HAADF-STEM Study of Mo/V Distributions in Mo-V-Te-Ta-O M1 Phases and Their Correlations with Surface Reactivity

TL;DR

This study uses HAADF-STEM imaging to analyze how different synthesis methods affect Mo/V distribution in MoVTeTaO M1 phases and how these variations influence surface reactivity in propane ammoxidation.

Contribution

It reveals the impact of synthesis methods on Mo/V distribution and correlates these distributions with catalytic activity and selectivity in propane ammoxidation.

Findings

Synthesis methods significantly influence Mo/V distribution.

Higher V content correlates with increased ACN yield.

Surface V sites are linked to catalytic activity.

Abstract

The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and microwave-assisted HT synthesis methods (MW) employing two different Ta precursors, Ta ethoxide and a custom-made Ta oxalate complex. The profile intensity analysis of the HAADF-STEM image of M1 phases oriented along [hk0] directions from the surface to bulk region of HAADF-STEM images indicated that the chemical composition of surface ab planes is very similar to their composition in the bulk. The HAADF-STEM image analysis showed that synthesis methods have a significant impact on the Mo/V distribution in the MoVTeTaO M1 phases and their reactivity in propane ammoxidation. Enhanced acrylonitrile (ACN) yield and 1st order irreversible reaction rate constants for propane consumption, normalized to the estimated surface ab plane areas, correlated with increased V content in the proposed catalytic center (S2-S4-S4-S7-S7). These observations lend further support to the idea that multiple VOx sites present in the surface ab planes may be responsible for the activity and selectivity of the M1 phase in propane ammoxidation.