Elsevier

Catalysis Today

Volume 362, 15 February 2021, Pages 2-10
Catalysis Today

The role of heterogeneous catalysts in the plasma-catalytic ammonia synthesis

https://doi.org/10.1016/j.cattod.2020.06.074Get rights and content

Highlights

  • Plasma-catalytic NH3 synthesis investigated using 16 different transition metal and oxide catalysts supported on γ-Al2O3.

  • Optimum N2/H2 feed ratio = 1 or 2, depending on the catalyst, substantially above the stoichiometric ratio of 0.33.

  • With 2 wt% Rh catalyst, 1.43 vol% NH3 was produced at the energy efficiency of 0.94 g kWh-1.

  • Increasing reaction temperature or decreasing gas flow rates activated the NH3 decomposition pathway.

  • Combination of gas-phase and surface reactions: synergism from activation of H2 on the catalyst surface and N2 in plasma.

Abstract

Ammonia, being the second largest produced industrial chemical, is used as a raw material for many chemicals. Besides, there is a growing interest in the applications of ammonia as electrical energy storage chemical, as fuel, and in selective catalytic reduction of NOx. These applications demand on-site distributed ammonia production under mild process conditions. In this paper, we investigated 16 different transition metal and oxide catalysts supported on γ-Al2O3 for plasma-catalytic ammonia production in a dielectric barrier discharge (DBD) reactor. This paper discusses the influence of the feed ratio (N2/H2), specific energy input, reaction temperature, metal loading, and gas flow rates on the yield and energy efficiency of ammonia production. The optimum N2/H2 feed flow ratio was either 1 or 2 depending on the catalyst – substantially above ammonia stoichiometry of 0.33. The concentration of ammonia formed was proportional to the specific energy input. Increasing the reaction temperature or decreasing gas flow rates resulted in a lower specific production due to ammonia decomposition. The most efficient catalysts were found to be 2 wt% Rh/Al2O3 among platinum-group metals and 5 wt% Ni/Al2O3 among transitional metals. With the 2 wt% Rh catalyst, 1.43 vol% ammonia was produced with an energy efficiency of 0.94 g kWh−1. The observed behaviour was explained by a combination of gas-phase and catalytic ammonia formation reactions with plasma-activated nitrogen species. Plasma catalysts provide a synergetic effect by activation of hydrogen on the surface requiring lower-energy nitrogen species.

Introduction

Reactive nitrogen in the form of ammonia is an essential element for life on Earth; [1,2] it is the second-largest chemical compound produced and is essential for the global economy [3]. Ammonia is used as a starting material for the production of many chemical compounds such as fertilisers, explosives, and in many industries such as pulp and paper, refrigeration, pharmaceuticals, fibres and plastics, mining and metallurgy. Ammonia is produced on a major scale via the Haber-Bosch process at 450−600 °C and 150−350 bar in the presence of a catalyst in megaton-scale centralised production facilities [4]. With the global population set to exceed 9 billion by 2050, ammonia is destined to become a focal point for intensification of agriculture and chemical industries [5,6].

In addition to these well-established applications, ammonia attracts attention as an energy storage chemical, [7] fuel [8], and as a reducing agent in selective catalytic reduction (SCR) of NOx produced by automobiles. [9] These applications demand distributed ammonia production under mild process conditions and on a smaller scale using electricity from renewable sources [1,10,11] because the Haber-Bosch process is not a viable option. Therefore, it is imperative to look for an alternative ammonia production process with zero CO2 emissions, close to electricity generation, and at a point-of-use, as demanded by these emerging applications [6,12,13].

Over the last century, several alternative approaches have been developed for CO2-free ammonia synthesis under mild operating conditions such as electron-driven electro- and photo-catalysis, homogeneous and enzyme catalysis [14]. Among these alternative approaches, non-thermal plasma (NTP) generated by renewable electricity is an appealing option for small scale and distributed production [[15], [16], [17], [18], [19]]. NTPs are characterised by an excessively high electron temperature, while the bulk of the gas remains at a mild temperature. These non-equilibrium properties enable thermodynamically unfavourable chemical reactions under mild conditions such as atmospheric pressure and low temperature [20,21]. In addition, NTP offers an opportunity to use catalysts to benefit surface reactions and increased selectivity towards the desired product [[22], [23], [24]]. Such a combination of NTP with a catalyst (plasma catalysis) often yields synergetic effects with the efficiency of the combined system exceeding that of the constituent parts [[23], [24], [25], [26], [27]].

