Research Article pubs.acs.org/journal/ascecg
Enhancement of Ammonia Synthesis on a Co3Mo3N‑Ag Electrocatalyst in a K‑βAl2O3 Solid Electrolyte Cell J. Díez-Ramírez,† V. Kyriakou,‡,§ I. Garagounis,‡,§ A. Vourros,‡,§ E. Vasileiou,‡,§ P. Sánchez,† F. Dorado,† and M. Stoukides*,‡,§ †
Departamento de Ingeniería Química, Facultad de Ciencias y Tecnologías Químicas, Avenida Camilo José Cela 12, 13071 Ciudad Real, Spain ‡ Department of Chemical Engineering, Aristotle University, Thessaloniki 54124, Greece § Chemical Processes & Energy Resources Institute, CERTH, Thessaloniki 56071, Greece ABSTRACT: The electrochemical promotion of ammonia synthesis by potassium ions (K+) on a Co3Mo3N-Ag electrocatalyst was studied in a K-β″-Al2O3 solid electrolyte cell. The effect of temperature (400−550 °C), PH2/PN2 feed composition (1.0, 3.0 and 6.0) and applied voltage was explored in detail. The catalyst was prepared by ammonolysis of the mixed oxide and was characterized by XRD and SEM. A volcano-type behavior was found, i.e., around 1% of potassium per total moles of Co3Mo3N improved the ammonia formation rate by as much as 48%. However, high percentages of potassium act as poison for the reaction, possibly due to the formation of K−N−H compounds that block the active sites. Faradaic efficiency (Λ) values close to 300 are for the first time reported in NH3 synthesis. KEYWORDS: Ammonia synthesis, Nitride catalysts, Electrochemical promotion, K-β″-Al2O3, Cation electrochemical promotion of catalysis
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synthesis.8 The good performance of Co3Mo3N is mainly attributed to the ability of molybdenum to dissociate the dinitrogen molecule.9,10 The activity of molybdenum increases significantly when it is alloyed with iron, nickel or cobalt (which have excellent hydrogen adsorption properties), with the latter exhibiting the optimum behavior.10 Furthermore, if an alkali metal, such as potassium,10−12 rubidium11 or cesium8,11 is added to the nitride, the reaction rate is further enhanced. As to the reaction mechanism of these promoted nitrides, it is considered that initially an electron is transferred to the active center from the alkali promoter and then to the adsorbed dinitrogen, which then dissociates to atomic nitrogen.10 Kojima et al. reported that 2% of cesium and 5% of potassium were the optimum amounts of alkali addition. The use of higher quantities of the alkali promoter led to the decomposition of the Co3Mo3N active phase into Co and Mo2N and the loss of catalytic activity.10 On the other hand, chemical reactions can also be promoted electrochemically by employing the catalysts as working electrodes in a solid electrolyte cell reactor (SECR).13 Numerous studies of electrochemical promotion of catalysis (EPOC) with different electrochemical promoters (Oδ‑, Hδ+, Naδ+, Kδ+) have been conducted in the last three and half decades.14−16 Specifically, in ammonia synthesis, EPOC, also
INTRODUCTION Ammonia is one of the most important chemicals with a worldwide production of 146 million tons in 2015.1 Its uses cover a broad range of different industrial sectors, such as fertilizers, refrigeration, explosives and pharmaceuticals.2 In the past years, the quest for a clean energy future has also made ammonia an ideal candidate as an energy carrier, due to its high hydrogen density and zero carbon emissions during oxidation.3 Nowadays, ammonia is produced via the well-known Haber− Bosch process, which was developed one hundred years ago and is considered one of the most significant scientific discoveries of mankind.4 In this energy-intensive process, ammonia is produced from its elements at temperatures between 400 and 500 °C and at pressures of 100−150 bar:2,4,5 N2 + 3H 2 ↔ 2NH3
