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Article

Catalyst Coatings for Ammonia Decomposition in Microchannels at High Temperature and Elevated Pressure for Use in Decentralized and Mobile Hydrogen Generation

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 2024, 14(2), 104; https://doi.org/10.3390/catal14020104
Submission received: 16 November 2023 / Revised: 11 January 2024 / Accepted: 19 January 2024 / Published: 26 January 2024
(This article belongs to the Special Issue Catalysts for Ammonia Decomposition)
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Abstract

:
We report an investigation of catalyst performance for the decomposition of ammonia under industrially relevant conditions (high temperatures of up to 800 °C and an elevated pressure of 5 bar) with further emphasis on their stability at high reaction temperatures. The catalysts were applied and tested as coatings in 500 µm wide channels of microreactors. Nickel-based catalysts were studied and compared to a ruthenium-based catalyst supported on SiO2. The effect of the support on the catalytic performance was investigated, and CeO2-supported nickel catalysts were found to exhibit the highest activity. Promoters were applied to increase the NH3 decomposition activity of the Ni/CeO2 catalysts. The addition of cesium led to a slight reduction in activity, while lanthanum, calcium, and barium doping resulted in increased activity. In particular, the barium-doped Ni/CeO2 catalyst showed very high ammonia conversion and closed the activity gap with respect to ruthenium catalysts at reactor temperatures of 650 °C and higher. The hydrogen production rates achieved in this work were compared to values in the literature and were shown to exceed values found earlier for both nickel- and ruthenium-based catalysts. Furthermore, the ruthenium-based catalysts under investigation were rapidly deactivated at 700 °C, while the nickel-based catalysts did not show deactivation after 220 h on time on stream at 700 °C.

