J. Phys. Chem. 1986,90, 3137-3140
3137
Catalytic Reduction of CO with Hydrogen Sulfide. 4. Temperature-Programmed Desorption of Methanethlol on Anatase, Rutile, and Sulflded Rutile D. D. Beck: J. M. White,* Department of Chemistry, University of Texas, Austin, Texas 78712
and C. T. Ratcliffet Exxon Research and Engineering Co., Annandale, New Jersey 08801 (Received: June 28, 1985)
The interaction of methanethiol with anatase, rutile, and sulfided rutile was studied by temperature-programmed desorption. Dissociative adsorption occurs on rutile but is insignificant on anatase. Decomposition products are dominated by H2 on rutile and by CH4 on sulfided rutile. In both cases desorption occurs between 550 and 775 K. The 5- and 4-coordinate sites on the (1 10) face of rutile are proposed as the active sites for decomposition. The dominance of methane on a sulfided surface is attributed to the relatively large supply of highly mobile surface hydrogen atoms.
Introduction Hydrodesulfurization catalysts have been the subject of extensive study owing to their industrial In a recent steady-state study of CO reduction with H2S on rutile Ti0214 mercaptans dominated the hydrocarbon products between 523 and 630 K,while methane dominated at T 1 630 K. From this and other data,5 methanethiol and dimethyl sulfide were proposed as intermediates in the formation of methane. Pretreatment of rutile with H$3 at 673 K was required for catalyst activation! In a later study, catalysts so treated (“sulfided”) were characterized by TPD and AES (temperature-programmed desorption and Auger electron spectroscopy),5 In this paper, methanethiol adsorption and decomposition on anatase, clean rutile, and sulfided rutile are characterized by using TPD. Experimental Section TPD experiments were carried out in a two-chamber ultrahigh-vacuum system described previously,6 except that an Auger electron spectrometer was added to the analysis chamber. A detailed description of the Ti02catalysts used here has been given? The anatase (A2) sample was synthesized from titanium isopropoxide, and rutile (R2)from TiCL. Both samples were oxidized in O2 (100 Torr) at 700 K for 1 h to remove surface carbon. A C/Ti AES intensity ratio of CH4 > CH3CH3. CS2and SO2 (not shown) were also detected above 900 K, but in insignificant amounts. In a third experiment, rutile was presulfided with H2Sat 1 X lo-’ Torr for 2000 s at 673 K.5 It was then exposed to CH3SH at 1 X lo4 Torr for 300 s at 140 K. In TPD (Figure 4) molecular desorption (CH3SH) was similar to nonsulfided rutile, except the high-temperature increase begins at a lower temperature. Near 625 K there was a relatively intense CH4 peak but neither a C O nor H2peak. H2 desorption grew monotonically at T, 1 700 K. Other products (HIS, CH3SCH3,and CH3CH3)were detected with intensities of 120% of CH4. The SO2 signal (not shown) increased above 900 K but was no different from TPD on sulfided rutile prior to CH3SH exposure. A very small amount of CS2 (not shown) was detected above 900 K. To examine the role of acid sites in adsorption of methanethiol on clean anatase and rutile, they were poisoned by dosing with various amounts of pyridine. Various doses were used, ranging from 0 to 3000 s at 1 X lo4 Torr a t 140 K. After evacuation at 140 K the sample was dosed with 3000 L of CH3SH. On
-
~~~~
(1) (2) (3) (4)
F. E. Massoth, Ado. Catal., 27, 265 (1978).
P. Grange, Catal. Rev-Sci. Eng., 21, 135 (1980). P. C. H. Mitchell, Spec. Period. Rep.: Catalysis, 4, 175 (1981). C. T. Ratcliffe and P. J. Tromp, US.Patent 4517 171, 1985. ( 5 ) D. D. Beck J. M. White, and C. T. Ratcliffe, submitted for publication to J. Phys. Chem. ( 6 ) D. D. Beck and J. M. White, J . Phys. Chem., 88, 2764 (1984).
0 1986 American Chemical Society
Beck et al.
3138 The Journal of Physical Chemistry, Vol. 90, No. 14, 1986 -I
I
I
I
I
I
I
1
I
I
co H p x 10
cos
a 5 100
300
7 00
500
x 10 W I-
cos
w-
900
, x 1
0
1
I O
T (K) Figure 1. TPD of CH3SH adsorbed on anatase. Sample was dosed at 1 X IO” Torr for 300 s at 140 K.
