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Article

Ammonia Decomposition over Ru/SiO2 Catalysts

by
Ho Jin Lee
1 and
Eun Duck Park
1,2,*
1
Department of Energy Systems Research, Ajou University, Suwon 16499, Korea
2
Department of Chemical Engineering, Ajou University, Suwon 16499, Korea
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1203; https://doi.org/10.3390/catal12101203
Submission received: 15 September 2022 / Revised: 29 September 2022 / Accepted: 3 October 2022 / Published: 9 October 2022
(This article belongs to the Special Issue Catalysis on Stable Molecules (CO2, CO, CH4, N2, NH3) Activation and Their Transformation)
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Abstract

:
Ammonia decomposition is a key step in hydrogen production and is considered a promising practical intercontinental hydrogen carrier. In this study, 1 wt.% Ru/SiO2 catalysts were prepared via wet impregnation and subjected to calcination in air at different temperatures to control the particle size of Ru. Furthermore, silica supports with different surface areas were prepared after calcination at different temperatures and utilized to support a change in the Ru particle size distribution of Ru/SiO2. N2 physisorption and transmission electron microscopy were used to probe the textural properties and Ru particle size distribution of the catalysts, respectively. These results show that the Ru/SiO2 catalyst with a high-surface area achieved the highest ammonia conversion among catalysts at 400 °C. Notably, this is closely related to the Ru particle sizes ranging between 5 and 6 nm, which supports the notion that ammonia decomposition is a structure-sensitive reaction.

Graphical Abstract

1. Introduction

Climate change caused by increasing CO2 concentrations in the atmosphere is a serious threat to human survival on Earth. This is directly related to the current carbon-based energy systems, in which coal and petroleum are the primary energy sources. Recently, renewable energy sources, including solar and wind power, have been considered as potential solutions. However, intermittent power generation, which is a typical characteristic of solar and wind energy, requires an energy-storage system. Along with battery systems, power-to-gas (PtG) systems are also regarded as effective methods. In the PtG system, H2 is first produced through water electrolysis with renewable electricity, stored, transported, and finally used to produce heat and electricity. Various routes have been proposed for hydrogen transportation, including high-pressure H2 gas, liquefied H2, liquid organic H2 carriers, and ammonia. For intercontinental hydrogen transportation, ammonia has an advantage, especially from a practical perspective, owing to its commercial use.
Ammonia is a promising hydrogen carrier because of its high gravimetric (17.8 wt.% H2) and volumetric (121 kg m–3 in the liquid form) H2 density. [1,2,3,4,5]. Hydrogen production through ammonia decomposition can meet the near-zero carbon H2 production requirement, resulting in a very low carbon footprint [6,7,8,9,10]. Hydrogen can be produced from ammonia via the following reaction (Equation (1)):
2NH3 (g) ↔ N2 (g) + 3H2 (g) ∆H⁰ = 46.22 kJ/mol
The generally accepted reaction mechanism for ammonia decomposition is as follows: (1) the adsorption of ammonia on the catalyst surface, (2) the successive cleavage of N–H bonds on adsorbed ammonia to release hydrogen atoms, and (3) the recombinative desorption of N and H atoms to form gaseous nitrogen and hydrogen molecules [11]:
NH3 + * → NH3 *(i)
NH3 * + * → NH2 * + H *(ii)
NH2 * + * → NH * + H *(iii)
NH * + * → N * + H *(iv)
N * + N *→ N2(v)
H * + H * → H2(vi)
Where * represents the number of active sites on the catalyst surface.
In general, the recombinative desorption of N is the rate-determining step in ammonia decomposition [5,12,13,14]. A broad range of metals have been tested for ammonia decomposition, such as Ru [15,16,17,18,19,20,21,22,23,24], Ni [15,24,25,26,27,28], Rh [15,29], Co [30,31,32], Fe [15,31,33], Pt [15,34,35], and Pd [15,36]. Among them, Ru has been the most studied metal as it is the most active catalyst. Ammonia decomposition over Ru-based catalysts is recognized as a structure-sensitive reaction [37,38]. Sensitivity is induced by active B5-type surface sites. These B5 sites have been recognized as being highly active for both the dissociation of N2 and the association of N because of their unique geometric configuration and electronic properties [7,39,40].
In addition to active metals, the selection of support is important for supported metal catalysts, as its surface acidity or basicity, surface oxygen vacancies, redox properties, and metal–support interaction can enhance catalytic performance. To date, the catalytic performance of Ru has been reported to be dependent on supports such as activated carbon [23,41], carbon nanotubes [15,19,20,21,22,42], Al2O3 [16,17,18,20,28,43], SiO2 [24,25,39,40,44,45,46], MgO [16,18,20,47,48], ZrO2 [22,49,50], TiO2 [50], and CeO2 [16,47,51]. Unlike other supports [52], SiO2 is an inert support, and the role of the active metal can be clearly discussed in the case of SiO2-supported metal catalysts. The effect of the surface area of SiO2 and pretreatment conditions on the catalytic performance can be interpreted in terms of the distribution and sizes of the Ru particles.
In this study, ammonia decomposition was performed on Ru catalysts (Ru/SiO2(SC)) supported on SiO2 supports of different surface areas obtained by varying the calcination temperature. The effect of Ru particle size on the catalytic activity for ammonia decomposition was also examined over Ru/SiO2 catalysts (Ru/SiO2(C)) calcined at different temperatures. Characterization techniques such as N2 physisorption and transmission electron microscopy (TEM) were used to relate the catalytic performance to the textural properties of the catalysts and the Ru particle size distribution.

