Functionalized Iron Hydroxyapatite as Eco‐friendly Catalyst for NH₃‐SCR Reaction: Activity and Role of Iron Speciation on the Surface

Abstract

Eco‐friendly catalysts have been obtained by functionalizing hydroxyapatite (HAP) with iron (Fe/HAP), according to three preparative methods (flash ionic exchange, deposition‐precipitation, and impregnation). Fe/HAP samples (ca. 2–7 wt.% Fe) have been tested in the reaction of NOx reduction by ammonia (NH3‐SCR) in the 120–500 °C interval with different NH3/NO ratios (0.6–2) at fixed contact time (0.12 s). All Fe/HAP samples were active and selective in the NH3‐SCR reaction starting from ca. 350 °C. Better performances have been observed on catalysts prepared by deposition‐precipitation and impregnation (about 70 % of NOx conversion and selectivity to N2 higher than 95 % at 350 °C), where α‐Fe2O3 and 3D‐Fe2O3 nanoclusters were present, as indicated by Mössbauer and UV‐Vis‐DR spectroscopies. On the opposite, paramagnetic Fe3+ centres were the predominant species on samples prepared by flash ionic exchange. Further characterization techniques (XRPD, N2‐physisorption, acidity by NH3 adsorption, and H2‐TPR) have concurred to elucidate Fe‐sitting HAP and structure‐activity relationships.

Environmental Reactions of Air-Quality Protection on Eco-Friendly Iron-Based Catalysts

“…Iron phase was supported on hydroxyapatite with concentration in a wide range (2 < wt.% Fe < 13) according to a flash ion exchange procedure, already presented in the literature [26,27]. In this procedure the operative pH (ca.…”

“…More recently, our group has investigated the hydroxyapatite functionalization with Cu and Fe species to obtain effective catalysts for NH 3 -SCR reactions [26][27][28]. The experimental conditions adopted during the functionalization strongly affected the sitting, local environment, and structure of metal phase.…”

“…In addition, the copper salt used as metal phase precursor (e.g., nitrate, chloride and acetate) played a key role in inducing changes in textural properties and surface composition, which unavoidably influenced the catalytic behavior [28]. In the case of Fe-loaded hydroxyapatite (Fe/HAP), great attention has been devoted to develop a deposition technique able to assure high metal dispersion with controlled nuclearity of iron species and limited aggregation [26]. An innovative flash ion exchange procedure, characterized by an acidic solution containing the Fe-salt and very short contact time with HAP, has demonstrated its effectiveness in highly dispersing and stabilizing iron in the form of isolated or oligonuclear species on the surface.…”

Incorporation of Iron(II) and (III) in Hydroxyapatite—A Theoretical Study

“…[20], thereby making exchanged HAp efficient catalytic systems. Accordingly, the synthesis and study of HAp-based catalytic systems with iron impurities have attracted great interest in recent years [21][22][23][24][25][26]. As reviewed by Goldberg et al [27,28], Fe-HAp and Fe oxide nanoparticles demonstrated high efficiency as a catalyst in the field of heavy oil purification via oxidative desulfurization and aquathermolysis methods, oil asphaltene extraction, etc.…”

“…[20], thereby making exchanged HAp efficient catalytic systems. Accordingly, the synthesis and study of HAp-based catalytic systems with iron impurities have attracted great interest in recent years [21][22][23][24][25][26].…”