| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 105 === Metal oxide supported gold catalysts readily catalyze CO oxidation at sub-ambient temperature, wherein moisture can influence the activity typically with a volcano-shape dependence. The abundant OH density on metal oxide surface hinders the study of how moisture may influence the CO oxidation over Au catalysts. In this work, we use hexagonal Boron Nitride (h-BN), a hydrophobic material, as the support for gold catalyst to study room temperature CO oxidation with various partial pressure of water (PH2O). A deposition method was conducted for preparing 1wt%Au/BN. Two commercial h-BN supports are compared and the effect of pretreatment conditions is examined for understanding the observed moisture-enhanced catalytic activity.
The first strategy of this work is using spectroscopic methods to explore roles of water co-catalyst with Au/BN in RT CO oxidation. Au/BN catalyst shows a quick increasing CO oxidation with increase of moisture concentration upto 100% relative humidity (RH). How water can enhance RT CO oxidation over Au/BN is found related to intermediate species of hydroperoxyl (*OOH) and CO(H2O)2 complex upon O2-H2O and CO-H2O cofeedings, respectively. Injection of isotope labeled H218O demonstrates that OH from H2O takes part in the process of CO2 formation and proton transfer to oxygen leading production of CO2.
The second focus in this work is the effect of calcination conditions on water adsorption behaviors and catalyst activity in RT CO oxidation over Au/BN. Ex situ XPS analysis demonstrates only metallic gold (Au0) dominated on Au/BN1 and Au/BN2 after calcination at temperatures from 300 to 600oC, with the exception of Au/BN1-300 and Au/BN2-600. The existence of Au+ (plus charged gold) on Au/BN1-300 and Au/BN2-600 presents less active CO oxidation in nominal dry and wet conditions. Increasing calcination temperature results in increase of both physisorption and chemisorption of water on Au/BN1 and Au/BN2, which can be described by Henry’s model and non-dissociative Langmuir model, respectively. This suggests that H2O adsorbs probably at the interface between Au and BN and that COad on Au surface may interact with the adsorbed H2O to form CO-(H2O)n and *OOH species. That also regards to the significant enhancement of moistened RT CO oxidation over Au/BN at elevated calcination temperature. Experimental evidences in this section indicates the interfacial perimeter of Au/BN is involved in the activation of CO and O2.
The third achievement of this work examines the nature of active site on Au/BN catalyst. We examined how an additional H2 treatment at 300oC influence Au/BN1-600, Au/BN2-450 and Au/BN2-600. Only Au0 governed on Au/BN1 and Au/BN2 as proven by in situ DRIFTS of dry CO adsorption. Only a slight increase in nominal dry CO oxidation activity was found over Au/BN after the additional H2 treatment. The catalytic activity of wet RT CO oxidation is almost the same over Au/BN1-600 with/without the additional H2 treatment. However, a noticeable increase in wet CO oxidation rate over Au/BN2-600 was found after H2 treatment. This suggests that Au0 is more likely the active phase than Au+ in water co-feed RT CO oxidation over Au/BN.
Another contribution of this work is to propose a possible mechanism that is consistent with the spectroscopic data and the kinetic data. The kinetic study was performed by varying space velocity and concentration of CO, O2 and H2O and then the kinetic data was reported. The reaction order on CO, O2 and H2O was found as 0.5 - 0.9, 0.5 – 0.5, and 0.6 – 1.5, respectively, which are not much influenced by catalyst pretreatment and type of BN (exception of H2O reaction order of 0.3 and 1.04 for Au/BN2-600 and Au/BN2-600-H2, respectively). A mechanism with good species balance is proposed which may explain how water acts as a co-catalyst in RT CO oxidation over Au/BN. Au0 is the surface site for COad and O2ad while the interface of Au-BN involved the sites for molecular H2Oad. CO(H2O)n complex is formed from the interaction between COad and H2Oad and the rate-determining step is the reaction between CO(H2O)n and O2ad.
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