(1)
The high pressure is dictated by the reaction stoichiometry according to the Le Chatelier principle. On the other hand, the operating temperature is a trade-off solution between faster reaction rates and higher equilibrium conversion. The catalyst usually employed is a potassium promoted iron oxide (magnetite).4 Aside from the widely used Fe-based catalyst, Ru-based materials are considered the most effective for the process.6,7 However, their prohibitive cost and the limited availability of Ru prevent their widespread application in industry. More recently, nitrides and specifically Co3Mo3N have been identified as promising catalytic composites for ammonia © 2017 American Chemical Society
Received: May 23, 2017 Revised: July 31, 2017 Published: August 16, 2017 8844
DOI: 10.1021/acssuschemeng.7b01618 ACS Sustainable Chem. Eng. 2017, 5, 8844−8851
Research Article
ACS Sustainable Chemistry & Engineering called the non-faradaic electrochemical modification of catalytic activity (NEMCA) effect, has been reported in proton conducting cells.17,18 An impressive 1300% increase in ammonia formation and a maximum faradaic efficiency of 600% (Λ = 6) has been reported by Yiokari et al.17 in a high pressure (operating at 50 bar) electrochemical reactor, when protons were pumped to the catalyst surface.17 The results were attributed to the supply of the electropositive protons to the catalyst surface, which enhanced the chemisorptive binding energy of the electron acceptor, nitrogen.17 It should be pointed out, however, that Yiokari et al. observed this significantly high NEMCA effect when the conversion to ammonia was much lower than that corresponding to thermodynamic equilibrium. In the opposite case, a very weak effect was observed.18 This is because the electrochemically supplied protons not only modify the catalytic properties of the electrode, but also serve as the carriers of the electrical power required for the nonspontaneous ammonia synthesis reaction.19 The effect of electrochemical promotion is more clearly observed when the promoting species are neither reactants nor products of the catalytic reaction. In the last two decades, several catalytic reactions have been shown to be electrochemically promoted in alkaline SECRs.15,16,20 In these studies, some unique advantages of the electrochemical vs the chemical promotion of alkali metals have been identified, which include among others, the in situ optimization of the alkali promoter coverage and tuning of product selectivities, as well as the permanent promotion effect.21 Despite the aforementioned advantages, to this day, no alkaline cationic conductor has been employed for the study of EPOC in ammonia synthesis. Hence, the aim of this work is to investigate the electrochemical enhancement of the ammonia synthesis rate on a Co3Mo3N nitride catalyst by using a K-β″Al2O3 potassium conductor under atmospheric total pressure. Potassium is used because it constitutes a common promoter for ammonia synthesis. The Co3Mo3N was mixed with Ag in order to improve its adherence with the solid electrolyte and its electronic conductivity so it can be used as an electrode. The effect of various parameters, such as the H2/N2 feed ratio, the operating temperature and the applied potential is examined and discussed.
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Figure 1. Schematic diagram of the single-chamber K+-conducting cell reactor used for the electrochemical promotion of ammonia synthesis.
