J . Phys. Chem. 1992, 96, 2230-2235
2230
conduction band to yield '0,species which combine with protons to form 'OOH radicals. Particularly in acidic media, the 'OOH radicals could be one of the predominant oxidants in the initial photodegradation stages: O2 e- *02-
+
--
'02+ H+ 'OOH After addition of 'OH radicals to the aromatic group, the hydroxylated ring is opened to finally yield COz gas via many oxidation steps implicating such species as aldehyde and carboxylate intermediates (as evidenced by NMR measurement4). Similarly, in the photodegradation of the long alkyl chain, the highly reactive *OH radical attacks the a,0, or w position to form hydroxyl or carbonyl intermediates; ultimately it also evolves C 0 2 gas. The attack of 'OOH radicals to a surfactant can produce the peroxide intermediates. It further decomposes to hydroxyl or carbonyl species.
Conclusions Anionic DBS and cationic BDDAC surfactants together with
their reference compounds BS, DS, BTAC, and HTAB were degraded in irradiated T i 0 2dispersion. The photodegradation rates in initial stages are fitted to the simple Langmuir-Hinshelwood equation. The DBS surfactant is decomposed more slowly than BS having no long alkyl chain. The aromatic moiety in the DBS structure is photodegraded more rapidly than the alkyl chain. The cationic DBBAC system exhibits the same tendency. The anionic surfactant DBS is degraded faster than the cationic BDDAC. The ESR results indicate the existence of 'OH radicals in irradiated Ti02dispersions. The surfactant is attacked by 'OH and/or 'OOH and further degraded to ultimately evolve C 0 2gas via some oxidized species of peroxides, aldehydes, and carboxylates. Acknowledgment. This work was financed by the Chemical Materials Research and Development Foundation, which we gratefully acknowledge. We appreciate Prof. M. Gratzel (EPFL, Switzerland) for his generous encouragement as well as Miss K. Nohara, Mr. N. Awato, Mr. K. Kitamura, Mr. H. Nagatsuka, and Mr. A. Morii for technical assistance. J.Z. also thanks Watanabe's Foundation for a scholarship.
Adsorption of H2S on ZSMB Zeolites Cristina L. Garcia and Johannes A. Lercher* Institut fur Physikalische Chemie und Christian Doppler Laboratorium fur Heterogene Katalyse, Getreidemarkt 9, A-1 060 Vienna, Austria (Received: May 13, 1991)
The adsorption of H2Son H-ZSM5 and Na-ZSM5 was investigated by means of IR spectroscopy, temperature-programmed desorption, and gravimetry. The surface chemistry for equilibrium pressures of H2Sfrom to 0.5 mbar was studied. H2S was found to be hydrogen bonded to the SiOHAl groups of H-ZSM5, oriented in two different positions toward the surface. On the alkali-metal-exchangedform of ZSM5, H,S adsorbed coordinatively on the alkali-metal cation via the sulfur atom.
Introduction The adsorption of H2Son oxides was studied by several groups over the past years to understand the surface chemistry during the Clauss process or on hydrodesulfurization catalysts.l-" Upon adsorption on 7- and 7-A1203, S O 2 ,and hydrogen forms of faujasite zeolites, H2Swas found to be mainly hydrogen bonded to the OH surface groups.*' On metal-cation-exchanged faujasites and on alkaline and transition metal forms of 4A and 5A zeolites, heterolytic dissociative adsorption of H2Swas reported.3-6*8-'0 The formation of new OH acid groups was observed as the result of the dissociation. Additionally, dissociative HzS adsorption via a radical mechanism was proposed to occur on NaY samples.8s1i Infrared spectra of adsorbed HzSrevealed bands between 2500 and 2600 cm-' which were either assigned to the SH stretching vibration of SH- groups attached to the cations for dissociative adsorption or to the HSH stretching vibrations for nondissociative adsorption. Up to now, however, the bands of the stretching vibrations of molecularly adsorbed H2S are not unequivocally attributed to specific modes. It was suggested that the bands are (1) Heinemann, H. Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981, Vol. I, Chapter 1. (2) Schuman, S.; Shalit, H. C a r d Reu. 1971, 4, 245. (3) Deo, A. V.; Dalla Dana, I. G.; Habgood, H.W. J. Card. 1971, 21, 270. (4) Meyer, C.; Bastick, J. Bull. Soc. Chim. Fr. 1974, 1-2, 59. (5) Liu, C.; Chuang, T.; Dalla Lana, I. J. Cutal. 1972, 26, 474. (6) Karge, H.; Rasko, J. J. Colloid Interface Sci. 1978, 64(3), 522. (7) Slager, T.; Amberg, C. Can. J. Chem. 1972, 50, 3416. (8) Karge, H.; Zi6lek, M.; Laniecki, M. Zeolites 1987, 7, 197. (9) Forster, H.; Schuldt, M. J . Colloid Interface Sci. 1975, 5 2 ( 2 ) , 380. (10) Howard, J.; Kadir, Z. Spectrochim. Acta 1985, 41A(6), 825. (11) Laniecki, M.; Zmierczak, W. Zeolites, 1991, 11, 18.
