Influence of Cation Substitutions Based on ABO3 Perovskite Materials

Research Article pubs.acs.org/journal/ascecg

Influence of Cation Substitutions Based on ABO3 Perovskite Materials, Sr1−xYxTi1−yRuyO3−δ, on Ammonia Dehydrogenation Hyunmi Doh,†,⊥ Hyo Young Kim,‡,⊥ Ghun Sik Kim,† Junyoung Cha,† Hyun S. Park,† Hyung Chul Ham,†,§ Sung Pil Yoon,† Jonghee Han,† Suk Woo Nam,† Kwang Ho Song,*,‡ and Chang Won Yoon*,†,§,∥ †

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ‡ Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea § Division of Energy and Environment Technology, KIST School, Korea University of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ∥ KHU-KIST Department of Converging Science and Technology, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Republic of Korea S Supporting Information *

ABSTRACT: In order to screen potential catalytic materials for synthesis and decomposition of ammonia, a series of ABO3 perovskite materials, Sr1−xYxTi1−yRuyO3−δ (x = 0, 0.08, and 0.16; y = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) were synthesized and tested for ammonia dehydrogenation. The influence of A or B site substitution on the catalytic ammonia dehydrogenation activity was determined by varying the quantity of either A or B site cation, producing Sr1−xYxTi0.92Ru0.08O3−δ and Sr0.92Y0.08Ti1−yRuyO3−δ, respectively. Characterizations of the as-synthesized materials using different analytical techniques indicated that a new perovskite phase of SrRuO3 was produced upon addition of large amounts of Ru (≥12 mol %), and the surface Ru0 species were formed simultaneously to ultimately yield Ruz(surface)/Sr0.92Y0.08Ti1−yRuy−zO3−δ and/or Ruz−w(surface)/SrwRuwO3/Sr0.92−wY0.08Ti1−yRuy−zO3−δ. The newly generated surface Ru0 species at the perovskite surfaces accelerated ammonia dehydrogenation under different conditions, and Sr0.84Y0.16Ti0.92Ru0.08O3−δ exhibited a NH3 conversion of ca. 96% at 500 °C with a gas hourly space velocity (GHSV) of 10 000 mL gcat−1 h−1. In addition, Sr0.84Y0.16Ti0.92Ru0.08O3−δ further proved to be highly active and stable toward ammonia decomposition at different reaction temperatures and GHSVs for >275 h. KEYWORDS: Ammonia dehydrogenation, Ammonia synthesis, Ruthenium, Perovskite, Hydrogen production

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(NH3BH3)7−9 have been proposed as capable chemical hydrogen storage materials that can potentially store a large quantity of hydrogen in a safe manner. More recently, liquid and/or gaseous chemical hydrogen storage materials have also been studied as hydrogen carriers because the physical nature of these materials could likely allow cost-effective hydrogen storage and delivery at desired sites using the existing infrastructures. Among such candidates, ammonia (NH3) has attracted significant attention because it has a considerably high gravimetric hydrogen storage density of 17.7 wt % and can provide a high energy density of 3 kWh/kgNH3.10,11 Moreover, ammonia can easily be liquefied under relatively moderate conditions (20.0 °C and 0.86 MPa), which allows economical hydrogen storage and delivery. Furthermore, regeneration of

INTRODUCTION Owing to the global energy and environment crises caused by the extensive use of carbon-based fuels, clean and sustainable energy sources are constantly being explored.1−4 Hydrogen has recently been recognized as a promising alternative to fossil fuels because it has a significantly high gravimetric energy density of 142 MJ kg−1, which is ca. 3 times higher than that of gasoline.4 In addition, utilization of hydrogen via fuel cells releases water as the only byproduct, which is beneficial to the environment. Despite these advantages, the low volumetric energy density4 of 10.1 MJ L−1 makes hydrogen less attractive for a number of stationary applications such as hydrogen refueling stations, off-grid power generators, and energy storage systems. Therefore, it is necessary to develop economically viable hydrogen storage systems with high gravimetric and/or volumetric storage densities for a variety of purposes. In order to address this issue, boron containing compounds such as sodium borohydride (NaBH4)5,6 and ammonia borane © 2017 American Chemical Society

Received: July 17, 2017 Revised: August 28, 2017 Published: August 29, 2017 9370

DOI: 10.1021/acssuschemeng.7b02402 ACS Sustainable Chem. Eng. 2017, 5, 9370−9379

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ACS Sustainable Chemistry & Engineering

sulting materials showed high activity and durability for NH3 dehydrogenation under different conditions.

