Ammonia is an important chemical feedstock, and more than 80% of the synthesized ammonia is used to produce fertilizer. Ammonia is also being considered as an energy carrier and hydrogen source (1) because it has a high energy density (12.8 GJ m-3) and a high hydrogen content (17.6 wt%), (2) because infrastructure for ammonia storage and transportation is already established, and (3) because carbon dioxide is not emitted when ammonia is decomposed to produce hydrogen. If ammonia could be efficiently produced from a renewable energy source, such as solar or wind energy, problems related to the global energy crisis could be mitigated. Ammonia is commonly produced by the Haber-Bosch process, which accounts for more than 1% of global energy consumption. In this process, ammonia is produced at high temperatures (>450 oC) and pressures (>20 MPa) over an iron-based catalyst. A catalyst that yields ammonia with high efficiency under milder reaction conditions than are required by iron-based catalysts are desirable for future energy and material source. We herewith present rare earth oxides supported Ru catalysts, where Ru has specific interaction with the supports, shows high ammonia synthesis rate under mild conditions (300-400 oC, 1.0 MPa).
Rare earth oxide supports were prepared by precipitation and used after calcination at 700 oC. 5wt% Ru was loaded on the supports by wet impregnation method.
Ammonia synthesis rate was measured using a conventional flow system. The catalysts were reduced in H2 flow at different temperatures at 0.1 MPa and ammonia synthesis rate was measured at 300-400 oC at 1.0 MPa.
Results & Discussion
Ru/Pr2O3 exhibited highest ammonia-synthesis activity among simple rare earth oxide supported Ru catalysts, Ba/Ru/A.C. and Cs+/Ru/MgO at 1.0 MPa at 400 oC1. STEM analysis revealed that the Ru was loaded as a low-crystalline nano-layer. The unique structure of Ru formed by strong interaction with the support as well as the strong electron-donating character of Pr2O3 were found to synergistically accelerate cleavage of the N≡N bond, which is the rate-determining step.
Next, we investigated influence of reduction temperature on the ammonia-synthesis activity of Ru/La0.5Ce0.5O1.75, a catalyst consisting of Ru supported on a La0.5Ce0.5O1.75 solid solution2. After pre-reduction at the unusually high temperature of 650 oC, the catalyst exhibited significantly high ammonia synthesis activity from 300 to 400 oC at 1.0 MPa; the activity was the highest among oxide supported Ru catalysts. This catalyst consisted of fine Ru particles anchored on a heat-tolerant complex-oxidic support. During pre-reduction, the particle size of the Ru particles remained unchanged, but the particles became partially covered with partially reduced La0.5Ce0.5O1.75. A strong interaction between the Ru active sites and the reduced support accelerated cleavage of the N≡N bond during ammonia synthesis.
And also, Ru/La0.5Pr0.5O1.75 that had the same strategy of catalyst design as Ru/La0.5Ce0.5O1.75 performed comparable activity to Ru/La0.5Ce0.5O1.75.3
We succeeded in designing highly active ammonia synthesis under mild condition using rare earth oxides as catalyst carrier. These catalysts have advantages of being easy to prepare and stable in atmospheric air, which makes it easy to load it into reactor.
Sato, K. Imamura, Y. Kawano, S.I. Miyahara, T. Yamamoto, S. Matsumura, K. Nagaoka, Chemical Science, 8 (2017) 674.
Ogura, Katsutoshi Sato, S. Miyahara, Y. Kawano, T. Toriyama, T. Yamamoto, S. Matsumura, S. Hosokawa, K. Nagaoka, Chemical Science, 9 (2018) 2230.
Yuta Ogura, Kotoko Tsujimaru, Katsutoshi Sato, Shin-ichiro Miyahara, Takaaki Toriyama, Tomokazu Yamamoto, Syo Matsumura, Katsutoshi Nagaoka, ACS Sustainable Chemistry & Engineering, 6 (2018) 17258