2. Results and Discussion

2.1. Physicochemical Properties of Ru/SiO2

The textural properties of the various Ru/SiO2 catalysts were probed via N2 physisorption. The N2 adsorption and desorption isotherms of Ru/SiO2(SC) and Ru/SiO2(C) are shown in Figure S1. All Ru/SiO2(C) and Ru/SiO2(SC) catalysts showed Type IV(a) physisorption isotherms and type H2(b) hysteresis loops, except for Ru/SiO2(SC950). The pore size distributions of the Ru/SiO2(SC) catalysts in Figure S2a reveal that a single main peak appears in the pore size distribution for all the catalysts, except for Ru/SiO2(SC950). Ru/SiO2(SC950) appears to have very small amounts of N2 adsorption; therefore, there is no noticeable peak in the pore size distribution. The physical properties of these catalysts are listed in Table 1. The BET surface areas and pore volumes of the Ru/SiO2(SC) catalysts decreased with increasing calcination temperatures from 700 to 950 °C. There were no noticeable changes in the BET surface area and pore volume of Ru/SiO2(C100), Ru/SiO2(C300), and Ru/SiO2(C500). These values appeared to decrease when the SiO2 support was calcined at 700 °C or higher. The average pore diameter of Ru/SiO2 catalysts is approximately 6 nm as long as the calcination temperature does not exceed 900 °C. However, its value increases when further increasing the calcination temperature above 900 °C.
To measure the particle size of Ru metal in the Ru/SiO2 catalysts, TEM images were obtained for Ru/SiO2(C) and Ru/SiO2(SC) catalysts after reduction with H2 at 350 °C. The average Ru particle size can be calculated for Ru/SiO2(C) catalysts, which have relatively well-dispersed Ru nanoparticles. The average particle size of Ru metal in Ru/SiO2(C100), Ru/SiO2(C300), Ru/SiO2(C500), and Ru/SiO2(C700) were determined as 2.3 ± 0.72, 6.0 ± 1.9, 5.4 ± 1.4, and 5.6 ± 2.2 nm, respectively (Figure 1 and Table 1). Ru/SiO2(SC700) and Ru/SiO2(SC800) also have well-dispersed Ru nanoparticles, and their average sizes were 2.0 ± 0.51 and 2.1 ± 0.39, respectively (Figure 2 and Table 1). On the other hand, Ru catalysts supported on SiO2 calcined at temperatures higher than 800 °C possess irregular, extremely large Ru agglomerates. Therefore, the average particle size of Ru metal could not be estimated for Ru/SiO2(SC900), Ru/SiO2(SC930), and Ru/SiO2(SC950). However, they still had some Ru nanoparticles, and their average particle sizes were calculated to be 3.8 ± 1.0, 3.7 ± 2.0, and 3.4 ± 2.0, respectively (Figure 2 and Table 1). These results imply that the SiO2 support with a small surface area is not beneficial for obtaining well-dispersed Ru nanoparticles. However, it can be said that the oxidation of Ru catalysts supported on SiO2 with a large surface area has the potential to increase the Ru particle size to a certain degree without severe agglomeration. It is worth mentioning that large, irregular Ru lumps were observed even in the 1 wt.% Ru/SiO2(SC900) with a BET surface area of 102 m2/g. More Ru lumps with extremely large sizes can be found in the Ru/SiO2 catalysts with smaller BET surface areas than in Ru/SiO2(SC900). Note that relatively well-dispersed Ru nanoparticles with an average Ru particle size of approximately 6 nm were obtained for Ru/SiO2 catalysts calcined at 300, 500, and 700 °C. This was related to the high surface area of the SiO2 support.
The temperature-programmed reduction with H2 (H2-TPR) revealed that all Ru oxides in Ru/SiO2 samples calcined at 300, 500, and 700 °C could be reduced below 200 °C (Figure S3). Doublet peaks were observed in temperatures ranging from 100 and 200 °C for all samples. The H2-TPR peak at 110 °C strengthened, but the other H2-TPR peak at 135 °C weakened when increasing the calcination temperature from 300 to 700 °C.
The X-ray diffraction (XRD) patterns were obtained for Ru/SiO2(C) catalysts and are displayed in Figure S4. The presence of RuO2 was confirmed in all catalysts except for the Ru/SiO2(C100) catalyst (Figure S4a). The crystallite sizes of RuO2 in Ru/SiO2(C300), Ru/SiO2(C500), and Ru/SiO2(C700) were determined to be ~12 nm. All these XRD peaks due to RuO2 disappeared after reduction with H2 at 350 °C, but new XRD peaks owing to Ru appeared in all catalysts except for the Ru/SiO2(C100) catalyst (Figure S4b). The crystallite sizes of Ru in Ru/SiO2(C300), Ru/SiO2(C500), and Ru/SiO2(C700) were determined to be ~8 nm. This implies that RuO2 in the calcined catalyst can be reduced into Ru after reduction with H2 at 350 °C, which is in line with the results of H2-TPR.