ammonia is readily achievable by the well-established Haber− Bosch process, making an ammonia-based hydrogen storage and release cycle feasible for power generation. In addition to the commercial synthetic process, metal hydride mediated synthesis of ammonia12 and electrochemical synthesis13−15 of ammonia utilizing electrical energy input from renewable energies have recently reported as an alternative method. However, for ammonia to be useful as a hydrogen carrier for polymer electrolyte membrane fuel cells (PEMFCs), a drastically high ammonia conversion close to 100% is required to prevent stack deactivation and to build a lightweight hydrogen purification system with high efficiency. Therefore, it is necessary to elucidate the various controlling factors associated with NH3 decomposition in order to develop highly capable catalysts. A number of heterogeneous catalysts that possess distinct transition metals such as Fe, Ru, Co, Rh, Ir, and Ni have been extensively reported for ammonia dehydrogenation (2NH3 → N2 + 3H2).11,16,17 Among these materials, Ru based compounds have been proven to be excellent catalysts for the targeted reaction.18,19 The desired activities with different catalysts have been found to be dependent on the type of support. For instance, Ru/carbon nanotubes, Ru/MgO, Ru/TiO2, Ru/ZrO2, Ru/Al2O3, and Ru/SiO2 showed different activities toward ammonia decomposition, emphasizing the importance of the metal−support interactions on the rate of H2-release from ammonia.19 In line with these previous results, we likewise found support-dependent activity toward ammonia dehydrogenation with Ru/La(x)−Al2O3 catalysts (x = 0−50 mol %).20 The activities for ammonia dehydrogenation increased as the La doping level increased in the catalytic support, La(x)−Al2O3, and a new perovskite phase, LaAlO3 was formed upon increasing the La doping level to 50 mol %. The resulting Ru/LaAlO3 catalyst showed a superior catalytic activity as well as an excellent durability even at 500 °C, which was explained by the improved interactions between the Ru active sites and the perovskite LaAlO3 support. In fact, Ru catalysts supported on numerous perovskite materials have been found to catalyze ammonia synthesis. For example, Ru/BaZrO3,21 Ru/BaTiO3,22 and Ru/BaCeO323 exhibited activities toward ammonia synthesis, which again resulted from strong metal−support interactions. Furthermore, control over A and B site elements in ABO3 perovskite materials was demonstrated to improve activities for several catalytic reactions.24,25 Besides, Kojima et al. demonstrated that perovskite materials, SrTiO3 and BaTiO3 could decompose ammonia to produce hydrogen.26 Our preliminary findings of the Ru catalyst supported on a perovskite LaAlO3 support, in conjunction with the previous reports of ammonia synthesis and decomposition as well as different catalytic reactions based on perovskite materials, have prompted us to investigate other perovskite phases as potential supports for ammonia dehydrogenation. Here we investigate possible roles of Sr1−xYxTi1−yRuyO3−δ, a highly versatile material for high temperature fuel cells (e.g., solid oxide fuel cell), in facilitating ammonia dehydrogenation by adding different quantities of A and B site substituents. Numerous analyses suggested that the original SrTiO3 material was incorporated with yttrium and/or ruthenium to finally give metallic Ru0 species supported on diverse perovskite materials as R u z (surface)/Sr 0 . 9 2 Y 0 . 0 8 Ti 1 − y R u y − z O 3 − δ and/or Ruz−w(surface)/SrwRuwO3/Sr0.92−wY0.08Ti1−yRuy−zO3−δ. The re-

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EXPERIMENTAL DETAILS

Materials and Characterization. Titanium(IV) isopropoxide (Ti(OC3H7)4, Junsei), ruthenium chloride hydrate (RuCl3·xH2O, Sigma-Aldrich), strontium nitrate (Sr(NO3)2, Sigma-Aldrich), yttrium nitrate (Y(NO3)3, Sigma-Aldrich), ethylene glycol (Sigma-Aldrich), and citric acid (Sigma-Aldrich) were used as received unless noted otherwise. In order to characterize a synthesized material, several different analytical techniques were employed. Chemical compositions of the as-synthesized materials were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Varian 5100 Agilent). The specific surface area, pore size, and pore volume of a sample were measured using a Brunauer−Emmett−Teller (BET) instrument (Micrometrics ASAP2000) at 77 K. Prior to the measurements, the materials were degassed at 473 K for 12 h to remove surface impurities and moisture. X-ray diffraction (XRD) studies were conducted by Rigaku, MiniFlex with Cu Kα radiation at 40 kV and 20 mA. Continuous scanning at a rate of 4° min−1 was conducted in the 2θ range of 10°− 90°. X-ray photoelectron spectroscopy (XPS) was employed to determine the chemical states of a sample by using Thermo Scientific K-alpha+ instrument with a source of Al Kα radiation calibrated by the Au 4f7/2 (83.96 eV), Ag 3d5/2 (368.21 eV), and Cu 2p3/2 (932.62 eV) energies with a background pressure of 5.0 × 10−7 Pa. In order to clarify each chemical state of a sample, the obtained broad peaks were deconvoluted with Lorentzian and Gaussian functions (Lorentzian:Gaussian = 20:80) using a software, XPS peak 4.1. For Ru 3d XPS analyses, only Ru 3d5/2 peak of a sample was used for comparison because the C 1s and Ru 3d3/2 peaks were overlapped in the range of 282−286 eV. Morphological studies in conjunction with composition analysis, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, and energy dispersive X-ray spectroscopy (EDX) elemental mapping were conducted by transmission electron microscopy (TEM, FEI, Talos F200X). Temperature-programmed reduction (TPR) experiments were carried out using 5% H2/Ar with a flow rate of 40 mL/min by scanning the temperature from 30 °C to 1000 °C at a heating rate of 5 °C/min. Each sample was calcined at 300 °C for 1 h to remove impurities under Ar atmosphere prior to the measurements. Hydrogen consumption was monitored by a thermal conductivity detector (TCD). M a t e r i a l s P r e p a r at i o n . T h e p e r o v s k i te m a te r ia l s, Sr1−xYxTi1−yRuyO3−δ, were prepared by a modified Pechini method.27 In a typical experiment, a desired amount of titanium(IV) isopropoxide was added into ethylene glycol with stirring in a glass beaker. Citric acid was added into the solution, producing titanium(IV) citrate complexes in situ in the mixture. A clear and yellowish liquid appeared shortly after stirring the mixture. A given amount of ruthenium chloride hydrate dissolved in deionized water (100 mL) was then added to the yellowish solution. Next, strontium nitrate and yttrium nitrate were added into the solution. When the Sr, Y, Ru, and Ti containing precursors were fully dissolved, the color of the aqueous solution became black. The resulting homogeneous solution was heated at 80 °C with stirring for 100 min, followed by aging at 110 °C for 10 h. After the black liquid solution had fully aged, the solution was calcined at 300 °C for solidification; i.e., to remove water, the liquid was heated to up to 300 °C at a rate of 5 °C min−1 and continued to heat at this temperature for a couple of hours. The obtained powders were then sieved using a 250 μm diameter grinder in order to homogenize their composition and size. The sieved powders were uniformly blended and calcined again at 650 °C for 10 h at a heating rate of 2.5 °C min−1 to obtain the desired Sr1−xYxTi1−yRuyO3−δ materials. The detailed compositions of each precursor containing a desired element are summarized in Table S1. In the as-prepared Sr1−xYxTi1−yRuyO3−δ materials, the molar ratios x and y were controlled as follows. The molar ratios between the Sr and 9371


