Znd. Eng. Chem. Res. 1996,34, 3675-3677
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KINETICS, CATALYSIS, AND REACTION ENGINEERING A DRIFTS Study of the Surface and Bulk Reactions of Hydrogen Sulfide with High Surface Area Zinc Oxide J. Michael Davidson* and Khalid Sohail Department of Chemical Engineering, King's Buildings, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JL, U.K.
Diffise reflectance FTIR has been used to elucidate the mechanism of the reaction of HzS with high surface area ZnO prepared from hydrozincite and, hence, containing some residual COZ. Hydrogen sulfide is converted into ZnS by successive proton transfers to chemisorbed hydroxyls on the Zn- and O-polar hexagonal faces. Much product molecular water remains adsorbed on the prism faces or combined with residual carbonate as hydrozincite. Counterdiffision of Zn2+ and H+ is proposed as the transport mechanism in the reaction of bulk solid ZnO.
Introduction High surface area zinc oxide is widely used as an absorbent for low-level hydrogen sulfide (down to 0.1 ppm) (Carnell, 1988). In an earlier paper (Davidson et al., 19951, we reported a study of the rate of ambient temperature absorption of hydrogen sulfide by high surface area zinc oxide. Three kinetic regimes became evident in the course of the investigation, and in the present paper, diffise reflectance FTIR is used to extend the level of our understanding of the nature of the absorbent and its surface and near-surface reactions, which constitute the fast stage and about 4-20% conversion of the ZnO. In particular, we find that the DRIFTS method is extremely well-suited t o studies of such reactions under conditions that closely parallel those applicable to a single pellet in an industrial absorber. The DRIFTS cell employed was the Spectratech environmental cell which has a ceramic cup which holds a cylindrical sample (3 mm x 3 mm) usually in powdered form but which can also be an absorbent pellet. Calcining operations can be carried out directly in the cup while the temperature is monitored by a microthermocouple with a junction within the powdered sample. The interferogram is collected from the circular surface layer of the cylinder as a result of reflectionabsorption, while the sample can be bathed in reactive gas or swept with inerts. The Spectratech cell is particularly well-matched to the current investigation because the partial pressures of HzS and HzO of commercial interest are too low to cause interference through infrared absorption by water vapor. The DRIFTS spectra derive from successive reflectionabsorption events in the upper layer of the sample and involve the near-surface layers of the grains or crystallites (Kortum, 1969). This is precisely the zone that is of interest from the point of view of the fast-stage performance of the absorbent in a reactor. Experimental Section Equipment. Diffuse reflectance FTIR spectra were recorded using a Digilab FTS-7 instrument equipped
* To whom correspondence should be addressed (E-mail:
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with an MCT detector and a Spectratech high-temperature environmental cell and mirror system. The cell had zinc selenide windows, a gas path length of 2 cm, and a gas volume of about 2 cm3. The powdered sample (about 50 mg) was held in a ceramic cup which could be heated to high temperature rapidly while the body of the cell was cooled by an internal water flow. Gas flows (-60 cm3/min set by means of capillary restrictors) were delivered from a system of mainly glass construction with a few Teflon tube connections and with Teflon plugs in taps. An aluminum cylinder with a stainless steel regulator was used to prepare and supply HzS (125-6000 ppm) and/or COZ(1000 ppm), whose streams could be further diluted with dinitrogen. Protium water (3000 ppm) was obtained from a saturator held a t 0 "C; DzO vapor was similarly supplied. The effluent from the DRIFTS cell was discharged through a ZnO dump bed and then a rotameter. For calcining operations, dinitrogen was dried by passing it first through a packed bed of activated 5-81 molecular sieve and then through zinc oxide. Freshly heat-treated ZnO (IC1 Puraspec grade; 300 cm3of 3-mm pellets pretreated at 300 "C for 3-4 h) was indispensable, as it had a proportion of highly active sites with greater drying power than the zeolite. Experimental