organometallic paste (Fuel Cell Materials, ref-231001) in two consecutive stages, first at 300 °C for 1 h and second at 800 °C for 2 h (heating and cooling ramps of 5 °C/min). The working electrode (WE) was prepared by mixing the as-prepared Co3Mo3N catalyst with Ag (at a weight ratio of Co3Mo3N/Ag = 60/40). The Co3Mo3N-Ag powder was mixed with an ink vehicle (terpineol based, Fuel Cell Materials) until a homogeneous paste was formed and was screen printed (40 mg) on the K-β″-Al2O3. The electrode was heated at 650 °C in nitrogen for 45 min. Gold wires were used for current collection and connection of the cell with the external circuit. Characterization Techniques. A Phenom ProX scanning electron microscope was used to acquire images of the Co3Mo3NAg electrode. This instrument was also equipped with an energy dispersive X-ray spectroscopy (EDX) analyzer to determine the average composition of the samples. The X-ray diffractograms were obtained using a Philips X’Pert instrument using nickel-filtered Cu Kα beam operating at 40 kV, 40 mA and a scan rate of 0.02° in the 2θ range of 30°−60°. All the electrochemical measurements were carried out using a VersaSTAT 4 electrochemical workstation (Princeton Applied Research) and the corresponding VersaStudio software for data processing. Cyclic voltammograms were obtained in the range of UWR = −2.0 to +2.0 V with a scan rate of 50 mV/s. Three to five cycles are necessary before two consecutive ones are identical. Before any electrochemical promotion measurement, the WE was “cleaned” of potassium ions by applying a UWR of 2.0 V for 2 h (the current reached zero). Ammonia Measurements. The experiments were carried out under atmospheric pressure, at temperatures between 400 and 550 °C and with a total flow rate of 75 mL·min−1. Under these operating conditions, N2 conversions were differential (typically less than 0.15%). The reactant gases were H2 and N2 (99.999% purity, Air Liquide Hellas) and their flows were controlled with mass-flow controllers (Bronkhorst High Tech). The ammonia concentration in the outlet stream of the reactor was measured by two independent methods; one photometric and one with a real-time analyzer. The main method of measuring the ammonia was via an EAA-24r-EP online, real-time analyzer (Los Gatos Research), with a Cavity Ringdown Spectroscopy (CRDS) technique. These measurements were confirmed by a photometric method in which the products were bubbled through a H3BO3 solution (10 mL and 4.5 pH) to collect the produced ammonia. The solution was then measured in a Pcompact photometric analyzer (Aqualytic), at a wavelength of 660 nm. EPOC Parameters. In the electrochemical promotion of catalysis, there is a correspondence between the different potentials applied and the amount of ions electrochemically transferred to the metal film. Several parameters are commonly used to quantify the EPOC effect: The rate enhancement ratio, ρ, defined by eq 1:
EXPERIMENTAL SECTION
Catalyst Preparation. The catalyst was prepared according to Hargreaves and McKay.22 The intermediate oxide was obtained through the coprecipitation method. An aqueous solution of cobalt nitrate (Co(NO3)2, Merck) was mixed with an aqueous solution of ammonium dimolybdate ((NH4)2Mo2O7, Alfa Aesar) in the proper molar ratio (Co/Mo = 1). The solution was heated under stirring until all the water was evaporated and a dark purple powder was received. Then, the powder was dried at 100 °C overnight and calcined in static air at 500 °C for 6 h to obtain the desired oxide. To obtain the Co3Mo3N nitride, the oxide powder was milled in a roll-mill for 24 h (HDPE vessel, YSZ balls of 5 mm diameter, ethanol) followed by high-temperature ammonolysis22 (100 mL·min−1, 99.99% NH3, Air Liquide Hellas). At room temperature, the flow was switched from ammonia to pure nitrogen (100 mL·min−1) for 1 h. The bulk oxidation was prevented by overnight treatment with 0.1% O2 in N2, prior to its exposure to the atmosphere. Cell Construction. The electrochemical single chamber cell (Figure 1) consisted of a 19 mm-diameter, 1 mm-thick K-β″-Al2O3 pellet (Ionotec) as the solid electrolyte (SE). In the electrochemical cell, the Au counter (CE) and reference (RE) electrodes were first deposited on the one side of the electrolyte by firing a gold
ρ = r /r0
(2)
where r is the electropromoted catalytic rate and r0 is the unpromoted rate. The apparent faradaic efficiency, Λ, defined by eq 3:
Λ i = Δrcatalytic/(I /F ) 8845
(3) DOI: 10.1021/acssuschemeng.7b01618 ACS Sustainable Chem. Eng. 2017, 5, 8844−8851
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ACS Sustainable Chemistry & Engineering where Δrcatalytic is the current- or potential-induced observed change in catalytic rate and I is the average current. In this work, the latter is calculated by dividing the total charge transported at each potensiostatic step by the duration of the step. For this reaction system, it implies: Λ NH3 = 3ΔrNH3/(I /F )
of EDX analysis (Figure 2b). The crystallography of the Co3Mo3N-Ag electrode was examined by XRD analysis (Figure 3). The main peaks observed correspond to the cubic phase of
(4)
where 3ΔrNH3 is given in mol/s of H+. The promotion index (PIK+), calculated by the following equation: Δrcatalytic r0
PIK+ =
θ K+
(5)
where θK is the potassium to catalyst molar ratio calculated from the integration of the current (I) versus time (t) curves according to Faraday’s law: +
θ K+ =
∫0
t
|I |d t nFNG
(6)
where n is the potassium ion charge, i.e., +1, F is the Faraday constant (96484.6 C/mol) and NG is the moles of Co3Mo3N.