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ACS Sustainable Chemistry & Engineering Y containing metal precursors were adjusted according to their known valence states, ionic radii, and A-site (Sr) or B-site (Ti) occupancy. Given that Y displaces the Sr site,28 the relative molar ratios between Sr and Y were adjusted such that the total amount of these elements was fixed at 50 mmol. Likewise, owing to their valence states and sizes, Ru ion would replace Ti ion upon formation of the perovskite structure. The relative molar ratios of Ti and Ru were also varied while the total quantity of these elements was maintained to 50 mmol. The quantities of the metal precursors employed in this study are also summarized in Table S1. Ammonia Dehydrogenation. The performance of an assynthesized catalyst was assessed at atmospheric pressure by using a fixed-bed quartz reactor under pure NH3 gas flow (Figure S1). In a typical experiment, a desired catalyst (0.1 g) with a particle size of ca. 250 μm was placed in the reactor and reduced by using 50% H2/N2 at a flow rate of 80 mL min−1 at 600 °C for 2 h, followed by flushing with N2 for 1 h prior to the ammonia dehydrogenation reaction. The ammonia decomposition reaction was then conducted by using pure NH3 under a GHSVNH3 of either 10 000 or 20 000 mL gcat−1 h−1 initially at a temperature of 350 °C, which was then increased stepwise to 600 °C. During the reaction, the gas effluents were analyzed by using a tunable diode laser ammonia gas analyzer (Model Airwell+7, KINSCO Technology) in order to determine the quantity of unreacted ammonia. The obtained temperature dependent rate data were further used to calculate the apparent activation energies with the catalytic materials for NH3 dehydrogenation based on the Arrhenius equation. The ammonia conversions were calculated using eqs 1 and 2 given below (in mL).

Sr 0.92 Y 0.08 Ti 0 .9 6 Ru 0. 0 4 O 3− δ , Sr 0.92 Y 0.08 Ti 0 .9 3 Ru 0. 0 7 O 3− δ , Sr0.92Y0.08Ti0.88Ru0.12O3−δ, Sr0.92Y0.08Ti0.83Ru0.17O 3−δ, and Sr0.92Y0.08Ti0.74Ru0.26O3−δ. Likewise, a different series of Sr1−xYxTi1−yRuyO3−δ samples with the fixed amounts of Ti and Ru (y = ca. 8 mol %) were synthesized in order to determine the effect of yttrium doping level on catalytic activity. Since yttrium could be incorporated into the perovskite structure with the maximum content of ca. 14 mol % with respect to Sr,31 three samples having different Y contents (x = 0, 8, and 16 mol % relative to Sr) were prepared, yielding Sr 1 Y 0 Ti 0.91 Ru 0.09 O 3−δ , Sr 0.92 Y 0.08 Ti 0.93 Ru 0.07 O 3−δ , and Sr0.84Y0.16Ti0.92 Ru0.08O3−δ. The chemical compositions of the as-synthesized Sr1−xYxTi1−yRuyO3−δ materials were determined by ICP-OES, and it was found that the obtained atomic molar ratios were close to those predicted based on the initially added metal quantities (Table S2). The textural properties of the as-synthesized materials were determined by using the nitrogen (N2) adsorption−desorption method. The measured BET surface areas, pore diameters, and pore volumes are summarized in Table 1. The surface area of Table 1. Textural Properties of a Series of Sr1−xYxTi1−yRuy O3−δ, Determined by the BET Method control parameter

unreacted NH3 = analyzer detected NH3 (ppm) =

(inletNH3 − reactedNH3) (mL) (inletNH3 + balance N2 − reactedNH3 + produced H + produced N ) (mL) 2

× 106

=

(inletNH3 − reactedNH3) (mL) (inletNH3 + balance N2 + reactedNH3) (mL)

conversion NH3 (%) =

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reactedNH3 (mL) inletNH3 (mL)