U
I-
O ’ W n v)
u -
m
m v)
4
CH3CH3x
5
I
10
I 300
100
1 500
I
I-
I
I
I
700
I 900
T (K)
5 2 .
TPD of CH3SH adsorbed on rutile. Sample was dosed at IO“ Torr for 300 s at 140 K.
Figure 3.
t
a 4 a t m
X
1
a
a
Y
7 1 8 0 K PEAK
d(
w
4
1000
0
2000
I
1
/
a i .
4000
3000
5000
EXPOSURE (L)
Methanethiol TPD peak area as a function of exposure in langmuirs (1 langmuir = 1 X lod Torr s) on anatase. The result was very similar for rutile and sulfided rutile. Figure 2.
anatase and rutile, the 320 K methanethiol TPD peak was unaffected.
Discussion Reversible Adsorption. The methanethiol TPD in the lowtemperature range (140-500 K) is very similar to the result after H2S d ~ s i n g .The ~ TPD peak at T , = 180 K does not saturate and is thus assigned to weak molecular adsorption into islands more than one layer thick. As discussed for H2S, these lowtemperature peaks do not involve the formation of bulk multilayers since the vapor pressure, particularly of H2S, is high (- 1 Torr for H2S and Torr for CH3SH a t 140 K). A shoulder in 215 K is not well-resolved but does the TPD spectrum a t T , saturate with increasing dosage. It is thought to be connected with the 180 K peak and may represent the first layer of adsorbate in the island. The TPD peak at T , = 320 K is assigned to molecular species either bound through hydrogen bonding to a surface oxygen anion or coordinated to surface acid sites (Ti4+) through a lone pair of electrons on sulfur. (A combination of the two is possible.) The two bonding configurations are the following:
-
CH
‘X
77
7
VI
CH3SCH
m
a I
-
/
CH3CH3
I
100
I
300
I
I 500
I
I
700
I
I 900
T (K)
TPD of CH3SH adsorbed on sulfided rutile. Sample was pretreated with HIS at 1 X Torr for 2000 s at 673 K and then dosed Figure 4.
with CH,SH at 1 X 10” Torr for 300 s at 140 K.
Lavalley and co-workers7studied the adsorption of H2S, CH,SH, and CH3SCH3on alumina using IR, finding that CH3SH adsorbs molecularly in these two modes with hydrogen bonding to the surface being preferred by a 5:3 ratio. They also suggested that CH3SH can be molecularly bound through hydrogen bonding to a surface hydroxyl group. Since Ti02 is not as acidic as alumina, we expect that the hydrogen-bonded species is even more predominant over the species coordinated to Ti4+. This is supported
H-C\s/H
I
(7) 0. Saw, T. Chevreau, J. Lamotte, J. Travert, and J.-C. Lavalley, J . Chem. SOC.,Faraday Trans. I , 11,427 (1981).
Catalytic Reduction of C O with H2S
The Journal of Physical Chemistry, Vol. 90, No. 14, 1986 3139
by the fact that pyridine, which chemisorbs on acid. sites (Ti4+), does not poison the 320 K methanethiol TPD peak. Confirming this view, Khrstaleva and co-workers studied the adsorption of H2Son rutile using ESR and found no evidence for adsorption on acid sites (Ti4+).* In view of these arguments, the TPD peak at T, = 320 K is assigned to molecular CH3SH hydrogen bonded to surface oxygen anions. Dissociative Adsorption. Either of the above configurations can act as a precursor state leading to dissociation. Because methanethiol is relatively acidic, the S-H bond cleaves, leaving a thiolate (-SCH3) group bound on an acid site and a proton bound to lattice oxygen' (which we denote as type I cleavage). Breakage of the S-C bond leaving an adsorbed methyl group on lattice oxygen (methoxide) and a sulfhydryl (-SH) group on Ti4+ (which we denote as type I1 cleavage) is also a possibility based on methanol decomposition on Ti029J0and other Types I and I1 species are illustrated:
type 1
type
I1
Type I is expected to be the predominant decomposition pathway because of the acidity of methanethiol. This preference has been demonstrated in adsorption studies on clean and modified tungsten surfaces,14 on A1203,' and on anatase,15 the latter probably containing significant amounts of rutile. Comparison to methanol adsorption suggests that formation of type I species can lead to CH3SH by reassociation of -SCH3 and -H bound in adjacent sites, to CH3SCH3from reassociation between adjacent -SCH3 and -CH3, and to H2.12 Type I1 species decompose mainly to CO because the T i 0 2 surface is better able to stabilize -CH3 (to lattice oxygen) than -SCH3 (which is weakly attached to acid sites through an ionic bond). The presence of a high-temperature CO TPD peak ( T , 625 K) on clean anatase and clean