due either to strongly overlapped bands of the asymmetric and symmetric HSH stretching vibrations3 or to the fundamental v 3 asymmetric stretching vibration of H2S,6,9the symmetric mode having a very low extinction coefficient. In this paper we describe qualitative and quantitative aspects of the adsorption and surface chemistry of H2S on Na- and H-ZSM5 zeolites. IR spectroscopy, thermogravimetry, and temperature-programmed desorption were used as experimental means to characterize the adsorption system.
Experimental Section HZSM5 zeolite (Si/Al = 35.5) was provided by Mobil. NaZSM5 was obtained by ion exchange of the hydrogen form in 1 M NaNO, solution at 363 K. For H-ZSM5 an acid site density of 2.42 strong Bronsted acid sites (SiOHAl) per unit cell was determined gravimetrically by pyridine adsorption-desorption experiments. The samples were investigated by means of transmission-absorption IR spectroscopy (Bruker IFS 88, 4 cm-' resolution). Self-supporting disks of the samples (8-10 mg/cm2) were pressed into wafers and subsequently placed in a sample holder at the center of a small furnace in the IR beam. The IR cell was evacuated to pressures below 10-6 mbar. For activation, the sample was heated in situ to 873 K with a heating rate of 10 K-min-I. The adsorption experiments were carried out in situ at room temperature, at HIS partial pressures of 10-5-0.5 mbar. HzS diluted in He (5 mol %) was used. The pressure was kept constant during equilibration by means of differential pumping of the adsorption manifold. The bands of the lattice vibrations between 2090 and 1740 cm-I were used as standards to normalize the IR spectra.I2 A curve
0022-3654/92/2096-2230%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2231
Adsorption of H2S on ZSMS Zeolites 1
3745
a
1
a
absorbonce
6 5 - 4 0 C
9 3 D v)
O 2
02
1
-I
e d
fl 0
C
b a
0 I
3500
3500
3000 2500 2000 1500 wavenumbers/cm- 1
41,
?745 A3725
1
b
obsorbonce O1
Oo5
wovenumbers/cm-
'
Figure 1. IR spectra of the activated samples: (a) H-ZSMS activated at 873 K (b) Na-ZSMS activated at 873 K.
fit program (LABCALC, Galactic Ind. Corp.) was used to deconvolute overlapping bands. After evacuation at room temperature, temperature-programmed desorption (TPD) was carried out in situ (in the IR cell) with a temperature increment of 10 Kqmin-' up to 873 K. The gas phase was analyzed with a quadrupole mass spectrometer (Balzers QMG 31 1) directly connected to the vacuum system. For the gravimetric measurements a Cahn RG electrobalance was used. For these experiments, the samples were pretreated as described for the IR experiments. The concentrations of adsorbed species were determined for equilibrium pressures from to 0.5 mbar.
