Transitional Metal Catalysts Prepared By Ball Milling And Their Activities For The CO Oxidation

CO oxidation has been applied to prepare CO sensors,purify the gas and clean up the automobile exhaust.The copper-and noble metal-based catalysts are two kinds of common catalysts for the CO oxidation.To obtain the catalysts,almost preparations are operated in the solvents,which always cause damages to the environment,consume a lot of time and need tedious preparation procedures.At the same time,they also require catalyst’s precursors to be soluble in solvents,limiting the scope of precursors.Considering this,the current works employed the ball milling methods to synthesize a series of highly porous metal oxide,well dispersed noble metal and highly thermal stable catalysts.These developed methods can overcome the drawbacks of solution preparation and have the potential applications in catalyst preparation.We introduce our works as follows:（1）Embedding porosity in the transitional metal catalysts can advance their catalytic activities because the porous structures facilitate the diffusion of reactants which enhances the contact between the reactants and active sites.Herein,employing the ball milling method,a series of highly porous metal oxides were successfully synthesized using the commercial porous silica as template.Their specific surface areas are comparable to the reported metal oxides with high surface areas,ZrO₂:293 m² g^-1,Fe₂O₃:163 m² g^-1,CeO₂:211 m² g^-1,CuO_x-CeO_y catalyst:237 m² g^-1 and CuO_x-CoO_y-CeO_z catalyst:202 m² g^-1.And CuO_x-CeO_y and CuO_x-CoO_y-CeO_z are active for the CO oxidaton.This developed method is named mechanochemical nanocasting.The mechanochemical method can complete the casting procedure within 60 mins,much shorter than that of soulution process（2-3 days）.Moreover,no slovents are needed in the mechanochemical nanocasting.Hence this developed technology in principle can ignore the solubility issue of metal precusors,and tolerate metal precursors with poor or even no solubility.（2）The noble metal catalysts show excellent catalytic activities for the CO oxidation,especially for the highly dispersed noble metal.In the current work,employing the ball milling technique,a supported Pd-P25（P25 is the a commercial TiO₂）heterogeneous catalyst was prepared with the aid of NaCl.The structural analysis reveals that the Pd is highly dispersed in the P25,and the P25 structure can be maintained in the preparation process with the aid of NaCl.The as-synthesized catalyst displays excellent activity for the CO oxidation.This developed method is also successfully used to prepare the highly dispersed Pt-and Ru-P25 catalysts.For the solution procedure,the synthesis of highly dispersed noble metal catalysts heavily depends on the solvents,accurate modulation of synthesis conditions and protection reagents,resulting in tedious and high-cost preparation process.Our developed method is operated in the solvent free condition,and the anchoring process can be complete within 30 mins.To obtain the very active catalysts,the non-toxic and low-cost NaCl is only needed.Compared to the solution process,the developed method has the potential applications in the synthesis of noble metal catalysts.（3）The above works have demonstrated the advantages of ball milling method for the synthesis of porous metal oxide and highly dispersed noble metal catalysts.Thus,we try to prepare noble metal/porous metal oxide catalysts.Herein,ball milling the mixture of noble metal precursors,ZrCl₄ and Na Cl,then calcinating the mixture and removing NaCl with amount of water,the Pd,Pt,Ru-ZrO₂ catalysts were prepared.The structure analysis reveals that the noble metals are highly dispersed on porous ZrO₂ and these catalysts exhibit high specific surface areas（180 m²/g）.And these catalysts show excellent catalytic activities for CO oxidation.To obtain the highly porous noble metal supported catalysts,the traditional methods always need two steps,the preparation of porous supports and anchoring of noble metals.For the solution preparation,the two steps are all tedious and consume a lot of time.While the developed method can finish these in one step and only needs 30 mins.This provide an efficient route for the synthesis of porous supported noble metal catalysts.（4）To clean up the CO gas in the automobile exhaust demands the catalysts not only possessing high catalytic activities but also exhibiting highly thermal stability.The ball milling method presents many advantages to synthesize catalysts for the CO oxidation.And we also notice that it also has been utilized to synthesis of ceramics with outstanding properties.Considering this,we try to synthesize highly thermal stable catalysts for the CO oxidation.The CuO_x-CoO_y-CeO_z catalyst（CCC）prepared by co-precipiation has been reported to show excellent CO oxidation activity.Herein,empolying the ball milling method,a CuO_x-CoO_y-CeO_z catalyst（CCC-B）were prepared by using the Cu（OAc）₂,Co（OAc）₂ and Ce（OAc）₃ as the precursors.The CO oxidation experiments reveal that activity of CCC-B is similar with the CCC catalyst after 600 ^oC calcination.When rising the calcination temperatures to 900 and 1000 ^oC,the CCC-B displays higher activity than the CCC,indicating that the catalyst prepared by ball milling is more thermal stable than that prepared by co-precipitation.The structure analysis reveals that the ball milling synthesis can impede the growth of CuO,inhibit the decomposition of Co₃O₄ and maintain the surface lattice oxygen.This provides a new route for the synthesis of highly thermal stable catalysts towards the CO oxidation.