An important reaction for carbon monoxide removal is the Water Gas Shift (WGS) reaction, the catalysts of which have been investigated over the past three decades. Studies have revealed that noble metals improve the activity of WGS catalysts, yet the price of these metals is a cause for concern. This is the reason why a noble free metal catalyst is needed for Water Gas Shift reaction.
Activity tests were examined with various pre-reduction conditions using K/Co3O4. A Q-Mass (Model QGA; Hiden Analytical Ltd.) was used to make the measurements. It is to be noted that if K/Co3O4 is not pre-reduced, it will not show any activity. Either H2 or CO+H2 was used to conduct pre-reduction. The K/Co3O4 catalyst that was pre-reduced with syngas (CO+H2) exhibited high and stable WGS activity. However, K/CO3O4 pre-reduced by H2 did not show high WGS activity levels. Also, it is to be mentioned that K/Co3O4 that was reduced by syngas (CO+H2) showed high selectivity towards CO2 and H2 (more than 99%), and carbon deposition occurred under the detection limit of temperature-programmed oxidation apparatus.
Figure 1. Catalytic activities over various K-loaded catalysts
Next, the effects of the loading amount of potassium on WGS catalytic activity were investigated. In addition to a 0.78wt%K/Co3O4, 0.20wt%, 3.93wt%, and 5.89wt%K/Co3O4 catalyst were also prepared, and activity tests were conducted under the same conditions (syngas was used for pre-reduction). 0.20wt%K/Co3O4 exhibited the least activity among all the samples. Apparently a threshold to the loading amount of potassium exists, yet increasing the loading amount to 3.93wt% enhanced the CO conversion from about 65% (0.78wt%K) to >80%. However, the activity of 5.89wt%K/Co3O4 was observed to be lower than that of 3.93wt%K/Co3O4, leading to the conclusion that the optimum loading amount of potassium for this catalyst to exhibit high WGS activity was between 0.78 and 3.93wt%K.
To analyze the structure of the support after reduction by CO, H2, and CO+H2, XRD measurements were performed. All peaks were attributed to Co metal after H2 reduction, but several peaks showing CO2C were observed after CO or CO+H2 reduction. Also, the spectra after CO reduction included some peaks that were attributed to CoO. Therefore, it was concluded that H2 has a very strong reduction property. The low initial activity was caused by CO pre-reduction for 3.93wt%K/Co3O4 and hence, pre-treatment by syngas is more favorable than pre-treatment by CO. Using K/CoO without pre-reduction, activation tests were also conducted to verify whether CoO exhibits WGS activity or not, and it was found that CoO shows no activity at all. Considering these results, it can be seen that CO2C is an important active species to proceed with WGS reaction.
EXAFS measurements were investigated to understand the fine structure of Co more clearly. The reference sample used was a Co foil. The most noticeable peak on the Co foil shows the Co-Co bond to be close to 0.2 nm. The same peak appeared for 0.78wt%K/Co3O4 as well as 3.93wt%K/Co3O4. However, the peak was stronger as the loading amount of K decreased. This trend is same as the results shown in the XPS spectra for CO2p3. Also, with the decrease in the loading amount of K, the bulk structure is also reduced to Co metal, and carbide formation becomes difficult.
To conclude, the activity of a noble metal free Co-system catalyst for a Water Gas Shift (WGS) reaction was investigated. While K/Co3O4 catalyst exhibits no WGS activity without pre-treatment, it exhibits high stable activity after pre-reduction with syngas for 200 minutes. Potassium loading stabilizes the structure of the catalyst in a reduction atmosphere, and has an electron-donating effect. Excess reduction of Co-oxide to metallic Co is suppressed by loading potassium.
Figure 2. Analyses images using Q-Mass
Project summary by:
3-4-1, Okubo, Shinjuku,
Tokyo 169-8555 Japan
T. Majima, E. Kono, S. Ogo, Y. Sekine (2016) “Pre-reduction and K loading effects on noble metal free Co-system catalyst for water gas shift reaction”, Applied Catalysis A: General 523, 92-96
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
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