Procedures. Zinc oxide was prepared within the DRIFTS cell by heat treatment of hydrozincite in dry Nz as described previously. Calcining was normally carried out a t 653 K for 1 h, after which the sample was cooled in a continuing flow of dry Nz. During calcining, the evolution of H20 and COZ could be monitored by the absorption spectrum of the gas, while the intensity of the hydroxyl and carbonate lattice modes was greatly reduced. Spectra were collected during the absorption experiments in which the dry Nz flow was replaced by the reactive gas. Spectra were frequently recorded at 8-cmP1resolution to reduce the scan time during data collection in transient rate experiments. Most changes were easily identified in the single-beam spectrum, whereas other weak features emerged more strongly in the Kubelka-Munk (KM) mode. KM spectra (not shown) were obtained by ratioing spectra obtained in the course of the reaction with the initial spectrum. Transient behavior was
0 1995 American Chemical Society
3676 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
3800 -1600 3430 3200 Wavenuniber
2500
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(cm-I)
Figure 1. Calcining of hydrozincite (HZ) and subsequent DzO treatment; (a) HZ flushed with dry Nz for 30 min, (b) zinc oxide obtained after calcining of HZ in situ a t 573 K for 30 min followed by cooling to 298 K in dry Nz flow and (c) after subsequent DzO treatment (0.6% DzO vapor in Nz, flow rate = 60 c d m i n ) for 100 min, (d) residual COz formed during the sulfiding of HZ (125 ppm HzS diluted with Nz, flowrate = 60 cdmin) for 15 min, and (e) gaseous C02 evolved during the calcining of HZ.
readily monitored using, say, 125 ppm HzS in dry carrier (a “dry run”) or after various pretreatments.
Results and Discussion The high surface area zinc oxide was prepared by calcining hydrozincite ( Z ~ ~ ( C O ~ ) Z ( in OH situ ) ~ usually ) at 653 K in a stream of very dry Nz. In the course of the operation (Figure l),the very strong absorptions due to hydroxyl (3350-cm-l) and carbonate (1400- and 1550cm-l) bands are substantially reduced and replaced by residual bulk modes with overlying sharp bands due to surface OH- and C032-. The latter species (about 2 mol %) are partly on the surface and partly in the bulk of which the surface carbonate can be decomposed to COZ by treatment with water vapor at ambient temperature. Second and subsequent heating, cooling, and water treatment cycles cause further diffusion to the surface of the residual Cos2-, which is thereby steadily reduced. The spectrum in the 0-H stretching region (Figure 1) corresponds to partially dehydroxylated hexagonal zinc oxide (see Appendix 1). The sharp high frequencies, 3670 and 3620 cm-l, have been attributed to the isolated OH groups on the zinc-polar faces and oxygenpolar faces that are free of the effects of hydrogen bonding. Bands a t 3450 and 3555 cm-l are due to OH on the prism faces and stepped faces which are broadened by hydrogen bonding and broaden further with coalescence on physisorption of water. A further band appears a t 3645 cm-l if the calcining reaction is carried out in 0 2 , rather than in vacuo or under dry N2. Dioxygen treatment during calcining causes the development of a stepped face on ZnO, which proves t o be highly reactive toward HzS, but influences the rate and stoichiometry only in the very early stages of sulfiding. These experimental procedures parallel the method of commercial production and result in predominently hexagonal platelets or truncated prisms with partially dehydroxylated surfaces. On treatment with dry H2S (125 ppm), the hydroxyl groups on the polar faces (Figure 2) are removed by reaction yielding water ( Y Z bending mode = 1620 cm-l), some of which evidently becomes bound to the hydroxyls on the prism faces and stepped faces, the bands at 3450 and 3555 cm-l becoming broader. As the reaction
3600
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Wnveiruiiiber
1600
1400
1200
(cti1-l)
Figure 2. Sulfiding of partially dehydroxylated zinc oxide using 125 ppm HzS; (a)partially dehydroxylated zinc oxide obtained after heat treatment of an ambiently aged zinc oxide sample at 653 K for 30 min in dry Nz flow followed by cooling to 298 K and (b-0 after treatment of (a) with 125 ppm of HzS in Nz for 30, 40, 60, 90, and 160 min, respectively.
