Study On The Properties Of Hg ⁰ Catalytic Oxidation Of TiO 2 Submicron Carbon Nanotubes (MWCNTs) Supported Transition Metal

The mercury emissions from the coal-fired source threatened the environment and human health seriously and caused an increasing attention in the field of energy and environmental protection. The microporous carbon materials with its abundant pore structure and good surface chemical properties, have been considered to be the excellent catalyst carrier for mercury adsorption. The titanium-based photocatalyst doping transition metal ions has the unique catalytic oxidation performance. Therefore, this paper combined the two aspects to explore a novel preparing method for the composite catalytic adsorbent with high performance using titanium-based photocatalyst doping transition metal, such as Cu, Mn, to modify the multi-walled carbon nanotubes (MWCNTs). The mercury removal experiments were carried out under the simulated flue gas conditions and the adsorption-catalytic oxidation ability of the novel adsorbent was determined.The different adsorption-catalytic oxidation adsorbents, including TiO2, TiO2 doped with Mn ion (Mn(x)-TiO2), the TiO2 supported by MWCNTs (MWCNTs/TiO2) and the multi-walled carbon nanotubes with the titanium-based photocatalyst doping transition metal, were prepared by the sol-gel method and doping process. The microscopic features of the Mn(x)-TiO2, MWCNTs/TiO2, MWCNTs/Cu-TiO2 and MWCNTs/Mn-TiO2 were characterized using BET surface area analysis (BET), Transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and Ultraviolet-visible spectrophotometer (UV-Vis). The characterization results showed that the introduction of Mn ion increased the specific surface area of Mn(x)-TiO2 and improved pore structure, while inhibited the grain growth of TiO2, the Mn(x)-TiO2 was mainly consisted of anatase. MnO2 was the main occurrence form of Mn in Mn(x)-TiO2. Compared with original TiO2, the chemisorbed oxygen functional group on Mn(x)-TiO2 increased obviously. The characterization results of MWCNTs/TiO2, MWCNTs/Cu-TiO2 and MWCNTs/Mn-TiO2 revealed that MWCNTs were virtually covered by TiO2 in the form of clusters. MWCNTs/TiO2, MWCNTs/Cu-TiO2 and MWCNTs/Mn-TiO2 were mainly consisted of anatase. The introduction of MWCNTs, Cu ion and Mn ion inhibited the growth of TiO2 grain. 10%MWCNTs/Cu-TiO2 and 0.5%MWCNTs/Mn-TiO2 have strong adsorption intensity in the UV region. The introduction of MWCNTs narrowed the band gap of TiO2. Cu and Mn existed in multivalent states on catalyst surface.The performance of the mercury removal by the catalytic oxidation of Mn(x)-TiO2 was investigated in the fixed-bed adsorption reactor. Based on this experiment, the performance of photocatalysts (MWCNTs/TiO2, MWCNTs/Cu-TiO2 and MWCNTs/Mn-TiO2) for Hg0 photocatalytic oxidation removal were studied. The experimental results showed that with the increasing of the Mn content, the removal performance of Hg0 by Mn(x)-TiO2 was improved. MnO2 participated in the reaction of Hg0 catalytic oxidation and was reduced to Mn2O3, Hg0 was oxidized to Hg2+. O2 and NO promoted the removal of Hg0. However, SO2 and H2O had an inhibiting effect. SO2 limited Hg0 removal obviously, while in the presence of NO, the inhibition effect was weakened. In N2, N2+O2, N2+SO2 and N2+NO atmosphere, Hg0 was oxidized to Hg2+by Mn(x)-TiO2, furthermore, Hg2+was adsorbed on the surface of Hg2+, However, when NO and SO2 were both present in the flue gas, a portion of adsorbed Hg2+ was desorbed. In N2, N2+6%O2, N2+600ppmSO2 and N2+500ppmNO atmosphere, after catalytic oxidation by Mn(x)-TiO2, mercury speciation was mainly in the form of Hg0, there was almost no Hg2+ in the flue gas, in N2+600ppmSO2+500ppmNO atmosphere, mercury speciation was mainly in the form of Hg2+, the concentration of Hg2+ increased to 11.7μg/m3, the concentration of Hg0 was 6.8μg/m3.Compared with original TiO2, the removal performance of Hg0 by MWCNTs/TiO2 was improved significantly under the irradiation of the UV light, the removal efficiency increased from 54% to 86%. O2 promoted the removal of Hg0 with MWCNTs/TiO2. However, SO2, NO and excessive H2O showed a negative effect. The inclusion of Cu and Mn ion changed the electronic structure and crystalline size of TiO2, which resulted in the higher Hg0 removal efficiencies of MWCNTs/Cu-TiO2 and MWCNTs/Mn-TiO2. In the (1%、3%、10%)MWCNTs/Cu-TiO2 and (0.5%、1%、 3%)MWCNTs/Mn-TiO2,10%MWCNTs/Cu-TiO2 and 0.5%MWCNTs/Mn-TiO2 showed the best Hg0 removal performance, respectively.