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activated carbon at low temperature
Journal Pre-proof Role OF H2O and O2 during the reactive adsorption OF H2S ON CuO-ZnO/Activated carbon at low temperature S. Cimino, L. Lisi, A. Erto, F.A. Deorsola, G. de Falco, F. Montagnaro, M. Balsamo PII:
S1387-1811(19)30808-X
DOI:
https://doi.org/10.1016/j.micromeso.2019.109949
Reference:
MICMAT 109949
To appear in:
Microporous and Mesoporous Materials
Received Date: 23 October 2019 Revised Date:
4 December 2019
Accepted Date: 9 December 2019
Please cite this article as: S. Cimino, L. Lisi, A. Erto, F.A. Deorsola, G. de Falco, F. Montagnaro, M. Balsamo, Role OF H2O and O2 during the reactive adsorption OF H2S ON CuO-ZnO/Activated carbon at low temperature, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/ j.micromeso.2019.109949. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
CRediT author statement Stefano Cimino: Conceptualization, Investigation, Writing- Reviewing and Editing, Supervision, Funding Acquisition Luciana Lisi: Investigation, Validation, Writing - Original Draft, Supervision Alessandro Erto: Data Curation, Writing - Original Draft Fabio A. Deorsola: Investigation, Formal Analysis, Writing - Original Draft Giacomo de Falco: Investigation, Data Curation Fabio Montagnaro: Formal analysis, Writing - Original Draft Marco Balsamo: Investigation, Data Curation, Formal analysis, Writing - Original Draft
ROLE OF H2O AND O2 DURING THE REACTIVE ADSORPTION OF H2S ON CuO-ZnO/ACTIVATED CARBON AT LOW TEMPERATURE S. Ciminoa1, L. Lisia, A. Ertob, F.A. Deorsolac, G. de Falcod, F. Montagnarod, M. Balsamod a
b
Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy
Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy c
d
Dipartimento Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant'Angelo, 80126 Napoli, Italy
Abstract The reactive adsorption of H2S on CuO-ZnO dispersed onto activated carbon was investigated in different gas streams, containing either O2, H2O or O2/H2O, at room temperature and in a lab-scale fixed-bed reactor. Sorbents performances were analyzed in terms of H2S capture rate and capacity, and results correlated with the sulphur species formed upon adsorption under different experimental conditions. Temperature Programmed Desorption (TPD), porosimetric and XPS analyses were performed in order to support the adsorption dynamic tests. The co-presence of O2 and H2O caused a remarkable increase in H2S adsorption capacity, in particular for Cu-containing sorbents, favouring the formation of various sulphur species such as sulphides, sulphates and, above all, elemental sulphur. The analysis of the experimental data set showed the occurrence of two main H2S oxidation mechanisms with different rates: the first one quickly formed metal sulphates utilizing lattice oxygen from metal oxide clusters or superficial oxygen species; the second process was
1
Corresponding author: [email protected]
1
slower and required moisture and molecular oxygen in the feed gas to catalytically form elemental sulphur chains that progressively filled-up the sorbent microporosity.
Keywords: biogas purification; catalytic oxidation; activated carbons; sorbent
1. Introduction The elimination of hydrogen sulphide (H2S) from high-value gas streams such as reformate gas, natural gas and raw biogas is one of the mandatory steps for their valorization as fuels for energy production. H2S is a toxic gas for human beings, corrosive for engines and for fuel processing devices, and poisonous to catalysts. Hence, its maximum concentration in gaseous streams must be kept below 1 ppmv with appropriate treatments [1]. In the industrial practice, adsorption has gained high esteem as downstream gas treatment, due to a general versatility and simplicity of the process. Currently, many research efforts are devoted to the development of high-performance sorbents capable to efficiently remove large quantities of H2S from various gas streams already at room temperature: to this aim, active phases mainly based on metal oxides (e.g. ZnO, CuO, Fe2O3, MgO) are finely dispersed on porous supports [2−6]. Activated carbons are often adopted as supports due to their favourable textural properties, such as high specific surface area and pore volume, and because they are also able to promote the oxidation of H2S in the presence of even small amounts of surface oxygen, which can provide a significant contribution to adsorption capacity [6,7]. The definition of the adsorption mechanism, along with the punctual individuation of the species formed upon H2S reactive adsorption, assumes fundamental importance for design purposes, process optimization and for the possible individuation of effective regeneration strategies of the spent sorbents. When ZnO and CuO are used as active phase for H2S capture,
2
the indications retrievable from the pertinent literature are conflicting. Some authors reported experimental evidence of the formation of the correspondent metal sulphides [3,8]. However, the presence of heteroatoms, oxygenated surface functional groups and inorganic matter on activated carbon can also promote oxidation reactions mainly leading to elemental sulphur and sulphur poly-chains (Sx), which in turn can react to give further sulphur compounds (e.g. SO2) [8,9]. Moreover, a synergistic effect in the presence of metals can be also present, the metal acting as a catalyst for H2S oxidation [10]. Nguyen-Thanh and Bandosz [7] hypothesized that copper dispersed on activated carbon and bound to the carbonaceous surface can activate the oxygen locally present, leading to the formation of SO2, elemental sulphur and copper complex (e.g. Cu2S) in significantly larger quantities than on the raw carbon (i.e. without copper deposition). In our previous works, we investigated the use of ZnO/CuO supported onto activated carbon under dry feed conditions. Experimental results clearly showed the formation of sulphates during the initial phases of the reactive adsorption process likely promoted by the presence of copper as oxygen donor. Cu and Zn sulphides started to be formed with slow overall kinetics only after the corresponding sulphates, probably due to the lack of oxygen availability on the sorbent [11]. The complexity of surface reactions increases with multicomponent gas mixtures, which unquestionably represent more realistic gas streams. For instance, a typical biogas composition mainly includes CH4 (40–75 %), CO2 (25–40 %), N2 (0.5–2.5 %), O2 (0.1–1 %), H2S (0.1–0.5 %) and humidity, even if it can significantly vary depending on organic content of residues, retention time and digester operating conditions [12]. The effect of gaseous O2 on H2S capture is equivalent to that exerted by oxygen functional groups on carbonaceous supports, leading to the formation of elemental sulphur and SO2 and is enhanced by the co-presence of humidity [10,12]. On the contrary, the effect of water is still debated and likely depends on the specific adsorption system. In most of the
3
cases, the presence of a water film adsorbed on activated carbon surface is claimed to enhance the H2S reactive adsorption, as it can lead to a faster dissociation of H2S and the subsequent oxidation to SO2, elemental sulphur and sulphuric acid. It was highlighted that H2S dissociation is promoted by a basic local pH, due to the presence of either basic surface functional groups [13] or metal oxides [14]. Similarly, the hydroxylation of metal oxides (e.g. CuO) by reaction with water is reported to enhance H2S adsorption by dissociation of H2S (giving HS− and S2−) and subsequent reaction with the metal cation to give the correspondent sulphide [15]. On contrary, Huang et al. [8] reported that water presence is detrimental for H2S adsorption on Cu-supported activated carbon due to possible competition phenomena, while Yang and Tatarchuk [16] working on ZnO/SiO2 and Cu-ZnO/SiO2 sorbents found a negligible effect of water on H2S adsorption. In order to elucidate the role of O2 and H2O in the adsorption mechanism of H2S from gas streams onto CuO-ZnO dispersed onto activated carbon, we set out a new experimental study based on the results previously obtained on the same adsorption system, operated under inert, dry conditions [4,11]. H2S dynamic adsorption tests from gaseous streams containing O2, H2O and H2O + O2 were carried out at room temperature in a lab-scale fixed-bed reactor operated at short contact times in order to elucidate kinetic effects of the process. Temperature Programmed Desorption (TPD) tests, as well as XPS and porosimetric analyses on partially exhausted sorbents, were carried out in order to investigate the nature of sulphur species formed upon adsorption, retrieving information on the overall H2S reaction mechanism. The final target was a thorough comprehension of the intertwining between solid properties (e.g. specific surface area, Cu/Zn ratio, etc.) and gas stream composition in the H2S reactive adsorption.