proceeds, a weak future develops a t 2530 cm-l which we attribute to SH groups on the polar surfaces (Reding and Hornig, 1957). The spectrum of gaseous HzS is very weak (Allen and Plyler, 1956) and does not interfere. On flushing the sulfided sample with dry Nz, much physisorbed water desorbs, while the SH frequency was observed to persist for up to 15 h. When water vapor was admitted in the feed after sulfiding under dry conditions, the SH band disappeared after 1min. Thus, it seems that HzS requires the agency of chemisorbed hydroxyls as the means of transfer for both protons. Autocatalysis arises through the provision of these hydroxyls by chemisorption of HzO. Conversely, it can be concluded that sulfiding does not involve direct proton transfer from H2S to the lattice or surface oxide. A further experiment was carried out under the conditions (0.18% HzS; 0.30% HzO) of the previously reported reactor studies with pretreatment using 0.30% HzO. As under dry conditions, absorption due to SH appears at 2530 cm-’, while a strong broad OH frequency developed at 3420 cm-l, which was only partially removed in dry NZand gave only partial H/D exchange. On flushing with pure dry Nz, the SH and surface OH intensities both decreased, confirming that the second stage of sulfiding is proton transfer from SH to OH. The fate of the water and carbon dioxide during sulfiding appears to be crucial in the understanding of the reaction. It is evident from the spectra that molecular water can become bound at several different types of sites during pretreatment and sulfiding. These include physical adsorption on the prism faces and water more strongly bound in the partially sulfided zinc oxide lattice. Physisorbed water also becomes strongly bound after prolonged ambient aging in air. The spectra show that such water does not desorb up t o 473 K. Part of the residual carbonate is also decomposed to carbon dioxide during sulfiding. Infrared assignments for carbonate are given in Appendix 2. Some COz desorbs into the gas stream as shown by the doublet centered at 2350 cm-l, and some reacts with Zn2+and water to form a hydrozincite-like surface phase identified by bands at 1410,1550,1620, and 3745 cm-l. Yet more COZgives rise to a singlet frequency (2345 cm-l) that appears and grows at the center of the P-R branch
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3677 doublet during calcining and sulfiding of hydrozincite (Figure 1)and sulfiding of ZnO (Figure 2). This form of COZis, like some HzO, very strongly held and is not completely removed below 653 K. We assign the singlet frequency to trapped or matrix-isolated molecular COS in the oxide lattice. A singlet asymmetric stretching mode with quenched rotation has previously been reported for the Cu20 system at high temperatures (London and Bell, 1973) and from COZ adsorbed on sodium chloride a t low temperatures (Kozirovsky and Folman, 1964).
coordination unsaturation in the surface will be relieved by retaining HzO, COS, and 0 2 by chemisorption. Atherton et al. (1971) described the surface hydroxyl infrared spectra arising from chemisorption of water on low surface area zinc oxide. By means of rapid H/D exchange using DzO, they were able to separate surface spectra from bulk modes. Their assignments were as f;llows: 3670 cm-' (sharp) (0001)
zinc polar face
3620 cm-' (sharp) (OOOi)
oxygen polar face
Conclusions Absorption of hydrogen sulfide by zinc oxide is autocatalytic and promoted by product water and water supplied in the carrier. The absorbent is ineffective under dry conditions. The central feature of the mechanism appears to be chemisorption of water as hydroxyls on the polar faces followed by proton transfer from hydrogen sulfide to hydroxyl in successive stages. Some of the product water becomes physisorbed on nonpolar faces, while some appears t o be trapped in the lattice. Although the nature of the surface reaction is apparent from this investigation, it is still necessary t o postulate a transport mechanism to account for the bulk reaction. We discount the possibility of diffusion of ZnO from the bulk to the reactive interface at these low temperatures. A reasonable alternative appears to be counterdiffusion of the small cations, Zn2+and H+,allowing the formation of ZnS as an overlayer. Some protons then react with 02- and CO& in subsurface layers, yielding trapped HzO and COa. The above DRIFTS experiments show the structure sensitivity of the reaction between H2S and high surface area zinc oxide. From the results, it can be concluded that in the production of ZnO absorbents, temperature and environmental conditions should be chosen to maximize the exposure of hexagonal polar faces in the grains of the solid.
