Research Article pubs.acs.org/journal/ascecg
Strong Interaction of Ruthenium Species with Graphite Structure for the Self-Dispersion of Ru under Solvent-Free Conditions Wenfeng Han,* Linhui Li, Haiyu Yan, Haodong Tang, Zhi Li, Ying Li, and Huazhang Liu* Institute of Industrial Catalysis, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310032, Zhejiang, P.R. China S Supporting Information *
ABSTRACT: Carbon materials-supported Ru catalysts are some of the most efficient catalysts for ammonia synthesis. Usually they are prepared by impregnation for the dispersion of the Ru metal. In the present study, we report that a strong interaction of ruthenium species (with triruthenium dodecacarbonyl as the precursor) with a graphite structure enables the self-dispersion of Ru under solvent-free conditions. Via simple mixing and heat treatment, sub-nano Ru particles over high surface area graphite (HSAG) are obtained with uniform distribution. The ammonia yields or ammonia synthesis rate is almost 50% higher than the catalysts prepared via impregnation whether the support is activated carbon or HSAG. During catalyst preparation, Ru3(CO)12 is adsorbed on the surface of HSAG via CO groups in Ru3(CO)12 and surface oxygen groups on HSAG. Subsequently, Ru3(CO)12 undergoes decarbonylation reaction at very low temperatures. Then, CO has a dismutation reaction over the Ru surface leading to the formation of RuO2 at temperatures between 180 and 190 °C. During heat treatment, RuO2 was partially reduced by carbon at elevated temperatures, and the resulting Ru has strong interaction with HSAG, which further leads to the dispersion and stabilization of Ru nanoparticles. KEYWORDS: Ammonia synthesis, Ruthenium, Ball milling, Self-dispersion, Graphite, Interaction between metal and support
■
ammonia synthesis.15 Third, it is well accepted that ammonia synthesis over Ru is a structure-sensitive reaction. A nanoparticle size of about 1−2 nm is suitable for the formation of active site. Consequently, special attention needs to be paid to the dispersion and control of Ru size. More recently, Hosono et al. reported that electrides are excellent supports for Ru catalysts as electrides donate electrons to Ru. Increased electron cloud density of Ru facilitates the dissociation of N2, which is the rate-determining step in ammonia synthesis.16−18 Similarly, with a graphite structure as the support, enhancement of catalytic activity was observed as the strong electron conductivity of graphite.19 However, the low surface area limits its application as the catalyst support. Previously, we discovered that high surface area graphite (HSAG) materials are potential candidates as they are composed of a fine graphite crystalline featured with high surface area and developed mesopores.20 However, as a hydrophobic material of graphite, it is difficult to use for the deposition of Ru precursor over HSAG. In the present work, we report the strong interaction of ruthenium species with a graphite structure for the selfdispersion of Ru under solvent-free conditions which enables
INTRODUCTION Ammonia synthesis play a key role both in industry and the development of catalysis science. Although the catalytic ammonia synthesis was discovered more than 100 years ago, it is still a “never ending story”.1 After well consolidated iron catalysts, ruthenium is considered to be the new generation of ammonia synthesis catalysts which show much higher activity.2,3 Since the discovery of Ru catalysts, various supports have been tested, such as Al2O3, MgO, zeolite, BaTiO3, C3N4, and BN.4−9 Among them, activated carbon was confirmed to be one of the most effective supports.10 However, the chemical and physical properties pose significant challenges for the development of Ru catalysts for ammonia synthesis. First, carbon tends to react with feeding H2 during ammonia synthesis resulting in the gradual degradation of activated carbon support.11 To inhibit the methanation reaction, carbon supports are usually treated in inert atmosphere at temperatures above 1900 °C achieving graphitization.12,13 However, this treatment results in a huge loss of surface area of carbon support. Second, activated carbon usually contains small amounts of ash which is unfavorable for the activity of catalysts. Therefore, prior to the loading of Ru, treatment of activated carbons with HNO3 is usually suggested.10,14 In addition, hydrogen treatment at high temperatures was found to be effective for the improvement of activity as it removes the elements such as P and Cl which function as poisons for © 2017 American Chemical Society
