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Preparation of Ru/ZrO2 catalysts by NaBH4 reduction and their catalytic activity for NH3 decomposition to produce H2 Takeshi Furusawa, Masayuki Shirasu, Keita Sugiyama, Takafumi Sato, Naotsugu Itoh, and Noboru Suzuki Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03265 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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Industrial & Engineering Chemistry Research

Submitted to Industrial and Engineering Chemistry Research

Preparation of Ru/ZrO2 catalysts by NaBH4 reduction and their catalytic activity for NH3 decomposition to produce H2 Takeshi Furusawa1, 2*, Masayuki Shirasu2, Keita Sugiyama1, Takafumi Sato1, Naotsugu Itoh1, Noboru Suzuki1,2

1

Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya

University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan 2

Department of Advanced Interdisciplinary Sciences, Graduate School of Engineering, Utsunomiya

University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan

CORRESPONDING AUTHOR FOOTNOTE Takeshi Furusawa Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan TEL & FAX. +81-28-689-6160 E-mail: [email protected] ACS Paragon Plus Environment


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Abstract ZrO2-supported Ru catalysts were prepared by an impregnation method and by NaBH4 reduction from RuCl3·3H2O and a commercial ZrO2 support. The catalyst prepared by NaBH4 reduction showed high activity (around 60%) at 673 K for NH3 decomposition, whereas the catalyst prepared by the impregnation method exhibited low activity (10%) under the same reaction conditions. X-ray fluorescence spectrometry indicated that the catalyst prepared by NaBH4 reduction was Cl-free, whereas the catalyst prepared by the impregnation method contained more than 5 wt % of Cl from the Ru precursor. This was main factor that caused the differences in catalytic activity. In addition, the Ru/ZrO2 catalyst prepared by NaBH4 reduction was active for NH3 decomposition without H2 pretreatment, and its activity was slightly higher than the catalyst pretreated with H2. The transmission electron microscopy and H2 pulse chemical adsorption results demonstrated that Ru metal particle size also affected the activity. Keywords: H2 production, NH3 decomposition, Ru/ZrO2 catalyst, NaBH4 reduction method, Cl-free

1. Introduction Fossil fuels are consumed worldwide, depleting recoverable reserves of fossil fuels and causing environmental problems such as global warming.1 Renewable energy resources such as biomass are being developed as alternatives to fossil fuels because biomass is a carbon-neutral material.2 H2 is also expected to be a next-generation energy carrier, although it is currently produced from various fossil fuels such as oil, coal, natural gas, and hydrocarbons.3 H2 produces only water when upon combustion and has a high energy conversion efficiency; therefore, it would be a suitable energy carrier if it could be produced from environmentally friendly resources, such as by splitting water or from biomass.4 There are further challenges in developing methods to store and transport H2 from production sites to the end-user.3-4 For example, H2 gas compressed at 70 MPa with a hydrogen storage capacity of 100 2 ACS Paragon Plus Environment

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wt % could be transported in cylinders.4-5 However, the amount of H2 carried would be small owing to its low volumetric density (39.0 kg m-3 at 70 MPa).4 Because liquefied H2 has a higher volumetric density (71.2 kg m-3) it is another candidate for carrying hydrogen energy, although it is produced at low temperatures (20 K) resulting in high production costs.4-7 Other methods for transporting hydrogen energy are hydrogen energy carriers such as methylcyclohexane (MCH) and NH3.6-13 These materials could be easily liquefied at room temperature under atmospheric pressure for MCH and 0.8 MPa for NH3. Therefore, hydrogen energy carriers6-8,

10-13

and the catalysts for hydrogenation and

dehydrogenation of the carriers7, 13-18 have been investigated. We have focused on NH3 because it has a higher hydrogen storage capacity (17.6 wt %) than liquid organic hydrides, such as a MCH (6.2 wt %), and a higher hydrogen volumetric density (106.1 kg m-3) than compressed H2 gas and liquefied H2.10-11, 13, 19-21

Moreover, NH3 decomposition with the catalyst produces COx-free H2, indicating that NH3 does

not contribute to global warming. NH3 transported from the production site must be decomposed at the end-use site. It is desirable for on-site H2 production to require low temperatures for NH3 decomposition because the reaction heat is typically provided by waste heat from nearby facilities or the combustion heat of H2 produced by NH3 decomposition. There are two main strategies to achieve a low reaction temperature for NH3 decomposition.19-41 The first is developing catalysts with sufficiently high activity at low temperatures.19-34 Choudhary et al. reported catalytic NH3 decomposition to produce COx-free H2 in fuel cell systems.22 They found that supported Ru catalysts showed higher activity than Ni and Ir catalysts. Yin et al. also reported that a multiwall carbon nanotube (CNT)-supported Ru catalyst showed the best performance for the catalytic decomposition of NH3 among catalysts prepared with various supports (CNTs, active carbon, Al2O3, MgO, TiO2, ZrO2) and metals (Ru, Rh, Pt, Pd, Ni, Fe).23 In addition, the activity of supported metal catalysts was improved by adding K, Ba, and Cs as promoters,19, 24-26 modifying the support with metal hydroxides or metal oxides or nitrogen,13, 20, 27-30 and by using different starting materials for preparing the support.21 Although Ru-based catalysts exhibited 3 ACS Paragon Plus Environment