Results Activated ZSM5 Samples. H-ZSMS activated at 873 K exhibited four IR bands in the region of the OH stretchingvibrations, at 3745,3725 (as a shoulder), 3610, and approximately at 3384 cm-I (Figure la). These bands are assigned to SiOH groups terminating the lattice (3745 cm-I), OH at framework defects (3725, 3384 cm-I), and SiOHAl bridging groups, Le., strong Briinsted acid sites (3610 cm-1).13.14 Na-ZSM5 activated at 873 K exhibited a sharp band due to SiOH vibrations at 3745 cm-l and unresolved bands between 3700 and 3500 cm-I (water on extralattice material and hydrogen bonded SiOH groups) (Figure lb). The lack of the band at 3610 cm-l (attributed to the stretching vibration of the SiOHAl group) shows that the cation exchange was complete. The decrease in the intensity of the bands at 3725 and 3384 cm-l as compared to the hydrogen form of the ZSMS suggests that some of the H+ at these OH groups, i.e., weak Bransted acid sites, were also exchanged by Na+ cations. J. A. Stud. Surf. Sci. Carol. 1989, 46, 585. (13) Jacobs, P.;von Ballmoos, R.J . Phys. Chem. 1982,86, 3050. (14) Qin, G.; Zhcng, L.; Xie, Y.;Wu, C . J . Carol. 1985, 95, 609. (12) Jentys, A.; Lcrcher,
3000 2500 2000 wavenumbers/cm- 1
1500
b
1
-
1639 3680
3500
I
3000 2500 2000 1500 wavenumbers/cm- 1
Figure 2. Adsorption of H2S on Na-ZSM5: (a) differences between the spectra of Na-ZSM5 in contact with H+3 at several equilibrium pressures and the spectrum of the activated sample: (a) lo4 mbar; (b) lo-' mbar; (c) lW2 mbar; (d) IO-' mbar; (e) 0.5 mbar; (b) changts in the IR spectra induced by 1 order of magnitude increment of H,S partial pressure, from lo4 to lo-' mbar.
Adsorption of Ha on Na-ZSMS. The differences between the spectra of H2S in contact with Na-ZSMS and the spectrum of the activated sample are plotted in Figure 2a. Bands pointing upwards increased and those pointing downwards decreased in intensity, compared to the spectrum of the activated sample. Equilibration of Na-ZSMS with lo4 mbar of H2S caused the appearance of a weak and asymmetric band between 2625 and 2500 cm-', attributed to SH stretching vibrations. Band deconvolution indicated three overlapping bands at 2563,2580, and 2593 cm-I. Additional bands were observed at 3583, 3680,2020, and 1639 cm-'. These can be better observed in Figure 2b, in which the difference between the spectrum recorded at lW3 mbar partial pressure and that recorded at lo4 mbar is shown. This mode of presentation emphasizes the changes induced in the IR spectra because of the increase in pressure from lo4 to mbar. In agreement with other author^,^**-'^ we assign the bands at 3583 and 3680 cm-' to the OH stretching and the one at 1639 cm-I to the deformation vibration of molecular water adsorbed on the zeolite surface. The intensity of the band at 1639 cm-' increased with increasing H# partial pressure. We speculate that this water originates from the oxidation of H2S by oxygen traces in the gas phase, according to H20 + S H2S + (1/2)02 The sharp band at 2020 cm-' cannot be unequivocally assigned to a specific vibration. ,It must be noted that the adsorption experiment was repeated several times and the band at 2020 cm-I was always observed. It is known that the sulfur formed during the oxidation reaction might undergo further reaction with H2S to form a polysulfide species (H2S,, x > 2). The formation of this type of compounds at room temperature was previously rep ~ r t e d . ~Tentatively, ~.~~ we propose that this species is responsible
Garcia and Lercher
2232 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 TABLE I: H2S Adsorbed on Na-ZSMS: Band Positions and Normalized Integral Intensities for the Deconvoluted SH Stretching Bands (Region 2500-2600 cm-') normalized integral press./mbar band position/cm-' intensity 10-3 2566 0.004 2580 0.0013 2593 0.0002 10-2 2568 0.009 2580 0.006 2594 0.0018 lo-' 2569 0.013 258 1 0.0 13 2594 0.003 0.5 2572 0.017 258 1 0.02 2595 0.007
