Alloyed Ni-Fe nanoparticles as catalysts for NH3 decomposition
Graphical abstract
Highlights
► Non-noble metal Ni-Fe alloy catalysts for NH3 decomposition. ► Promising catalysts for high temperature NH3 decomposition. ► Structural sensitivity: increasing activity for decreasing particle sizes. ► Of various support oxides: Al2O3-based support materials give the best performance.
Introduction
Hydrogen can be used as an environmentally clean energy carrier to power for example fuel cells and internal combustion engines [1]. However, serious challenges related to hydrogen as energy vector, such as the development of safe infrastructures for hydrogen storage and distribution must be addressed. In this relation, NH3 has a number of favorable attributes, the primary one being its high capacity for H2 storage [2], [3], and that NH3 can be safely stored in salts such as MgCl2 with a high density of H2 compared to liquid H2 [4]. Ammonia is the second largest chemical in terms of production volume. More than 130 million tons of NH3 is produced each year, bulk of which is used as fertilizer and the infrastructure for transport of NH3 in large scale already exists [2].
To be able to use H2, stored in NH3, it is necessary to decompose the NH3 into H2 and N2, and this is done most efficiently by using a catalyst [5]. The most active catalysts for NH3 decomposition are based on noble metals like Ru, Rh, Pt, Ir as well as the non-noble metals Ni and Fe [6], [7], [8], [9], [10], [11], [12], [13]. The catalytic performance of these metals is typically enhanced by distributing the metals on a high surface area support material, such as an oxide [6], [7], [10], [11], [12], [13] or carbon-based material [6], [7], [8], [9], [12], [13]. The catalytic activity may further be enhanced by the use of promoters such as K, Na, Li, Ce, Ba, La, Ca, Cs [6], [7], [9], [10], [11], [12], [13].
Theoretical models, describing the activity of catalysts for ammonia synthesis as a function of the N2 dissociative chemisorption energy for transition metals in so-called volcano plots [14], have been shown to be applicable to the ammonia decomposition reaction as well [15]. The volcano plot presents Ru near the top of the curve, as indicated by Fig. 1 which is reprinted from Ref. [15]. Ni and Fe are less catalytically active and are found to the right and left side, respectively, of Ru in the volcano plot (Fig. 1). The shape of the volcano plot implies that there is an optimum for the adsorption energy, which reflects the compromise that a good catalyst should be able to activate the reactants, but not bind the reaction intermediates and products too hard. Previous studies have used a rational approach to design catalysts, where the idea was to combine metals with high and low binding energies to obtain an alloy with optimal interaction strength, resulting in a high catalytic activity [16]. For the ammonia synthesis reaction a Co-Mo-based catalyst with a relatively high activity was designed by using this approach [16]. Similarly, a highly active ammonia decomposition catalyst could possibly be developed by alloying metals from both sides of the peak of the volcano curve (Fig. 1), e.g. Ni and Fe. A few alloys have already been developed as catalysts for NH3 decomposition, for example related to NOx emissions in fossil-fuel fired power plants [17], but also related to the conversion of NH3 into H2 [18], [19], [20]. These are, however, only described in patents from which the exact alloy composition and the catalytic performance is not fully transparent.
A factor that complicates the approach of finding an alloy with an optimal peak position in the volcano plot, is that the peak position depends on the reaction conditions [5], [15]: for high NH3-to-H2 ratios (e.g. 99% NH3) the peak position is between the nitrogen binding energies for Ru and Ni, while a decrease in the NH3-to-H2 ratio results in a movement of the peak to the left toward the nitrogen binding energy for Fe (Fig. 1). This is also the reason why the optimal catalyst for ammonia decomposition is not identical to the optimal catalyst for ammonia synthesis [15].
A different and significant effect of a decreasing NH3-to-H2 ratio is the slowing down of the decomposition reaction resulting from an established equilibrium between adsorbed N atoms, gas-phase NH3 and gas-phase H2 [8]. This effect is a challenge if one aims for an end product with close to 100% H2 and only traces of NH3, for example in relation to PEM fuel cell applications where NH3 poisons the fuel cell [3].
In the present study we apply the rational design approach based on the volcano curve to develop an alloyed catalyst for decomposition of NH3. We focus on Ni and Fe, since these are relatively inexpensive and abundant. We test the dependence of the activity of the alloyed phase as a function of several parameters such as the Ni-to-Fe ratio, type of Ni-Fe alloy phase, metal loading and type of the oxide support. To describe the influence of the gas environment on the catalytic performance, all tests were performed both at a high and a low NH3-to-H2 ratio in the feed gas. In agreement with the theoretical considerations it is found that the alloyed Ni-Fe/Al2O3 catalyst has a high activity compared to Ni/Al2O3 or Fe/Al2O3 catalysts in the hydrogen rich gas environment. The catalytic activity per active site is found to increase with decreasing average particle size.
Section snippets
Catalyst preparation
The catalysts were prepared by incipient wetness impregnation of spherical γ-Al2O3 pellets with a diameter of 1 mm (Sasol Germany GmbH) with a aqueous solution of Ni and Fe nitrates. The samples were dried at RT for at least 48 h. Further drying and calcination was carried out in air at 100 °C for 10 h and at 450 °C for 4 h in a tube furnace. Three different types of catalysts were prepared in this way: Ni/Al2O3, Fe/Al2O3 and alloyed Ni-Fe/Al2O3 with a desired metal loading of 10 wt.% for all three
Results and discussion
To determine the catalytic activity, a series of Ni-Fe/Al2O3 catalysts with varying Ni concentration in the active phase were tested. Fig. 2 presents the conversion of NH3 for these catalysts as a function of reaction temperature for (a) test A and (b) test B.
Conclusions
The developed Ni-Fe catalysts for NH3 decomposition are based on relatively inexpensive and abundant materials compared to the noble metal-based catalysts needed to achieve a comparable catalytic activity. The catalyst could be a promising candidate to replace more expensive noble metal-based catalysts for decompositions of NH3 in systems where a high concentration of H2 (corresponding to low concentrations of NH3) is needed. It is found that small Ni-Fe nanoparticle sizes are crucial for an
Acknowledgements
Center for Individual nanoparticle Functionality (CINF) is funded by the Danish National Research Foundations. The authors would like to thank the EUROSTARS program for funding this research project, Dmitry E. Dorokin, DTU CASE and Irek Sharafutdinov, DTU CASE for assistance in the catalyst test lab and Christian D. Damsgaard, DTU CEN for assistance with the in situ XRD measurement.
References (37)
- et al.
Int. J. Hydrogen Energy
(1994) - et al.
J. Catal.
(1992) - et al.
Appl. Catal. A-Gen.
(2002) - et al.
Appl. Catal. A-Gen.
(2004) - et al.
J. Catal.
(2004) - et al.
Appl. Catal. B-Environ.
(2004) - et al.
Appl. Catal. A-Gen.
(2004) - et al.
Appl. Catal. A-Gen.
(2010) - et al.
J. Catal.
(2001) - et al.
J. Catal.
(2005)
Catal. Today
Appl. Catal. A-Gen.
J. Catal.
J. Catal.
J. Catal.
Nature
J. Mater. Chem.
J. Mater. Chem.
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