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A novel tungsten-doped magnetic iron oxide catalyst was fabricated for selective catalytic reduction of NOx with NH3 (NH3-SCR), which was supported by carbon-included magnetic materials of Fe2O3–-H synthesized by the solution combustion followed by the oxidization of hydrogen peroxide. As a consequence, the thermal stability of γ-Fe2O3 crystallites located in the magnetic support is significantly enhanced. Meanwhile, the irreversible transformation of Fe2O3 crystallites from γ-phase to α-phase during the annealing process at high temperatures (i.e. 400 °C) is restrained to a large extent. In addition, the magnetic W/Fe2O3 catalyst presents a superb porous structure with higher BET surface area and pore volume due to the anti-sintering effect of tungsten doping on the magnetic Fe2O3. In this regard, the surface acidity as well as the molar ratio of the surface-adsorbed oxygen is substantially augmented, leading to a remarkable improvement of the redox ability of the active bulk species in the catalyst. It is worth mentioning that the promotions in thermal stability and redox ability is affected by the doping amount of tungsten in the catalyst, the molar ratio of glucose/urea/iron, and the ignition temperature for synthesizing the magnetic iron oxide support. The experiments revealed that with a doping amount of 7.5 wt%, a molar ratio of glucose/urea/iron of 5:20:8, and an ignition temperature of 400 °C, the NH3-SCR activity of the novel catalyst could be optimized.

A highly active catalyst of cerium–tungsten–titanium mixed oxide was synthesized by introducing Ce4+ and H2O2 in the base sample of Ce20W10Ti100Oz–Ce3+. As a consequence, the NH3-SCR activity of Ce20W10Ti100Oz–Ce3+ is significantly improved as the additives of Ce4+ and H2O2 enlarge the Brunauer–Emmett–Teller (BET) surface area by refining its pore size. Meanwhile, the introduction of Ce4+ increases the Lewis acid sites of Ce20W10Ti100Oz–Ce3+ and decreases its low-temperature Bronsted acid sites. The further addition of H2O2 improves the Bronsted acid sites and dispersion of cerium/tungsten species, and thereby enhances the concentrations of the adsorbed oxygen (Oα) and the adsorbed oxygen $${O'_\alpha }$$ due to the activation of chemisorbed water on the surface of the catalyst. The addition of Ce4+ and H2O2 shows a synergistic promotional effect, which is due to the largest BET surface area and the highest concentrations of Oα or/and $${O'_\alpha }$$ . Ce20W10Ti100Oz–Ce3+:Ce4+ = 17.5:2.5 + H2O2 exhibits the highest catalytic activity compared with the conventional ones (Fig. 5).

Two kinds of magnetic iron oxides (Fe2O3-PO and Fe2O3-AO) are synthesized via the solution combustion coupled with the chemical oxidization of hydrogen peroxide. The magnetic carbon-containing Fe2O3-AO obtained at the absence of air presents the main γ-Fe2O3 crystal with smaller crystallite size and larger BET surface area, but not the aggregated and sintered α-Fe2O3 crystal of Fe2O3-PO. The doping of tungsten has no effect on the growth of α-Fe2O3 in Fe2O3-PO during the annealing process at 400 °C. However, the doping of tungsten effectively restrains the irreversible transformation of γ-Fe2O3 into α-Fe2O3 crystal and enhances the anti-collapse of the pore structure of Fe2O3-AO under the same annealing process at 400 °C by inducing a stronger interaction between tungsten species and γ-Fe2O3 crystal. In addition, the doping of tungsten improves the ratio of surface adsorbed oxygen, the Fe2+/(Fe2++Fe3+) ratio and the surface acidity of Fe2O3-PO-400 and Fe2O3-AO-400. Therefore, the doping of tungsten promotes the catalytic performance of Fe2O3-PO-400, especially its high-temperature activity, but exhibits a better promotional effect on the low-temperature activity of Fe2O3-AO-400. Magnetic 5W/Fe2O3-AO-400 shows the highest BET surface area, the largest Fe2+/(Fe2++Fe3+) ratio, the strongest surface acidity and the appropriate redox ability compared with the other tested catalysts, thus exhibits the highest low-temperature NH3-SCR activity and better resistance to SO2 and H2O than 5W/Fe2O3-PO-400. The formation of low crystallinity and high dispersive γ-Fe2O3 is an important reason on the good catalytic performance of magnetic 5W/Fe2O3-AO-400, which is also confirmed by the influence of annealing temperature. Furthermore, the results of steady-state kinetic experiments demonstrate that the 5W/Fe2O3-AO-400 catalyst mainly follows the Eley-Rideal mechanism.