rutile suggests that some dissociation of methanethiol via type I1 intermediates does take place. The absence of this C O peak in the TPD on sulfided rutile can be explained by the following argument. A significant number of surface anions have been replaced by sulfur, and type I1 cleavage of methanethiol catalyzed by an acid-base site pair yields -CH3 bound to sulfur. Since -CH3is more weakly held to lattice sulfur than to lattice oxygen, it is more likely to react with a neighboring bound species than to decompose. Perhaps the most striking result in this study is that methanethiol decomposition takes place more readily on rutile than on anatase. Lavalley and co-workersI5 proposed two types of acid sites formed by coordinatively unsaturated Ti4+, one of which carries two anion vacancies (i.e., 4-coordinate). We suggest, on the basis of previous TPD studies of H 2 0 and H2Sadsorption on Ti02,5 that such sites are present in small numbers on anatase but in much larger numbers on rutile, particularly on the (1 10) face. The most stable of the low-index surface planes in rutile is the (1 10) plane,16which is thought to comprise as much as 60% of the surface of rutile particle^.'^ These coordinatively unsaturated acid sites form terminal or bridge bonds to the sulfur lone pairs in sulfhydryl or thiolate, and at higher temperatures loss
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(8) Y . A. Zarifyants, S. N. Karyagin, J. E. Kiselev, and S. V. Khrstaleva, Kinet. Katal., 15, 1077 (1974). (9) I. Carrizosa, G. Munuera, and S. Castanar, J. Catal., 49,265 (1977). (10) I. Carrizosa and G. Munuera, J . Catal., 49, 174, 189 (1977). (1 1) Y . Soma, T. Onishi, and K. Tamaru, Trans. Faraday SOC.,65, 2215 (1969). (12) T. Matsushima and J. M. White, J . Catal., 44, 183 (1976). (13) D. C. Foyt and J. M. White, J . Cafal.,47, 260 (1977). (14) J. B. Benzinger and R. E. Preston, private communication. ( 1 5) H. Saussey, 0.Saur, and J.-C. Lavalley, J . Chim. Phys. Phys.-Chim. Biol., 81, 261 (1984). (16) V. E. Henrich and R. L. Kurtz, Phys. Rev. E Condens. Matfer, 23, 6280 (1981). (17) G. D. Parfitt, Prog. Surf. Membr. Sci., 11, 181 (1976).
of the hydrogen or methyl group occurs, respectively, leaving a sulfur anion which can then be incorporated into the lattice (Le., ~ulfiding).~ The large H2 TPD peak observed after adsorption of methanethiol on clean rutile gives additional evidence that decomposition occurs primarily via a type I pathway. H2 desorption probably originates from recombination from fragments formed by the decomposition of two methanethiol molecules in a type I pathway. H2 formation via this route is consistent with an earlier study of H2S decomposition on rutile, in which repeated H,S dosing followed by TPD resulted in a H2 TPD peak ( T , 650 K) gradually replacing the H 2 0peak ( T , 600 K).5 That work pointed to a decomposition mechanism in which chemisorbed H2S lost hydrogen atoms sequentially. H2desorption from methoxide decomposition is probably not important, since relatively little CH4 or CO accompanies H2. Methyl groups which are part of the thiolate bound to the surface, or which are part of a bound methoxy group, are unable to migrate like hydrogen, which explains the relatively small CH4 desorption in the temperature region of the H2 TPD peak (500-750 K). Sulfur-containing molecules (H2S, CH3SH, and CH,SCH,) desorbing at high temperature ( T > 500 K) can form by reaction of adjacent bound type I or type I1 species in various combinations. The gradual increase at T > 700 K in the signal of all sulfur- and carbon-containing species and H2 can be attributed to titration of various fragment species left on the surface after methanethiol decomposes. Presulfiding the rutile sample had a significant effect on the hydrocarbon distribution. The predominance of CH4 over other hydrocarbons in the TPD is attributed to an abundant supply of atomic hydrogen at or near the surface which can migrate on or through the lattice and hydrogenate bound thiolates or methyl groups. We have shown that significant amounts of atomic sulfur and hydrogen are incorporated in the rutile lattice during the sulfiding treatment (exposure to H2Sat 673 K).5 A similar result occurs during the sulfiding of rutile by methanethiol decomposition. Diffusion to the surface of