Precisely Tailoring Metal-oxide Interfaces For Improved Catalytic Performance In Preferential Oxidation Of Carbon Monoxide

Atomic layer deposition?ALD?is a technique for producing thin films which develops rapidly in recent years.ALD is similar to chemical vapor deposition,except that ALD has the characteristics of self-limiting growth,which relies on self-limiting binary reactions between gaseous precursor molecules and a substrate.By varying the number of ALD cycles,the thickness of deposited films can be precisely controlled at the atomic level.Thus,this film growth technique has been widely used in many fields,including catalysis.Compared to conventional methods of synthesis of heterogeneous catalyst,a method of atomically precise control over supported catalysts is highly desirable to understand of reaction mechanisms and rational design of catalysts with high performance.Metal-oxide interfaces play essential roles in many catalytic reactions.For example,in preferential oxidation of CO in hydrogen?PROX?reation,the interfaces beteween platinum group metals and oxide is considered to be the active center.However,it is difficult to precisely control the metal-oxide interfaces by the conventional methods of catalyst synthesis to optimize catalytic performance.In this work,ALD is used to "bottom-up" construct metal-oxide intrefaces with near atomic precision.Based on the selective deposition strategy,FeO_x or CoO_x was selectively deposited onto the metal nanoparticle surface in supported Ir?Pt?metal catalyst with different number of ALD cycles toprecisely tailer the metal-oxide interfaces for maximizing the catalytic performance.The main results of this study are as follows:?1?We successfully selectively deposited FeO_x onto SiO₂ supported Iridium catalysts using low-temperature ALD,where FeO_x was selectively deposited onto Ir nanoparticles,but not on the SiO₂ support.By varying the number of FeO_x ALD cycles,the loading of iron was precisely controlled as well as the Ir-FeO_x interfaces.Compared to Ir/SiO₂ catalyst,we found that only adding a small amount of FeO_x,the activities of the resulting Ir-FeO_x/SiO₂ catalysts in PROX reaction was significantly improved,among which the Ir-FeO_x/SiO₂ catalyst with two cycles of FeO_x ALD,showed the highest activity by achieving a complete CO conversion within a temperature range from 60 ? to 180 ?.Increasing the number of FeO_x ALD,the catalytic activities of Ir-FeO_x/SiO₂ catalysts sharply decreased.In addition,we also found that decreasing the Ir particle size would allow further improving the catalyst activities by broadening the temperature range of CO complete conversion.?2?Ir-Co/SiO₂ catalysts were prepared by the same strategy described above,where CoO_x was selectively deposited on Ir nanoparticles to precisely tailer the Ir-CoO_x interfaces.We found that after depositing one cycle of CoO_x ALD on Ir/SiO₂ yielded the highest catalytic activity by showing a complete CO conversion within a temperature range from 83 ? to 122 ?,while increasing the number of CoO_x ALD,the catalytic activitives decreased.Meanwhile,the pretreatment conditions were found to have a significant effect on the catalytic performance.?3?Finally we further investigated the support effect on catalytic activities of Pt-Fe catalysts for PROX reaction,although Pt-FeO_x interface was thought to be the active sites.In the part of work,one cycle of FeO_x was deposited on Pt/C,Pt/Al2O3 and Pt/TiO₂ catalysts using ALD,respectively.Preliminary results demonstrated that cataltyst supports have a large impact on the catalytic perforamce of Pt-Fe catlaysts by showing a completely different temperature range of complete CO conversion.