Acknowledgment We thank the U.K. Engineering and Physical Sciences Research Council for their generous support of this work.
Appendix 1 The marked structure sensitivity of the sulfiding reaction reported in this investigation derives from the hexagonal close-packed (wurtzite) arrangement of the zinc oxide. The faces normal to the hexad axis are called Zn-polar (0001) and 0-polar (OOOl), respectively, because a surface plane overlying the bulk structure would expose gxclusively Zn or 0 atoms in three coordination. The (1010) prism or nonpolar faces have equal exposure of Zn and 0 atoms. Cleavage planes that intersect both meridial and polar axes are called stepped faces. For example, (1011) forms a ridged surface alternating between two and three coordination. Of necessity, faces and defects develop in order to achieve electrical neutrality in the crystal, but it can be anticipated that
3555 cm-' (broad) ( l O i l ) and (1011) stepped faces 3450 cm-' (broad) (lOi0)
prism face
Appendix 2 Carbonate groups on the surface of zinc oxide can be formed either by reaction of COZ with strongly dehydroxylated ZnO or from residual carbonate by heat treatment (1330, 1390, 1470, and 1515 cm-l). The sharp 1330- and 1515-cm-l bands are favored by high calcining temperatures. The formation of hydrozincitelike phases is characterized by simultaneous appearance of bands at 1410-1440 and 1550-1555 cm-l. The latter appear first as shoulders and increase in intensity t o give broad bands.
Literature Cited Allen, H. C.; Plyler, E. K. Infrared Spectrum of Hydrogen Sulfide. J . Chem. Phys. 1966,25, 1132-1136. Atherton, K.; Newbold, G.; Hockey, J. A. Infra-red Spectroscopic Studies of Zinc Oxide Surfaces. Discuss. Faraday SOC.1971, 52, 33-43. Carnell, P. J. H. Feedstock Purification. In Catalyst Handbook, 2nd ed.; Twigg, M., Ed.; Wolfe Scientific: London, 1988. Davidson, J. M.; Lawrie, C. H.; Sohail, K. Kinetics of the Absorption of Hydrogen Sulfide by High Purity and Doped High Surface Area Zinc Oxide. Znd. Eng. Chem. Res. 1995,34,29812989. Kortiim, G. Reflectance Spectroscopy; Springer-Verlag: Berlin, 1969. Kozirovsky,Y.; Folman, M. Infrared Studies of Molecules Adsorbed on Alkali-Halide Films. J. Chem. Phys. 1964,41, 1509-1510. London, J. W.; Bell, A. J. Infrared Spectra of Carbon Monoxide, Carbon Dioxide, Nitric Oxide, Nitrogen Dioxide, Nitrous Oxide, and Nitrogen Adsorbed on Copper Oxide. J . Catal. 1973, 31, 32-40. Reding, F. P.; Hornig, D. F. Vibrational Spectra of Molecules and Complex Ions in Crystals X H2S and D2S. J . Chem. Phys. 1967, 27, 1024-1050.
Received for review April 24, 1995 Accepted J u n e 27, 1995@ IE9405100
Abstract published in Advance A C S Abstracts, September 15, 1995. @
