The role of heterogeneous catalysts in the plasma-catalytic ammonia synthesis
Graphical abstract
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.
References (69)
- et al.
Plasma N2-fixation: 1900–2014
Catal. Today
(2015) - et al.
Ammonia and related chemicals as potential indirect hydrogen storage materials
Int. J. Hydrogen Energy
(2012) - et al.
Wind-powered ammonia fuel production for remote islands: a case study
Renew. Energy
(2014) - et al.
Low-cost small scale processing technologies for production applications in various environments — Mass produced factories
Chem. Eng. Process. Process. Intensif.
(2012) - et al.
Small scale, modular and continuous: a new approach in plant design
Chem. Eng. Process. Process. Intensif.
(2012) - et al.
Industrial applications of plasma, microwave and ultrasound techniques : nitrogen-fixation and hydrogenation reactions
Chem. Eng. Process. Process. Intensif.
(2013) - et al.
A review of the existing and alternative methods for greener nitrogen fixation
Chem. Eng. Process. Process. Intensif.
(2015) - et al.
NH3 decomposition for H2 generation: effects of cheap metals and supports on plasma–catalyst synergy
ACS Catal.
(2015) - et al.
Can plasma be formed in catalyst pores? A modeling investigation
Appl. Catal. B Environ.
(2016) - et al.
Tubular membrane-like catalyst for reactor with dielectric-barrier-discharge plasma and its performance in ammonia synthesis
Appl. Catal. A Gen.
(2004)
Low temperature plasma-catalytic NOx synthesis in a packed DBD reactor: effect of support materials and supported active metal oxides
Appl. Catal. B Environ.
N. S. F. XPS, FTIR and TPR characterization of Ru/Al2O3 Catalysts
Appl. Surf. Sci.
Preparation of highly dispersed W/Al2O3 hydrodesulfurization catalysts via a microwave hydrothermal method: effect of oxalic acid
Arab. J. Chem.
Temperature programmed reduction of alumina-supported iron, cobalt, and nickel bimetallic catalysts
Appl. Catal.
The dissociation energy of the hydrogen molecule
J. Mol. Spectrosc.
Improved oxidation of air pollutants in a non-thermal plasma
Catal. Today
Optimal catalyst curves: connecting density functional theory calculations with industrial reactor design and catalyst selection
J. Catal.
From the sabatier principle to a predictive theory of transition-metal heterogeneous catalysis
J. Catal.
Synthesis of ammonia using microwave discharge at atmospheric pressure
Thin Solid Films
Nitrogen fixation
Ullmann’s Encyclopedia of Industrial Chemistry
Ammonia, 2. Production Processes. Ullmann’s Encyclopedia of Industrial Chemistry
The Haber-Bosch heritage: the ammonia production technology
Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production
Techno-economic feasibility study of renewable power systems for a small-scale plasma-assisted nitric acid plant in Africa
Processes
Plasma assisted nitrogen fixation reactions
Sustainable Ammonia Synthesis Exploring the Scientific Challenges Associated With Discovering Alternative, Sustainable Processes for Ammonia production DOE Roundtable Report SUSTAINABLE AMMONIA SYNTHESIS
The 2012 plasma roadmap
J. Phys. D Appl. Phys.
Modern Plasma Technology for Nitrogen Fixation: New Opportunities?
Plasma assisted nitrogen oxide production from air: using pulsed powered gliding arc reactor for a containerized plant
AIChE J.
Plasma (Catalyst) – Assisted Nitrogen Fixation : Reactor Development for Nitric Oxide and Ammonia production
Plasma Chemistry
Principles of Plasma Discharges and Materials Processing
Ambient temperature hydrocarbon selective catalytic reduction of NOx using atmospheric pressure nonthermal plasma activation of a Ag/Al2O3 catalyst
ACS Catal.
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