Article pubs.acs.org/IECR
Preparation of Highly Dispersed and Small-Sized ZnO Nanoparticles in a Membrane Dispersion Microreactor and Their Photocatalytic Degradation Cui Huang, Yujun Wang,* and Guangsheng Luo The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Highly dispersed ZnO nanoparticles were prepared by membrane dispersion using ZnSO4 and NH4HCO3 aqueous solutions as raw materials. The particle size and aggregation of ZnO were reduced by intensifying the mixing performance and using CO2 bubbles. The properties of the ZnO nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, and Brunauer−Emmett−Teller analysis. The relationship between the mixing performance of the microreactor and the size and monodispersion of ZnO nanoparticles was investigated. The particle size decreased with the decrease in segregation index Xs. Monodispersion appeared to be related to the micromixing performance and the ratio R of dispersed feed flow rate to continuous feed flow rate. ZnO nanoparticles with size of 11 nm and agglomeration coefficient CF of 1.181 were successfully prepared when R = 2.0 and Xs = 0.0004. Smaller-sized ZnO nanoparticles showed greater photocatalytic activity toward methyl orange with a degradation of 97.5% under UV light radiation for 90 min.
1. INTRODUCTION ZnO nanoparticles can be used as photocatalysts to degrade organic pollutants, such as hydrocarbon, halogenated organic compound, dyestuff, and pesticides into inorganic substances, such as carbon dioxide and water.1−4 ZnO is also a candidate host for various catalysts, among which Ni/ZnO is typically used in the reformation and hydrogenolysis of polyatomic alcohol5 and the desulfurization of sulfocompounds6 or adsorption of hydrogen sulfide.7 An unagglomerated spherical particle with a small size is the preferred state for applications of nanoparticles. Li. et al.6 found that smaller ZnO particle size contributes to higher desulfurization activity and sulfur capacity for Ni/ZnO. However, agglomeration control of ZnO particles is rarely considered, especially when particle sizes are particularly small and have high surface energy. Efficient mixing performance and homogeneous supersaturation environment are very important in controlling the nucleation and growth progress. Several synthesis routes are available; however, aqueous-phase synthesis is one of the more popular methods because of the convenience involved in controlling particle size, morphology, and composition. Hong et al.8 described a direct precipitation route to synthesize 30 nm ZnO nanoparticles. Kaluza et al.9 successfully synthesized ZnO nanometer particles using Na2CO3, K2CO3, and Zn(NO3)2 as raw materials by a novel continuous precipitation method. They found that the porosity and surface areas of the resulting material and the property of the precipitate highly depend on the sequence of unit operations applied after precipitation as well as on the precipitating agent. Chen et al.10 used zinc nitrate and ammonium carbonate aqueous solutions as precursors to prepare ZnO powder with a mean diameter of 35.3 nm. They proposed an index CF to describe the agglomeration degree of particles which was defined as d C F = BET DXRD (1)
specified in section 2.2.3.1. The agglomeration coefficient CF was smaller, and the agglomeration was less evident. If CF = 1, then particles can be considered ideally monodispersed, and if CF > 1, then the agglomeration cannot be ignored. A 1.61 value of CF of the prepared ZnO particles indicates that serious conglomeration exists in the ZnO powder, which is against the application demand. In our previous work, Zhang11 prepared ZnO nanoparticles using ZnSO4 and NH4HCO3 aqueous solutions as feedstock in a membrane dispersion microstructured reactor. The mean diameter size of the ZnO obtained under calcination temperature of 400 °C was 13.2 nm, showing a great advantage over other products prepared by traditional direct precipitation method. However, monodispersion of ZnO nanoparticles is not achieved with the CF agglomeration coefficient of 1.69. Moreover, the relationship between the micromixing performance of the microstructured reactor and the ZnO product properties has been not been sufficiently investigated. In this study, two new methods were developed to improve dispersion and reduce diameter. The first method increases the flow rate of the NH4HCO3 solution, which is dispersed by a piece of membrane medium to improve mixing performance. The micromixing performance was expressed with a segregation index, which can be used to measure the micromixing efficiency by chemical selectivity. Xs = 0 and 1 indicate that perfect micromixing and total segregation, respectively. Generally, micromixing of actual reactors is partially segregated so the value of Xs should be between 0 and 1. Chen et al.12 tested homogeneous mixing performance in a microreactor with the Dushman reaction and found that the Xs can reach
