Ammonia decomposition over citric acid chelated

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Short Communication

Ammonia decomposition over citric acid chelated g-Mo2N and Ni2Mo3N catalysts Sharif F. Zaman*, Lateef A. Jolaoso, Seetharamulu Podila, Abdulrahim A. Al-Zahrani, Yahia A. Alhamed, Hafedh Driss, Mohammad M. Daous, Lachezar Petrov Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia

article info

abstract

Article history:

A novel synthesis route, using citric acid as a chelating agent, for the formation of g-Mo2N

Received 28 April 2018

and pure phase of Ni2Mo3N catalysts and their application for NH3 decomposition reaction

Received in revised form

for clean hydrogen production, have been performed. Successful formation of a pure bulk

6 July 2018

phase of Ni2Mo3N was confirmed by using XRD, XPS, HRTEM techniques and found that

Accepted 12 July 2018

Ni2Mo3N is not air sensitive. Ni2Mo3N catalyst showed very high catalytic activity for NH3

Available online 10 August 2018

decomposition reaction having ~97% conversion of NH3 at 525 C at 6000 h1 GHSV, better than previously reported results on any non-promoted, non-precious catalysts, which is

Keywords:

mainly due to the formation of pure phase and high surface area for this catalyst using a

Ammonia decomposition

chelating method of preparation.

Hydrogen

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

g-Mo2N Ni2Mo3N Citric acid Chelating agent

Introduction Hydrogen is the ideal fuel that can mitigate environmental problems connected with energy production as water is the only combustion product. The use of clean hydrogen containing low 10 hrs) at elevated temperature (600 C), may undergo the structural change which is evidenced by a shift in peak position. In the diffractogram of Ni2Mo3N catalyst, a pure phase of Ni2Mo3N with diffraction peaks (PDF#01-089-7952) was recorded. No other oxidic or separate Ni or Mo nitride phases were detected, which confirms that Ni2Mo3N phase is quite stable in the presence of oxygen. This was also observed by other researchers [29,31]. Also Fig. 1 shows the XRD patterns of calcined (oxidic precursor), fresh and spent Ni2Mo3N catalyst. Ni2Mo3N catalyst maintained its phase composition after the reaction of NH3 decomposition (600 C) and upon exposure to air, which shows the stability of the catalyst. The stable structure was maintained even after over 60 h of long run and exposure to ambient atmosphere without oxygen passivation treatment. This unique characteristic of Ni2Mo3N catalyst might be used to catalyze several reactions. In the Supplementary Information S3, crystallite size calculation for the fresh and spent catalysts shows that there is no considerable amount of change in crystallite size.

XPS analysis The XPS analyses were done using freshly prepared nitride catalysts in order to gain insight into the oxidation states of the catalyst surface. The XPS spectra of g-Mo2N and Ni2Mo3N, which are displayed in Mo 3d region are shown in Fig. 2a.

The predominant surface species in the g-Mo2N catalyst are the Mo2þ and/or Modþ (d ¼ 4 or 6) oxidation states. Similar results were reported by Podila et al. [32]. Previous reports [33,36] showed that Mo2þ and Modþ oxidation states represent molybdenum nitride and Mo4þ state correspond to MoO2 while the Mo6þ depicts the oxidized form of g-Mo2N phase since XRD did not reveal the presence of any other phase than these two phases. As was previously described in the preparation section, the pyrophoricity nature of g-Mo2N requires that it should be passivated in 1% O2/Ar mixture. During the passivation process, a protective layer of MoO2 phase was formed on the surface of the bulk sample. After that, it is possible to expose the sample to the atmosphere and used for the activity test and characterization. For Ni2Mo3N catalyst, Mo 3d and Ni 2P XPS profile are reported in Fig. 2b. Mo3d XPS spectra showed typical mix oxidation state of molybdenum. Higher oxidation states Mo5þ and Mo6þ are more pronounced compared to the case of the gMo2N catalyst. The deconvoluted peaks for Mo3d5/2 positioned at 227.2 eV, 228.33 eV and 230.83 eV correspond to Moo, Moþ2/ d and Moþ4 oxidation states, which may correspond to the existence of MoO3 surface phases [37]. For Ni 2P profile (Fig. 2c) clearly shows the formation of metallic Nio and also Ni2þ oxidation states. Binding energy (BE) of 852.58 eV corresponds to metallic nickel (Nio) and BE of 856.38 eV correspond to Niþ2 as NiO. BE of 862.0 eV corresponds to Niþ2 as of Ni(OH)2 oxidation state [38]. Supplementary Table [ST1 and ST2] lists the different BE's of the oxidation state of Mo and Ni for these catalysts.

