The specific Cu0 surface areas of the samples were measured by N2O decomposition using reactive frontal chromatography (RFC) method. This method is based on the oxidation of the exposed Cu0Â surface atoms of a reduced sample by switching the feed from an inert gas stream to a mixture containing N2O as an oxidizing species. The amount of N2O consumed in the reaction is used to determine the surface area of Cu0Â as given below (Eq.1)
2Cu + N2O = â€œCu2Oâ€ + N2
Before measurements, the CZC samples were activated in a stream of MeOH/H2O = 2:1, with a flow rate of 36 cm3min-1Â at 523 K. In order to diminish the amount of adsorbed molecules on the Cu surface, the samples were subsequently purged in pure He at 50 cm3min-1Â at 523 K for 1 h and then cooled down to 313 K, where the measurements were performed in a 0.5% N2O/He stream of 15 cm3min-1. Further, the catalysts were diluted with boron nitride to provide a significant sample bed of ca. 25 mm height, with a thermocouple positioned directly inside the powder bed. The material was placed onto a quartz frit in a tubular quartz reactor. For the RFC method the area under the ion current trace (m/e = 28 for N2) was directly used to determine the Cu0Â surface area. For the current experiments, however, the same MS signal obtained from the amount of N2Â produced for the CZC samples was found to be insufficient for evaluation. Therefore, the amount of N2Â formed during decomposition was indirectly estimated from the retarded evolution of N2O with mass to charge of 44 as compared with blank experiments performed with boron nitride. However, earlier investigations for Cu/ZnO samples have revealed a good agreement between the Cu0Â surface areas determined by N2O RFC with those obtained from the crystallite size of Copper. The Cu0Â surface area was calculated by assuming a stoichiometry of N2O/Cu as 0.5 and a value of 1.47Ã—1019Â Cu atoms/m2Â determined for the surface density.