Preparation Of Carbon-supported ZnO Composite And Their Synergistic Mechanism For VOCs Degradation

The excessive release of volatile organic compounds（VOCs）seriously threatens the ecological environment and human health.VOCs emission is widespread in wood and its products,particularly in wood-based panels.China is the largest producer,consumer,and trader of wood-based panels over the world.So it is essential to upgrade the green manufacturing and application of wood-based panels.Photocatalytic technology based on traditional semiconductors has been widely studied for VOCs treatment due to its simple equipment and sustainability.However,traditional semiconductors usually have the instinct disadvantages,which limit their performance under many conditions:1)the high recombination rate of photogenerated carriers;2)narrow light response range;3)weak adsorption capacity for VOCs;and 4)accumulation of intermediate products on their surface.Therefore,this paper aims to degrade the main VOCs in the wood-based panel industry,based on the steps of VOCs degradation of "adsorption,mass transfer,photocatalytic reaction,desorption" .The wide-band gap semiconductor ZnO was used as the catalytic center,and the one-dimension nano-biochar（nBC）,two-dimension Ti₃C₂T_X and S-doped nanosheets（SCNs）were used as carriers to improve the activity of ZnO for achieving efficient photocatalytic removal of VOCs.The enhancement mechanisms of photocatalytic performance were study.In addition,the environmental factors of VOCs in the adsorption-photocatalytic degradation process were systematically studied.The main conclusions of this paper are as follows:（1）Cellulose nanocrystals（CNCs）with high crystallinity and high surface area were used as the synthesis template for synthesis of ZnO/CNCs composite.The nanoscale ZnO was uniformly deposited on the surface of CNCs in a rod-like arrangement,and its dispersion was significantly improved.Subsequently,ZnO/nBC was obtained after carbonizaation at 550℃.The skeleton structure of CNCs was preserved,and the nano ZnO was still uniformly loaded on the surface of nBC.ZnO/nBC composites showed significantly enhanced performance in the VOCs degradation process,especially the optimal nBC addition amounts of 1.0%and 2.0%（i.e.,ZnO/nBC1.0 and ZnO/nBC2.0）.The degradation rate of formaldehyde by ZnO/nBC composite reached 89%within60 min UV light irradiation,and the apparent rate constant(k_obs)of pseudo-first-order kinetics was2.67 times that of pure ZnO.In addition,ZnO/nBC1.0 composite exhibited high photocatalytic degradation efficiency for methanol and acetone,which were 77%and 71%,respectively,indicating the universality of ZnO/nBC composite for the VOCs of wood-based panel industry.After 5 cycles,the photocatalytic degradation rate of ZnO/nBC1.0 for formaldehyde only decreased by 6%.（2）A hierarchical structured Ag/ZnO/nBC photocatalyst was constructed by decorating the surface of ZnO/nBC with silver nanoparticles（Ag NPs）that possesses local surface plasmon resonance（LSPR）effect.The nBC can act as a carrier to provide more active sites for the growth and adhesion of ZnO and Ag NPs.Spherical ZnO with an average diameter of～460 nm is attached to nBC to form a tight bond,and is enveloped by Ag NPs with an average diameter of～10 nm.The specific surface area of Ag1.0/ZnO/nBC composite(32.43 m²·g^-1)is significantly higher than that of ZnO(14.47 m²·g^-1)and ZnO/nBC(15.76 m²·g^-1),which promotes the adsorption and mass transfer of VOCs on the catalyst surface.In addition,the plenty oxygen-containing functional groups（–OH,–C=O,–CO）on the surface of Ag/ZnO/nBC photocatalyst also promote the adsorption and diffusion of VOCs into the internal hierarchical channels under non-equilibrium conditions,achieving dynamic adsorption process.Subsequently,the LSPR effect induced by Ag NPs and the multi-pathway synergy derived from the hierarchical nanostructure realize visible light capture performance and high electron density,and further generating more active oxygen species（such as·OH and·O₂^-）.Correspondingly,the degradation efficiency of formaldehyde by Ag1.0/ZnO/nBC is significantly improved,with the k_obs of 0.0374 min^-1 for the simulated solar light degradation of formaldehyde,which is 7.8 times,4.6 times,and 5.6 times higher than those of ZnO,ZnO/nBC,and Ag/ZnO catalysts,respectively.Ag1.0/ZnO/nBC also has universality in the degradation of various VOCs,with degradation efficiencies of methanol and acetone reaching92%and 81%,respectively.The degradation efficiency of formaldehyde by Ag1.0/ZnO/nBC can still reach more than 80%after 5 cycles,indicating its stability and repeatability.