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the best performance, their activities at reaction temperatures below 673 K were not sufficient for on-site H2 production. Recently, catalysts with high catalytic activities (NH3 conversion of 52–92%) at 623 K with gas hourly space velocities (GHSVs) of 3000 mL-NH3 g-cat-1 h-1 have been developed.31-34 Although these catalysts are promising for on-site H2 production from NH3 decomposition, they require either complex preparation procedures for the support and catalyst, or they use precious elements for the catalyst, and thus are not suitable for industrial scale production. The second method is accelerating the NH3 decomposition reaction rate by incorporating a selective H2 permeation membrane wall into the reactor packed with the catalyst.35-41 Because the H2 produced selectively permeates through the membrane, the thermodynamic equilibrium of the reaction shifts toward the product side. Consequently, the reaction rate increases and the temperature at which the same NH3 conversion rate is achieved decreases. Zhang et al. reported the first trial of a H2 generation system (membrane reactor), which consisted of a Ni-based catalyst and a Pd membrane separation wall.35 The NH3 conversion increased in the presence of the Pd membrane wall inside the reactor, and high purity (99.96%) H2 was collected with a high H2 recovery yield (77%) at 773 K. Other examples of a similar increase in reaction rate from inserting a membrane wall into a reactor have been reported.36-41 García-García et al.36 examined catalytic NH3 decomposition for H2 production with a highly active Ru/carbon catalyst coupled with a Pd membrane, and found that thermodynamic equilibrium conversion was exceeded at temperatures higher than 630 K with this system. Li et al.37-39 found that NH3 conversion increased with H2 extraction by using a γ-Al2O3-supported Ru catalyst combined with a silica separation layer. We also reported a 15% increase in NH3 conversion compared with conventional packed reactor and a H2 recovery of 60% at 723 K for a Ru/SiO2 catalyst combined with a Pd membrane (thickness 200 µm).42 We proposed that further increases could be achieved by using a thinner Pd membrane and a more active catalyst. Although a higher NH3 conversion and high purity H2 could be obtained with a multifunctional membrane reactor, the performance of all these systems could be increased by more active catalysts. 4 ACS Paragon Plus Environment

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Catalysts should be prepared by a simple method from a conventional oxide support and a cheap Ru precursor, and the catalyst should show activity for NH3 decomposition comparable to previously reported catalysts.31-34 In a preliminary study,43 we prepared several Ru-based catalysts by a chemical reduction method and by an impregnation method from a conventional oxide supports and a RuCl3 precursor, and we used the catalysts in the NH3 decomposition reaction. Cl from the Ru precursor decreases the catalytic performance for NH3 synthesis and decomposition reactions.27, 44-47 Moreover, a reducing agent, such as NaBH4, hydrazine, or LiAlH4, can help to remove Cl from the catalyst surface during preparation.45-47 Consequently, our results also showed that Cl-free Ru-based catalysts could be prepared by a chemical reduction method using NaBH4 as a reducing agent and that these catalysts showed higher activity than those prepared by the impregnation method. In addition, the Ru/CeO2, Ru/MgO, and Ru/ZrO2 catalysts exhibited high activities for NH3 decomposition to produce H2. Thus, in this work, we investigate Ru/ZrO2 further. First, Ru/ZrO2 catalysts are prepared by the chemical reduction and impregnation methods, and then they are used in the NH3 decomposition reaction to investigate the effect of reaction temperatures on their activities. The effect of H2 pretreatment on the catalytic performance of the Ru/ZrO2 catalyst is also discussed based on the characterization results.

