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Article

Effect of the Active Metal on the NOx Formation during Catalytic Combustion of Ammonia SOFC Off-Gas

by
Tobias Weissenberger
*,
Ralf Zapf
,
Helmut Pennemann
and
Gunther Kolb
Fraunhofer Institute for Microengineering and Microsystems IMM, Carl-Zeiss-Straße 18-20, 55129 Mainz, Germany
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1186; https://doi.org/10.3390/catal12101186
Submission received: 30 August 2022 / Revised: 26 September 2022 / Accepted: 28 September 2022 / Published: 7 October 2022
(This article belongs to the Special Issue Advanced Oxidation Catalysts)
Browse Figures
Versions Notes

Abstract

:
Catalytic combustion of hydrogen and ammonia containing off-gas surrogate from an ammonia solid oxide fuel cell (SOFC) was studied with a focus on nitrogen oxides (NOx) mitigation. Noble and transition metals (Pt, Pd, Ir, Ru, Rh, Cu, Fe, Ni) supported on Al2O3 were tested in the range of 100 to 800 °C. The tested catalysts were able to completely convert hydrogen and ammonia present in the off-gas. The selectivity to NOx increased with reaction temperature and stagnated at temperatures of 600 °C and higher. At low temperatures, the formation of N2O was evident, which declined with increasing temperature until no N2O was observed at temperatures exceeding 400 °C. Over nickel and iridium-based catalysts, the NOx formation was reduced drastically, especially at 300 to 400 °C. To the best knowledge of the authors, the current paper is the first study about catalytic combustion of hydrogen-ammonia mixtures as a surrogate of an ammonia-fed SOFC off-gas.