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Figure 3. X-ray diffraction pattern of the Co3Mo3N-Ag electrode after exposure to reaction conditions for 24 h. (T = 500 °C, PH2/PN2 = 3.0).
RESULTS AND DISCUSSION Cell Characterization. SEM micrographs and EDX analysis of the Co3Mo3N-Ag catalyst-electrode are shown in Figure 2a,b,
Co3Mo3N (JCPDS No. 04-008-1301)23 and the cubic phase of metallic silver (JCPDS No. 87-0720). The average particle size for the Co3Mo3N catalyst on the K-β″-Al2O3 solid electrolyte was 103 nm, whereas that of Ag is 185 nm. These values were calculated according to the Scherrer method24 and are reasonably close to those observed in the SEM images of Figure 2. Electrochemical Promotion Studies. The electrochemical studies with the Co3Mo3N-Ag/β-Al2O3/Au cell were carried out at temperatures between 400 and 550 °C for PH2/ PN2 = 1.0, 3.0 and 6.0. First, the fresh sample was treated with reactant gas (PH2/PN2 = 3.0, T = 500 °C) for 6 h to eliminate oxygen compounds originating from the passivation step and to activate the catalyst surface.10 Moreover, prior to all studies, a constant cell voltage of UWR = 2.0 V was applied for 2 h in order to clean the catalyst surface from potassium ions. Then, a new lower voltage was applied and usually after 30−40 min a new steady-state was achieved and ammonia was measured again. Figure 4 shows the effect of the reaction temperature on the rate of NH3 formation (rNH3) under unpromoted (UWR = 2.0 V) and promoted conditions (UWR = 1.0 V). The Co3Mo3N-Ag electrode was very active for ammonia synthesis, with rates as high as 8 × 10−4 mol·g−1·h−1 observed at 500 °C. Under unpromoted conditions, i.e., UWR = 2.0 V, the optimum ratio for ammonia synthesis was PH2/PN2 = 1.0. This is not unusual because the ammonia rates, though high, are still far from those corresponding to the equilibrium conversions and thus the optimum feed ratio can be different from the stoichiometric. Upon application of a promoting potential (UWR = 1.0 V), the formation rates of ammonia are improved for ratios of PH2/PN2 = 1.0 and 3.0 and at temperatures higher than 450 °C. A maximum in rNH3 was observed at 500 °C both for the promoted and unpromoted catalyst-electrode. The latter observation is due to the fact that the reaction rate is controlled by kinetics up to 500 °C, but at higher temperatures the reverse reaction, i.e., NH3 decomposition, dominates, according to the Le Chatelier principle.25 The electrocatalytic activity of the working electrode was studied by varying the applied voltage from UWR = +2.0 to −0.4
Figure 2. SEM images (a) and elemental mapping (b) of the Co3Mo3N-Ag electrode after exposure to reaction conditions for 24 h. (T = 500 °C, PH2/PN2 = 3.0).
respectively. The images were obtained after exposing the sample to a PH2/PN2 = 3.0 mixture at 500 °C for 24 h. It can be seen that the electrode shows a relatively homogeneous structure with particle diameters between 70 and 150 nm. The electrode also has a high porosity, which facilitates the diffusion of the reactants and products. Moreover, a uniform distribution of the catalyst elements without significant particle agglomeration was verified by means 8846
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Figure 4. Effect of reaction temperature on the rate of NH3 synthesis under unpromoted (UWR = 2.0 V) and promoted (UWR = 1.0 V) conditions.