2. Experimental 2.1 Catalyst preparation Two methods were used to prepare Ru/ZrO2 catalysts in the present study. In both cases, the ZrO2 support (JRC-ZRO-3, Catalysis Society of Japan, 113 m2 g-1) was dried at 383 K overnight prior to catalyst preparation. For the impregnation method, the Ru/ZrO2 catalyst was prepared by impregnating the dried ZrO2 powder with an aqueous solution of RuCl3‧3H2O (Kanto Chemical Co.) at 398 K under stirring followed by overnight drying at 383 K. This catalyst is denoted as Ru/ZrO2-imp1. A further calcination in Air at 773 K for 5 h was conducted for Ru/ZrO2-imp1 catalyst, and the obtained sample is denoted as Ru/ZrO2-imp2 in this study. For the chemical reduction method, the dried ZrO2 powder was 5 ACS Paragon Plus Environment


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stirred in an aqueous solution of RuCl3‧3H2O at room temperature for 1 h and NaBH4 (Kanto Chemical Co., 0.374 g) in ethanol (10 mL) and water (10 mL) was added to the mixture. The mixture was stirred at room temperature for 2 h and heated at 333 K with the addition of water (100 mL). After 2 h the product was separated from the mixture by filtration, washed with hot water (333 K) several times, and dried at 383 K overnight. This catalyst is denoted as Ru/ZrO2-lpr. The Ru loading was adjusted to 5 wt % by controlling the amounts of ZrO2 powder and the aqueous solution of RuCl3‧3H2O.

2.2 Characterization Nitrogen adsorption isotherms of samples (0.1 g) were measured by using a surface area analyser (Nova 1200e, Quantachrome Instruments Japan G.K.) to calculate the specific surface area by the BET method. The samples were degassed at 473 K for 2 h before the measurements and the adsorption isotherms were recorded at 77 K in an N2 atmosphere. The amounts of Ru and Cl in the catalysts were measured by X-ray fluorescence spectrometry (XRF; ZSX primus II, Rigaku Co.). X-ray photoelectron spectroscopy (XPS) measurements (ESCA PHI5000 Versa Probe II, ULVAC-PHI Inc.) were taken with an Al Kα X-ray source operating at 25 W (15 kV, 1.7 mA). We measured the Ru/ZrO2-imp1 and Ru/ZrO2-lpr catalysts treated in Ar at 773 K for 1 h and reduced with H2 at 723 K for 2 h, and the Ru/ZrO2-lpr catalyst treated in Ar at 773 K for 1 h, followed by 100% NH3 feed gas at 623 K for 5 min. The samples were mounted on an indium film fixed to the specimen stage by carbon tape, and were placed in the main chamber at 3.5 × 10-7 Pa. The specimens were analysed at an electron take-off angle of 45° with respect to the surface plane. For all samples, the survey scans (binding energy range 0–1400 eV, pass energy 58.70 eV) and the high-resolution spectra (pass energy 58.70 eV) in the regions of Ru 3p3/2 and Zr 3d5/2 were recorded. Binding energies were normalized to the In 3d5/2 binding energy at 443.9 eV. The Ru metal dispersion and Ru metal particle size were determined by H2 pulse chemical adsorption (BELCAT-B, Microtrac BEL Co.) at 323 K assuming a 1/1 H/Ru stoichiometry. In the preliminary tests, 6 ACS Paragon Plus Environment

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the dispersion and particle size of Ru on the as-prepared catalyst and the catalyst treated in Ar at 773 K for 1 h could not be directly evaluated by H2 pulse chemical adsorption techniques. The same behaviour was observed for the catalyst collected after the NH3 reaction at 673 K for 5 h. Therefore, various combinations of H2 treatment and O2 treatment temperatures were tested to decide the temperatures prior to H2 pulse chemical adsorption experiments for these catalysts. Table 1 summarizes the results of H2 pulse chemical adsorption tests conducted for the as-prepared Ru/ZrO2-lpr catalyst treated with H2 and O2 combinations at different temperatures. As can be seen, Ru dispersion was gradually increased with a decrease in treatment temperatures until 423 K for H2 reduction, resulting in the decrease of Ru particle size from 15.6 to 8.2 nm. It suggests that a higher reduction temperature causes the aggregation of Ru particles. In contrast, an excess decrement of treatment temperature to 373 K for H2 reduction gave a lower Ru dispersion and a higher Ru particle size. This is probably due to incomplete reduction of Ru particles at 373 K. Based on the results shown in Table 1 the pretreatment reduction with H2 at 423 K for 15 min, oxidation with O2 at 473 K for 15 min, and reduction with H2 at 423 K for 15 min, was performed for the as-prepared catalyst, the catalyst treated in Ar at 773 K for 1 h, and the catalyst collected after the NH3 reaction at 673 K for 5 h. In some measurements, the catalyst treated in situ at 773 K for 1 h under Ar flow was exposed to 100% NH3 flow at 623 K for 5 min to investigate the effect of the NH3 feed on the Ru. The catalyst powder (0.05 g) collected at different steps (as-prepared, after Ar treatment, after reaction) was loaded into a specific cell and it was pretreated as described above. The Ru metal dispersion and particle size of the catalyst treated in Ar at 773 K for 1 h followed by in situ H2 reduction at 723 K for 2 h were also calculated based on the results obtained by H2 pulse chemical adsorption. The size and morphology of the catalyst collected in the different steps were observed by transmission electron microscopy (TEM; JEM-2010, JEOL).