1. Introduction

The maritime transport of goods emits less CO2 per ton of cargo and km compared to other modes of transport. However, since a large part of international trade is transported by ship, the CO2 emissions caused by international maritime shipping are immense [1,2].
Thus, global shipping is responsible for about 3% of the global greenhouse gas emissions (GHG) [3]. To reduce the negative impact of the shipping industry, the International Maritime Organization has set the goal to reduce the global CO2 emission of the maritime shipping sector by at least 50%, compared to the emissions in the year 2008 [4].
To achieve this massive reduction in GHG emissions associated with shipping, environmentally friendly fuels and power systems must be introduced.
Fuel cells convert chemical energy directly into electrical energy, which results in a much higher efficiency compared to internal combustion engines [5]. The most common energy carrier used in fuels cells is hydrogen, which allows the operation of the fuel cell without emissions of CO2 or other pollutants. Therefore, fuel cells are a very promising technology for the green energy generation, especially in transport applications such as maritime shipping.
However, when used as transportation fuel, the low energy density and high flammability of hydrogen represents major challenges. To increase the energy density, hydrogen must be stored at very high pressures, often exceeding 300 bar, or in liquid state at cryogenic temperatures [6]. High pressure and cryogenic hydrogen storage come with much higher energy demand for compression and cooling, respectively. Further, the necessary high-pressure tanks and compressors as well as the cooling system increase the investment costs drastically.
An attractive alternative for the direct hydrogen storage is the use of hydrogen carriers, which can be handled and stored under atmospheric or near atmospheric pressures and ambient temperatures [7].
One promising hydrogen carrier is ammonia due to advantages in terms of energy density, flammability and its possible direct use in solid oxide fuel cells (SOFC) [8]. Ammonia can be liquefied at 20 °C at a moderate pressure of 8.6 bar [9,10]. The lower storage pressure makes the tanks for liquefied ammonia much cheaper than the high-pressure tanks necessary for storage of compressed hydrogen or the cryogenic equipment necessary for storage of liquefied hydrogen. Therefore, the cost of ammonia storage per kWh would be much cheaper compared to storage of pure hydrogen [11].
Another advantage of ammonia as hydrogen carrier is the possibility of its direct conversion in SOFCs without the requirement for cracking reactor. The ammonia is decomposed into hydrogen and nitrogen directly in the SOFC due to its high operation temperature and the nickel catalyst present. Therefore, ammonia-fed SOFCs have attracted much interest for energy generation [12,13,14].
Recently, the European Union awarded funding for the ShipFC project to convert the world’s first offshore vessel to run on ammonia-powered fuel cells under its Fuel Cells and Hydrogen Joint Undertaking (FCH JU) of the Research and Innovation program Horizon 2020. The ShipFC project consists of a consortium of 14 European companies and institutions and aims to install a 2 MW ammonia solid oxide fuel cell system on a shipping vessel to demonstrate that zero emission large-scale shipping is feasible. Thus, current SOFC systems will be scaled up to 2 MW and installed on the vessel Viking Energy in 2023 and operated for 3000 h during a one-year period. In addition to shipping, ammonia-powered SOFCs are an emerging technology for other mobile and stationary applications and can help to store and distribute energy form renewable sources such as wind and solar energy [15].
SOFCs and all other fuel cells cannot convert the fuel completely. Therefore, the fuel cell anode off-gas contains small quantities of unreacted hydrogen, even when a recycle of the off-gas back into the fuel cell is installed. The remaining hydrogen in the off-gas must be removed before releasing it to the atmosphere, which is commonly performed by combustion in a catalytic afterburner which also generates heat [14]. The heat generated in the afterburner can be used to preheat the ammonia and air feed before the fuel cell and excess heat can be used for heating on the vessel itself, increasing the overall efficiency of the SOFC system. In addition to hydrogen, the off-gas also contains traces of unconverted ammonia which must be removed in the catalytic afterburner too. Combustion of the ammonia traces would lead to relatively low NOx emissions, due to the low NH3 concentration in the feed. Nevertheless, selective catalytic oxidation of the ammonia to water and nitrogen is much preferred. Hence, the catalytic afterburner must be able to completely remove hydrogen and ammonia via combustion, while minimizing the formation of NOx species.
In the literature, different studies about the catalytic ammonia combustion can be found. The studies could demonstrate that different catalysts are effective for the selective catalytic oxidation (SCO) of ammonia into nitrogen and water rather than NOx, with nitrogen selectivities of up to 99% [16]. The tested catalysts include supported noble metals such as platinum [17], palladium [18], iridium [19], ruthenium [20], rhodium [21], silver [22] and gold [23]. Platinum-based catalysts showed relative low nitrogen selectivities while rhodium, palladium and silver reached selectivities of up to 97%. Another class of highly active NH3 SCO catalysts are transition metals, e.g., iron [24], copper [25,26], cobalt [27], nickel [28], manganese [29], molybdenum [30] and vanadium [31]. Copper-based catalysts showed by far the highest nitrogen selectivities of the transition metal-based catalysts, converting up to 99% of the ammonia into nitrogen [32]. Bimetallic catalysts based on Pt/Rh and Pt/Pd have also been studied and have proven to be effective NH3 SCO catalysts [33,34]. Further, catalysts based on mixed oxides such as Cu-Ce-Zn, Fe-Mg-Al or Cu-Mg-Al have also been studied for NH3 SCO [35,36,37]. Common support materials for NH3 SCO catalysts are Al2O3 [38], SiO2 [25], TiO2 [39], Nb2O5 [40], and zeolites such as CHA and ZSM-5 [41,42,43].
However, all of the NH3 SCO studies found in the literature are dedicated to catalytic combustion of ammonia. No reports can be found about the catalytic combustion of hydrogen ammonia mixtures or anything reassembling an ammonia SOFC off-gas. Thus, the results might not apply to the SOFC off-gas combustion.
Here, we report investigations about the combustion of ammonia SOFC off-gas surrogate over different catalysts with focus on mitigating NOx formation. Microstructured reactors coated with different supported noble and transition metal catalysts were used for the off-gas combustion and their performance in terms of NOx formation was evaluated and compared at different temperatures.

