Morphologies of fresh and spent gold catalysts

The morphology of the as-prepared Au/SS5-C after calcination can be seen as shown in Figure 4.6 of Chapter IV. Most of the small particles remain on the surface of large Stöber silica globules with diameter about 490 nm. Only small amount of Au-NPs are aggregated after the sample after calcination at 30 ^oC for 4 h. The major of the small particles still exists (4.4 nm ± 0.7 nm). However, this material does not possess obvious activity for the CO oxidation until 300^oC, although the sizes of Au-NPs are sufficient to active the reaction. The metal addition (molar ratio of M: Au is 1: 1, about 0.3 wt% of M to SiO2) can greatly increase the catalytic activity for CO oxidation. Different from the previous work, in which the addition amount of Fe or Mg oxidized components are as high as 6wt%,^{[16, 35]} the addition amounts here is amazingly low. The reason why is urgently required. It is also wondered that dose the specially morphology account somewhat for the catalytic properties of the catalysts?

5.4.1 Morphologies of the fresh Au/SS5@Cu-C

In the following work, we only take the Au/SS5@Cu-C sample with the higher CO oxidation activity as typical sample. The TEM technique is performed for understanding the growth of Au-NPs during CO oxidation, and the interaction between gold, copper and silica components.

We also try to understand the mechanism of metal addition influencing the structure and activity of catalysts.

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Figure 5.8 The TEM image (a) and size distribution based on 200 particles (b) of the fresh Au/SS5@Cu-C catalyst, and the high resolution of one single particle showing different crystal faces (c-d).

Comparing the TEM image and size distribution of Au/SS5-C sample without copper addition as shown in Figure 4.6 of Chapter IV, it can be clearly observed that the average particle size increases from 4.4 to 5.3 nm after copper addition, in which the particles larger than 8 nm is more frequently observed in the Au/SS5@Cu-C sample. There are also particles as large as 19 nm in the Au/SS5@Cu-C sample, which reveals a more intensive aggregation of the particles in this catalyst. In fact, some of reports always gave the assertions that the addition of metal (oxide) into supported Au-NPs could induce the high dispersion of Au-NPs due to some kind of synergetic interaction,^{[8, 36]} which is seems not the case here. In order to understand the dispersion of gold and copper components and the formation process of larger aggregations, the high resolution TEM images are operated. The larger particles are mainly focused. Very interesting, two kinds of crystal faces in one large particle are observed sometimes. The particle in Figure 5.8(c-d) is the same one. The remarkable part is that two different interplanar spacing

50 nm

0 2 4 6 8 10 12 14 16 18 20 22 0

10 20

X Axis Title

Size (nm)

%

2.36 Å 2.47 Å

2.48 Å 5.3 ± 3.15 nm

2 nm 2 nm

a b

c d

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are revealed. The crystal faces in Figure 5.8c with the interplanar spacing of 2.36 Å can be ascribed to the (111) crystal face of metallic Au.^[37] While the crystal faces in Figure 5.8(d) with the interplanar spacing of about 2.47 Å is resulted from the (111) crystal faces of Cu2O.^[38]

Some investigators reported that the gold and copper components in the catalyst can form AuCu alloy under special conditions (e.g. under heat treatment under at least 300^oC and then annealed in H2-N2 mixture at 800 ^oC),^{[39, 40]} the interplanar spacings of which might ranges from 2.1 to 2.3 Å depending on the Au/Cu ratio. Under this point of view, it can be inferred that there is generally no AuCu alloy formed but only separated or conjugated Au and Cu (Cu₂O) particles or aggregations under current conditions. Combining the two TEM images of the same large particle, it is considered that the copper components partially encapsulates the Au-NPs and form one large aggregation, which might happened during either the preparation or the calcination process.

5.4.2 Morphologies of the spent Au/SS5@Cu-C

The Au/SS5@Cu-C displays an much improved catalytic activity for CO oxidation comparing to the Au/SS5-C, and the activity lasts for 7 h with only slight decrease of the activity, although the addition amount of copper is tiny. The spent Au/SS5@Cu-C sample is also observed by the TEM technique as shown in Figure 5.9. It is remarkable that the nanoparticles in the spent Au/SS5@Cu-C sample seem to be more dispersive with fewer large particles. The size distribution based on large amounts of particles is shown in Figure 5.9(d).

Comparing the fresh Au/SS5@Cu-C sample, the percentage of nanoparticles smaller than 3 nm do not change obviously in the spent Au/SS5@Cu-C sample. Only partial of the nanoparticles (≤ 2 nm) evolves into larger particles. However, the amount of nanoparticles smaller than 5 nm in the spent Au/SS5@Cu-C sample increases from 63.1% to 72.1% after reaction. The high-resolution TEM images over spent Au/SS5@Cu-C display some single spherical particles which are confirmed to be gold or Cu2O particles by their corresponding interplanar spacing. During the observation, some interesting calabash-like particles are also observed as

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shown in Figure 5.9(c). Combining with the XPS showing the appearance of Cu²⁺ species, it is considered that the calabash-like particle is consisted by the conjunction of Au-NPs and CuO patch, although only the Au(111) face is seen due to the observation angle.

Figure 5.9 The TEM image (a-c) and size distribution based on 200 particles (d) of the spent Au/SS5@Cu-C catalyst.