× 10

Ru content (y value)

2

6

(1) Y content (x value)

× 100

(2)

catalyst

SBET (m2/g)

pore volume (cm3/g)

pore diameter (nm)

Sr0.92Y0.08Ti1 Ru0O3−δ Sr0.92Y0.08Ti0.96Ru0.04O3−δ Sr0.92Y0.08Tio.93Ru0.07O3−δ

41.2 37.5 37.3

0.280 0.301 0.245

25.7 31.2 25.2

Sr0.92Y0.08Ti0.88Ru0.12O3−δ Sr0.92Y0.08Ti0.83Ru0.17O3−δ Sr0.92Y0.08Ti0.74Ru0.26O3−δ Sr1Y0Ti0.91Ru0.09O3−δ Sr0.92Y0.08Ti0.93Ru0.07O3−δ Sr0.84Y0.16Ti0.92Ru0.08O3−δ

33.5 37.3 24.5 31.1 37.3 48.8

0.297 0.262 0.166 0.187 0.245 0.345

34.1 27.3 26.0 23.3 25.2 27.6

the material doped with Y (8 mol %) and containing no Ru metal (Sr0.92Y0.08Ti1Ru0O3−δ) was determined to be 41.2 m2/g, which is in good agreement with a previous report of SrTiO3 perovskite materials.32 The surface areas of Sr1−xYxTi1−yRuyO3−δ with Ru contents ranging between 4 and 17 mol % were found to be similar (33.5−37.5 m2/g) whereas that of materials with a high Ru content of 26 mol % decreased slightly to 24.5 m2/g. This result can be attributed to the formation of a new phase, SrRuO3 (vide infra), which has a decreased surface area compared to SrTiO3.32 Morphological analyses of Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) materials were then conducted in order to understand the influence of Ru content. The TEM images of Sr0.92Y0.08Ti1−yRuyO3−δ show the presence of Ru nanoparticles (NPs) of different average sizes depending on the Ru content (Figure 1a). These Ru NPs appear to be located on the support surface. In addition, the amount of the Rucontaining phases increased with increasing content of Ru. The Sr0.92Y0.08Ti0.96Ru0.04O3−δ material showed small sized Ru NPs over the support. The formed Ru NPs were clearly visible on the Sr0.92Y0.08Ti1−yRuyO3−δ materials with the added Ru content of ≥7 mol %), clearly evidenced by the HAADF-STEM images in which the Ru-containing phases are shown as bright dots (Figure S2). In particular, the Sr0.92Y0.08Ti0.74Ru0.26O3−δ materials showed some aggregated Ru NPs. In contrast, other elements (Sr, Y, Ti, and O) were found to be uniformly

RESULTS AND DISCUSSION Materials Preparation and Characterization. Strontium titanate (SrTiO3) is a material with a ABO3 perovskite structure, and its A (Sr) and B (Ti) sites can be readily substituted with other elements. For instance, the Sr site can be doped with Y to produce Sr1−xYxTiO3−δ. This material possesses increased ionic and electrical conductivities compared to SrTiO3 and has therefore been widely employed as an electrode material for high temperature fuel cells.29,30 In the Sr1−xYxTiO3−δ structure, the Ti site can likewise be displaced by Ru to generate Sr1−xYxTi1−yRuyO3−δ that could potentially be an active material for ammonia decomposition. Note that the notations Sr1−xYxTi1−yRuyO3−δ and Sr1−xYxTi1−yRuyO3−δ employed here show the relative stoichiometry with the elements added upon preparing the materials, rather than their active site compositions; e.g., if a sufficient amount of Ru is added during preparation of the material, then a different phase, Ruz(surface)/Sr1−xYxTi1−yRuy−zO3−δ may be formed and these metallic surface Ru species can likely be active sites. In order to make an initial assessment of the influence of the amount of Ru in the perovskite material on ammonia dehydrogenation, a series of Sr1−xYxTi1−yRuyO3−δ samples with the fixed amounts of Sr and Y (x = 8 mol %) were prepared. Thus, the following materials with known varying concentrations of Ru were prepared: Sr0.92Y0.08Ti1Ru0O3−δ, 9372


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Figure 1. TEM images of the as-synthesized materials (after reduction): (a) Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) and (b) Sr1−xYxTi0.92Ru0.08O3−δ (x = 0 and 0.16).