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2. Experimental Adsorbents based on CuO and ZnO highly dispersed onto a commercial granulated activated carbon (Darco G40, Norit) were produced by incipient wetness impregnation using aqueous solutions of metal nitrates, and finally heat-treated at 250 °C under inert flow [4]. The total metal loading was set to 10% wt. and formulations included samples with only Zn or Cu as well as equimolar amounts of the two metals (sorbents termed as Zn/AC, Cu/AC and Cu0.5Zn0.5/AC, respectively). The textural properties of the sorbents were determined by N2 adsorption at –196 °C starting from P/P0=10–5 using a Quantachrome Autosorb 1-C instrument. Before analysis, the samples were degassed under high vacuum at 120 °C for 18 h. The BET method (P/P0 range from 0.01 to 0.11) was adopted for the calculation of the specific surface area (SBET), while the micropore volume (VM) and the total pore volume (VTOT) were computed by means of the Dubinin–Astakov and Gurvitsch’s rule (at P/P0=0.995), respectively. The pore size distribution (PSD) was evaluated via the Quenched Solid Density Functional Theory (QSDFT). XPS analysis was carried out on a XPS PHI 5000 Versa probe apparatus, using band-pass energy of 187.85 eV, a 45° take-off angle and a 100.0 µm diameter X-ray spot size for survey spectra. High-resolution XP-spectra were recorded in the following conditions: pass energy of 20 eV, resolution of 1.1 eV, and step of 0.2 eV. Sample charging effects were eliminated by referring to the spectral line shift to the C 1s binding energy (BE) value of 284.8 eV. XPspectra were analysed by means of commercial software (CasaXPS, version 2.3.16), by applying mixed Gaussian–Lorentzian (70–30 %) profiles. The assignation of XPS signals was based on the NIST compilation of spectral data [17] unless otherwise stated. H2S dynamic adsorption tests were carried out at atmospheric pressure and T=30 °C, in a lab-scale experimental apparatus based on a quartz fixed-bed reactor with an annular section,
5
already described elsewhere [18,19]. A weighted amount of sorbent (20–200 mg) with a particle size range of 125–200 µm was mixed with quartz to obtain a total mass of the bed of ) and the total flow rate (Qt) were set at 100
200 mg. The inlet concentration of H2S (
ppmv (in high purity, dry N2, reference case) and 20 SL h–1, corresponding to a contact time as low as 3.4–34 ms, referred to the volume of the bed. Notably, those conditions were selected to investigate the kinetic aspects of the reactive adsorption process rather than to estimate the equilibrium saturation capacity of the sorbents. The role of O2 and H2O in the feed gas was studied by adding to the feed 2500 ppmv of O2 and/or by passing part of the gas stream through a water saturator before feeding H2S, thus achieving a relative humidity (RH) content in the range 20–50%. Gas analysis was performed with a continuous analyser (ABB Optima Advance Limas 11 UV) for the simultaneous measurement of H2S and SO2, with cross-sensitivity correction and accuracy better than 1% of span (200 ppmv). The cumulative H2S adsorption capacity,
[mg g–1], was determined through a
material balance over the adsorption column: ad s
=
Q t C in H 2S ρH 2S m
t∗ 0
where Qt [L s–1] is the total gas flow rate;
1−
C out H 2 S (t) C in H 2S
dt
(1)
[-] and
fractions at the inlet and outlet of the bed, respectively;
[-] are the H2S volumetric [mg L–1] is H2S density; m [g] is
the sorbent mass; t* [s] represents the capture time. By convention, breakpoint and saturation conditions refer to capture times for which the outlet H2S concentration equals 5% and 99% of its inlet value, respectively. Prolonged dynamic capture tests were run over several days with a discontinuous operation (shut down during night-time). Temperature programmed desorption (TPD) tests were carried out in the same lab-scale set-up used for dynamic adsorption tests. In a typical TPD test, the sorbent (just after the adsorption test, without being exposed to air) was treated under a flow of 20 SL h–1 of high
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purity N2 (99.999%) and heated at 10 °C min–1 up to a maximum of 620 °C. The specific amount of desorbed sulphur-based species was evaluated in a similar way as in Eq. (1) by integrating the temporal profiles of H2S+SO2.
3. Results and Discussion Figure 1 shows the breakthrough curves, i.e. the temporal profiles of
/
, for the
three sorbents under investigation (Cu/AC, Cu0.5Zn0.5/AC and Zn/AC) and raw AC for comparison. In particular, sub-figures (a), (b) and (c) refer to operating conditions with (a) presence of O2 only, (b) presence of H2O only and (c) co-presence of H2O+O2. The corresponding values of H2S adsorption capacity, ωads (Eq. (1)), are listed in Table 1 where, for comparison, the data previously obtained in absence of both H2O and O2 are reported as well [11]. In the latter case (dry N2 environment), with respect to the very low value shown by the raw AC (ωads=2.7 mg g–1), it was highlighted the promoting role for H2S capture increased in following the order Zn< Cu < Zn + Cu (e.g. ωads as high as 34 mg g–1 for Cu0.5Zn0.5/AC). In the presence of O2, the capture capacity increased: O2 can be able to foster (i.e., to “catalyse”) the H2S reactive adsorption process, favouring the formation of sulphates, sulphides and elemental sulphur. The effect of oxygen was particularly evident when copper was present. In the presence of H2O, the values for ωads were even higher (up to 56.8 mg g–1). After adsorption into sorbent pores, water vapour can contribute to the dissociation of sulphidric acid thus promoting the formation of elemental sulphur. The two mechanisms jointly act and result in better adsorption performance when both H2O and O2 are present in the reactive environment (again, particularly for Cu-containing sorbents). Notably, in the co-presence of H2O + O2, the relative ranking between Cu/AC and Cu0.5Zn0.5/AC is reversed with respect to the one retrieved in the other tested conditions, i.e.
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ωads was higher for Cu/AC (129.2 mg g–1) than for Cu0.5Zn0.5/AC (118 mg g–1). Moreover, the occurrence of a very long tail for both Cu/AC and Cu0.5Zn0.5/AC (see Figure 1(c)) made difficult the rigorous application of Eq. (1) for the calculus of ωads. For this reason, the integral was calculated up to t=20000 s instead of t*. These aspects will be further discussed in the following. Figure 2 reports the trends of the specific H2S removal rate, r, against the cumulative amount of H2S captured by Cu0.5Zn0.5/AC sorbent during the tests of Figure 1, in comparison with the reference case represented by a feed of H2S in N2 (dry). As a general consideration, r is initially equal to the H2S molar feed rate and significantly decreases after the breakpoint has been reached. This was related to the formation of oxidized sulphur species that partially hinder the pore structure of the adsorbent, so limiting the diffusion of H2S [11]. In particular, in the reference case (H2S in dry N2), the reaction rate shows a vertical asymptote in correspondence of the saturation capacity, probably due to the complete consumption of the oxygen contained in the active phase and on the surface of the AC. The addition of 2500 ppmv of O2 in the feed has no impact during the first part of the capture process, leading to an almost identical breakthrough capture capacity (Table 2). However, the reaction rate does not drop abruptly but it shows a clear inflexion point, suggesting that O2 can induce a residual H2S capture activity via a relatively slower mechanism of reactive adsorption. Under humid N2 feed conditions, the breakthrough capacity is enhanced and, thereafter, the capture rate is faster than in dry conditions, but it eventually drops due to saturation once again for the lack of available oxygen. Notably, the further addition of molecular oxygen to a humid feed has little effect on the breakthrough adsorption capacity (Table 2) but it significantly enhances the residual capture rate, which decreases slowly along with the progressive increase in the amount of sulphur without reaching a clear saturation during the tested adsorption times.