Graphical Abstract

1. Introduction

Hydrogen (H2) is widely considered to have the potential to become a major green energy vector [1]. However, the storage and transport of hydrogen are major challenges [2]. Hydrogen possesses the lowest density among all elements, resulting in a very poor energy density of only 3 Wh per liter. Therefore, for storage and transportation, hydrogen must be compressed to a high pressure (up to 700 bar) or liquefied to a cryogenic temperature of −253 °C. But even in its liquid form, the energy density of hydrogen is comparably low due to its low liquid density of only 75 kg/m3.
Ammonia is emerging as a hydrogen carrier due to its high hydrogen storage density of 17.7 wt.%, high energy density of 3 kWh/kgNH3, and the fact that it can be liquified at a moderate pressure of only 0.86 MPa at 20 °C. Hydrogen then can be generated through the decomposition of carbon-free ammonia into hydrogen and nitrogen. The synthesis of ammonia consumes less than 10% of the energy required to produce hydrogen through water electrolysis [3].
Approximately 150 Mt of ammonia is produced worldwide each year [4]. Technologies for the synthesis, storage, and transportation of ammonia are well established and widely available. However, the technology for hydrogen generation through ammonia decomposition and subsequent hydrogen purification is not as advanced. Thus, the development of advanced catalysts and efficient reactor technology for ammonia decomposition is required for the future application of ammonia as a hydrogen carrier [4].
Decentralized ammonia decomposition units can provide hydrogen at facilities that are not connected to the hydrogen infrastructure, e.g., for hydrogen fueling stations, use in chemical processes, or in energy or heat generation via fuel cells or direct combustion. Other applications for small decomposition units include mobile applications, for example, in the automotive or maritime sector [5].
Many applications make the operation of the decomposition catalyst at an elevated pressure necessary. For example, if the produced hydrogen will be purified through pressure swing adsorption or via membrane separation, a pressure of 10 bar is required. To increase the pressure by an additional compressor downstream of the decomposition reactor causes several disadvantages.
Another example is a process of partial splitting of ammonia to utilize a hydrogen–ammonia mixture in an internal combustion engine, where the injection pressure may also be about 10 bar.
Operating a decomposition reactor at elevated pressure has many practical advantages but unfortunately shifts the chemical equilibrium towards ammonia, hence reducing the obtainable conversion level. To shift the chemical equilibrium towards greater ammonia conversion, operation at higher temperatures is necessary. For example, at a reactor pressure of 5 bar, a temperature of over 600 °C is required to achieve an ammonia conversion rate exceeding 99% [6]. However, studies on ammonia decomposition found in the literature were almost exclusively conducted at atmospheric pressure. Only a few studies at elevated pressure can be found [7,8,9].
Various metals are active in ammonia decomposition. Studies have outlined that the activity of different metals follows the order Ru > Ni > Rh > Co > Ir > Fe [10]. Therefore, the most commonly applied catalysts for ammonia decomposition are based on ruthenium (Ru) and nickel (Ni).
Ruthenium exhibits the highest activity for ammonia decomposition [11]. Many different supports have been studied, including carbon nanotubes (CNTs), Pr6O11, CeO2, TiO2, Al2O3, MgO, perovskites, fumed silica and mesoporous silica (MCM-41, SBA-15) [12,13,14,15,16,17,18,19]. Doping ruthenium catalysts with strong basic alkali, alkaline earth elements, and rare earth elements has resulted in enhanced performance [12,15,20,21,22,23,24]. The increase in efficiency is often attributed to the facilitation of nitrogen desorption due to electron transfer from the basic dopant to the ruthenium [11].
Ruthenium is very expensive, has a high global warming power (GWP), and might suffer from deactivation at elevated temperatures [25,26]. Nickel-based catalysts are cheaper than ruthenium and can reach good activities at higher temperatures. Additionally, there is much experience with nickel catalysts in high-temperature applications, such as methane steam reforming. Nickel-based catalysts are usually deposited on oxides such as silica [14], rare earth oxides [27,28], mixed oxides [29], zeolites [30], perovskites [31], and alumina [32], and carbon nanotubes have been used as support [33]. Promoters have been successfully applied to boost the ammonia decomposition activity of nickel and ruthenium catalysts. For example, alkali and alkaline earth metals have shown a promotive effect on the ammonia decomposition activity of ruthenium catalysts [11,34]. For nickel catalysts, the addition of rare earth oxides such as La2O3 and CeO2, as well as alkali and alkaline earth elements, significantly enhanced the ammonia decomposition activity [11,34,35,36,37,38].
We report the catalytic performance of nickel-based catalysts for ammonia decomposition under elevated pressures and high temperatures for use in microstructured reactors. Different supports were studied, and the loading of the active metal was varied. Additives were applied to increase the activity. Furthermore, the long-term stability of a selected catalyst was studied for 220 h.
The ammonia decomposition system for which the catalysts reported in the current study were developed consisted of a microstructured catalytic wall reactor with integrated heating through catalytic combustion, an evaporator and heat exchanger for heat recovery, and hydrogen purification through pressure swing adsorption. The plant was designed to produce hydrogen at 2.9 kg/h, which is sufficient to power a 50 kWh PEM fuel cell.

2. Results

2.1. Catalyst Characterization

Figure 1 shows a photograph of a catalyst-coated reactor plate with visible channels. The connection points for the inlet and outlet capillaries are visible on the left and right sides of the reactor. Two reactor plates were combined with the inlet and outlet capillaries and assembled by laser welding.
The surface areas of the different supports and catalysts are shown in Table 1. The specific surface areas of the supports ranged from 156 m2/g−1 for Al2O3 and over 103 m2/g−1 for SiO2 to 37 m2/g−1 for CeO2. The corresponding catalysts showed a slight reduction in surface area compared to the pure supports due to the high density and low surface area of the nickel particles. The XRF analysis showed that the nickel loadings were in the range of 4.35 to 4.86% and were slightly lower than the set values of 5%. The loading of the additives barium, calcium, lanthanum, and cesium was very close to the loading of 1% set during catalyst preparation (see Table S1 in the Supplementary Materials).
The nickel particles were hard to distinguish from the supports in the TEM images shown in Figure 2. The Al2O3 support consisted of rod-like particles in different sizes. The SiO2 support displayed spherical particles with diameters of about 40 nm, representing the particle size and shape of the colloidal silica. The CeO2 support showed crystallites of different sizes and shapes.
Since the distribution of the nickel particles could not be determined using TEM, volumetric hydrogen chemisorption experiments were carried out. Table 1 lists the estimated nickel dispersions, nickel surface area, and particle sizes. The corresponding isotherms can be found in the Supplementary Materials. The data show that nickel supported on alumina exhibited the highest dispersion of 7.98% and the smallest particles with an average diameter of 12.7 nm, followed by nickel supported on silica, which showed the second highest dispersion of 5.91%. The lowest dispersion of 4.29% was observed for nickel supported on CeO2, resulting in an average particle diameter of 23.5 nm.
The catalysts with different nickel loadings showed a slight reduction in metal dispersion at higher loadings. The dispersion dropped from 4.29 to 4.06% for 5 and 15% nickel loadings, respectively. The addition of lanthanum (La) resulted in a small increase in dispersion to 5.8% from 4.3% for Ni/CeO2. The addition of lanthanum during the catalyst preparation might have resulted in smaller nickel particles due to changes in pH or by altering the metal–support interactions. The addition of cesium (Cs) resulted in a reduction in the nickel dispersion (3.48%), indicating larger nickel particles. The addition of barium (Ba) showed no significant effects on the nickel dispersion compared to that of the Ni/CeO2 catalyst without the addition of a promoter.
The EDX mappings of the cesium-, lanthanum-, and barium-doped Ni/CeO2 samples are displayed in Figure 3. The mappings displayed an even distribution of the additives over the CeO2 supports. Nickel was present in larger particles and was not as well dispersed as the additives. For both the cesium- and barium-doped catalysts, smaller and larger nickel particles were visible in the EDX mappings. The catalyst with lanthanum showed slightly smaller nickel particles and better dispersion of the particles compared to those of the barium- and cesium-doped catalysts, which was in good accordance with the H2 chemisorption results.
The nickel-based catalysts on different supports were also investigated using X-ray photoelectron spectroscopy. The peaks were attributed to NiO and Ni(OH)2 species according to the literature [39]. The data revealed that nickel was present as NiO and Ni(OH)2 species in the calcined samples (see Figure 4). Differences could be measured in the fractions of NiO and Ni(OH)2 in the different samples. The Ni/Al2O3 sample showed the largest fraction of Ni(OH)2 species with 59.7%, followed by Ni/SiO2 with 29.8% Ni(OH)2. Ni/CeO2 showed the smallest fraction of Ni(OH)2 at 19.4%. This was also evident in the oxygen measurements (available in the Supplementary Materials), where a prominent Ni(OH)2 peak was visible for the Al2O3 sample, and a less prominent peak was visible for the SiO2 sample. The sample supported on CeO2 did not show a Ni(OH)2 peak there.

2.2. Catalytic Tests

2.2.1. Nickel Catalysts on Different Supports

The catalyst support played an important role, as was evident in the ammonia conversions shown in Figure 5. As a reference, a ruthenium catalyst supported on silica was chosen, since this support showed the best performance among the tested ruthenium catalysts. Compared to the silica-supported ruthenium-based catalyst, the nickel catalysts showed lower activities, especially at lower temperatures. At higher reactor temperatures, the ammonia conversion increased, and the nickel-based catalysts reached similar conversion rates to that of the ruthenium-based catalyst. The nickel catalysts on different supports differed greatly in their degree of ammonia conversion. The aluminum-supported catalyst showed the lowest rate of ammonia conversion. The silica-supported catalyst showed a similarly low rate of conversion at lower temperatures. Only at temperatures of 700 °C and higher did the silica-supported catalyst surpass the aluminum-supported catalyst. The nickel catalyst supported on cerium dioxide showed much higher activity, especially at low temperatures. For example, at 650 °C, the nickel catalysts supported on silica and alumina reached around 20% ammonia conversion, while the ceria-supported catalyst reached 70% ammonia conversion. Generally, for the different supports, the order of activity was found to be CeO2 > SiO2 > Al2O3. Interestingly, the activity of the catalysts was inversely proportional to the dispersion of the nickel particles determined through H2 chemisorption (see Table 1).
Since the activity did not correlate with the nickel particle size, other effects of the support likely caused the differences in activity. Further research is planned to gain insight into the mechanism behind the observed catalytic performance. In the literature, several possible explanations for the higher activity of the CeO2-supported catalyst compared to the SiO2- and Al2O3-supported catalysts are reported. Lucentini et al. also found that Ni/CeO2 and Ru/CeO2 catalysts were more active in ammonia decomposition than Ni/Al2O3 and Ru/Al2O3 were [40]. For nickel-based catalysts, they also observed a higher nickel dispersion for Al2O3-supported catalysts (average particle diameter of 3.3 nm) compared to that of a CeO2-supported catalyst (average particle diameter of 5 nm), but they found a much higher activity for the CeO2-supported catalyst. The difference in activity was attributed to a particular metal–support interaction.
CeO2 is often used as a support or promoter for ammonia decomposition catalysts due to its large number of oxygen vacancies [41]. Do and coworkers found that CeO2 showed effects on NiO reducibility, surface oxygen vacancies, basicity, and the nickel–support interaction [42]. A support with high basicity can donate electrons to nickel particles and, thus, lower the binding energy of nickel and nitrogen, which facilitates the associative desorption of nitrogen and results in enhanced activity in ammonia decomposition [42].
The morphology of the support can also change the activity of the catalyst. Chuanqing Huang and coworkers found that the activity of Ni/CeO2 and Co/CeO2 catalysts in ammonia decomposition was significantly altered by the morphology of the CeO2 support [43].