hydrogen atoms contained in the lattice is the probable source of the broad, intense H2 desorption peak above 600 K. Also, surface lattice oxygens have been replaced by sulfur in the sulfided sample, so the formation of surface thiolate species becomes more likely, and surface methoxide formation becomes rare. As mentioned above, methyl groups are more weakly bound to lattice sulfur than to lattice oxygen, making hydrogenation of the former species to CH4 easier. The absence of any CH4 desorption above 750 K may indicate that -CH3 groups present on the surface at this temperature are either inactive to hydrogenation or are not accessible to hydrogen which leaves the surface at T > 750 K. Diffusion of hydrogen atoms from the bulk to surfaces other than the rutile (1 10) surface plane, where most of the methyl groups are formed, is kinetically more favored, according to a model study.I8 On the basis of this argument, we conclude that the CHI, which desorbs at 600 K, is the result of methanation of surface methyl groups by hydrogen atoms located primarily on the surface. Studies of H2S adsorption on Ti025 and other s ~ r f a c e s l ~ - ~ ~ provide evidence for water formation by transfer of hydrogen from sulfur to surface oxygen anions. The same adsorption mechanism for methanethiol would lead to H 2 0 , C H 3 0 H , and CH30CH3 products on unsulfided rutile. Of these, only water was observed ( T , 780 K with relatively low intensity). Acidic cation surface sites induce basic character in neighboring anion sites, enhancing type I dissociation.I2 This is one likely cause for the preference of H 2 0over methanol or dimethyl ether in the reaction with lattice oxygen. We suggested previously that donation of hydrogen atoms from H2S to oxygen anions (to form water) occurs in two steps;5
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-
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(18) J. B. Bates, J. C. Wang, and R. A. Perkins, Phys. Rev. B: Condens. Matter, 19, 4130 (1979). (19) A . V. Deo and I. G.Dalla Lana, J . C a r d . , 21, 270 (1971). (20) T. L. Sager and C. H. Amberg, Can. J . Chem., 50, 3416 (1972). (21) A. M. Deane, D. L. Griffiths, I . A. Lewis, A. J. Winter, and A. J. Tench, J . Chem. SOC.,Faraday Trans. 1 , 71, 1005 (1975).
3140
J. Phys. Chem. 1986, 90, 3 140-3 148
formation of CH30Hor CH30CH3in a similar fashion is sterically hindered, also contributing to the absence of these products. The low intensity of the water desorption is a result of surface oxygen being more tightly bound than surface sulfur. It has been proposed that methanethiol can oxidatively adsorb on sulfided tungsten to form a surface di~ulfide.'~In this mechanism lattice vacancy sites are not required for dissociation. Although thiols are easily oxidized by weak oxidizing agents22 (such as a metal surface), rutile and sulfided rutile are considered to be very weak acids" and thus are probably unable to oxidize thiols to disulfides. Summary TPD studies of methanethiol adsorption were carried out on anatase, rutile, and sulfided rutile. Molecular adsorption on all surfaces resembles H2Sand occurs through hydrogen bonding to (22) E. E. Reid, Organic Chemistry of Bimetallic Sulfur, Vol. 1, Chemical Publishing Co., New York, 1958.
surface oxygen anions. Dissociative adsorption occurs on rutile but is not significant on anatase. Decomposition products desorb at relatively high temperatures ( T , = 550-775 K) and are dominated by H2 on clean rutile and by CH, on sulfided rutile. The proposed dominant pathway involves decomposition of methanethiol to form a thiolate (-SCH3) in adjacent 5- and 4-coordinate Ti sites on rutile (1 lo), which can accommodate bridge bonding of sulfur. In another less important pathway, sulfhydryl and methoxide groups are formed. Recombination of hydrogen lost in the initial decomposition step on clean rutile results in H2 desorption, while reactions between bound species (-H, -SH, -CH3, -SCH3) lead to hydrocarbon products. The dominance of methane desorption on sulfided rutile is attributed to the large supply of bound hydrogen left by the presulfiding step and the relatively easy migration of hydrogen to methyl groups on a sulfided surface, facilitating hydrogenation. Registry No. CH,SH, 74-93-1; Ti02, 13463-67-7; CO, 630-08-0; H,S, 7783-06-4.