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To get more information about the reactive mechanisms in the initial stage of the adsorption process, TPD tests were carried out on sorbents previously exposed to H2S during dynamic tests that were stopped at the corresponding breakpoint times. Table 2 lists the values of the specific amount of sulphur desorbed as SO2. H2S was never detected, thus confirming that the adsorption was characterised by a chemical (oxidation) reaction, rather than a barely physical process. However, only a partial recovery of the adsorbed sulphur can be obtained during TPD, due to the possible desorption of species not detectable by the UV analyser (“ghost species” such as elemental or organic sulphur and termed as other S-species in Table 2), as well as to the formation of metal sulphides, which are stable up to 1000 °C under inert flow [4,11]. The TPD peaks in Figure 3 were identified according to the literature by comparison to reference materials [11]. The broad peak around 500 °C is typical of the decomposition of ZnSO4 supported on activated carbon, while the peak around 340 °C is assigned to the first decomposition peak of CuSO4/AC. Under reference inert dry feed conditions, the formation of ZnSO4 is largely favoured during the early stages of the capture process, with CuO showing a significant promoting effect as oxygen donor [11]. The addition of O2 to the feed mix causes minor modifications in the relevant TPD profile of Cu0.5Zn0.5/AC, in agreement with the results presented in Figure 2. It can be argued that H2S capture still proceeds via reaction with the lattice oxygen from the mixed Cu-Zn oxides, mainly forming ZnSO4. Molecular O2 appears to have a limited effect to promote the formation of CuSO4 and no effect on the formation of other S-species. The presence of humidity (within an inert feed) marginally affects the total amount of sulphate species found by TPD, while it slightly promotes the formation of CuSO4. Notably, the main effect of water addition is to increase the amount of ghost species formed during the early stage of the process. This result agrees well with previous reports on H2S capture on (impregnated) activated carbons and is generally ascribed to the higher reactivity of HS−
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species deriving from the dissolution of H2S in a liquid film leading to the formation of elemental sulphur [5]. Table 2 indicates that the simultaneous presence of O2 and humidity can further speed up the early formation of those S species that are not detectable during our TPD experiments: indeed, ghost S-species represent almost 53% (on a mass basis) of the total sulphur-based compounds stored up to the breakpoint. Moreover, Figure 3 shows a more intense peak related to CuSO4, whereas the amount of ZnSO4 is not significantly affected. Therefore, it can be concluded that CuO, rather than ZnO is able to activate a different reaction mechanism in the presence of humidity and O2. Notably, in the presence of H2O and O2, all sorbents containing some Cu do not completely saturate their adsorption capacity during the time scale of the experiments (see the long tails in Figure 1c) but rather achieve a pseudo-steady state. This result clearly suggests the occurrence of an additional H2S capture process with a relatively slower rate. Therefore, dynamic capture experiments were repeated for the best performing materials (namely Cu/AC and Cu0.5Zn0.5/AC) using a larger quantity of sorbent and/or prolonging the total time on stream over several days of discontinuous operation (shut down during night-time). An example of the typical capture profiles vs time for Cu/AC sorbent is reported in Figure 4. First, it is observed that the breakpoint time does not linearly scale with the mass of sorbent since it jumped from 3.5 min (Figure 1c) up to 212 min by increasing the sorbent dose from 20 to 200 mg under similar feed conditions. During the downtime, the sorbent was able to partially restore some of the initial capture activity, but, for longer times on stream, all profiles seem to merge in a single master line, once again showing an almost asymptotic trend. The restoration of the active CuO sites via re-oxidation of metal sulphides (by dissolved oxygen) or via reduction of metal sulphates by reaction with additional HS−, is likely the rate-
10
limiting step and, therefore, it is at the origin of the abovementioned partial recovery of initial activity recorded during downtimes. Figure 5 reports the specific H2S capture rate measured during the first capture experiment and at the end of each subsequent test as a function of the cumulative quantity of H2S adsorbed for various experimental conditions (in terms of sorbent mass and relative humidity). In agreement with our previous results [11], it is observed that the initial capture rate is quite fast so that the reactive adsorption proceeds under external mass transfer limitations. After the probable formation of sulphates on the sorbent, the capture rate declines, approaching a pseudo-steady value around 5·10–4 mg g–1 s–1. During those prolonged tests, it was possible to attain a quite large cumulative adsorption capacity (as much as 330 mg g–1 for Cu/AC). However, this value is not representative of the true saturation capacity, which seems possibly upper limited only by the total (micro)pore volume of the sorbent, which progressively fills-up with S-species [5]. Figure 5 also shows that, for identical experimental conditions, the sorbent containing only copper slightly outperforms its counterpart with equimolar amounts of Cu and Zn, most probably because ZnO is not a catalyst for the oxidation of H2S to S. In agreement with previous results in the literature, from data of Figure 5 it can be inferred that a larger relative humidity content of the gas stream has a positive effect on the capture rate. Cu/AC and Cu0.5Zn0.5/AC sorbents loaded with ca 120 mg g–1 of S (Table 3) during experiments in the presence of 50% humidity and 2500 ppmv of O2 were recovered to air and subjected to XPS and porosimetric analyses in order to further shed light on the capture mechanism and evaluate microstructural modifications of the sorbents occurring upon reactive adsorption. Figure 6 depicts results of XPS analysis on Cu0.5Zn0.5/AC sample; results for Cu/AC sample are here not reported for the sake of brevity, as Cu and S spectral lines are
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qualitatively similar to those retrieved for Cu0.5Zn0.5/AC. Results confirm that a significant fraction of surface Zn (Figure 6a) is present in the sample as ZnSO4, as testified by the highintensity 2p3/2 peak with BE=1022.26 eV. Minor contributions of both ZnS (Zn 2p3/2 spectral line peaked at 1021.70 eV) and zinc bound to organic functional groups of the support (Zn 2p3/2 spectral line centred at 1024.33 eV) can be also detected [17]. A close inspection of the Cu 2p spectral region highlights that copper mainly occurs as CuO and CuSO4 (vide Figure 6 b). More specifically, copper oxide can be identified by the 2p3/2 and 2p1/2 lines centred at 932.45 and 951.92 eV, respectively. Copper sulphate displays the characteristic 2p3/2 band with a maximum located at 934.56 eV and the 2p1/2 peak with BE=953.93 eV. The deconvolution analysis also allowed to detect a slightly greater fraction of CuO with respect to CuSO4, with a CuO to CuSO4 ratio of peaks area equal to 1.2. The large amount of CuO detected from XPS analysis is possibly due to a re-oxidation of surface CuS or Cu2S by reaction with oxygen present in the feed gas or during air exposure after spent sorbent recovery [20]. The deconvolution analysis of the 2p spectral region of S (Figure 6c) allows to highlight the ranking in terms of relative abundance is elemental sulphur+organic sulphur species (including sulphide (C-S-C), thiol (C-S-H), thiophene (C4S-H) and disulphide (C-S-S-C)) > metal sulphates > metal sulphides. In particular, elemental sulphur and organic sulphur species produce the S 2p3/2 and S 2p1/2 peaks observed at binding energies of 164.05 eV and 165.23 eV, respectively [5,17,21]. The occurrence of copper and zinc sulphates is testified by S 2p3/2 and 2p1/2 spectral lines centred at 168.50 eV and 169.68 eV; the S 2p3/2 and 2p1/2 bands peaked at 162.35 and 163.53 eV are linked to metal sulphides. Interestingly, XPS analysis performed on the same sample tested for H2S capture under dry conditions (cf. [11]) revealed that S-based species mainly occurred as metal sulphates. The remarkable increase in the
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sorbent adsorption capacity in the presence of oxygen and water is associated with the formation of elemental sulphur/organic sulphur compounds. It can be argued that, apart from the rapid formation of metal sulphates, in the presence of water and O2, CuO active sites are also capable to catalyse the oxidation of H2S to elemental sulphur [22,23] via a vapour-liquid-solid mechanism: H2S is firstly dissociated into HS− in the film of adsorbed water at the carbon surface, and then oxidized over CuO sites. The latter process can also induce the formation of CuS/Cu2S species [23], which are then reoxidized by molecular oxygen to restore CuO sites. Furthermore, the formation of elemental sulphur can also take place on copper sulphates, as demonstrated by Laperdrix et al. [24] who observed the same performance of CuO/Al2O3 towards H2S wet oxidation also for a pre-sulphated catalyst. They also proposed that oxygen of CuSxOy species, after promoting H2S oxidation, is regenerated by molecular oxygen through a Mars van Krevelen mechanism. Table 3 shows the results of porosimetric analyses performed on selected sorbent samples, both before and after H2S adsorption either in dry N2 or in the co-presence of H2O/O2, which resulted in different values of ωads. It was already demonstrated that the incipient wetness impregnation method guaranteed a high dispersion of the active phase largely preserving the textural features of the pristine AC. Nevertheless, Zn and Cu oxides showed a different tendency to be dispersed onto the surface of micro and mesopores, respectively [4,11]. After H2S adsorption, all sorbents experienced a significant reduction of specific surface area and porosity due to the formation and accumulation of different S-bearing compounds. However, considering the quantity of stored S, the loss of specific surface area and microporosity was proportionally more significant for samples saturated under dry inert conditions and this effect is even more pronounced for the Zn containing sorbent. In fact, the initial formation of metal sulphates was shown to be responsible for the partial occlusion of sample porosity upon the reactive adsorption of H2S under inert conditions [11], due to the
13
much larger ratio of molar volumes for zinc and copper sulphates with respect to their corresponding oxides (average value=3.3). The additional amount of sulphur captured by the sorbents when the feed gas contains also H2O and O2 is stored as elemental/organic species characterized by a smaller molar volume. These observations are also confirmed by the pore size distributions reported in Figure 7 where it is highlighted a decrease of the micropore volume for both the materials (Cu/AC and Cu0.5Zn0.5/AC) after adsorption in the reactive environment (saturated samples denoted by the suffix -s) when compared to their fresh counterparts. This seems to be associated with the large formation of elemental sulphur, able to accumulate in the micropores [25]. On the other hand, in the mesopore region, no substantial differences were observed for Cu/AC before and after adsorption, at odds with the behaviour shown by Cu0.5Zn0.5/AC. All results of characterization of S-loaded sorbents converge towards the occurrence of two different H2S reactive adsorption mechanisms, which could be sequential or simultaneous: the first one quickly forms metal sulphates utilizing lattice oxygen from metal oxide clusters or superficial oxygen species; the second, slower one, requires H2O and molecular oxygen in the feed gas to catalytically form elemental sulphur chains and organic sulphur species that progressively fill-up the micropores of the activated carbon.
4. Conclusions In this work, H2S reactive adsorption tests were carried out at room temperature in a labscale fixed-bed reactor starting from gaseous streams also containing O2, H2O and H2O + O2 mixtures. Highly dispersed CuO-ZnO sorbents were prepared via impregnation onto commercial activated carbon support at constant total metal loading (10% wt) but different formulations in term of amounts of the two metals (Cu/AC, Cu0.5Zn0.5/AC and Zn/AC).