2. Results

2.1. Catalyst Characterisation

The metal-loadings of the calcined catalysts are close to the values targeted during preparation, as it gets evident from the XRF data summarized in Table 1. Only the rhodium and ruthenium-based catalysts contain higher amounts of the noble metals than the desired loading of 5 wt.-% active metal, with 5.79 and 6.50 wt.-%, respectively.
The XRD pattern (see Figures S1 and S2 in the supporting information) reveals no differences in the lines observed for alumina, thus it is evident that the aluminium support was not altered by the impregnation with the corresponding metals. The metal phases display distinctive diffraction lines in the XRD patterns. The identification of the metal phases according to the XRD pattern reveal that other that platinum, which is in elemental state, all other metals are present as oxides (palladium oxide PdO, rhodium oxide Rh2O3, ruthenium oxide RuO2, iridium oxide IrO2, nickel oxide (bunsenite) NiO, iron oxide (hematite) Fe2O3 and copper oxide (tenorite) CuO.
The nitrogen sorption isotherms of the different catalysts and the pure support Al2O3, shown in Figure S5 in the supporting information, are of IUPAC type II typical for mesoporous materials. The metal deposition did not alter the isotherms and thus did not change the textural properties significantly. This is also evident in the BET surface areas (see Supporting Information Table S1) of 147 to 154 m2 g−1, which are slightly lower than the surface area of the pure Al2O3 support of 156 m2·g−1. The small reduction in surface area is likely caused by the lower specific surface area of the added metals. The surface area of the copper sample is further reduced to 140 m2·g−1 due to higher metal content.
The transmission electron microscopy (TEM) micrographs are shown in Figure 1 (higher magnification micrographs can be found in Figure S3 in the supporting information). The TEM images of all samples display amorphous Al2O3 particles as well as rod shaped Al2O3 crystallites. The metal phases as well as support material were identified using selected area electron diffraction (SAED). The SAED pattern and TEM images with the interplanar distances are shown in Figure S4 in the supporting information. Pt and Rh particles were determined to be in metallic state while all other metals are present as oxides (PdO, RuO2, IrO2, NiO, Fe2O3 and CuO). These results are in good accordance to the XRD data. Only exemption is rhodium which was identified as Rh2O3 by XRD and metallic rhodium by TEM.
In case of Pt, Pd, Rh, Ru and Ir-based catalysts, the particles of the active metal phase are well dispersed over the support and can be seen easily due to their high molecular mass and thus high contrast. The average crystallite diameters can be found in Table 1. The CuO particles are hard to see and distinguish from the Al2O3 support, which made the measurement of a particle size distribution impractical. For the Ni and Fe-based catalysts, only a small numbers of large particles of the active phase could be seen on the TEM images.

2.2. Catalytic Tests

The evaluation of the catalytic activity was carried out under identical conditions applying a surrogate of ammonia SOFC off-gas with a composition as shown in Table 2. For the presented studies, the off-gas surrogate composition was chosen according to simulation results. For the simulation, the presumption was made that the off-gas will not be recycled in the SOFC, thus still contains a relative high concentration of hydrogen. Since the off-gas is not recycled, no separation of the water in the off-gas by condensation is necessary. Hence, the water content in the off-gas surrogate is very high. The model off-gas was mixed with air using an air-to-fuel ratio λ = 4 and then fed into the reactor.

2.2.1. Noble Metal Catalysts

The noble metals platinum, palladium, rhodium, ruthenium and iridium supported on Al2O3 were tested as catalysts for the ammonia SOFC off-gas combustion. The hydrogen and ammonia concentrations as observed for different reaction temperatures are shown in Figure 2. Differences in low temperature activity of the catalysts tested are obvious.
For all tested noble metal-based catalysts, the hydrogen and ammonia conversions increase with increasing reaction temperature and reach complete conversion at about 400 °C latest. The lowest light off temperatures for hydrogen and ammonia are evident for the platinum catalyst, with full conversion of hydrogen at 150 °C and ammonia at 200 °C, respectively. The palladium-based catalyst shows a slightly reduced low temperature activity, reaching full conversion of hydrogen at 200 °C and ammonia 400 °C. The rhodium and iridium-based catalysts display comparable low temperature activity and reach full conversion of hydrogen and ammonia at 400 °C. For the ruthenium-based catalyst, the lowest activity can be observed, with complete ammonia conversion at 400 °C and complete hydrogen conversion at 500 °C, respectively. Thus, the observed light off temperatures for hydrogen and ammonia increase in the order Pt < Pd < Rh < Ir < Ru.
The concentrations of the nitrogen oxides (nitric oxide NO and nitrogen dioxide NO2), shown in Figure 3a, reveal that ammonia is oxidized and forms nitrogen oxides (NOx). Generally, once the reaction temperature is high enough for catalytic ammonia combustion, NOx can be detected at the reactor outlet. Therefore, at low temperatures the tested catalysts show first NOx formation according to their activity for ammonia combustion. With increasing reaction temperature, the observed NOx concentrations increase and then stagnate at reaction temperatures exceeding 500 °C for all tested noble metal catalysts. At high temperatures in the range of 700 °C–800 °C, the NOx concentrations increase in the order Ru < Rh < Ir < Pd < Pt.
The different noble metal catalysts display differences in NOX selectivity as summarized in Figure 3b. Interestingly, the measured NOx concentrations at high temperatures exceed the ammonia concentration present in the off-gas feed of the reactor resulting in NOx selectivities exceeding 100% if referred only to the NH3 present in the feed. This indicates that the ammonia is completely converted into NOx and a small fraction of the nitrogen is oxidized to NOx as well, resulting in formation of additional NOx. Especially the platinum catalyst displays a high additional NOx formation, giving rise to a NOx selectivity of about 170%. For the other noble metal catalysts high temperature NOx selectivities between 115% and 140% were observed.
At lower temperatures, the observed NOX molar flows are often lower than the converted ammonia resulting in NOX selectivities lower than 100%. Therefore, it can be assumed that at lower reaction temperatures a certain fraction of the ammonia is not converted into NOx, but very likely into nitrogen. The selectivities to NOx at 300 and 400 °C increase in order Ir<Ru<Rh<Pd<Pt. The lowest NOx selectivity at full ammonia conversion of the tested noble metal catalysts was observed for the iridium catalyst with a NOx selectivity of 63% at 400 °C.
The formation of nitrous oxide (N2O) shows a different behaviour compared to the NOx formation (see Figure 4). The N2O concentrations have a maxima at 150 or 200 °C and with further increase in reaction temperature the N2O concentration decreases. At 500 °C almost no N2O is detectable at the reactor outlet for all tested noble metal catalysts.
The highest N2O concentrations are evident for the platinum catalyst, followed by palladium and rhodium. For the ruthenium and iridium catalysts almost no N2O formation is observable, which can be traced back to the low activity of these catalysts at temperatures below 300 °C.
The selectivity to N2O (see Figure 4b) follows the same trend as the N2O concentrations, with exception of the palladium catalyst which shows an increased N2O selectivity at 150 °C. The highest observed N2O selectivity is 17% for the platinum catalyst. For all other noble metal catalyst N2O selectivities below 10% can be observed.
Among the tested noble metals, iridium displays the most promising catalytic properties for the off-gas combustion. At a temperature of 400 °C, the iridium catalyst obtains complete hydrogen and ammonia conversion, low N2O formation and comparatively low selectivity to NOx.