HRTEM and SAED analysis HRTEM and selected-area electron diffraction (SAED) pictures of Ni2Mo3N are shown in Fig. 3. The HRTEM image of Ni2Mo3N


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Fig. 2 e XPS analysis of Mo3d of g-Mo2N (a) and Ni2Mo3N (b) and Ni2p for Ni2Mo3N (c) catalysts.

Fig. 3 e High-resolution TEM image (a) and SAED image (b) of fresh Ni2Mo3N catalyst.

with an average particle size of 3e4 nm was clearly observed with the interlayer distance of 2.2 A (221 plane), the major plane was also seen as a sharp and tall peak in XRD pattern, which confirmed the presence of Ni2Mo3N in the analyzed sample. The selected-area electron diffraction (SAED) pattern of a small representative of Ni2Mo3N particle is shown in the

bright-field image. In the pattern, both spots and rings belong to Ni2Mo3N. The SAED patterns established the characteristic (110), (210), (221), (310), (311), (320), (321), (410), (330) and (421) planes of Ni2Mo3N are numbered between 1 and 10. HRTEM sample analysis clearly established the formation of Ni2Mo3N phase in the prepared catalyst.

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The SEM and HRTEM images of fresh g-Mo2N catalyst were previously presented and explained by our group in several publications [32,33,39] and also provided in the Supplementary Fig. SF2.

Catalytic activity results The catalytic activities of g-Mo2N and Ni2Mo3N catalysts for ammonia decomposition reaction were investigated as a function of temperature at atmospheric pressure. Obtained results are depicted in Fig. 4a. As expected, ammonia decomposition being an endothermic reaction, there was an increase in activity with the increase in temperature. A sharp increase of the activity was observed at 350 C for Ni2Mo3N catalyst and above 450 C for g-Mo2N. Interestingly at 525 C the registered degree of conversion was 97.7% for Ni2Mo3N which is the best conversion for nitride catalysts that have ever been reported at this temperature. ~99% conversion of ammonia over Ni2Mo3N was achieved at 550 C catalyst while for g-Mo2N sample the required temperature was 600 C. A comparison table for the activity of Ni2Mo3N and g-Mo2N catalysts in recent past is shown in Table 1. Srifa et al. [29] reported that Ni3Mo3N and gMo2N catalysts demonstrated almost 83% and 69% conversion respectively at 550 C. In this work, the catalysts gave better conversion at the same temperature. In our case, we have successfully prepared Ni2Mo3N phase using a chelating agent

