Donath, Advances in Catalysis Vol. 8, p 245 (1956), Academic Press, New York. H. Friz, Zeit. Fur. Electrochimie, 54 (1950) 538. J. Decrue and 8. Susz, Helv. Chim. Acta., 39 (1956) 619. M.G. Gonikberg and 1.1. Levitskii, Izvest. Akad. Nauk. S.S.S.R., Otdel. Khim. Nauk.. (19601 1170. 6 E.H.M: Badger, R.H. Griffith and W.8.S. Newling, Proc. Roy. Sot., (1949) 184. 7 Neutron Beam Facilities at the High Flux Reactor, Institut Laue-Langevin,Grenoble, France (1974). 8 A.H. Baston and D.H.C. Harris, Neutron Beam Instruments at Harwell, A.E.R.E. R9278. H.M.S.O. (1978). A. Hansen and H.L. Gruber, J. Catal., 20 (1971) 97. 1: L.J. Bellamy, The infrared spectra of complex molecules, Methuen and Co. Ltd., London, p 351, (1958). P. Ratnasamy and J.J. Fripiat, Trans. Faraday Sot.. 66 (1970) 2897. ;: CO;iekel, H.G. Reznik, R. Schollhorn and C.J. Wright, J. Chem. Phys., 70 (1979) C.J.'Wright, J. Chem. Sot. Faraday Trans II, 73 (1977) 1497. U.A. Jayasooriya,M.A. Chesters, M.W. Howard, S.F.A. Kettle, D.B. Powell and N. Sheppard, Surface Sci., 93 (1980) 526. 15 R.J.H. Voorhoeve and J.C.M. Stuiver, J. Catal., 23 (1971) 243. 16 S.J. Tauster, T.A. Pecoraro and R.R. Chianelli, J. Catal., 63 (1980) 515. ;: