Tailoring amorphous alumina catalysts with enriched five-coordinated aluminum via pH control

Abstract

Amorphous alumina is known to contain a high fraction of five-coordinated aluminum species (Al(V)), which are recognized as active sites and serve as anchoring sites for noble metals. In this study, we aimed to synthesize highly active amorphous alumina enriched in Al(V) by controlling the pH during the gelation of aluminum hydroxide using buffers with different concentrations (0.1, 1, 10, and 100 mM). After dehydration, the resulting alumina was characterized and evaluated for VOC decomposition. While the Al(V) fraction increased with buffer concentration (42–53% at 650 °C), no direct correlation was observed between activity and the parameter Al(V) ratio in terms of specific surface area. Instead, deconvolution of the ²⁷Al NMR spectra revealed that Al(V) consists of sharp (trigonal bipyramidal) and broad (square pyramidal) components, the latter exhibiting a strong correlation with VOC decomposition when specific surface area was taken into consideration (R² = 0.77). Experimental results and insights from previous research suggest that high pH suppresses hydroxide precipitation, yielding small amorphous particles prone to aggregation, whereas low pH promotes nitrate incorporation into the lattice, leading to structural disorder and site blocking on pyramidal Al(V). Among the tested conditions, the 1 mM buffer produced amorphous alumina with an optimal balance of surface area and Al(V) broad content, achieving for the first time 100% VOC decomposition using pure amorphous alumina. These findings highlight pH control as a promising strategy for tailoring active Al(V) sites in alumina catalysts.