The Effect Of Rare Earth Ions Doping On Photo-catalytic Properties And Activity For Volatile Organic Compounds (Benzene, Toluene, Ethyl-benzene, And O-xylene) Photo-degradation

The La³⁺, Nd³⁺, Ce³⁺ (Ln³⁺) doped TiO₂ photo-catalysts were prepared by means of sol-gel process. The photo-catalytic properties including crystal pattern, lattice parameter, the specific surface area, chemical state of Ti 2p, O Is on the surface of photo-catalysts, the electronic structure and valence band structure were tested by means of XRD, BET and XPS. The absorption capacity, the first-order reaction kinetic and the removal efficiency of VOCs including benzene, toluene, ethyl-benzene and O-xylene (BTEX) were tested in a 200L-static reactor and 57L continuous reactor respectively. The photo-catalytic degradation process and intermediates were tested by means of GC-MS and FTIR and then the degradation pathways were proposed. The relationship between the structure and properties of photo-catalysts and photo-catalytic activity for BTEX degradation was discussed and investigated in order to disclose the mechanisms of the enhancement effect of rare earth ions.The crystal transformation and crystallization process were hindered and the thermal stability was enhanced owing to Ln³⁺ doping. And the crystal size of Ln³⁺ doped TiO₂ decreased while their specific surface area increased with the increase of dosage. The lattice parameter "a" and "c" of Nd³⁺-TiO₂ hardly influenced and the value of "c" of La³⁺-TiO₂ and Ce³⁺-TiO₂ decreased slightly. The results imply that Nd³⁺ should not enter into the lattice of TiO₂ and Ce³⁺ and La³⁺ should enter into the lattice of TiO₂.The content of Ti³⁺ on the surface increased greatly with the increase of Ln³⁺ dosage. The reason of Ti³⁺ increase includes (1) The formation of solid solution matrix La_xTi_1-xO₂, Nd_xTi_1-xO₂ and Ce_xTi_1-xO₂ promote Ti⁴⁺ to be reduce to Ti³⁺;(2) Ce³⁺ and La³⁺ enter the lattice of TiO₂, and the formation of Ti³⁺ make the charge be balance;(3) The redox reaction occurred between Ce³⁺ and Ti⁴⁺ , then Ti⁴⁺ was reduced to Ti³⁺ during heating treatment.The ratio of O/Ti increased with the increase of Nd³⁺, La³⁺ dosage respectively while that decreased with the increase of Ce³⁺ dosage. Nd₂O₃ andNd(OH)3 present on the surface of Nd3+-TiO2, La2O3 on the surface of La3+-TiO2, Ce2O3 and CeO2 present on the surface of Ce3+-Ti02.The absorption percentage of BTEX increased significantly owing to rare earth ion doping. A higher dosage leads to a higher absorption capacity. The enhancement of absorption capacity was attributed to (1) a higher specific surface area of Ln3+ doped TiO2, (2) the formation of a composite between rare earth ions and BTEX molecular.The first-order reaction of BTEX photo-degradation was promoted and the first-order kinetic constants increased owing Ln3+ doping. The first-order kinetic constants increased when Ln3+ dosage was less than 1.2% while that decreased when Ln3+ dosage was more than 1.2%. That imply the optimal dosage of Ln3+ was 1.2%. The first-order kinetic constant of Ln3+-Ti02 was 4-6 fold of that of pure TiO2. The order of first-order kinetic constant for BTEX from large to small listed as O-xylene, ethyl-benzene, toluene and benzene.In order to investigate in detail the effect of Ln3+ doping on the photo-catalytic activity of BTEX photo-degradation and to test the probability of photo-catalytic process applied for indoor air pollution control, a series of tests were carried out in a 57L continuous photo-reactor under 75 seconds of residence time and 1.5% of relative humidity and with the initial concentration of BTEX of 22-24 ppb(v). As a result, BTEX could be removed efficiently and the concentration of BTEX decreased rapidly in the first hour and the removal percentage increased rapidly simultaneously. And the removal percentage of BTEX trend to keep stable after 2 hours. The same as the first-order reaction, the removal percentage increased when Ln3+ dosage was less than 1.2% while that decreased when Ln3+ dosage was more than 1.2%. That imply the optimal dosage of Ln3+ was 1.2%. The removal percentage for BTEX from large to small listed as O-xylene, ethyl-benzene, toluene and benzene.The photo-catalytic activity for BTEX enhanced by Ln3+ doping might be attributed to reasons as follows: (1) the absorption capacity increased with the increase of Ln3+ dosage;(2) the concentration of Ti3+ on the surface of photo-catalysts increased with the increase of Ln3+ dosage. And a suitable Lndosage leads to a suitable concentration of Ti3+, which act as the hole-trap and accelerates the electron to be transferred to oxygen and promotes the separation of electron-hole pairs. However, an excessive Ln3+ dosage leads to an excessive concentration of Ti3+, which act as the recombination center and then leads to the decrease of photo-catalytic activity and removal percentage of BTEX.The intermediates of BTEX photo-degradation were measured by means of GC-MS and FTIR. And the probable pathways of BTEX photo-degradation were proposed.The key origins of this investigation include:(l)Photo-catalysts chacterization and photo-catalytic reaction were carried out to discuss the relationship between the structure, properties and photo-catalytic activity and to disclose the mechanisms of the effect of Ln3+ doping on the enhancement of photo-catalytic activity.(2)The photo-degradation of BTEX with the initial concentration at indoor environment carried out by using Ln3+-Ti02 with a series of dosage could be supplied some beneficial experimental data to promote the application of photo-catalytic process for indoor air pollution control.(3)The presence of Ti3+ and defect level proved by means of XPS might be the key factor to promote the separation of electron-hole pairs and to enhance the photo-catalytic activity.(4)The mechanisms of the formation of Ti3+ were proposed and compared on the surface of La3+, Nd3+, Ce3+ doped TiO2 photo-catalysts. For Ce3+-Ti02, the formation of Ti3+ was promoted owing to the formation solid solution matrix CexTii-xO2, Ce3+ entering the lattice of TiO2, and the redox reaction occurring between Ce3+ and Ti4+. And then Ti4+ was reduced to Ti3+ during heating treatment.