Methane Reforming on Ni

This reaction is of great technological interest in the area of solid oxide fuel cells (SOFC) since it is catalyzed by the Ni surface of the Ni-stabilized Zr02 cermet used as the anode material in power-producing SOFC units.60,61 The ability of SOFC units to reform methane internally , i.e. in the anode compartment, permits the direct use of methane or natural gas as the fuel, without a separate external reformer, and thus constitutes a significant advantage of SOFC in relation to low temperature fuel cells. 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i =0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

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Blaylock, D.W., Ogura, T., Green, W.H., Beran, G.J.O. Computational investigation of thermochemistry and kinetics of steam methane reforming on Ni(lll) under realistic conditions. J. Phys. Chem. C 2009, 113, 4898-908. 

Methane reforming on Ni is of great technological interest in the area of solid oxide fuel cells (SOFCs) because methane reforming is catalyzed... 

The ability of bimetallic systems to enhance various reactions, by increasing the activity, selectivity, or both, has produced a great deal of interest in understanding the different roles and relative importance of ensemble and electronic effects. Deposition of one metal onto the single-crystal face of another provides an advantage by which the electronic and chemical properties of a well-defined bimetallic surface can be correlated with the atomic structure.5 22 23 Besenbacher et al.24 used this method to study steam reforming (the reverse of the CO methanation process) on Ni(l 11) surfaces... 

Fig. 13.2 DFT-calculated potential energy surfaces for various elementary steps associated with methane steam reforming on Ni(lll) [15, 16]...

Figure 2.6. Potential energy diagram for the steam methane reforming over Ni(lll) and Ni(211) surfaces based on the density functional theory (DFT) study. Adapted from Rostrup-Nielsen et al.10...

Ethanol and methane steam reforming reactions were studied assuming that the exit composition of the ethanol reformer depends on the steam reforming of methane. The competition for the same active site for ethanol and methane reforming maximizes the H2 and C02 production and minimizes the CO formation Catalysts were prepared by incipient wet impregnation. 20 wt% Ni supported on ZnO exhibited better performance compared to that supported on La203, MgO and A1203... 

In addition to the activity, other important requirements for the catalyst are the capability to start the reaction rapidly without the necessity for previous reduction with hydrogen and to perform effectively with intermittent operation these are essential properties for the catalyst in reformers, especially for portable and small-scale stationary fuel cell applications. In this respect, Dias and Assaf [61] focused on the potential of Pd, Pt and Ir to promote fast and intermittent ignition of methane ATR in Ni/y-Al203. They concluded that the three metals are very good promoters of the reduction of the nickel catalyst with methane, but the lower cost of palladium makes this metal more suitable than Pt and Ir for small fuel cells. 

Reaction Studies. Despite the large body of work on the effect of sulfur on Ni-based and precious metal catalysts for reforming of methane, there is relatively little work on sulfur deactivation in the reforming of liquid fuels. 

Problem of coke formation on Ni/Zr02 catalysts during the carbon dioxide reforming of methane... 

Oh, Y., Roh, H., Jun, K., Baek, Y. (2003). A highly active catalyst, Ni/Ce-Zr02/9-AljOj, for on-site Hj generation by steam methane reforming pretreatment effect. Int.. Hydrogen Energy 28,1387-1392. 

Yokata, O., Oku, Y., Sano, T., Hasegawa, N., Matsunami, J., Tsuji, M., Tamura, Y. (2000). Stoichiometric consideration of steam reforming of methane on Ni/A1203 catalyst at 650°C by using a solar furnace simulator. Int. 1. Hydrogen Energy 25,81-86. 

Ni/MgO did not so high activity, but rather stable. 3 mol% Ni/AljOj catalyst deactivated very rapidly and finally the reactor was plugged with the deposited carbon. We measured the amount of total carbon species on the catalyst surface after the reaction for 120 h. The results were 0.1 wt% on Ni o,o3Mgo.970,1.6 wt% Ni/MgO and 10 wt% on Ni/AljOj catalysts. From these results, it was found that Ni Mg j O catalyst has high resistance to carbon deposition in CO 2 reforming of methane. 

Fig. 4. Steam reforming of methane on Ni/MgAUO. Turnover frequency and sulfur passivation. Specific activity Tj. is referred to total Ni-area. (J.R. Rostrup-Nielsen, reproduced with the permission of Academic Press).

Nichio N, Casella ML, Santori GF, Ponzi EN, Ferretti OA (2000) Stability promotion of Ni/ alpha-AljOj catalysts by tin added via surface organometallic chemistry on metals - application in methane reforming processes. Catal Today 62 231... 

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