V. The effect of the cell voltage (bottom axis) and potassium to catalyst ratio (top axis) on the ammonia formation rate expressed in mol·g−1·h−1, is shown in Figure 5 for PH2/PN2 = 3.0
adsorption. Though the former is supported by the cyclic voltammograms of Figure 10, there is as yet no evidence that the latter does not also contribute to the observed decline in reaction rate. The volcano-type behavior for alkali addition has also been observed in catalytic studies of ammonia synthesis. Kojima et al.10,26,27 studied cobalt molybdenum bimetallic nitride catalysts in great detail and showed that the addition of up to 5 mol % of potassium led to a significant increase in the ammonia production.10 Higher quantities of impregnated potassium, however, led to surface structural changes detrimental to ammonia synthesis. Yunusov et al.12 also reported a volcanotype behavior when varying the alkali content of potassium carbonyl ruthenate catalysts. The authors demonstrated that the increased electron density on Ru atoms due to potassium addition was beneficial to dinitrogen activation.12
Figure 5. Effect of the cell voltage (bottom axis) and potassium to catalyst ratio (top axis) on the ammonia formation rate in μmol·h−1· g−1 for PH2/PN2 = 3.0.
and for temperatures between 400 and 500 °C. It can be seen that the catalyst exhibits a volcano-type behavior with the cell potential. The highest activity is achieved between 1.25 and 0.95 V vs RE, which correspond to a K+ to Co3Mo3N ratio between 0.01 and 0.015. In other words, by pumping a small amount of potassium ions to the Co3Mo3N-Ag electrode, its catalytic activity toward ammonia is significantly enhanced. Specifically, at 450 °C, rNH3 increased from 214 to 317 μmol· g−1·h−1 and at 500 °C it increased from 640 to 817 μmol·g−1· h−1. The observed increase is attributed to the decrease of the catalyst work function that weakens the chemical bond between the catalyst active sites and the electron donor adsorbate (H2) and at the same time strengthens the bond with electron acceptor (N2).17 However, when the cell voltage decreases further, a decrease in rNH3 is observed. There are two possible reasons for this “poisoning” effect: (a) a considerable fraction of the active surface area is occupied by the alkali species, which inhibits adsorption and/or reaction of the gaseous species, (b) “over-promotion” of the catalyst-electrode, i.e., an excessive strengthening of the N-catalyst site bond that then prevents H2
Figure 6. Dependence of rNH3 on the applied voltage and potassium to catalyst ratio at 450 °C, for various PH2/PN2 ratios.
Figure 6 shows the effect of the cell voltage (bottom axis) and potassium to Co3Mo3N (top axis) on rNH3 at 450 °C, for various PH2/PN2 feed ratios (PH2/PN2 = 1.0, 3.0 and 6.0). When the catalyst surface was clear of K+ (UWR = 2.0 V), the highest rate of 220 μmol·g−1·h−1 is observed for an equimolar feed composition. Under potassium pumping conditions, the maximum rate for ammonia synthesis increases to 317 μmol· 8847
