Chapter 3: Characterisation 3.0 Introduction
3.6 TEM Analysis
This technique can identify if there are smaller active particles on the catalysts which are order of magnitudes smaller than the particles and clusters observed by SEM analysis.
Figure 3.43 TEM images of 1:1 CuAu/SiO2 calcined. (C978/19A)
TEM analysis was investigated for CuAu/SiO2 calcined in air (Figure 3.43) and reveals large
clusters on the support. EDX analysis (Figure 3.44) confirms that the large clusters are Au- rich although there is a small amount of Cu present.
200nm 50nm
160
Figure 3.44 TEM/EDX analysis of calcined CuAu/SiO2 (C978/19A). The background
161
Figure 3.45 TEM images of CuAu/SiO2 reduced only part of the Sinfelt method
162
Figure 3.46 TEM/EDX analysis of reduced only part of the Sinfelt method for CuAu/SiO2
163
Figure 3.47 TEM/EDX analysis of reduced only part of the Sinfelt method for CuAu/SiO2
(C978/101A). The Ni is observed because it is the grid used to load sample.
TEM images of CuAu/SiO2 catalyst reduced at 315 oC for 2 h in H2 showed the average
164
degrees (Figure 3.45). EDX analysis showed that the more open clusters contained both Cu and Au particles (Figure 3.46). However, the more compact clusters were rich in Cu content but only contained a little Au (Figure 3.47).
TEM analysis showed that the interaction between copper and gold was high and that alloy formation was present as supported by the linescan (Figure 3.48). The EDX line scan clearly shows that some of the particles have surface layers of copper rich and their interior regions contain both Cu and Au
.
165
166
Figure 3.50 TEM/EDX CuAu/SiO2 made by the Sinfelt method (C978/101A). The Ni is
observed because it is the grid used to load sample.
167
Figure 3.51 TEM-EDX analysis of CuAu/SiO2 Sinfelt method (C978/101A). The supported
material is predominantly Cu. The Ni observed arises from the grid used to support the sample.
TEM-EDX analysis shows that, after high temperature calcination the structure of the catalyst is different with irregular shaped copper particles of between 20-40 nm. The high temperature de-alloys the catalyst, leading to bimetallic Au and Cu particles. Clusters were observed which appeared to be rich in copper, as shown by the EDX analysis (Figure 3.50 and 3.51). The gold particles seem to have little interaction with the support.
168
Figure 3.52 TEM image of CuAu/SiO2 prepared by the Sinfelt method using a copper
chloride precursor (C978/65A)
TEM analysis for a CuAu/SiO2 catalyst by Sinfelt route but with copper nitrate replaced by
copper chloride can be seen in Figure 3.52. CuAu alloy particles (Figure 3.53 and 3.54) can be found of micron size and the EDX line scan shows that some of the particles have surface layers that are copper rich, with their interior regions consisting of Cu and Au (Figure 3.55).
169
Figure 3.53 TEM/EDX analysis for CuAu/SiO2 catalyst made by the Sinfelt route with
170
Figure 3.54 TEM/EDX analysis for CuAu/SiO2 catalyst made by the Sinfelt route with
171
Figure 3.55 TEM/line scan analysis for CuAu/SiO2 prepared by the Sinfelt method with
172
(a) (b)
Figure 3.56 TEM images of CuAu/SiO2 reduced in NaBH4 (C978/80A) (a) low
173
Figure 3.57 EDX HAADF and line scan for CuAu/SiO2 reduced by NaBH4 (C978/80A).
TEM analysis showed that there was alloy formation (Figure 3.57) but some particle illustrated Au concentration variation. Particle clusters (Figure 3.56 b) were seen occasionally on the surface of silica particle
.
174
(a) (b)
(c)
Figure 3.58 TEM images of HDC Cu + Au DP (C978/87) (a) at high magnification (b) at low magnication (c) at lower magnication
175
Figure 3.59 EDX HAADF and line scan for CuAu/SiO2 made by HDC Cu + Au DP
(C978/87).
Cu and Au form alloys as seen as well as clusters on the silica support. Some copper rich patches have been found (Figure 3.59) on the support silica. Moreover, the CuAu particle size distribution is bimodal. The majority of the particles are small but some particles are several times larger.
176
177
Figure 3.61 EDX HAADF and line scan for CuAu/SiO2 made by HDC Cu + Au IW
.(C978/90)
TEM analysis shows some pure Cu patches. Wormlike big CuAu particles (Figure 3.60) are common in addition to small alloy particles.
178
(a) (b)
179
180
Figure 3.64 EDX HAADF and line scans for CuAu/SiO2 reduced in NaBH4 and calcined in
air
TEM analysis of the CuAu/SiO2 catalyst, reduced in NaBH4 and calcined in air (Figure 3.63),
showed that Cu and Au seemed to be separated as two phases, although some Cu particles were seen in contact with Au particles, as seen in the EDX line scan.
Sample Mean Minimum Maximum
Reduced NaBH4(C978/80A) 24 11 42
HDC Cu + Au DP (C978/87) 9 2 71
HDC Cu + Au IW (C978/90) 11 3 107
Reduced NaBH4 and calcined in
air 24 11 33
181
Table 3.4 displays the particle size variations for four different preparation methods. CuAu/SiO2 catalyst reduced in NaBH4 has the largest particle diameter of 24 nm. The
smallest particle size is formed as a result of a high dispersion route for Cu followed by Au deposition precipitation method, with an average size of 9 nm. This agreed much better with the sizes observed by XRD which showed smaller particles compared with SEM which showed bigger particles and if bulk Au then they are probably not contributing to the catalytic activity. More than 500 particles were counted for TEM particle size data whilst for XRD analysis, more than 1000 particles were counted. It is important that many particles are measured for these characterization techniques so that statistically reliable mean size data can be presented.
182