Effect of the Catalyst Support on the NOX Formation During Combustion of NH3 SOFC Off-Gas

Abstract

Ammonia has attracted much interest as a potential green and renewable hydrogen carrier or energy vector. Compared to hydrogen, ammonia offers several advantages. For example, ammonia has a significantly higher energy density and can be liquefied at room temperature at a moderate pressure of 8 bars. While ammonia can be cracked to supply hydrogen, it is also possible to convert it directly into high-temperature solid oxide fuel cells (SOFCs) to generate electricity. The Ship-FC project aims to install an ammonia-fed 2MW SOFC system on board the vessel Viking energy to demonstrate the feasibility of zero CO₂ emission shipping. For this NH₃ SOFC system, a catalytic afterburner is required to remove the hydrogen and ammonia present in the SOFC off-gas and to recover heat. The current study analysed the effects of different catalyst supports, with a focus on NO_X formation through the combustion of an SOFC off-gas surrogate. The study investigated the performance of catalysts based on the active metals, platinum and iridium, as well as the catalyst supports, Al₂O₃, SiO₂, and TiO₂. The results were correlated with catalyst characterisation data and ammonia TPD results. The investigations showed that the formation of NO_X was clearly affected by the nature of the catalyst support. The highest selectivity towards NO_X was observed for Al₂O₃, followed by SiO₂, and the lowest selectivity was observed for TiO₂. This trend was evident for the supported platinum and iridium catalysts and for the samples exclusively containing the support. The trend for N₂O formation was opposite to that of NO_X formation (TiO₂ > SiO₂ > Al₂O₃) in both the presence and absence of platinum or iridium.

1. Introduction

Currently, ammonia cracking and, in general, ammonia as an energy or hydrogen carrier has gained much interest. However, the hydrogen clean-up required for ammonia cracking with NH₃-fed SOFCs produces ammonia-containing off-gasses. Combustion of these gas mixtures results in the formation and emission of NO_X, negatively affecting the environmental footprint. Therefore, reducing the NO_X formation originating from the catalytic combustion of ammonia-containing off-gasses is mandatory to reduce the environmental impact of these systems.

The Ship-FC project, funded by the Fuel Cells and Hydrogen Joint Undertaking (FCH JU) of the Research and Innovation program Horizon 2020, is aimed at installing a 2 MW ammonia-powered fuel cell system on board the off-shore vessel Viking Energy by 2023 [1]. The operation of the ship with electricity generated by the fuel cell system for 3000 h will prove that shipping without CO₂ emissions is feasible.

The CO₂ emissions of maritime shipping per ton of cargo and kilometre are rather low compared to other modes of transport, but due to the sheer scale of international shipping, the resulting emissions are immense [2,3]. Shipping currently accounts for 3% of the world’s annual greenhouse gas emissions [4].

The Ship-FC system will rely on solid oxide fuels cells which are fuelled by ammonia and therefore do not produce any CO₂ emissions. The ammonia cracking to hydrogen and nitrogen occurs directly in the SOFC due to the high operation temperature and the nickel-based catalyst and, consequently, no additional cracking reactor is necessary [5]. Therefore, ammonia SOFCs have great potential for energy generation [6,7,8]. The fuel cell converts the chemical energy of the in situ generated hydrogen and oxygen from the air directly into electrical energy. Thus, the SOFC system has a much higher efficiency compared to internal combustion engines [9].

Compared to hydrogen, ammonia has several advantages as an energy carrier, including higher energy density and lower flammability. Ammonia can be stored in liquid form at a moderate pressure of 8.6 bar at 20 °C [10,11]. In comparison, hydrogen must be stored at very high pressures, normally exceeding 300 bars, or in liquidised form under cryogenic temperatures in order to achieve comparable energy densities [12]. Therefore, ammonia storage per kWh is less costly and energy consuming compared to the storage of hydrogen [13].

SOFCs do not fully convert the ammonia and hydrogen, even when an off-gas recycling system is installed. Hence, the SOFC off-gas contains unreacted hydrogen and traces of unreacted ammonia.

