The specific Cu0 surface areas of the samples were measured by N2O decomposition

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 Cu⁰ 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 Cu^0 as given below (Eq.1)

2Cu + N₂O = “Cu₂O” + N₂

Before measurements, the CZC samples were activated in a stream of MeOH/H2O = 2:1, with a flow rate of 36 cm3min^-1 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^-1 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 Cu⁰ surface area. For the current experiments, however, the same MS signal obtained from the amount of N₂ produced for the CZC samples was found to be insufficient for evaluation. Therefore, the amount of N₂ 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 Cu⁰ surface areas determined by N₂O RFC with those obtained from the crystallite size of Copper. The Cu⁰ surface area was calculated by assuming a stoichiometry of N₂O/Cu as 0.5 and a value of 1.47×10¹⁹ Cu atoms/m^2 determined for the surface density.