2.2.2. Transition Metal Catalysts

In addition to noble metals, the transition metals iron, nickel and copper supported on alumina were tested as catalysts for the SOFC off-gas combustion and compared to the platinum catalyst. Both hydrogen and ammonia conversion follow the same trend as observed for the noble metal-based catalysts (see Figure 5). The differences in light off temperatures, however, are more pronounced for the transition metal catalysts. Again, with increasing reaction temperature both hydrogen and ammonia conversion increase for all tested transition metal-based catalysts. The nickel-based catalyst displays the highest activity of the tested transition metal catalyst, with complete hydrogen and ammonia conversion at 400 °C. The iron-based catalyst is slightly less active with full hydrogen and ammonia conversion at 400 °C and 600 °C, respectively. The copper-based catalyst shows a very low activity and thus high light off temperatures for both hydrogen and ammonia, despite its higher active metal loading of 10% compared to 5% for all other tested metals. To reach complete hydrogen and ammonia conversion over the copper-based catalyst a reaction temperature of about 600 °C is necessary.
The NOx concentrations observed for the tested transition metal catalysts are much lower compared to the platinum catalyst (see Figure 6a). The trend however is the same as for the noble metal catalysts. Once the reaction temperature is sufficiently high for catalytic ammonia combustion, NOx can be detected at the reactor outlet. The NOx concentrations and selectivities observed for the transition metal-based catalysts increase with reaction temperature and then stagnate at temperatures of 500 °C and above.
The nickel and iron-based catalysts display similar NOx concentrations. However, since nickel is more active at low temperatures compared to iron, the nickel catalyst has a lower selectivity to NOx below 500 °C as summarised in Figure 6b. The copper-based catalyst shows fairly low NOx concentrations too, but the NOx selectivity of the copper catalyst is higher than observed for iron and nickel. Again, at higher temperatures the NOx concentrations surpass the ammonia concentration present in the feed gas and thus the NOx selectivities are larger than 100%. This is a clear indication that at high temperatures NOx is formed by ammonia combustion as well as by oxidation of nitrogen present in the off-gas.
Different studies have reported that copper oxides are good catalysts for ammonia combustion in terms of low NOx selectivity. In our study the copper catalyst showed a much higher selectivity to NOx compared to literature. However, the literature results were obtained for combustion in the absence of hydrogen. The presence of hydrogen could for example alter the oxidation state of copper in the catalyst which was proven to play an important role [26].
The formation of N2O follows the same trend as observed for the noble metal catalysts (see Figure 7). At low temperatures the concentrations and selectivities increase with increasing ammonia conversions and reach a maximum at 200 °C to 300 °C. With further temperature increase the N2O formation decreases and no N2O can be overserved at temperatures exceeding 500 °C for all tested transition metal-based catalysts.
The nickel-based catalyst shows lower N2O concentrations and selectivities compared to the platinum-based catalyst despite its relative high activity at 200 and 300 °C. At 400 °C no N2O was observed for the nickel catalyst. The iron catalyst displays even lower N2O concentrations but also lower activity at low temperatures resulting in slightly higher N2O selectivity at 300 and 400 °C. No N2O can be observed for the copper catalyst, likely because it is almost inactive at temperatures below 400 °C, thus preventing N2O formation as well.
Under the tested transitions of metal-based catalysts, the nickel-based catalyst displays the best performance. At a temperature of 300 °C, the nickel catalyst shows complete hydrogen and ammonia conversion with a rather low NOx selectivity of 51%.