An afterburner can be used to generate heat and remove the hydrogen and ammonia before off-gas emission [8]. Utilising the hydrogen in the off-gas to generate heat and pre-heat, e.g., the ammonia feed entering the SOFC enhances the overall efficiency of the SOFC system.

While the combustion of hydrogen is inherently clean, the combustion of ammonia can cause the formation of nitrogen oxides (NO_X). Because of the low ammonia concentration in the feed, the resulting NO_X concentrations would be rather low. The conversion of ammonia to nitrogen (selective catalytic oxidation; SCO) is preferable to reduce the pollution emitted by an SOFC system. A comprehensive review by Chmielarz and Jabłońska provided a good overview of the recent studies regarding catalysts for the ammonia SCO reaction [14]. However, the results cannot be directly translated to the combustion of the SOFC off-gas because the SCO investigations were carried out in the absence of hydrogen. The tested ammonia SCO catalysts included supported noble metals, e.g., platinum [15], palladium [16], iridium [17], ruthenium [18], rhodium [19], silver [20], and gold [21], as well as supported transition metals, e.g., iron [22], copper [23,24], cobalt [25], nickel [26], manganese [27], molybdenum [28], and vanadium [29]. Further, supported bimetallic catalysts [30,31] and catalysts based on mixed oxides have also been studied for ammonia SCO [32,33,34].

The support material plays an important role and is often reported to have a large effect on the selectivity towards nitrogen [35]. Catalyst supports which have been investigated include Al₂O₃ [36], SiO₂ [23], TiO₂ [37], Nb₂O₅ [38], and zeolites such as CHA and ZSM-5 [39,40,41].

Within this publication, investigations regarding the combustion of an ammonia SOFC off-gas surrogate over platinum- and iridium-based catalysts supported by different materials are reported. The work aims to elucidate the effect of the support on catalytic performance, with a focus on mitigating NO_X formation.

The presented work shows a distinct correlation between NO_X formation and the catalyst support, which is important knowledge for the design of catalysts for the combustion of NH₃-SOFC off-gas. The data show that supports with lower acidity such as SiO₂ or TiO₂ result in lower NO_X formation during the combustion of ammonia containing SOFC off-gas. Microstructured reactors coated with platinum or iridium supported on Al₂O₃, SiO₂, and TiO₂ were investigated for the combustion of an SOFC off-gas surrogate, and their performance in terms of NO_X formation was evaluated and compared at different temperatures.

2. Results

2.1. Catalyst Characterisation

The concentration of the active platinum species was detected by XRF, see Table 1. The values meet the target concentration of 5 wt.% quite well. The XRD pattern of the catalysts can be found in the Supplementary Materials. Phase identification reveals that the platinum in all the platinum-based catalysts is present as elemental Pt⁰, while the iridium in the iridium-based catalysts is present in the form of iridium oxide IrO₂.

The supports appear to be unaltered by the impregnation with iridium and platinum precursors and the following calcination. The aluminium oxide is amorphous in all the prepared catalysts. The titanium oxide support consists mostly of anatase TiO₂, with a small amount of rutile TiO₂. The silica support is present in the form of cristobalite.

The textural properties of the pure supports differ greatly as evident in the nitrogen sorption results (see Supplementary Materials). The used alumina shows the highest surface area (155.9 m²g⁻¹), followed by silica (92.3 m²g⁻¹), while TiO₂ has the smallest surface area of only 38.7 m²g⁻¹.

Nitrogen sorption isotherms of the platinum-based catalysts (Figure 1) reveal the same differences in the textural properties as observed for the pure support materials. The BET specific surface areas listed in Table 1 reveal that the alumina support has the highest specific surface area (150.2 m²g⁻¹), followed by silica (90.4 m²g⁻¹) and titania (36.4 m²g⁻¹).

The nitrogen sorption isotherms of the iridium catalysts are shown in Figure 2. The specific surface areas are again close to the surface areas of the pure supports (Ir/Al₂O₃ = 151.6 m²g⁻¹, Ir/SiO₂ = 87.0 m²g⁻¹, Ir/TiO₂ = 36.9 m²g⁻¹), which is consistent with the values observed for the platinum-based catalysts. The deposition of iridium and platinum particles generally reduced the specific surface areas of the samples due to the low surface area but high density of the metal compared to the support.

