Influence of the Oxygen Storage Capacity

The cerium oxide catalysts are known for their high oxygen storage capacity, and it is clear that the cerium oxide has a direct role in the catalytic activity. However, the function of ceria and how the metal component promotes the WGS reaction is not clear. Some research groups [26, 27, 32] have claimed that the redox mechanism and oxygen storage capacity have a direct role in the WGS reaction. The other mechanism was proposed [28, 43] to proceed through the formation of surface-formate intermediates. Nevertheless, the oxygen storage capacity of the copper-ceria catalyst can be observed in Figure 8.12. Copper-ceria catalysts have oxygen stored in the catalyst lattice, as described by the formula Cu0.2Ce0.8O2-y, where the oxygen storage capacity is reported to be y = 0.17 [34].

The physisorbed oxygen reacts quickly with CO in the gas flow to form CO2, as can be seen in Figure 8.12 (open squares). When this takes place, no water is consumed (open circles). As soon as all oxygen stored in the catalyst has reacted, the WGS reaction takes over. As can be observed in the figure, the WGS equilibrium line is crossed (open squares), which unfortunately makes it clear that this is CO oxidation and not WGS reaction. Hence the catalyst had to be pretreated in the actual WGS feed stream before starting to observe the WGS reaction. In the above experiment, it took more than 70 min to discharge all of the oxygen stored in the catalyst to get stable results for measuring the WGS reaction, which probably means that oxygen from the crystalline bulk material also was liberated at these temperatures (up to 673 K) due to reducing atmosphere [44]. This was observed for both copper-ceria catalysts and always appeared after the catalyst was left in oxygen-containing

Ceria Oxygen Storage

Temperature (K)

FIGURE 8.12 Oxygen storage capacity over the Cu0.2Ce0.8O2-y catalyst. Open squares represent conversion of CO and open circles the conversion of H2O (which is none) in the CO oxidation reaction. Filled squares and filled circles are CO and H2O conversions, respectively, in the WGS reaction. The dotted line represents the WGS equilibrium curve. The solid line is model fit assuming first-order reversible kinetics in WGS.

Temperature (K)

FIGURE 8.12 Oxygen storage capacity over the Cu0.2Ce0.8O2-y catalyst. Open squares represent conversion of CO and open circles the conversion of H2O (which is none) in the CO oxidation reaction. Filled squares and filled circles are CO and H2O conversions, respectively, in the WGS reaction. The dotted line represents the WGS equilibrium curve. The solid line is model fit assuming first-order reversible kinetics in WGS.

atmosphere. The catalyst reoxidation process at atmospheric conditions was nonpy-rophoric. After the initial treatment with WGS feed (see Figure 8.2), the catalysts exhibit stable operation, with little or no deactivation (filled squares [CO] and filled circles [H2O]).

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  • hamid
    How is oxygen a renewable resource?
    8 years ago
  • Robert Young
    How is oxygen a renewable resources?
    8 years ago

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