(421d) Influence of Reductant Composition on NH3 Synthesis from Exhaust NO Gas Using NO-CO-H2O-H2 Reaction

Despite being released to the atmosphere in unprecedented quantity by industrial and agricultural sectors, as well as possessing much higher global warming potentials than carbon dioxide and methane, reactive nitrogen has received relatively little attention in efforts to combat climate crisis. While NO_x in the exhaust gas is usually reduced into nitrogen that can be released without harm to the environment, this thermo-catalytic reduction process requires relatively high energy consumption. Moreover, some NO_x abatement processes (i.e. fast selective catalytic reduction) use NH₃, which is a valuable product, to reduce NO_x into N₂. On the other hand, process technology of reduction of NO_x into ammonia that could be used as a carbon-free fuel presents an attractive alternative to conventional selective catalytic reduction system and closes the nitrogen cycle.

Dissociation energy required to cleavage the N=O dual bond is much lower than, for example, triple bond in nitrogen for case of conversion of nitrogen to ammonia. Therefore, a mild operating condition for production of ammonia can be expected. Moreover, the equilibrium constant of the NO reduction is typically very favorable towards conventional synthesis of ammonia, which opens a possibility of much higher conversion and selectivity towards ammonia. Several NO_x-to-ammonia (hereinafter referred as NTA) processes have been proposed in the literature. Electrochemical reduction methods by using excess electricity from renewables have been proposed recently [1-3], however it has been deemed energetically inefficient and expensive even with assumption of declining renewable prices. On the other hand, we have reported the viability of near-complete conversion of NTA under thermocatalytic reduction process using Pt/TiO₂ catalyst at 473-523 K and atmospheric pressure [4-5]. The main reaction is shown as follows.

NO + 2.5 CO + 1.5H₂O ↔ NH₃ + 2.5CO₂ (R1)

Assuming that reductant CO is supplied from the steam methane reforming (SMR) process, it is considered that produced hydrogen in the SMR could be also utilized as reductant for the NTA process.

NO + 2.5H₂ ↔ NH₃ + H₂O (R2)

The use of hydrogen is expected to reduce carbon dioxide emissions in the NTA process. However, to best of our knowledge, behaviour of the NO-CO-H₂O-H₂ reaction over Pt/TiO₂ catalyst has not been fully elucidated. Thus, we investigate influence of reductant composition on ammonia synthesis using the NO-CO-H₂O-H₂ reaction by experiments and numerical analysis.

First, behavior of NO-CO-H₂O reaction on Pt/TiO₂ were investigated with fixed NO concentration of 5000 ppm, as well as CO and H₂O concentrations of 5000-20000 ppm and 8000-35000 ppm, respectively. The result showed that CO and H₂O supply above the stoichiometric level was important to guarantee conversion towards ammonia above 90%. Prior works [4-5] revealed that the reaction proceeds through formation of intermediate COOH group from CO and OH. The COOH intermediate is subsequently decomposed into CO₂ and H, of which the latter reacts with NH_x to generation of ammonia. As assuming the abovementioned reaction mechanism, we derived reaction rate model based on the Langmuir-Hinshelwood mechanism, and subsequently estimated values of parameters for the model by using the experimental data. Numerical analysis for surface coverage of intermediate on the catalyst showed that COOH coverage was constantly below 5% at relevant temperature and composition. It is estimated that an excessively high COOH coverage indicates reaction bottleneck, which limits the quantity of H to allow formation of ammonia.

Next, we investigated behavior of NO-CO-H₂O-H₂ reaction on Pt/TiO₂ by changing ratio of flow rate for H₂ under constant total flow rate of CO and H₂. NO and H₂O concentration was 5000 ppm and 7500 ppm, respectively. GHSV was set at 20000 h^-1. In a case when reaction temperature was 498 K, it was seen that the conversion of NO and the conversion of CO increase with increase in the flow rate ratio for H₂. It was also found that the insufficient feed of CO (below the stoichiometric proportion) yielded sub-optimal (< 50%) conversion of NO to NH₃ in either case of NO-CO-H₂O reaction and NO-CO-H₂O-H₂ reaction. Furthermore, it was shown that the selectivity of NO to NH₃ was constant at almost 100% in the presence of H₂. It was also estimated that the yield of NH₃ increased with increase in the flow rate ratio for H₂. When the reaction temperature was changed from 498 K to 473 K and 448 K, the consumption of NO declined.

It is notable that while formation of other reactive nitrogen species was investigated, production of was NO₂ not observed. On the other hand, production of N₂O, which has a higher greenhouse effect than CO₂, was confirmed. In a case when reaction temperature was 448 K, selectivity of NO to N₂O was seen to decrease with increase in the flow rate ratio for H₂. On the other hand, in a case when reaction temperature was 498 K, the selectivity of NO to N₂O was lower than the case for 448 K and increased with increase in the flow rate ratio for H₂.

Hence, it was found that in the NO-CO-H₂O reaction, the addition of H₂ has a positive effect on NH₃ production, and the effect changes depending on the reaction temperature. The above experimental results suggest that competitive adsorption of CO, H₂O and H₂ (and the resultant intermediate species) is influential towards the conversion of NO to NH₃. On the other hand, the decreasing trend of conversion towards NH₃ with increasing CO in case of fixed total flow rate of CO and H₂ implies that a correct consideration of NO-CO-H₂O composition balance is very important. Thus, we derived two reaction rate models for R1 and R2 based on the Langmuir-Hinshelwood mechanism, and subsequently estimated values of parameters for the models by using the experimental data. There was a good agreement between the calculated conversion by using estimated values of parameters and experimental data in a range of high flow rate ratio for H₂. On the other hand, in the cases of lower ratio for H₂, it was seen that calculated conversion of NO was up to ~20% lower than that of the experimental data. It is suggested more complex reaction mechanism beyond abovementioned assumptions might be at play and is a subject for further more rigorous kinetic analysis.

The presented results herein demonstrate the effectiveness of NO-CO-H₂O reaction, particularly with H₂ reductant addition, to develop more efficient NTA process. It is thought that the NTA process based on NO-CO-H₂O-H₂ reaction will become more environmentally friendly by complementarily using green hydrogen derived from renewable energy. Therefore, the future works in this study should be focused on plant-wide techno-economic analysis.

References:

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[2] Long, J., Chen, S., Zhang, Y., Guo, C., Fu, X., Deng, D., & Xiao, J. (2020). Angewandte Chemie International Edition, 59(24), 9711-9718.

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