Research Of Electrochemical Growth Process In Solution And Its Application In Catalyst Synthesis Based On Microelectrode Chips

Platinum group metal nanomaterials are widely applied as catalysts in a variety of fields such as fuel cell, petrochemical industry, automobile exhaust purification and chemical industry. Due to the rare reserves and the high price of platinum group metal, how to further improve their activity, stability and efficiency has been an important scientific and engineering issue. Therefore, the study of practical platinum group nanocatalysts with high-performance is of great scientific significance and application value.For this reason, scientists have used a variety of strategies to improve the performance of catalyst. Reducing the size of the nanoparticles and loading the nanoparticals on the carrier can effectively improve the activity and utilization efficiency of the catalyst. In addition, the crystal faces with high density of step atoms perform excellent catalytic activity and selectivity, and the properties of the metal nanocatalysts can be significantly improved by regulating the surface structure of the nanoparticles. In general, metal nanocatalysts can be prepared by chemical methods and electrochemical methods. In chemical controlled synthesis, surface active agents are often used as stabilizer, which would pollute the crystal surfaces and reduce its catalytic activity. By contrast, electrochemical synthesis system is simple and the surface structure can be conveniently regulated by adjusting the potential. Moreover, the surface of obtained particles by electrochemical method is clean as surfactants are not needed to be added during the synthetic process. However, the yield is limited by the two-dimensional surface of electrode, which restricts the application of electrochemical synthesis in industry. Therefore, how to improve the electrochemical synthesis method to amplify the yield and maintain the intrinsical advantages is an important issue to be solved.Through long-term research, our group has developed an electrochemical flow reaction system (EFRS), which breaked the limits of two-dimensional growth on surface and realized the electrochemical synthesis of carbon supported Pt catalysts with high-density of surface step atoms (HDSA-Pt/C). The EFRS is expected to achieve mass production of catalyst with high performance. However, the electrochemical behavior and the involved reaction mechanism of the system remains to be explored further.In this thesis, we use the microelectrode chips prepared by micro-electro-mechanical system (MEMS) to study the electrochemical behavior of conductive particles in solution under an electric field environment, and explore a new principle of three-dimensional electrochemical synthesis in solution. Besides, we expanded the range of applications of EFRS to prepare other kinds of metal nanocatalyst. The main results are listed as below:1. Based on the requires of the experiments and the structural characteristics of electrochemical flow reactor (EFR), we designed and fabricated two kinds of microelectrode chips by MEMS technology for researching the electrochemical behavior in EFR.2. Built the multi-channel test system and used the microelectrodes chips to measure the potential distribution of conductive particles in the EFR. We discovered that the locally-enhanced interfacial potential (LEIP) existed near the conductive particles in the solution under electric field environment, which was considered as the driving force for the growth of metal nanoparticles on carbon support. External electric field is the direct factor which inducing the LEIP of conductive particles, and the intensity and phase of LEIP can be modulated by external electric field. SEM and EDS characterization results confirmed that under the electric field environment, non-contact metal electrochemical deposition on the micron-sized electrodes can be achieved in the liquid phase. Based on the above results, the potential distribution of conductive particles in solution was obtained, and reasonable explanation of HDSA-Pt/C’s growth mechanism was stated.3. The EFRS was applied to treat the commercial carbon supported platinum catalyst (Pt/C-JM) by square wave potential to prepare HDSA-Pt/C successfully. The parameters are listed as follow:E+= 1.75 V, E-=-1.2 V,f= 1 Hz, the processing time is 10 min. Electrochemical characterization results showed that compared with untreated Pt/C-JM, the nanoparticles of HDSA-Pt/C catalyst have high-density of surface step atoms and its electrocatalytic performance towards ethanol electro-oxidation is significantly increased. Moreover, we also used EFRS to prepare PtPd/C alloy catalysts and studied the influences of atomic ratio of Pt:Pd and square-wave treatment potential on their electrocatalytic performance towards methanol electro-oxidation. The results showed that when the atomic ratio of Pt:Pd is 9:1 and the square-wave potential is controlled as E+=1.75 V,E-=-1.2 V, the prepared alloy catalyst exited the best catalytic activity and stability.Herein, the exploration of the reaction mechanism involved in EFR is of great importance in the study of the electrochemical reaction in 3D solution. Using EFRS to synthesize single metal and alloy supported catalysts can expand the scope of application of EFRS, which is valuable to fuel cells and other electronic areas.