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μmol·h−1·g−1 is obtained. The voltage is then decreased to 1.3 V and a substantial increase in NH3 formation is observed. After about 1 h, the voltage is again reduced and a further increase in rNH3 is observed. This continues until the applied potential reaches 1 V, where rNH3 = 317 μmol·h−1·g−1 (an increase of 48%), after which any further reduction in the voltage leads to lower ammonia rates. At a voltage somewhere between 0.4 and 0 V, the NH3 formation rate drops below the initial steady value of 214 μmol·h−1·g−1, and it becomes even lower at −0.4 V. However, when a potential of 2 V is once again applied to the cell the ammonia rate quickly returns its original value, confirming the reversibility of the process. EPOC Parameters. To quantify the EPOC effect in studies that were conducted at different conditions, the rate enhancement ratio (ρ), the promotion index (PIK+) and the apparent faradaic efficiency (ΛNH3) were evaluated. Figures 8 and 9 show the effect of applied potential (UWR) and potassium to catalyst ratio (θK+) on ρ, PIK+ and ΛNH3, at various temperatures (400−
g−1·h−1, but this maximum rate is attained at a feed ratio of PH2/ PN2 = 3.0. In general, the voltage at which the maximum ammonia rates are obtained is shifted to lower values (1.19, 0.98 and 0.82 V for PH2/PN2 = 1.0, 3.0 and 6.0, respectively) when the inlet H2 concentration increases. As was described above, by pumping potassium ions to the working electrode, the adsorption of the N2 molecule (electron acceptor) is enhanced at the expense of H2 (electron donor). Thus, on a surface where adsorbed hydrogen is the dominant species, more K+ must be pumped in order to remove these species from the surface (higher potassium to catalyst ratio, or more negative voltages) and facilitate the (dissociative) adsorption of N2. The effect of electrochemical promotion presented here can be compared to conventional catalytic alkali promotion. Kojima et al.10 improved their Co3Mo3N performance by 33% (from 650 to 869 μmol·h−1·g−1 at 400 °C for PH2/PN2 = 3.0) with an impregnation of 5 mol % of potassium in the nitride. In this work, the maximum formation rate obtained was slightly higher, reaching 915 μmol·h−1·g−1, and was achieved at 500 °C for PH2/ PN2 = 1.0, corresponding, however, to a rate enhancement of less than 20%. On the other hand, an up to 48% improvement of the formation rate was achieved at 450 °C where the unpromoted ammonia formation was quite lower (reached 317 from 214 μmol·h−1·g−1 at PH2/PN2 = 3.0). The main advantage of the electrochemical system, however, resides is the fact that the promoter addition can be controlled in situ and reversibly. Furthermore, the quantity of promoter can be adjusted depending on the conditions used. In the present work, the different conditions (temperature and PH2/PN2 ratio) strongly affected the amount of potassium needed to reach the highest values of ammonia production. The ability to actively control the percentage of promoter on the catalyst surface can be very important in decentralized applications where feed conditions may vary widely and the catalyst will have to be adapted to them. The response of the NH3 formation rate with time, for a typical experiment is presented in Figure 7. The data in this case were obtained at a temperature of 450 °C and under a stoichiometric hydrogen to nitrogen feed ratio (3.0/1.0). For the first 2 h, 2 V are applied across the cell to completely remove K+ from the working electrode, and a steady rNH3 of 214
Figure 8. Dependence of (a) the rate enhancement ratio (ρ), (b) the promotion index (PIK+) and (c) the apparent faradaic efficiency (ΛNH3) on the applied potential (UWR) and potassium to catalyst ratio (θK+) for PH2/PN2 = 3.0 at T = 400, 450 and 500 °C.