for the narrow band at 2020 cm-l. Further, we conclude that most H2S molecules are adsorbed molecularly, because we did not observe an increase in any of the bands attributed to surface OH groups. Subsequent increases of the equilibrium pressure to lO-l, and 0.5 mbar resulted in the increase of the intensity of all previously mentioned bands (Figure 2a). A significant decrease of the intensity of the band corresponding to the SiOH groups at 3745 cm-l was not detected at the different partial pressures employed in this study. Consequently, we attribute the bands between 2625 and 2500 cm-l, assigned to SH stretchingvibrations, to H2Sspecies interacting with different Lewis acid sites. It should be noted that adsorption of H2S on Si02 (Aerosil380) under the same experimental conditions did not lead to a noticeable change of the IR spectrum of the activated sample in the whole range of pressure investigated in the experiment (10-5-0.5 mbar). Table I summarizes the band positions and the normalized integral intensities of all bands in the 2500-2600-~m-~region for loh3, lO-l, and 0.5 mbar. All the bands correspond to a Lorentzian curve shape. As an example, the fitting for the spectrum measured after equilibration of Na-ZSM5 with mbar H2S is shown in Figure 3a. The intensity of the bands at 2580 and 2594 cm-l increased in parallel (see Figure 3b). We assign the bands at 2580 and 2594 cm-l to the symmetric and asymmetric SH modes of H2Smolecules interacting with the Na+ cations. Because the band at 2566 cm-' was observed at very low partial pressures (p < lo" mbar) and the intensity did not increase after equilibration at pressures higher than mbar, we attribute it to SH stretching vibrations of H2S adsorbed on extra lattice material. We conclude that H2Sis coordinatively bound through the lone electron pair of the sulfur atom to cations. The structure proposed for the adsorbed complex is schematically presented in Figure 4a. The 50-cm-l half-width of the band at 2566 cm-l suggests some perturbation of the adsorbed species by weak lateral interactions. As the intensity of the bands of the silanol groups decreased slightly, we speculate that the OH groups at defect sites (close to the extralattice material) are responsible for the perturbation. In contrast, band half-widths of 17 and 10 cm-' were observed for the bands at 2580 and 2594 cm-l, respectively. Subsequent to adsorption, the sample was outgassed 40 min at room temperature. The bands at 2594 and 2580 cm-l disappeared after evacuation. Thus, we conclude that at ambient temperature H2S is reversibly adsorbed on Na+ cations. In contrast, the band at 2566 cm-l was observed after evacuation, indicating stronger interaction between H2S and the extralattice material. The bands at 3680, 3583, and 1639 cm-' (attributed to water adsorbed on cations) were also still present after evacuation at room temperature. Note that this accords well with previously reported results.'' (15) Steijns, M.; Derks, F.; Verloop, A.; Mars, P. J. Cutul. 1976,42, 87. (16) Golyland, S.; Krapivina, T.; Lazarev, V. Russ. J . Phys. Chem. 1966, 36, 700.
0)
P0 .os
e
In
D 0
-02
.01
0 I
2610
2590
2600
2580
2570
2560
wovenumbers/cm-
'
2550
2540
normaliied intensity 0.02
0.015
b
0.01
0.005
0
0 001
0 01
05
01
pressure/mbar
Figure 3. Deconvolution effected on the 2620-25OO-cm-' region for H2S in contact with Na-ZSM5: (a) deconvoluted bands for the difference spectrum between the spectrum collected at mbar of H2S and the spectrum of the activated Na-ZSM5 sample; (b) intensity increase of the SH bands as a function of the H2S partial pressure.
a
H,
/
b
H
H
S
/
Si
H 'S'
0
d
0 *...
AI
C
I
L
f
/Si\
/O...
.
,A'\
/ O\
,y,
Figure 4. Proposed models for the adsorbed structures: (a) H2S on cationic sites; (b) linear hydrogen bonded HIS on bridging OH groups; (c) cyclic hydrogen bonded H2S on bridging OH groups.
Subsequently, TPD was carried out from room temperature to 773 K. H2S and H 2 0 were detected by mass spectrometry in the gas phase as evolving from the surface (see Figure 5). Judged (17) Jentys, A.; Warecka, G.; Derewinski, M.; Lercher, J. A. J. Phys. Chem. 1989, 93, 4837.
Adsorption of H2S on ZSM5 Zeolites m.s.response (arb.units)
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2233
424K
a 0.2 \
\\ \
:.03
\
\..
0 C
"2O m/e = 18
0.1
x
?
