Acid-base properties of alumina-oxide support modified with europium and platinum ions for preparation of three-route catalysts

Abstract

Three-route catalysts promote the simultaneous conversion of CO, C_xH_y, and NO_x into CO₂, N₂, and H₂O. They consist of a ceramic or metal block with a cellular structure, the inner surface of which is coated with a catalytically active coating containing oxide carriers (aluminum, zirconium and cerium oxides), precious metals (Pt, Pd and Rh) and modifiers.

The oxide carrier must have a developed specific surface area and a sufficient concentration of acid-base centers, which is typical for the low-temperature modification of γ-Al₂O₃. The main reasons for deactivation of the catalyst are the blocking of active sites, erosion of the substrate, and a decrease in the specific surface area of Al₂O₃. One of the effective ways to increase the resistance of a catalyst to thermal decontamination is to modify it with rare earth metals (REM). In this work, Eu³⁺ ions were used to modify the surface of platinum-containing aluminum oxide. The suspension was crushed in a bead mill, dried on a polymer substrate at 150°C for 4 hours, sintered at 500°C, and examined by IR spectroscopy, TEM, and the indicator method.

Two methods were used to study the acid-base properties of the powder surface: the IR Fourier spectroscopy method for determining Lewis centers and the Gamete indicator method for detecting Brensted centers.

Pyridine was used as a probe molecule to determine the number of Lewis acid centers on the surface of experimental samples. The concentration of acid centers was estimated from the integral intensity of the absorption bands corresponding to these centers in the spectra of adsorbed pyridine. IR spectra were taken on samples of aluminum oxide suspensions modified with platinum and europium ions and sintered at temperatures of 500°C. The sample modified with platinum and europium ions has the highest concentration of Lewis acid centers (0.28 mmol/g). Acid centers on the surface of the automotive catalyst will promote the sorption and conversion of hydrocarbons contained in exhaust gases into carbon dioxide and water.

The content of Brensted adsorption centers was determined by the change in the optical density of standard indicator solutions at wavelengths corresponding to their absorption maxima. It follows from the results of the indicator analysis that the concentration of Brensted centers in the Pt⁴⁺/ γ-Al₂O₃ sample decreases with increasing pK_a of the indicators. The concentration of Brensted centers in the Eu³⁺, Pt⁴⁺/ γ-Al₂O₃ sample first increases, then decreases. There is a large amount of interlayer water in the Pt⁴⁺/γ-Al₂O₃ sample, since the Brensted acid centers are associated with this. The addition of europium ions reduces the concentration of interlayer water and will prevent the sintering of aluminum oxide grains during the operation of the catalyst.

According to transmission electron microscopy, the samples are nanostructured. Sponge platinum nanoparticles in the Pt⁴⁺/γ-Al₂O₃ sample and Eu₂O₃ nanorods are located on the surface of elongated aluminum oxide nanoparticles; sponge platinum nanoparticles and Eu₂O₃ nanorods are present in the Eu³⁺, Pt⁴⁺/ γ-Al₂O₃ samples. The addition of Eu, Pt modifiers causes a decrease in the particle size of aluminum oxide by 5-7 nm. The size of platinum particles practically does not change in the samples Eu³⁺, Pt⁴⁺/γ-Al₂O₃ and Pt⁴⁺/γ-Al₂O₃. The length of the europium oxide nanorods decreases from the Eu³⁺/γ-Al₂O₃ sample to the Eu3+, Pt⁴⁺/ γ-Al₂O₃ sample.

Synthesized samples of catalysts were tested on the stand for determining the catalytic activity of Ecoalliance LLC. The redox processes of converting gases HCN, CO, NO_x into end products CO₂, H₂O, N₂ on a catalyst of the composition Eu³⁺, Pt⁴⁺/ γ-Al₂O₃ occur more efficiently than on a Pt⁴⁺/ γ-Al₂O₃ catalyst, the degree of conversion increases by 3-4%.

The results obtained are a foundation for the continuation of work on the modification of the alumoplatin catalyst with ions of rare earth elements for effective exhaust gas purification.