Study on Preparation, Characteristics of MWNTs and TiO2 Composite Catalyst with Its Photocatalytic Redox Properties
|Course||Physical and chemical|
|Keywords||Composite catalyst Carbon nanotubes TiO2 The light catalytic restore hydrogen production Photocatalytic oxidation of methyl orange|
The photocatalytic disciplines multidisciplinary fusion of chemistry, physics, materials and environment to form a new interdisciplinary. From the development status, photocatalysis can be broadly divided into energy photocatalytic (light water splitting and photocatalytic renewable resource hydrogen production) and environmental photocatalysis (advanced oxidation technology for environmental pollution control) two main research areas. The two study areas are closely related to the sustainable social and economic development, thus has broad application prospects and enormous social and economic benefits. Various modified methods to improve the activity of the catalyst to increase the catalyst sunlight utilization is an important aspect of this technology to practical application. In recent years, a number of reports in the literature using the sol - gel method, carbon nanotubes (MWNTs) introduced into the titanium dioxide photocatalyst found this composite photocatalyst has a good photocatalytic oxidation of organics activity. Detailed studies have shown that the synergies between carbon nanotubes and titanium dioxide prompted titanium dioxide photo-generated electron transfer to the carbon nanotubes, to extend the life of the photo-generated electron - hole pairs, also visible light absorption capacity of the composite catalyst . This article envisioned a similar interaction between the carbon nanotubes and titanium dioxide are equally effective for the photolysis of water into hydrogen system. In order to confirm the assumptions, we first investigated using the the usual sol - gel method to prepare carbon nanotubes and titanium dioxide composite catalyst, and then the impregnation the metal Ni, measuring its activity of photocatalytic reduction of water to produce hydrogen. It was found that carbon nanotubes doped will block of TiO 2 is phase change and the particle diameter grew; increase in the doping amount of the carbon nanotubes, the catalyst has a specific surface area increases; When the carbon nanotube doping amount is 5% and the load of 1% of Ni composite catalyst UV catalytic reduction of water into hydrogen activity maximum, reached 122μmol / h than the undoped nearly doubled. Doped carbon nanotubes TiO 2 photocatalytic activity generally increased, and indeed there is some kind of process interactions directly affect the photocatalytic reaction between the carbon nanotubes and titanium dioxide. Because this interaction is to occur in the tight junctions between the two, so we speculated try to increase the connection between the two points should be conducive to the improvement of photocatalytic activity. Research direction, this paper further study to design polymeric network legitimate - chemical vapor deposition method to prepare composite catalyst. The reason for this design is the clever use of Ni as either a carbon nanotube growing point, but also as a co-catalyst to produce hydrogen. The selected polymerization complexation able to Ni is uniformly dispersed in TiO 2 matrix, through the high-temperature calcination generates NiTiO 3 , and then hydrogen from the body phase restore them surface into a small metal Ni as carbon nanotubes growing point. Since the conventional methane cracking chemical vapor deposition method is mainly used to the preparation of carbon nanotubes, so step growth method hope that the yield of carbon nanotubes can be as high as possible. However, in accordance with the previous design, can know the carbon nanotubes does not require long is too long, but the carbon nanotubes and the titanium dioxide contact points should be more, to increase the synergistic effect of the two, to improve the photocatalytic activity. In order to achieve this object, this study Ni surface of the amorphous carbon is removed using hydrogen methanation situ in , of TiO 2 surface repeatedly growing carbon nanotubes possibility. SEM, Raman characterization results demonstrate improved chemical vapor deposition method can in , of TiO 2 surface growth of a layer of graphite structure complete, with an average diameter of about 28 nm, multi-wall carbon nanotubes. Secondary growth of carbon nanotubes, this composite catalyst of UV and visible light catalytic reduction of water into hydrogen activity to achieve the best, up to 2076μmol / h, and 38.1μmol / h respectively. Stability Test after repeated 5 times, a total of 20 h, the activity loss is less than 5% of the composite catalyst, no significant deactivation phenomenon. Combination of XPS, UV-Vis DRS, PL a variety of different means of characterization, from semiconductor theoretical analysis of the composite catalyst activity, and that the growth of carbon nanotubes can be effectively transferred TiO 2 conduction band the photo-generated electrons, but also makes the composite catalyst of ultraviolet and visible light absorption capacity greatly increased, and at the same time generate hydrogen reduction step in the preparation of the Ti 3 sup> is also conducive to the improvement of the photocatalytic activity. Also attempted in this paper on the basis of the above study, from the molecular level to explain the Ni complex catalysts photocatalytic hydrogen production reaction activation energy changes its surface hydrogen micro reaction process. Calculated by varying the reaction temperature, the secondary growth of carbon nanotubes under UV excitation the composite catalysts apparent activation energy was 18.7 KJ / mol, apparent quantum yield up to 4.8%. Kinetic experiments show that, with methanol as the sacrificial reagent Ni Department the composite catalytic dehydrogenation activity is much higher than other alcohols as sacrificial reagent, the order of methanol> ethanol> isopropanol = n-propanol> n-butyl alcohol. Methanol dissociative adsorption on the catalyst surface is the first step reaction occurs be photogenerated holes oxidation with subsequent removal of H step together as the overall reaction rate controlling step. Meanwhile, it is considered that the growth of carbon nanotubes from the kinetically accelerated after the adsorption of the methanol dissociation steps, such that the activity of the composite catalyst to greatly improve the removal of H. According to the characteristics of the semiconductor photocatalyst, photocatalytic reduction and oxidation reactions are to reflect the performance of the catalyst from one side, so the necessary composite catalyst photocatalytic reduction and oxidation of the performance of the two in order to more fully understand the performance of the photocatalyst, research. Therefore, the second half of this dissertation is to proceed from the performance of the composite catalyst oxidation to examine the photocatalytic oxidation of methyl orange activity. Composite catalyst metal Ni Ni system before the experiment must first go with pickling metal Ni with methyl orange snatch photogenerated oxidative free radicals and thus greatly reduce the photocatalytic oxidation activity. Next, the further activity of the test results show that the same secondary growth of carbon nanotubes composite catalyst methyl orange decolorization rate is the highest, 1.5 hours of UV Decolorization rate of 100%, the kinetic constant k a 0.066 min -1 sup>; 99% of visible light decolorization rate of 3.5 hours, the reactions kinetic constants k a 0.017 min -1 sup>. In order to understand the detailed mechanism of the photocatalytic oxidation process, that investigated whether · O 2 - sup> or · OH initiation reaction paper added to the reaction system O 2 CCl 4 t-BuOH and other additives, compared to their activity. The experimental results of composite catalyst under visible light irradiation by carbon nanotubes as a photosensitizer, stimulated transfer of photogenerated electron TiO 2 conduction band, and then further generate · O 2 < sup> - sup> led visible light photocatalytic oxidation of methyl orange reaction; composite catalyst under UV irradiation by the TiO 2 stimulated to generate price band holes and conduction band electrons, valence band holes further generate · OH, the conduction band electrons further generate · O2 - sup>, the two co-dominant UV catalytic oxidation of methyl orange reaction.