Figure 7. Dynamic response of rNH3 to step changes in the applied voltage, given at the top of the graph, at 450 °C and inlet PH2/PN2 = 3.0. 8848
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“poisoned” by the addition of excessive amounts of the promoting species. On the other hand, in studies such as this where the promoting ion does not take part in the electrode reaction Λ is a less meaningful parameter than PIK+.14,29,30 Although, in this work it has been defined somewhat differently and essentially represents the efficiency of the moles of K+ transported to the catalyst, in promoting NH3 formation. It also helps in the comparison with other works in electrochemical ammonia synthesis. A large number of papers on the electrochemical synthesis of ammonia have been published in the past two decades.2,5,7 Nevertheless, in only a few of them a gaseous mixture of hydrogen and nitrogen, rather than pure nitrogen, was introduced at the cathode. Therefore, very few studies focused on the EPOC effect either for the forward reaction of NH3 synthesis 17,18,31,32 or for the reverse reaction of NH 3 decomposition.33−35 Yiokari et al.17 used a single-chamber reactor and studied the reaction on a commercial Fe catalyst. Upon “pumping” H+ to the catalyst surface, the reaction rate could increase by as much as 1300% (ρ = 14). When H+ were removed from the catalyst, a complete, though reversible, deactivation of the catalyst was observed.17 A very weak effect (Λ < 3, ρ < 2) was observed by Marnellos et al.18 and by Ouzounidou et al.,32 who studied the reaction on Pd and Fe electrodes, respectively. It should be pointed out, however, that in the work of Yiokari et al., the conversions both with and without applying the voltage, were well below those predicted by thermodynamic equilibrium. An explanation for the weak NEMCA effect in reactions with limited conversion was proposed by Garagounis et al.19 The role of protons on equilibrium limited reactions is both, electrochemical and catalytic. The pumped protons not only modify the catalytic properties of the working electrode but also carry the electrical power required for ammonia synthesis.19 That is likely one of the reasons that makes the Λ values reported herein the highest found in literature for ammonia synthesis. The EPOC effect on catalytic decomposition of NH3 has been studied on Pd,18 Ru,34 Ag34,35 using proton (H+) conducting cells and on Fe using both, H + and K + conductors.33 On a Ag electrode, although Λ remained near unity, reaction rate enhancements (ρ) as high as 57 were achieved.35 Pitselis et al.33 studied the reaction using Fe films interfaced either with CaZr0.9In0.1O3−α (a H+ conductor) or with K 2 YZr(PO 4 ) 3 (K + conductor). The rate of NH 3 decomposition decreased significantly upon decreasing the catalyst potential, i.e., upon pumping H+ or K+ to the Fe surface. The effect of K+ was more pronounced than that of H+ and could result in a complete poisoning of the reaction.33 Cyclic Voltammetry. The results of the cyclic voltammetry study were obtained at 450 °C with different PH2/PN2 ratios and are presented in Figure 10. The cyclic voltammograms display cathodic peaks when the scan goes to lower potentials (UWR = +2.0 to −2.0 V) and anodic peaks when the scan goes to higher potentials (UWR = −2.0 to +2.0 V). It has been widely established36−41 that the cathodic peaks are related to species formed on the surface of the catalyst when the ions of the electroactive support migrate to the catalyst surface and interact with the various absorbed reactants. The anodic peaks are related to the decomposition of these species. Hence, in the present study, if the cathodic and anodic peaks have the same area, the formed compounds have undergone decomposition upon application of positive polarization. In other words, the
Figure 9. Effect of the applied potential (UWR) and potassium to catalyst ratio (θK+) on (a) the rate enhancement ratio (ρ), (b) the promotion index (PIK+) and (c) the apparent faradaic efficiency (ΛNH3) at T = 450 °C for PH2/PN2 = 1.0, 3.0 and 6.0.
500 °C) and reactant compositions (1 < PH2/PN2 < 6). The highest ρ value was 1.5 and it was obtained at 450 °C for PH2/ PN2 = 3.0. Under these conditions, the PIK+ values were around 60. The promotion index is positive for promoters and negative for poisons.28 In this sense, as was discussed above, when the potential applied decreases beyond the optimum, the potassium ions begin to act as a poison for the catalyst. The highest ΛNH3 was 310 and was obtained at 500 °C for a feed ratio of 1. Of these three parameters, the promotion index, PIK+, is the most representative in studies like this one, i.e., when a cationic promoter (K+) is used. The physicochemical meaning of this parameter is the increase in the reaction rate upon a differential increase in the surface coverage of the promoting species (in our case of the potassium to catalyst molar ratio).14 Figures 8 and 9 show that an increase in θK+ from 0 to about 0.01 has a positive effect on the reaction rate at which point a maximum rate is attained. When further increasing θK+, a decrease in the reaction rate is observed because the catalyst surface is 8849
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sites of the catalyst, leading to a decline in the ammonia formation rate, as observed in the electrochemical experiments.