2.2.3. Catalyst Stability

Since the catalyst will be operated for prolonged times, catalytic stability was tested. A platinum catalyst was chosen for the experiments. The reaction was carried out at 800 °C with frequent cold starts from 100 to 800 °C.
The hydrogen conversion over time on stream in Figure 8a reveals that the catalyst fully converts the hydrogen even after prolonged reaction time at 800 °C. However, the hydrogen conversion during the cold start experiments (see Figure 8b) reveals catalyst deactivation. Initially during the first 24 h of reaction the catalyst loses activity as evident in the reduced hydrogen conversion at lower temperatures compared to the fresh catalyst.
With prolonged time on stream, no further reduction in hydrogen conversions can be observed. This indicated that the catalyst deactivation has stopped after the initial deactivation. Due to the observed catalyst deactivation the light-off temperature for hydrogen increases from below 100 °C to about 270 °C. Hence the preheating temperature necessary to start up the afterburner would be around 300 °C, which is still feasible.

3. Discussion

The light off temperatures for all tested catalysts, calculated at a 50% conversion level, for hydrogen and ammonia are summarized in Table 3. Despite the different homogenous light off temperatures for ammonia and hydrogen, the differences observed between hydrogen and ammonia light off are relatively small over all tested catalysts.
Overall, platinum, nickel and palladium show low light-off temperatures, below 200 °C. For iridium, rhodium, iron and ruthenium the light–off temperatures are around 200 to 300 °C. For the copper-based catalyst the highest light-off temperature exceeding 400 °C can be observed, despite its higher loading of active metal on the catalyst support.
The selectivity to NOx at 600 °C is close to the maximum, since the NOx formation usually reaches a plateau at 600 °C. Here, the selectivity increases in the order Rh < Cu < Ni < Fe < Ru < Ir < Pd < Pt. The lowest selectivity to NOx at full ammonia conversion (XNH3 > 98%) also corresponds to the lowest temperature necessary for full ammonia conversion since the NOx selectivities increased with reaction temperature. The lowest value can be found for nickel and increases in the order Ni < Ir < Pt < Ru < Cu < Rh < Fe < Pd.

Comparison to Internal Combustion Engines

Compared to conventional internal combustion engines used for shipping, the presented NOx emissions of the SOFC system are much lower.
NOx emissions of maritime diesel engines are limited by regulation 13 of the MARPOL Annex VI of the International Maritime Organization (IMO). The current Tier II regulations set the NOx limit at 14.4 gNOx/kWh for slow running engines with rotational speeds of under 130 rpm and 7.7 gNOx/kWh for engines running at over 2000 rpm.
The 2016 Tier III regulations by the IMO set more stringent NOx limits in defined Emission Control Areas. In this areas, which include the Baltic and northern sea, the NOx emission limits are 3.4 and 1.96 gNOx/kWh, for engines operating at <130 and >2000 revolutions per minute, respectively. NOx emission according to Tier II can be archived by modern diesel engines, Tier III however makes a deNOx process such as ammonia-SCR necessary.
The presented system of NH3 SOFC and afterburner emits 0.18 g NOx per kWh (electric) in the worst case of complete NH3 conversion to NO2 at full load. Thus, the SOFC system only emits 9% of the NOx emitted by Tier III conventional diesel engines. For the iridium and nickel-based catalysts developed in this work, values of about 5% of the specific NOx emission of Tier III diesel engines can be obtained. The NOx emission of the SOFC system were calculated at full load. Emissions are measured according to ISO 8178 at different loads which would lower the emissions even further.
Direct use of ammonia in internal combustion engines is possible too. Ammonia is partially split to hydrogen, since pure ammonia is not suitable for use in internal combustion engines. The ammonia/hydrogen mixture can be combusted at lower temperatures compared to diesel engines (CO2 formation) and thus NOx formation can be reduced. Studies have shown that engines fuelled by hydrogen ammonia mixtures can achieve NOx concentrations as low as 1−2 g NOx per kWh depending of the ammonia concentration in the feed gas, which is still at least 10 times higher than the NOx emissions of the NH3 SOFC system.
Additionally, the tested catalysts were able to obtain complete conversion of ammonia at feasible operation temperatures for the afterburner, thus preventing ammonia slip from the SOFC system which can be a problem for ammonia-fuelled internal combustion engines.

