Chapter V Impacts of metal addition into Au/SS5-C applied as catalysts
5.2 Au/SS5@M-C samples applied for CO oxidation
The preparation method can be seen in Part 2.1.5 of Chapter II. CO oxidation is a famous and unfailing reaction model for estimating the catalytic activity of supported Au-NPs, which is also one of the earliest reactions found to be efficiently catalyzed by gold catalysts [17].
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Figure 5.1 CO conversion during CO oxidation as a function of temperature over 50 mg Au/SS5@M-C catalysts: The first-time run (a) and the second-time run (b) of CO oxidation.
“C” means calcination in air at 300 oC for 4 h.
The series of gold materials are tested by CO oxidation from room temperature to 300oC with 1oC/min rate as shown in Figure 5.1. 50 mg of samples are used as catalysts. The samples are tested by the CO oxidation for three times, the first run and second run of reaction results are shown. The Au/SS5-C catalyst without metal addition performs barren activity until 300oC.
The CO oxidation over 50 mg Cu/SS5-C (Cu loading is 0.3 wt% to silica) is also tested as comparison, unfortunately no activity is displayed (not shown). As it can be seen from the previous TEM images in Figure 4.6 of Chapter IV, the Au-NPs are dispersed on the surface of huge SS5 globules after calcination, the major parts of the small Au-NPs still exist, and only spots of gold aggregations are formed. In fact, the particle sizes of Au-NPs around 3 nm fall in the range of sizes active for the CO oxidation[18]. However, the Au/SS5-C is nearly futile during the CO oxidation below 300oC, demonstrating that the size of Au-NPs is not the reason for the deactivation of Au/SS5-C. In other word, the sizes of Au-NPs are not the only critical parameter influencing the catalytic activity even over the inner silica carrier supported Au-NPs.
The Au/SS5@M-C catalysts display much better activity than either the Cu/SS5-C or Au/SS5-C for CO oxidation. In the first run of CO oxidation, the CO conversion raises rapidly
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160 180 200 220 240 260 280 300 0
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along with the temperature, the T10 (light-off temperature, temperature at 10% conversion) over Au/SS5@Co-C, Au/SS5@Fe-C and Au/SS5@Cu-C are 235, 227 and 192oC, respectively.
When the CO conversion reaches about 50%, the further increase of CO conversion with temperature becomes gentle. The final CO conversion under tested temperature (300oC) do not arrive 100%. The Au/SS5@Cu-C possessing better activity gets about 78% CO conversion at 300oC. The CO conversion at 300oC over Au/SS5@Co-C and Au/SS5@Fe-C are similar of 67% and 64%. The activity behavior of Au/SS5@Co-C and Au/SS5@Fe-C differ not obviously. The Au/SS5@Fe-C is slightly preferable.
The three samples in Figure 5.1(a) are tested for the second run of CO oxidation.
Comparing the first and second run of CO oxidation over the Au/SS5@M-C catalysts, they perform generally hysteretic catalytic activity during the first run, the T10 of which are about 40oC higher than the second run. The second run of reaction as shown in Figure 5.1(b) over Au/SS5@M-C catalysts is accelerated at the beginning. But at the end of the reaction (300oC), the corresponding CO oxidation are similar for each run over the same catalyst. During the second run and the third run of CO oxidation, the CO conversion over each sample maintains at the same level. It is a worth note that the amount of doping metal here is only 0.005Mr wt%
to SS5 (about 0.3wt%, Mr is the mass of metal). However, the metal addition plays great importance for improving the catalytic activity of CO oxidation. The Au/SS5@Fe-C shows slightly lower light-off temperature and T50 (the temperature corresponding 50% CO oxidation) than Au/SS5@Co-C. The Au/SS5@Cu-C displays better catalytic activity than the other samples. The light-off temperature and T50 are 160 and 220oC respectively, which is about 40oC lower than the Au/SS5@Fe-C and Au/SS5@Co-C catalysts.
However, none of these gold catalysts arrive 100% conversion of CO until 300oC. Thus the CO oxidation under the same operation conditions over 100 mg Au/SS5@Cu-C catalyst is carried out as comparison. By using different amount of Au/SS5@Cu-C catalyst (50 mg or 100 mg, 1 wt% of Au, atomic ratio of Au: Cu= 1: 1), the corresponding CO conversion varies not obviously below 150oC, whilst the T50 decreases from 215 to 180oC. The CO conversion
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reaches its maximum (100%) at about 220oC. The previous work of Liu et al.[19] focused on the pretreatment of Au-Cu/SBA-15 catalysts with total metal loading of 6 wt% (Au: Cu molar ratio of 1: 1) for CO oxidation. It was reported that the 60 mg Au-Cu/SBA-15 catalysts after calcination without further pretreatment possesses T50 and T100 at 205 and 220oC, respectively.
Ramírez-Garza et al.[20] also investigated the performance of Au/HMS-Fe (HMS: hexagonal mesoporous silica) catalyst for CO oxidation. For catalyst with 1.2 wt% Au loading and 10 wt%
Fe after O2 pretreatment, the T50 is about 210oC. However, the amount of catalyst for reaction is as high as 300 mg, which is 6 fold higher of the Au/SS5@Fe-C in our work. The current Au/SS5@M-C catalysts displayed no inferior activity comparing with the reported work although the reaction condition slightly differs, not to mention the much lower amounts of gold and metal contents in this work. No matter how, the Au/SS5@M-C catalysts with only 1 wt% gold loading after metal addition (about 0.3 wt% of metal to silica) is definitely effective for improving the catalytic activity of Au/SS5-C catalysts under the current operation system.