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CONCLUSIONS The following conclusions can be drawn from this study: • The catalytic activity of nitride catalysts, such as Co3Mo3N was electrochemically enhanced in a K+ conducting cell. • The highest values of ammonia synthesis under unpromoted conditions (UWR = 2.0 V) were obtained, at PH2/PN2 = 1.0. The optimum feed ratio shifts to PH2/ PN2 = 3.0 under promoted conditions (UWR = 1.0 V). • The catalyst exhibited a volcano-type behavior with applied voltage, indicating that a small quantity of potassium ions improves the activity of Co3Mo3N-Ag toward ammonia synthesis. However, when the voltage decreases further and more K+ are pumped to the catalyst (working electrode), the rate of ammonia formation decreases due to the high surface concentration of potassium ions. The poisoning effect is attributed to blocking of active sites by K+ and the formation of K− N−H poisoning compounds at high potassium to catalyst ratios (lower voltages). • By pumping small amounts of potassium ions to the Co3Mo3N-Ag electrode the formation rate could be enhanced by as much as 48% at 450 °C. Moreover, the in situ control of the amount of potassium pumping can help in adapting the system to different operation parameters (temperature, feed ratio). • Faradaic efficiencies up to 300 were obtained, and they are the highest found in the literature for ammonia synthesis.
Figure 10. Cyclic voltammograms recorded at 450 °C for various PH2/ PN2 ratios.
applied potential of UWR = 2.0 V was high enough to electrochemically decompose or remove the surface promoter compounds, and that is why it was selected as a reference unpromoted state. The Λ values reported herein are the highest found in literature for ammonia synthesis. This can be largely attributed to the size of the conducting ion, as in reference 33, where, because a K+ cation is 30 times the size of a H+ ion, it was expected to have a much more pronounced EPOC effect. Another reason is that K+ do not take part in the reaction being studied.19 However, compared to other reactions studied in alkali conducting cells29,30 these values are far from impressive. One reason for this is the slight change in the definition of Λ, whereas another is the very nature of the reaction being studied.19 Another important reason, as evidenced by Figure 10 is the formation of compounds between the potassium promoter species and the adsorbed reactant species. In fact, the cathodic peaks begin at about 1.0 V, that is, the voltage at which the maximum NH3 formation rates were obtained. In other words, the formed compounds block the active sites on the catalyst surface and gradually negate the promotion effect observed at more positive voltages. The formation of these compounds also generates higher currents than would be observed if they were not formed, leading to reduced Λ values (eq 4). The areas of the peaks in the cyclic voltammograms depend on the PH2/PN2 ratio. The ratio that gives the highest anodic and cathodic area was 1.0 (50% H2−50% N2). An increase in the H2 feed concentration decreases the cathodic and anodic area, as does a higher inlet concentration of N2. This means that although the formation of K−N or K−H compounds is possible, the formation of potassium compounds of both reactants, i.e., K−N−H, is more likely. However, the identity of these compounds is purely speculative. According to previous studies, three K−N−H compounds are possible candidates: (i) K+ NH3 molecular complexes42 with a weak interaction between the potassium ion and the ammonia molecule, which could explain the decomposition at UWR = 2 V, (ii) K2NH2,42 which has a strong ionic character and is formed between two negatively charged lone pair electrons of a nitrogen atom and the positively charged potassium ions, and (iii) KNH2 compounds,43 which can be formed via the reaction: 1 K + NH3 → KNH 2 + H 2 (7) 2 These compounds, whose formation is obviously enhanced by the increased presence of K (or K+), gradually block the active
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AUTHOR INFORMATION
Corresponding Author
*M. Stoukides. Tel.: +30 2310 996165; Fax:+30 2310 996145; E-mail address:
[email protected]. ORCID
J. Díez-Ramírez: 0000-0002-5357-5006 M. Stoukides: 0000-0002-4360-4846 Notes
The authors declare no competing financial interest.
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REFERENCES
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