The selected area electron beam diffraction (SAED) pattern shown in the Supplementary Materials confirm that platinum is present as Pt⁰ in all the catalyst samples.

The transmission electron microscopy (TEM) images in Figure 3 display the variations in the size and dispersion of platinum particles among the different supports, despite all of them being prepared using the same method. The platinum-based catalysts supported on Al₂O₃ and SiO₂ reveal well-dispersed particles with average diameters of 2.35 nm and 2.37 nm, respectively. However, when supported on TiO₂, the platinum particles are even smaller, with an average particle diameter of only 1.36 nm. These observations highlight the effect of different support materials on the size and dispersion of platinum particles present in the catalysts.

TEM-based electron beam diffraction results mostly confirm that the previously discussed XRD results in that the iridium is present as iridium oxide on the Al₂O₃ and SiO₂ supports (see SAED pattern in the Supplementary Materials). For the titania catalyst, the iridium crystallites were identified as Ir⁰ by SAED; however, the XRD data suggest the Ir to be present as IrO₂.

The size of iridium particles depends on the support material, as depicted in Figure 4. When supported on Al₂O₃, large and poorly dispersed iridium particles are observed, with diameters exceeding 6 nm. The small number of particles makes a reliable estimation of an average particle diameter impossible. In contrast, iridium particles supported on SiO₂ show good dispersion, with an average particle diameter of 3.72 nm. Similar to platinum, iridium particles supported on TiO₂ appear smaller compared to the other investigated supports, with an average diameter of 1.6 nm.

Ammonia temperature-programmed desorption (TPD) was used to study the acidic properties of the supports. Pure support samples were treated and calcined in the same way as the washcoated catalyst samples to allow for a comparison with the catalyst samples.

The ammonia TPD graphs in Figure 5 show distinctive differences in the ammonia desorption behaviour of the three support materials. The Al₂O₃ sample shows a very large desorption peak at about 220 °C, a much smaller second peak at about 400 °C, and a large increase in signal at 550 °C.

The first peak can be attributed to ammonia adsorbed to weak Lewis acid sites. The second peak at 400 °C could indicate the presence of weak Brønsted acid sites. The peak at 550 °C is likely caused by the decomposition of aluminium–ammonia complexes and the subsequent desorption of ammonia [42].

The TPD graph of the SiO₂ support shows two peaks at about 170 °C and 510 °C, as well as a further increase at high temperatures >550 °C. The rather small peak at 170 °C indicates a small number of weak Lewis acid sites present on the SiO₂ support. The second peak at 510 °C is much more pronounced, showing large quantities of strong Brønsted acid sites on the SiO₂ support. Again, the further increase in desorption at >550 °C could be caused by decomposition and the subsequent ammonia desorption of silica–ammonia complexes.

The TiO₂ sample shows a low temperature peak at about 190 °C, a shoulder at about 275 °C, and a high temperature peak at 550 °C. The first peaks are likely caused by the desorption of ammonia from weak and strong Lewis acid sites. The peak at 550 °C hints at a large number of strong Brønsted acid sites in the TiO₂ support.

The number of Brønsted acid sites present in the different supports follows the order SiO₂ > TiO₂ > Al₂O₃, while the weak acid sites follow the order Al₂O₃ > TiO₂ > SiO₂.

2.2. Catalytic Tests

The catalytic testing was carried out under identical conditions to compare the effects of different supports. The following gas composition was chosen according to the simulation results of the Ship-FC system without off-gas recirculation: 25 Vol.-% N₂, 15 Vol.-% H₂, 60 Vol.-% H₂O, and 100 Vol.-ppm NH₃. The SOFC off-gas surrogate was mixed with air using an air-to-fuel ratio λ = 4, resulting in a feed composition of 56.8 Vol.-% N₂, 13.2 Vol.-% O₂, 24.0 Vol.-% H₂O, 6 Vol.-% H₂, and 36 Vol.-ppm NH₃. The air-to-fuel ratio was chosen based on process simulation results to obtain the desired afterburner temperature (adiabatic temperature rise for the combustion).