4. Materials and Methods

4.1. Catalyst Preparation and Reactor Assembly

The investigations were carried out using microreactors containing microchannels coated with catalyst.
The reactors consisted of two plates with 14 channels each. The channels are 500 μm in width, 250 μm deep and 25 mm long. Once the two plates were welded together, channels with a depth of 500 µm were obtained.
The preparation and assembly consisted of three main steps: preparation of the catalyst powder, wash coating of the catalyst in the microchannels and finally assembly of the reactor via laser welding.
Catalyst powder preparation: The catalyst powders were prepared by impregnation of the Al2O3 support (Puralox, Sasol) with the calculated amount of an aqueous solution containing the corresponding metal precursor to archive the desired catalyst composition. After impregnation the samples were calcined at 450 °C for 6 h and the powders were milled and used for the wash coating. The metal precursors used were: Pt(NH3)4(NO3)2 (Alfa Aesar), Ni(NO3)2·6H2O (Sigma-Aldrich), Rh(NO3)3 solution (chemPUR), IrCl3·xH2O (Alfa Aesar), Fe(NO3)3·9H2O (Sigma-Aldrich), Cu(NO3)2·3H2O (Honeywell / Fluka), Pd(NO3)2 solution (Sigma-Aldrich), Ru(NO)(NO3)3 (Alfa Aesar).
Wash coating: The microchannels were coated by using a suspension containing the catalyst powder, polyvinyl alcohol as binder, acetic acid and deionised water. The polyvinyl alcohol was dissolved in water at 65 °C for 3 h before 1 wt.-% acetic acid and the catalyst powder were added. The suspension was then stirred at 65 °C for additional 3 h followed by stirring at room temperature for 2–3 days to obtain a homogeneous suspension.
The microchannels were filled with the suspension and excess suspension was removed using a blade. Then the plates were dried at room temperature and calcined at 450 °C. The wash coating process was described in greater detail in previous publications [44].
Reactor assembly: The coated plates as well as the inlet and outlet capillaries were assembled via laser welding to form the microreactor. More details can be found in a previous publication [45,46].

4.2. Catalytic Testing

The catalyst coated microstructured reactors were placed in a steal heating block equipped with two heating cartridges and thermocouples and connected to the test rig.
The gas mixture consisting of nitrogen, hydrogen and 250 ppm ammonia was mixed with air and steam before it was fed into the reactor. The gases were dosed via Bronkhorst mass flow controllers. Steam was delivered by dosing liquid water using a Bronkhorst Cori-flow into an evaporator. All lines were heated to 180 °C to avoid condensation.
Ammonia concentrations were measured using a MKS FTIR spectrometer. The hydrogen concentration was measured using an Agilent µGC.
To obtain a weight hourly space velocity (WHSV) of 600 L/g h a catalyst mass of 20 to 25 mg was used and the flow rate adjusted accordingly. For example, for a catalyst mass of 20 mg the flow rates were set to 120.1 mL/min air, 31.98 mL/min off-gas surrogate and 2.31 g/h water, resulting in a total flow rate of 200 mL/min.

4.3. Catalyst Characterization

X-ray diffraction (XRD): X-ray diffraction measurements were carried out using a D8-Advance diffractometer (Bruker) equipped with Cu-Kα radiation source and LYNXEYE XT-E detector in Bragg-Brentano geometry from 5–90° 2Theta with a step size of 0.02°.
X-ray fluorescence (XRF): The elemental composition of the samples was measured by XRF spectroscopy on an ED-XRF spectrometer model 1510 (Canberra Packard, USA). As radiation source Cd-109 (22 keV) and Am-241 (60 keV) were used.
Transmission electron microscopy (TEM): the transmission electron microscopy images were taken on a Zeiss Libra 120 instrument operating at an accelerating voltage of 120 kV. Before the measurements, the catalyst samples were suspended in ethanol, dropped onto the TEM grids and dried.
Nitrogen physisorption: nitrogen sorption experiments were carried out at −273 °C using an Anton Parr Autosorb iQ. The samples were degassed for 12 h at 250 °C under vacuum before measurement.