2.2.2. Effect of the Nickel Loading

Since CeO2 showed the best performance, the following investigations were carried out using the ceria support. The effect of the nickel loading on the degree of ammonia conversion can be seen in Figure 6. As expected, the ammonia conversion increased with increasing nickel loading. The biggest differences were observed for reaction temperatures in the range of 550 to 650 °C. At temperatures of 700 °C and higher, the differences between the nickel loadings became rather small.
However, if one considers the significant increase in nickel loading, the gain in ammonia conversion was modest. As was evident in the H2 chemisorption data, a higher loading resulted in slightly larger nickel particles and, thus, reduced metal dispersion. Nevertheless, the available nickel surface areas estimated through H2 chemisorption increased from 1.43 m2/gcat to 4.07 m2/gcat with greater nickel loading. Thus, the reduced nickel dispersion could not fully explain the modest increase in activity of the catalysts.

2.2.3. Effects of Dopants on Nickel Catalysts Supported on CeO2

The effects of different additives, such as alkaline metal (Cs), alkaline earth metals such as magnesium (Mg), calcium (Ca), and barium (Ba), lanthanum (La), and yttrium (Y), on the performance of Ni/CeO2 catalysts were studied. Figure 7 shows the ammonia conversion over the reactor temperature for ceria-supported nickel catalysts with different additives.
Doping with cesium (Cs) did not enhance the catalysts performance and even led to a small reduction in ammonia conversion over nearly the entire temperature range. Only at high temperatures of 750 and 800 °C were comparable conversions achieved. A possible explanation for this reduction in activity due to the addition of Cs was the reduced nickel dispersion observed for this catalyst in the H2 chemisorption data.
The lanthanum-doped catalyst exhibited slightly increased ammonia conversion in the temperature range of 600 to 800 °C compared to that of the non-doped catalyst. At lower temperatures, a small reduction in ammonia conversion was evident in the data. The magnesium-doped catalyst showed a very similar performance to that of the lanthanum-doped catalyst. Again, a small decrease in activity at low temperatures and an increase in activity at higher temperatures was observed for the magnesium-doped catalyst.
The addition of yttrium had almost no effect on the catalytic performance. Only a small reduction in ammonia conversion at lower temperatures was evident. The ammonia conversion at temperatures of 600 °C and higher was similar to that of the Ni/CeO2 catalyst without a dopant.
The addition of calcium showed a pronounced effect at higher reaction temperatures of 600 °C and higher, where the conversion was significantly increased compared to that of the catalyst without a dopant. At lower temperatures, the addition of calcium did not alter the catalytic performance.
Barium was the best out of the tested additives, as it had increased conversion over the entire temperature range. Especially at higher temperatures, the effect of barium addition was very pronounced. At 600 and 700 °C, the conversion when using the barium-doped catalyst was 35% higher than that observed for the catalyst without a dopant. Furthermore, the barium-doped catalyst surpassed the ammonia conversion of the aforementioned Ru/SiO2 catalyst at 700 °C and higher. The strong promotive effect of barium on nickel-based catalysts was also previously reported for Ni/Al2O3 and Ni/Y2O3 catalysts [38].
Kaname Okura et al. studied barium- and strontium-modified nickel catalysts in NH3-TPSR and detected less strongly adsorbed nitrogen atoms for the modified samples than for a non-modified Ni/Y2O3 catalyst. They concluded that this could be attributed to enhancement in the electron density of the nickel due to the interaction with the strong basic barium or strontium promoter [11].