Ru content was further increased to 7 mol %, the corresponding Ru0 peaks were intensified. In contrast, the intensities of the Ru peaks decreased upon increasing the Ru content from 7 mol % to 26 mol %, and the peak with a maximum intensity centered at 46.5° was shifted to a lower degree (46.2°) to give an XRD pattern that was quite similar to that of SrRuO3 (JCPDS 85-1907). Considering the TEM results with Sr0.92Y0.08Ti1−yRuyO3−δ (≥12 mol % Ru; Figure 1a and Figure S2); however, it is conceivable that Ru0 (surface) species were still existed on these materials. In order to clarify the observed phenomena, the perovskite materials before reduction were also analyzed by XRD (Figure S4), and showed the presence of RuO2 in the samples. The intensities of the peaks corresponding to RuO2 increased as the amount of added Ru increased until 7 mol %. These results imply that the initially added Ru metals were mainly located at the perovskite surface under the employed conditions, which is again in line with TEM results. These surface RuO2 species were then transformed into metallic Ru species upon reduction with H2 (i.e., Ru z O 2 z (surface)/Sr 0 . 9 2 Y 0 . 0 8 Ti 1 − y Ru y − z O 3 − δ → Ruz(surface)/Sr0.92Y0.08Ti1−yRuy−zO3−δ). Upon addition of higher Ru contents (≥12 mol %), however, the intensities of RuO2 peak decreased. The disappearance of the RuO2 peaks with high Ru contents can partly be interpreted by the formation of a different Ru-containing phase of SrYRuO3; i.e., the B site (Ti) vacancy located near the surface became occupied with surface Ru atoms and produced a different species containing SrYRuO3 (e.g., Ruz−w(surface)/SrwRuwO3−δ/ Sr0.92−wY0.08Ti1−yRuy−zO3−δ). The formation of SrYRuO3 was further supported by the observed asymmetric peaks of Sr0.92Y0.08Ti0.74Ru0.26O3−δ, which corresponds to (211) and (310) facets of SrRuO3 wherein a portion of the Ru atoms were

distributed over the materials (Figure S2f). These results may indicate that the as-found Ru species existed as Ruy0(surface)/ S r 1 − x Y x T i 1 R u 0 O 3 − δ a n d / o r R u z0 ( s u r f a c e ) / Sr1−xYxTi1−yRuy−zO3−δ. The Sr1−xYxTi0.92Ru0.08O3−δ (x = 0, 0.08, and 0.16) materials were also employed in order to elucidate the effect of Y content. In the Y-undoped material, Sr1Y0Ti0.92Ru0.08O3−δ, small sized Ru NPs with uniform distribution were found (Figure 1b). However, upon increasing the Y quantity to Sr0.92Y0.08Ti0.92Ru0.08O3−δ and Sr0.84Y0.16Ti0.92Ru0.08O3−δ, increased amounts of Ru NPs were found to decorate the support surfaces. Moreover, HAADF-STEM and EDX mapping images showed increased Ru quantities as the Y content increased (Figure S3). These results likewise suggest the formation of Ru0z (surface)/Sr1−xYxTi0.92Ru0.08−zO3−δ. In addition, the obtained results imply that yttrium-incorporated perovskite lattice likely facilitated the formation of surface Ru0 species. This phenomenon is attributed to the n-doping with Y at the A site (Sr). In other words, because of the higher electron density induced from Y substitution,31 increased amounts of Ru ions could be reduced near the support surface. The crystal structures of the as-synthesized Sr0.92Y0.08Ti1−yRuyO3−δ materials after reduction were then characterized by XRD (Figure 2a). Figure 2a shows that the XRD patterns of Sr0.92Y0.08Ti1−yRuyO3−δ after reduction were nearly identical to those of the SrTiO3 perovskite structures, with the peak position being shifted only slightly to higher degrees. Owing to the smaller ionic radius of yttrium ion (Y3+) compared to that of the strontium ion (Sr2+), yttrium doping (8 mol %) merely affected the lattice parameters of the pristine SrTiO3 material. Upon increasing the Ru content to 4 mol %, the XRD peaks attributed to metallic Ru0 appeared. When the 9373


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(Figure 1 and Figure S3), as well as to the incorporation of Ru atoms into the lattice. Notably, the metallic Ru0 peaks appear to increase as the Y content increased. These results suggest that with increasing Y content, an increased surface electron density induced by Sr2+ substitution with Y3+ ions facilitated the reduction of Ru at the surface. XPS studies with Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) were further conducted in order to examine the chemical states of each element in the materials. Figure 3a depicts the Ru 3d5/2 peaks of Sr0.92Y0.08Ti1−yRuyO3−δ

Figure 2. XRD patterns of the as-prepared materials (after reduction): (a) Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) and (b) Sr1−xYxTi0.92Ru0.08O3−δ (x = 0, 0.08, and 0.16).

incorporated in the lattice (Figure S5), in contrast to the symmetrical peaks of Sr0.92Y0.08Ti1Ru0O3−δ wherein no Ru metal was present. Next, the perovskite materials with varying yttrium contents, Sr1−xYxTi0.92Ru0.08O3−δ were characterized using XRD. It is worth noting that the Sr1−xYxTi0.92Ru0.08O3−δ material, employed as a base material to determine the effect of Y content, has surface Ru0 species. Upon increasing the yttrium doping level from 0 up to 16 mol %, the XRD peaks at 46.5° corresponding to the (200) facet shifted toward higher degrees (Figure 2b), indicating more substitution of Sr2+ ions with the smaller sized Y3+ ions in the lattice. The peaks attributed to the metallic Ru0 species were also affected by the amount of the added Y3+ ions. No metallic Ru0 peaks appeared in the Yundoped Sr1Y0Ti0.91Ru0.09O3−δ material whereas they could be clearly observed in the Y-doped Sr0.92Y0.08Ti0.93Ru0.07O3−δ and Sr0.84Y0.16Ti0.92Ru0.08O3−δ materials. The absence of Ru0 phase in the XRD pattern with Sr1Y0Ti0.91Ru0.09O3−δ is attributable to the formation of small sized Ru NPs, as observed by TEM

Figure 3. XPS (Ru 3d) spectra: (a) Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, black; 0.04, red; 0.07, blue; 0.12, magenta; 0.17, green; and 0.26, navy) and (b) Sr1−xYxTi0.92Ru0.08O3−δ (x = 0, wine; 0.08, blue; and 0.16, pink).

