Catalytic removal of SO2, NO and HCl from incineration flue gas over activated carbon-supported metal oxides

Activated carbon-supported copper, iron, or vanadium oxide catalysts were exposed to incineration flue gas to investigate the simultaneous catalytic oxidation of sulfur dioxide/hydrogen chloride and selective catalytic reduction of nitrogen oxide by carbon monoxide. The results show that AC-supported catalysts exhibit higher activities for SO₂ and HCl oxidation than traditional γ-Al₂O₃-supported catalysts and the iron and vanadium catalysts act as catalysts instead of sorbents, and can decompose sulfate with evolution of SO₃ and then regenerate for more SO₂ adsorption to take place. The AC-supported catalysts also display a high activity for NO reduction with CO generated from a flue gas incineration process and the presence of SO₂ in the incineration flue gas can significantly promote catalytic activity. Using CO as the reducing agent for NO reduction is more effective than using NH₃, because NH₃ may be partially oxidized in the presence of excess O₂ (12 vol%. in the incineration flue gas used) to form N₂, which can decrease the overall extent of NO reduction.     

Study of SO2 adsorption and thermal regeneration over activated carbon-supported copper oxide catalysts

The mechanisms of SO₂ adsorption and regeneration over activated carbon-supported copper oxide sorbent/catalysts were analyzed. Studies were carried out in a fixed-bed reactor equipped with a non-dispersive infrared gas analyzer to detect the reaction products and by using X-ray powder diffraction (XRPD) and temperature-programmed desorption (TPD) experiments to characterize the nature of the sulfate species and surface oxygen complexes. The results indicate that SO₂ was catalytically oxidized to SO₃ over a copper phase in the presence of gaseous oxygen, and then reacted with a copper site to form a sulfate linked to copper without desorption into the gas phase. The activated carbon support did not participate in this sulfation reaction. After the adsorption of SO₂, the exhausted sorbent/catalysts could be regenerated by direct heat treatment in inert gas at temperatures between 260 and 480 °C, while the neighboring surface oxygen complexes on the carbon surface were acting as the reducing agents to reduce CuSO₄ to Cu. During the subsequent adsorption process, the copper is rapidly oxidized by oxygen in the flue gas.     

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.     

Adsorption of SO2 using vanadium and vanadium–copper supported on activated carbon

Activated carbon impregnated with precursor salts of Ba, Co, Cu, Fe, Mg, Mn, Ni, Pb and V and their binary mixtures was used for adsorption of SO₂ at 20 °C. The most promising materials for SO₂ removal are carbons doped with V, Cu and mainly their binary mixtures, which show a synergetic effect. Kinetic curves and isotherms of SO₂ adsorption were obtained at 20 °C. These isotherms are reasonably well fitted by the Langmuir model and the respective parameters were determined. TPD experiments show that adsorption of SO₂ increases the oxygenated groups on the carbon surface. The sample doped with V, after SO₂ adsorption at 20 °C, presents an increase of basic oxygenated groups, which may be responsible for the observed extra adsorption of SO₂.     

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.     

Carbon materials as catalyst supports for SO2 oxidation: catalytic activity of CuO-AC

Copper catalysts supported on acid treated activated carbon (AC) were prepared, characterized and tested in terms of their SO₂ oxidation activity. Reactions of CuO-AC in flow systems with sulfur dioxide, oxygen and nitrogen streams were investigated to determine the types of chemical interactions that occur on the sorbent surface. The effects of reaction temperature, acid treatment, metal loading, support particle size, SO₂ concentration and O₂ concentration on SO₂ oxidation activity were evaluated. It was found that carbon materials used as catalyst supports for copper oxide catalysts provided a high catalytic activity for adsorbing SO₂ from flue gas and oxidizing it. Acid pretreatment of the carbon supports increased the content of specific surface chemical groups to enhance the catalytic activity for SO₂ oxidation. Metal loading, as well as support particle size, have a significant influence on the SO₂ activity. The supported metals rather than surface oxygen functional groups on AC may be the active sites for adsorbing SO₂.     

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.     

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.     

Cobenefit of SO3 reduction on mercury capture with activated carbon in coal flue gas

Parametric experiments were carried out to study the interactions of mercury, SO₃, and injected activated carbon (AC) in a coal flue gas stream. The levels of SO₃ vapor in flue gas were altered by individually varying flue gas temperature, moisture, or sodium fume injection in the flue gas. Meanwhile, mercury emissions with AC injection (ACI) upstream of an electrostatic precipitator (ESP) were evaluated under varied SO₃ concentrations. SO₃ measurements using a condensation method indicated that low temperature, high moisture content, and sodium fume injection in flue gas shifted SO₃ partitioning from the vapor to particulate phase, subsequently improving mercury capture with ACI. 0.08 g/m³ of DARCO^® Hg-LH injection only provided approximately 20% mercury reduction across the ESP in a bituminous coal flue gas containing 28 ppm SO₃, but mercury capture was increased to 80% when the SO₃ vapor concentration was lowered less than 2 ppm. Experimental data clearly demonstrate that elevated SO₃ vapor is the key factor that impedes mercury adsorption on AC, mainly because SO₃ directly competes against mercury for the same binding sites and overwhelmingly consumes all binding sites.     

