Construction Of Pd/MTiO_x Catalysts And Its Catalytic Performances For Formaldehyde At Room Temperature

Volatile organic compounds,such as formaldehyde,benzene,methylbenzene,dimethylbenzene,are one of the major pollutants that may cause human health and environment problems.By so far,the catalytic oxidation of VOCs has been considered as the most promising method for the chemical transformation of indoor HCHO.This way can completely transform VOCs into harmless CO2 and H2O in the presence of catalysts at low temperature.The key point of catalyzing VOCs lies in the study of catalysts with high activity and stability.Currently,the catalysts of VOCs elimination can be classified into two main types:supported noble metals(such as Pt,Pd,Rh,Ir,Ru,Au)and non-noble metal oxides(such as oxides of MnO2,TiO2,CuO and so on).In the past decades,supported noble metal catalysts have received much attention due to its high activity at low temperature.Furthermore,a recent study reported that alkali ions can efficient stabilize noble metal nanoparticles and enhance its disperation.Based on the above,we synthesized a titanate MTiOx(M=K,Na)via hydro-thermal method and subsequent ion exchange.Based on this support,Pd nanoparticles with uniform small size and high dispersion were successfully loaded.We applied this one-dimension structure catalysts to the room-temperature oxidation of formaldehyde.The main contents are summarized as follows:1.The preparation of Pd/Na2Ti3O7 catalysts and its performance in room-tempe-rature oxidation of formaldehyde.The purchased P25 was synthesized into one-dimention structured Na2Ti3O7 nanobelts via hydro-thermal method.Noble metals with small particle size(?2?m)and high dispersion were subsequently loaded on the as-synthesized nanobelts and characterized by SEM and TEM.The SEM images shows that the prepared Na2Ti307 catalysts have smooth and neat nanobelts which crossed with each other,shaping a three-dimention hierarchically porous structure.From HR-TEM images,it can be seen that noble particles have strong metal-support interation(SMSI)with Na2Ti3O7 nanobelts.XPS reveals that the active component of as-prepared catalysts is Pd instead of PdO.Only a small part of metal Pd was oxidized into Pd2+ during the process of formaldehyde oxidation,indicating the good stability of as prepared Pd/Na2Ti3O7 catalysts.Besides,we have explored many other factors influencing the performance of catalysts in formaldehyde catalytic oxidation.We found that the catalytic activity at 20?varied in the following order for the supported noble metal catalysts:Pd?Pt>>Au?Na2Ti3O7.The conversion of formaldehyde is proportional to its initial concentration below some certain value but remain almost unchanged above this value.Moderate RH(relative humidity)is beneficial for the catalytic oxidation of formaldehyde due to the abundance of surface hydroxyl,which has changed the path of formaldehyde catalytic process and lowered the activation energy.But a RH beyond 75%is harmful for the catalytic process because of the blocking of active sites.The transformation efficiency of formaldehyde into CO2 and H2O decrease with the increase of gas hourly space velocity(GHSV).2.The preparation of Pd/MTiOx catalysts and its performance in room-tempera-ture oxidation of formaldehyde.On the basis of Na2Ti3O7 nanobelts,ion exchange between Na+ and K+ was used to generate partially nanowire included one-dimention titanate mixture.TPR and XPS results reveal that the active component of as synthesized catalysts are metallic Pd,only a small amount of PdO was discovered.And there is SMSI between metal Pd and Na2Ti3O7 support.SEM and XRD spectrogram prove that the as-synthesized K2Ti6O13 nanowires mixed with Na2Ti3O7 nanobelts have disordered arrays.The newly formed architecture is pretty favorable for the mass transport of both reactants and products owing to the three-dimention porous structure with nanowires and nanobelts penetrated with each other.The mixed architecture with high porosity was subsequently employed in the catalytic oxidation of formaldehyde.A high conversion of 90%was achieved at room temperature.And the mixed titanate structure has better stability compared with Na2Ti3O7 supported catalysts.