2.2.1. Effect of the Catalyst Support on the NO_X Formation over Platinum-Based Catalysts

The platinum-based catalysts exhibit good low-temperature activities and thus low light-off temperatures, as displayed in Figure 6. All platinum catalysts exhibit complete hydrogen conversion at ≥150 °C and complete ammonia conversion at ≥200 °C. At low temperatures of 100 °C and 150 °C, differences in hydrogen and ammonia conversion can be observed. For hydrogen combustion at low temperatures, the TiO₂-supported catalyst shows the highest activity, followed by SiO₂ and Al₂O₃. For the ammonia conversion, the differences between the catalyst supports are mostly insignificant; only the Al₂O₃-supported catalyst displays almost no activity at 100 °C.

Figure 7 displays the NO_X concentrations and selectivities, revealing notable differences in the catalytic performance of the tested supports.

Generally, regardless of the catalyst support, an increase in reactor temperature leads to higher NO_X concentrations and hence NO_X selectivities. As the reactor temperature surpasses 600 °C, the formation of NO_X becomes stagnant and levels off between 700 °C and 800 °C. At these high temperatures, the NO_X concentrations exceed the NH₃ concentration in the feed (35 ppm), and the selectivities surpass 100%. This indicates that nitrogen in the combustion air is oxidised and converted to NO_X, further increasing the total NO_X formation rate.

Notably, this effect is more pronounced for the Al₂O₃-supported catalysts and decreases in the order Al₂O₃ > SiO₂ > TiO₂.

The platinum catalyst supported on Al₂O₃ exhibits the highest NO_X selectivities, with exception of a reactor temperature of 100 °C were its low activity leads to no detectable NO_X formation. In contrast, the platinum catalyst supported on SiO₂ exhibits lower NO_X selectivities across the entire temperature range. The catalyst supported on TiO₂ shows the lowest NO_X selectivities, indicating significantly reduced NO_X formation at both high and low reaction temperatures.

N₂O is an especially problematic pollutant because it is a very potent greenhouse gas. Therefore, N₂O is not included in the NO_X concentration but considered separately.

The N₂O concentration and selectivities of the platinum-based catalysts are shown in Figure 8. For all catalysts, the formation of N₂O is highest at low reaction temperatures and then decreases. At temperatures of over 500 °C, no N₂O formation can be detected. The N₂O concentrations are generally below 10 ppm.

The highest N₂O formation is observed for the TiO₂-supported catalyst. The N₂O concentration increases with temperature, reaches the maximum at 200 °C, and then steeply declines. The Al₂O₃-supported platinum displays a low initial N₂O formation, due to its low catalytic activity at 100 °C. At higher temperatures, the N₂O formation increases and reaches the second highest value of all the tested catalysts. The catalysts supported on SiO₂ display a high initial N₂O formation, which then declines until it reaches zero at a reactor temperature of 500 °C. The N₂O concentrations and selectivities decrease in the order TiO₂ > Al₂O₃ > SiO₂.

2.2.2. Effect of the Support on the NO_X Formation over Iridium-Based Catalysts

In the previous studies, iridium-based catalysts showed promising properties such as low NO_X formation at low temperatures. Therefore, the effect of a catalyst support on ammonia combustion over iridium-based catalysts was studied.

The hydrogen and ammonia conversion over the iridium-based catalysts are summarised in Figure 9. The iridium catalysts exhibit lower activity for both ammonia and hydrogen conversion compared to the platinum-based catalysts. As expected, both hydrogen and ammonia conversion increase with reaction temperature for all the catalysts.

The highest activity at low temperatures is evident for the Al₂O₃-supported iridium catalyst, followed by the TiO₂-supported catalyst. The catalyst supported on SiO₂ shows the smallest activity at low temperatures. All tested iridium catalysts obtain full hydrogen and ammonia conversion at 500 °C and 400 °C, respectively.

The NO_X concentration and NO_X selectivities of the iridium-based catalysts are shown in Figure 10. The iridium based catalysts exhibit lower NO_X formation compared to the platinum-based catalysts, which is in good accordance with the previous studies [43].