5. Conclusions

The present study shows that it is possible to use an afterburner catalyst to completely combust both hydrogen and ammonia present in an ammonia SOFC off-gas stream. However, all tested catalysts oxidized at least parts of the ammonia to nitrogen oxides. The selectivity to NOx increases with reaction temperature and reaches a plateau at about 600 °C. The formation of N2O increases initially with increasing reaction temperature and reaches a maximum at about 200 to 300 °C. With further temperature increases, the N2O formation decreases and no N2O is detectable at temperatures between 400 and 500 °C, depending on the catalyst.
Compared to other studies reported in the literature, much higher NOx selectivities were observed for the catalysts tested in this study. However, the studies in literature were dedicated to ammonia combustion without any other fuel present in the feed gas. The relative high concentration of hydrogen in the SOFC off-gas clearly alters the nitrogen selectivities of the catalysts.
Another difference is the WHSV used for the catalytic tests. The afterburner for the Ship-FC project needs to be cost effective and relatively small and light for use onboard a shipping vessel. Thus, the used WHSV of 600 L/gcat h is high compared to the WHSV reported in literature.
Nevertheless, by selecting a suitable catalyst it is feasible to reduce the NOX formation drastically, especially at low temperatures. Nevertheless, a complete mitigation of NOX formation was not found possible using the presented catalyst systems.
Very promising candidates are nickel and iridium-based catalysts. The nickel-based catalyst shows the lowest NOx selectivity in the low temperature range, as well as good cold start performance. However, operating the afterburner at low temperatures results in the formation of small quantities of N2O. The observed N2O concentration at 300 °C is below 2 ppm, and could be reduced by further catalyst optimization. For complete N2O mitigation, iridium is the most promising active metal tested in this study.
The iridium-based catalyst has the smallest NOx selectivities at 400 and 500 °C of all catalysts tested in this work. The selectivity to NOx at 400 °C is higher compared to the nickel-based catalyst at 300 °C, but due to higher reaction temperature, the N2O formation is suppressed completely. One drawback is the reduced cold start performance of iridium, with higher light off temperatures for both hydrogen and ammonia, compared to nickel, palladium and platinum-based catalysts.
In future, further reduction in NOx emissions could be achieved by reducing the ammonia concentration in the off-gas, by further advancements in catalyst development or by removal of the formed NOx. Studies to determine the effect of different catalyst supports, other active metals and bi-metallic catalysts could be beneficial to further reduce the NOx formation in the afterburner. For the off-gas cleaning, well-established processes such as catalytic selective reduction (SCR) of NOx to N2 with ammonia are available and could be implemented to lower NOx emissions further.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12101186/s1, Figure S1: Powder XRD pattern of noble metal-based catalysts; Figure S2: Powder XRD pattern of transition metal-based catalysts; Figure S3: TEM micrographs of the used catalysts with two different magnifications; Figure S4: TEM micrographs with interplanar distances and selected area electron beam diffraction images used for identification, Figure S5: Nitrogen sorption isotherm of Al2O3 support; Figure S6: Nitrogen sorption isotherms of the used catalysts, Figure S7: Particle size distribution of different catalysts determined by TEM, Table S1: Specific surface area (BET method) of the used catalysts.

