Low Temperature H2S Removal with Metal-Doped Nanostructure ZnO

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Low Temperature H2S Removal with Metal-Doped Nanostructure ZnO Sorbents: Study of the Origin of Enhanced Reactivity in Cu-Containing Materials Jonathan Skrzypski, Igor Bezverkhyy,* Olivier Heintz, and Jean-Pierre Bellat Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR5209 CNRS, Universite de Bourgogne, 9 avenue A. Savary, BP47870, 21078 Dijon Cedex, France ABSTRACT: Sulfidation of pure and metal-doped ZnO nanostructure sorbents (M0.03Zn0.97O, M = Fe, Co, Ni, Cu) was studied in order to clarify the effect of metal on the transformation kinetics at 200350 °C. The solids were prepared by coprecipitation from metal nitrate solution followed by calcination at 400 °C. Reaction with H2S was studied by thermal gravimetric analysis (TGA) using a gas mixture containing 0.2 vol % H2S in equimolar H2N2. It was found that at 350 °C the TGA sulfidation profiles of all studied samples are similar, with the interface reaction being the main rate-determining step. After lowering the temperature to 250 °C the transformation of Cu0.03Zn0.97O continues to be controlled by the interface reaction with only a slightly decreased rate. In contrast, for all other samples the diffusion resistance appears, provoking a significant drop of their transformation rates. This finding shows that during sulfidation of Cu-doped ZnO the diffusion is faster than for all other sorbents. The same effect was observed for the sample prepared by impregnation of ZnO powder and containing supported Cu species. In order to understand the origin of this effect, the sulfided sorbents were characterized by XRD and N2 physisorption, and no correlation was found between the sulfidation rate and textural properties of formed sulfides. This result indicates that sulfur transport during sulfidation occurs by solid state rather than gas phase diffusion. Also XPS has shown that Cu2SZnS solid solution is formed during sulfidation of the Cu-doped solids. We thus suggest that diffusion enhancement in the presence of copper is brought about by sulfur vacancies created through charge compensation of Cuþ replacing Zn2þ.

1. INTRODUCTION Hydrogen sulfide is a well-known poison for metallic catalysts, and its concentration in feedstocks should be decreased to sub parts per million level before their use. To accomplish this task, ZnO-based sorbents have been successfully employed for decades in different domains of the chemical industry. Given a large variety of process schemes and feedstocks, the optimum desulfurization temperature and the composition of ZnO-based sorbents must be adapted correspondingly. Thus from hydrocarbon feedstocks, used for H2 production by steam reforming, hydrogen sulfide (H2S) can be removed at 350450 °C using pure ZnO.1 In contrast, when eliminating H2S from syngas, obtained by carbon gasification, higher temperatures are preferable (500700 °C) and the atmosphere is strongly reducing. Since under these conditions the major problem is the reduction of ZnO and the volatilization of metallic Zn, zinc titanates 2,3 or zinc ferrite 4 instead of ZnO must be used. Despite the differences in composition, the ZnO-based sorbents used in current industrial applications have similar textures characterized by micrometer-sized particles. Utilization of nanostructure ZnO does not make much sense in these cases since nanoparticles would rapidly sinter at high temperatures used during absorption and/or regeneration of sorbents. The process conditions are however different in some emerging technologies. Thus, H2S scrubbing from syngas in the FischerTropsch process,5 from reformate, or from hydrogen used in low temperature fuel cells (PEMFC)6,7 can be realized at low temperatures (100300 °C) and in some circumstances without on-site sorbent regeneration. Under such conditions the nanostructure r 2011 American Chemical Society