2.3 Catalytic tests

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Catalytic tests were conducted with the atmospheric flow experimental system. The reactor was a straight stainless tube (i.d., 7 mm) and the catalyst bed was supported by two quartz wool plugs. The catalyst was pelletized, crushed, and sieved to 0.50–1.00 mm before use, and was heated under an Ar flow at 10 K min-1 to 773 K and held at 773 K for 1 h. The catalyst was subsequently cooled to 723 K in H2/Ar (50/50) flow and was reduced with 100% H2 at 723 K for 2 h. However, Ru/ZrO2-imp2 catalyst was treated in Ar flow at 773 K for 1 h followed by the reduction with 100% H2 at 873 K for 2 h. After reduction, the catalyst bed was cooled to the reaction temperature (573–673 K) under Ar flow and 100% NH3 was passed through the catalyst bed at a feed rate of 10 mL min-1, which resulted in a GHSV of 2000 mL-NH3 g-cat-1 h-1 for 0.3 g of catalyst. The effluent gases from the reactor were fed through a six-port sampling valve and the gases, including unreacted NH3, H2, and N2, were analysed by gas chromatography (GC4000, GL Sciences, equipped with a thermal conductivity detector, a Porapak T column, a 5 Å molecular sieve column, and a Porapak Q column). Unreacted NH3 was trapped in H2SO4 aqueous solution before the gas was emitted and the flow rate of the products (H2 and N2) was measured by a film flow meter (STEC VP-2, HORIBA) at the end of the pipeline. After the reaction was complete, the reactant was replaced by Ar by switching the valve at the reaction temperature. The catalyst was cooled to room temperature and used for further characterization. In preliminary tests, the catalytic activity for NH3 decomposition was evaluated by three methods. Although NH3 conversion could be calculated by using the amount of unreacted NH3 detected by a Porapak Q column during gas chromatography or the product flow rate determined by film flow meter, we calculated it from the amounts of N2 and H2 detected by the 5 Å molecular sieve column because the value calculated by the individual method was similar. Therefore, NH3 conversion was calculated based on the amount of products by using Eqs. 1 and 2:

Unreacted NH3 [%] = 100 - (H2 concentration [%] + N2 concentration [%] in the effluent gases) NH3 conversion [%] = 100 - unreacted NH3 [%]

(1) (2). 8

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When the effect of reaction temperatures on the catalytic activity was investigated, kinetic measurements were taken at temperature intervals of 50 K between 573 and 973 K. The catalyst was kept at each temperature for 2 h to obtain measurements under a steady state.

3. Results and Discussion 3.1 Catalytic activities of the reduced Ru/ZrO2 catalysts for the decomposition of NH3 Figure 1 shows the catalytic performance of reduced Ru/ZrO2-imp and reduced Ru/ZrO2-lpr catalysts for the decomposition of NH3 at 673 K with GHSV of 2000 mL-NH3 g-cat-1 h-1. The products were only N2 and H2 over a reaction time of 5 h, and NH3 conversion was stable in all cases. However, a higher conversion was observed with the reduced Ru/ZrO2-lpr catalyst than with the reduced Ru/ZrO2-imp1 and Ru/ZrO2-imp2 catalysts. It has been proposed that the presence of Cl derived from a Ru precursor decreases the catalytic performance because of the decrease in the exposed metal surface area (active sites) and the interference with metal-support or metal-promoter interactions.27, 44-47 Therefore, XRF measurements of the reduced catalysts were conducted to estimate the amounts of Ru and Cl in the catalysts (Table 2). The Ru loadings in all catalysts were similar to those calculated before preparation (5.3–5.6 wt %), although the amounts of Cl left on the catalyst were different. No Cl was observed on the reduced Ru/ZrO2-lpr catalyst and Ru/ZrO2-imp2 catalyst, whereas a large amount (5.6 wt %) of Cl was left on the reduced Ru/ZrO2-imp1 catalyst. In addition, the XRF results (data not shown) for the as-prepared Ru/ZrO2-imp catalyst showed that the amount of Cl remaining on the catalyst was 5.6 wt %. Therefore, Cl left on the as-prepared Ru/ZrO2-imp catalyst was not removed with Ar treatment at 773 K for 1 h and H2 reduction at 723 K for 2 h. In contrast, the as-prepared Ru/ZrO2-lpr catalyst was Cl-free, suggesting that reduction with NaBH4 completely removed Cl from the catalyst.45-47 The reduction treatment at higher temperature (873 K) helps to decrease the amount of Cl remained on the catalyst (Table 2; Ru/ZrO2-imp2); however, a lower NH3 conversion was observed with this catalyst than with