.-_. \.-----
-
lo
D 0
\----
.02
m/e = 34
0 315
364
414
464
514
564
614
.01
664
714
764
temperature/K
Figure 5. TPD profiles of H2S and H 2 0 evolving from Na-ZSMS, sample outgassed at room temperature. wovenumbers/cm- 1
absorbance
normalized intensity
b
T
hydrogenbonded 0.004
--
linear structure
1 0.002
3800
3400
3000
1
--
2200
2600
wavenumbers/cm-
2596cm-1
1
Figure 6. Differences between the spectra of H-ZSMS in contact with H2S at several equilibrium pressures and the spectrum of the activated sample: (a) mbar; (b) lo-' mbar; (c) 0.5 mbar.
by IR spectroscopy and gravimetric experiments, the desorption was complete. Adsorption of H2S on H-ZSMS.The differences between the spectra of H-ZSMS in contact with H2S and that of the activated sample are plotted in Figure 6, for equilibration pressures from to 0.5 mbar. Besides a minor interaction of the H2S with the terminal silanol groups (evidenced from a slight decrease in the intensity of the band at 3745 cm-l), no significant interaction of the adsorbed molecule with the zeolite surface was observed at mbar. Bands that could be attributed to perturbed OH vibrations of the silanol groups interactingwith H2Sor to adsorbed H2S species were not observed. After the equilibrium pressure was increased to lo4 mbar, no additional interaction of the H2Swith the OH groups was detected. A very weak absorption band, similar in shape to the one observed upon adsorption of H2Son Na-ZSM5, was observed at 2560 cm-'. Consequently, we assigned this band to the SH vibration of species interacting with extralattice material. At mbar, H2Swas concluded to interact with the SiOHAl because of the decrease in the intensity of the band at 3610 cm-l and the appearance of a flat broad band at 3100 cm-l. The latter band was assigned to the stretching vibration of the SiOHAl groups perturbed by hydrogen-bonded H2S. In parallel, a band appeared at 2580 cm-l. The intensity of the band at 2560 cm-' further increased. After the equilibrium pressure was increased to mbar the intensity of the band at 3610 cm-l decreased substantially indicating further interaction of the H2S molecule with the SiOHAl groups (see Figure 6). Additionally, the intensities of the bands at 3 100 and 2580 an-' increased and an additional band appeared at 2596 cm-l. The asymmetry of the broad band at 3100 cm-l suggests that it is composed of more than one band.
-I
0.01
0.1
0.5
pressure/mbar
Figure 7. (a) Deconvoluted bands for the difference spectra between the spectrum at 0.5 mbar equilibrium pressures of H2S and the spectrum of the activated H-ZSMS. (b) Intensity increase of the S H bands as a function of the H2S partial pressure.
After equilibration at lo-' mbar, further adsorption of H2S on the SiOHAl (Bronsted acid) sites was indicated by the decrease of the intensity of the band at 3610 cm-l. In parallel the intensity of the bands at 3100,2596, and 2580 cm-l increased further and a new broad band appeared at 2440 cm-l. The band at 2560 cm-' did not further increase in intensity after subsequently increasing the pressure. Raising the equilibrium pressure to 0.5 mbar again caused increasing intensities of all previously mentioned bands. At this equilibrium pressure the coverage of the Bronsted acid sites estimated from the decrease of the band at 3610 cm-l reached 54%. A stoichiometry of one molecule per occupied Bronsted acid site was determined gravimetrically. Deconvolution of the overlapping bands observed between 3 100 and 2800 cm-l and between 2600 and 2300 cm-' was carried out for the equilibrium pressures of lO-l, and 0.5 mbar. The bands were fitted with a Lorentzian curve shape. The positions and normalized integral intensities of all fitted bands are summarized in Table 11. As an example, the deconvolution for the spectrum collected at 0.5 mbar equilibrium pressure is plotted in Figure 7a. Because we observe two perturbed OH bands at 3102 and 2944 cm-l, we conclude that the adsorbed H2S molecules assumed two different adsorption structures at the OH Bronsted acid sites. One of the structures can be visualized as a hydrogen-bonded linear form, in which the sulfur atom acts as an electron donor (EPD) toward the proton of the SiOHAl site and the hydrogen atoms of the H2S are pointing away from the surface (see Figure 4b). Accordingly, the two bands at 2580 and 2596 cm-l are assigned