materials are stable, and they should exhibit naturally a higher reactivity toward H2S in comparison with classical sorbents consisting of micrometer-sized ZnO particles. The enhanced activity has recently been demonstrated for ZnO-based nanostructure adsorbents prepared by alkoxide synthesis,8,9 matrixassisted method,10,11 citrate method, 12,13 or solgel autocombustion.13 A fine dispersion of ZnO particles can also explain an enhancement of a low temperature reactivity observed when zinc oxide particles are supported on SBA-1514 or entrapped into glass fibers.15 The reactivity of nanostructure ZnO-based sorbents at low temperatures can further be improved by doping with transition metals. It was shown that addition of Mn, Fe, Co, Ni, or Cu oxides to ZnO allows an increase of the breakthrough capacity during room or moderate temperature H2S absorption.1623 In spite of the promising properties of metal-doped ZnO nanoparticles in H2S absorption, the detailed kinetic studies of the sulfidation are not available in the literature, so the origin of the enhanced reactivity is not clear. Frequently invoked higher dispersion of doped solids can indeed explain a better surface reactivity; however, it is more difficult to understand how this parameter can increase the bulk reactivity of ZnO particles. In order to fill this gap in the present work, we synthesized pure nanostructure ZnO as well as its doped counterparts Received: July 27, 2010 Accepted: March 22, 2011 Revised: March 18, 2011 Published: March 22, 2011 5714

dx.doi.org/10.1021/ie200240e | Ind. Eng. Chem. Res. 2011, 50, 5714–5722

Industrial & Engineering Chemistry Research

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Table 1. Composition and Textural Properties of the Solids Used in This Work SBET, m2/g

molar ratio M/Zn sample

calculated from XRD

D(ZnO),a nm

29

51

21.2

63

104

10.3

0.41

37

58

18.5

0.059

0.40

38

60

18.0

0.032

0.50

44

74

14.5

Vp, cm3/g

measured

0.39 0.54

0.048

0.029 0.030

chemical analysis

XPS

Fe3-ZnO

0.029

0.031

Co3-ZnO

0.030

Ni3-ZnO Cu3-ZnO

ZnO

a

Calculated from a full profile refinement of the XRD patterns.

containing 3 mol % Fe, Co, Ni, or Cu (M0.03Zn0.97O). After a detailed characterization of their structural and textural properties we studied the reactivity of the samples toward H2S (0.2 vol %) in H2/N2 (1:1) flow by thermal gravimetric analysis (TGA). For Cu-containing samples the breakthrough curves were also measured at 200350 °C. Analysis of TGA conversion profiles allowed determination of the rate-limiting step of sulfidation for the sorbents. Also, in order to understand the structural changes underpinning the increase of sulfidation rate, the structure and texture of the sulfided Cu-containing solids were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and N2 physisorption.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Metal-doped ZnO sorbents were synthesized by coprecipitation of Zn(NO3)2 3 6H2O and corresponding metal nitrates (reactive grade, Aldrich) by sodium carbonate. Amounts of 0.0191 mol of zinc nitrate and 5.91 104 mol of dopant precursor were dissolved in 124 mL of water. Coprecipitation was done by dropwise addition of the solution containing the metal salts to a stoichiometric amount of 0.5 M solution of Na2CO3 under vigorous stirring. The obtained suspension was stirred during 4 h, and then the precipitate was filtered, thoroughly washed with water, and oven dried overnight at 100 °C. The thus obtained solid (without any grinding) was annealed in air flow at 400 °C for 4 h (heating rate 2 °C/min). After calcination the solids were ground in an agate mortar and sieved to obtain the fraction of 90125 μm, which was used in all experiments. The pure ZnO sample was prepared in the same way without the addition of dopant. 2.2. Sample Characterization. X-ray diffraction patterns of the samples were recorded using a position sensitive INEL CPS120 detector using the Cu KR monochromatic radiation. The Rietveld full profile refinement of the patterns was realized using FullProf software package.24 In order to determine correctly the particle size, the instrument contribution to a peak broadening was measured with a BaF2 standard and subtracted from the integral peak breadth. Transmission electron microscopy (TEM) micrographs were obtained on a JEOL JEM 2100 instrument. The samples were ground and dispersed in ethanol, and after sonication the obtained suspension was supported on a carbon-covered Cu grid. Scanning electron microscopy (SEM) images were taken using a JEOL JSM6400F instrument. N2 physisorption isotherms were measured on a BEL Mini apparatus, and pore size distributions were calculated using the BarrettJoynerHalenda (BJH) method implemented in the apparatus software package. For chemical analysis the samples were dissolved in 1 wt % HNO3 and the obtained solution was

analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Varian apparatus. X-ray photoelectron spectra (XPS) were recorded with a PHI 5000 VersaProbe spectrometer using Mg anode. 2.3. Characterization of Solids Reactivity toward H2S. For thermal gravimetric study of the reaction kinetics a homemade setup was used. The sample was placed in a quartz pan and suspended in a quartz tube in which an updown gas flow passed. The weight of the sample during reaction with H2S was recorded using a SETARAM TGA92 balance head which was continuously purged with nitrogen. Before the reaction, the solids were heated in N2 flow at 350 °C for 1 h. The reactive mixture containing 0.2 vol % H2S in H2/N2 (1:1) was used during measurements. The sample mass of 2 mg and a reactive gas flow of 150 mL/min were chosen for recording the sulfidation profiles. These conditions were found to ensure the absence of mass transfer limitations from the gas phase to the solid and between the powder particles. During measurements of the breakthrough curves the samples were diluted with silica gel (100 mg of sample þ 30 mg of SiO2) to avoid the formation of preferential pathways for gas flow in the adsorbent bed after a partial sulfidation. It was checked that silica gel does not adsorb H2S under used conditions. The diluted sample was placed on a fused silica support inside a Pyrex U-tube reactor (inside diameter = 6 mm) and heated under N2 flow at 350 °C for 1 h. Then, the reactor was brought to the desired temperature and the reactive mixture containing 0.2 vol % H2S in H2/N2 (1:1) was directed to the reactor with a flow of 50 mL/min. The H2S concentration at the reactor outlet was automatically analyzed every 5 min using an HP6890 online gas chromatograph equipped with an HP-1ms capillary column and a mass spectrometry detector (MSD 5972). The ChemStation software package was used for treating the chromatographic data.

3. RESULTS AND DISCUSSION 3.1. Composition and Structural and Textural Properties of the Samples. Composition and structural and textural

properties of the samples used in the work are presented in Table 1. The particle sizes were obtained from a Rietveld full profile refinement of the XRD patterns, and a satisfactory quality of the fit (Figure 1) was achieved using isotropic peak broadening. This fact indicates that the shape of ZnO crystallites is close to a spherical one and therefore the approximation of the spherical particles is justified when fitting the sulfidation profiles with the unreacted shrinking core model (see below). The important question about the prepared sorbents is whether the dopant oxides form solid solutions with ZnO or they form a separate phase. According to the literature data, 5715

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Figure 2. TEM (a) and SEM (b) micrographs of the pure ZnO sample used in this work.

Figure 1. XRD patterns of the solids used in the work (dots, experimental data; line, full profile fit).

coprecipitation of Zn with Fe, Co, Ni, or Cu generally allows obtaining the solid solutions without formation of dopant's own phase if its content is not very high (ca. 23 mol %).2528 The same seems to be true for our samples since the XRD patterns of Fe3-ZnO, Co3-ZnO, and Cu3-ZnO do not contain any peaks of dopant oxide phases, allowing us to suppose that in these solids the dopant atoms replace Zn2þ in the structure of ZnO. For Ni3ZnO a low intensity peak characteristic of NiO at 2θ ∼ 42.5° can be distinguished in the diffraction pattern (Figure 1), indicating that a part of the dopant segregates in the form of NiO. More detailed information about localization of the dopant atoms was obtained from the XPS (Table 1). Indeed, if the dopant oxide phases are present but undetectable by XRD, they have to be finely dispersed on the surface of ZnO and therefore the surface layer, analyzed by XPS, should be enriched by the dopant. We notice that in accordance with XRD this is indeed the case for Ni3-ZnO, for which the surface Ni/Zn ratio is twice that

