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Cite this: Catal. Sci. Technol., 2018,

8, 6302

Received 30th June 2018,Accepted 5th November 2018

DOI: 10.1039/c8cy01355a

rsc.li/catalysis

Microwave catalytic synthesis of ammonia frommethane and nitrogen†

Xinwei Bai, a Sarojini Tiwari,a Brandon Robinson,a Casey Killmer,a

Lili Li*b and Jianli Hu *a

This study presents our recent findings that under microwave

irradiation and/or microwave plasma conditions, nitrogen can

react with methane to form ammonia and other value-added by-

products, hydrogen and carbon nanotubes, at atmospheric pres-

sure. Microwave catalysis alters current industrial processes to en-

able most difficult reactions to take place under mild conditions.

Since the microwave effect was discovered in the 1980s, theeffect of microwave irradiation (MW) on synergizing chemicalreactions, including oxidative/non-oxidative methanecoupling, aromatization, etc., has been reported.1–9 TheHaber–Bosch ammonia synthesis, developed by Fritz Haberand Carl Bosch, has been commercialized for more than 100years and this process has achieved a single-pass-conversionof 15%.10 The Haber–Bosch process requires high tempera-tures (400–570 °C) and high pressures (100–300 atm) in thereactor, which dramatically increase the capital and operatingcosts. Therefore, novel ammonia synthesis processes havebeen studied worldwide to devise a sustainable energy effi-cient methodology. To date, most of the research is focusedon ammonia synthesis using nitrogen and hydrogen. In al-most all of these previous studies, the activation of nitrogen(N2) is a huge barrier due to its N–N triple bond. Several acti-vation schemes have helped to overcome this hurdle such asthe use of nitrogen activating catalysts and non-thermalplasma.11–15 In industry, natural gas reforming (or coal gasifi-cation) is the main source of hydrogen (H2). For instance,about 50% of the cost in ammonia plants is on the hydrogenproduction from methane steam reforming. Direct naturalgas conversion to value-added products without steamreforming can lead to a huge economic impact. As a result,direct catalytic ammonia synthesis from CH4 and N2 is an at-tractive alternative to not only better utilize CH4 but also pro-

duce H2 as a valuable by-product. However, there has been al-most no investigation about this process due to the highchemical stability of N2 and CH4 molecules. One valuablestudy which mentioned CH4 conversion co-fed with N2 wasreported in 1994.16 Although only trace amounts of ammoniawere reported, it inspired the nitrogen activation by micro-wave plasma. In a traditional fixed-bed reactor without cata-lysts, either the methane conversion or ammonia selectivity islow.17 Using plasma without a catalyst seems to result in lowammonia selectivity which increases the separation costdownstream.18

Driven by the demand for high selectivity of CH4/N2 am-monia synthesis under atmospheric pressure, we used amicrowave-assisted catalytic reactor system to investigate thereaction between these two stable molecules. The main goalof this paper is to synthesize ammonia directly from methaneand nitrogen and simultaneously produce valuable by-prod-ucts, crystalline carbon nanotubes, to increase the processeconomics. Cobalt and cobalt–iron supported on gammaalumina (γ-Al2O3) were used as catalysts. As a catalystsupport, γ-Al2O3 was experimentally observed to have betterabsorption of microwave energy. The simplified diagram thatrepresents the microwave reaction system is shown in Fig. 1.

Fig. 2 illustrates the reaction performance for all four ex-periments, and Table S1 (ESI†) numerically summarizes theexperimental result. For all four experiments, the reactionwas carried out at 600 °C and 1 atm, with a weight hourly

6302 | Catal. Sci. Technol., 2018, 8, 6302–6305 This journal is © The Royal Society of Chemistry 2018

a Chemical & Biomedical Engineering Department, West Virginia University,

Morgantown, WV, USA. E-mail: john.hu@mail.wvu.edubCollege of Life Science and Agronomy, Zhoukou Normal University, Zhoukou,

Henan, China. E-mail: 13672165360@163.com

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy01355a Fig. 1 Simplified diagram of the microwave-catalysis reaction system.

