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Insight into the governing factors for ammonia synthesis in the electric field using a DFT study

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(1)2B08. Insight into the governing factors for ammonia synthesis in the electric field using a DFT study (Waseda University *, Myongji University **, National Institute of Materials Science ***, Nippon Shokubai ****) 〇K. Murakami*, Y. Tanaka*, S. Hayashi*, R. Sakai*, Y. Hisai*, S. Ogo*, J. G. Seo**, A. Ishikawa ***, H. Tsuneki *, ****, Y. Sekine*. 1.. Introduction The development of an efficient ammonia synthesis process at low-temperature and low-pressure is required for achieving hydrogen energy economy. We have reported an enhancement of ammonia synthesis rate by applying an electric field on Cs/Ru/SrZrO3 catalyst.1) It is revealed that N2 dissociation is promoted by H+ in the electric field.2), 3) Efficient ammonia synthesis in the electric field proceeds via an “associative mechanism” in which N2 dissociates through N2H intermediate. In this work, the effect of doping (Ba, Ca, Y and Al) to SrZrO3 on the ammonia synthesis activity in the electric field was examined using activity tests and density functional theory (DFT) calculations. Through the investigation, governing factors of supports for ammonia synthesis in the electric field were revealed.. calculated EN2H formation. Hence, the factors which govern the EN2H formation were considered. Figure 1 shows the relationship among the hydrogen adsorption energy over support (EH adsorption), the N2H adsorption energy over Ru rod (EN2H adsorption) and EN2H formation. The bubble size depicts the EN2H formation in which smaller bubble means advantage for N2H formation. Circles become smaller as the EH adsorption decreases, and EN2H adsorption increases. The trend indicates that the proton-donating ability of supports is required for ammonia synthesis in the electric field. The N2H stability is the second governing factor. Hence, the role of the supports in this factor was considered. The results revealed that the negatively charged Ru is suitable for N2H adsorption. It means that the electron-donating ability of supports is necessary for N2H stability. In summary, the coexistence of proton-donating and electron-donating ability of supports is essential for ammonia synthesis in the electric field. Ba and Ca addition to SZO enhanced both factors, leading to the higher ammonia synthesis rate.. 2. Experimental We prepared 5wt%Ru/Support catalysts using an impregnation method. SrZrO3 (SZO), Sr0.875Ba0.125ZrO3 (SBZO), Sr0.875Ca0.125ZrO3 (SCZO), SrZr0.875Y0.125O3- (SZYO) and SrZr0.875Al0.125O3- (SZAO) supports were prepared by a complex polymerization method. Ammonia synthesis rates were evaluated using a fixed bed flow type reactor. Stainless steel rods were attached on the catalyst bed for applying 6 mA direct current. DFT calculations were conducted using the Vienna ab initio simulation package (VASP) 5.4.1. Ru/SZO models were constructed by loading Ru rod over 4×4 SZO slab with Sr-O (001) termination. The dopant effects were considered by replacing the uppermost Sr (Zr) into Ba and Ca (Y and Al). 4) 3.Results and Discussion The ammonia synthesis rate in the electric field were compared among all supports (SZO, SBZO, SCZO, SZYO and SZAO). Results revealed that SBZO and SCZO showed much higher activity than SZO. 4) The governing factors for this trend were considered using DFT calculations. At first, the N2H formation energies (EN2H formation) were calculated over Ru/Support models. The results revealed the good correlation between the experimental ammonia synthesis rate and the. Fig. 1. The relationship among the hydrogen adsorption energy over support (EH adsorption), the N2H adsorption energy over Ru rod (EN2H adsorption) and EN2H formation. The size of the bubble means the EN2H formation. 1) R. Manabe et al., Chem. Sci. 8 (2017) 5484. 2) K. Murakami et al., Catal. Today 303 (2018) 271. 3) K. Murakami et al., Catal. Today in press doi: 10.1016/j.cattod.2018.10.055. 4) K. Murakami et al., J. Chem. Phys. 151 (2019) 064708.. -46-.

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Fig. 1. The relationship among the hydrogen adsorption  energy over support (  E H adsorption ), the N 2 H adsorption  energy over Ru rod (  E N2H adsorption ) and   E N2H formation

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