2.2.4. Variations in the Barium Loading of Barium-Promoted Ni/CeO2 Catalysts

The addition of barium showed the most pronounced improvement in catalytic performance; hence, the effect of barium loading was investigated. The catalysts with 1 and 2% barium showed comparable catalytic performance, which was made evident by the ammonia conversion shown in Figure 8. Only at lower temperatures did the higher loading of 2% lead to a marginal improvement in ammonia conversion. A further increase in barium loading to 4% led to a decrease in catalytic performance compared to that of the 1 and 2% barium loadings.

2.2.5. Variations in the Lanthanum Loading of Lanthanum-Promoted Ni/CeO2 Catalysts

The effect of the lanthanum loading on the catalytic performance of nickel catalysts supported on CeO2 was studied (see Figure 9). With the exception of the 1% lanthanum catalyst at low temperatures, increasing the lanthanum loading resulted in increased ammonia conversion. At a reaction temperature of 650 °C, the catalysts doped with 1, 2, and 4% lanthanum reached 73.5, 76.8, and 85.6% ammonia conversion.

2.2.6. Long-Term Catalytic Stability

Stability tests were carried out at a reaction temperature of 700 °C for up to 225 h. A Ru/SiO2 catalyst was compared to a Ni/CeO2 catalyst. Both catalysts initially showed a stable ammonia conversion rate of over 99.5% (see Figure 10). The ruthenium-based catalyst started to be deactivated and lose activity after about 5.5 h of time on stream. The catalyst deactivation proceeded rapidly, and the ammonia conversion dropped by nearly half after 9 h of time on stream. In the literature, ruthenium catalysts were found to be deactivated by sintering and Ru volatilization at temperatures as low as 450 °C [40]. In contrast, the nickel-based catalyst showed no signs of deactivation, even after 225 h of time on stream at 700 °C.

3. Discussion

The hydrogen production rates for the tested catalysts at a reactor temperature of 650 °C are shown in Figure 11. The catalysts supported by CeO2 showed much higher hydrogen production rates than those of the catalysts supported by SiO2 and Al2O3. The highest hydrogen production rates were evident for CeO2-supported nickel catalysts that were doped with either barium or calcium.
The hydrogen production rates at 600–650 °C achieved in this work were very high compared to values found in the literature, as displayed in Table 2.
The comparison might have been flawed, since the catalysts in this work were applied as coatings, while most studies in the literature tested the catalysts as powders in a fixed-bed reactor. The thin catalyst coatings resulted in much higher catalyst utilization, and microreactors commonly show a much better temperature distribution than that of fixed-bed reactors. Hence, the good performance might have been caused not only by the high activity of the developed catalysts, but also by the advantageous microreactor technology.

