Temperature-programmed desorption of NH3 (NH3-TPD) was used to assess the acidity of the catalysts. In a typical TPD protocol, the reduced sample was placed in a quartz flow reactor and heated at 500 °C under He flow for 1 h to remove impurities from the surface. Adsorption of NH3 was carried out at 100 °C for 15 min. Physically adsorbed NH3 molecules were removed by treating the sample with He for 15 min at 100 °C before the TPD experiment was carried out from room temperature to 500 °C with a 25 °C min -1 temperature ramp (TCD detector).