The different supports show less pronounced differences in NO_X selectivities, especially at low reactor temperatures. With increasing temperature, the NO_X concentration and selectivity increase, before they stagnate at 600 °C. At the plateau between 600 °C and 800 °C, differences in NO_X selectivity are evident. The NO_X selectivities of the different iridium-based catalysts at high temperatures increase in the order TiO₂ < SiO₂ < Al₂O₃. This is consistent with the results for the platinum-based catalysts where the same behaviour in terms of NO_X formation was evident for the supports under investigation.

Again, N₂O formation was considered separately from the other NO_X species. The results for N₂O concentration and resulting selectivity are shown in Figure 11. All iridium-based catalysts showed low activity at reaction temperatures below 300 °C, where high N₂O formation was observed. Therefore, the iridium-based catalysts show much lower N₂O formation compared to the platinum-based catalysts.

As already observed for the platinum catalysts, the highest N₂O formation was evident for the TiO₂-supported catalyst. The iridium catalyst supported on Al₂O₃ shows N₂O formation at 300 °C and 400 °C, while N₂O formation is only evident at 400 °C for the other iridium-based catalysts. With increasing temperature, N₂O formation declines; for all tested iridium-based catalysts, N₂O formation was not observed at temperatures exceeding 400 °C.

2.2.3. Effect of Different Supports on the NO_X Formation During Combustion in Absence of an Active Metal

To study the effect of the support in the absence of active metals, pure supports were tested for the off-gas combustion. The pure support samples were coated in the reactors and calcined, following exactly the same procedure as applied for the catalyst samples and then tested under the same reaction conditions.

The NH₃ and NO_X concentrations as determined at different reactor temperatures are shown in Figure 12 and reveal interesting effects. The activity and the NO_X formation are impacted considerably by the supports.

As expected, the conversion of ammonia is much lower for the different supports compared to the platinum- and iridium-containing catalysts. Nevertheless, significant ammonia conversion can be observed at temperatures exceeding 600 °C. Interestingly, the first ammonia conversion in a blank reactor (without any catalyst or support) can only be observed once ammonia’s ignition temperature of 650 °C is surpassed. Hence, the supports without an active metal might have some catalytic effects.

Above 600 °C, the ammonia conversions increase for all the support materials. Again, a comparison of the ammonia conversion for the different supports also shows that the presence of the support has an impact on the ammonia conversion. In the case of pure supports, SiO₂ shows the lowest NH₃ conversions, followed by TiO₂. The highest NH₃ conversions can be observed for Al₂O₃.

At 800 °C, all supports reach similar ammonia conversions of about 90%. The noticeable ammonia slip of about 10% can probably be attributed to the short residence time in the reactor, which does not leave sufficient time at high temperature for complete homogeneous combustion before the gas is cooled down after leaving the reactor.

The presence of different support materials also influences the formation of NO_X (see Figure 12b). As the reactor temperature increases, the NO_X concentration rises for all the supports. However, once the temperature exceeds 700 °C, the NO_X concentrations reach a plateau. Among the supports, Al₂O₃ exhibits the highest formation of NO_X at high temperatures, followed by SiO₂, while TiO₂ shows the lowest NO_X formation at high temperatures. The much higher NO_X formation over Al₂O₃ compared to SiO₂ and TiO₂ is consistent with the observations made for both the iridium- and platinum-based catalysts.

3. Discussion

When comparing the results of activity testing with the characterisation results, different correlations are evident.

The particle size of both the platinum- and iridium-based catalysts was smallest on the TiO₂ support. The platinum particles supported on SiO₂ and Al₂O₃ were of similar size, while for iridium, the particles were larger when supported on Al₂O₃. The size of the platinum and iridium particles correlated partially with the low-temperature activity of the catalysts. The highest dispersion and smallest platinum particles were observed for TiO_2, and these catalysts showed the highest low-temperature activities. The low-temperature activity of the platinum catalysts supported on Al₂O₃ and SiO₂ was comparable, which correlated well with their similar particle size. However, this was not the case for the iridium-based catalysts on Al₂O₃ and SiO₂, which showed no distinct correlation between the particle size and their activity at lower temperatures. Further explanations for the highest performance of TiO₂ in terms of low temperature activity could be strong metal–support interactions (SMSIs) often attributed to the electronic defects on TiO₂ and the activation of oxygen by the strong adsorption onto oxygen vacancies over TiO₂ [44].