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the reduced Ru/ZrO2-lpr catalyst. This is probably depending on the size of Ru metal particles formed after H2 reduction treatment, and the results will be shown later. The state of Ru species on the reduced catalyst was also determined to clarify its effect on the catalytic performance of Ru/ZrO2 catalysts (Fig. 1). The reduced Ru/ZrO2-imp1 catalyst and the reduced Ru/ZrO2-lpr catalyst were analysed by XPS (Fig. 2). For the reduced Ru/ZrO2-imp1 catalyst (Fig. 2(a)), the electron emission peak at 463.25 eV in the Ru 3p3/2 region was assigned to RuCl3 according to the literatures which reported that the electron emission of this phase was observed at 463.3–463.9 eV.44, 48 In contrast, the emission peak at 462.25 eV assigned to Ru metal30, 44, 49 was observed for the reduced Ru/ZrO2-lpr catalyst (Fig. 2(b)). These narrow XPS profiles suggested that Cl remained on the reduced Ru/ZrO2-imp1 catalyst and most Ru existed as RuCl3, whereas Ru metal was formed on the reduced Ru/ZrO2-lpr catalyst via NaBH4 reduction and H2 pretreatment. H2 pulse chemical adsorption results (Table 2) indicated that the amount of chemisorbed H2 on the catalyst was too low to calculate the exposed Ru metal surface area, resulting in a low apparent Ru dispersion and high apparent Ru particle size. These results also showed that most of the Ru on the reduced Ru/ZrO2-imp1 catalyst was present as RuCl3 and that little Ru metal was formed despite the H2 reduction treatment at 723 K for 2 h. H2 reduction at 723–773 K is sufficient to remove Cl from catalysts prepared by the impregnation method with RuCl3 as a precursor27, 47, 50-52; however, a long treatment time (6–24 h),47, 51-52 slow heating rate (2 K min-1),47, 52 or two step reduction50 is necessary for complete removal. The short treatment time (2 h) and rapid heating rate (10 K min-1) in this study may explain the insufficient removal of Cl. In contrast, the amount of chemisorbed H2 on the reduced Ru/ZrO2-imp2 catalyst could be calculated, indicating that the reduction treatment at 873 K for 2 h is sufficient for the removal of Cl from the catalyst and the formation of Ru metal particles. The dispersion and particle size of Ru metal on this catalyst were estimated to be 0.14% and 931.6 nm, respectively. Similarly, Ru metal was formed on the reduced Ru/ZrO2-lpr catalyst, and the dispersion and particle size of Ru metal were estimated to be 11.8% and


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11.4 nm, respectively. These results suggest that Ru metal particle size affected the activity for NH3 decomposition reaction (Fig. 1). The XRF, XPS, and H2 pulse chemical adsorption results showed that the complete removal of Cl derived from the Ru precursor, and the formation of Ru metal on the catalyst are important for high catalytic performance in the decomposition of NH3. Because the reduced Ru/ZrO2-lpr catalyst exhibited higher catalytic activity than the reduced Ru/ZrO2-imp catalysts, we investigated the catalyst prepared by chemical reduction further.

3.2 Effect of reaction temperatures on catalytic activities The decomposition of NH3 is endothermic reaction; thus, NH3 conversion is increased by increasing the reaction temperature.13, 19, 22-23, 25, 27, 32, 41-42 Although the reduced Ru/ZrO2-lpr catalyst showed NH3 conversion of around 60% at 673 K with GHSV of 2000 mL-NH3 g-cat-1 h-1, we expected that a higher conversion would be obtained by increasing the reaction temperature. Thus, kinetic measurements were performed at temperature intervals of 50 K from 573 to 973 K (Fig. 3). NH3 conversion increased with the reaction temperature to 723 K and remained around 100% between 723 and 973 K. Detail NH3 conversion obtained between 723 and 973 K were 98.11, 99.29, 99.35, 99.74, 99.85, and 99.95% at 723, 773, 823, 873, 923, and 973 K, respectively. According to the thermodynamic analysis for this reaction,23 NH3 conversion reaches more than 98% at 623 K, remains around 99.5% to 773 K, and never reaches 100% due to thermodynamic limitations, even when the temperature reaches 973 K. Considering the thermodynamic equilibrium conversion, the catalytic activity of the reduced Ru/ZrO2-lpr catalyst was insufficient between 573 and 673 K with GHSV of 2000 mL-NH3 g-cat-1 h-1, although its performance was excellent at temperatures above 723 K.