determined by chemical analysis. For Cu3-ZnO and Fe3-ZnO the surface metal content is equal to the bulk one, showing that these two metals are completely dissolved in ZnO matrix. A considerable surface enrichment observed for Co3-ZnO indicates that a fraction of Co forms Co oxide but its particle size and/or amount is too low to be detected by XRD. It follows from the data given in Table 1 that the coprecipitation allowed obtaining well-dispersed ZnO-based sorbents. For all materials the measured values of surface areas are lower by ∼ 40% than those calculated from the crystallite size determined by XRD. This effect can be due either to a polycrystalline nature of the particles or to aggregation of the monocrystalline grains which decreases the accessible surface area. The analysis of TEM and SEM images for ZnO (Figure 2) seems to confirm the second possibility. According to the TEM images the solid consists of particles of 2030 nm, which is close to the XRDderived size. It can be noticed however that these monocrystalline grains form aggregates which can be distinguished in the SEM images (Figure 2b) as spheres of 7080 nm. Agglomeration of ZnO particles gives rise to the interparticle voids whose size distribution is centered at ∼30 nm, as follows from the BJH 5716



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3.2. TGA Study of Sulfidation of ZnO and M-ZnO. Sulfidation at 350 °C. The sulfidation profiles of the solids recorded at

350 °C are compared in Figure 4a. In order to obtain quantitative information on the sulfidation kinetics of different sorbents, we applied the shrinking unreacted core model to fit the experimental data.29 Application of this simple model in our case is justified by the fact that the used sorbents consist of nonporous spherical ZnO grains. Two possible limiting steps were considered: the interface reaction and the diffusion through the product layer. According to this model, when both steps have comparable rates, the timeconversion data for spherical particles obey the following equation: FR kR 1=3 2=3 1 ð1 XÞ þ ½1 3ð1 XÞ þ 2ð1 XÞ t ¼ kC 6D ð1Þ

Figure 3. N2 adsorptiondesorption isotherm of the pure ZnO sample. Inset: BJH pore size distribution.

where F is the ZnO molar density (0.069 mol/cm3), R is the particle radius determined from XRD (cm), k is the rate constant (cm/s), C is the concentration of H2S in the gas phase (mol/ cm3), D is the diffusion coefficient of H2S in the sulfide layer (cm2/s), and X is the conversion degree of ZnO. It follows from eq 1 that the ratio kR/6D gives a measure of the relative importance of the rate of interface reaction (kR) and that of diffusion (D) and its value determines which step must be taken into account when describing a conversion profile. If it is close to unity (0.1 < kR/6D < 10), both the interface reaction and diffusion must be accounted for and eq 1 must be used. If kR/ 6D , 1, the interface reaction becomes a unique rate-determining step and eq 1 is reduced to t ¼

FR ½1 ð1 XÞ1=3 kC

ð2Þ

If kR/6D . 1, the transformation is controlled by diffusion and eq 1 becomes t ¼

Figure 4. Sulfidation profiles of M-ZnO samples recorded at 350 (a) and 250 °C (b) (symbols, experimental data; lines, fit with the shrinking core model; see details in the text).

pore size distribution (Figure 3). Similar textural properties were observed for doped ZnO samples (data not shown). The extent of aggregation of monocrystalline grains is the same for all solids as it follows from similar ratios of measured to calculated surface areas (Table 1). However, due to smaller crystallite size the doped samples exhibit higher surface areas than ZnO.

FR 2 ½1 3ð1 XÞ2=3 þ 2ð1 XÞ 6CD

ð3Þ

It is generally accepted that sulfidation of ZnO at moderate temperatures is controlled by diffusion through the sulfide layer.30,31 However, application of eq 3 to the conversion profile of pure nanostructure ZnO used in our study did not give a satisfactory fit. Hence, we tried the complete eq 1 using the following approach. As at low conversion (X < 0.2) the product layer is virtually absent, the transformation is controlled only by the interface reaction, and therefore the application of eq 2 to this domain permits the determination of k. Then this value was fixed and D was varied to achieve the best fit of the whole curve. We found that in this way a satisfactory fit of the conversion profile of the undoped ZnO sample could be achieved with kR/6D = 0.4 (Figure 4a). The necessity to take into account both the interface reaction and diffusion in our case can be explained by a nanometer size of ZnO particles. In such a case even if the diffusion coefficient remains the same, the ratio kR/6D becomes smaller so that one passes from the range corresponding to the diffusion control (kR/6D . 1) to the range corresponding to the mixed control (0.1 < kR/6D < 10). The value of the diffusion coefficient obtained from the fit (3.4 1010 cm2/s) compares fairly well with those reported by Lew et al.30 (2.7 1010 cm2/s at 400 °C) or by Efthimiadis et al.31 (2 109 cm2/s at 400 °C). 5717