4. Materials and Methods

Preparation of catalysts and assembly of microreactors: The microreactor that was used with 14 channels can be seen in Figure 1 (the channel dimensions of the assembled reactor were 500 µm × 500 µm, with a channel length of 25 mm).
Catalyst powder preparation: The first step of the catalyst powder preparation was impregnating the support (Al2O3 Puralox, Sasol, SiO2 Ludox AS-40 (Merck, Darmstadt, Germany) CeO2 nanopowder, <25 nm, Sigma Aldrich, St. Louis, MO, USA) with an aqueous solution of the metal precursor (Ni(NO3)2*6H2O, Sigma Aldrich or Ru(NO)(NO3)3, Alfa Aesar, Haverhill, USA) and the dopant precursor to obtain the desired catalyst composition. The precursors for the dopants were Ca(NO3)3×4H2O (Sigma Aldrich), Y(NO3)3×6H2O (Alfa Aesar), Ba(NO3)2 (Alfa Aesar), Mg(NO3)2×6H2O (Sigma Aldrich), La(NO3)3×6H2O (Sigma Aldrich), and CsNO3 (Sigma Aldrich). The impregnated powders were then calcined at 450 °C for 6 h in an air atmosphere and then milled.
Wash-coating of the catalyst: Details on the wash-coating procedure can be found in our previous work [53]. For the wash-coating, a suspension of the catalyst power, deionized water, polyvinyl alcohol, and acetic acid was used. The microchannels of the reactor plates were filled with the homogeneous catalyst suspension before the excess was removed with a blade, and the plates were dried at room temperature. After drying, the plates were calcined at 450 °C in an air atmosphere.
Two reactor plates coated with the catalyst were sandwiched together and assembled via laser welding to form a microreactor, as described in previous papers [54].
Catalytic testing: Catalytic tests were carried out using a test rig with online analytics. The microreactor was placed in a stainless-steel heating block with two 300 W heating cartridges and different thermocouples. Gaseous ammonia (5.0 purity) and argon were dosed via Bronkhorst mass flow controllers. Then, 10% argon was added as an internal standard for the calculation of the mass balance. All lines were heat-traced.
The pressure was controlled by a back-pressure regulator downstream of the reactor (Equilibar® Research Serie LF, Equilibar, Fletscher, United States).
The gas was analyzed online via a Gasmet FTIR spectrometer (used for the measurement of the ammonia concentration) and an Agilent (Santa Clara, CA, USA) µGC (used for the measurement of hydrogen, nitrogen, and argon concentrations).
Before every experiment, bypass measurements were carried out to measure the feed concentration. The reactors contained between 20 and 25 mg of catalyst. For the catalytic measurements, the flow rates were adjusted according to the catalyst mass (ammonia and argon flow rates of 60–75 and 6.7–8.3 mL/min, respectively) to realize a weight hourly space velocity (WHSV) of 200 L/gcatalyst h.
Catalyst characterization: The loadings of the active metals and dopants were analyzed by using XRF spectroscopy (ED-XRF 1510 Canberra Packard Central Europe GmbH, Schwadorf, Austria) equipped with Cd-109 (22 keV) and Am-241 (60 keV) as an electron source).
For the transmission electron microscopy (TEM) measurements, a Libra 120 (Zeiss, Oberkochen, Germany) instrument was used. Nitrogen sorption isotherms were measured at −273 °C on a Autosorb iQ (Anton Paar, Graz, Austria) instrument after degassing the sample at 250 °C for 12 h.
X-ray photoelectron spectroscopy (XPS) investigations were carried out using a multi-chamber UHV system (PREVAC). The spectra were measured using a hemispherical Scienta R4000 electron instrument (Scienta Omicron AB, Uppsala, Sweden) equipped with an Al Kα X-ray source (1486.6 eV, 0.8 eV band) and XM 650 X-ray Monochromator (Scienta Omicron AB, Uppsala, Sweden) (0.2 eV band). The pass energies were set to 200 eV and 50 eV for the survey and high-resolution spectra, respectively. The pressures during the measurements were below 3 × 10−8 mbar.
H2 chemisorption: Measurements were carried out using a Autosorb TPX instrument (Anton Paar, Graz, Austria). Before measurement, the samples were degassed under a helium flow at 120 °C for 30 min and reduced under a pure hydrogen flow at 600 °C for 120 min. After the reduction, the remaining hydrogen was removed through evacuation at 600 °C for one hour. The measurements were conducted at 45 °C. One combined isotherm was measured (strong and weak adsorption); then, the sample was evacuated for 1 h to remove weakly adsorbed hydrogen. Then, another isotherm was measured to obtain the amount of weakly adsorbed hydrogen. The chemisorption isotherm was obtained by subtracting the weak isotherm from the combined isotherm.