Catal. Sci. Technol., 2018, 8, 6302–6305 | 6303This journal is © The Royal Society of Chemistry 2018

space velocity of 120 ccm per gram catalyst. In our analysis,we only consider ammonia and hydrogen as our gaseousproducts in the outlet stream. As shown in Fig. 2, when Co/γAl2O3 was used as a catalyst operated under conventionalheating conditions (no MW, no plasma), the ammonia pro-duction rate was only 1.7 × 10−8 mol g−1 s−1 with the maxi-mum methane conversion being 19.7%. Upon applyingmicrowave irradiation and using the same Co/γAl2O3 catalyst(MW only), the ammonia production and methane conver-sion were enhanced significantly, reaching 40.8 × 10−8 molg−1 s−1 and 73.8%, respectively. By calculating the total NH3

production during the 30 min reaction period, it was ob-served that NH3 production under microwave plasma condi-tions (MW plasma) was 143.5% (383.1 × 10−8 mol) higherthan that with microwave irradiation only (157.3 × 10−8 mol).By comparing the three experiments using Co/γAl2O3 as thecatalyst, we observed that microwave irradiation does acceler-ate the chemical reaction rate, enhancing the ammonia yieldand methane conversion compared with the conventionalfixed-bed process. When microwave irradiation began to gen-erate plasma from the feedstock, the ammonia yield was fur-ther increased.

High concentration of hydrogen was detected at the reac-tor outlet (see the ESI†), indicating that carbon formationwas significant in this reaction. As shown in Fig. 2, the accu-mulation of carbon deposits directly inhibits the ammonia

production and methane conversion. Since the microwave ir-radiation and/or microwave plasma were sustained duringthe reaction period, the decrease of ammonia productionand methane conversion illustrated the role of the cobalt cat-alyst. According to the Raman analysis, the carbon depositon Co/γAl2O3 was observed to be mainly low quality amor-phous carbon (high ID/IG value, see the ESI†). In order to fur-ther improve the process economics of ammonia synthesisfrom methane and nitrogen, increasing the selectivity to highvalue carbon becomes imperative. Based on our previous re-search, some metal promoters exhibited unique properties inproducing higher value carbon products such as carbonnanotubes (CNTs).19 Therefore, an iron promoter (0.5 wt%)was introduced onto the Co/γAl2O3 catalyst, and the catalystwas evaluated for the direct conversion of methane and nitro-gen. As shown in Fig. S4 in the ESI,† the ID/IG value in theRaman spectra has been improved. The TEM images in Fig. 3confirm the formation of carbon nanotubes. Meanwhile, it isobserved that the maximum ammonia production ratereached 53.9 × 10−8 mol g−1 s−1 and the total ammonia pro-duction was 441.5 × 10−8 mol, which was 180.7% higher thanthat of the non-promoted cobalt catalyst under the same re-action conditions of microwave irradiation only (Fig. 2a).Also, compared with the Co/γAl2O3 catalyst, a higher ammo-nia selectivity was observed (Table S1†).

To better illustrate how metal particles of the catalystsevolve along with the reaction, the particle size for each cata-lyst sample, including fresh (reduced) catalysts and spent (15min and 30 min time-on-stream) catalysts, was measuredaccording to the TEM images. Meanwhile, their distributionhistograms are reported in the ESI† (Fig. S6–S8). Statisticalanalysis reveals how the plasma, iron promoter and/or reac-tion time affect metal particle agglomeration. According toindividual t-tests (Table S4†) from analysis of variance(ANOVA), the existence of plasma does not have a significanteffect on the change in the average particle size. However,the addition of the iron promoter is a key factor for metalparticle agglomeration. According to energy-dispersive X-rayspectroscopy, the component of the metal particles in theCo–Fe catalyst was confirmed to be cobalt–iron alloys,resulting in bigger average metal particle size. Reaction is

Fig. 2 Reaction performance shown by (a): ammonia production inmol NH3 per gram catalyst per second (mol g−1 s−1) and (b): methaneconversion in percentage.

Fig. 3 TEM images of CNTs on spent Co–Fe/γAl2O3. Reaction wascarried out under conditions of microwave irradiation only. Both basegrowth (a) and tip growth (b) reaction mechanisms are present.

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6304 | Catal. Sci. Technol., 2018, 8, 6302–6305 This journal is © The Royal Society of Chemistry 2018

another key factor for agglomeration. According to the boxplots shown in Fig. 4, the particle size distribution at 15 mintime-on-stream is similar to that at 30 min time-on-stream,but the particle size of the spent catalyst is significantlylarger than that of the fresh, reduced catalyst. Given the factthat after 15 minutes the conversion of methane and the pro-duction of ammonia drop significantly (Fig. 2), it can be con-cluded that the higher the reaction rate, the larger the parti-cle size will be formed.

