Catalysis mechanism of Pd(II)@PVA membrane catalyst studied from the aspect of molecular level micro-defects by positron annihilation spectroscopy. (March 2019)

• Record Type: Journal Article
• Authors: Xu, Mengdie; Zhao, Jing; Luo, Chao; Liu, Qi; Zeng, Minfeng; Qi, Chenze; Xia, Rui; Cao, Xingzhong; Wang, Baoyi
• Is Part Of: Radiation physics and chemistry. Volume 156(2019:Mar.)
• Page Start: 128
• Page End: 136
• Publication Date: 2019-03
• Subjects: Positron annihilation -- Micro-defects -- Catalysis mechanism -- Polyvinyl alcohol -- Heck reactions; Radiation chemistry -- Periodicals; Radiometry -- Periodicals; Radiation -- Periodicals; Chimie sous rayonnement -- Périodiques; 539.2
• DOI: 10.1016/j.radphyschem.2018.10.007
• Languages: English
• ISSNs: 0969-806X

Abstract

Abstract: In this study, an insightful understanding of the correlations of the molecular level micro-defects structure and catalytic performance of palladium species supported on polyvinyl alcohol (Pd(II)@PVA) membrane heterogeneous catalyst have been studied with positronium probe and other structural characterization methods. The micro-defects size both in crystalline region ( D c ) and amorphous region ( D a ) of PVA matrix increases gradually with the Pd species loading and membrane catalyst recycle times. As a result of a synergism of micro-defects enlargement caused by solvents swelling and highly thermal motions of macromolecular segments at high temperature, a number of instantaneously connected micro-defects channel rather than a single micro-defect in the real reaction state might provide enough open spaces for the coupling reactions. This catalysis mechanism from the aspect of molecular level micro-defects structure is powerfully supported by comparing the catalytic performances of a common Pd(II)@PVA membrane catalyst and a PVA layer sealed Pd(II)@PVA membrane catalyst applied in Heck reactions. As the recycle times increases, the compactness and regularity of PVA molecular chain segments stacking decreases obviously, which promotes the Pd leaching and causes gradually decrease in catalytic efficiency of the recycled Pd(II)@PVA membrane catalyst. The mechanical properties, morphology, crystalline behavior, and swelling properties of the Pd(II)@PVA membrane catalysts are also characterized and explained in view of the changes of molecular level micro-defects structure. Highlights: Positronium probe is used in the characterization of molecular level micro-defects of Pd(II)@PVA catalyst. A number of instantaneously connected micro-defects channel provides open spaces for the reactions. The proposed catalysis mechanism in molecular level is powerfully supported by the catalytic performance assessments.