2234 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 TABLE 11: H2S Adsorbed on H-ZSM5 Band Positions and Normalized Integral Intensities for the Deconvoluted OH and SH Stretching Bands, Regions 3200-2800 and 2600-2500 cm-', Respectively normalized integral press./mbar band position/cm-' intensity 10-2 2562 0.0022 4.4 x 10-5 2580 2938 0.0024 3078 0.0045 lo-' 2564 0.0022 0.0013 2580 0.0003 2596 0.0039 2440 0.04 2944 0.13 3093 0.5 2573 0.0029 2581 0.005 0.00 15 2596 0.041 2440 0.24 2944 0.6 3102
to the symmetric and asymmetric stretching vibrations of the S-H bonds. This structure is equivalent to the one proposed for the H2S interaction with metal cations (compare Figure 4 a and b). The parallel increase of the intensity of both bands at 2580 and 2596 cm-I as a function of the equilibrium pressure support this conclusion (see Figure 7b) . The other structure proposed is a hydrogen-bonded cyclic form, in which the sulfur atom acts as electron donor (EPD)to the proton of the SiOHAl acid site, and one of the hydrogen atoms of the H2S interacts with the neighboring oxygen atom of the zeolite framework (see Figure 4c). This is concluded from the appearance of a strongly perturbed S H vibration at 2440 cm-I, and a strongly perturbed OH vibration at 2944 cm-I. Band deconvolution suggests that the band at 2573 cm-l is composed of two bands at 2562 and at 2574 cm-I. The latter of the bands was attributed to the vibration of the free SH group of the cyclic structure. This explains the shift of 9 c m - I to higher wavenumbers and the slight intensity increase of the band at 2562 cm-' noted at 0.5 mbar equilibrium pressure. H2S was reversibly adsorbed at room temperature on the H-ZSMS sample. A Langmuir type adsorption isotherm was determined from the IR intensities of the SH stretching mode for H2S on both H- and Na-ZSMS (see Figure 8). The shape of the curves evidence weak type of interactions between the adsorbate molecule and both surfaces.
Garcia and Lercher
-..-
Na-ZSMS
normalized intensity
H-ZSMS I
normalized intensity
0.05
0.015
0.04 -
0.03
. 0,010
.
0.02.
. 0.005 0.01 -
000
f 00
0 000 02
01
03
04
05
pressurelmbar
Figure 8. Langmuir adsorption isotherms for H$ on Na- and H-ZSM5 (IR normalized intensity vs pressure).
was observed (1639 cm-I). All these modes can be assigned to water molecularly adsorbed on Na+ cations.I7 This water is speculated to be the result of the oxidation of HIS by O2traces according to H2S (1/2)02 S + H20
+
-
The presence of adsorbed water was confirmed by evolution of H20from the Na-ZSMS sample during TPD. On the contrary, oxidation did not occur if H S was contacted with H-ZSM5. The same distinctive behavior was previously reported for NaY and HY zeolite^.^ Upon adsorption on the H-ZSMS sample, specific interactions of H2S with the SiOHAl sites are concluded from the changes in the IR spectra of both the OH groups of the zeolite and SH groups of the adsorbed molecules. The intensity of the 3610-cm-l band (OH stretching vibration) decreased upon H2S adsorption while, in parallel, two broad bands appeared at lower wavenumbers. These bands were attributed to perturbed OH stretching vibrations of SiOHAl groups. The enhanced half-width, enhanced extinction coefficients, and the shift to lower wavenumbersof these bands indicate the formation of hydrogen bonds between the H2S molecule and the surface SiOHAl Bronsted sites. Hydrogen bonding of H2S was previously reported for H2S adsorption on 1- and 7-A1203,Si02 and on H-faujasite-like Au of the order of 150,250-285, and 350-450 cm-' were observed on Si02,y'A1203, and HY, respectively. The Au of approximately 600 cm-l we observed upon adsorption on H-ZSMS indicates a higher strength of interaction between H2S and the SiOHAl groups of H-ZSMS in comparison to HY. In the case of water adsorption on H-ZSM5, protonized water molecules hydrogen bonded to framework oxygen were found to Discussion be formed upon adsorption on SiOHAl groups (strong Brbnsted acid sites).I7J9 Ab initio calculations suggested that water is Dissociative and nondissociative adsorption of H2S on different in two different types of hydrogen-bonded structures oxide surfaces have been discussed by different a ~ t h o r s . ~ , ~ , ~ - stabilized '~ on these sites.20 In one of the structures the adsorbed