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For Cu containing sorbents, the simultaneous presence of O2 and H2O in the feed determines an increase in H2S capture rate and adsorption capacity, favouring the extensive formation of different sulphur species, mainly elemental and organic sulphur, in addition to metal sulphates, which are formed also under inert dry feed conditions. In fact, H2S capture tests at short contact times indicate that CuO (but not ZnO) sites can promote the activation of a catalytic oxidation process in the presence of humidity and molecular oxygen. Tests carried out prolonging the total time on stream over several days of discontinuous operation suggest that the capture process attains a pseudo-steady rate and the total H2S adsorption capacity corresponds to the filling-up of microporosity with elemental and organic S species. TPD-SO2 and XPS analyses of partially saturated sorbents indicate the occurrence of Zn as ZnSO4; on the contrary, in the presence of oxygen and moisture, Cu occurr as CuSO4 and CuO, suggesting a catalytic redox mechanism of reaction leading to the selective oxidation of H2S to elemental S, with the reformation of CuO active sites by molecular oxygen being a kinetic limiting step.
Acknowledgements The financial support of the Italian Ministry for Economic Development (MiSE) under MiSE-CNR Agreement on National Electrical System is acknowledged.
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[10] H.B. Fang, J.T. Zhao, Y.T. Fang, J.J. Huang, Y. Wang, Selective oxidation of hydrogen sulfide to sulfur over activated carbon-supported metal oxides, Fuel 108 (2013) 143– 148.
16
[11] G. de Falco, F. Montagnaro, M. Balsamo, A. Erto, F.A. Deorsola, L. Lisi, S. Cimino, Synergic effect of Zn and Cu oxides dispersed on activated carbon during reactive adsorption of H2S at room temperature, Micropor. Mesopor. Mater. 257 (2018) 135– 146. [12] R. Sitthikhankaew, D. Chadwick, S. Assabumrungrat, N. Laosiripojana, Effects of humidity, O2, and CO2 on H2S adsorption onto upgraded and KOH impregnated activated carbons, Fuel Process. Technol. 124 (2014) 249–257. [13] F. Adib, A. Bagreev, T.J. Bandosz, Analysis of the relationship between H2S removal capacity and surface properties of unimpregnated activated carbons, Environ. Sci. Technol. 34 (2000) 686–692. [14] A. Bagreev, T.J. Bandosz, On the mechanism of hydrogen sulfide removal from moist air on catalytic carbonaceous adsorbents, Ind. Eng. Chem. Res. 44 (2005) 530–538. [15] J. Wang, L. Wang, H. Fan, H. Wang, Y. Hu, Z. Wang, Highly porous copper oxide sorbent for H2S capture at ambient temperature, Fuel 209 (2017) 329–338. [16] H. Yang, B. Tatarchuk, Novel-doped zinc oxide sorbents for low temperature regenerable desulfurization applications, AIChE J. 56 (2010) 2898–2904. [17] A.V. Naumkin, A. Kraut-Vass, S.W. Gaarenstroom, C.J. Powell, NIST Standard Reference Database 20, Version 4.1, 2012. http://dx.doi.org/10.18434/T4T88K. [18] M. Balsamo, F. Rodríguez-Reinoso, F. Montagnaro, A. Lancia, A. Erto, Highlighting the role of activated carbon particle size on CO2 capture from model flue gas, Ind. Eng. Chem. Res. 52 (2013) 12183–12191. [19] S. Cimino, L. Lisi, M. Tortorelli, Low temperature SCR on supported MnOx catalysts for marine exhaust gas cleaning: Effect of KCl poisoning, Chem. Eng. J. 283 (2016) 223–230.
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[20] Y. Duan, L. Pirolli, A.V. Teplyakov, Investigation of the H2S poisoning process for sensing composite material based on carbon nanotubes and metal oxides, Sensor Actuat. B-Chem. 235 (2016) 213–221. [21] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database, Wiley Interscience, New York, 1992. [22] E.K. Lee, K.D. Jung, O.S. Joo, Y.G. Shul, Liquid-phase oxidation of hydrogen sulfide to sulfur over CuO/MgO catalyst, React. Kinet. Catal. Lett. 87 (2006) 115–120. [23] N. Haimour, R. El-Bishtawi, A. Ail-Wahbi, Equilibrium adsorption of hydrogen sulfide onto CuO and ZnO, Desalination 181 (2005) 145–152. [24] E. Laperdrix, G. Costentin, O. Saur, J.C. Lavalley, C. Nedez, S. Savin-Poncet, J. Nougayrede, Selective oxidation of H2S over CuO/Al2O3: Identification and role of the sulfurated species formed on the catalyst during the reaction, J. Catal. 189 (2000) 63– 69. [25] T.J. Bandosz, On the adsorption/oxidation of hydrogen sulfide on activated carbons at ambient temperatures, J. Colloid Interface Sci. 246 (2002) 1–20.
18
Table 1. Specific H2S adsorption capacity ωads (mg g–1) for Cu/AC, Cu0.5Zn0.5/AC, Zn/AC and raw AC at 30 °C under: dry N2, presence of O2, presence of H2O, co-presence of H2O+O2. Feed gas composition H2S 100 ppmv in: *
N2 + O2 (2500 ppmv) + H2O (50% RH) + O2, H2O (2500 ppmv, RH 50%) *
AC 2.7 3.06 5.10 11.6
Saturation capacity ω ads (mg g–1) Cu/AC Cu0.5Zn0.5/AC Zn/AC 29.9 34.0 24.8 41.1 43.5 25.2 47.6 56.8 29.6 129.2**
118.0**
30.9
Data from ref. [11]
**
Data obtained integrating the breakthrough curve up to t=20000 s rather than up to 99% saturation.
Table 2. Specific amount of sulphur captured ωbr during H2S adsorption tests carried out up to the breakpoint over Cu0.5Zn0.5/AC at 30 °C under: dry N2, presence of O2, presence of H2O, co-presence of H2O+O2. Corresponding amount and percentage of sulphur desorbed as SO2 during the subsequent TPD analysis up to 620 °C in pure N2.
Feed gas composition H2S 100 ppmv in: N2 + O2 (2500 ppmv) + H2O (50% RH) + O2, H2O (2500 ppmv, RH 50%)
H2S adsorption at breakpoint ωbr
Desorption as SO2
mg S g–1
mg S g–1
%
12.8 13.1 16.6
8.6 9.2 9.3
67.5 70.7 55.8
25.0
11.9
47.4
19
Table 3. Textural properties for fresh Cu/AC and Cu0.5Zn0.5/AC and after H2S adsorption tests in dry N2 conditions or in the presence of O2 (2500 ppmv) and H2O (RH=50%) with indication of the corresponding amount of sulphur captured by the sorbent. Sorbent AC Fresh Cu/AC Cu0.5Zn0.5/AC Cu/AC Exhausted in dry N2 Cu0.5Zn0.5/AC Cu/AC Exhausted in O2 + H2O Cu0.5Zn0.5/AC
SBET m2 g–1 641 559 570 426 348 361 254
VTOT cm3 g–1 0.81 0.67 0.76 0.58 0.55 0.55 0.48
VM cm3 g–1 0.23 0.22 0.20 0.16 0.13 0.14 0.10
ωads mg S g–1 ---46 50 127 122
20
Figure Captions Figure 1. Breakthrough curves (where C is the H2S concentration) for Cu/AC, Cu0.5Zn0.5/AC, Zn/AC sorbents (20 mg) and, for comparison, raw AC. H2S adsorption at 30 °C in (a) 2500 ppmv O2; (b) 50% RH; (c) 2500 ppmv O2 and 50% RH. Plots have been cut at t=3000 s.
Figure 2. Specific H2S removal rate at 30 °C as a function of cumulative amount of H2S adsorbed onto Cu0.5Zn0.5/AC from a gas stream containing either only N2, 2500 ppmv O2 in N2, H2O in N2 (RH=50%) or 2500 ppmv O2 and 50% RH in N2.
Figure 3. TPD-SO2 for Cu0.5Zn0.5/AC sorbent previously exposed to H2S during dynamic capture tests under various atmospheres that were stopped at the corresponding breakpoint.
Figure 4. Temporal profile of the outlet H2S concentration during a prolonged capture test over Cu/AC sorbent (200 mg); feed conditions: 100 ppmv H2S, 2500 ppmv O2 and 25% RH in N2 at 30 °C.
Figure 5. Specific H2S removal rate as a function of cumulative amount of H2S adsorbed onto Cu/AC and Cu0.5Zn0.5/AC in the presence of H2O and O2 (RH and sorbent dose in the legend).
Figure 6. XPS analysis on used Cu0.5Zn0.5/AC sorbent (feed conditions: 100 ppmv H2S, 2500 ppmv O2 and 50% RH in N2 at 30 °C) recovered in air: 2p spectral regions of (a) Zn, (b) Cu, (c) S.
