Investigating the Surface Chemical Composition of Metal Oxide Catalysts

Heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Generally, heterogeneous catalysts include an active material distributed upon a support material.

This active material is usually a dispersion of transition metal particles that may be either in a partially or completely reduced form. The support material is usually an alumina or other metal oxide and may play a key role in the catalytic procedure either directly or by affecting the active material.

Catalysts can come in a precursor form, which must be activated before usage. Given that industrial process largely depends on permanent operating conditions, the right precursor must be used to realize a successful process.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is an analytical technique that is extensively used for catalyst analysis. It helps in extracting both chemical state and elemental data. Despite the fact that the lateral resolution of XPS is beyond 10µm, this technique is extremely surface selective.

The information thus obtained represents the outer 10nm of the sample, which means that while separate particles cannot be determined, it is possible to measure and contrast the average chemistry of the catalyst’s surface area with that acquired from bulk-sensitive techniques.

In this article, a couple of catalyst samples were used for analysis. The first sample was taken from a batch that showed the predicted performance in a process, and the second sample was obtained from a batch that showed poor performance.

Thermo Scientific’s K-Alpha+ XPS system (Figure 1) was utilized to determine the chemical state of the components identified as well as the elemental composition of the material.

K-Alpha+ system

Figure 1. K-Alpha+ system

In order to understand the performance differences, identification and quantification of the variations between the two samples was made through the XPS technique.

Experimental Method

The samples that were used in this analysis were in a coarse powder form. These samples were loaded directly onto a K-Alpha+ sample plate of 60 × 60mm size by pressing onto a conductive carbon tape. Then, the samples were mounted into the K-Alpha+ fast entry airlock (FEAL) and automatically pumped down.

The K-Alpha+ automatically shifts the samples into the analysis chamber as soon as the vacuum level in the FEAL meets the preferred level. This usually occurs within 10 minutes, provided the samples do not outgas extensively. Subsequently, individual sample was studied to determine the elemental composition and ultimately the chemical state of the elements identified.

A ‘survey’ scan was used to detect the elemental composition that covers the complete accessible energy range and aids in detecting and quantifying the elements at the surface of the sample. Using the results obtained from the survey scan, scans of high resolution were obtained and utilized to measure the chemistry of the sample surface.

Results and Discussion

Similar elemental compositions were observed in both samples (Table 1), except for the differences observed in the oxygen and carbon concentrations. This outcome could be a major factor, but at these levels of concentrations this could be attributed to the slight changes in surface contamination.

Table 1. Elemental composition of the “good” and “bad” samples based upon analysis of the survey spectra

Peak Peak BE/eV At. % (Good) At. % (Bad)
Si2p 99.35 26.56 25.54
Cl2p 199.48 2.60 2.69
C1s 284.94 36.42 41.39
01s 532.78 32.45 28.11
Cu2p3 933.36 1.61 1.68
Zn2p3 1022.17 0.36 0.59

High resolution scans were obtained based on the elemental components detected. Figure 2 shows the Cu2p spectra for the good and bad samples. It can be seen that the copper is fully oxidized.

However, an obvious variation can be seen in the spectrum structure, indicating that the combined oxidation states are quite different. In the good and bad samples, the ratios of Cu(I):Cu(II) were 3:2 and 1:3, respectively.

Cu2p spectra for (a) the “good” sample, and (b) the “bad” sample.

Cu2p spectra for (a) the “good” sample, and (b) the “bad” sample.

Figure 2. Cu2p spectra for (a) the “good” sample, and (b) the “bad” sample.

This difference in the surface oxidation state of copper can be attributed to the different performance of the two samples. Such types of catalyst are often reduced prior to usage, and the typical reduction procedure may have led to non-optimal surface conditions with regard to the bad sample.

Conclusion

XPS is a surface selectiveoxidation state at the surface of the bad sample was greater in comparison to the good sample. This was specifically seen in the Cu2p XPS spectrum, which revealed that more Cu(II) was detected in the bad sample. This variation in the level of oxidation attributes to the different performance of the two samples.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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