3.3 Catalytic performance with or without H2 pretreatment



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A supported metal catalyst is generally activated by H2 reduction treatment to expose the metal phase to the reactants. Because H2 pretreatment is a necessary step for NH3 decomposition with Ru-based catalyst, the origin of the H2 for the pretreatment should be considered for practical applications. A simple solution is using the H2 produced by NH3 decomposition; however, this decreases the net amount and recovery yield of H2. Thus, a catalyst that does not require H2 pretreatment prior to NH3 decomposition reaction is highly desirable. Moreover, although in the Ru/ZrO2-lpr catalyst Ru metal was formed by chemical reduction with NaBH4 followed by reduction with H2 at 723 K for 2 h, it was not clear in which step the Ru metal phase was formed. If Ru metal was formed by chemical reduction with NaBH4 during the preparation, the as-prepared Ru/ZrO2-lpr catalyst or the Ru/ZrO2 catalyst treated with Ar (773 K, 1 h) could be used directly for the NH3 decomposition reaction without H2 pretreatment. Thus, the Ru/ZrO2-lpr catalyst treated with Ar and the reduced Ru/ZrO2-lpr catalyst were tested for NH3 decomposition at 623 and 673 K with GHSV of 2000 mL-NH3 g-cat-1 h-1 and the results are shown in Fig. 4. Surprisingly the Ru/ZrO2-lpr catalyst treated with Ar at 773 K for 1 h also exhibited NH3 conversion at both reaction temperatures, and a higher NH3 conversion was obtained by using the catalyst without H2 pretreatment than the reduced Ru/ZrO2 catalyst. This result indicated that the catalyst could be activated by NH3 itself or H2 produced by initial NH3 decomposition on the catalyst. Once the catalyst was activated, NH3 conversion was stable over 5 h at both reaction temperatures. In addition, the NH3 conversion increased with temperature for both catalysts. This observation was explained by the increase in reaction rate with the reaction temperature and the formation of Ru metal depending on the temperature of the NH3 feed. These results are discussed in more detail in relation to other characterization results. The effect of H2 pretreatment on the catalytic performance of the Ru/ZrO2-lpr catalyst was observed, and the reduced catalyst showed a lower NH3 conversion than the catalyst without pretreatment. Thus, these catalysts were characterized by TEM and H2 pulse chemical adsorption techniques. Figure 5 shows the TEM images and particle size distribution of the as-prepared catalyst, the catalyst treated with 12 ACS Paragon Plus Environment

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Ar at 773 K for 1 h, the catalyst reduced at 723 K for 2 h, the catalyst used for NH3 decomposition at 623 K for 5 h, the catalyst used for NH3 decomposition at temperature intervals of 50 K from 573 to 973 K, and the catalyst treated with Ar at 773 K for 1 h after it was used for NH3 decomposition at 623 K for 5 h. In the TEM images, Ru particles were visible as small black spots, and the average Ru particle size was calculated by counting 200 particles in the TEM images. In addition, the distribution of particle size estimated from each TEM image is also shown in Fig. 5. The average Ru particle size was estimated to be 5.5 nm from the image of the as-prepared catalyst (Fig. 5(a)) and 6.3 nm for the catalyst treated with Ar at 773 K for 1 h (Fig. 5(b)). This indicates that Ar treatment had a negligible effect on the particle size of Ru on the catalyst. In contrast, the average Ru particle size increased to 9.3 nm after H2 pretreatment at 723 K for 2 h (Fig. 5(c)) suggesting that the Ru metal particles aggregated during H2 treatment of the Ru/ZrO2 catalyst. These observations were confirmed by the H2 pulse chemical adsorption results (Table 3). The as-prepared catalyst, the catalyst treated with Ar, and the catalyst used for the NH3 reaction at 673 K for 5 h were subjected to the pretreatment. As shown in Table 1, a H2 treatment temperature should be more than 423 K and the O2 treatment should be 50 K higher to expose all the Ru metal surfaces on the catalyst. However, H2 reduction treatment above 473 K caused the Ru metal on the catalyst to aggregate. Thus, for the Ru/ZrO2-lpr catalyst, although H2 reduction above 473 K formed the Ru metal phase, it also caused the sintering of the Ru metal particles. Therefore, the aggregation of Ru metal particles probably occurred after H2 reduction pretreatment at 723 K for the catalyst treated in Ar at 773 K. Table 3 shows that the Ru particle sizes calculated from the H2 pulse chemical adsorption results before and after Ar treatment was similar (5.1 vs. 5.6 nm), whereas H2 reduction at 723 K for 2 h increased the size from 5.6 to 11.4 nm. These results were consistent with the TEM observations in Fig. 5. The TEM and H2 pulse chemical adsorption results demonstrate that the increase in Ru metal particle size decreased NH3 conversion (Fig. 4) because a lower dispersion of Ru metal particles decreases the number of active sites for NH3. The Ru metal phase was formed above 423