2.2. Catalytic Performance for Ammonia Decomposition

The catalytic activity for ammonia decomposition was evaluated for Ru/SiO2 catalysts. Ammonia conversion as a function of the reaction temperature over different catalysts is shown in Figure 3. In the case of the Ru/SiO2(SC) catalysts (Figure 3a), Ru/SiO2(SC700), Ru/SiO2(SC800), and Ru/SiO2(SC900) showed similar ammonia conversions with Ru/SiO2(C100) at the same temperature. This can be attributed to the similar Ru particle sizes of Ru/SiO2(SC700), Ru/SiO2(SC800), and Ru/SiO2(C100). Notably, Ru/SiO2(SC900) had both Ru nanoparticles with an average particle size of 3.8 nm and large Ru lumps. Ru/SiO2(SC930) had similar catalytic activity with Ru/SiO2(C100) at low temperatures but became inferior to Ru/SiO2(C100) at high temperatures. Note that Ru/SiO2(SC950) was also vastly inferior to Ru/SiO2(C100) at 450 °C and higher. As these catalysts have both Ru nanoparticles and extremely large Ru lumps, it was difficult to establish any correlation between the catalytic activity and Ru particle size. However, it was clear that extremely large Ru agglomerates, which can easily form on a SiO2 support with a small surface area, are not plausible for ammonia decomposition.
Figure 3b shows Ru/SiO2(C300), Ru/SiO2(C500), and Ru/SiO2(C700) demonstrating similar catalytic activities, and their superiority to Ru/SiO2(C100) at all reaction temperatures. These catalysts had relatively well-dispersed Ru nanoparticles with an average particle size of 6 nm, larger than those of Ru/SiO2(C100). This implies that Ru particle size is an important factor that affects the catalytic activity for ammonia decomposition. As ammonia decomposition is a structure-sensitive reaction, the larger 6 nm-sized Ru particles performed better than the smaller 2 nm-sized ones, which is consistent with previous claims [53,54].
The H2 formation rates based on the amount of Ru metal were obtained at 400 °C over Ru/SiO2(C) and Ru/SiO2(SC) catalysts. Figure 4a shows that the initial catalytic activities were maintained over all the Ru/SiO2(SC) catalysts, and their rates decreased in the following order: Ru/SiO2(SC900) > Ru/SiO2(SC700) > Ru/SiO2(SC800) > Ru/SiO2(SC930) > Ru/SiO2(SC950)~Ru/SiO2(C100). The H2 formation rates over the Ru/SiO2(C) catalysts decreased in the following order: Ru/SiO2(C500)~Ru/SiO2(C300) > Ru/SiO2(C700) >> Ru/SiO2(C100). Note that the H2 formation rates decreased slowly with time on stream over Ru/SiO2(C300), Ru/SiO2(500), and Ru/SiO2(700), even though their H2 formation rates were much higher than that of Ru/SiO2(C100).
Kinetic data at different temperatures were obtained over Ru/SiO2(C500) and Ru/SiO2(C100), as shown in Figure S5. The apparent activation energies were determined as 108 and 146 kJ/mol, respectively. The kinetic data were compared to those reported previously (Table 2). The H2 formation rate over Ru/SiO2(C100) at 400 °C was similar to that over Ru/SiO2 [46,55]. The apparent activation energies over Ru/SiO2(C100) were much higher than those over Ru/κ-Al2O3 and Ru/θ-Al2O3 catalysts, even though the average Ru particle size ranged from 2 nm to 3 nm for all catalysts. However, Ru nanoparticles of approximately 5 nm can be formed in Ru/SiO2(C500), which has similar apparent activation energy as the Ru/κ-Al2O3 and Ru/θ-Al2O3 catalysts. Ru/SiO2(C500) showed a higher H2 formation rate than Ru/κ-Al2O3, Ru/θ-Al2O3, and Ru/α-Al2O3. However, its H2 formation rate was lower than that of Ru/θ-Al2O3(C300). Notably, the average Ru particle size increased from 2.6 to 9.8 nm when Ru/θ-Al2O3 calcined in air at 300 °C. This implies that Ru particle size is critical to catalytic activity for ammonia decomposition, as supported by previous studies [43,55].

