| Chromium is a common chemical product.It enters the environment with the discharge of various waste water.Chromium mainly exists in the form of hexavalent chromium and trivalent chromium in the environment.Among them,hexavalent chromium is a typical environmental pollutant with significant danger.higher than trivalent chromium;and hexavalent chromium is the cause of various diseases in humans,including inhalation dermatitis,lung cancer,and nasopharyngeal cancer.Therefore,its removal from the environment is the subject of current research.Supported catalysts consist of a carrier and active components.Commonly used supports have some limitations,such as small specific surface area or relatively weak metal-support force,which leads to larger particle size or lower dispersion of active components in the catalyst.UiO-66 has a high specific surface area and a special chemical structure,which can make the active components more dispersed and smaller in particle size.In this paper,the supported Pd/UiO-66 catalysts were synthesized by impregnation and adsorption methods,and the catalysts were analyzed by FT-IR,XRD,XPS,BET,TEM and other characterization methods.Combined with the characterization results and catalytic activity results,the catalytic hydrogenation reduction reaction of Cr(Ⅵ)was studied.The main research results are as follows:1.The supported Pd/UiO-66 catalyst was synthesized by impregnation method,and the catalytic hydrogenation reduction reaction of Cr(Ⅵ)with formic acid as the hydrogen source was studied.The results of FT-IR and XRD showed that the supported Pd/UiO-66 catalyst synthesized by impregnation method did not destroy the chemical structure of the carrier itself.XPS results show that the Pd in the supported Pd/UiO-66 catalyst is mainly metallic Pd,and the content of metallic Pd increases with the increase of the loading;the possible reason is that there are more metallic Pd in the catalyst.Electron transfer occurs between Pd and the carrier,and the metal-carrier interaction is stronger.The catalytic hydrogenation reduction of Cr(Ⅵ)by three catalysts,Pd/Al2O3,Pd/Si O2 and Pd/UiO-66,was studied.The experimental results show that the catalytic activity of the Pd/UiO-66 catalyst is significantly higher than that of the Pd/Al2O3 and Pd/Si O2 catalysts.The main reason is that the supported Pd/UiO-66 catalyst has a higher specific surface area,which can make the active components dispersed more uniformly,the particle size is smaller.Using formic acid as the hydrogen source to catalyze the reduction of Cr(Ⅵ),with the increase of formic acid concentration,the initial reaction activity also increased,indicating that the initial formic acid concentration was positively correlated with the initial reaction activity.The Langmuir-Hinshelwood model was used to correlate the relationship between the initial concentration of formic acid and the initial activity of the reaction.The results showed that 1/r and 1/C had a good linear relationship(R2=0.98),indicating that the increase in the initial concentration of formic acid was in line with the adsorption-controlled reaction mechanism.2.A series of supported Pd/UiO-66 catalysts were synthesized by adsorption method,and their catalytic hydrogenation reduction reaction of Cr(Ⅵ)with hydrogen as hydrogen source was studied.XRD and TEM results show that when the supported Pd/UiO-66 catalyst is synthesized by adsorption method,the stronger the acidity of the prepared solution,the smaller the particle size of the active components and the better the dispersibility.XPS results show that the Pd in the supported Pd/UiO-66 catalyst synthesized by adsorption method is mainly cationic Pd,and with the increase of the p H value of the preparation solution,the content of cationic Pd in the catalyst gradually decreases;the main reason is that The metal-support force is weakened.During the catalytic reduction of Cr(Ⅵ),with the increase of the initial concentration of pollutants,the initial activity of the reaction continued to decrease,indicating that the concentration of Cr(Ⅵ)conforms to the adsorption inhibition mechanism in the catalytic hydrogenation reduction reaction.The effect of p H value of the reaction solution on the catalytic hydrogenation reduction of Cr(Ⅵ)was further explored.The experimental results show that the low p H value is more favorable for the catalytic hydrogenation reduction reaction of Cr(Ⅵ);under the condition of low p H value,the reaction solution contains a large amount of H+,which is favorable for the conversion of Cr(Ⅵ)to Cr(III);And at p H≤2,Cr(Ⅵ)mainly exists in the form of Cr O4-,which is more active than other forms of Cr(Ⅵ). |
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