製備Au/Y觸媒應用於0℃ CO氧化反應

Abstract

本研究以氯化金酸(HAuCl4)為前趨物，利用沸石Y特殊的孔洞結構，製備具催化0℃ CO氧化反應活性的Au/Y觸媒。藉由不同擔體沸石Y (HY)表面處理過程以及改變Au/Y觸媒製備程序，瞭解各製程參數對金觸媒載負量與催化CO氧化反應活性的影響，提出Au/Y觸媒的載負機制。本研究亦藉各製程參數以及反應中加入水氣對Au/Y觸媒催化CO氧化反應活性的影響，探討水氣對Au/Y觸媒的影響以及反應中Au/Y觸媒活性衰退的原因。觸媒性質的鑑定包含：AA、ICP、TPR、XRD、UV-Vis、HRTEM。 研究結果顯示，HY的表面處理過程和處理溫度(Tadjust)決定Y上的Na+含量，Y上的H+和Na+含量有一適當比例，使其加入金溶液後有較多金錯離子載負在擔體Y上。擔體Y上含較多H+離子，易促使載負於Y上的金再與鄰近的金錯離子聚合成較大的離子，影響觸媒的金載負量、均勻分散程度、催化CO氧化反應活性與反應中觸媒的穩定性。Au/Y觸媒製備程序中，起始金溶液濃度、NaOH滴加方法、金溶液調整後的pH值，混合攪拌時間(Mixing time)、升溫過程和乾燥方法是影響金觸媒載負和反應活性的主要製備變因。金溶液pH值為決定金錯離子載負比例的因素，pH值越高溶液中的OH基取代氯化金酸上Cl數目越多，產生較多中性的Au(OH)3化合物且促使金與金聚合反應產生較大的金錯離子，混合攪拌時間對金載負量影響不大，但混合攪拌時間越長取代Cl數目越多，Au/Y觸媒最終CO轉化率較高。乾燥觸媒的過程會影響觸媒的Au/Y催化CO氧化反應活性，室溫乾燥的觸媒含較多水分，可延緩金與金之間的聚集，反應轉化率較高，60℃烘箱乾燥過程可能已使部分離子金還原成金屬金，造成金與金之間燒結成較大的顆粒而反應活性較低。0℃CO氧化反應活性來源為金屬金，金顆粒尺度的增加與CO2毒化為觸媒反應中活性衰退的原因。Au/Y觸媒製備程序控制不佳與擔體Y結構的崩壞，皆造成金觸媒並無受到孔洞結構的限制，因放熱反應造成Au/Y觸媒表面局部過熱，加速燒結成較大顆粒造成活性下降。反應中加入水氣有利帶走毒化金觸媒的CO2，使反應活性提升。 Au/Y觸媒的載負為金溶液中帶正電的金錯離子與Y擔體上的H+或Na+離子，在高溫下行離子交換反應載負於Y上，以金溶液調整至pH=6為例反應式如下： [Au(OH)2]+ + Hx(Na)1-xY 以本研究的研究結果，建議沸石HY表面處理的條件為：在控溫Tajust=30℃下以鹶液調整pH值至6，表面處理3h後過濾無水洗幹燥。Au/Y觸媒的製備條件為：起始金濃度為1.6×10-3M，在調整溫度Tadjust=30℃下，以取代3.2個CL(上標 -)經驗值的鹶液，一次滴加使滴加金溶液pH值為6，加入擔體後均勻攪拌升溫40分鐘至80℃，持溫一小時後過濾水洗，室溫下不照光幹燥6h。

Au/Y catalysts, with high catalytic activity for CO oxidation at 0℃, were prepared in chloroauric acid solutions (HAuCl4). A possible deposition mechanism of gold supported on Y-type zeolite (Au/Y) was developed, according to the experimental results obtained from this research. The effects of water vapor on catalytic activity of Au/Y and the reason for deactivation during the reaction were studied. The catalysts were characterized by AA, ICP, temperature-programmed reduction (TPR), XRD, UV-Vis, HRTEM. It was found that an optimum ratio of H+ and Na+ in zeolite Y, from the pretreatment process of HY and the pretreatment temperature, caused high gold loading. However, more H+ on zeolite Y (HY) would enhance the aggregation of gold species during deposition on Au/Y and result the lower gold loading and the lower catalytic activity. The effects of preparation conditions (i.e. chloroauric acid concentration, NaOH(aq) addition process, pH of gold solution, mixing time, deposition temperature and drying temperature) on gold loading and catalytic activity of Au/Y were significant. In a solution with higher pH, the gold species contained less chlorine, which would generate more gold polymers for deposition. Therefore, Au particles on Au/Y were larger. Gold loading and catalytic activities were lower. In a solution with higher gold concentration, more gold species were deposited on the surface of Y, which caused dramatic deactivation during CO oxidation. Drying temperature of Au/Y catalysts can highly affect the activity of Au/Y for CO oxidation. Some gold species can be reduced during drying at 60℃, which causes the sintering of gold particles and the decreasing of catalytic activity. The main reason for deactivation of Au/Y during CO oxidation were CO2 poison and increasing the gold particle size. It is proposed in this study that during the deposition process, chloride dissociated from AuCl(OH)2 to form [Au(OH)2]+, which then reacted with the zeolite as follows: [Au(OH)2]+ + Hx(Na)1-xY