3. Experiment

3.1. Catalyst Preparation

All Ru catalysts were prepared via a wet impregnation method using an aqueous solution of ruthenium nitrosyl nitrate (Ru(NO)(NO3)3·6H2O, Sigma-Aldrich Co. Llc., St. Louis, MO, USA) and SiO2 (ZEOprep 60, Zeochem Co. Ltd., Uetikon, Switzerland). Before the Ru precursor was impregnated onto the support, silica was calcined in air at different temperatures to obtain different surface areas. The prepared catalysts were dried in an oven at 100 °C for 12 h. The calcination temperature of SiO2 is denoted by the sample name. For example, Ru/SiO2(SC700) indicates that SiO2 was calcined in air at 700 °C before the impregnation step.
The dried Ru/SiO2(C100) catalyst was calcined in air at different temperatures before the reduction step. These samples were denoted as Ru/SiO2(C100), Ru/SiO2(C300), Ru/SiO2(C500), and Ru/SiO2(C700), where the numbers in parentheses indicate the calcination temperature in °C. We refer to these catalysts as Ru/SiO2(C). All the catalysts were reduced using H2 at 350 °C for 3 h before the activity test. The Ru contents for all catalysts were intended to be 1 wt.%.

3.2. Catalytic Performance

The activity of the ammonia decomposition reaction was evaluated in a packed-bed reactor at atmospheric pressure. In general, catalyst powder of 100 mg was loaded in the middle of a quartz reactor (O.D. = 9.525 mm, I.D. = 7.745 mm) and retained by quartz wool. Before the activity test, the prepared catalyst sample was first reduced with pure H2 gas at a flow rate of 30 mL/min at 350 °C, if not specified, for 1 h, followed by cooling to 25 °C. H2 gas was then switched with He gas to purge the residual hydrogen at a flow rate of 30 mL/min for 30 min. Then, the feed gas, which was composed of 25 mol% NH3, 70 mol% He, and 5 mol% CH4, was fed into the reactor at a total flow rate of 100 mL/min. CH4 was used as an internal standard. The catalytic activity for ammonia decomposition was monitored by increasing the temperature from 300 to 600 °C under atmospheric pressure. The reactants and products were separated using a Porapak Q column and analyzed using a thermal conductivity detector (TCD) in a gas chromatograph (GC, ChroZen, YOUNGIN chromass, Anyang, South Korea). The conversion of NH3 ( X N H 3 ) and the H2 formation rate ( r H 2 ) were calculated using the following equations:
X N H 3   ( % ) = F N H 3 , i n − F N H 3 , o u t F N H 3 , i n × 100
r H 2 ( m o l H 2 / m o l R u / m i n ) = F N H 3 , i n × X N H 3 × 0.015 × 101.07 m c a t . × 0.01
where F N H 3 , i n , F N H 3 , o u t , and m c a t . are the molar flow rates of NH3 ( m o l N H 3 / m i n ) in the feed and outlet gases and the mass of catalyst (g), respectively.
To obtain the kinetic data for ammonia decomposition, separate experiments were performed by mixing small amounts of the catalyst and a diluent SiO2 (ZEOprep 60, Zeochem Co. Ltd., Uetikon, Switzerland) in order to keep the NH3 conversion below 20%. The apparent activation energy (Ea) of each catalyst was calculated using the following equation:
k = A exp ( − E a R T )
where k is the reaction rate constant, A is the pre-exponential factor, R is the gas constant, and T is the absolute temperature.