water Dissociative adsorption was observed exclusively on alkaline and molecule acts as a proton acceptor toward the Bronsted acid site transition metal cation exchanged forms of faujasites."8-10J8 The and as a proton donor toward the neighboring oxygen of the main argument to support that dissociation occurs has been the framework (cyclic structure). The other structure is a type of appearance of new IR bands attributed to OH groups generated ion pair structure, where the protonated water molecule (hydroby the attachment of protons released from the H2S molecule nium ion) is strongly hydrogen bonded through two of the protons according to H2S H+ SH-. to the SiO-A1 framework site. Accordingly, bands detected between 2500 and 2550 c m - I were Considering the higher acidity of H2S compared to H20, we assigned to the stretching mode of SH- groups affiliated to the do not expect the formation of a protonated H3S+molecule. In zeolite cations. However, the tendency to dissociation was said fact, we had not seen spectroscopic evidence for protonation of to decrease with increasing Si/Al ratio. Karge et a1.8 found that the adsorbed molecule. However, the existence of two different for Na-faujasite-like zeolites, H2S dissociation did not occur for hydrogen-bonded adsorbed species of H2S on the Bronsted acid Si/Al ratios greater than 2.5. sites is evidenced by the two overlapping OH perturbed stretching Under our experimental conditions we conclude that molecular vibration bands deconvoluted in the 3200-2800-~m-~ region. adsorption of H2S takes place on both Na- and H-ZSM5 zeolite Furthermore, this is also confirmed by the different rate of growing surfaces. As previously described, the new OH stretching vibration of the S H bands in the 2600-2000-cm-I region. We propose a bands at 3583 and 3680 cm-I upon adsorption on Na-ZSM5 cannot be assigned to new OH groups resulting from simple dissociation as describe above, because also a HOH bending mode (19) Jentys, A.; Mirth, G.; Schwank, J.; Lercher, J. A. Zeolites: Facts,
-
+
(18) Kallo, D.; Onyestyak, G. Y.; Papp, J. J . Mol. Catal. 1989, 51, 329.
Figures, Future; Elsevier: New York, 1989; p 847. (2O),Sauer, J.; Horn, H.; HBser, M.; Ahlrichs, R. Chem. f h y s . Lett., to be published.
J. Phys. Chem. 1992, 96,2235-2241 linear and a cyclic hydrogen-bonded adsorbed structures on the basis of the SH vibrations observed (see Figure 4). It must be noted that hydrogen bonds to atoms with two lone electron pairs are characterized by flexibility with respect to angular variations in one plane. Consequently, the electron donor molecule can be rotated in the plane determined by the directions of the two electron pairs without loss in the energy of interaction. Additionally, hydrogen-bonded bifurcated and cyclic arrangements are as stable as the linear ones in the case of molecules with weak tendency to hydrogen bonds, like HzS.21 It should be emphasized at this point that the “linear structure” of adsorbed HIS does not exist with oxygen-containing polar molecules, while we have observed this type of adsorbate structure for thiols. It is unclear whether this is due to the lower EPD strength of sulfur in comparison with oxygen or to the larger size of sulfur. In the case of the linear structure (also found in interaction with cations) we assigned the bands at 2580 and 2596 cm-’ to the HSH fundamental stretching vibrations uI (symmetric) and u3 (asymmetric), respectively. The HSH fundamental symmetric stretching vibration uI was The HSH reported to appear at 26 14 cm-l in the gas fundamental asymmetric stretching u3 has never been observed, but it was expected to appear 13 cm-’at higher wavenumbers that the symmetric mode. Matrix studies showed that the position and intensity of the u3 mode, both relative to the uI mode, depend on the type of matrix used.2s Consequently, a matrix offering an asymmetric trapping site (i.e., Nz or CO) resulted in a Au (u3 (21) Schuster, P. The Hydrogen Bond, Schuster, Zundel, Sandorfy, Eds.; North Holland: Amsterdam, 1976; Vol. I. (22) Allen, H.; Blaine, L.; Plyler, E. J . Chem. Phys. 1956, 24(1), 35. (23) Allen, H.; Plyler, E. J . Chem. Phys. 1956, 25(6), 1132. (24) Sprague, A,; Nielsen, H. J. Chem. Phys. 1937, 5(2), 85. (25) Barnes, A.; Howells, J. J . Chem. SOC.,Faraday Tram. 2 1971, Part
7, 729.
