Environment-friendly magnetic Fe–Ce–W catalyst for the selective catalytic reduction of NO_x with NH₃: influence of citric acid content on its activity-structure relationship

Abstract

The enhancement of critic acid amount strengthened the sol–gel reaction between critic acid and metal ions, showed an important role on the structure properties of magnetic Fe–Ce–W mixed oxide catalyst, thereby affected its NH3-SCR activity.

The Role of Nitrate on the Sol-Gel Spread Self-Combustion Process and Its Effect on the NH3-SCR Activity of Magnetic Iron-Based Catalyst

“…As illustrated in Figure 8A, after NH3 adsorption and N2 purge at 250 °C, Fe85Ce10W5-CP-CA showed several bands located at 1188, 1406, 1587, 3256, and 3351 cm −1 . The bands located at 1188 and 1587 cm −1 can be attributed to the coordinated NH3 on the Lewis acid sites, and the band located at 1406 cm −1 can be ascribed to the ionic NH4 + bound to the Brønsted acid site, while the bands of 3256 and 3351 cm −1 corresponded to the N-H stretching modes of the coordinated NH3 connected to the Lewis acid sites [27,[44][45][46][47][48]. After the introduction of NO + O2 for 10 s, the adsorption peaks of the coordinated NH3 on the Lewis acid sites (located at 1188 and 1587 cm −1 ) and the N-H stretching modes of coordinated NH3 connected to the Lewis acid sites (located at 3256 and 3351 cm −1 ) disappeared, and the intensity of ionic NH4 + bound to the Brønsted acid site at 1406 cm −1 also became weakened.…”

“…Meanwhile, the intensity of diffraction peaks attributed to the γ-Fe 2 O 3 crystallite in Fe 85 Ce 10 W 5 -CP-CA is stronger than that of Fe 85 Ce 10 W 5 -CP-CA(NA 1.0 ). Therefore, the removal of nitrate promotes the formation of γ-Fe 2 O 3 crystallite, which is usually thought to be an important active specie for NH 3 -SCR reaction [27]. The re-introduction of nitrate also weakens the crystallization of CeO 2 in Fe 85 Ce 10 W 5 -CP-CA.…”

“…After the introduction of NH 3 for 10 s, the intensity of adsorption peaks of M-NO 2 (1353 cm −1 ) and bridging nitrate (1621 cm −1 ) became weakened, and the band at 1560 cm −1 ascribed to the bidentate nitrates disappeared. The IR bands assigned to the coordinated NH 3 on the Lewis acid sites (1189 and 1587 cm −1 ), the ionic NH 4 + bound to the Brønsted acid site (1439 cm −1 ), and the N-H stretching modes of the coordinated NH 3 connected to the Lewis acid sites (3255 and 3364 cm −1 ) appeared [27,[47][48][49][50]. Figure 8C shows the experimental results of the introduction of NH 3 + NO+O 2 gases over Fe 85 Ce 10 W 5 -CP-CA at 250 • C. It can clearly be seen that the IR bands ascribed to the coordinated NH 3, the N-H stretching modes of the coordinated NH 3 and the ionic NH 4 + appeared, and their intensity gradually became stronger with the increase in the NH 3 + NO + O 2 introduction.…”

“…ascribed to the bidentate nitrates disappeared. The IR bands assigned to the coordinated NH3 on the Lewis acid sites (1189 and 1587 cm −1 ), the ionic NH4 + bound to the Brønsted acid site (1439 cm −1 ), and the N-H stretching modes of the coordinated NH3 connected to the Lewis acid sites (3255 and 3364 cm −1 ) appeared [27,[47][48][49][50]. Figure 8C shows the experimental results of the introduction of NH3 + NO+O2 gases over Fe85Ce10W5-CP-CA at 250 °C.…”

“…However, the sol-gel spread self-combustion method, also called low-temperature combustion synthesis (LCS), which takes advantage of organics (citric acid, glucose, urea, and so on) as reactants and nitrates from metal nitrates as oxidants, has been widely used to obtain nano-particles, ultrafine powders, and metal oxide catalysts [21][22][23][24][25][26]. In our precious studies, a novel Fe-Ce-W mixed oxide catalyst synthesized via the citric acid sol-gel spread self-combustion method exhibited high catalytic activity of NO x reduction with high-dispersive γ-Fe 2 O 3 crystallite formed in it [27]. However, a large amount of heat is released during the spread self-combustion of the citric acid dried gel, and brings about a high temperature that acts on the obtained composite oxide catalyst, which might destroy the physical structure of catalyst, thereby influencing its catalytic performance of NO x reduction even though the duration of high temperature is short.…”