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Table 2. Rate Constants of the Interface Reaction for M-ZnO Sorbents at 350°C Obtained from Fitting the Sulfidation Profiles with the Shrinking Core Model k 104, cm/s

sample ZnO

8.4

Fe3-ZnO

5.6

Co3-ZnO

7.3

Ni3-ZnO

9.9

Cu3-ZnO

11.0 (350 °C), 7.6 (250 °C)

In contrast to pure ZnO, the sulfidation profiles of the doped solids recorded at 350 °C can be successfully fitted taking into consideration only the interface reaction (eq 2). This difference can stem from two different effects. First, the decrease of the particle size in the doped solids (Table 1) can diminish the ratio kR/6D so that only the interface reaction should be taken into account to describe the sulfidation profile. Second, addition of metal dopant can increase the coefficient of diffusion, further decreasing the ratio kR/6D. At the same time, the interface reaction seems to not be strongly influenced by doping as it follows from similar values of the rate constant k for doped and pure ZnO (Table 2). Sulfidation at 250 °C. Lowering the reaction temperature results in a drastic change in the shape of sulfidation profiles for all samples except Cu3-ZnO (Figure 4b). The sulfidation profiles of ZnO, Fe3-ZnO, Ni3-ZnO, and Co3-ZnO recorded at 250 °C cannot be fitted with a simple shrinking core model. The reason for this seems to be a change of the rate-determining step in the course of reaction. This change, taking place at X ∼ 0.3, can be inferred from a very different effect of the temperature on the reaction before and after this point. Indeed we note that the initial region of the conversion profiles (X < 0.3) remains almost unchanged after lowering the temperature from 350 to 250 °C, while the region corresponding to X > 0.3 changes dramatically. We suppose that at 250 °C for X < 0.3 the sulfidation of nanostructure ZnO is controlled only by the interface reaction having low activation energy. For X > 0.3 the reaction is fully controlled by diffusion whose activation energy is higher, and when the temperature is lowered to 250 °C, the decrease of the rate is much more pronounced. The absence of any diffusion resistance until X = 0.3 can be attributed to a small grain size in the used sorbents and consequently higher exposed surface and/ or higher reactivity of the surface layer. The conclusion about the surface transformation at low conversion is consistent with the observation that for the sample having the highest surface area (Fe3-ZnO) the change occurs at X ∼ 0.45 while for other samples with lower areas this change takes place at X ∼ 0.3 (Figure 4b). In contrast to other sorbents the sulfidation rate of Cu-doped ZnO changes only slightly after the temperature is decreased from 350 to 250 °C (Figure 4b). Moreover, the shape of the conversion profile of Cu3-ZnO remains the same and it can be successfully fitted with eq 2, pointing out that for this sorbent the interface reaction remains a unique rate-determining step even at 250 °C. This result clearly shows that faster transformation of Cu3-ZnO is due to a more rapid diffusion and is not simply provoked by higher dispersion of the solid. This conclusion is supported by comparison with Fe3-ZnO sample which consists of smaller particles than Cu3-ZnO (Table 1), but it reacts with H2S much less rapidly.

Figure 5. Sulfidation profiles of pure ZnO and Cu-containing sorbents recorded at 250 (a) and 200 °C (b).

