Catalytic Degradation Properties Of Formaldehyde On Transition Metal Oxides Catalysts

1.Preparation and characterization of catalystsA series of different TiO₂ content semiconductor photocatalysts TiO₂/Al₂O₃ were prepared by the impregnation method, using tetrabutyl titanate as Ti source and Al₂O₃ as the support. They were prepared at following steps. Add a certain amount of tetrabutyl titanate to a certain volume of anhydrous ether and then the ether solution including tetrabutyl titanate was formed. Then the supports Al₂O₃ which were calcined at 400℃for 4 hours in advance were added to the ether solution. TiO₂/Al₂O₃ photocatalysts with the range of TiO₂ loading from 1.7% to 8.4% were obtained after calcining them for 4h at 400℃. The physical properties of the TiO₂/Al₂O₃ catalysts were characterized using XRD,SEM and TEM methods. The XRD results showed that the anatase phase of TiO₂ appeared at 25.16°on the diffraction patterns. And the anatase phase of TiO₂ was obvious and their crystal form was perfict when TiO₂ content was 5.0%. The SEM results showed that TiO₂ were homodisperse on the surface of Al₂O₃ and Al₂O₃ kept good hexagonal crystal form when TiO₂ content was 5.0%. The TEM results also showed that TiO₂ were homodisperse on the surface of Al₂O₃ and the dispersion of catalysts was nice when TiO₂ content was 5.0%.The nanometer Ni(OH)2 were prepared by solid state reaction with NiSO4·6H2O and NaOH at ambient temperature. Then the black powders NiO were received by calcining Ni(OH)2 at different temperature for 4h. The MnO₂/NiO catalysts were synthesized by dipping Ni(OH)2 carriers with Mn(NO3)2 solution. The samples were characterized using XRD,TEM,SEM,DTA and TG- DSC methods. The XRD,TEM,SEM results showed that the nanometer NiO powders were nanometer and uniform distribution of grainsize with the average size of 9nm. There was a endothermic peak at 287℃on DTA curve of Ni(OH)2 and it was ascripted to Ni(OH)2 breaking down to NiO. The diffraction peak appeared at 19o,54o on XRD diffraction pattern of MnO₂/NiO and that was the characteristic peak ofγ-MnO₂. The intensity of diffraction patterns was the biggest when MnO₂ content was 6.0%, which indicated that the crystal form of MnO₂ was perfict at this time. There was a endothermic peak at 93.1℃on DSC curve and 5.08% weight loss corresponding on TG curve of Mn(NO3)2(6.0%MnO₂)/Ni(OH)2, which meant the desorption of water on the surface. There was a another endothermic peak at 291.0℃on DSC curve which indicated that the decomposition temperature of Ni(OH)2 rised because of the interposition of Mn and 18.10% weight loss corresponding on TG curve which was appreciably under 19.41% of theory amout of the weight loss of Ni(OH)2 because of the impregnation of Mn(NO3)2. There was a third endothermic peak at 384.3℃and 3.49% weight loss corresponding which was ascripted to Mn(NO3)2 breaking down to MnO₂. So it indicated that the product of Mn(NO3)2/Ni(OH)2 was MnO₂/NiO after calcined at 400℃which accorded with the XRD results.2. Potocatalytic degradation properties of formaldehyde on TiO₂/Al₂O₃ catalystsThe photocatalytic degradation of formaldehyde was taken as the probe reaction to evaluate catalytic activity of TiO₂/Al₂O₃ catalysts. A series of factors were studied in the experiment to test the impact of TiO₂/Al₂O₃ catalysts on the degradation of formaldehyde: the calcination temperature of the photocatalysts, the amount of photocatalysts, the loading of TiO₂, the reaction temperature, reaction time and the stablility of catalysts. It was shown that the best calcination temperatue of photocatalyst was 400℃during the range of 300℃～600℃. The degradation rate of formaldehyde rised firsrly and dropped later with the TiO₂ loading from 1.7% to 8.4%. The result showed that 5.0% was the best loading and the degradation rate of formaldehyde reached 58.4%. The catalytic activity of photocatalysts lowered with the reaction temperature rised, from 58.4% at 25℃to 4.8% at 50℃. The illumination quickened the degradation rate of formaldehyde when catalysts existed and the degradation rate rised as the illumination time increased. The degradation rate of formaldehyde was 58.4% after reaction for 4h and 39.1% for 1h.We also studied the stablility of catalysts. The result of reusing of catalysts for 6 times indicated that the degradation rate lowered as the reaction times rised, but it still kept 50% for the first three times.3. The catalytic oxidation properties of formaldehyde on NiO catalystsThe catalytic oxidation of formaldehyde was taken as the probe reaction to evaluate the catalytic activity of NiO and Mn-doped catalysts. A series of factors were studied in the experiment to test the impact of NiO catalysts on the degradation of formaldehyde: the calcination temperature of catalysts, the amount of catalysts, the reaction temperature and the reservation time of catalysts. It was shown that the best calcination temperatue was 400℃during the range of 350℃～550℃. The degradation rate of formaldehyde rised as the catalyst mount increased from 0.5 to 5.0g/L. The catalytic activity of catalysts increased with the reaction temperature rised, from about zero at normal temperature to 49.6% at 80℃. The catalytic activity of catalysts fell when they were reserved for long time because of the adsorption of impurity on catalysts. On the optimum conditions different MnO₂-doped NiO catalysts all had higher formaldehyde degradation rate than singe NiO catalysts. We also found 6.0% loading catalysts had best catalytic activity with the loading amount from 2.0% to 8.0% and the degradation rate of formaldehyde could reach 66.0% after reaction for12h at 80℃. It was also favorable of MnO₂-doped NiO catalysts for formaldehyde degradation when reaction temperature rised. It optimized reaction condition when MnO₂ was added and the degradation rate of formaldehyde could reach 52.9% after reaction for12h at 60℃which made the reaction condition gentle, so the catalytic oxidation properties of catalysts improved obviously.