Adsorption of H2S or SO2 on an activated carbon cloth modified by ammonia treatment

The aim of this research is to investigate how ammonia treatment of the surface can influence the activity of a viscose-based activated carbon cloth (ACC) for the oxidative retention of H₂S and SO₂ in humid air at 25 °C. Surface basic nitrogen groups were introduced either by treatment with ammonia/air at 300 °C or with ammonia/steam at 800 °C. The pore structure of the samples so prepared was examined by adsorption measurements. Changes in the surface chemistry were assessed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and temperature programmed desorption (TPD). The change of ACC activity could not be merely attributed to surface nitrogen groups but to other changes in the support. Ammonia/steam treatment improved ACC performance the most, not only by introducing nitrogen surface groups, but also by extending the microporosity and by modifying the distribution of surface oxygen groups. Successive adsorption-regeneration cycles showed important differences between oxidative retention of H₂S and SO₂ and the subsequent catalyst/support regeneration process.     

The effect of H2 treatment on the activity of activated carbon for the oxidation of organic contaminants in water and the H2O2 decomposition

In this work, hydrogen peroxide reactions, i.e. H₂O₂ decomposition and oxidation of organics in aqueous medium, were studied in the presence of activated carbon. It was observed that the carbon pre-treatment with H₂ at 300, 500, 700 and 800 °C resulted in an increase in activity for both reactions. The carbons were characterized by BET nitrogen adsorption, thermogravimetric analyses (TG), temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), iodometric titration and determination of the acid/basic sites. TPR experiments showed that activated carbon reacts with H₂ at temperatures higher than 400 °C. The treatment produces a slight increase in the surface area. EPR analyses indicate the absence of unpaired electrons in the carbon. Iodometric titrations and TG analyses suggested that the treatment with H₂ generates reduction sites in the carbon structure, with concentration of approximately 0.33, 0.53, 0.59, 0.65 and 0.60 mmol/g for carbons treated at 25, 300, 500, 700 and 800 °C, respectively. It was also observed the appearance of basic sites which might be related to the reduction sites. It is proposed that these reducing sites in the carbon can activate H₂O₂ to generate HO^* radicals which can lead to two competitive reactions, i.e. the hydrogen peroxide decomposition or the oxidation of organics in water.     

Kinetics of methane oxidation on Pt catalysts in the presence of H2S and SO2

The kinetic parameters of the lean oxidation of CH₄ over alumina and ceria-alumina supported platinum catalysts in the presence of H₂S and SO₂ were derived in a stagnation point flow reactor at atmospheric pressure and at temperatures up to . A novel methodology similar to that of plug-flow reactors was devised to calculate the best-fit values for frequency factors and activation energies for the proposed heterogeneous gas-solid reactions of oxidation. Doping the N₂-diluted CH₄/air reactant flow with H₂S or SO₂ concentrations between 30 and had a significant promotional effect on the methane combustion rate. Al₂O₃ and CeO₂/Al₂O₃ were shown to be inert with respect to the oxidation of CH₄ to CO₂, and also in the oxidation of SO₂ to SO₃ in air. Pt catalysed the oxidation of CH₄, SO₂ and that of H₂S. A temperature window of conversion of SO₂ to SO₃ on the Pt-supported catalysts was found experimentally, and could be of practical use in combustion exhaust clean-up techniques. The repeated use of the foil resulted in a slight ageing effect for Pt/Al₂O₃. The presence of ceria in the washcoat helped prevent the loss of activity in CH₄ oxidation by mitigating the extent of sintering of the Pt particles upon ageing. One-step and two-step chemical reaction mechanisms are proposed for the CH₄ and the SO₂ lean oxidations, respectively.     