Taking into account a practical importance of the acceleration of ZnO sulfidation in the presence of Cu, we have done additional experiments in order to assess how some preparation parameters influence the low temperature sulfidation of Cucontaining samples. 3.3. Influence of the Preparation Parameters on Sulfidation Profiles of Cu-Containing ZnO. First, we verified if an increase of Cu content can further enhance the low temperature reactivity of the sorbents. To check this point, we tested the solid prepared by coprecipitation and containing 6 mol % Cu (Cu0.06Zn0.94O, denoted as Cu6-ZnO). Another experiment was done to understand whether a simpler preparation method, ZnO impregnation, can bring a similar increase of the reactivity. For this purpose, we prepared the sample containing 3 mol % Cu by impregnating pure ZnO with Cu(NO3)2 3 2.5H2O and calcining at 400 °C (denoted as Cu/ZnO). The conversion profiles of all Cu-containing samples recorded at 250 °C are compared to that of ZnO in Figure 5a. The profiles of Cu6-ZnO and Cu/ZnO (for X < 0.75) can be successfully fitted with eq 2, indicating that for these samples the interface reaction is the unique rate-determining step as for Cu3-ZnO. The most surprising observation concerns the sorbent obtained by impregnation: it follows that for X < 0.75 this sample is as reactive as the sorbent containing Cu atoms in the bulk of ZnO particles. This means that even being located on the surface of ZnO particles Cu dopant atoms can accelerate the sulfidation. After the temperature is lowered to 200 °C (Figure 5b), the sulfidation of all samples slows down due to appearance of the diffusion resistance after a partial transformation. However, the transformation degree after which the reaction slows down is 5718



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Table 3. Breakthrough Capacities of Pure and Cu-Containing ZnO Sorbents breakthrough capacity, mol of S/mol of ZnO 350 °C

sample

250 °C

200 °C

ZnO

0.71

0.28

0.10

Cu3-ZnO

0.79

0.70

0.47

0.76

0.61

Cu6-ZnO (Cu0.06Zn0.94O)

Figure 6. Breakthrough curves of pure ZnO and Cu-containing sorbents measured at 350 (a), 250 (b), and 200 °C (c).

higher for the sorbent containing more Cu (Figure 5b), showing that increase of Cu content allows further improvement of the low temperature reactivity of ZnO. 3.4. Breakthrough Curves for Pure and Cu-Containing ZnO. In order to verify if a higher sulfidation rate of Cu-containing samples results in better performance in H2S absorption, we measured the breakthrough curves for pure ZnO and Cu-containing samples at 350, 250, and 200 °C (Figure 6). The curves represent the concentration of H2S at the reactor outlet as a function of the molar ratio of H2S passed through the reactor to the amount of ZnO in the adsorbent bed. The breakthrough threshold in our study corresponded to 1 ppmv (104 vol %) H2S. One can see that the performances of the sorbents follow the trend observed in TGA sulfidation profiles. At 350 °C the breakthrough capacity of the Cu-doped sample is higher than that of pure ZnO (Table 3), in accordance with the more rapid transformation measured by TGA (Figure 4a). The difference becomes more important at lower temperatures at which the Cudoped solids show much higher breakthrough capacities. Thus, at the lowest studied temperature (200 °C, Figure 6c) the capacity is multiplied by a factor of 6 when passing from ZnO to Cu6-ZnO (Table 3). Our measurements confirm therefore the increase of a low temperature sulfidation capacity of ZnO after doping with Cu, which has already been reported 23 and attributed to better dispersion. Our interpretation of this effect is however different. Even if the specific area of ZnO increases after doping with Cu (Table 1), our TGA experiments show that the main reason for

Figure 7. XRD patterns of sorbents after sulfidation at 250 °C: (a) Cu/ ZnO, (b) Cu3-ZnO, and (c) ZnO. s, peaks of sphalerite; w, peaks of wurtzite. Peak marked with asterisk is due to the sample support.

the increased breakthrough capacity is an acceleration of H2S diffusion during ZnO sulfidation in the presence of Cu. Taking into account the considerable practical significance of this phenomenon, it would be interesting to understand the origin of improved H2S diffusion in Cu-containing samples. If we assume that H2S transport through ZnS layer occurs in the gas phase through the pores, the acceleration observed in Cu-doped ZnO should be provoked by some modification of the microstructure (particle size and surface area) of ZnS when it is formed in the presence of Cu. To get information on this point, we compared the textural and structural properties of pure ZnO and different Cu-containing samples after sulfidation to different degrees at 250 °C. 3.5. Characterization of Sulfided Cu-Doped and Pure ZnO Sorbents. The XRD patterns of fully sulfided ZnO, Cu3-ZnO, and Cu/ZnO are compared in Figure 7. We note that all patterns contain the peaks corresponding to both cubic (sphalerite) and hexagonal (wurtzite) modifications of ZnS. In both cases Zn atoms are located in tetrahedral interstices of the close-packed sulfur layers, which are stacked in cubic sequence in sphalerite (ABCABC...) and hexagonal sequence in wurtzite (ABAB...). At 250 °C the thermodynamically stable modification is sphalerite, 5719