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K under H2, and consequently the increase in NH3 conversion with reaction temperature (Fig. 4) probably arose from the increase in reaction rate with temperature. A further H2 pulse chemical adsorption experiment was performed for the catalyst used in NH3 decomposition at 673 K for 5 h and the catalyst exposed to 100% NH3 at 623 K for 5 min. The particle size of Ru metal on the catalyst used in NH3 decomposition decreased from 11.4 to 7.0 nm. The TEM image (Fig. 5(d)) of the catalyst used for NH3 decomposition also indicated that the particle size of Ru metal was decreased from 9.3 to 4.5 nm after the reaction. The average Ru metal particle size was estimated as 7.7 nm from TEM image (Fig. 5(e)) for the catalyst used for NH3 decomposition at 50 K intervals from 573 to 973 K. These results suggest that the Ru metal particles were redispersed during NH3 decomposition regardless of the reaction temperature. Although it is not clear whether NH3 or H2 redispersed the Ru metal particles during the reaction, the redispersion would explain the stability of NH3 conversion over 5 h, whereas the BET surface area of the catalyst decreased after the reaction (Table 3). The Ru 3P3/2 XPS emission peak at 462.25 eV assigned to Ru metal30, 44, 49 was observed for the catalyst treated in Ar followed by exposure to 100% NH3 at 623 K for 5 min (Fig. 2(c)). Thus, Ru metal particles were formed by brief exposure to NH3 at 623 K. The particle size of the Ru metal formed on the catalyst was calculated to be 6.7 nm from the H2 chemical adsorption results (Table 3). These results indicated that the Ru/ZrO2-lpr catalyst treated with Ar could be activated by exposure to 100% NH3 at the reaction temperature, although it was not clear whether NH3 or H2 produced from NH3 decomposition reduced the catalyst. Furthermore, the TEM image (Fig. 5(f)) of the catalyst used for NH3 decomposition at 623 K for 5 h showed that the average Ru particle size was 4.1 nm, which was smaller than that of the catalyst exposed briefly to NH3 at 623 K calculated from the H2 chemical adsorption result (6.7 nm, Table 3). The result also indicates that the Ru metal particles were redispersed during the NH3 decomposition reaction at 623 K.


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4. Conclusion Ru/ZrO2 catalysts were prepared by two methods, and the activity of the catalysts for the NH3 decomposition reaction was tested. The catalyst prepared by chemical reduction with NaBH4 displayed higher catalytic activity than the catalyst prepared by the impregnation method. The characterization results indicated that the important factors for high NH3 conversion were complete removal of Cl from the catalyst surface and formation of Ru metal. The catalytic activity increased with increasing reaction temperature. Surprisingly, the Ru/ZrO2 catalyst prepared by chemical reduction with NaBH4 was active for NH3 decomposition without H2 pretreatment, and the results indicated that the catalyst was activated by NH3 or by H2 produced by the NH3 decomposition reaction. Moreover, the size of the Ru metal particles formed on the catalyst affected the NH3 conversion, and the Ru metal particles were redispersed during the NH3 decomposition reaction regardless of reaction temperature from 573 to 973 K.

5. Acknowledgements This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program, “Energy Carrier” (funding agency: JST).

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Figure captions Figure 1 Catalytic performance of the reduced Ru/ZrO2 catalysts for the NH3 decomposition reaction at 673 K with GHSV of 2000 mL-NH3 g-cat-1 h-1 as a function of time on stream: ■; Ru/ZrO2-lpr, ○; Ru/ZrO2-imp1, ●; Ru/ZrO2-imp2. Figure 2 XPS narrow spectra (Ru 3p3/2 region) of (a) the reduced Ru-ZrO2-imp1 catalyst, (b) the reduced Ru/ZrO2-lpr catalyst, and (c) the catalyst treated with Ar at 773 K for 1 h followed by exposure to 100% NH3 feed gas at 623 K for 5 min. Figure 3 NH3 conversion profile obtained by using the reduced Ru/ZrO2-lpr catalyst and increasing the reaction temperature from 573 to 973 K under GHSV of 2000 mL-NH3 g-cat-1 h-1: ------ thermodynamic equilibrium line. Figure 4 Catalytic performance of the Ru/ZrO2 catalysts with or without H2 pretreatment at 723 K for 2 h for NH3 decomposition reaction at 623 or 673 K under GHSV of 2000 mL-NH3 g-cat-1 h-1 as a function of time on stream: ■; 623 K with H2 pretreatment, □; 623 K without H2 pretreatment, ●; 673 K with H2 pretreatment, ○; 673 K without H2 pretreatment. Figure 5 TEM images and Ru particle size distribution of Ru/ZrO2-lpr catalysts: (a) as-prepared, (b) after pretreatment at 773 K for 1 h under Ar, (c) after Ar treatment and H2 reduction at 723 K for 2 h, (d) after NH3 decomposition at 623 K for 5 h, (e) after NH3 decomposition at temperature intervals of 50 K from 573 to 973 K, and (f) after using the catalyst pretreated under Ar for NH3 decomposition at 623 K for 5 h.