3.3. Characterization

The Ru content of each catalyst was confirmed to be 1 wt.% with an inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo Fisher scientific, Waltham, MA, USA) performed with an OPTIMA 5300 DV instrument. N2 physisorption was analyzed using a Micromeritics ASAP 2020 system in which the supports and catalysts were degassed under vacuum at 200 °C for 4 h. The specific surface areas (SBET) of the samples were calculated using the Brunauer–Emmett–Teller (BET) method. The pore size distributions of the catalysts were obtained using the Barrett–Joyner–Halenda (BJH) method. High-resolution transmission electron microscopy (HRTEM, TitanTM 80–300, FEI, Hillsboro, OR, USA) and HRTEM (JEM-2100F, JEOL Ltd., Tokyo, Japan) were used to characterize the average particle size of Ru in the catalysts. These samples were deposited on a Cu grid covered with a holey carbon film. Temperature-programmed reduction with H2 (H2-TPR) was performed using a Autochem 2920 instrument (Micromeritics Instrument Corp., Norcross, GA, USA). After loading 0.10 g of the sample, the temperature was increased from 30 to 800 °C while feeding 10 mol% H2/Ar at a flow rate of 30 mL min−1 monitoring the TCD signal. X-ray diffraction (XRD, Rigaku Smartlab, Tokyo, Japan) patterns were obtained using a Rigaku D/Max instrument with a Cu Kα source to assess the bulk crystalline structure of the samples. The primary crystallite size of RuO2 and Ru in the samples was determined using the Scherrer equation [56]:
L = 0.9 λ K α 1 β 2 θ c o s θ m a x
where L is the average particle size, β 2 θ is the full width at half maximum (FWHM) of the peak, λ K α 1 is the wavelength of the X-ray radiation (0.15406 nm), and θ m a x is the angular position of the (211) peak maximum of RuO2 or the (101) peak maximum of Ru.

4. Conclusions

Various Ru/SiO2 catalysts were prepared via calcination at different temperatures to obtain supports with different surface areas or via calcination of the dried Ru/SiO2 catalyst at different temperatures to determine the effect of the Ru particle size on the catalytic activity for ammonia decomposition. Many Ru lumps were obtained, with supports having a small surface area, resulting in a slight change in the catalytic activity for ammonia decomposition. The high fraction of large Ru agglomerates observed in the Ru catalyst supported on supports with very small surface areas is not favorable for catalytic activity, especially at high temperatures. On the other hand, well-dispersed and relatively large Ru nanoparticles could be obtained after calcining Ru/SiO2 at temperatures from 300 to 700 °C, which might be due to the high surface area of SiO2. Catalytic activity was enhanced when the Ru particle size increased from 2.3 to 5.4 nm in Ru/SiO2. This finding supports the notion that ammonia decomposition is a structure-sensitive reaction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12101203/s1, Figure S1: (a) N2 adsorption (filled points) and desorption (unfilled points) isotherms of Ru/SiO2(C100) (○), Ru/SiO2(SC700) (△), Ru/SiO2(SC800) (▽), Ru/SiO2(SC900) (☆), Ru/SiO2(SC930) (□), and Ru/SiO2(SC950) (◇) and (b) N2 adsorption (filled points) and desorption (unfilled points) isotherms of Ru/SiO2(C100) (○), Ru/SiO2(C300) (△), Ru/SiO2(C500) (▽), Ru/SiO2(C700) (☆); Figure S2: (a) Pore size distribution of Ru/SiO2(C100) (○), Ru/SiO2(SC700) (△), Ru/SiO2(SC800) (▽), Ru/SiO2(SC900) (☆), Ru/SiO2(SC930) (□), and Ru/SiO2(SC950) (◇) and (b) pore size distribution of Ru/SiO2(C100) (○), Ru/SiO2(C300) (△), Ru/SiO2(C500) (▽), Ru/SiO2(C700) (☆); Figure S3: Temperature-programmed reduction with H2 (H2-TPR) patterns of Ru/SiO2 calcined at different temperatures, such as Ru/SiO2(C300), Ru/SiO2(C500), and Ru/SiO2(C700); Figure S4: X-ray diffraction (XRD) patterns for Ru/SiO2 catalysts calcined at different temperatures (a) and Ru/SiO2 catalysts calcined at different temperatures and then reduced with H2 at 350 °C (b); Figure S5: Arrhenius plots for ammonia decomposition over Ru/SiO2(C100) and Ru/SiO2(C500).

