Abstract:

In this paper, the effects of calcination temperatures (350-1000 ℃) and metal loadings (5-15% Cu) on copper species of Cu/γ-Al_2O_3 catalysts are studied by means of XRD and EXAFS. For the catalysts, high dispersion state of the copper species could occur at calcination temperature below 700 ℃. The Cu-O coordination peaks at close to 0.16 nm were fitted according to Cu~(2+) taking octahedral (Oh) and tetrahedral (Td) sites. The results of two coordination shell fitting show that the average oxygen coordination numbers of Cu~(2+) (Oh) baries between 5.1 and 6.0. For 5CuY (Y: 350-700 ℃), the ratio of Cu~(2+) (Oh)/Cu~(2+) (Td) decreases gradually as calcination temperature increases, which means that Cu~(2+) ions in Oh move into Td site, and the coordination distance and average coordination number of Cu~(2+) (Oh) increase, indicating the Cu~(2+) (Oh) diffuse from open distorted Oh environment into symmetry one. As for 5Cu900, high temperature calcination makes Cu~(2+) ions diffusing into support phase, partly forming CuAl_2O_4. The value of Cu~(2+) (Oh)/Cu~(2+) ITd) is the same as that in CuAl_2O_4 at higher temperature (1000 ℃) or high loadings (10, 15% Cu).

Key words: Cu/γ-Al₂O₃ catalyst, Calcination temperature, EXAFS