Synergic effect of Zn and Cu oxides dispersed on activated carbon during reactive adsorption of H2S at room temperature
Graphical abstract
Introduction
Hydrogen sulphide (H2S) is a toxic compound found in a variety of fuel resources such as crude petroleum, natural gas and biogas [1]. A few ppm of H2S can corrode pipelines [2], poison catalysts used in fuel cells [3] and emission of sulfur compounds can contribute to the production of acid rains [4]. As an example Bao et al. [5] studied the effect of the concentration of H2S on the poisoning of a fuel cell based on solid oxides, concluding that even 1 ppmv of H2S causes an immediate performance loss. Adsorption is one of the most suitable methods for the removal of H2S from a gas stream, as it allows a deep purification (down to H2S concentration less than 1 ppmv) and a cost effective approach. However, highly performing adsorbent materials are required to obtain a deep purification particularly at low (i.e. room) temperature [6], [7]. Currently, research efforts are mainly devoted to sorbents based on metal oxides, hydroxides and carbonates supported onto a matrix of high superficial area such as zeolites, mesoporous silicas, activated carbons and graphene/graphite oxides [8], [9]. In particular activated carbons (AC) are extensively used as support for the removal of H2S as their properties, such as surface area and surface chemistry, play an important role [10], [11], [12].
The identification of the compounds produced upon the adsorption process of H2S is of paramount importance to select optimal chemical formulations and textural properties of sorbents and to set-up potential regeneration strategies for exhausted materials. Many literature studies focused on the formation of metal sulphides as a product of the reaction between metal oxides (supported or not) and H2S [13], [14]. Hernández et al. [15] employed two different commercial AC supporting 10% wt. of ZnO to evaluate the adsorption capacity in presence of a stream containing 200 ppmv of H2S in N2 at 28 °C. They detected, by means of X-ray photoelectron spectroscopy (XPS) analysis, the sole formation of ZnS. Huang et al. [16] studied the behaviour of Cu(OH)2 supported on AC assuming the sole formation of CuS after adsorption tests in presence of a stream containing 270 ppmv of H2S in He and with relative humidity of 40–80%. The assumption of the formation of metal sulphides, though relevant, should be integrated by other considerations, taking into account that the process of H2S adsorption might likely involve the formation of a complex variety of compounds such as elemental sulphur, sulphur dioxide, sulphuric acid, metal sulphates. In particular, some authors reported experimental evidence of partial formation of metal sulphates during the H2S capture on metal oxides/hydroxides [17], [18], [19]. Zhang et al. [20] highlighted the formation of metal sulphates together with elemental sulphur using spheres of mesoporous carbon impregnated with MgO for catalytic oxidation of H2S at room temperature. Those authors ascribed the sulphate formation (detected by means of XPS analysis) to the oxidation of H2S to acidic SO3 species, the latter being able to neutralise basic MgO. Mabayoje et al. [21] found the formation of copper sulphate, by thermogravimetric measurement, after H2S adsorption onto copper (hydr)oxychlorides supported on graphene/graphite oxide. In this case, the formation of sulphate is the result of the ability of copper to activate oxygen, and the oxidised graphene can contribute to this process as well [21].
Following this path, in our recent work [22] we inferred the formation of both Zn and Cu sulphates by temperature programmed desorption (TPD) tests upon reactive adsorption of H2S on mixed oxides (ZnO–CuO) supported on commercial activated carbon. Moreover, we found that the partial substitution of CuO for ZnO within a given range, at fixed total loading of metal (10% wt.), induced a progressive increase in the sorption capacity of H2S. The highest adsorption capacity was observed for the sorbent having Cu:Zn molar ratio equal to one. The present paper extends and complements the previous investigation with respect to the promoting role of copper in mixed ZnO–CuO supported systems on the H2S removal mechanism, focusing on the interrelationships among properties of functionalized activated carbon, operating conditions and process outcomes. Moreover, a significant effort was made in order to identify the adsorbed sulphur species produced during capture process, so to shed light on the complexity of the surface reactions.
H2S removal experiments (100–3000 ppmv in N2) under dynamic conditions at room temperature were complemented by BET and pore size distribution analysis, SEM-EDX and XPS analysis for the characterisation of fresh and spent sorbents. Additionally, a TPD/TPO experimental protocol was established for the speciation analysis of sulphur species formed at different stages during the reactive adsorption process depending on the specific formulation of the active phase.
Section snippets
Adsorbents
Hybrid adsorbents based on ZnO–CuO highly dispersed onto a commercial 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. More details on the preparation procedure were already reported elsewhere [22]. The nominal loading of metals (Zn + Cu) in functionalized sorbents was set to 10% wt., whereas the relative content of Zn and Cu was varied, including formulations with
H2S adsorption tests at high concentration
Table 1 compares the performance of H2S capture of CuxZn1-x/AC sorbents previously reported in Balsamo et al. [22] with the results obtained for the sorbent containing only copper oxide as active phase (Cu/AC), in terms of breakpoint time tbr (assumed as the time for which = 0.05) and , for tests carried out at = 3000 ppmv and T = 30 °C. The saturation adsorption capacity of Cu/AC was approximately 7% lower than the value retrieved for the best-performing sample (Cu
Conclusions
Sorbents for the deep removal of H2S from gas streams at room temperature were prepared by dispersing mixed oxides (ZnO–CuO) onto a commercial granulated activated carbon at fixed 10% wt. total metal loading and Cu:Zn molar ratio varying in the range from 0:1 to 1:0. Pore size distribution and SEM-EDX analyses indicated that the incipient wetness preparation method guaranteed a high dispersion of the active phase largely preserving the textural features of the pristine AC. Nevertheless, Zn and
Acknowledgments
Mr. Luciano Cortese and Mr. Fernando Stanzione (IRC-CNR) are gratefully acknowledged for their help in carrying out SEM-EDX and Ion Chromatography characterisations.
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