| Formaldehyde(HCHO) is one of the major indoor air pollutants, which emitted from various building materials and many household products. Long-term exposure to HCHO with concentrations exceeding safety limitations may cause serious health problems. Therefore, research and development air purification technique on the effective removal of HCHO has become an important subject. Among various kinds of air purification technologies, catalytic oxidation of HCHO has been considered as one of the most effective approaches, which only yields harmless products of CO2 and H2 O at ambient temperature. It has been thought as one of the most promising approaches because this process is environmentally friendly and energy-saving. Pt-based catalysts have been used for catalytic oxidation removal of volatile organic compounds(VOCs) because of their outstanding ability for activation of oxygen molecules, which exhibited excellent catalytic performance for catalytic oxidation of HCHO at room temperature. At present, how to combine the carrier selection and preparation condition optimization, to design more efficient supported Pt catalysts, for low-temperature catalytic oxidation formaldehyde has become a subject widely concerned by researchers. In addition, intensive study and exploration on the nature of the activated center of the catalysts and catalytic mechanism are still the attractive issues in the field of catalysis.Based on the above situation, various preparation strategies were employed to developing highly active Pt-based catalysts for HCHO catalytic oxidation at ambient temperature in this thesis. The versatile supports such as Al2O3, Fe3O4 and Ti O2 were employed as supports, and a series of supported Pt-based catalysts were obtained by controlling the dispersion of Pt nanoparticles(NPs) and the metal-support interaction. These Pt-based catalysts exhibit relative high redox activity for molecular oxygen activation and high catalytic activity for catalytic oxidation of HCHO. The physicochemical properties of the obtained Pt-based catalysts were deeply studied by a series of characterization means. In addition, the correlation between the catalytic activity and physico-chemical properties of the catalysts as well as the catalytic mechanism of the reaction were also discussed. The main contents and results are as follows: 1. Catalytic performance of HCHO oxidation over Pt-Fe Ox/Al2O3 catalystsA series of Pt-Fe Ox/Al2O3 catalysts were prepared through a colloid deposition route using the versatile supports Al2O3 as support. The concrete role of the different Fe/Pt atom ratios and the addition of water vapor in the catalytic performance of Pt-Fe Ox/Al2O3 catalysts was investigated in detail. Among them, the sample with a Fe/Pt ratio of 1.0 exhibits the highest activity, which can efficiently convert formaldehyde to CO2 at ambient temperature. A variety of characterization results showed that both Pt NPs and Fe Ox species are highly dispersed on the surface of the Al2O3, and there are relatively strong interactions between Fe Ox and Pt NPs, the Pt-1.0Fe Ox/Al2O3 catalyst have more accessible active sites, thus showing high activity for the oxidation of formaldehyde under ambient conditions. The activity for the oxidation of formaldehyde decreases gradually with the increase of Fe/Pt atom ratios, suggesting that a certain amount of Pt NPs have been covered by the introduced Fe Ox species and resulting the accessible active sites decrease. In addition, The catalytic activity of the Pt-Fe Ox/Al2O3 catalyst can be further improved by the addition of water vapor into the feed stream. The introduced H2 O molecules are easily transformed to –OH groups by interacting with the Fe Ox species located at the surface of Pt-Fe Ox/Al2O3 catalysts. And the resultant –OH groups located at the interface of Pt NPs and Fe Ox species(like Fe3+ –OH–Pt) may act as additional active sites for the oxidation of HCHO or the intermediates(like formates) to produce CO2 and H2 O, thus improving the catalytic activity of the Pt-Fe Ox/Al2O3 catalyst for low-temperature oxidation of HCHO. When the relative humidity is beyond some critical concentration, water vapor will lead to severe catalytic deactivation due to its strong adsorption on the active sites, hindering O2 and HCHO adsorption. 2. Catalytic performance of HCHO oxidation over Pt/Fe3O4 catalystsThe catalytic properties of Ferroferric oxide supported platinum catalysts(Pt/Fe3O4), prepared by the impregnation method or co-precipitation method were investigated for the complete oxidation of formaldehyde. The effects of calcination temperature and the reaction atmosphere on the catalytic performance of Pt/Fe3O4 catalysts were studied. Among them, the Pt/Fe3O4-CD catalyst prepared by co-precipitation method exhibits the highest activity, which yield a HCHO conversion efficiency of 94.5% to CO2 and H2 O at ambient temperature. Characterization result shows that small Pt particles distributed evenly on the smooth surface of octahedral Fe3O4, and there are relatively strong interactions between Fe3O4 and Pt NPs. The Pt/Fe3O4-CD catalyst with abundant surface hydroxyl groups has more accessible active sites, thus showing high activity for the oxidation of formaldehyde under ambient conditions. Besides, there is a close relationship between catalytic activities and calcination temperature of catalysts. The valence states of Pt and Fe species, the amount of surface hydroxyl groups change somewhat with increasing the calcination temperatures. The HCHO conversion decreased gradually as the calcination temperature rising. In addition, the hydroxyl groups and water molecules could be more easily adsorbed on the relatively stable(111) crystal plane of Fe3O4 octahedral, where the water molecules easily occur dissociative adsorption, thus forming the metal–oxygen or metal–hydroxyl species. The catalytic activity of the Pt/Fe3O4-CD catalyst can be further improved by the addition of water vapor into the feed stream. In the presence of water, some new active sites like(Pt)-O-Fe-OH may be easily formed, which can more efficiently activate HCHO molecules, thus considerably improving the catalytic activity of Pt/Fe3O4-CD catalyst. 3. Ti O2 nanobelts with Pt decoration for HCHO oxidationPt/Ti NBs and Pt/Ti NBs-ac catalysts were prepared by a traditional impregnation method using Ti O2 nanobelts(Ti NBs) and H2SO4-treated Ti NBs(Ti NBs-ac) as supports, which were prepared through hydrothermal treatment of commercial P25 samples. A variety of characterization results show that the Ti NBs-ac has the characteristics of mesoporous structure and rough surface after treating by H2SO4. The mesoporous support with a high-surface-area not only facilitates the fast diffusion and transport of reactants and products, but also provides more active sites for HCHO adsorption. The evaluation of catalyst results show that Pt/Ti NBs-ac exhibits relative high redox activity for molecular oxygen activation under ambient conditions. It could work as an efficient catalyst for HCHO oxidation with O2. A 91.6 % conversion of HCHO was obtained, which is much higher than that of Pt/Ti NBs catalyst without H2SO4 treatment as well as Pt/P25 catalyst(HCHO is 42.3% and 39.1%, respectively). The characterization results suggest that Pt/Ti NBs-ac possess abundant oxygen vacancies which are generated at the metal-support interface of Pt/Ti NBs-ac, which can easily adsorb and activate molecular oxygen, thus significantly improving the catalytic activity of the Pt/Ti NBs-ac catalyst for oxidation of HCHO. In addition, a large number of surface hydroxyl groups existed on defective sites of Ti NBs-ac(110)surfaces can facilitate the adsorption of HCHO, thus being favorable for the enhancement of HCHO oxidation. The addition of water vapor can promote the formation of surface OH group on oxygen vacancies. The resultant-OH groups located at the interface of Pt NPs and Ti O2 species(like Ti4+ –OH–Pt) may act as the main active sites for directly participating in the activation of HCHO. |
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