The selectivities towards NO_X and N₂O over the different supports draw a much clearer picture. The formation of NO_X and N₂O over the different supports follow the same trend for both the iridium- and platinum-based catalysts and also for the samples without an active metal. NO_X formation increases with reaction temperature and reaches selectivities exceeding 100% at temperatures exceeding 600 °C. At high temperatures (≥700 °C), the NO_X concentrations observed in the product gas follow the order Al₂O₃ > SiO₂ > TiO₂ and are similar for the same support material in the presence of platinum and iridium, as well as for the pure support without an active metal as shown in Figure 13.

This emphasises the importance of the support to minimise the formation of NO_X species during the catalytic combustion of SOFC off-gas.

The acidity of the support might be an important factor for NO_X formation. In the literature, the effects of both types of acid sites were analysed. Strong acid sites have often been reported to enhance the selectivity to N₂, while Lewis acid sites promote the formation of NO_X species [45].

The measured NO_X formation and acid sites of the Al₂O₃ supports show a clear agreement with the results from the literature. The Al₂O₃ support has the largest number of Lewis acid sites promoting NO_X formation and a limited number of strong Brønsted acid sites. The NO_X formation over all the Al₂O₃ catalysts is therefore comparably high. The other supports exhibit a higher number of Brønsted sites and lower numbers of weak acid sites. SiO₂ has the highest number of strong acid sites and almost no peaks for weak acid sites are visible in the TPD data. The NO_X formation over this support is consequently comparably low, especially when there is no active metal present. TiO₂ shows a slightly higher number of weak acid sites as compared to SiO₂, along with fewer strong acid sites. The TiO₂ support shows the lowest NO_X formation in the presence of both active metals but also without any active metal, which may hint at additional effects such as different metal–support interactions or a different number of oxygen vacancies.

The oxygen defects in the catalyst support are frequently linked with their catalytic performance in an NH₃ SCO reaction. Larger numbers of oxygen vacancies, as present in TiO₂, can increase the N₂ selectivity of catalysts for selective ammonia combustion [46].

N₂O formation is also clearly affected by the support, and the effect of the support is again similar for both the platinum- and iridium-based catalysts. The formation of N₂O increases with temperature up to 400 °C and then declines until no N₂O can be observed at temperatures exceeding 500 °C. This volcano plot-like behaviour has already been reported for platinum-based catalysts [47].

Compared to NO_X formation, the effect of the support on N₂O selectivity is the opposite, with the highest N₂O formation over TiO₂, followed by the SiO₂- and Al₂O₃-supported catalysts. The enhanced N₂O selectivity of the TiO₂-supported platinum catalysts was also observed in the literature. Sebu et al. attributed the increased N₂O selectivity of TiO₂-supported catalysts to their high density of oxygen vacancies [48].

4. Materials and Methods

Catalyst preparation: All catalysts were tested as coatings in microstructured reactors. Firstly, the catalyst powders were prepared, then the catalysts were washcoated into the channels of the reactor plates and, finally, the reactors were assembled by laser welding.

The catalyst powders were prepared by the impregnation of the Al₂O₃, and TiO₂ powders with the metal precursor, as well as by the mixing of the colloidal SiO₂ with the metal precursor. TiO₂ (TiO₂ nano powder; Alfa Aesar GmbH, Karlsruhe, Germany), Al₂O₃ (Puralox, Sasol Germany GmbH, Hamburg, Germany), and colloidal silica (Ludox AS 40; Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany) were used as received. The supports were then impregnated or mixed with a calculated amount of an aqueous solution of the corresponding metal precursors Pt(NH₃)₄(NO₃)₂ (Alfa Aesar GmbH) and IrCl₃*xH₂O (Alfa Aesar GmbH) to obtain the desired catalyst composition. After the impregnation, the samples were calcined at 450 °C for 6 h, and the resulting catalyst powders were milled by hand using a pestle and mortar.

The catalyst powders were then used for the washcoating [49]. Firstly, a suspension was prepared by dissolving polyvinyl alcohol in deionised water at 65 °C for 3 h. Then, acetic acid and the previously prepared catalyst powder were added and stirred for 3 h at 65 °C. Afterwards, the suspension was stirred for two to three days at room temperature until a homogeneous suspension was achieved. The catalyst suspension was then used to coat the channels of the microreactor.