Metal Atom Based Growth Kinetics of Low Nuclearlty Metal Clusters in Liquid Poly(dlmethyls1loxane-co-methylphenylslloxane). 2 Mark P. Andrewst and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, Chemistry Department, University of Toronto, Toronto, Ontario Canada M5S 1 A1 (Received: January 30, 1985; In Final Form: October 21, 1985)
A need has arisen to develop kinetic and spectroscopic methods to trace the fate of metal atoms deposited into reactive oligomeric and polymeric liquids and to pursue product distributions as a function of the amount of metal atoms deposited into these media. Our first attempts at simulating what are probably complex processes in a liquid poly(dimethylsi1oxane-comethylphenylsiloxane) assume, on one hand, a simple step-by-step addition of metal atoms to an initially formed bis(arene)M complex. The latter compound probably forms from encounters with favorably oriented intra- and interchain phenyl pairs (part 1). By suitably eliminating the time variable in the rate expressions, equations have been developed which predict linear behavior for appropriately correlated series-parallel reactions. A second analysis is developed as a method for inferring metal cluster nuclearity from the metal atom concentration dependence of observed spectroscopic features. The predictions of these analyses have been compared with some success with the experimental results obtained for M/DCSIO (M = V, Cr, or Mo) quantitative metal atom titrations.
Introduction We present two approaches to describe the initial growth behavior of the species giving rise to the metal-concentration-dependent absorption spectra which originate from polymer-anchored metal clusters and which absorb to the red of the MLCT band in DC5 10 thin-film microscale metal vapor experiments.' For molybdenum, chromium, and vanadium atoms we refer to the spectra and growth curves given in Figure 1-5. The first approach is necessarily simple due to the complexity of the process which should increase after the initial stages, as the viscosity of the medium (due to cross-linking) and competitive colloid growth increase. In fact, the residual unreacted phenyl, the observation that the growth curve characterizing a particular species maximizes at different total metal loadings depending on the rate of metal deposition (at fixed temperature), and the eventual leveling out of the growth curves at some total metal loading in the film all attest to the complexity of the system. Thus it is only feasible to treat the initial stages of the growth process based on some plausible assumptions. The second approach takes as its point of departure the quenched reaction model of Moskovits and H ~ l s eapplicable ,~ to rare gas matrix-isolated metal clusters and cluster complexes. We 'Current Address: AT&T Bell Labs, Murray Hill, NJ.
0022-3654/86/2090-3140$01.50/0
present a method which can be considered potentially useful in assigning metal cluster nuclearities in thin-film liquid oligomer and polymer microscale metal vapor experiments. (a) Series-Parallel Reactions In the first place, a stepwise formation of the metal sandwich compound is reasonable so that we can write ki
M+L-ML ML
+L
k2 +
(1)
ML,
Equations 1 and 2 are acceptable in view of our identification of processes v + C6H.5 V(C6H6) and V(C6H6) + C6H6 ---* v-+
(1) (a) Francis, C. G.; Huber,H.X.; Ozin, G. A. Inorg. Chem. 1980, 19, 219. (b) J. Am. Chem. Soc. 1979,10, 1250. (c) Angew. Chem., Int. Ed. Engl. 1980, 19,402. (d) US.Patent, 4292253, Sept 1981. (e) Francis, C. G.; Ozin, G. A. J . Mol. Sfrucf.1980,59, 55. J . Macromol. Sci. Chem., A ( l ) 1981, 16, 167. (f) Ozin, G. A.; Andrews, M. P. Angew. Chem. 1982, 94, 219. (g) Angew. Chem. Suppl. 1982, 1255. (h) Inorg. Synth. 1983, 22, 116. (i) Ozin, G. A. CHEMTECH 1985,488. (2) Pearson, R. G. J . Phys. Chem. 1977, 81, 2323. (3) Moskovits, M.; Hulse, J. E. J . Chem. Sor., Faraday Trans. I 1977. 73, 471.
0 1986 American Chemical Society
