Decomposition Of Adsorbed Toluene Using Non-thermal Plasma Combined With Adsorptive Catalysts

For the removal of VOCs with low concentration,non-thermal catalytic technology has a high removal efficiency,simple process,low operation cost,rapid start and stop compared with the traditional processing method.However,the undesirable byproducts and low-energy efficiency are the problems limits its application.To solve this problem,through the optimization of catalysts and system process,the first adsorption and then plasma oxidation technology was used to explore a kind of high efficiency,low pollution processing method for a deep purification of low concentration of VOCs,which will provide the theoretical and design basis for the industrial application.Firstly,a series of Ag-Mn/γ-Al₂O₃ were prepared under different Ag/Mn impregnation sequence and tested in the sequential adsorption-plasma catalytic removal of toluene.When Mn was impregnated first,the resulting catalyst,Ag–Mn（F）/γ-Al₂O₃,had longer breakthrough time,gave less emission of toluene,had higher CO₂ selectivity,and had better carbon balance and CO_x yield compared to catalysts prepared via other impregnation sequences.After 120 min of NTP treatment,the carbon balance of Ag-Mn（F）/γ-Al₂O₃ was 91%,with 87% as CO_x contributions.A Brunauer – Emmett–Teller（BET）analysis and X-ray photoelectron spectroscopy（XPS）results show that,the impregnation sequence impacts the BET surface area and the ratio and existing state of Ag on the surface of the catalysts.The longer breakthrough time when using Ag–Mn（F）/γ-Al₂O₃ as catalyst is attributed to the large amount of Ag⁺ on the surface.Ag⁺ is a new active site for toluene adsorption.When Ag was impregnated first（Ag（F）-Mn/γ-Al₂O₃）or Ag and Mn co-impregnated（Ag-Mn-C/γ-Al₂O₃）,the predominant species was Ag⁺.Both Ag⁰ and Ag⁺ species were detected on Ag–Mn（F）/γ-Al₂O₃.Ag⁰ cooperation with MnO_x may promote the migration of surface active oxygen.This would facilitate the oxidation of adsorbed toluene with C=C bond already weakened by Ag⁺ and would result in higher CO₂ selectivity and better carbon balance as seen in the Ag–Mn（F）/γ-Al₂O₃ system.Secondly,gas flow-through and gas circulation systems were compared in the same reactor with Ag-Mn（F）/γ-Al₂O₃ as catalyst for adsorbed toluene oxidation.After 90 min of plasma treatment,the mineralization of adsorbed toluene reached 68% in the O₂ flow-through system with some emission of desorbed toluene.For the O₂ circulation system,the mineralization of adsorbed toluene increased to 78% without any emission of desorbed toluene.Furthermore,compared to the gas flow-through system,the CO₂ selectivity in the gas circulation system was improved in both air and oxygen plasma.Moreover,the circulation system was more efficient than the gas flow-through system for O₃ decomposition,resulting in better mineralization of adsorbed toluene and CO₂ selectivity.The energy yield of the gas circulation system was higher than that of gas flow-through system.When oxygen circulated for 60 min,the energy yield of the system was increased from 0.68 g·kWh-1 to 4.02 g·kWh-1 with the adsorbed amount of toluene increased from 0.35 mmol to 1.06 mmol.Thirdly,adsorbed benzene/toluene on Ag-Mn/γ-Al₂O₃ and Ag-Mn-Co/γ-Al₂O₃ catalysts was oxidised by non-thermal plasma with gas circulation.And the change rules and move ways of the by-products in the system were analyzed.After the gas was circulated for 90 minutes,compared to γ-Al₂O₃,the introduction of Ag-Mn/γ-Al₂O₃ improved the CO_x yield by 33% with CO₂ selectivity close to 100%,and reduced the amount of O₃ by 51%,while Ag-Mn-Co/γ-Al₂O₃ inhibited N₂O formation by 38% in comparison with γ-Al₂O₃.The elimination of O₃ was mainly through its decomposition into active oxygen atoms on Ag-Mn,which would promote the oxidation of absorbed organics.The reduction of N₂O was mainly due to its decomposition on Co catalyst.The organics on the surface of the catalysts was first increased and then decreased.The adsorbed benzene and toluene was first transformed into organic intermediates and then further oxidised into CO₂ and water.Since the rate of the former is greater than the latter,the intermediates were accumulated on the surface of the catalysts.With discharge continues,the amount of adsorbed toluene was reduced,most of the active species were used in the degradation of organic intermediates,which lead to reduction of organic intermediates.Finally,a sequential adsorption and plasma oxidation system was employed to remove the toluene from the simulated dry air under atmospheric pressure and temperature with small particles γ-Al₂O₃,HZSM-5,or a mixture of the two materials as adsorbents.The results showed that,compared to HZSM-5,γ-Al₂O₃ had better carbon balance（⁷5%）.However,the CO₂ yield of γ-Al₂O₃ was only ⁵0%,and the breakthrough time of γ-Al₂O₃ was shorter than that of HZSM-5;the amounts of byproducts O₃ and N₂O produced by γ-Al₂O₃ were higher than that produced by HZSM-5.Moreover,some toluene was desorbed when γ-Al₂O₃ was used.When a mixture of HZSM-5 and γ-Al₂O₃ with mass ratio of 1/2 was used,the carbon balance could reach as high as 90%,of which 82% was CO₂.The reason for the better CO_x yield of the mixed adsorbents was that in the mixed supports,the HZSM-5 with higher BET surface are and smaller pore diameter was mainly used to trap the adsorbed toluene in the discharge region,while γ-Al₂O₃ was used to produce more micro-discharges and more active species for toluene oxidation.Finally,the mechanism for the catalysts deactivation after 7 cycles of sequential adsorption and plasma oxidation was studied by characterizing the supports before and after deactivation by SEM,BET method,XRD and GC-MS.There were 3 main reasons responsible for the deactivation of the catalyst: 1)the discharge changed the structure of the supports and reduced its surface area;2)the organic intermediates or carbon deposition covered the active sites of the catalysts;3)the state of the active component was changed by the discharge,which lead to the deactivation of the catalysts.By temperature progress oxidation（TPO）method,the deactivated supports could be 97.6% regenerated,while deactivated catalysts could be 76% recovered.