as a function of Ru content. The observed broad peak in the range of 277−282 eV was deconvoluted into four peaks with maxima centered at ca. 279.1, 280.0, 280.6, and 281.4 eV which correspond to SrTiO3 (Sr 3p1/2), Ru0 (Ru 3d5/2), RuO2 (Ru 3d5/2), and SrRuO3 (Ru 3d5/2), respectively, based on the previous reports.33−35 When Ru content was 4 mol %, the peak attributed to the metallic Ru0 appears to have relatively higher intensity than those of other perovskite materials. The peak corresponding to Ru0 showed a decreased intensity but a new peak with a maximum centered at ca. 281.4 eV appeared after increasing Ru content to 7 mol %, implying that some of the Ru species began to incorporate into the lattice to form the 9374


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Figure 4. (a) Catalytic ammonia decomposition over Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, black; 0.04, red; 0.07, blue; 0.12, magenta; 0.17, green; and 0.26, navy) as functions of temperature and Ru content at different GHSVs of: (a, b) 10 000 mLNH3/(gcat·h) and (c, d) 20·000 mLNH3/(gcat·h).

XRD results that the formation of the surface Ru species was favored when a high Ru content was employed. The Sr 3d5/2 XPS peaks were shifted to higher binding energies as Y content increased (Figure S7a), which likely originates from the relatively higher electronegativity of Y3+ ion compared to that of the Sr2+ ion.31 However, the extent of shift of the XPS peaks for Y 3d were insignificant with increasing Y content, indicating that the added Y3+ ions are fully incorporated into the lattice (Figure S7b). Similarly, the peak positions for Ti 2p3/2 were nearly identical while Y content increased (Figure S7c). Reducibility of the Sr0.92Y0.08Ti1−yRuyO3−δ materials before reduction was analyzed by TPR (H2-TPR) experiments, and the obtained TRP profiles are illustrated in Figure S8. The reduction peaks with maxima centered at 100−250 °C are typically attributed to the reduction of RuO2 to Ru0,19 which is further supported by the fact that Sr0.92Y0.08Ti1Ru0O3−δ showed no reduction peak. The Sr0.92Y0.08Ti0.96Ru0.04O3−δ material showed multiples reduction peaks, and the corresponding reduction peaks for Sr0.92Y0.08Ti1−yRuyO3−δ (Ru ≥ 7 mol %) were shifted into higher reduction temperatures when Ru content increased. In addition, the multiple peaks seem to merge into a single and broad peak upon utilization of a high Ru content (26 mol %), which is highly associated with the formation of a different perovskite phase.36 These reduction behaviors may be associated with metal−support interaction since perovskite supports likely interact with active sites. Reduction of metal oxide species supported on supports could be hindered or facilitated depending on the nature of metal− support interactions.37 The increased reduction temperatures of

SrRuO3 phase. The observed partial phase transition to SrRuO3 became apparent when Ru content increased further from 7 mol % to 26 mol %. In parallel, the quantity of metallic Ru0 species increased with increasing Ru content, which is consistent with the TEM results (Figure 1a and Figure S2). Moreover, it was found that the intensities of the peak corresponding to RuO2 seem to increase as Ru content increases, which results from the formation of SrRuO3 and/or Sr0.92Y0.08Ti1−yRuyO3−δ. The observation is further supported by XPS (Sr 3d5/2, Y 3d5/2, and Ti 2p3/2) spectra (see Figure S6 in the Supporting Information). The Sr 3d5/2 peak at 133.3 eV observed in the XPS spectra of the Sr0.92Y0.08Ti1‑yRuyO3−δ materials support the formation of the SrRuO3 phase. Notably, the Sr0.92Y0.08Ti1−yRuyO3−δ materials showed decreased binding energies for Sr 3d5/2, Y 3d5/2, and Ti 2p3/2 with increasing Ru content except the peak at the Ru content of 4 mol %, where the peak was shifted to the higher binding energy. These results, in conjunction with the TEM and XRD results, indicate that the Ru content plays an important role in generating different surface species. Subsequently, additional XPS analyses were conducted using Sr1−xYxTi0.92Ru0.08O3−δ (x = 0, 0.08, and 0.16) as a function of the Y content (Figure 3b). The observed broad peak in the XPS spectra was likewise deconvoluted and presented four peaks with maxima centered at ca. 279.1, 280.0, 280.6, and 281.4 eV corresponding to SrTiO3 (Sr 3p1/2), Ru0 (Ru 3d5/2), RuO2 (Ru 3d5/2), and SrRuO3 (Ru 3d5/2), respectively. The intensity of the peak corresponding to the metallic Ru0 species increased with increasing Y content, which agrees well with the TEM and 9375