Bimetallic Pt–Sn catalysts supported on activated carbon. II. CO oxidation

A commercial activated carbon (AC) was used as a catalyst support either in its original form or after two different oxidation treatments, namely air oxidation and HNO₃ oxidation, aiming at the enhancement of its textural and surface chemical characteristics. These properties were determined by N₂ adsorption and temperature programmed desorption (TPD), respectively. Monometallic Pt and bimetallic Pt–Sn catalysts were prepared on the AC supports. Impregnation was used in the preparation of the monometallic samples. For the bimetallic samples, coimpregnation and a sequential impregnation procedure, in which the Sn precursor is introduced prior to Pt, were used. The Pt load was kept fixed as 1 wt.% for all monometallic and bimetallic samples. Two different Sn loads, 0.25 and 0.50 wt.%, were used for the bimetallic samples in order to investigate the effects of Sn load on the catalytic properties. The catalyst samples were characterized by H₂ adsorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and structure insensitive benzene hydrogenation. The activities of all samples were measured in CO oxidation. The results indicate the strong effects of the surface chemistry of the AC supports, the Pt:Sn ratio, the preparation procedure and the reduction procedure, on the CO oxidation activities of the catalysts.     

Removal of SO2 by activated carbon fibers in the presence of O2 and H2O

This work describes the potential capability of activated carbon fibers (ACFs) in continuously removing SO₂ from inert atmosphere without requiring further regeneration. A tubular reactor packed with ACF was used to study the conversion of SO₂ into H₂SO₄ in the presence of O₂ and H₂O with varying concentrations of SO₂ (3000-10,000 ppm), O₂ (10-20%), and H₂O (10-70%) and temperatures (313-348 K). The experiments were carried out on two precursors (viscose rayon and phenolic resin) based ACFs. The breakthrough data revealed that the steady-state SO₂ concentration levels at the reactor exit increased with increasing inlet SO₂ concentration and decreased with increasing concentration levels of O₂ as well as H₂O. Increase in the reaction temperature was found to moderately enhance the steady-state exit concentration levels of SO₂. The viscose rayon-based ACF exhibited higher SO₂ removal activity in comparison to the phenolic resin-based ACF. A mathematical model was developed to predict the gas concentration profiles in the reactor, incorporating the mass transfer in the bed as well as within the ACF pores, along with the surface reactions on the ACF. The model predictions agreed reasonably well with the data.     

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.     

Characterization and reactivity of pure TiO2–SO4 SCR catalyst: influence of SO4 content

The kinetics of the reaction of NO, N₂O and CO₂ with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N₂O and CO₂ was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N₂O conversion the main products obtained were N₂, N₂O, CO and CO₂ but for CO₂ conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V₂O₅/V₆O₁₃.     

The role of carbon surface chemistry in N2O conversion to N2 over Ni catalyst supported on activated carbon

The effect of acidic treatments on N₂O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N₂ adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N₂O-carbon reaction catalyzed by Ni catalyst. HNO₃ treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N₂O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed.     

Adsorption and reduction of NO2 over activated carbon at low temperature

The reactive adsorption of NO₂ over activated carbon (AC) was investigated at 50 °C. Both the NO₂ adsorption and its reduction to NO were observed during the exposure of AC to NO₂. Temperature programmed desorption (TPD) was then performed to evaluate the nature and thermal stability of the adsorbed species. Adsorption and desorption processes have been proposed based on the nitrogen and oxygen balance data. The micropores in AC act as a nano-reactor for the formation of -C(ONO₂) complexes, which is composed by NO₂ adsorption on existing -C(O) complexes and the disproportionation of adsorbed NO₂. The generated -C(ONO₂) complexes are decomposed to NO and NO₂ in the desorption step. The remaining oxygen complexes can be desorbed as CO and CO₂ to recover the adsorptive and reductive capacity of AC.     

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.     

Treatment of chlorophenols-bearing wastewaters through hydrodechlorination using Pd/activated carbon catalysts

The catalytic hydrodechlorination of 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol in aqueous solution over Pd/activated carbon catalysts (0.5% w/w Pd) was studied in a fixed bed reactor. The reactor was fed with a 100 mg/l solution of chlorophenol and a H₂/N₂ (1:1) gas stream. The ranges studied for temperature, pressure and space-time were 25-100 °C, 1.8-6.0 bar and 14-55 kg h/mol, respectively. A commercial and some home-made catalysts were tested. The carbon supports were subjected to oxidation with nitric acid and sodium persulfate. Chlorophenols conversion was found to peak for pressure values ≈2.4 bar. In these conditions, an increase of reaction temperature increases conversion. In the runs carried out at high space-time both the reactants conversion and the selectivity towards end chain reaction products was enhanced. The oxidation of the carbon support with nitric acid prior to impregnation improves conversion. At the optimum conditions (2.4 bar, 75 °C and nitric acid oxidation of carbon support) conversion values over 95% were reached for all chlorophenols. As a result of this treatment the toxicity of the initial solutions was reduced by more than 90%.     

Influence of CO-evolving groups on the activity of activated carbon fiber for SO2 removal

An investigation was made into the influence of CO-evolving and CO₂-evolving groups on the activities of activated carbon fibers (ACFs) for the oxidative conversion of SO₂ into aq. H₂SO₄ in the presence of O₂ and H₂O. The results indicated that the amount of evolved CO determined the SO₂ removal activity of ACFs, whereas, the amount of evolved CO₂ did not correlate with the ACFs activity for SO₂ removal. A direct proportionality between the amount of evolved CO and the enhanced activity of SO₂ removal was confirmed by using different oxidizing agents for changing the types and amount of oxygen functional groups in ACFs.