Carbon dioxide methanation is well known to offer some advantages and be catalyzed by Ru, Rh, Pd, and Ni. In this study, Ni catalysts supported on various metal oxides were fabricated and their catalytic activity for CO2 methanation was evaluated. The CO2 conversion for most of catalysts drastically increased at 225-250 ºC and reached a maximal value at 300-350 ºC. The order of CH4 yield at 250 ºC was as follows; Ni/Y2O3 > Ni/Sm2O3 > Ni/ZrO2 > Ni/CeO2 > Ni/Al2O3 > Ni/La2O3. The catalytic activity could be partly explained by the basic property of the catalysts. Moreover, the chemical species formed on the catalyst surface during CO2 methanation were examined by in situ infrared spectroscopy. From the obtained results, the difference in the activity depending on the support material of Ni catalysts was discussed.
In recent years, solid oxide fuel cells fueled with ammonia have been attracting intensive attention. In this work, ammonia fuel was supplied to the Ni/yttria-stabilized zirconia (YSZ) cermet anode at 600 and 700 °C, and the change of electrochemical performance and microstructure under the open-circuit state was studied in detail. The influence of ammonia exposure on the microstructure of Ni was also investigated by using Ni/YSZ powder and Ni film deposited on a YSZ disk. The obtained results demonstrated that Ni in the cermet anode was partially nitrided under an ammonia atmosphere, which considerably roughened the Ni surface. Moreover, the destruction of the anode support layer was confirmed for the anode-supported cell upon the temperature cycling test between 600 and 700 °C because of the nitriding phenomenon of Ni, resulting in severe performance degradation.
The electrochemical oxidation of ammonia over Pt electrode in alkaline aqueous solutions was studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. In 0.1 M NH3-1 M KOH, the band ascribable to the HNH bending mode of adsorbed NH3 was confirmed at 1662-1674 cm(-1) in the potential range of 0.1-1.1 V. The intensity of this band decreased continuously with a rise in potential, indicating the oxidative consumption of adsorbed ammonia. In response to this behavior, the band at 1269 cm(-1) appeared alternatively above 0.2 V, and its intensity reached the local maximal value at ca. 0.4 V. Note that this potential of ca. 0.4 V agreed well with the onset potential of ammonia oxidation, ca. 0.45 V, in the linear sweep voltammogram. This 1269 cm(-1) band was assigned to the NH2 wagging mode of N2H4, which was one of the active intermediates, N2H(x+y,ad) (x = 1 or 2, y = 1 or 2), according to the mechanism proposed by Gerischer and Mauere. To the best of our knowledge, this is the first report for the detection of N2H4 as a reaction intermediate over Pt electrode. Furthermore, the formation of bridged NO was also observed above the onset potential of ammonia oxidation, ca. 0.5 V. Such adsorbed NO species probably inhibit the electrochemical reaction due to the occupation of reaction sites at higher potential.
The recent development of anion exchange membranes (AEMs) has increased the potential of anion exchange membrane fuel cells (AEMFCs). Although highly active electrocatalysts for specific reactions have been successfully developed by placing the most importance on the fuel species, only a few studies have focused on OH ad (hydroxyl adsorbed species), which is known to be a common reactive species in alkaline environments. In this study, highly oxophilic CeO 2 was selected as a surface modifier for a Pt electrode. We first applied in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to ionomercoated Pt and CeO 2 -modified Pt surfaces for clarifying the adsorption behavior of OH ad . As a result, a distinct change in adsorption behavior of OH ad was confirmed in blank KOH solution. These peculiar characteristics were applicable to various electrochemical oxidation reactions. During the ammonia oxidation reaction, the acceleration of the formation of NO ad species was observed in CeO 2 -modified Pt, suggesting the enhancement of OH adsorption. Furthermore, the degree of activity enhancement by CeO 2 addition was investigated for the CO oxidation reaction, methanol oxidation reaction, and ethanol oxidation reaction. Under basic conditions, each of these reactions exhibited distinct activity enhancement. In contrast, under acidic conditions, the promoting effect on these reactions was not observed. These results strongly indicate the potential of our catalyst design strategy and the importance of OH ad species as reactive species in alkaline environments.
Please cite this article as: Kaname Okura, Takeou Okanishi, Hiroki Muroyama, Toshiaki Matsui, Koichi Eguchi, Promotion effect of rare-earth elements on the catalytic decomposition of ammonia over Ni/Al2O3 catalyst, Applied Catalysis A, General http://dx. Graphical abstract Graphical abstract Graphical abstract Graphical abstractHighlights Highlights Highlights Highlights • The Ni/Al2O3 catalysts modified by rare-earth elements were investigated.• The addition of rare-earth elements promoted ammonia decomposition reaction.• La-modified Ni/Al2O3 was most active in this study.• The hydrogen inhibition phenomenon was alleviated by the modification. AbstractAmmonia decomposition has attracted much attention as an efficient method for the on-site generation of hydrogen. In this study, alumina-supported nickel catalysts (Ni/Al2O3) modified by rare-earth elements were prepared by the impregnation method and their catalytic activity for the ammonia decomposition was investigated. The addition of rare-earth elements promoted the decomposition reaction over catalysts and the La-modified catalyst achieved the highest ammonia conversion in this work. For the modified catalysts, the adsorbed hydrogen, which is known to be an inhibitive species for the ammonia decomposition, desorbed at lower temperature compared to the unmodified one. Therefore, the effective alleviation of hydrogen inhibition would be responsible for the activity enhancement for the modified catalysts. The reaction kinetics study also supported this proposed mechanism. For the La-modified catalyst, the optimal pretreatment condition was investigated to enhance the catalytic activity.The catalyst calcined at 400ºC followed by reduction at 600ºC exhibited the highest ammonia conversion of 94% at 550ºC.
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