Catalytic testing: The gas feed consisted of a gas mixture (nitrogen, hydrogen, and ammonia), synthetic air, and steam. The steam was added to the off-gas surrogate because the off-gas from fuel cells contains large quantities of steam, and no condensation prior to the afterburner was planned in the practical system. The individual gasses were dosed by mass flow controllers (Bronkhorst High-Tech B.V., AK Ruurlo, The Netherlands). The steam was generated by dosing water into an evaporator using a CoriFlow^TM (Bronkhorst High-Tech B.V.).

The testing reactors consisted of two plates with 14 channels each, with channel dimensions of 500 μm × 250 μm × 25 mm. The channels were filled with the suspension, and the excess was removed using a blade. The plates were dried at room temperature and then calcined at 450 °C. Then, the two coated plates were assembled with 1/8″ inlet and outlet capillaries to a reactor by applying laser welding [50,51]. The reactors were installed between two heating blocks, each equipped with a thermocouple and heating cartridge. All relevant lines were heat traced to prevent condensation.

The applied weight hourly space velocity (WHSV) of 600 L/g h results from the catalyst mass of 20 mg and the flow rates of 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. The flow rates were adjusted to compensate for slight deviations in the catalyst mass.

The NH₃, NO, NO₂, N₂O, and H₂O concentrations were determined using a Multigas 2030 FTIR spectrometer (MKS Instruments Deutschland GmbH, München Germany). The concentrations of hydrogen and nitrogen were measured using a 490 Micro gas chromatograph (Agilent, Santa Clara, CA, USA).

The catalyst selectivity towards NO_X and N₂O was calculated as follows:

(1)$S_{NOx}={\displaystyle \frac{{\dot{n}}_{NO+NO2,out}}{{\dot{n}}_{NH3, in}-{\dot{n}}_{NH3, out}}}$

(2)${ S}_{N2O}={\displaystyle \frac{{\dot{n}}_{N2O}}{{\dot{n}}_{NH3, in}-{\dot{n}}_{NH3, out}}}$

Catalyst characterisation: Details of the catalyst characterisation by X-Ray diffraction (XRD), X-Ray fluorescence (XRF), nitrogen physisorption, and transmission electron microscopy (TEM) can be found in our previous work [43].

Hydrogen chemisorption: Static, volumetric hydrogen chemisorption was carried out to determine the dispersion of the platinum-based catalyst samples using an Autosorb iQ TPX instrument (Anton Paar Germany GmbH, Ostfildern, Germany). Details can be found in our previous work [52].

Ammonia temperature-programmed desorption (TPD) experiments were conducted using an Autosorb iQ TPX instrument (Anton Paar Germany GmbH, Ostfildern, Germany) equipped with a thermal conductivity detector. The samples were degassed at 400 °C for 30 min under a dry nitrogen flow before the sample was loaded with ammonia using a 50 mL/min flow of 5 wt.% ammonia in helium at 100 °C for 30 min. After this loading procedure, non-adsorbed ammonia was removed by a helium flow of 40 mL/min for 60 min. Ammonia TPD measurements were then carried out by ramping the temperature up to 600 °C with a 10 K min⁻¹ heating ramp.

5. Conclusions

The effects of different supports on the performance of iridium- and platinum-based catalysts for off-gas combustion have been studied, with a focus on NO_X formation.

The catalyst support showed a strong effect on NO_X formation. A general trend for NO_X formation over the different supports was found to be independent of the active metal dispersed on the support. This trend was also evident in the absence of an active metal. For all tested catalysts with and without an active metal, the highest NO_X formation was observed for the Al₂O₃ support, while SiO₂ and TiO₂ showed a reduced NO_X formation.

N₂O formation was much lower compared to NO_X formation and increased with reactor temperature, reaching a maximum at 400 °C and dropping to zero when the temperature increased further. The N₂O selectivity over the iridium- and platinum-based catalysts on different supports showed a clear trend which was opposite to the trend observed for the NO_X formation. The highest N₂O formation was observed for TiO₂, followed by SiO₂ and Al₂O₃.