Author Contributions

Conceptualization, G.K., H.P. and T.W.; investigation, T.W and R.Z.; writing—original draft preparation, T.W.; writing—review and editing, T.W., R.Z., H.P. and G.K.; visualization, T.W.; supervision, G.K.; project administration, H.P. and G.K.; funding acquisition, G.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the ShipFC project which has received funding from the Fuel Cells and Hydrogen Joint Undertaking under grant agreement No 875156. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation program and from Hydrogen Europe.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01186 g001 550
Figure 1. TEM micrographs of the different catalysts.
Figure 1. TEM micrographs of the different catalysts.
Catalysts 12 01186 g001
Catalysts 12 01186 g002 550
Figure 2. (a) Hydrogen and (b) ammonia conversion vs. reaction temperature for different noble metal catalysts, WHSV = 600 L/g h.
Figure 2. (a) Hydrogen and (b) ammonia conversion vs. reaction temperature for different noble metal catalysts, WHSV = 600 L/g h.
Catalysts 12 01186 g002
Catalysts 12 01186 g003 550
Figure 3. (a) NOX concentration and (b) NOX selectivities vs. reaction temperature for different noble metal catalysts, WHSV = 600 L/g h.
Figure 3. (a) NOX concentration and (b) NOX selectivities vs. reaction temperature for different noble metal catalysts, WHSV = 600 L/g h.
Catalysts 12 01186 g003
Catalysts 12 01186 g004 550
Figure 4. (a) N2O concentration and (b) N2O selectivities over reaction temperature for different noble metal catalysts, WHSV = 600 L/g h.
Figure 4. (a) N2O concentration and (b) N2O selectivities over reaction temperature for different noble metal catalysts, WHSV = 600 L/g h.
Catalysts 12 01186 g004
Catalysts 12 01186 g005 550
Figure 5. (a) Hydrogen and (b) ammonia conversion vs. reaction temperature for different transition metal catalysts, the platinum-based catalyst is shown for comparison, WHSV = 600 L/g h.
Figure 5. (a) Hydrogen and (b) ammonia conversion vs. reaction temperature for different transition metal catalysts, the platinum-based catalyst is shown for comparison, WHSV = 600 L/g h.
Catalysts 12 01186 g005
Catalysts 12 01186 g006 550
Figure 6. (a) NOX concentration and (b) NOX selectivity vs. reaction temperature for different transition metal catalysts, platinum-based catalyst shown for comparison, WHSV = 600 L/g h.
Figure 6. (a) NOX concentration and (b) NOX selectivity vs. reaction temperature for different transition metal catalysts, platinum-based catalyst shown for comparison, WHSV = 600 L/g h.
Catalysts 12 01186 g006
Catalysts 12 01186 g007 550
Figure 7. (a) N2O concentration and (b) N2O selectivity over reaction temperature for different transition metal catalysts, platinum-based catalyst shown for comparison, WHSV = 600 L/g h.
Figure 7. (a) N2O concentration and (b) N2O selectivity over reaction temperature for different transition metal catalysts, platinum-based catalyst shown for comparison, WHSV = 600 L/g h.
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Catalysts 12 01186 g008 550
Figure 8. (a) Hydrogen conversion over time on stream with frequent cold start experiments and (b) hydrogen conversion over temperature for the corresponding cold start experiments after different times, catalyst 5% Pt/Al2O3, TReactor = 800 °C (cold starts from 100 to 800 °C), WHSV = 600 L/g h.
Figure 8. (a) Hydrogen conversion over time on stream with frequent cold start experiments and (b) hydrogen conversion over temperature for the corresponding cold start experiments after different times, catalyst 5% Pt/Al2O3, TReactor = 800 °C (cold starts from 100 to 800 °C), WHSV = 600 L/g h.
Catalysts 12 01186 g008
Table
Table 1. Characterization results (XRD and TEM) for the prepared catalysts.
Table 1. Characterization results (XRD and TEM) for the prepared catalysts.
CatalystMetal Loading (XRF)/wt.-%Particle Size (TEM)/nm
Pt/Al2O35.282.35
Pd/Al2O34.904.30
Rh/Al2O35.791.02
Ru/Al2O36.5050.3
Ir/Al2O34.803.72
Ni/Al2O35.01>30 nm
Fe/Al2O34.87>25 nm
Cu/Al2O310.35not measured
Table
Table 2. Composition of the model off-gas surrogate and feed gas used for the catalytic tests.
Table 2. Composition of the model off-gas surrogate and feed gas used for the catalytic tests.
ComponentSOFC Off-Gas SurrogateReactor Feed
N225%56.8%
H215%5.9%
NH3100 ppm35.8 ppm
H2O60%23.9%
O2-13.2%
Table
Table 3. Comparison of the catalytic performance of different metal-based catalysts tested in this study.
Table 3. Comparison of the catalytic performance of different metal-based catalysts tested in this study.
CatalystH2 Light-Off
TX=50%		NH3 Light-Off
TX=50%		Lowest SNOx at XNH3> 98%S NOx at 600 °C
Pt/Al2O3<10012581.1 (200 °C)160.3
Pd/Al2O3178214117.2 (400 °C)135.9
Rh/Al2O3243244113.6 (400 °C)102.9
Ru/Al2O330326690.2 (400 °C)116.4
Ir/Al2O326727563.1 (400 °C)120.3
Ni/Al2O318317751.4 (300 °C)110.7
Fe/Al2O3257254115.0 (600 °C)115.0
Cu/Al2O3412419109.6 (600 °C)109.6
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Weissenberger, T.; Zapf, R.; Pennemann, H.; Kolb, G. Effect of the Active Metal on the NOx Formation during Catalytic Combustion of Ammonia SOFC Off-Gas. Catalysts 2022, 12, 1186. https://doi.org/10.3390/catal12101186

AMA Style

Weissenberger T, Zapf R, Pennemann H, Kolb G. Effect of the Active Metal on the NOx Formation during Catalytic Combustion of Ammonia SOFC Off-Gas. Catalysts. 2022; 12(10):1186. https://doi.org/10.3390/catal12101186

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Weissenberger, Tobias, Ralf Zapf, Helmut Pennemann, and Gunther Kolb. 2022. "Effect of the Active Metal on the NOx Formation during Catalytic Combustion of Ammonia SOFC Off-Gas" Catalysts 12, no. 10: 1186. https://doi.org/10.3390/catal12101186

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