The ease of catalyst screening and simplicity of operation motivated many plasma catalysis studies for ammonia synthesis in a dielectric barrier discharge (DBD) reactor [[28], [29], [30], [31]]. Most of the reported literature studies use relatively inert (or ferroelectric) materials conventionally considered as catalyst support [[31], [32], [33], [34], [35]]. These materials, nevertheless, show significant improvements in nitrogen fixation due to their effect on the plasma formation [31,36]. Mizushima et al. [37] showed that loading active metals on the support can yield higher ammonia concentration and can also improve the energy yield. The ammonia yield was reported to increase in the following order Ru > Ni > Pt > Fe > only Al2O3 [37]. Recently, Peng et al. [38] used 10 % Ru + Cs/MgO catalyst and Hong et al. [39] reported ammonia over a functional carbon-coated catalyst. Iwamoto et al. [40] and Mehata et al. [41] attempted to explain the differences in reactivity between various metals and evaluated it through DFT calculations and microkinetic modelling.

Even though the plasma-assisted ammonia syntheses have long been investigated using various plasma catalysts, the nature of the support-catalyst effect is largely unknown; the performance of different supported catalysts and reasons of their synergy is scarcely available. Moreover, no detailed studies exist that explain the influence of the feed composition, temperature and metal loading. In this paper, we aim to study the role of an active component in heterogeneously-catalysed plasma-catalytic ammonia synthesis. To achieve this and distinguish between the electrical and curvature effects of various materials, we used supported catalysts with low metal loading deposited on the same particles of γ-alumina.

Section snippets

Experimental section

Extrudates of γ-Al2O3 was purchased from Mateck GmbH, crushed, and sieved into a 250−350 μm fraction. The particle size fraction was selected to have optimum performance in the reactor, low-pressure drop, and high product yield [31,36]. The supported catalysts were prepared using wet impregnation and calcined at 450 °C, more details are in the Supplementary S1. The catalysts obtained are referred by the weight loading of the active metal and its type, for example, 2 Ru for 2 wt% Ru/ γ-Al2O3.

Oxidation state of the catalyst and feed ratio optimisation

Before carrying out plasma ammonia synthesis, we studied the effect of hydrogen plasma treatment on the catalysts. The results of the temperature-programmed reduction experiments of the catalysts before and after the plasma exposure are shown in Supplementary S3, summarised in Table 1.

The platinum-group catalysts were completely or mostly reduced by the H2 plasma. For example, a low-temperature reduction peak below 200 °C in the 2 Pt catalyst disappeared after plasma treatment. The reduction of

Conclusions

A range of catalysts has been systematically screened for plasma-assisted ammonia synthesis in a packed dielectric barrier discharge (DBD) reactor at atmospheric pressure. The efficiency of plasma-catalytic ammonia formation depends significantly on the initial N2/H2 feed ratio which should be in the range of 1–2 for maximum ammonia production. The gas feed ratio is substantially above the stoichiometric ratio of 0.33 due to the possible energy dissipation by activation of hydrogen. The most

CRediT authorship contribution statement

Bhaskar S. Patil: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft. Nikolay Cherkasov: Investigation, Writing - review & editing. Nadadur Veeraraghavan Srinath: Investigation. Juergen Lang: Funding acquisition, Conceptualization. Alex O. Ibhadon: Writing - review & editing. Qi Wang: Funding acquisition. Volker Hessel: Supervision, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This research is funded by the EU project MAPSYN: Microwave, Acoustic and Plasma SYNtheses, under the grant agreement no. CP-IP 309376 of the European Community’s Seventh Framework Program.

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