and confirmed by XRD and HRTEM results and this active phase makes the difference in catalytic activity results [comparison activity curve shown in Fig. 4a]. From the experimental data, it is clear that Ni2Mo3N catalyst has shown higher activity compared to g-Mo2N catalyst indicating that the reaction was actually promoted with the addition of Ni species. Consequently, we can come to the conclusion that adding Ni onto the molybdenum nitride structure is the key to the high activity in ammonia decomposition reaction. The work reported by Leybo et al. [30] showed that with the increase of the content of Ni2Mo3N phase (they had a mixed phase of Ni-Mo-N) in their prepared catalysts, there was an increase in the activity of NH3 decomposition. Thus, they identified the active phase in the reaction, though their catalyst required a temperature of 600 C to have a complete conversion. In our case, the pure bulk phase of Ni2Mo3N showed the lowest possible temperature over ternary nitride based catalysts reported, to demonstrate >97% conversion at 525 C. The high activity of Ni2Mo3N phase may be ascribed to its higher surface area compared to other reported Ni-Mo-N catalysts. Also, the ternary nitrides follow the Mars-VanKrevelen mechanism for ammonia decomposition/synthesis [25]. The structural nitrogen involved in the reaction, facilitate the N-N combination step on the surface, which is the rate limiting step for NH3 decomposition reaction at low temperature [40]. Also in another way, incorporation of Ni in Mo2N structure will reduce the nitrogen adsorption energy and thus

Fig. 4 e [a] Catalysts activity data [b] Arrhenius plot of Mo2N and Ni2Mo3N catalysts.

Table 1 e Reaction rate parameters and % conversion of ammonia over different Ni based catalysts at atmospheric pressure. Catalyst g-Mo2N Ni2Mo3N g-Mo2N Ni3Mo3N La-Ni/Al2O3 Ni/Y2O3 NiMoN/a-Al2O3 Ni-Mo-N (82% Ni2Mo3N) Ni2Mo3N

Temperature ( C)

GHSV (h1)

Conversion %

Activation Energy, Ea KJ mol1

Reference

550 525 550 550 550 550 600 600 600

6000 6000 6000 6000 6000 6000 3600 21,600 30,000

71.9 97.7 69.0 83.0 95 75 79 99 98

131.3 66.1 97.0 83.8 e e e e 66.1

This work This work [29] [29] [43] [44] [42] [30] This work


more surface sites are available for further reaction [29,41]. Fig. 4b shows the Arrhenius plot (conversion vs 1/T) for ammonia decomposition over Ni2Mo3N and g-Mo2N catalysts. A dramatic decrease in activation energy was observed for Ni2Mo3N catalysts. The activation energies obtained are 131.0 kJ mol1 for g-Mo2N and 66.0 kJ mol1 for Ni2Mo3N catalysts. Long run stability test over Ni2Mo3N catalyst was also performed for 60 h at three different temperatures of 500 C, 550 C and 600 C (15 h at each temperature) and GHSV (gas hourly space velocity) of 30,000 h1, shown in Supplementary Fig. SF3. The obtained conversions of ammonia were 58.0, 81.0 and 98.0% and the corresponding hydrogen productivity were 122.0, 170.0 and 206.0 mol H2 g cat1 h1. A much higher hydrogen productivity was observed for Ni2Mo3N catalyst compared to other non-precious catalysts, i.e. Ni, Co, Mo2N or MoC2 based catalysts [23,29]. Stable performance and reproducible activity were clearly observed when the temperature was dropped down from 600 C to 550 C. The catalyst showed stable and reproducible performance during the run. Another stability test was performed at 550 C at GHSV of 6000 h1 and 12,000 h1for 30 h each, showing 99% and 97% conversion respectively [Supplementary Figs. # SF4 and SF5], which signify that, this catalyst is capable of producing H2 at a very high rate.

Conclusions g-Mo2N and Ni2Mo3N (pure phase) catalysts were successfully prepared using a chelating agent, citric acid, and been tested as catalysts for NH3 decomposition reaction. Pure Ni2Mo3N phase had attained a high surface area compared to other reported ones in the literature. The Ni2Mo3N catalyst showed better catalytic activity compared to g-Mo2N and 97.7% NH3 decomposition was achieved at 525 C, whereas only ~50% NH3 conversion was achieved for bulk g-Mo2N. At 600 C the hydrogen production rate can be reached to 206 mol H2 g cat1 h1, which is very promising for onboard hydrogen generation for fuel cell application.

Acknowledgments This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no (9-135-36-RG). The authors, therefore, acknowledge with thanks DSR technical and financial support.

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.07.085.

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