Figure 7. Pore size distribution analysis for Cu/AC (a) and Cu0.5Zn0.5/AC (b), before and after adsorption tests at 30 °C, 100 ppmv H2S, 2500 ppm O2, 50% RH.
21
H2S removal rate r, mg/(g s)
1e-1
1e-2
H2O+O2 O2
1e-3
H2O
N2
1e-4
0
20
40
60
80
100
cumulative ωads, mg/g
120
140
SO2 signal, a.u.
H2O+O2
H2O O2 N2 ZnSO4/AC
CuSO4/AC
100
200
300
400
Temperature, °C
500
600
100 day 1
day 2
day 3
day 4
46 48 50 52
70 72 74 76
out
C /C
in
80 60 40 20 0 0 2 4 6
2224262830
time, h
H2S removal rate r, mg/(g s)
1e-1
Cu0.5Zn0.5/AC 20mg RH=50% Cu0.5Zn0.5/AC 150mg RH=50% Cu/AC 150mg RH=50% Cu/AC 200mg RH=22% Cu/AC 40mg RH= 35%
1e-2
1e-3
1e-4
0
100
200
300
cumulative ωads, mg/g
400
dV(d), cm3/(g Å)
0.04
Cu/AC Cu/AC-s in H2O/O2
(a)
0.03 0.02 0.01 0.00
dV(d), cm3/(g Å)
0.04
5
10
100
Cu0.5 Zn0.5/ACÅ pore diameter, Cu0.5Zn0.5/AC-s in H2O/O2
(b)
0.03 0.02 0.01 0.00 5
10
100
pore diameter, Å
•
Enhanced H2S capacity of CuO-ZnO/AC sorbents with O2 and H2O
•
Reactive adsorption mechanism starts with the initial fast formation of metal sulphates
•
CuO catalyzes H2S oxidation to elemental S in the presence of H2O and O2
•
Large sorption capacity related to the filling up of micropores with elemental S
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1387-1811(19)30808-X
DOI:
https://doi.org/10.1016/j.micromeso.2019.109949
Reference:
MICMAT 109949
To appear in:
Microporous and Mesoporous Materials
Received Date: 23 October 2019 Revised Date:
4 December 2019
Accepted Date: 9 December 2019
Please cite this article as: S. Cimino, L. Lisi, A. Erto, F.A. Deorsola, G. de Falco, F. Montagnaro, M. Balsamo, Role OF H2O and O2 during the reactive adsorption OF H2S ON CuO-ZnO/Activated carbon at low temperature, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/ j.micromeso.2019.109949. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
CRediT author statement Stefano Cimino: Conceptualization, Investigation, Writing- Reviewing and Editing, Supervision, Funding Acquisition Luciana Lisi: Investigation, Validation, Writing - Original Draft, Supervision Alessandro Erto: Data Curation, Writing - Original Draft Fabio A. Deorsola: Investigation, Formal Analysis, Writing - Original Draft Giacomo de Falco: Investigation, Data Curation Fabio Montagnaro: Formal analysis, Writing - Original Draft Marco Balsamo: Investigation, Data Curation, Formal analysis, Writing - Original Draft
ROLE OF H2O AND O2 DURING THE REACTIVE ADSORPTION OF H2S ON CuO-ZnO/ACTIVATED CARBON AT LOW TEMPERATURE S. Ciminoa1, L. Lisia, A. Ertob, F.A. Deorsolac, G. de Falcod, F. Montagnarod, M. Balsamod a
b
Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy
Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy c
d
Dipartimento Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant'Angelo, 80126 Napoli, Italy
Abstract The reactive adsorption of H2S on CuO-ZnO dispersed onto activated carbon was investigated in different gas streams, containing either O2, H2O or O2/H2O, at room temperature and in a lab-scale fixed-bed reactor. Sorbents performances were analyzed in terms of H2S capture rate and capacity, and results correlated with the sulphur species formed upon adsorption under different experimental conditions. Temperature Programmed Desorption (TPD), porosimetric and XPS analyses were performed in order to support the adsorption dynamic tests. The co-presence of O2 and H2O caused a remarkable increase in H2S adsorption capacity, in particular for Cu-containing sorbents, favouring the formation of various sulphur species such as sulphides, sulphates and, above all, elemental sulphur. The analysis of the experimental data set showed the occurrence of two main H2S oxidation mechanisms with different rates: the first one quickly formed metal sulphates utilizing lattice oxygen from metal oxide clusters or superficial oxygen species; the second process was
1
Corresponding author: [email protected]
1
slower and required moisture and molecular oxygen in the feed gas to catalytically form elemental sulphur chains that progressively filled-up the sorbent microporosity.
Keywords: biogas purification; catalytic oxidation; activated carbons; sorbent
1. Introduction The elimination of hydrogen sulphide (H2S) from high-value gas streams such as reformate gas, natural gas and raw biogas is one of the mandatory steps for their valorization as fuels for energy production. H2S is a toxic gas for human beings, corrosive for engines and for fuel processing devices, and poisonous to catalysts. Hence, its maximum concentration in gaseous streams must be kept below 1 ppmv with appropriate treatments [1]. In the industrial practice, adsorption has gained high esteem as downstream gas treatment, due to a general versatility and simplicity of the process. Currently, many research efforts are devoted to the development of high-performance sorbents capable to efficiently remove large quantities of H2S from various gas streams already at room temperature: to this aim, active phases mainly based on metal oxides (e.g. ZnO, CuO, Fe2O3, MgO) are finely dispersed on porous supports [2−6]. Activated carbons are often adopted as supports due to their favourable textural properties, such as high specific surface area and pore volume, and because they are also able to promote the oxidation of H2S in the presence of even small amounts of surface oxygen, which can provide a significant contribution to adsorption capacity [6,7]. The definition of the adsorption mechanism, along with the punctual individuation of the species formed upon H2S reactive adsorption, assumes fundamental importance for design purposes, process optimization and for the possible individuation of effective regeneration strategies of the spent sorbents. When ZnO and CuO are used as active phase for H2S capture,
2
the indications retrievable from the pertinent literature are conflicting. Some authors reported experimental evidence of the formation of the correspondent metal sulphides [3,8]. However, the presence of heteroatoms, oxygenated surface functional groups and inorganic matter on activated carbon can also promote oxidation reactions mainly leading to elemental sulphur and sulphur poly-chains (Sx), which in turn can react to give further sulphur compounds (e.g. SO2) [8,9]. Moreover, a synergistic effect in the presence of metals can be also present, the metal acting as a catalyst for H2S oxidation [10]. Nguyen-Thanh and Bandosz [7] hypothesized that copper dispersed on activated carbon and bound to the carbonaceous surface can activate the oxygen locally present, leading to the formation of SO2, elemental sulphur and copper complex (e.g. Cu2S) in significantly larger quantities than on the raw carbon (i.e. without copper deposition). In our previous works, we investigated the use of ZnO/CuO supported onto activated carbon under dry feed conditions. Experimental results clearly showed the formation of sulphates during the initial phases of the reactive adsorption process likely promoted by the presence of copper as oxygen donor. Cu and Zn sulphides started to be formed with slow overall kinetics only after the corresponding sulphates, probably due to the lack of oxygen availability on the sorbent [11]. The complexity of surface reactions increases with multicomponent gas mixtures, which unquestionably represent more realistic gas streams. For instance, a typical biogas composition mainly includes CH4 (40–75 %), CO2 (25–40 %), N2 (0.5–2.5 %), O2 (0.1–1 %), H2S (0.1–0.5 %) and humidity, even if it can significantly vary depending on organic content of residues, retention time and digester operating conditions [12]. The effect of gaseous O2 on H2S capture is equivalent to that exerted by oxygen functional groups on carbonaceous supports, leading to the formation of elemental sulphur and SO2 and is enhanced by the co-presence of humidity [10,12]. On the contrary, the effect of water is still debated and likely depends on the specific adsorption system. In most of the
3
cases, the presence of a water film adsorbed on activated carbon surface is claimed to enhance the H2S reactive adsorption, as it can lead to a faster dissociation of H2S and the subsequent oxidation to SO2, elemental sulphur and sulphuric acid. It was highlighted that H2S dissociation is promoted by a basic local pH, due to the presence of either basic surface functional groups [13] or metal oxides [14]. Similarly, the hydroxylation of metal oxides (e.g. CuO) by reaction with water is reported to enhance H2S adsorption by dissociation of H2S (giving HS− and S2−) and subsequent reaction with the metal cation to give the correspondent sulphide [15]. On contrary, Huang et al. [8] reported that water presence is detrimental for H2S adsorption on Cu-supported activated carbon due to possible competition phenomena, while Yang and Tatarchuk [16] working on ZnO/SiO2 and Cu-ZnO/SiO2 sorbents found a negligible effect of water on H2S adsorption. In order to elucidate the role of O2 and H2O in the adsorption mechanism of H2S from gas streams onto CuO-ZnO dispersed onto activated carbon, we set out a new experimental study based on the results previously obtained on the same adsorption system, operated under inert, dry conditions [4,11]. H2S dynamic adsorption tests from gaseous streams containing O2, H2O and H2O + O2 were carried out at room temperature in a lab-scale fixed-bed reactor operated at short contact times in order to elucidate kinetic effects of the process. Temperature Programmed Desorption (TPD) tests, as well as XPS and porosimetric analyses on partially exhausted sorbents, were carried out in order to investigate the nature of sulphur species formed upon adsorption, retrieving information on the overall H2S reaction mechanism. The final target was a thorough comprehension of the intertwining between solid properties (e.g. specific surface area, Cu/Zn ratio, etc.) and gas stream composition in the H2S reactive adsorption.
