Redox Kinetics of Ceria‐Based Mixed Oxides in Selective Hydrogen Combustion

Abstract

Ceria-based mixed oxides, in which about 10 mol % of the cerium is replaced by another metal, catalyze the selective combustion of hydrogen from a mixture of hydrogen, propane, and propene at 550 degrees C. This makes them attractive catalysts for the oxidative dehydrogenation of propane. Hydrogen combustion shifts the equilibrium to the products side, supplies energy for the endothermic dehydrogenation, and simplifies product separation. The type of metal added has an important effect on the catalytic properties. To gain insight into the process, a set consisting of six mixed oxides was synthesized and the catalytic properties and redox behavior were tested. The mixed oxides generally release more oxygen than plain ceria. Mixed oxides containing Bi, Cu, Fe, Pd or Ca release between 1.6 and 2.0 mg of oxygen per 100 mg sample (compared to only 1.2 mg for plain ceria). This result is important for reactions in which the catalyst acts as an oxygen reservoir, such as selective hydrogen combustion. The temperature at which oxygen is released is generally lower for the mixed oxides, and varies from 110 degrees C (for Cu-CeO2) to 550 degrees C (for Ca-CeO2), which enables catalytic applications over a wide temperature range. The reduction rate at 550 degrees C is related to the reduction onset of the catalysts. Those catalysts with a relatively low reduction temperature, such as Cu-, Mn-, Bi-, and Pb-CeO2, show a high reduction rate, whereas those with a high reduction temperature, such as Ca-CeO2, Fe-CeO2, and plain ceria, reduce at a slower rate. The latter catalysts also have a low selectivity towards hydrogen combustion. The influence of the catalyst composition and crystallite size on the activity and selectivity is discussed.

Bismuth‐Doped Ceria, Ce0.90Bi0.10O2: A Selective and Stable Catalyst for Clean Hydrogen Combustion

“…Previously, we showed that the reduction rate at 550 8C is related to the reducibility of the catalyst, as determined by H 2 -TPR. [42] Catalysts with a low TPR reduction temperature, have a high reduction rate at 550 8C. Conversely, catalysts with a high reduction temperature (close to 550 8C), have a low reduction rate at 550 8C.…”

“…and a solid mixed metal nitrate formed. This was converted into the mixed oxide by calcining in static air at 700 8C for 5 h. [42] Importantly, these catalysts were not prepared by impregnating a cerium oxide support. Rather, the co-melting of the cerium nitrate with the nitrate or chloride of the appropriate metal yields a liquid precursor.…”

“…We found that the most promising dopants are Cu, Mn, K, Cr, Pb, Sn and Bi, with selectivities ranging from 80-98%. [41,42] Of the set of promising dopants, the highest activities were obtained using Pb and Bi. The activity is very important for practical application, since it governs the reactor volume (the amount of SOR needed) and the process cycle time.…”

Selective Hydrogen Oxidation Catalysts via Genetic Algorithms

“…[11,37] After the precursor has liquefied, the pressure was lowered and a solid mixed-metal nitrate formed. This was converted into the mixed oxide by calcining in static air at 700 8C for 5 h. The following notation is used: Gn-m, where n is the generation number and m is the catalyst number.…”

“…[11,37] The formulations are synthesised and tested, and the fitness value is calculated from the obtained activity and selectivity values. From these data, a set of new catalyst formulations ("virtual catalysts"), is generated, using three steps: selection (based on the fitness value), cross-over (the exchange of genes) and mutation (a random alteration of a small amount of genes).…”