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ACS Sustainable Chemistry & Engineering the materials with high Ru contents suggest that the Ru0 species may have interacted strongly with perovskite structures, SrRuO3 and Sr1−xYxTi1−yRuyO3−δ, and could further act as an active site for ammonia decomposition. To further elucidate potential interaction between Ru0 and the surface of SrRuO3 and/or Sr1−xYxTi1−yRuyO3−δ, additional analyses such as EXAFS and XANES are needed. Ammonia Dehydrogenation over the As-Synthesized Perovskite Materials. The catalytic performances of the prepared Sr1−xYxTi1−yRuyO3−δ materials in ammonia dehydrogenation (2NH3 → N2 + 3H2) were initially studied as functions of temperature and Ru content. First, pure ammonia gas was supplied into a continuous reactor at a GHSVNH3 of 10 000 mL gcat−1 h−1 at different reaction temperatures. Figure 4a depicts the activities of Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, 4, 7, 12, 17, and 26 mol %). It was found that the catalytic activities increased at a particular temperature in the following order: Sr 0.92 Y 0.08 Ti 1 Ru 0 O 3−δ < Sr 0.92 Y 0.08 Ti 0.96 Ru 0.04 O 3−δ < Sr0.92Y0.08Ti0.93Ru0.07O3−δ < Sr0.92 Y0.08 Ti0.88Ru0.12O 3−δ < Sr0.92Y0.08Ti0.84Ru0.17O3−δ < Sr0.92Y0.08Ti0.74Ru0.26O3−δ. The observed dependence between Ru content and activity (Figure 4b) indicates that the Ru species are the active sites and Sr0.92Y0.08Ti1Ru0O3−δ likely acted only as a support. This explanation is consistent with the observation that nearly no decomposition with Sr1−xYxTi1Ru0O3−δ was occurred even at 600 °C. When GHSVNH3 was increased to 20 000 mL gcat−1 h−1, conversion of NH3 at a particular temperature decreased in comparison to those when GHSVNH3 was 10 000 mL gcat−1 h−1 (Figure 4c). It is worth noting that based on TEM, XRD, and XPS analyses, Ruz(surface)/Sr0.92Y0.08Ti1−yRuy−zO3−δ and/or Ruz−w(surface)/SrwRuwO3/Sr0.92−wY0.08Ti1−yRuy−zO3−δ materials may be formed in particular when a high Ru content is used during their preparation (vide supra), and the formed surface Ru0 species and/or the newly produced surface Ru0 species interacting with the perovskite structures are likely the main active sites for ammonia decomposition. Ammonia conversions were also found to be enhanced with increasing temperature for all the materials, which agrees well with the endothermic nature of ammonia dehydrogenation. For instance, Sr0.92Y0.08Ti0.84Ru0.17O3−δ gave the NH3 conversion of 58% at 450 °C, which increased to 97% at 500 °C. Based on the temperature dependent data, Arrhenius plots for the Sr0.92Y0.08Ti1−yRuyO3−δ (y = 0, 4, 7, 12, 17, and 26 mol %) catalysts provided activation energies in the range of 31−120 kJ mol−1 depending on the surface Ru quantities (Figure S9a). In addition to the temperature and Ru content, the Y content can likewise affect the activity of the Sr1−xYxTi0.92Ru0.08O3−δ (x = 0, 0.08, and 0.16) materials in ammonia decomposition. Figure 5a demonstrates that the dehydrogenation activity at the GHSV of 10 000 mLNH3/(gcat·h) increased as the Y content increased in the temperature range between 350 and 600 °C. The materials Sr0.92Y0.08Ti0.92Ru0.08O3−δ and Sr0.84Y0.16Ti0.92Ru0.08O3−δ gave ca. 68% and 96% conversion at 500 °C, respectively. When the GHSV increased to 20 000 mLNH3/(gcat·h), NH3 conversion decreased (Figure 5b). However, the influence of Y content on catalytic activity is not as significant as that of Ru content. Given that the TEM, XRD, and XPS analyses indicate an increased amount of surface Ru0 species with increasing added Ru content, it was found that the quantity of added Y also played an important role in generating surface Ru0 species as active sites. The obtained temperature dependent data gave activation energies of 63−90 kJ mol−1 with Ea being decreased as Y content increased

Figure 5. Catalytic ammonia decomposition over Sr1−xYxTi0.92Ru0.08O3−δ (x = 0, wine; 0.08, blue; and 0.16, pink) as functions of temperature and Y content (x) at the GHSV of: (a) 10 000 mLNH3/(gcat·h) and (b) 20 000 mLNH3/(gcat·h).

(Figure S9b, GHSV of 10 000 mLNH3/(gcat·h)). The results again originate partly from electronic perturbation induced by Y incorporation. Notably, the Sr0.84Y0.16Ti0.92Ru0.08O3−δ materials presented the lowest activation energy of 63 kJ mol−1, which is comparable or lower than those of other previously reported Ru catalysts.19,38−40 Further experiments were conducted using Sr 0.92 Y 0.08 Ti0.92Ru0.08O3−δ to determine the effect of calcination temperatures (650, 750, 1000, and 1300 °C) applied for the material synthesis on ammonia dehydrogenation. It was found that catalytic activity decreased as the calcination temperature increased from 650 to 1300 °C (Figure S10). The result is presumably attributable to the evaporation of RuO4 produced in situ during calcination,41 which is strongly supported by XRD patterns showing that RuO2 phase was disappeared upon utilization of calcination temperatures of ≥750 °C (Figure S11); instead, a new Y2Ti2O7 phase appeared. Thus, the observed decreased activity for ammonia decomposition with increasing calcination temperatures above 750 °C comes from the loss of Ru active sites. The elemental compositions, surface areas, pore sizes, Ru particle sizes, and catalytic activities of the prepared catalysts are summarized comprehensively in Table S3. Catalytic activities of the recently developed heterogeneous catalysts for ammonia decomposition are further compared in Table S4. Long-Term Stability of the As-Synthesized Materials for Ammonia Dehydrogenation. Long-term durability is 9376


