Identifying the location of Cu ions in nanostructured SAPO-5 molecular sieves and its impact on the redox properties.

Jörg Radnik,Vikram Singh Raghuwanshi,Thi Thuy Hanh Dang,Armin Hoell,Dragomir Tatchev,Suresh Gatla

doi:10.1039/c8ra10417d

Abstract

Combining X-ray Absorption Fine Spectroscopy (XAFS) with Anomalous Small-Angle X-ray Scattering (ASAXS) determines the location of Cu2+ ions in silicoaluminophosphate (SAPO-5) frameworks prepared by hydrothermal crystallization or impregnation. As expected, for the hydrothermally prepared sample, incorporation in the SAPO-5 framework was observed. For the first time preferential location of Cu2+ ions at the inner and outer surfaces of the framework is determined. Temperature-Programmed Reduction (TPR) and X-ray Photoelectron Spectroscopy (XPS) investigations demonstrated that such Cu2+ is stable in an argon (Ar) atmosphere up to 550 °C and can only be reduced under a hydrogen atmosphere. In contrast, Cu2+ deposited by impregnation on the pure SAPO-5 framework can be easily reduced to Cu+ in an Ar atmosphere. At lower Cu amounts, mononuclear tetrahedrally coordinated Cu species were formed which are relatively stable in the monovalent form. In contrast, at higher Cu amounts, CuO particles were found which change easily between the mono- and bivalent species.

Highlights

Molecular sieves as supports of transition metal ions (TMIs) offer new opportunities in the preparation of nanostructured materials, because their ordered structure of regular channels or pores in the sub- or low nanometre range allows preparing nanoparticles with more homogeneous size distributions in well-de ned sizes
Combining X-ray Absorption Fine Spectroscopy (XAFS) with Anomalous Small-Angle X-ray Scattering (ASAXS) determines the location of Cu2+ ions in silicoaluminophosphate (SAPO-5) frameworks prepared by hydrothermal crystallization or impregnation
Summarizing, depending on the Cu amount and the preparation method, different Cu components could be created: CuO nanoparticles are only available at high Cu amount, low Cu content leads to species with isolated Cu atoms: a er impregnation isolated mononuclear Cu2+ species coordinated by four oxygen atoms with no hints for the second shell in the Extended X-ray Absorption Fine Structure (EXAFS) oscillations, whereas the hydrothermal synthesis leads to Cu2+ incorporated into the framework

Summary

Introduction

Molecular sieves as supports of transition metal ions (TMIs) offer new opportunities in the preparation of nanostructured materials, because their ordered structure of regular channels or pores in the sub- or low nanometre range allows preparing nanoparticles with more homogeneous size distributions in well-de ned sizes. Combining X-ray Absorption Fine Spectroscopy (XAFS) with Anomalous Small-Angle X-ray Scattering (ASAXS) determines the location of Cu2+ ions in silicoaluminophosphate (SAPO-5) frameworks prepared by hydrothermal crystallization or impregnation.

Results

Conclusion