A common feature of plasma-assisted reactions is the acti-vation of chemical bonds and subsequent formation of radi-cals or activated molecules or atoms. Other than heating thematerial effectively, microwave irradiation can cause dielectricpolarization and deliver electromagnetic energy onto activesites, which is able to lower the activation energy in catalyticreactions when compared to conventional heating.20–22 Thisis largely because microwave irradiation increases the internal

energy of reactant molecules. The activated species henceformed interaction with the catalyst surface to form desiredproducts.23 Nitrogen is a very stable molecule and requireshigh dissociation energy to change into reactive species. Dur-ing a conventional ammonia synthesis process, the adsorbedN2 dissociates to N species on the catalyst surface triggeredby high temperature and pressure conditions.24 In contrast,microwave plasma first activates N2 molecules to N2

+ andthen forms N+ and N species on the catalyst surface.16 Theinteraction of these active species on the catalyst surface inthe presence of microwave plasma is an area yet to be investi-gated. Similar to the N2 molecule, the tetrahedral structure ofthe methane molecule and the high dissociation energy ofthe C–H bond make methane activation difficult. Conven-tional methods used to activate CH4 molecules such as cata-lytic oxidation are highly energy intensive and non-selective.Microwave plasma can activate CH4 molecules under compar-atively mild conditions due to electron collisions, leading todirect ionization and catalytic C–H bond cleavage.16,25

Based on the theories mentioned, the reaction mechanismwas postulated by analyzing the product distribution. Fig. 5illustrates the reaction pathways occurring during the micro-wave catalytic conversion of CH4 and N2. High H2 productionrepresents decomposition of CH4. It is possible that micro-wave irradiation lowers the activation energy of CH4 decom-position which produces active methane species. The meth-ane decomposition and the subsequent release of dihydrogenmolecules occur at the catalyst surface, while carbon deposi-tion deactivates the catalyst. Addition of the iron promotercan activate the CNT formation and the scheme is shown inFig. 6. Under microwave irradiation, the formation of acti-vated hydrogen atoms is likely. Simultaneously, dinitrogenmolecules are activated and form activated nitrogen atoms.These activated nitrogen atoms combine with hydrogenatoms to form ammonia on the metal of the catalyst surface.Microwave irradiation can potentially enhance the formationof the metal–nitrogen–hydrogen intermediate which leads tohigher ammonia production. It is noticed that when plasmawas generated, there was a very small amount of C2 productsin the outlet stream. This indicates the presence of compara-tively higher concentration of CHx species upon generationof plasma. However, the concentration is so small that wecan neglect it in our product analysis. With the decomposi-tion of methane, the carbon deposit covers the catalyst sur-face that causes catalyst deactivation. The research on attenu-ating the catalyst deactivation and increasing the selectivityto higher value carbon allotropes or C2 olefins is ongoingand the result will be reported shortly.

Fig. 4 Box plots for the particle size distribution at different reactiontimes-on-stream, (a): Co/γAl2O3 and (b): Co–Fe/γAl2O3.

Fig. 5 Scheme of postulated reaction pathways.

Fig. 6 Scheme of carbon species formation on the catalyst.

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Catal. Sci. Technol., 2018, 8, 6302–6305 | 6305This journal is © The Royal Society of Chemistry 2018

Conclusions

In conclusion, this novel process integrates system elementsof electromagnetically sensitive catalysts and a microwave re-actor design to convert methane and nitrogen to value addedproducts. The results indicate that stable molecules such asCH4 and N2 can be activated by microwave irradiation underappropriate reaction conditions to produce NH3. It is shownthat the conversion of CH4 and the yield of NH3 are en-hanced in the presence of microwave plasma. By adding ametal promoter, a higher ammonia yield can be achieved andvalue-added CNTs can be produced at the same time. Takingthe advantages of “state-of-the art” non-equilibrium micro-wave plasma technology, other novel reaction pathways canbe accomplished due to its potential to be scalable and eco-nomically feasible in future industrial processes.

Conflicts of interest

There are no conflicts to declare.

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