4
2. Experimental Adsorbents based on CuO and ZnO highly dispersed onto a commercial granulated activated carbon (Darco G40, Norit) were produced by incipient wetness impregnation using aqueous solutions of metal nitrates, and finally heat-treated at 250 °C under inert flow [4]. The total metal loading was set to 10% wt. and formulations included samples with only Zn or Cu as well as equimolar amounts of the two metals (sorbents termed as Zn/AC, Cu/AC and Cu0.5Zn0.5/AC, respectively). The textural properties of the sorbents were determined by N2 adsorption at –196 °C starting from P/P0=10–5 using a Quantachrome Autosorb 1-C instrument. Before analysis, the samples were degassed under high vacuum at 120 °C for 18 h. The BET method (P/P0 range from 0.01 to 0.11) was adopted for the calculation of the specific surface area (SBET), while the micropore volume (VM) and the total pore volume (VTOT) were computed by means of the Dubinin–Astakov and Gurvitsch’s rule (at P/P0=0.995), respectively. The pore size distribution (PSD) was evaluated via the Quenched Solid Density Functional Theory (QSDFT). XPS analysis was carried out on a XPS PHI 5000 Versa probe apparatus, using band-pass energy of 187.85 eV, a 45° take-off angle and a 100.0 µm diameter X-ray spot size for survey spectra. High-resolution XP-spectra were recorded in the following conditions: pass energy of 20 eV, resolution of 1.1 eV, and step of 0.2 eV. Sample charging effects were eliminated by referring to the spectral line shift to the C 1s binding energy (BE) value of 284.8 eV. XPspectra were analysed by means of commercial software (CasaXPS, version 2.3.16), by applying mixed Gaussian–Lorentzian (70–30 %) profiles. The assignation of XPS signals was based on the NIST compilation of spectral data [17] unless otherwise stated. H2S dynamic adsorption tests were carried out at atmospheric pressure and T=30 °C, in a lab-scale experimental apparatus based on a quartz fixed-bed reactor with an annular section,
5
already described elsewhere [18,19]. A weighted amount of sorbent (20–200 mg) with a particle size range of 125–200 µm was mixed with quartz to obtain a total mass of the bed of ) and the total flow rate (Qt) were set at 100
200 mg. The inlet concentration of H2S (
ppmv (in high purity, dry N2, reference case) and 20 SL h–1, corresponding to a contact time as low as 3.4–34 ms, referred to the volume of the bed. Notably, those conditions were selected to investigate the kinetic aspects of the reactive adsorption process rather than to estimate the equilibrium saturation capacity of the sorbents. The role of O2 and H2O in the feed gas was studied by adding to the feed 2500 ppmv of O2 and/or by passing part of the gas stream through a water saturator before feeding H2S, thus achieving a relative humidity (RH) content in the range 20–50%. Gas analysis was performed with a continuous analyser (ABB Optima Advance Limas 11 UV) for the simultaneous measurement of H2S and SO2, with cross-sensitivity correction and accuracy better than 1% of span (200 ppmv). The cumulative H2S adsorption capacity,
[mg g–1], was determined through a
material balance over the adsorption column: ad s
=
Q t C in H 2S ρH 2S m
t∗ 0
where Qt [L s–1] is the total gas flow rate;
1−
C out H 2 S (t) C in H 2S
dt
(1)
[-] and
fractions at the inlet and outlet of the bed, respectively;
[-] are the H2S volumetric [mg L–1] is H2S density; m [g] is
the sorbent mass; t* [s] represents the capture time. By convention, breakpoint and saturation conditions refer to capture times for which the outlet H2S concentration equals 5% and 99% of its inlet value, respectively. Prolonged dynamic capture tests were run over several days with a discontinuous operation (shut down during night-time). Temperature programmed desorption (TPD) tests were carried out in the same lab-scale set-up used for dynamic adsorption tests. In a typical TPD test, the sorbent (just after the adsorption test, without being exposed to air) was treated under a flow of 20 SL h–1 of high
6
purity N2 (99.999%) and heated at 10 °C min–1 up to a maximum of 620 °C. The specific amount of desorbed sulphur-based species was evaluated in a similar way as in Eq. (1) by integrating the temporal profiles of H2S+SO2.
3. Results and Discussion Figure 1 shows the breakthrough curves, i.e. the temporal profiles of
/
, for the
three sorbents under investigation (Cu/AC, Cu0.5Zn0.5/AC and Zn/AC) and raw AC for comparison. In particular, sub-figures (a), (b) and (c) refer to operating conditions with (a) presence of O2 only, (b) presence of H2O only and (c) co-presence of H2O+O2. The corresponding values of H2S adsorption capacity, ωads (Eq. (1)), are listed in Table 1 where, for comparison, the data previously obtained in absence of both H2O and O2 are reported as well [11]. In the latter case (dry N2 environment), with respect to the very low value shown by the raw AC (ωads=2.7 mg g–1), it was highlighted the promoting role for H2S capture increased in following the order Zn< Cu < Zn + Cu (e.g. ωads as high as 34 mg g–1 for Cu0.5Zn0.5/AC). In the presence of O2, the capture capacity increased: O2 can be able to foster (i.e., to “catalyse”) the H2S reactive adsorption process, favouring the formation of sulphates, sulphides and elemental sulphur. The effect of oxygen was particularly evident when copper was present. In the presence of H2O, the values for ωads were even higher (up to 56.8 mg g–1). After adsorption into sorbent pores, water vapour can contribute to the dissociation of sulphidric acid thus promoting the formation of elemental sulphur. The two mechanisms jointly act and result in better adsorption performance when both H2O and O2 are present in the reactive environment (again, particularly for Cu-containing sorbents). Notably, in the co-presence of H2O + O2, the relative ranking between Cu/AC and Cu0.5Zn0.5/AC is reversed with respect to the one retrieved in the other tested conditions, i.e.
7
ωads was higher for Cu/AC (129.2 mg g–1) than for Cu0.5Zn0.5/AC (118 mg g–1). Moreover, the occurrence of a very long tail for both Cu/AC and Cu0.5Zn0.5/AC (see Figure 1(c)) made difficult the rigorous application of Eq. (1) for the calculus of ωads. For this reason, the integral was calculated up to t=20000 s instead of t*. These aspects will be further discussed in the following. Figure 2 reports the trends of the specific H2S removal rate, r, against the cumulative amount of H2S captured by Cu0.5Zn0.5/AC sorbent during the tests of Figure 1, in comparison with the reference case represented by a feed of H2S in N2 (dry). As a general consideration, r is initially equal to the H2S molar feed rate and significantly decreases after the breakpoint has been reached. This was related to the formation of oxidized sulphur species that partially hinder the pore structure of the adsorbent, so limiting the diffusion of H2S [11]. In particular, in the reference case (H2S in dry N2), the reaction rate shows a vertical asymptote in correspondence of the saturation capacity, probably due to the complete consumption of the oxygen contained in the active phase and on the surface of the AC. The addition of 2500 ppmv of O2 in the feed has no impact during the first part of the capture process, leading to an almost identical breakthrough capture capacity (Table 2). However, the reaction rate does not drop abruptly but it shows a clear inflexion point, suggesting that O2 can induce a residual H2S capture activity via a relatively slower mechanism of reactive adsorption. Under humid N2 feed conditions, the breakthrough capacity is enhanced and, thereafter, the capture rate is faster than in dry conditions, but it eventually drops due to saturation once again for the lack of available oxygen. Notably, the further addition of molecular oxygen to a humid feed has little effect on the breakthrough adsorption capacity (Table 2) but it significantly enhances the residual capture rate, which decreases slowly along with the progressive increase in the amount of sulphur without reaching a clear saturation during the tested adsorption times.
