In situ diffraction monitoring of nanocrystals structure evolving during catalytic reaction at their surface

With decreasing size of crystals the number of their surface atoms becomes comparable to the number of bulk atoms and their powder diffraction pattern becomes sensitive to a changing surface structure. On the example of nanocrystalline gold supported on also nanocrystalline CeO(2 )we show evolution of (a) the background pattern due to chemisorption phenomena, (b) peak positions due to adsorption on nonstoichiometric CeO2-x particles, (c) Au peaks intensity. The results of the measurements, complemented with mass spectrometry gas analysis, point to (1) a multiply twinned structure of gold, (2) high mobility of Au atoms enabling transport phenomena of Au atoms to the surface of ceria while varying the amount of Au in the crystalline form, and (3) reversible CeO2 peaks position shifts on exposure to He-X-He where X is O-2 , H-2 , CO or CO oxidation reaction mixture, suggesting solely internal alternations of the CeO2 crystal structure. We found no evidence of ceria lattice oxygen being consumed/supplied at any stage of the process. The work shows possibility of structurally interpreting different contributions to the multi-phase powder diffraction pattern during a complex physicochemical process, including effects of physi-, chemisorption and surface evolution. It shows a way to structurally interpret heterogeneous catalytic reactions even if no bulk phase transition is involved.

Automated summary

With decreasing size of crystals, the surface structure of nanocrystalline materials, such as nanocrystalline gold supported on ceria, becomes important and affects the powder diffraction pattern. This study reveals the changes in background pattern, peak positions, and Au peak intensity, and suggests a multiply twinned structure of gold and high mobility of Au atoms on the surface of ceria. The results also show reversible shifts in CeO2 peak positions, indicating internal alternations of the CeO2 crystal structure.