A gas reaction cell is used to study the oxidation of Cobalt Hydroxide in a pressurized oxygen environment, with the surface chemistry characterised by X-ray photoelectron spectroscopy.
Chemical and petrochemical industries use cobalt compounds for various applications, especially cobalt hydroxide (Co(OH)2) due to its outstanding physical properties that allow its widespread adoption in the fields of gas sensing, heterogeneous catalysis and Li-ion batteries. Co(OH)2 is also used as a precursor in the manufacture of Co3O4 and CoO(OH). The development of cost-effective Co(OH)2 catalysts as an alternative to expensive Pt in fuel cell electrodes for oxygen reduction reaction (ORR) has been the subject of intense research.1
This article discusses the use of the high temperature reaction cell for studying the processes occurring on a novel catalyst surface. Silica supported Co(OH)2 was studied before and after calcination in an oxygen atmosphere of 3 bar.
The advanced Kratos AXIS photoelectron spectrometerwas used to perform measurements. The gas reaction cell replicates the conditions of a normal reactor vessel, enabling the operator to reproduce reactor conditions and inspect the chemical changes occurring on the catalyst surface. Figure 1 shows a schematic diagram of the catalysis cell, where a fused quartz reactor vessel is placed within a stainless steel vacuum chamber. Here, using a simple transfer mechanism, experiments and analysis can be performed without exposing the sample to the atmosphere.
Figure 1. Schematic of the catalysis cell.
After preparing the sample, it was mounted and in-situ transferred to the sample analysis chamber (SAC) to acquire survey and high-resolution spectra. Oxygen and cobalt were clearly observed with a small amount of adsorbed carbonaceous material. A peak fitted high-resolution spectrum collected for the Co 2p region prior to reaction is shown in Figure 2. The most intense peak is visible at 780.8 eV – which is a characteristic of Co(OH)2.
Figure 2. Co 2p spectrum of prepared cobalt hydroxide catalyst.
Since both components of the Co 2p region reveal the same qualitative information, only the higher intensity Co 2p3/2 core level, including the shake-up satellites, were fitted. The additional spectral lines observed at higher binding energy have been related earlier to either coupling between unpaired electrons in the atom or to multiple electron excitation.3
After establishing the oxidation state of the prepared catalyst, the next step was to perform successive heating treatments in the reaction cell. Each treatment involved sample heating followed by introduction of oxygen gas to a measured pressure of 3 bar. After 10 minutes, the catalyst was removed and studied to observe any changes in its surface chemistry. However, the Co 2p region remained unchanged even after heating to 150 °C and 200 °C, but the C 1s signal decreased most likely due to oxidation and desorption of adventitious carbon as CO and CO2. When the sample was heated to 250 °C, the Co 2p envelope showed a distinct change (Figure 3).
Figure 3. Co 2p spectrum of oxidized cobalt hydroxide.
For the oxides of Co it is not possible to confidently identify the oxidation state simply by using the peak position of the Co 2p3/2 peak. However, according to a recent publication by Biesinger et al., the shake-up structure may identify the true oxidation state.4 This is common for many top-row transition elements and actinides, where the high binding energy shake-up feature, fitted with two components at 785.6 eV and 789.5 eV, serves as a fingerprint in determining the oxide as Co3O4.
The O 1s spectra before and after reaction in oxygen are presented in Figure 4, clearly showing a shift in the peak position of the O 1s orbital from 531.7 eV to 530.3 eV. This shift is characteristic of the switching of the oxygen environment from hydroxides to low oxides. The small low binding energy shoulder on the untreated sample may be caused by some surface contamination from adsorbed carbon or low percentages of oxyhydroxide species.
Figure 4. O 1s spectra of oxidized cobalt hydroxide catalyst before (black) and after (blue) oxidation reaction.
This article has demonstrated the analysis of cobalt hydroxide oxidation using the catalysis cell. Using the catalysis cell, the sample can be treated under high temperature, high pressure conditions similar to the environment in industrial reactors. Careful analysis of the XPS spectra acquired after each successive treatment reveals that once calcined above 200 ºC, the catalyst surface changes from hydroxide to oxide. Peak fitting the Co 2p region revealed Co to be present as Co3O4.
References and Further Reading
- B. Wang, J. Power Sources, 152, 2005, 1–15
- J. Yang, H. Liu, W. N. Martens, R. L. Frost, J. Phys. Chem. C 2010, 114, 111–119
- V. Vagvolgyi, R. L. Frost, M. Hales, A locke, A. Kristof, J. Therm. Anal. Calorim. 2008, 92, 893
- M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W. M. Lau, A. R. Gerson, R. St.C. Smart, Appl. Surf. Sci., 257, 2011, 2717–2730
This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical, Ltd.
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