8
To get more information about the reactive mechanisms in the initial stage of the adsorption process, TPD tests were carried out on sorbents previously exposed to H2S during dynamic tests that were stopped at the corresponding breakpoint times. Table 2 lists the values of the specific amount of sulphur desorbed as SO2. H2S was never detected, thus confirming that the adsorption was characterised by a chemical (oxidation) reaction, rather than a barely physical process. However, only a partial recovery of the adsorbed sulphur can be obtained during TPD, due to the possible desorption of species not detectable by the UV analyser (“ghost species” such as elemental or organic sulphur and termed as other S-species in Table 2), as well as to the formation of metal sulphides, which are stable up to 1000 °C under inert flow [4,11]. The TPD peaks in Figure 3 were identified according to the literature by comparison to reference materials [11]. The broad peak around 500 °C is typical of the decomposition of ZnSO4 supported on activated carbon, while the peak around 340 °C is assigned to the first decomposition peak of CuSO4/AC. Under reference inert dry feed conditions, the formation of ZnSO4 is largely favoured during the early stages of the capture process, with CuO showing a significant promoting effect as oxygen donor [11]. The addition of O2 to the feed mix causes minor modifications in the relevant TPD profile of Cu0.5Zn0.5/AC, in agreement with the results presented in Figure 2. It can be argued that H2S capture still proceeds via reaction with the lattice oxygen from the mixed Cu-Zn oxides, mainly forming ZnSO4. Molecular O2 appears to have a limited effect to promote the formation of CuSO4 and no effect on the formation of other S-species. The presence of humidity (within an inert feed) marginally affects the total amount of sulphate species found by TPD, while it slightly promotes the formation of CuSO4. Notably, the main effect of water addition is to increase the amount of ghost species formed during the early stage of the process. This result agrees well with previous reports on H2S capture on (impregnated) activated carbons and is generally ascribed to the higher reactivity of HS−
9
species deriving from the dissolution of H2S in a liquid film leading to the formation of elemental sulphur [5]. Table 2 indicates that the simultaneous presence of O2 and humidity can further speed up the early formation of those S species that are not detectable during our TPD experiments: indeed, ghost S-species represent almost 53% (on a mass basis) of the total sulphur-based compounds stored up to the breakpoint. Moreover, Figure 3 shows a more intense peak related to CuSO4, whereas the amount of ZnSO4 is not significantly affected. Therefore, it can be concluded that CuO, rather than ZnO is able to activate a different reaction mechanism in the presence of humidity and O2. Notably, in the presence of H2O and O2, all sorbents containing some Cu do not completely saturate their adsorption capacity during the time scale of the experiments (see the long tails in Figure 1c) but rather achieve a pseudo-steady state. This result clearly suggests the occurrence of an additional H2S capture process with a relatively slower rate. Therefore, dynamic capture experiments were repeated for the best performing materials (namely Cu/AC and Cu0.5Zn0.5/AC) using a larger quantity of sorbent and/or prolonging the total time on stream over several days of discontinuous operation (shut down during night-time). An example of the typical capture profiles vs time for Cu/AC sorbent is reported in Figure 4. First, it is observed that the breakpoint time does not linearly scale with the mass of sorbent since it jumped from 3.5 min (Figure 1c) up to 212 min by increasing the sorbent dose from 20 to 200 mg under similar feed conditions. During the downtime, the sorbent was able to partially restore some of the initial capture activity, but, for longer times on stream, all profiles seem to merge in a single master line, once again showing an almost asymptotic trend. The restoration of the active CuO sites via re-oxidation of metal sulphides (by dissolved oxygen) or via reduction of metal sulphates by reaction with additional HS−, is likely the rate-
10
limiting step and, therefore, it is at the origin of the abovementioned partial recovery of initial activity recorded during downtimes. Figure 5 reports the specific H2S capture rate measured during the first capture experiment and at the end of each subsequent test as a function of the cumulative quantity of H2S adsorbed for various experimental conditions (in terms of sorbent mass and relative humidity). In agreement with our previous results [11], it is observed that the initial capture rate is quite fast so that the reactive adsorption proceeds under external mass transfer limitations. After the probable formation of sulphates on the sorbent, the capture rate declines, approaching a pseudo-steady value around 5·10–4 mg g–1 s–1. During those prolonged tests, it was possible to attain a quite large cumulative adsorption capacity (as much as 330 mg g–1 for Cu/AC). However, this value is not representative of the true saturation capacity, which seems possibly upper limited only by the total (micro)pore volume of the sorbent, which progressively fills-up with S-species [5]. Figure 5 also shows that, for identical experimental conditions, the sorbent containing only copper slightly outperforms its counterpart with equimolar amounts of Cu and Zn, most probably because ZnO is not a catalyst for the oxidation of H2S to S. In agreement with previous results in the literature, from data of Figure 5 it can be inferred that a larger relative humidity content of the gas stream has a positive effect on the capture rate. Cu/AC and Cu0.5Zn0.5/AC sorbents loaded with ca 120 mg g–1 of S (Table 3) during experiments in the presence of 50% humidity and 2500 ppmv of O2 were recovered to air and subjected to XPS and porosimetric analyses in order to further shed light on the capture mechanism and evaluate microstructural modifications of the sorbents occurring upon reactive adsorption. Figure 6 depicts results of XPS analysis on Cu0.5Zn0.5/AC sample; results for Cu/AC sample are here not reported for the sake of brevity, as Cu and S spectral lines are
11
qualitatively similar to those retrieved for Cu0.5Zn0.5/AC. Results confirm that a significant fraction of surface Zn (Figure 6a) is present in the sample as ZnSO4, as testified by the highintensity 2p3/2 peak with BE=1022.26 eV. Minor contributions of both ZnS (Zn 2p3/2 spectral line peaked at 1021.70 eV) and zinc bound to organic functional groups of the support (Zn 2p3/2 spectral line centred at 1024.33 eV) can be also detected [17]. A close inspection of the Cu 2p spectral region highlights that copper mainly occurs as CuO and CuSO4 (vide Figure 6 b). More specifically, copper oxide can be identified by the 2p3/2 and 2p1/2 lines centred at 932.45 and 951.92 eV, respectively. Copper sulphate displays the characteristic 2p3/2 band with a maximum located at 934.56 eV and the 2p1/2 peak with BE=953.93 eV. The deconvolution analysis also allowed to detect a slightly greater fraction of CuO with respect to CuSO4, with a CuO to CuSO4 ratio of peaks area equal to 1.2. The large amount of CuO detected from XPS analysis is possibly due to a re-oxidation of surface CuS or Cu2S by reaction with oxygen present in the feed gas or during air exposure after spent sorbent recovery [20]. The deconvolution analysis of the 2p spectral region of S (Figure 6c) allows to highlight the ranking in terms of relative abundance is elemental sulphur+organic sulphur species (including sulphide (C-S-C), thiol (C-S-H), thiophene (C4S-H) and disulphide (C-S-S-C)) > metal sulphates > metal sulphides. In particular, elemental sulphur and organic sulphur species produce the S 2p3/2 and S 2p1/2 peaks observed at binding energies of 164.05 eV and 165.23 eV, respectively [5,17,21]. The occurrence of copper and zinc sulphates is testified by S 2p3/2 and 2p1/2 spectral lines centred at 168.50 eV and 169.68 eV; the S 2p3/2 and 2p1/2 bands peaked at 162.35 and 163.53 eV are linked to metal sulphides. Interestingly, XPS analysis performed on the same sample tested for H2S capture under dry conditions (cf. [11]) revealed that S-based species mainly occurred as metal sulphates. The remarkable increase in the
12
sorbent adsorption capacity in the presence of oxygen and water is associated with the formation of elemental sulphur/organic sulphur compounds. It can be argued that, apart from the rapid formation of metal sulphates, in the presence of water and O2, CuO active sites are also capable to catalyse the oxidation of H2S to elemental sulphur [22,23] via a vapour-liquid-solid mechanism: H2S is firstly dissociated into HS− in the film of adsorbed water at the carbon surface, and then oxidized over CuO sites. The latter process can also induce the formation of CuS/Cu2S species [23], which are then reoxidized by molecular oxygen to restore CuO sites. Furthermore, the formation of elemental sulphur can also take place on copper sulphates, as demonstrated by Laperdrix et al. [24] who observed the same performance of CuO/Al2O3 towards H2S wet oxidation also for a pre-sulphated catalyst. They also proposed that oxygen of CuSxOy species, after promoting H2S oxidation, is regenerated by molecular oxygen through a Mars van Krevelen mechanism. Table 3 shows the results of porosimetric analyses performed on selected sorbent samples, both before and after H2S adsorption either in dry N2 or in the co-presence of H2O/O2, which resulted in different values of ωads. It was already demonstrated that the incipient wetness impregnation method guaranteed a high dispersion of the active phase largely preserving the textural features of the pristine AC. Nevertheless, Zn and Cu oxides showed a different tendency to be dispersed onto the surface of micro and mesopores, respectively [4,11]. After H2S adsorption, all sorbents experienced a significant reduction of specific surface area and porosity due to the formation and accumulation of different S-bearing compounds. However, considering the quantity of stored S, the loss of specific surface area and microporosity was proportionally more significant for samples saturated under dry inert conditions and this effect is even more pronounced for the Zn containing sorbent. In fact, the initial formation of metal sulphates was shown to be responsible for the partial occlusion of sample porosity upon the reactive adsorption of H2S under inert conditions [11], due to the
13
much larger ratio of molar volumes for zinc and copper sulphates with respect to their corresponding oxides (average value=3.3). The additional amount of sulphur captured by the sorbents when the feed gas contains also H2O and O2 is stored as elemental/organic species characterized by a smaller molar volume. These observations are also confirmed by the pore size distributions reported in Figure 7 where it is highlighted a decrease of the micropore volume for both the materials (Cu/AC and Cu0.5Zn0.5/AC) after adsorption in the reactive environment (saturated samples denoted by the suffix -s) when compared to their fresh counterparts. This seems to be associated with the large formation of elemental sulphur, able to accumulate in the micropores [25]. On the other hand, in the mesopore region, no substantial differences were observed for Cu/AC before and after adsorption, at odds with the behaviour shown by Cu0.5Zn0.5/AC. All results of characterization of S-loaded sorbents converge towards the occurrence of two different H2S reactive adsorption mechanisms, which could be sequential or simultaneous: the first one quickly forms metal sulphates utilizing lattice oxygen from metal oxide clusters or superficial oxygen species; the second, slower one, requires H2O and molecular oxygen in the feed gas to catalytically form elemental sulphur chains and organic sulphur species that progressively fill-up the micropores of the activated carbon.