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Table 4. Textural Properties of Pure ZnO and Cu-Containing Sorbents after Sulfidation D(ZnS),a nm

BET, m2/g

sulfided ZnO

8.9

18

sulfided Cu3-ZnO sulfided Cu/ZnO

7.6 9.7

28 17

sample

a

Calculated from the broadening of the Bragg reflection at 2θ = 47.3°.

Figure 9. XRD patterns of Cu3-ZnO and ZnO after sulfidation to 50% at 250 °C.

Figure 8. Evolution of ratio of BET surface area of partially sulfided solids to surface area of pristine oxides for ZnO and Cu3-ZnO.

but due to the low energy of formation of stacking faults in these structures, many different polytypes with mixed stacking sequences can be formed.3234 The extreme case of this phenomenon is a random stacking of the layers which was observed previously in ZnS nanoparticles obtained in acidic solution.33 It was shown that in such a case the (102) peak of wurtzite (2θ = 39.6°) is absent and the (103) peak (2θ = 52°) forms a broad hump.32 We notice that the same is true for all our samples (Figure 7), which means that sulfidation at 250 °C results in formation of ZnS particles with a random layer stacking. The absence of the (102) peak of wurtzite makes impossible the application of a full profile treatment which by definition takes into account all peaks. Hence, to estimate the sulfide particle size, we used the broadening of the well-separated peak at 2θ = 47.3°, which is a superposition of sphalerite (220) and wurtzite (110) peaks. As l = 0 for both peaks, such an estimation corresponds to the size in the direction parallel to the layers and is not influenced by the disorder in the stacking. The corresponding data along with BET surface areas of the fully sulfided samples are reported in Table 4. Analysis of the textural properties of ZnO, Cu3-ZnO, and Cu/ ZnO samples sulfided at 250 °C shows that they can hardly be used to explain the reactivities of these sorbents toward H2S. Comparison between ZnO and Cu/ZnO is the most eloquent in this context. In fact, the sizes of sulfide particles and their surface areas are very similar for these two samples, while the sulfidation of Cu/ZnO is much faster. This discrepancy shows that a higher diffusion rate in the Cu-containing solids is not related to some particular textural properties (particle size or porosity) enhancing diffusion in the gas phase. In other words, the limiting step of ZnO sulfidation seems to be related to the transport of sulfur species in the solid state rather than in the gas phase (as it has yet been previously suggested30). To further support our conclusion about the absence of the correlation between textural properties and sulfidation rate, we

Figure 10. XPS spectra of Cu 2p in Cu3-ZnO in oxide form (a) and after sulfidation at 250 °C to different conversions: 17 (b), 50 (c), and 100% (d).

characterized Cu3-ZnO and ZnO after different sulfidation degrees at 250 °C by nitrogen adsorption and XRD. The BET surface areas of the partially sulfided solids normalized by the surface areas of the pristine oxides are compared in Figure 8. The similarity between the trends for pure and Cu-doped ZnO is again in sharp contrast to a much higher reactivity of the Cu3ZnO sample. Comparison of the XRD patterns of the solids sulfided to 50% (Figure 9) is in line with the BET data. Even if a detailed analysis of the patterns is not possible for the reasons invoked earlier, we note that the shapes of the sulfide part of the patterns (a broad peak with a shoulder at 2θ ∼ 28°) are rather close. Also the particle sizes determined from the broadening of the peak at 2θ = 47.3° are not very different: 8.0 nm for ZnO and 7.3 nm for Cu3-ZnO. These observations point out that not only the surface areas of the samples but also the crystallite sizes of formed ZnS are similar for partially sulfided Cu3-ZnO and ZnO. 5720