The results of catalyst characterisation indicate a strong correlation between support acidity and NO_X formation. The large number of Lewis acid sites and small amount of Brønsted sites in Al₂O₃ might lead to a high degree of NO_X formation. The other supports with much reduced numbers of Lewis acid sites and higher Brønsted acid sites showed a much-reduced NO_X selectivity.

For the afterburner, iridium supported on silica appeared to be a promising catalyst when operated at a temperature as low as 300 °C. At this temperature, low NO_X formation and almost no N₂O formation were observed. While the conversion of both hydrogen and ammonia was only around 80% at this temperature, this could be mitigated by reducing the WHSV (which results in an increased size of the reactor). However, the reactor dimensions are comparatively small considering the size of the overall SOFC system.

Changing the catalyst from Pt/Al₂O₃, which is commonly applied as a combustion catalyst, to Ir/SiO₂ and lowering the combustion temperature reduces the NO_X formation significantly. The necessary increase in reactor size and catalyst mass for Ir/SiO₂ increases the price of the afterburner, but the reduced NO_X formation might avoid additional exhaust gas purification. The removal of NO_X from the exhaust gas via Ammonia SCR introduces additional costs. Both CAPEX (additional reactor, ammonia dosing) and OPEX (additional ammonia demand, additional costs for maintenance) increase if an additional SCR system is needed. Hence, an Ir/SiO₂ catalyst might be the more cost-efficient option.

Author Contributions

Conceptualisation, 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.; visualisation, 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.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. Nitrogen sorption isotherms of platinum-based catalysts.

Figure 2. Nitrogen sorption isotherms of the iridium-based catalysts.

Figure 3. TEM micrographs of platinum-based catalysts.

Figure 4. TEM micrographs of iridium-based catalysts.

Figure 5. Ammonia temperature-programmed desorption (NH3-TPD) results for different supports (heating ramp 10 K min−1).

Figure 6. (a) Hydrogen conversion and (b) ammonia conversion vs. reaction temperature for 5 wt.% platinum on different supports, WHSV = 600 L/g h.

Figure 7. (a) NOX concentration and (b) NOX selectivities vs. reaction temperature for 5 wt.% platinum on different supports, WHSV = 600 L/g h.

Figure 8. (a) N2O concentration and (b) N2O selectivities over reaction temperature for 5 wt.% platinum on different supports, WHSV = 600 L/g h.

Figure 9. (a) Hydrogen and (b) ammonia conversion vs. reaction temperature for 5 wt.% iridium on different supports, WHSV = 600 L/g h.

Figure 10. (a) NOX concentration and (b) NOX selectivities vs. reaction temperature for 5 wt.% iridium on different supports, WHSV = 600 L/g h.

Figure 11. (a) N2O concentration and (b) N2O selectivities over reaction temperature for 5 wt.% iridium on different supports, WHSV = 600 L/g h.

Figure 12. (a) NH3 Conversion over temperature for different supports, (b) NOX concentrations over temperature for different supports, WHSV = 600 L/g h.

Figure 13. Measured NOX concentrations over different catalysts at 700 °C.

Table 1

Catalyst characterisation data.

Catalyst	Metal Concentration (XRF)/wt.%	BET SurfaceArea/m2g−1	Particle Size (TEM)/nm
Pt/Al2O3	4.82 ± 0.14	150.2	2.35
Pt/SiO2	4.68 ± 0.14	90.4	2.37
Pt/TiO2	4.81 ± 0.14	36.4	1.36
Ir/Al2O3	4.98 ± 0.15	151.6	>6
Ir/SiO2	4.80 ± 0.14	87.0	3.72
Ir/TiO2	5.37 ± 0.16	36.9	1.60

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15030196/s1, Figure S1: Powder XRD pattern of iridium and platinum based catalyst on different supports; Figure S2: Nitrogen sorption isotherm of Al₂O₃ support; Figure S3: TEM images and SAED pattern of (a) platinum and (b) iridium based catalysts; Figure S4: Particle size distribution measured using TEM images of (a) platinum and (b) iridium based catalysts. Table S1: Specific surface areas (BET method).

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