Author Contributions

Experimental investigation and data analysis, writing—original draft preparation, investigation, H.J.L.; supervision, writing—review and editing, project administration, funding acquisition, E.D.P. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (2015M3D3A1A01064899). This research was also supported by the H2KOREA, funded by the Ministry of Education (2022).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01203 g001 550
Figure 1. TEM images and particle size distributions of Ru metal for (a) Ru/SiO2(C100), (b) Ru/SiO2(C300), (c) Ru/SiO2(C500), and (d) Ru/SiO2(C700). All catalysts were calcined in air at different temperatures and subsequently reduced in H2 at 350 °C.
Figure 1. TEM images and particle size distributions of Ru metal for (a) Ru/SiO2(C100), (b) Ru/SiO2(C300), (c) Ru/SiO2(C500), and (d) Ru/SiO2(C700). All catalysts were calcined in air at different temperatures and subsequently reduced in H2 at 350 °C.
Catalysts 12 01203 g001
Catalysts 12 01203 g002 550
Figure 2. TEM images and particle size distributions of Ru metal for (a) Ru/SiO2(SC700), (b) Ru/SiO2(SC800), (c) Ru/SiO2(SC900), (d) Ru/SiO2(SC930), and (e) Ru/SiO2(SC950). All catalysts were reduced in H2 at 350 °C. The large Ru agglomerates were excluded in the particle size distribution.
Figure 2. TEM images and particle size distributions of Ru metal for (a) Ru/SiO2(SC700), (b) Ru/SiO2(SC800), (c) Ru/SiO2(SC900), (d) Ru/SiO2(SC930), and (e) Ru/SiO2(SC950). All catalysts were reduced in H2 at 350 °C. The large Ru agglomerates were excluded in the particle size distribution.
Catalysts 12 01203 g002
Catalysts 12 01203 g003 550
Figure 3. Ammonia conversion for ammonia decomposition over Ru/SiO2 catalysts. (a) Ru/SiO2(C100) (black), Ru/SiO2(SC700) (red), Ru/SiO2(SC800) (blue), Ru/SiO2(SC900) (green), Ru/SiO2(SC930) (pink), and Ru/SiO2(SC950) (brown). (b) Ru/SiO2(C100) (black), Ru/SiO2(C300) (dark green), Ru/SiO2(C500) (dark blue), and Ru/SiO2(C700) (dark pink). Reaction conditions: feed composition (25 mol% NH3, 70 mol% He, and 5 mol% CH4), WHSV = 60,000 mL gcat.−1 h−1.
Figure 3. Ammonia conversion for ammonia decomposition over Ru/SiO2 catalysts. (a) Ru/SiO2(C100) (black), Ru/SiO2(SC700) (red), Ru/SiO2(SC800) (blue), Ru/SiO2(SC900) (green), Ru/SiO2(SC930) (pink), and Ru/SiO2(SC950) (brown). (b) Ru/SiO2(C100) (black), Ru/SiO2(C300) (dark green), Ru/SiO2(C500) (dark blue), and Ru/SiO2(C700) (dark pink). Reaction conditions: feed composition (25 mol% NH3, 70 mol% He, and 5 mol% CH4), WHSV = 60,000 mL gcat.−1 h−1.
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Catalysts 12 01203 g004 550
Figure 4. Stability test over Ru/SiO2 catalysts at 400 °C. (a) Ru/SiO2(C100) (black), Ru/SiO2(SC700) (red), Ru/SiO2(SC800) (blue), Ru/SiO2(SC900) (green), Ru/SiO2(SC930) (pink), Ru/SiO2(SC950) (brown). (b) Ru/SiO2(C100) (black), Ru/SiO2(C300) (dark green), Ru/SiO2(C500) (dark blue), and Ru/SiO2(C700) (dark pink). Reaction conditions: feed composition (25 mol% NH3, 70 mol% He, and 5 mol% CH4).