2235
u l ) = 13 cm-I, similar to the gas phase, but in an enhanced extinction coefficient of the u3 mode ( ~ 3 / ~=l 2) compared to the gas phase (uJ/ul = 1/10). On the other hand, a matrix offering a symmetrical trapping site (i.e., noble gases), generated a Au (v3 - u l ) = 47 cm-I, while the intensity ratio u3/uI was comparable to that of the gas phase. It is interesting to note that in our case similarity with the asymmetrical matrix was found, considering that we observed a Au (u, - u I ) = 14 cm-I (comparable also to the gas phase) while the intensity ratio u 3 / u 1 approached 3/10 (enhanced compared to the gas phase proposed ratio).
Conclusions Molecular adsorption of H.$ was observed to take place on both Na- and H-ZSMS zeolites surfaces. In contrast to HzO adsorbed on H-ZSM5,17J9spectroscopic evidence of H3S+formation was not observed. This is consistent with the higher acidity of H2S compared to water. Two different types of adsorption structures account for the observed IR bands: (i) a linear structure in which the hydrogen atoms of the HzS molecule point away from the surface and (ii) a cyclic one, in which both hydrogen atoms point toward the zeolite surface. On H-ZSMS, hydrogen-bonding specific interactions with the SiOHAl sites was observed; both linear and cyclic structures were found to be present. On NaZSMS a coordinative bonding through the lone electron pair a t the sulfur atom and the Na+ cation results in only one type of adsorption structure, the linear form. It must be noted that in the case of the linear structure it was possible to distinguish between the corresponding HSH symmetric and asymmetric stretching vibration modes. Acknowledgment. This work was supported by the Fonds zur FBrderung der Wissenschaftlichen Forschung under project P7312CHE. C.L.G. acknowledges partial support from CONICET (Argentina). We are grateful to Dr. W. Haag from Mobil Oil Corp. for supplying us with the H-ZSMS sample.
Raman Spectroscopy and Thermal Desorption of NH, Adsorbed on TiO, (Anatase)-Supported V,05 Gregory T. Went, Li-Jen Leu, Stephen J. Lombardo, and Alexis T. Bell* Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California 94720-9989 (Received: June 3, 1991; In Final Form: November 1 I , 1991) The interaction of NH3 with Ti02 (anatase) and a series of TiO, (anatase)-supported V2OScatalysts has been investigated using thermal desorption and in situ Raman spectroscopy. NH3 adsorbs on Ti02(a)by coordinating to Ti4+Lewis acid sites. Upon heating, most of the adsorbed NH, desorbs intact, with only a small amount of N2 being produced. At V2OSloadings well below a monolayer, the dispersed vanadia is present on the surface in the form of monomeric vanadyl and polymeric vanadate species. NH3 adsorbs on vacant Ti4+sites not covered by the vanadia and V5+sites associated with the monomeric vanadyl species. Temperature-programmed desorption of NH, adsorbed on the VzOs loaded samples indicates that NH, is more strongly bound to the surface of these materials and displays a higher tendency to dissociate prior to desorbing. Increasing the coverage of the vanadia species to a monolayer results in a weaker NH,-surface bond. Desorption studies show that substantial quantities of NO and NzO are evolved by oxidation of the NH3 and reduction of the sample. Raman spectra recorded in the presence of NH3 at high temperatures indicate that the terminal oxygen atoms of the polyvanadate species and clustered monomeric species are removed preferentially, resulting in the reduction of these species. On the other hand, isolated monomeric vanadyls are not reduced by the adsorbing NH,. These results suggest that the dissociation of NH, is accelerated by the presence of adjacent V=O groups. Introduction Vanadium pentoxide supported on the anatase phase of titania is known to be an excellent catalyst for the selective reduction of NO by NH3 in the presence of OZ.l4 Previous studies have (1) Bosch, H.; Janssen, F. Coral. Today 1988, 2, 369. (2) Bauerle, G. L.; Wu, S. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17, 117.
shown that the activity of these catalysts depends on the weight loading of vanadia, with the optimal loading occurring near the m ” t ~ ~ d toecover d a monolayer of the Recent (3) Shikada, T.; Fujimoto, K.; Kunugi, T.; Tominaga, H.; Kaneko, S.; Kubo, Y. Ind. Eng. Chem. Prod. Res. Deu. 1980, 20, 91. (4) Pearson, I. M.;Ryu, H.; Wong, W. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Deo. 1983, 22, 381.
0022-3654/92/2096-2235%03.00/0 0 1992 American Chemical Society