4. Conclusions In this work, H2S reactive adsorption tests were carried out at room temperature in a labscale fixed-bed reactor starting from gaseous streams also containing O2, H2O and H2O + O2 mixtures. Highly dispersed CuO-ZnO sorbents were prepared via impregnation onto commercial activated carbon support at constant total metal loading (10% wt) but different formulations in term of amounts of the two metals (Cu/AC, Cu0.5Zn0.5/AC and Zn/AC).
14
For Cu containing sorbents, the simultaneous presence of O2 and H2O in the feed determines an increase in H2S capture rate and adsorption capacity, favouring the extensive formation of different sulphur species, mainly elemental and organic sulphur, in addition to metal sulphates, which are formed also under inert dry feed conditions. In fact, H2S capture tests at short contact times indicate that CuO (but not ZnO) sites can promote the activation of a catalytic oxidation process in the presence of humidity and molecular oxygen. Tests carried out prolonging the total time on stream over several days of discontinuous operation suggest that the capture process attains a pseudo-steady rate and the total H2S adsorption capacity corresponds to the filling-up of microporosity with elemental and organic S species. TPD-SO2 and XPS analyses of partially saturated sorbents indicate the occurrence of Zn as ZnSO4; on the contrary, in the presence of oxygen and moisture, Cu occurr as CuSO4 and CuO, suggesting a catalytic redox mechanism of reaction leading to the selective oxidation of H2S to elemental S, with the reformation of CuO active sites by molecular oxygen being a kinetic limiting step.
Acknowledgements The financial support of the Italian Ministry for Economic Development (MiSE) under MiSE-CNR Agreement on National Electrical System is acknowledged.
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Table 1. Specific H2S adsorption capacity ωads (mg g–1) for Cu/AC, Cu0.5Zn0.5/AC, Zn/AC and raw AC at 30 °C under: dry N2, presence of O2, presence of H2O, co-presence of H2O+O2. Feed gas composition H2S 100 ppmv in: *
N2 + O2 (2500 ppmv) + H2O (50% RH) + O2, H2O (2500 ppmv, RH 50%) *
AC 2.7 3.06 5.10 11.6
Saturation capacity ω ads (mg g–1) Cu/AC Cu0.5Zn0.5/AC Zn/AC 29.9 34.0 24.8 41.1 43.5 25.2 47.6 56.8 29.6 129.2**
118.0**
30.9
Data from ref. [11]
**
Data obtained integrating the breakthrough curve up to t=20000 s rather than up to 99% saturation.
Table 2. Specific amount of sulphur captured ωbr during H2S adsorption tests carried out up to the breakpoint over Cu0.5Zn0.5/AC at 30 °C under: dry N2, presence of O2, presence of H2O, co-presence of H2O+O2. Corresponding amount and percentage of sulphur desorbed as SO2 during the subsequent TPD analysis up to 620 °C in pure N2.
Feed gas composition H2S 100 ppmv in: N2 + O2 (2500 ppmv) + H2O (50% RH) + O2, H2O (2500 ppmv, RH 50%)
H2S adsorption at breakpoint ωbr
Desorption as SO2
mg S g–1
mg S g–1
%
12.8 13.1 16.6
8.6 9.2 9.3
67.5 70.7 55.8
25.0
11.9
47.4
19
Table 3. Textural properties for fresh Cu/AC and Cu0.5Zn0.5/AC and after H2S adsorption tests in dry N2 conditions or in the presence of O2 (2500 ppmv) and H2O (RH=50%) with indication of the corresponding amount of sulphur captured by the sorbent. Sorbent AC Fresh Cu/AC Cu0.5Zn0.5/AC Cu/AC Exhausted in dry N2 Cu0.5Zn0.5/AC Cu/AC Exhausted in O2 + H2O Cu0.5Zn0.5/AC
SBET m2 g–1 641 559 570 426 348 361 254
VTOT cm3 g–1 0.81 0.67 0.76 0.58 0.55 0.55 0.48
VM cm3 g–1 0.23 0.22 0.20 0.16 0.13 0.14 0.10
ωads mg S g–1 ---46 50 127 122
20
Figure Captions Figure 1. Breakthrough curves (where C is the H2S concentration) for Cu/AC, Cu0.5Zn0.5/AC, Zn/AC sorbents (20 mg) and, for comparison, raw AC. H2S adsorption at 30 °C in (a) 2500 ppmv O2; (b) 50% RH; (c) 2500 ppmv O2 and 50% RH. Plots have been cut at t=3000 s.
Figure 2. Specific H2S removal rate at 30 °C as a function of cumulative amount of H2S adsorbed onto Cu0.5Zn0.5/AC from a gas stream containing either only N2, 2500 ppmv O2 in N2, H2O in N2 (RH=50%) or 2500 ppmv O2 and 50% RH in N2.
Figure 3. TPD-SO2 for Cu0.5Zn0.5/AC sorbent previously exposed to H2S during dynamic capture tests under various atmospheres that were stopped at the corresponding breakpoint.
Figure 4. Temporal profile of the outlet H2S concentration during a prolonged capture test over Cu/AC sorbent (200 mg); feed conditions: 100 ppmv H2S, 2500 ppmv O2 and 25% RH in N2 at 30 °C.
Figure 5. Specific H2S removal rate as a function of cumulative amount of H2S adsorbed onto Cu/AC and Cu0.5Zn0.5/AC in the presence of H2O and O2 (RH and sorbent dose in the legend).
Figure 6. XPS analysis on used Cu0.5Zn0.5/AC sorbent (feed conditions: 100 ppmv H2S, 2500 ppmv O2 and 50% RH in N2 at 30 °C) recovered in air: 2p spectral regions of (a) Zn, (b) Cu, (c) S.
Figure 7. Pore size distribution analysis for Cu/AC (a) and Cu0.5Zn0.5/AC (b), before and after adsorption tests at 30 °C, 100 ppmv H2S, 2500 ppm O2, 50% RH.
21
H2S removal rate r, mg/(g s)
1e-1
1e-2
H2O+O2 O2
1e-3
H2O
N2
1e-4
0
20
40
60
80
100
cumulative ωads, mg/g
120
140
SO2 signal, a.u.
H2O+O2
H2O O2 N2 ZnSO4/AC
CuSO4/AC
100
200
300
400
Temperature, °C
500
600
100 day 1
day 2
day 3
day 4
46 48 50 52
70 72 74 76
out
C /C
in
80 60 40 20 0 0 2 4 6
2224262830
time, h
H2S removal rate r, mg/(g s)
1e-1
Cu0.5Zn0.5/AC 20mg RH=50% Cu0.5Zn0.5/AC 150mg RH=50% Cu/AC 150mg RH=50% Cu/AC 200mg RH=22% Cu/AC 40mg RH= 35%
1e-2
1e-3
1e-4
0
100
200
300
cumulative ωads, mg/g
400
dV(d), cm3/(g Å)
0.04
Cu/AC Cu/AC-s in H2O/O2
(a)
0.03 0.02 0.01 0.00
dV(d), cm3/(g Å)
0.04
5
10
100
Cu0.5 Zn0.5/ACÅ pore diameter, Cu0.5Zn0.5/AC-s in H2O/O2
(b)
0.03 0.02 0.01 0.00 5
10
100
pore diameter, Å
•
Enhanced H2S capacity of CuO-ZnO/AC sorbents with O2 and H2O
•
Reactive adsorption mechanism starts with the initial fast formation of metal sulphates
•
CuO catalyzes H2S oxidation to elemental S in the presence of H2O and O2
•
Large sorption capacity related to the filling up of micropores with elemental S
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: