Photocatalytic Oxidation Of Formaldehyde Over Plasmonic Au/TiO₂ Under LED Visible Light

Formaldehyde is a typical volatile organic compound?s??VOCs?of indoor air pollutants.Surface plasmonic resonance?SPR?photocatalysts can absorb visible light,which allows solar light driven photocatalytic removal of formaldehyde from air to occur at room temperature and ambient pressure.However,very few studies on Au/TiO₂ photocatalyst of SPR for removal of formaldehyde from air and its reaction mechanism are reported,of which the apparent kinetics and mechanism study is of significance for the design of SPR catalysts and the applications to removal of VOCs pollutants.This paper studied the kinetics and reaction mechanism of photocatalytic oxidation of formaldehyde over plasmonic Au/TiO₂ under LED visible light.The main results are below:Au/TiO₂ catalyst features about 4.5 nm Au nanoparticles in an average diameter.A strong absorption peak centered at 563 nm of wavelength due to SPR effect of Au nanoparticles was observed.It suggests that,under conditions of 13% relative humidity and 38.5 mW/cm2 of green light intensity,the formaldehyde conversion is 74.3%,almost 5 times that under dark.Effects of visible light sources with various wavelengths,light intensity and relative humidity of reaction gas on kinetics of visible-light photocatalytic oxidation of formaldehyde over Au/TiO₂ are investigated,and the apparent kinetics constant is achieved.?1?Under LED irradiations of red,green,blue and white light,the conversions of formaldehyde increases rapidly,slowly,and keeps steady with light intensity.Under the same conditons of light intensity?less than 42 m W/cm2?and relative humidity,the conversions of formaldehyde under LED red,green and blue light,are approximate,and the conversion under white light is relatively lower than that of the rest three.?2?At dry condition,the converion of formaldehyde is almost 0 %.However,at wet condition,the conversions under light or under dark exist?above zero?,and that under light is higher than under dark.The conversion as a function of relative humidity behaves similarily under various light sources.The conversion reaches the maximum at 21.9 % of relative humidity,followed almost being steady at 80%.Apparent kinetic parameters k?I?_H?k?H?_I and k?I,H?and the kinetic equations by fitting above data related to red,green,blue and white light are achieved and established.k?I,H?^R,G,B=?9.4-5.8×0.9^I?×(1-1.3×(5.79×10^-7)^H)k?I,H?^W=?10.2-6.6×0.96^I?×(1-1.2×(2.86×10^-6)^H)From the study of reaction mechanism of photocatalytic oxidation of formaldehyde over Au/TiO₂ under visible light,the results are summarized below: Only DOM can be formed on TiO₂ at dry or wet condition.For Au/TiO₂,DOM,formate and carbonate intermediates are obserbved at the dry condition under dark or light,however,there is no CO₂ in the gaseous products.The DOM disappears at wet condition,and formate and carbonate intermediates are also observed with their peak intensities approaching steady more quickly,and there is CO₂ evolution.Acceleration of rate determining steps such as formate oxidation to carbonate and carbonate decompositon are observed under visible light.The enhancement of visible light on decomposition of carbonate formed from adsorbed CO₂.In-situ DRIFT spectra under dark or under light are basically identical in terms of peak positions,indicating that the reaction features the same surface intermediates and proceeds via the same pathways under dark and under light.The moisture is indispensable to carbonate decomposition and also accelerates DOM oxidation to formate.