Adsorption and reaction behavior for the simultaneous adsorption of NO-NO2 and SO2 on activated carbon impregnated with KOH

This study examined the individual and simultaneous adsorption of NO_x (NO-NO₂) and SO₂ on activated carbon impregnated with KOH (KOH-IAC). For individual component adsorption, KOH-IAC showed a higher adsorption capacity in NO-NO₂ rich air than in SO₂-air. In the simultaneous adsorption of NO-NO₂-SO₂, SO₂ showed a greater adsorption affinity than NO-NO₂. The smaller the amount of NO-NO₂ adsorbed, the more SO₂ was adsorbed. XPS analysis of the adsorption of NO-NO₂ rich SO₂-air on KOH-IAC revealed that the adsorbed SO₂ was predominantly found on the external surface, producing mainly K₂SO₄ and, additionally, H₂SO₄ and K₂SO₃. Depth profile analysis showed that the amount of SO₂ adsorbed decreased regularly away from the surface, while the amount of adsorbed NO-NO₂ increased irregularly. We confirmed that the presence of the impregnant in KOH-IAC is a determining factor in the adsorption of NO-NO₂ and SO₂ by chemical reaction, clarifying the surface chemical behavior.     

Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO₂ by O₂/H₂O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO₂ adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO₂ was first adsorbed on the bare zigzag sites, followed by reaction with H₂O to form H₂SO₃, which was further oxidized by O₂ to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO₃ or H₂SO₃ as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO₂ adsorption (and oxidation) on the amount of surface oxygen.     

Kinetics of 1,3,6-naphthalenetrisulphonic acid ozonation in presence of activated carbon

Processes based on the simultaneous use of ozone and activated carbon have proven very effective for removing contaminants of high toxicity and low biodegradability. The present study is aimed to determine the kinetic constants involved in this purification process and their relationship with the surface chemistry of the activated carbon. For this purpose, the ozonation of 1,3,6-naphthalenetrisulphonic acid (NTS), selected as model compound, was carried out in the presence of different activated carbons. Determination of the Weisz-Prater parameter (C_WP) revealed that intraparticular diffusion limitations exist in the system for particles >500 μm. The degradation kinetics of NTS in the presence of activated carbon depends on the concentrations of both, the contaminant and the dissolved ozone, with a global reaction order of 2. The heterogeneous reaction constants were determined using a model that allowed quantification of the capacity of the activated carbon to increase the NTS degradation rate and of the chemical surface properties responsible for this increase. The basicity of the activated carbon is mainly responsible for the catalytic activity of the carbon in NTS ozonation, even though, mineral matter contributes positively to the catalytic activity.     

Preparation of photocatalytic TiO2 coatings of nanosized particles on activated carbon by AP-MOCVD

Anatase TiO₂ coatings on highly porous activated carbon were prepared by a novel method—atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). At a source temperature of 423 K, the TiO₂ particles were mostly coated on the external surface of activated carbon. These particles were well dispersed with their sizes ranging from 10 to 50 nm. The optimum loading of TiO₂ was found to be 12 wt%. The TiO₂ photocatalysts so prepared behave similarly to that of the pure commercial TiO₂ powder. The activated carbon supported TiO₂ catalyst could be easily separated from the treated water with its catalytic performance maintained even after 10 cycles, indicating that the TiO₂ coating was stable. It was observed that TiO₂ supported on activated carbon had a high capacity to mineralize pollutants. Consequently, activated carbon supported TiO₂ by AP-MOCVD is a promising photocatalyst for the photodegradation of pollutants in water.     

The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres

SO₂ removal from flue gases by carbonaceous materials is determined by their behaviour as catalysts for SO₂ oxidation into SO₃ or H₂SO₄ in the presence of O₂ or O₂ and steam, respectively. Previous studies have demonstrated that nitrogen (N) functional groups are active sites for the adsorption and oxidation of SO₂, although the nature of the N groups with the higher activity had not been established yet. For this reason, in the present work a number of activated carbons (AC) and activated carbon fibres (ACF) doped with N atoms have been prepared using different methods. The number and nature of these N groups have been assessed by XPS. The materials prepared have a wide range of nitrogen content, which is distributed into different chemical species. In this way, we were able to determine the effect of the N content and the role of the different N-containing functional groups on the catalytic activity for SO₂ oxidation. The results confirm that, although the pore volume and the pore size distribution strongly influence the catalytic activity, the presence of N species at the surface increases the catalytic activity. They also demonstrate that, among the different N functional groups, pyridinic nitrogen is the most active for this reaction.     