5. Conclusions

This study showed that nickel catalysts are promising for the decomposition of ammonia at high temperatures and elevated pressures. The nickel catalysts showed high activity at temperatures over 600 °C, and a doped nickel catalyst surpassed the ammonia conversion rate of a highly active ruthenium catalyst at 650 °C.
Operation at elevated pressure made a higher temperature necessary to achieve the goal of conversion exceeding 99%. At 700 °C, the ruthenium-based catalyst was rapidly deactivated, which disqualified it for high-temperature applications. The nickel catalysts were generally less active than ruthenium but showed a much-improved high-temperature stability. While the nickel-based catalysts were less active, this disadvantage vanished at high temperatures, and the optimized nickel catalyst surpassed the activity of ruthenium at temperatures of 700 °C and higher.
The addition of dopants from the elements in the alkali and earth alkali groups, as well as lanthanum, enhanced activity. However, some elements, such as La, Ca, and Ba, showed a much greater effect. Barium proved to be the best additive and enhanced the activity drastically. The loading of the dopant, affected the catalytic performance differently for barium and lanthanum. Increasing the lanthanum loading showed an increase in performance. The ammonia conversion when using barium-doped catalysts was at its maximum for a loading of 2 wt.% barium.
In conclusion, it can be stated that the combination of excellent catalyst stability, good activity, and low cost makes nickel-based catalysts a promising candidate for ammonia decomposition, especially in decentralized and mobile applications that require high conversion rates.
Further catalyst optimization led to highly stable and active catalyst formulations that were applied to the demonstrator plant (2.9 kg/h hydrogen) developed within the Ammonpaktor project. The results of the operation of this large decomposition unit will be disseminated shortly. The stability of the developed catalyst in the presence of traces of water (0.5%) in the ammonia fed into the decomposition reactor was also studied in the framework of this project and will also be disseminated.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14020104/s1, Table S1: XRF analysis data of the used catalysts; Figure S1: Additional TEM micrographs of nickel-based catalysts supported on Al2O3, SiO2 and CeO2; Figure S2: EDX mapping of 5%Ni, 1% Cs/CeO2 catalyst; Figure S3: EDX mapping of 5%Ni, 1% La/CeO2 catalyst; Figure S4: EDX mapping of 5%Ni, 1% Ba/CeO2 catalyst; Figure S5: Nitrogen sorption isotherms of the used catalyst supports; Figure S6: Oxygen XPS results for nickel catalysts on different supports; Figure S7: H2 Chemisorption isotherms of supported nickel 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

Financial support for the AMMONPAKTOR project was provided by the EFRE program of the German region Rhineland-Palatinate under grant no. 84009390.

