Chapter 5: Results and Discussion: Catalyst Characterization
5.2. Scanning Electron Microscopy (SEM)
The SEM images of CuO supported on alumina (CuO/Al2O3) are shown in Figures 5.2 (a) – (b). These images show that the catalyst is made up of ‘stacked’ particles with some regions having ‘flower-like’ particles existing amongst them. All these features put together create a ‘rough’ surface morphology.
Figure 5.2 (a) – (b): SEM images of CuO/Al2O3
The backscattered SEM images for this catalyst are shown in Figures 5.3 (a) – (b). As stated in Chapter 1, Section 1.4.3, those elements with the higher atomic number will appear brighter in the backscattered SEM image. Thus, in the images shown in Figure 5.3, the brighter regions of the catalyst are due to the CuO particles and the darker/grey regions correspond to the alumina particles. The CuO particles formed clusters and appear to be present on the surface of the alumina particles.
Figure 5.3 (a) – (b): Backscattered SEM images of CuO/Al2O3
(a) (b)
‘Flower-like’ particles
(a) (b)
The EDS composition scanning data for CuO/Al2O3 is shown in Figure 5.4 (a) – (d). The map data of the combination of the individual maps (Figure 5.4(b)) shows the presence of Cu-rich particles (CuO) on the Al-rich particle’s surface (Al2O3). The individual Cu map shows that there are local concentrations of the Cu due to the clustering of the CuO and it is these ‘clusters’ that display a ‘scattered’ distribution relative to the alumina as seen in Figure 5.4 (b).
The SEM images of CuO supported on silica (CuO/SiO2) are shown in Figures 5.5 (a) – (b).
These images show the presence of sponge-like particles that have a flakey surface morphology. Some particles that possess a ‘globular-like’ appearance are also seen. As with the CuO/Al2O3 catalyst, this catalyst displays a rough surface morphology, however, the appearance of the particles differs noticeably from that of the former mentioned catalyst.
Figure 5.4: (a) Secondary SEM image and (b) – (d) EDS composition map data for CuO/Al2O3
(a) (b) (a) (b)
‘Local
concentration’ of Cu (also shown in Figure 5.2.3 (c) as yellow dotted circle)
(c) Cu (d) Al
Figure 5.5 (a) – (b): SEM images of CuO/SiO2
The backscattered SEM images for the CuO/SiO2 catalyst are shown in Figures 5.6 (a) and (b). In a similar fashion as the former discussed catalyst, the CuO particles tend to exist as clusters on the surface of the silica particles. However, in contrast to the clusters of CuO on the alumina, the clusters of CuO on the silica surface appear to have a more spherically defined shape. In comparing the SEM and the backscattered SEM images of the silica supported CuO, it becomes apparent that some of the ‘globular-like’ particles seen in the SEM image are due to the CuO particles.
Figure 5.6 (a) – (b): Backscattered SEM images of CuO/SiO2
‘flower-like’
particles
‘sponge-like’
particles ‘globular-like’
particles
(a) (b)
From the elemental map data for CuO/SiO2 as shown in Figures 5.7 (a) – (d), it is seen that the Cu-rich particles are not highly dispersed relative to the silica support; instead a scattered distribution exists. The clusters of CuO are also observed in the map data for Cu (Figure 5.7 (c)). Even though a scattered distribution is seen for the alumina-supported and silica-supported catalysts, the manner in which this type of distribution appears differs between the two catalysts. This may be attributed to the different supports influencing the distribution (the same wet impregnation method preparative method was followed for the two). In addition to these observations, it is seen, upon comparing the individual maps for Cu and Si, that there are some Cu-rich particles that exist unsupported by the silica particles.
Figure 5.7: (a) Secondary SEM image and (b) – (d) EDS composition map data for CuO/SiO2
(a) (b)
(c) Cu (d) Si
Unsupported CuO Due to the unsupported CuO
(a) (b)
Cu
The SEM images for the CuO supported on chromia are shown in Figures 5.8 (a) – (b). The images show the presence of irregularly shaped particles that display a ‘mould-like’
surface morphology. The appearance of the catalyst particles does not share any obvious similarities with the CuO/SiO2 and the CuO/Al2O3 catalysts.
Figure 5.8 (a) – (b): SEM images of CuO/Cr2O3
The backscattered SEM image of the CuO/Cr2O3 catalyst is shown in Figure 5.9 and the entire surface of the particles appears brighter. This is due to the similarity in atomic number between Cu and Cr, causing these elements to backscatter electrons to nearly the same degree. Thus it is difficult to say with absolute certainty that the brightness seen is solely due to the CuO particles, since Cr-containing particles will also contribute to the brightness.
Figure 5.9: Backscattered SEM image of CuO/Cr2O3
(a) (b)
The EDS composition scanning data is shown in Figures 5.10 (a) – (d). The map data (Figures 5.10 (a) – (c)) show that there is a homogeneous distribution of both the Cu and Cr, as well as a uniform dispersion of Cu relative to the Cr. This collectively indicates a high degree of dispersion in this catalyst. It is probable that this dispersion characteristic of the CuO/Cr2O3 catalyst is primarily due to the method of co-precipitation utilized for its synthesis. Much like the SEM and backscattered SEM images for the CuO/Cr2O3, the EDS composition scanning data does not display any similarities with the two former discussed catalysts.
Figure 5.10: (a) Secondary SEM image and (b) – (d) EDS composition map data for CuO/Cr2O3. The color of the image seen in Figure (b) above is created by the
superimposition of the images in Figures (a), (c) and (d)
(a) (b)
(c) Cu (d) Cr