Received: May 6, 2017 Revised: June 17, 2017 Published: June 21, 2017 7195
DOI: 10.1021/acssuschemeng.7b01413 ACS Sustainable Chem. Eng. 2017, 5, 7195−7202
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic illustration of catalyst preparation procedure. by the mixture of N2 and H2 (H2:N2 = 3:1) at a pressure of 5.0 MPa and with a space velocity of 30,000 h−1 at 400, 425, 450, and 475 °C for 4, 8, 8, and 4 h, respectively. The synthesis gas (H2/N2 = 3) was derived from the decomposition of ammonia with deep removal of H2O, CO, CO2, and residual NH3 over Pd, 13X, and 5A molecular sieves and compressed by a compressor. Ammonia concentration was measured by titration with a sulfuric acid solution. Characterization of Support and Catalysts. BET specific surface areas were measured by nitrogen adsorption isotherms at 77 K on a Micromeritics ASAP 2020 instrument in static measurement mode. Before the measurement, the samples were degassed at 250 °C for 10 h. X-ray diffraction (XRD) was performed with a Thermo ARL X’TRA diffractometer using Cu Kα radiation (λ = 0.154056 nm), equipped with a Si(Li) solid detector at 40 kV/40 mA with a monochromator. The in situ experiments were carried out in an Anton Paar XRK 900 reactor built in the diffractometer with a TCU 750 temperature control unit; the temperature error is within ±1 °C. The in situ XRD performed at a stipulated heating rate in the range of 50−350 °C with 60 mL/min of N2 and 101 kPa of pressure. The XRD patterns tracing the phase transformation are carried out in a continuous scan mode, with a step of 0.02° from 30° to 90° at a speed of 2.5° per minute. Temperature-programmed decomposition of Ru3(CO)12 in an Ar atmosphere (Ar-TPD-MS) was performed on an in situ reaction device equipped with a MS detector. Here, a 100 mg sample was purged with Ar for 1 h to remove physically adsorbed species. Then, the furnace temperature was gradually increased to 600 °C with a ramp rate of 10 °C/min. The effluent gases were recorded by a mass spectrometer (m/e = 28 for CO and 44 for CO2). Methanation of the catalysts was carried out on a Micromeritics AutoChem 2910 instrument. The catalyst was reduced in a H2 flow at 450 °C for 4.0 h. The sample was subsequently purged with helium for 1.5 h to remove H2 adsorbed on the surface of the catalyst and cooled. Then, the reaction temperature was ramped to 800 °C with a rate of 5 °C in H2 with a flow rate of 30 L/min. The formation of CH4 during the reaction with H2 with a carbon support was monitored by an online mass spectrometer detector (m/e = 15). For X-ray photoelectron spectroscopy (XPS) experiments, a spectrometer from Kratos AXIS Ultra DLD photoelectron spectroscopy with a monochromatized microfocused Al X-ray source was employed. Charging of samples was corrected by setting the binding energy of adventitious carbon (C 1s) at 284.6 eV. Prior to the measurements, the powder sample, pressed into self-supporting disks, was loaded into a subchamber and then evacuated at 25 °C for 4 h. For the morphology investigation, SEM was carried out on a scanning electron microscope (FESEM, Hitachi S-4700) at an
the facile preparation of ruthenium nanoparticles supported on HSAG by simple mechano-mixing. Promoted with Ba and K, high activity and stability for ammonia synthesis were achieved. The catalyst explored in the present study was prepared by mechano-mixing of Ru3(CO)12 with HSAG300 (ball milling or hand grinding) followed by thermal treatment. Promoters of Ba and K can be introduced by impregnation or co-milling. In the catalyst, the weight contents of Ba, K, and Ru are 4 wt %, respectively. Ammonia synthesis was carried out at 10 MPa and 10,000 h−1 with feed of H2/N2 = 3.
■
EXPERIMENTAL SECTION
Catalyst Preparation. Ru3(CO)12 was prepared by the reaction of RuCl3·xH2O (Aladin, Ru content of 35.0−42.0%) with CO (Minxing Gases, China, 99.99%) in ethylene glycol (Sinopharm Chemical Reagent, 99.5%) in the presence of anhydrous Na2CO3 (Aladin, 99.5%) at 110 °C for 10 h.21 Then, synthesized Ru3(CO)12 was extracted by CH2Cl2 (Aladin, 99.9%) and purified by distillation, filtration, and washing. Finally, the paste was dried at 50 °C for 15 h leading to the pure and golden yellow Ru3(CO)12 powder. HSAG (Timcal Ltd. Group) in the form of powder (
