| Formaldehyde(HCHO)is a common and harmful indoor air pollutant,and its elimination is of great significance to protect public health.Catalytic oxidation has a wide application prospect due to its advantages of high removal efficiency,energy saving and mild reaction conditions.Nano Au and nano Ag catalysts have attracted much attention because of their unique catalytic properties.However,easy aggregation of their nanoparticles,poor dispersion,poor stability and high cost have limited the practical application of such catalysts.Based on the characters of mesoporous silica materials(such as high specific surface area,ordered pores and rich surface hydroxyl),organic functional groups modification,"H2-O2”two-step treatment and acid modification are taken for the preparation of highly dispersed Au,Ag and their nanoalloy metal catalysts in this article.The systematic research on the relationship between catalyst structure and performance,and "adsorption-activation-conversion" behaviors of HCHO over the catalyst are carried out.The specific research contents and results are displayed as follows:(1)Three sizes of AuAg alloy particles are synthesized on mesoporous silica SBA-15 by modification of support with different functionalized organic groups.With(CH3)3SiCl and P123 AuAg modified SBA-15,size of 3.1 nm can be obtained for the alloy particles,showing good oxygen activation ability,but reduction of number of surface hydroxyl lowers formaldehyde adsorption activation ability,completely oxidizing HCHO at 230℃.With blank SBA-15,size of 5.3 nm can be obtained for the alloy particles,showing good oxygen activation and formaldehyde adsorption activation behaviors,completely oxidizing HCHO at 130℃.The AuAg alloy particles with the size of 6.4 nm can obtained by P123 modified SBA-15,which shows a good formaldehyde adsorption and activation ability,however,the increase of particle size leads to the decrease of oxygen activation ability,completely oxidizing HCHO at 140℃.The synergistic effects of the alloy particle size and the amount of hydroxyl group on the surface of SBA-15 enhance the catalytic oxidation activity of formaldehyde.(2)The relationship between gold nanoparticle size and catalytic oxidation activity of formaldehyde is further studied by changing the preparation methods.Au/MCM-41 catalysts with three particle size distributions are prepared by post-grafting organic groups(PG),impregnation(IM)and liquid phase reduction(CR)methods.The average particle sizes of PG,IM and CR samples are 2.1 nm,2.5 nm and 4.1 nm,and the percentage of particles at 2-4 nm are 88%,75%and 34%,respectively.HCHO complete conversion temperatures are 55℃,90℃,and 200℃,respectively.The room-temperature conversions of HCHO are 10%,5%,and 3%,respectively.Small-sized Au nanoparticles show better HCHO oxidation performance.During HCHO adsorption,the number of hydroxyl consumptions on the catalyst are similar,while small-sized Au particles are beneficial for HCHO adsorption and activation and enhancing the intermediate formate formation.Therefore,highly dispersed and relatively homogeneous gold particles are important to accelerate the "adsorption-activation-conversion" of HCHO.(3)Aminopropyl groups are grafted onto the pore wall of the mesoporous silica molecular sieve to support Au.Efficient Au catalyst is activated by "H2-O2" two-step treatment to build highly dispersed Au nanoparticles and achieve the complete oxidation of HCHO at room temperature.The results show that Au-carbon nanostructure is formed from the precursor catalyst after hydrogen pretreatment at 500℃.Subsequent oxygen treatment(600℃)obtains highly dispersed Au particles(average size of 2.5 nm),more importantly,more Aunδ+ species are formed.The average particle size of the one-step oxidized sample(06)is 4.6 nm,only 60%conversion of formaldehyde at room temperature is shown.Highly dispersed Au particles and more Au.5+promote HCHO activation to form intermediate(formate,etc.)and oxygen activation.The two-step treatment of "H2-O2" provides a controllable method for preparing highly dispersed gold particles and active Aunapspecies.(4)Mesoporous silica MCM-41 is treated with dilute nitric acid solutions in different concentrations to regulate the surface hydroxyl structure of the molecular sieve and control the particle size of nano silver catalyst.It is found that the transformation of surface hydroxyl of MCM-41 does not change the valence state of silver particles,but plays an important role in the size of Ag particles and the formation of intermediate(formate).The surface of MCM-41 without acid treatment contains more geminal hydroxyl groups,which makes Ag particles to aggregate and form large particles with a size of 15.5 nm.At a low concentration(0.38 mol/L)of nitric acid,the isolated hydroxyl group on the surface of MCM-41 is increased,while the geminal hydroxyl group is decreased.At this time,the minimum size of Ag particles is 4.5 nm.The further increase of acid concentration(0.57 mol/L,0.76 mol/L)leads to the condensation of surface hydroxyl groups,resulting in the formation of more hydrogen-bonded hydroxyl groups on the surface of MCM-41,and the particle size respectively increases to 5.5 nm and 6.5 nm.In HCHO adsorption and activation,the consumption of hydroxyl on the blank MCM41 is similar.while the small-sized Ag particles promote the adsorption and activation of HCHO to form formate intermediates.The nano-silver catalyst with smaller particle size shows better catalytic oxidation performance of HCHO,achieving highly efficient oxidation of HCHO over nano Ag catalyst with low content(0.95 wt.%).The complete oxidizing temperature of HCHO is 75℃,decreasing by the temperature of 65℃ than that of the non-acid modified catalyst. |