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newly formed Ru0 species supported on the perovskite materials were proposed as the active sites for ammonia decomposition, and Sr0.84Y0.16Ti0.92Ru0.08O3−δ displayed high activity as well as durability (>275 h) under varying conditions for hydrogen production from ammonia. The observed high activity and stability of the Sr0.84Y0.16Ti0.92Ru0.08O3−δ material were proposed to have likely originated from the interactions between the formed Ru0 species and the perovskite support, which further needs to be investigated. The as-developed catalyst could potentially be applied for the synthesis of ammonia from nitrogen and hydrogen electrocatalytically. Indeed, the Sr0.84Y0.16Ti0.92Ru0.08O3−δ material demonstrated its capability to catalyze electrochemical ammonia synthesis (data are not shown). Given that Irvine et al. have recently reported the in situ growth of NPs at the surface of ABO3 perovskite materials through exsolution of the B site cations, which was driven by the A site deficiency,42,43 the newly formed metallic Ru0 NPs could also conceivably be generated by the exsolution process via optimization of the preparation method for Sr1−xYxTi1−yRuyO3−δ. The realization of uniform Ru0 NPs strongly interacting with perovskite supports through exsolution is expected to result in significantly enhanced activity toward ammonia dehydrogenation and further electrochemical ammonia synthesis, which would make these perovskite materials highly desirable for NH3 based energy applications with fuel cells.

one of the most important and desired properties for a catalyst. In order to determine the stability of the as-prepared materials for ammonia dehydrogenation, an experiment was performed with the Sr0.84Y0.16Ti0.92Ru0.08O3−δ material by randomly varying reaction temperatures (450−550 °C) and GHSVNH3 (10 000− 20 000 mL gcat−1 h−1). As illustrated in Figure 6, the

Figure 6. Long-term stability test results for ammonia decomposition over Sr0.84Y0.16Ti0.92Ru0.08O3−δ at different reaction temperatures with the GHSV of 10 000 mLNH3/(gcat·h) (blue line) and 20 000 mLNH3/ (gcat·h) (red line).

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ASSOCIATED CONTENT

S Supporting Information *

Sr0.84Y0.16Ti0.92Ru0.08O3−δ material, which can be considered more accurately as Ruz(surface)/Sr1−xYxTi0.92Ru0.08−zO3−δ, showed a high activity (ca. 90% NH3 conversion) for 100 h with the GHSV of 10 000 mL gcat−1 h−1 at 500 °C and this activity was maintained for an additional 100 h upon lowering the temperature to 450 °C, followed by increasing the temperature to 550 °C. The durability test was continued by increasing the GHSV up to 20 000 mL gcat−1 h−1 while changing the operating temperatures (550 → 500 → 450 → 550 °C). This methodology continuously released hydrogen without noticeable catalyst deactivation. The used material, Sr0.84Y0.16Ti0.92Ru0.08O3−δ was analyzed by TEM, and as depicted in Figure S12, the material morphology was not changed significantly. The observed high stability of the material may be associated with strong metal to support interaction. In summary, Ru-containing ABO3 perovskite materials Sr1−xYxTi1−yRuyO3−δ were prepared and tested as potential catalysts for ammonia decomposition. The resulting materials, Sr0.92Y0.08Ti1−yRuyO3−δ and Sr1−xYxTi0.92Ru0.08O3−δ, were characterized using different analytical methods. These techniques showed that the surface structure of the Sr1−xYxTi1−yRuyO3−δ materials was significantly altered to produce different surface phases depending on the Ru and Y content added upon the mixing and consecutive calcination steps under a reducing environment during their preparation. In the case of the material Sr0.92Y0.08Ti1−yRuyO3−δ, surface Ru0 active sites were generated to give Ruz(surface)/Sr0.92Y0.08Ti1−yRuy−zO3−δ and/ or Ruz−w(surface)/SrwRuwO3/Sr0.92−wY0.08Ti1−yRuy−zO3−δ following a partial phase transition near the surface. In the case of Sr1−xYxTi0.92Ru0.08O3−δ, surface Ru0 species supported on a perovskite material (Ruz(surface)/Sr1−xYxTi0.92Ru0.08−zO3−δ) were likewise produced as the content of Y increased. For both Sr0.92Y0.08Ti1−yRuyO3−δ and Sr1−xYxTi0.92Ru0.08O3−δ, the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02402. Additional experimental data including a schematic diagram of ammonia dehydrogenation reactor, compositions of materials analyzed by ICP-OES, XRD patterns, XPS spectra, TEM and EDX images, TPR profiles, and catalytic activities depending on calcination temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Kwang Ho Song). *E-mail: [email protected] (Chang Won Yoon). ORCID

Hyun S. Park: 0000-0002-7960-9729 Hyung Chul Ham: 0000-0003-0850-584X Kwang Ho Song: 0000-0002-3011-1495 Chang Won Yoon: 0000-0003-1221-6901 Author Contributions ⊥

These authors contribute equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20143030031360) 9377


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Influence of Cation Substitutions Based on ABO3 Perovskite Materials

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