Data Availability Statement

The data presented in this study are available.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00104 g001
Figure 1. Photograph of a catalyst-coated reactor before assembly. The scale is in cm.
Figure 1. Photograph of a catalyst-coated reactor before assembly. The scale is in cm.
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Catalysts 14 00104 g002
Figure 2. TEM micrographs of nickel-based catalysts supported on Al2O3, SiO2, and CeO2.
Figure 2. TEM micrographs of nickel-based catalysts supported on Al2O3, SiO2, and CeO2.
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Catalysts 14 00104 g003
Figure 3. EDX mappings for Cs-, La-, and Ba-doped nickel catalysts supported on CeO2.
Figure 3. EDX mappings for Cs-, La-, and Ba-doped nickel catalysts supported on CeO2.
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Catalysts 14 00104 g004aCatalysts 14 00104 g004b
Figure 4. Results of curve fitting for XPS spectra (Ni2p) for the nickel catalysts supported on different supports.
Figure 4. Results of curve fitting for XPS spectra (Ni2p) for the nickel catalysts supported on different supports.
Catalysts 14 00104 g004aCatalysts 14 00104 g004b
Catalysts 14 00104 g005
Figure 5. Ammonia conversion over reactor temperature for nickel catalysts on different supports (Ru/SiO2 as a reference). WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
Figure 5. Ammonia conversion over reactor temperature for nickel catalysts on different supports (Ru/SiO2 as a reference). WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
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Catalysts 14 00104 g006
Figure 6. Ammonia conversion over reactor temperature for nickel catalysts supported on CeO2 with different nickel loadings. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
Figure 6. Ammonia conversion over reactor temperature for nickel catalysts supported on CeO2 with different nickel loadings. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
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Catalysts 14 00104 g007
Figure 7. Ammonia conversion over reactor temperature for nickel catalysts supported on CeO2 with different additives. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 bar.
Figure 7. Ammonia conversion over reactor temperature for nickel catalysts supported on CeO2 with different additives. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 bar.
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Catalysts 14 00104 g008
Figure 8. Ammonia conversion over reactor temperature for barium-doped nickel catalysts supported on CeO2 with different barium loadings. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
Figure 8. Ammonia conversion over reactor temperature for barium-doped nickel catalysts supported on CeO2 with different barium loadings. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
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Figure 9. Ammonia conversion over reactor temperature for lanthanum-doped nickel catalysts supported on CeO2 with different lanthanum loadings. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
Figure 9. Ammonia conversion over reactor temperature for lanthanum-doped nickel catalysts supported on CeO2 with different lanthanum loadings. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
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Figure 10. Ammonia conversion over time on stream with nickel (5% Ni, 1% Ba/CeO2) and ruthenium catalysts. Treactor = 700 °C, WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
Figure 10. Ammonia conversion over time on stream with nickel (5% Ni, 1% Ba/CeO2) and ruthenium catalysts. Treactor = 700 °C, WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
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Figure 11. Hydrogen production rates at 650 °C for different catalysts. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
Figure 11. Hydrogen production rates at 650 °C for different catalysts. WHSV = 200 L gcat−1 h−1, 10% Ar, 90% NH3, and p = 5 barg.
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Table 1. Characterization data for different nickel-based catalysts and pure catalyst supports.
Table 1. Characterization data for different nickel-based catalysts and pure catalyst supports.
Sample
Loading in wt.%		Surface Area 1
m2 g−1		Metal Loading 2
wt.%		Ni Dispersion 3
%		dpNi 4
nm
Al2O3155.9---
SiO2102.7---
CeO237.1---
5% Ni/Al2O3147.54.867.9812.7
5% Ni/SiO297.24.705.9117.1
5% Ni/CeO233.74.674.2923.5
10% Ni/CeO238.3n.d.4.1324.3
15% Ni/CeO237.213.564.0624.8
5% Ni, 1% Cs/CeO236.54.933.4829.1
5% Ni, 1% La/CeO239.74.145.8317.4
5% Ni, 1% Ba/CeO240.34.354.3023.5
1 BET (Brunauer–Emmett–Teller Method) surface area determined through N2 sorption; 2 metal loading measured with XRF; 3 nickel dispersion determined through H2 chemisorption; 4 average particle diameters estimated with nickel dispersion assuming spherical particles.
Table 2. Comparison of different catalysts for ammonia decomposition.
Table 2. Comparison of different catalysts for ammonia decomposition.
Catalyst Temperature
°C		NH3 Conversion
%		H2 Production Rate
mmolH2 gcat−1 min−1
2.8% Cs-Ru/MgO [44]50088.259.1
5% K-Ru/CNTs [16]45097.332.6
0.97% Ru/Fe-C [45]60097.521.7
5% Ru/graphitic C [46]55095.029.1
9.6% Ru/SiO2 [47]50086.0345.5
15% Ni/Mica [48]65097.232.5
Ni0.6(Mg0.29Al0.57On) [49]60099.333.3
12% Ni/MRM [50]70095.532.0
Ni1/C-LDHs-ST [51]60098.811.0
5% Ni/ZSM [52]65097.632.7
19.3% Ni/Al1Ce0.05Ox [42]60081.849.3
5% Ru/SiO2 (this work)65099.0198.8
5% Ni, 4% La/CeO2 (this work)65088.1176.9
5% Ni, 2% Ba/CeO2 (this work)65098.5198.2
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Weissenberger, T.; Zapf, R.; Pennemann, H.; Kolb, G. Catalyst Coatings for Ammonia Decomposition in Microchannels at High Temperature and Elevated Pressure for Use in Decentralized and Mobile Hydrogen Generation. Catalysts 2024, 14, 104. https://doi.org/10.3390/catal14020104

AMA Style

Weissenberger T, Zapf R, Pennemann H, Kolb G. Catalyst Coatings for Ammonia Decomposition in Microchannels at High Temperature and Elevated Pressure for Use in Decentralized and Mobile Hydrogen Generation. Catalysts. 2024; 14(2):104. https://doi.org/10.3390/catal14020104

Chicago/Turabian Style

Weissenberger, Tobias, Ralf Zapf, Helmut Pennemann, and Gunther Kolb. 2024. "Catalyst Coatings for Ammonia Decomposition in Microchannels at High Temperature and Elevated Pressure for Use in Decentralized and Mobile Hydrogen Generation" Catalysts 14, no. 2: 104. https://doi.org/10.3390/catal14020104

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