Figure 4. Stability test over Ru/SiO2 catalysts at 400 °C. (a) Ru/SiO2(C100) (black), Ru/SiO2(SC700) (red), Ru/SiO2(SC800) (blue), Ru/SiO2(SC900) (green), Ru/SiO2(SC930) (pink), Ru/SiO2(SC950) (brown). (b) Ru/SiO2(C100) (black), Ru/SiO2(C300) (dark green), Ru/SiO2(C500) (dark blue), and Ru/SiO2(C700) (dark pink). Reaction conditions: feed composition (25 mol% NH3, 70 mol% He, and 5 mol% CH4).
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Table
Table 1. The physicochemical properties of the Ru/SiO2 catalysts.
Table 1. The physicochemical properties of the Ru/SiO2 catalysts.
CatalystSurface Area a
(m2/g)		Pore Volume a
(cm3/g)		Average Pore Size a
(nm)		Average Ru Particle Size b
(nm)
Ru/SiO2(C100)4280.676.22.3
Ru/SiO2(SC700)3390.485.72.0
Ru/SiO2(SC800)2830.466.52.1
Ru/SiO2(SC900)1020.166.33.8 c
Ru/SiO2(SC930)510.0947.43.7 c
Ru/SiO2(SC950)100.036123.4 c
Ru/SiO2(C300)4250.625.96.0
Ru/SiO2(C500)4360.656.05.4
Ru/SiO2(C700)3280.505.85.6
a Surface area, pore volume, and average pore diameter were calculated based on the N2 physisorption data. b The particle size is the result of analysis via transmission electron microscopy. c The particle size was calculated excluding large agglomerated Ru lumps.
Table
Table 2. Comparison of catalytic performance of Ru/Al2O3 and Ru/SiO2 catalysts for ammonia decomposition.
Table 2. Comparison of catalytic performance of Ru/Al2O3 and Ru/SiO2 catalysts for ammonia decomposition.
CatalystAverage
Ru Particle Size a
(nm)		WHSV
(mL/gcat/h)		 r H 2   b   ( m o l H 2 / m o l R u / m i n ) Ea c
(kJ/mol)		Ref.
Ru/CNT4.360,0001371[42]
Ru/α-Al2O34.260,0003983[43]
Ru/κ-Al2O33.060,00023103[43]
Ru/θ-Al2O32.660,00015106[43]
Ru/θ-Al2O3(C300)9.860,0007098[43]
Ru/SiO22.330,0009.3108[46]
Ru/K2SiO32.030,0004373[46]
Ru/c-MgO1238,7108776[48]
Ru/TiO2-60008.863[50]
Ru/ZrO2-60007.466[50]
Ru/CeO2<260007.183[51]
Ru/SiO22.8360,0008.5-[55]
Ru/SiO2(C100)2.360,0005.2146This work
Ru/SiO2(C500)5.460,00056108
a The particle size is the result of analysis via TEM. b The hydrogen formation rate was measured at 400 °C. c Activation energy was determined based on the kinetic data at different temperatures.
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Lee, H.J.; Park, E.D. Ammonia Decomposition over Ru/SiO2 Catalysts. Catalysts 2022, 12, 1203. https://doi.org/10.3390/catal12101203

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Lee HJ, Park ED. Ammonia Decomposition over Ru/SiO2 Catalysts. Catalysts. 2022; 12(10):1203. https://doi.org/10.3390/catal12101203

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Lee, Ho Jin, and Eun Duck Park. 2022. "Ammonia Decomposition over Ru/SiO2 Catalysts" Catalysts 12, no. 10: 1203. https://doi.org/10.3390/catal12101203

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