Industrial & Engineering Chemistry Research The partially sulfided Cu-containing samples were also analyzed by XPS to obtain information on Cu localization during sulfidation (Figure 10). In the spectrum of pristine Cu3-ZnO the position of the Cu 2p3/2 peak (933.5 eV) is characteristic of Cu2þ surrounded by oxygen.35 After even a partial sulfidation the peak becomes narrower and shifts to lower energy (932.6 eV), indicating the appearance of Cuþ in a sulfide environment.35 The Cu/Zn ratio measured by XPS in sulfided samples (0.045) is higher than in the initial oxide, showing that the ZnS layer formed during sulfidation is enriched with Cu. The same effect is observed for the solid containing supported copper (Cu/ZnO) in which the Cu/Zn ratio increases from 0.03 to 0.08 after sulfidation. Such a redistribution of Cu atoms during sulfidation occurs probably because of their high mobility in sulfide environment. This effect corroborates previously reported formation of Cu2S nanowires from metallic Cu at room temperature 36 or a rapid migration of Cuþ cations in the bulk of CdS particles in a Cu2S/ CdS junction.37 The XPS allows thus the conclusion that after sulfidation a Cu2SZnS solid solution is formed for both bulkand surface-doped ZnO material. Concerning the mechanism of diffusion acceleration in the presence of Cu, we suppose that it is brought about by increase of defect concentration in Cu2SZnS solid solution. Indeed, it is known that the charge compensation of Cuþ replacing Zn2þ in ZnS results in the appearance of sulfur vacancies.38 Given that the main mechanism of anion transport is the jumps into the neighboring vacant sites, the increase of number of sulfur vacancies must considerably accelerate the diffusion of O2 and S2. To determine whether the diffusion occurs in bulk or by grain boundaries, the value of diffusion activation energy can be used as a criterion. To estimate it, we calculated the rate of ZnO sulfidation at 200 and 250 °C for the range X > 0.4, in which the reaction is controlled only by diffusion. Using the Arrhenius equation, we found that the activation energy is about 60 kJ/mol, which is much lower than typical values observed for the bulk anionic diffusion (300400 kJ/mol).39 We conclude therefore that during ZnO sulfidation the diffusion occurs by the grain boundaries of forming ZnS crystallites.

4. CONCLUSIONS Reaction between H2S (0.2 vol % in H2N2 equimolar mixture) and nanostructure ZnO doped with metals (M0.03Zn0.97O, M = Fe, Co, Ni, Cu) was studied at 200350 °C by TGA. For Cu-containing samples this kinetic study was completed by breakthrough curve measurements. The sulfidation profiles recorded at 350 °C were successfully fitted with the unreacted shrinking core model, and it was shown that transformation of the doped solids is controlled only by the interface reaction, while for pure ZnO diffusion contribution should also be taken into account. At 250 °C the sulfidation of all samples except Cu0.03Zn0.97O slows down considerably after a transformation degree of 0.3 because of the appearance of diffusional resistance. In contrast, for Cu0.03Zn0.97O the interface reaction continues to be a unique ratedetermining step, indicating that in this case the diffusion is more rapid than in other samples. Such an improved reactivity of Cu-containing solids permits the achievement of much higher breakthrough capacities at low temperatures. Thus, at 200 °C the breakthrough capacity of the sample containing 6 mol % Cu (Cu0.06Zn0.94O) was found to be 6 times higher than that of pure ZnO.

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Characterization of sulfided Cu-doped and pure ZnO by XRD, N2 physisorption, and XPS has shown that the enhanced reactivity of Cu-containing solids is not related to higher porosity or smaller particle size in the formed sulfide. We concluded therefore that sulfur transfer through ZnS layer occurs not in the gas phase but rather by a grain boundary solid state diffusion. Its acceleration in the presence of Cu is due to a high concentration of anionic vacancies created after replacement of Zn2þ by Cuþ. The same phenomenon is shown to occur even in ZnO particles containing supported Cu due to a fast diffusion of Cu cations from the surface into the bulk of ZnS particles during sulfidation.

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Low Temperature H2S Removal with Metal-Doped Nanostructure ZnO

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