Influence of Alumina Precursor Properties on Cu-Fe Alumina Supported Catalysts for Total Toluene Oxidation as a Model Volatile Organic Air Pollutant

Tadej Žumbar,Albin Pintar,Nataša Zabukovec Logar,Goran Dražić,Nataša Novak Tušar,Janez Volavšek,Hristina Lazarova,Alenka Ristić

doi:10.3390/catal11020252

Abstract

The structure–property relationship of catalytic supports for the deposition of redox-active transition metals is of great importance for improving the catalytic efficiency and reusability of the catalysts. In this work, the role of alumina support precursors of Cu-Fe/Al2O3 catalysts used for the total oxidation of toluene as a model volatile organic air pollutant is elucidated. Surface characterization of the catalysts revealed that the surface area, pore volume and acid site concentration of the alumina supports are important but not the determining factors for the catalytic activity of the studied catalysts for this type of reaction. The determining factors are the structural order of the support precursor, the homogeneous distribution of the catalytic sites and reducibility, which were elucidated by XRD, NMR, TEM and temperature programed reduction (TPR). Cu–Fe/Al2O3 prepared from bayerite and pseudoboehmite as highly ordered precursors showed better catalytic performance compared to Cu-Fe/Al2O3 derived from the amorphous alumina precursor and dawsonite. Homogeneous distribution of FexOy and CuOx with defined Cu/Fe molar ratio on the Al2O3 support is required for the efficient catalytic performance of the material. The study showed a beneficial effect of low iron concentration introduced into the alumina precursor during the alumina support synthesis procedure, which resulted in a homogeneous metal oxide distribution on the support.

Highlights

Volatile organic compounds (VOCs) are classified, together with particulate matter, NOx, SOx, carbon monoxide and ammonia, as major or primary air pollutants with direct emission to the atmosphere and subsequent harmful effects [1]
We found out that the promotion of catalytic oxidation of toluene as a model VOC was due to the cooperative redox effect between CuO nanocrystals and finely dispersed Cu–oxo–Fe nanoclusters with defined Cu/Fe molar ratio required for the synergistic effect
Different alumina supports were prepared with presence of iron species, using different alumina precursors and techniques, which led to differences in the structure of supports’ precursors-aluminum hydroxides and dawsonite

Summary

Introduction

Volatile organic compounds (VOCs) are classified, together with particulate matter, NOx , SOx , carbon monoxide and ammonia, as major or primary air pollutants with direct emission to the atmosphere and subsequent harmful effects [1]. There are two main groups of catalysts for VOC oxidation reactions: noble metal and non-noble metal catalysts The former generally require lower operating temperatures, but their cost, their tendency to poisoning in the presence of sulphur compounds, and their geographical distribution, are the main reasons why the non-noble metals or transition metal oxides come into play [4,5]. Copper oxide is acknowledged as a prosperous catalyst for catalytic oxidation reaction and its catalytic properties were tested on various supports [8,9]. Activity in the total oxidation of VOC is connected with the interaction of aromatic electrons with metal ions in tetrahedral position, acting as Lewis acid sites, increasing the possibility of electrophilic attack of adsorbed oxygen and combustion of toluene molecules [12,13]. Copper proved to be the most active metal among several transition metals (Cr, Co, Mo, Ni, Mn, Fe, Cu) on alumina support tested, with the optimum concentration being more than 5 wt% and less than

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