Barium-promoted Ru/carbon catalyst for ammonia synthesis: State of the system when operating (original) (raw)

The Kinetics of Ammonia Synthesis over Ruthenium-Based Catalysts: The Role of Barium and Cesium

Journal of Catalysis, 2002

The effect of barium and cesium on the kinetic behavior of Ru/MgO in ammonia synthesis was studied. The activity measurements were performed in a differential reactor at 400 • C under a pressure of 6.3 MPa and supplemented with chemisorption measurements and temperature-programmed surface reaction experiments (TPSR). The latter were performed by titrating preadsorbed atomic nitrogen (N ads ) with hydrogen. Both promoted systems proved to be much more active in NH 3 synthesis than the unpromoted one: Ba-Ru/MgO > Cs-Ru/MgO Ru/MgO. The kinetic behavior of Ba-Ru/MgO was found to be different from that of Cs-Ru/MgO which was much less sensitive to changes in the ammonia content (x NH 3 ). The dependence of the turnover frequencies (TOF) on x NH 3 for Ba-Ru/MgO and Cs-Ru/MgO was found to be analogous to that for Ba-Ru/C and Cs-Ru/C, respectively. For Ba-Ru/MgO and Ba-Ru/C the differences in TOF did not exceed 10 to 30% over the range of x NH 3 studied. It is therefore suggested that cesium acts as an electronic promoter, whereas barium plays the role of a structural promoter that controls the concentration of active sites which are most likely B 5 -type sites, the effect of the support being negligible. The same onset temperature of ammonia formation during the TPSR experiment observed for Ru/MgO and Ba-Ru/MgO supports this hypothesis. Furthermore, a strong increase in activity was observed for Ru/MgO and Ba-Ru/MgO when heating in synthesis gas up to 520 • C. This can be attributed to the sintering of very small Ru particles and to the removal of water traces from both the MgO and the Ba + O adlayer. In contrast, the activity of Cs-Ru/MgO decreased significantly after heating at 520 • C. Thus, due to the very high activity, very high thermal stability, and absence of methanation problems, barium-promoted ruthenium catalysts supported on magnesia are considered excellent ammonia synthesis catalysts. c 2002 Elsevier Science

Exsolved Ru on BaCexOy Catalysts for Thermochemical Ammonia Synthesis

International Journal of Energy Research

Ammonia (NH3) is a carbon-free and hydrogen-rich (17.8 wt% H2) chemical that has the potential to revolutionize the energy sector. Compared with hydrogen (H2), NH3 can be easily liquefied, stored, and transported globally. However, the conventional thermocatalytic process to synthesize NH3 accounts for 2% of global energy consumption and 1.2% of CO2 emissions annually. To make the process further efficient, new catalysts must be developed to allow for NH3 synthesis in milder conditions with high thermal stability. To this end, we have developed ruthenium (Ru) supported on perovskite (BaCexOy) via a ball-milling-assisted exsolution method that allows for a more tunable morphology. Reactivity is compared with the catalyst prepared via the conventional impregnation technique. The as-synthesized catalysts are characterized by XRD, H2-TPR, TEM, XPS, and APT. The NH3 synthesis is carried out in a packed-bed tube reactor thermochemically. Using N2 instead of Ar as the carrier gas during ex...

Ammonia synthesis over the Ba-promoted ruthenium catalystssupported on boron nitride

Catalysis Letters, 2005

Barium promoted ruthenium catalysts deposited on the boron nitride supports were characterised (XRD, O2 and CO chemisorption) and tested in NH3 synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m2/g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m2/g) were heated in an ammonia stream at 700–800 °C for 120 h. As a result, the oxygen content was reduced from 7.0 at% (BNS) to 3.5 at% (BNSNH3) and from 3.8 to 2.7 at% (HCVNH3), as evidenced by XPS. The kinetic studies of NH3 synthesis (63 or 90 bar; H2:N2 = 3:1) revealed that the catalysts based on the modified supports were more active, respectively, than those derived from starting nitrides, the difference being especially pronounced in the case of BNS and BNSNH3. Studies of the catalysts activation have shown, in turn, that the stabilisation in a H2:N2=3:1 mixture at 1bar is very slow, i.e. the reaction rate increases slowly versus time on stream even at a high temperature of 550 – 600°C. Stabilisation is faster and the NH3 synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH3 in H2:N2=3:1) flowing under high pressure of 90 bar. It is suggested that boron oxide (an impurity) acts as a deactivating agent for Ba–Ru/BN and that the reaction between NH3 and B2O3 (B2O3+2NH3=2BN +3H2O) is responsible for the activity increase. A poisoning mechanism of B2O3 is discussed.

Carbon-based ruthenium catalyst for ammonia synthesis

Applied Catalysis A-general, 2003

A series of ruthenium catalysts deposited on graphitized carbon and promoted with barium, caesium or both Ba and Cs have been studied in ammonia synthesis. Under experimental conditions (90 bar, H 2 :N 2 = 3:1, 400 • C, 10% NH 3 ), the reaction rate over the co-promoted catalyst (9.1 wt.% Ru in Ru + C) was found to be higher than those over singly promoted specimens, the overall effect from Ba + Cs in Ba-Cs-Ru/C being almost as high as the sum of individual effects from Ba (Ba-Ru/C) and Cs (Cs-Ru/C), respectively. The co-promoted Ru catalysts, especially that of high ruthenium loading (23.1 wt.% Ru) were shown to be significantly more active in NH 3 synthesis than the conventional fused iron catalyst (KMI, H. Topsoe). The oxygen chemisorption studies have shown that the amounts of O 2 taken up by the Cs-containing samples (Cs-Ru/C, Cs-Ba-Ru/C) are considerably larger than those for Ru/C and Ba-Ru/C, thus indicating caesium to be in a highly reduced, most likely zero valent form, when operating under ammonia synthesis conditions. It is suggested that barium ((Ba + O) adlayer ) is located on the ruthenium surface and it acts as a structural or electronic promoter. Caesium (Cs 0 ) is suggested to be localised on the carbon surface: the Cs promotion occurs at contact points between Ru and the Cs atoms adsorbed on carbon ("hot ring promotion"-electronic).