Understanding the Redox Reactions of Ni and Co during Catalytic Combustion of Methane

1. Introduction

The catalytic combustion of methane (CCM) is a significant process for energy production and reduction of direct emission from methane¹. Our study uses an operando setup to explore the synergy between Ni and Co catalysts to understand the oxidation state of the transition metals. The idea is to develop a method where the retrieved in-situ knowledge on the oxidation state is used to improve the CCM activity.

2. Experimental

A series of well-controlled Ni-Co catalysts supported on a hydrotalcite(HT)-like structure have been synthesized using co-precipitation. The catalysts contain a total metal loading of 12wt% with various ratios of Ni and Co content. In addition, the pure Co catalyst was impregnated with 0.1wt% Pd. Several characterization techniques have been utilized to get at further understanding of the catalysts, e.g. hydrogen chemisorption, nitrogen adsorption, XRD, XPS, ICP-MS, TPR and TEM. The catalysts activities were analyzed in a fixed-bed quartz reactor equipped with a fiber optic probe. The probe is pointing directly towards the catalyst bed and connected to a UV-vis-NIR spectrometer giving valuable information on the in-situ state of the catalyst. The compositional gas changes were analyzed using a gas chromatograph (GC) and a mass spectrometer (MS). The experiment consists of alternating between O₂ and CH₄ conditions in 3 cycles to compare the oxidation and reduction rates of the catalysts. In addition, steady-state catalytic combustion was performed over 4 h. All analyses were performed at 670 °C to keep it at isothermal conditions.

3. Results and discussions

Figure 1A shows the activity during complete combustion for catalyst with different Ni/Co ratios, it is evident that increasing the amount of Co increases the activity. This could be explained by the oxygen conversion rate, where the results show that cobalt is a much better metal for converting oxygen from the feed compared to nickel. The results for the methane conversion rate show the opposite trend; cobalt has the lowest reduction rate. For all catalysts the oxidation rate is faster than the reduction rate which is also confirmed by the normalized Kubelka-Munk Function (KMF). The KMF is calculated from the reflectance found at 540 nm in the UV-vis spectra, which shows how Co⁰ goes to Co²⁺ and Ni⁰ goes to Ni²⁺ during combustion, see Figure 1B. It is evident that the activity is related to the oxidation of the catalysts. Due to this the pure Co was impregnated with Pd, which is an oxidative metal. The results clearly show that Pd enhances the methane conversion during combustion, which is found to be due to the prolongation of the oxidation time. A similar technique has also been done for oxychlorination on copper in our group^2-4. Figure 2 shows in more details the results from the 2^nd cycle of the transient reduction of 12Co, here the ion currents of the CO and CO₂ peaks from the MS are combined with the UV-vis results. The results show how the cobalt oxide is reducing from Co³⁺ to Co²⁺ to Co⁰, while the CO product is not forming until a certain fraction of the reducible cobalt is in the Co⁰ state. This shows that the presence of metallic Co is important for the CO formation, which can be an indication that Co⁰ instead of Co²⁺ or Co³⁺ can be applied to enhance the activity towards partial oxidation of methane. This technique is considered valid for several systems where transition metals are undergoing redox reaction. We believe that by utilizing these in-situ data from the UV-vis-NIR spectroscopy, that a method can be developed which relates the activity and selectivity with the oxidation state of the catalysts.

4. Conclusions

Based on catalysts with different Ni/Co ratios, the Co has the highest activity, however impregnating Co with Pd has a significant effect on the activity since Pd is an oxidative metal. The high activity is found to be due to the fast oxidation of Co in combination with high amounts of surface oxygen. The UV-vis-NIR spectroscopy combined with the MS signals also show that the selectivity is influence by the oxidation state of the catalyst, where CO is not produced on cobalt oxides.

References

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[2] Baidoo, M. F.; Fenes, E.; Rout, K. R.; Fuglerud, T.; Chen, D., Catalysis Today 2017.

[3] Rout, K. R.; Baidoo, M. F.; Fenes, E.; Zhu, J.; Fuglerud, T.; Chen, D., Journal of Catalysis 2017, 352 (Supplement C), 218-228.

[4] Rout, K. R.; Fenes, E.; Baidoo, M. F.; Abdollahi, R.; Fuglerud, T.; Chen, D., ACS Catalysis 2016, 6 (10), 7030-7039.