In situ X-ray powder diffraction ex

In situ X-ray powder diffraction experiments were performed using a STOE Bragg-Brentano diffractometer (Cu-Kα radiation, secondary monochromator, scintillation counter). Reduction of the calcined CuO/ZnO precursors was carried out in an in situ cell (Bühler HDK S1) as described above. XRD patterns were recorded at 523 K under methanol steam-reforming conditions in a 2θ range from 28.0◦ to 93.0◦ with a step width of 0.04◦ (counting time, 2 s/step). A sum of pseudo-Voigt profile functions and an appropriate background function were refined to the in situ XRD patterns of the Cu/ZnO catalysts according to the Pawley method (i.e., “full pattern refinement”). Lattice constants of Cu and ZnO, a linear zero shift in the 2θ scale, four coefficients of the background polynomial, peak intensity, and the Gaussian and Lorentzian parts of the pseudoVoigt profile ofeach individual hkl line were allowed to vary in the refinement. The volume-averaged copper crystallite size (diameter) and microstrain were determined from the Lorentzian and Gaussian parts of the individual profile functions [24].An alternative method for determining crystallite size and microstrain by analyzing X-ray diffraction line profiles has been described by Williamson and Hall [25]. However, an isotropic distribution of both size and strain is a prerequisite for a linear correlation between d∗ = 2sinθ/λ and β∗ = β cosθ/λ and the applicability of the Williamson–Hall analysis.