（3）ZnO/Ti₃C₂T_X composite with Schottky junction was prepared using two-dimensional Ti₃C₂T_X as a carrier.The single-layer Ti₃C₂T_X nanosheets have a negatively charged surface and a cross-linked,wrinkled structure that is conducive to increasing surface active sites and providing conditions for the electrostatic deposition of Zn²⁺ions and the adsorption of VOCs.ZnO spherical particles with a size of 250-300 nm are uniformly covered on the Ti₃C₂T_X surface,effectively inhibiting the aggregation of ZnO and the stacking of Ti₃C₂T_X.The specific surface area of ZnO/Ti₃C₂T_X is 54.32 m² g^-1,significantly higher than that of pure ZnO(27.78 m² g^-1).In addition,ZnO has a wide bandgap,which can generate enough active oxygen groups to oxidize VOC molecules,while Ti₃C₂T_X MXene nanosheets further provide abundant surface active sites for loading ZnO and adsorbing VOC molecules,thus forming continuous catalytic centers and obtaining a larger photocatalytic reaction interface and shorter photocarrier transfer distance.Therefore,the ZnO/Ti₃C₂T_X composite exhibits better photocatalytic performance for VOC degradation,corresponding 83%degradation rate of formaldehyde.In addition,the ZnO/Ti₃C₂T_Xcomposite material has universality and stability in the degradation of VOCs in wood-based panel,and the degradation rates of methanol and acetone can also reach 84%and 80%,respectively.And its morphology structure and photocatalytic activity can still be maintained after 5 cycles.（4）Using a "bottom-up" strategy,CNCs nanosheets via freeze-drying were used as precursors to prepare sulfur-doped carbon nanosheets（SCNs）by carbonization.Sulfur doping can simultaneously regulate the morphology and surface chemical state of SCNs.SCNs are assembled from thin sheets with cross-linked folds,which is favorable for preventing their aggregation and stacking.At the optimal carbonization temperature of 550℃,SCN-550 has a more regular graphite structure,a more abundant surface oxygen-containing functional group,and all S atoms exist in an oxidized state,increasing its surface hydrophilicity,which plays an important role in the adsorption of organic pollutants.The nano-sized ZnO particles with a spherical structure can uniformly load on the surface of SCN-550 through the electrostatic interaction.Based on the enhcnaced light absorption performance of SCN-550 and the efficient channel for electron transfer,the SCN-550/ZnO composite exhibits excellent photocatalytic activity under visible light,which can degrade 92%formaldehyde within 60 min.The degradation efficiency of SCN-550/ZnO for acetone and methanol is also as high as 79%and 84%,respectively.After 4 cycles,the efficiency of SCN-550/ZnO for formaldehyde decreased from 92%to 85%,indicating that SCN-550 can effectively prevent ZnO from photocorrosion.（5）After comprehensive comparison of the composites prepared in this paper,the Ag/ZnO/nBC with high photocatalytic degradation activity and stable structure was used as a photocatalyst for the photocatalytic degradation of formaldehyde under different environmental conditions and parameters.The results showed that the relative humidity,initial concentration of formaldehyde,formaldehyde flow rate,and catalyst dosage all had an effect on the photocatalytic degradation efficiency of formaldehyde.Among them,there were optimal conditions for relative humidity and initial concentration of formaldehyde,which were 60%±5%and 290 ppm,respectively.The photocatalytic degradation efficiency was negatively correlated with the gas flow rate and positively correlated with the catalyst dosage.