Removal of elemental mercury by activated carbon impregnated with CeO2

Mercury emission from coal-fired power plants becomes a great environmental concern due to its high toxicity and volatility in particularly for elemental mercury. Activated carbon adsorption is considered to be a potential technology to control elemental mercury emission. In this work, a novel CeO₂/AC (activated carbon impregnated with cerium dioxide) sorbent was studied with an attempt to produce economical and effective sorbent for capturing mercury. The influencing factors researched include loading values changing from 1 wt% to 10 wt% and adsorption temperature changing from 30 to 200 °C. Some physicochemical techniques such as BET and XRD were used to characterize the properties of the sorbents. The adsorption test results show that CeO₂ impregnation significantly enhanced the AC adsorption ability for elemental mercury. When the CeO₂ load was below 3%, Hg⁰ adsorption ability of ameliorated AC enhanced with the increase in the loading value, and then decreased at higher loading. The influence of temperature on the mercury removal efficiency was also studied, the trend of which was similar to the effect of loading value. The maximum removal efficiency was obtained at 100 °C.     

Oxidative dehydrogenation of ethylbenzene on activated carbon fibers

Activated carbon fibers from different precursors and with different degrees of activation were used as catalysts for the oxidative dehydrogenation of ethylbenzene. Within each group, the fibers exhibited similar surface chemistries, so that the observed catalytic performances could be interpreted exclusively in terms of their textural properties. Analysis of the catalytic results highlighted common trends. In particular, the fibers with an average micropore width larger than 1.2 nm were found to be the best catalysts for this reaction.     

Influence of surface properties and iron addition on the SO2 adsorption capacity of activated carbons

When iron derivatives are added to low ash activated carbons having basic surface characteristics (obtained by suitable oxidation at 800°C with 2% of O₂ in N₂), certain materials are obtained showing high SO₂ sorbent properties from gaseous mixtures having a composition close to that of the flue gases. This behaviour seems to be related to the simultaneous presence of both basic surface sites promoting the initial adsorption of SO₂ and iron promoting the transformation of the adsorbed SO₂ into other, more stable forms. The sorbent properties of these activated carbons are more stable, following consecutive cycles, in the processes of adsorption and desorption of SO₂, than those shown by similar carbons with different surface characteristics.     

TiO2 photocatalyst deposition by MOCVD on activated carbon

Activated carbon modified by HNO₃ was used as the support during the production of TiO₂ by metal organic chemical vapor deposition (MOCVD). The HNO₃ modification increased mesopores surface area of activated carbon indicating the size of pores increased. The concentration of surface oxygen bearing groups on the HNO₃ modified activated carbon was much higher than that of the original activated carbon. It was found that a modification of activated carbon by 6 mol/L HNO₃ increased the deposition rate of TiO₂ by 4.5 times. The modification of activated carbon by HNO₃ significantly raised the photocatalytic activity of TiO₂ resulting from the formation of smaller-sized TiO₂ particles well dispersed confirmed by the results of XRD patterns and N₂ adsorption-desorption isotherms.     

Performance of fixed-bed KOH impregnated activated carbon adsorber for NO and NO2 removal in the presence of oxygen

KOH-impregnated activated carbon (K-IAC) was used in this study. This paper contains observation the adsorption behavior of NO and NO₂ with/without oxygen and with different bed depths of adsorbent. The paper also defines surface chemical changes due to NO_x adsorption. By using a simple design of adsorber, the packed amount of adsorbent for NO_x abatement for 6 months on a pilot scale was calculated. When oxygen was present, NO and NO₂ had a great improvement in adsorptivity. Adsorption of NO₂ forms a oxide crystal on the surface of the K-IAC and at the same time produces NO, which acts to bring about increased adsorptivity. The higher the bed of adsorbent was, the more NO was produced and the longer the breakthrough time took. The adsorber was designed in a scale-up condition where NO, NO₂ and O₂ were applied to K-IAC. The adsorbate that consumed the least packed amount was NO₂-air followed by NO₂-N₂, NO-air and NO-N₂. The results of the experiment demonstrated that with regard to adsorption of NO and NO₂ on K-IAC, the presence of oxygen and the bed depth of adsorbent were the biggest variables to adsorptivity.     

