Preparation Of Alkali Titanate Supported Pd-based Catalysts For Room Temperature Formaldehyde Oxidation

Formaldehyde(HCHO)as a primary indoor air pollutant with high-risk of threatening people's health,its removal is necessary for reducing the risk of formaldehyde poisoning and public health.Thus,it is of great significance to develop a new catalytic material,for the complete oxidation of formaldehyde with efficient,safe and economical at room temperature.Supported noble metal catalysts are the most potential catalysts for realizing the removal of HCHO by room temperature oxidation,and the catalytic performances are often related to the supports because of the strong metal-support interaction.In recent years,several studies found the promoting effect of alkali metals on catalysis.For supported noble metal catalysts,the smaller size of Pd clusters/nanoparticles would give more active sites,hence promoting the catalytic performance.In most cases,it is postulated that the presence of alkali metal usually made metal species more dispersed,thus reducing the size of the loaded metal particles.At the present time,most of studies have focused on the addition of alkali metals from the outside,and few reports involve in directly using layered alkali metal titanate as support in noble metal catalysts.Consider that alkali metal titanates,such as sodium titanate and potassium titanate,have intrinsic interlayer Na+ and K+ ions between TiO6 octahedron planes in their crystals,which are expected to be ideal supports for active noble metal catalysts.In this investigation,the supported palladium clusters catalysts were prepared by four supports,including,titan acid nanobelts(HTO-NB),sodium titanate nanobelts(NTO-NB),hybrid nanostructures obtained though partially replacing Na+ in NTO-NB by K+(KNTO-NB),potassium titanate nanowires(KTO-NW).Then,they were tested for room temperature HCHO oxidation.The main research contents are as follows:(1)Preparation and catalytic performances of sodium titanate and potassium titanate nanobelts supported Pd-based catalysts for room temperature formaldehyde oxidation.Three supports were synthesized by a typical hydrothermal process,including titan acid nanobelts(HTO-NB),sodium titanate nanobelts(NTO-NB),and hybrid nanostructures obtained though partially replacing Na+in NTO-NB by K+(KNTO-NB).Pd clusters were deposited on the surface of the supports by deposition-precipitation method,after H2 reduction treatment at 400?,Pd/S(KNTO-NB,NTO-NB,TiO2-NB)catalyst were obtained.It can be found from TEM images,a large amount of tiny Pd clusters are uniformly distributed on supports.Compared to TiO2-NB supported palladium catalyst,the size of Pd clusters on NTO-NB and KNTO-NB are smaller.It indicates that the existence of interlayer Na+ and K+ ions in NTO-NB and KNTO-NB can increase the dispersion of Pd species.From XPS analysis,in Pd/S(KNTO-NB,NTO-NB,Ti02-NB)catalyst,Pd 3d orbits can be deconvoluted into two parts of metallic Pd and PdO.In comparison with Pdi/TiO2-NB sample,the Pd0 binding energies in Pdi/NTO-NB and Pdi/KNTO-NB shifted to lower values,indicating that the existence of K+ions and Na+ions can increase the electron density of Pd clusters.(2)The catalytic activity of Pd/S(KNTO-NB,NTO-NB,TiO2-NB)catalyst were studied with the oxidation of formaldehyde at room temperature.The results showed that Pd/NTO-NB and Pd/KNTO-NB catalysts are much more efficient for HCHO oxidation at room temperature compared to TiO2 nanobelts(TiO2-NB)supported palladium catalyst(Pd/TiO2-NB),and Pd/KNTO-NB catalyst gave the best catalytic performance.The conversion rate of 140 ppm formaldehyde is as high as 97%at 25?.O2-pretreated and all the freshly deposited samples without hydrogen reduction treatment samples gave poor HCHO conversion below-35%.The above results indicate that Pd0 species should be the active catalyst in Pd/S(KNTO-NB,NTO-NB,TiO2-NB)nanostructures.(3)Preparation and catalytic performances of potassium titanate nano wires supported Pd-based catalysts.Potassium titanate nanowires(KTO-NW)supports were synthesized by a typical hydrothermal process.And then Pd clusters were deposited on the surface of KTO-NW by deposition-precipitation method.After H2 reduction treatment at 400?,Pd/KTO-NW nanostructures were obtained.Pdx/KTO-NW(x=0.1,0.5,1 wt%)catalysts with different Pd loading can be prepared by adjusting the amount of PdC12 precursor.HAADF-STEM characterization show that tiny Pd clusters(<1nm)and single Pd atoms dispersed on the surface of KTO-NW,indicating that the existence of K+ ions in KTO-NB can increase the dispersion of Pd species.Pd-loading is lower,the density of single Pd atoms is higher.The results showed that Pd/KTO-NW are high-efficiency for HCHO oxidation at room temperature.Pdi/KTO-NW can catalyze almost complete oxidation of formaldehyde conversion to H2O and CO2 at room temperature,and the conversion rate remains unchanged with 30 hours.With the decrease of Pd-loading,the TOF value of Pd/KTO-NW for HCHO oxidation significantly increased,indicating that single Pd atoms is beneficial to the oxidation of formaldehyde.It is also shown that Pd0 species should be the active catalyst in Pd/KTO-NW nanostructures.