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Table 1 H2 pulse chemical adsorption results obtained for the as-prepared Ru/ZrO2-lpr catalyst treated with H2 and O2 combinations at different temperatures1) H2 pulse chemical adsorption results H2 reduction O2 treatment temperature [K] temperature [K] Ru dispersion [%] Ru particle size [nm] 573 623 8.6 15.6 473 523 10.9 12.3 423 473 16.5 8.2 373 423 12.6 10.7 1) The results displayed in this table were obtained using the Ru/ZrO2-lpr catalyst prepared in a different batch from one shown in Table 2 and Table 3.

Table 2 Characterization of the reduced Ru/ZrO2 catalysts prepared by chemical reduction and by impregnation XRF H2 pulse chemical adsorption Catalyst Ru [wt %] Cl [wt %] Ru dispersion [%] Ru particle size [nm] Ru/ZrO2-imp1 5.6 5.6 0.67 198.5 Ru/ZrO2-imp2 5.6 0.17 0.14 931.6 Ru/ZrO2-lpr 5.3 0 11.8 11.4

Table 3 Characterization results for as-prepared, pre-treated, and used catalysts for the NH3 decomposition reaction TEM H2 pulse chemical adsorption BET surface Material Step 2 -1 area [m g ] Average Ru Ru dispersion Ru particle particle size [nm] [%] size [nm] ZrO2 112.8 As-prepared 103.8 5.5 26.3 5.1 Ar treatment 6.3 24.1 5.6 H2 reduction 87.8 9.3 11.8 11.4 Ru/ZrO2-lpr NH3 67.7 19.0 7.0 decomposition1) NH3 feed2) 19.9 6.7 1) After using the reduced catalyst for the NH3 decomposition reaction at 673 K for 5 h. 2) After exposing the catalyst treated with Ar to 100% NH3 feed gas at 623 K for 5 min.


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Figure 1 Catalytic performance of the reduced Ru/ZrO2 catalysts for the NH3 decomposition reaction at 673 K with GHSV of 2000 mL-NH3 g-cat-1 h-1 as a function of time on stream: ■; Ru/ZrO2-lpr, ○; Ru/ZrO2-imp1, ●; Ru/ZrO2-imp2.



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Figure 2 XPS narrow spectra (Ru 3p3/2 region) of (a) the reduced Ru-ZrO2-imp1 catalyst, (b) the reduced Ru/ZrO2-lpr catalyst, and (c) the catalyst treated with Ar at 773 K for 1 h followed by exposure to 100% NH3 feed gas at 623 K for 5 min.


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Figure 3 NH3 conversion profile obtained by using the reduced Ru/ZrO2-lpr catalyst and increasing the reaction temperature from 573 to 973 K under GHSV of 2000 mL-NH3 g-cat-1 h-1: -----thermodynamic equilibrium line



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Figure 4 Catalytic performance of the Ru/ZrO2 catalysts with or without H2 pretreatment at 723 K for 2 h for NH3 decomposition reaction at 623 or 673 K under GHSV of 2000 mL-NH3 g-cat-1 h-1 as a function of time on stream: ■; 623 K with H2 pretreatment, □; 623 K without H2 pretreatment, ●; 673 K with H2 pretreatment, ○; 673 K without H2 pretreatment.


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Figure 5 TEM images and Ru particle size distribution of Ru/ZrO2-lpr catalysts: (a) as-prepared, (b) after pretreatment at 773 K for 1 h under Ar, (c) after Ar treatment and H2 reduction at 723 K for 2 h, (d) after NH3 decomposition at 623 K for 5 h, (e) after NH3 decomposition at temperature intervals of 50 K from 573 to 973 K, and (f) after using the catalyst pretreated under Ar for NH3 decomposition at 623 K for 5 h. 28 ACS Paragon Plus Environment

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Graphical abstract


Reduction and Their Catalytic Activity f - American Chemical Society

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