Adsorption of SO2 from flowing air by alkaline-oxide-containing activated carbons

Adsorption of SO₂ under dynamic conditions from an SO₂-air mixture at 298 and 573 K on alkaline-oxide-containing activated carbons has been studied. The adsorption capacity of these samples at 298 K was, in general, lower than that in the original activated carbons and mainly governed by their microporosity accessible to benzene. However, at 573 K, the alkaline-oxide-containing activated carbons adsorbed a greater amount of SO₂ than the original activated carbon, following the order Na ≥ K > Rb. At both adsorption temperatures, part of the SO₂ adsorbed formed H₂SO₄ and Me₂SO₄, where Me = Na, K or Rb. When the SO₂ adsorption was carried out at 573 K, this gas fixed additional oxygen complexes that evolved as CO₂ under heating up to 873 K in He flow, probably by reaction of SO₂ with carbon surface atoms of a basic nature that are not able to chemisorb oxygen from the air at the same conditions.     

Influence of HNO3 oxidation on the structure and adsorptive properties of corncob-based activated carbon

An investigation of the impact of strong oxidation with HNO₃ on the porosity and adsorption characteristics of char and activated carbons, derived from corncobs, is presented. Texture parameters, as obtained from N₂ adsorption at 77 K, showed a considerable decrease in surface area of the activated carbons with enhanced pore widening. The extent of porosity modification was found to depend on the scheme of activation of the precursor, simple carbonization, steam pyrolysis, steam gasification of the char, or chemical activation with H₃PO₄. Surface-chemical changes were detected by FTIR spectroscopy, where absorption bands assigned to carboxyl, carboxylate, carbonyl, and phenolic groups were observed. A SEM study demonstrated the erosive effect of HNO₃, detected by the presence of disintegration of the carbon grains, with the porous structure probably containing very large macropores. As a consequence of the oxidation process, elemental analysis showed high contents of O, H and N, and TG confirmed that the weight loss distribution in the thermogram becomes slower at higher temperatures. The removal of phenol decreased as a result of the formation of oxygen functionalities. Mono-nitrophenols were adsorbed in smaller amounts than phenol, and p-nitrophenol showed a relatively higher uptake than the other two mono-nitrophenols, whereas the uptake of Methylene Blue was improved. Removal of Pb²⁺ from aqueous non-buffered solution was considerably enhanced by chemical oxidation, which may be related to pore widening, increased cation-exchange capacity by oxygen groups, and the promoted hydrophilicity of the carbon surface.     

SO2 retention on CaO/activated carbon sorbents. Part II: Effect of the activated carbon support

The present paper analyses the role of the activated carbon (AC) properties on the SO₂ uptake capacity of CaO/AC sorbents prepared by AC impregnation or ionic exchange with calcium acetate water solutions. Gas adsorption and mercury porosimetry have been used for textural characterization of the AC and surface oxygen groups have been characterized by temperature programmed desorption (TPD). Thermogravimetry has been used for SO₂ retention tests and CO₂ chemisorption at 300 °C for CaO dispersion (d) determinations. The results show that the surface calcium on CaO/AC samples, determined as “Ca loading · CaO dispersion” (parameter Ca(%) · d), governs the SO₂ uptake. The surface oxygen content is the AC property that mainly controls both the calcium loading and surface calcium on CaO/AC samples, which could be explained by the fact that the surface oxygen lowers the hydrophobic character of the AC supports therefore favouring the interaction with the calcium acetate water solutions. The combination of high calcium loading and dispersion leads to SO₂ uptakes up to 123 mg SO₂/g. The textural properties of the supports have some influence in the calcium loading. However, the effect is masked by the blockage of AC porosity by the calcium loaded.