Bimetallic AgFe Systems on Mordenite: Effect of Cation Deposition Order in the NO Reduction with C3H6/CO

Abstract

Mono- and bimetallic systems of Ag, Fe, and Ag–Fe exchanged in sodium mordenite zeolite were studied in the reaction of NO reduction. The transition metal cations Ag and Fe were introduced by ion exchange method both at room temperature and 60 °C; modifying the order of component deposition in bimetallic systems. These materials were characterized by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), ultraviolet-visible spectroscopy (UV-Vis), X-Ray photoelectron Spectroscopy (XPS) and High-resolution transmission electron microscopy (HR-TEM). The XPS and UV–Vis spectra of bimetallic samples revealed that under certain preparation conditions Ag+ is reduced with the participation of the Fe2+/Fe3+ ions transition and is present in the form of a Ag reduced state in different proportions of Agm clusters and Ag0 NPs, influenced by the cation deposition order. The catalytic results in the NO reduction reaction using C3H6/CO under an oxidizing atmosphere show also that the order of exchange of Ag and Fe cations in mordenite has a strong effect on catalytic active sites for the reduction of NO.

Properties of Iron-Modified-by-Silver Supported on Mordenite as Catalysts for NOx Reduction

“…Different values are observed between NaMOR, AgMOR, and Fe-containing samples. Since it is impossible to estimate the number of iron cations those who enter in the framework and evaluate the ratio between the coexisting Fe 2+ and Fe 3+ cations [50] from the ICP-OES data, the limit values of EIEM-Fe 2+ and EIEM-Fe 3+ are calculated, considering that all the iron is entirely as cations of one of the possible oxidation states (Fe 2+ or Fe 3+ ). The real EIEM for our samples should approximate to an intermediate value between the two calculated idealized estimations:…”

“…For most samples, EIEM is close to 1, with the exception of NaMOR and FeMOR. The high EIEM value for NaMOR has been previously associated with the fact that NaOH, which is present during the synthesis of zeolite in the industrial process, is not completely removed by washing of the resulting material [50]. For FeMOR, the EIEM has a value of less than 1.…”

“…EXAFS Fourier transforms (Figure 2b) have shown that the spectra of FeMOR are closer to the FeO reference, while the shapes of EXAFS Fourier transforms for both bimetallic samples remain partially similar to those of the α-Fe 2 O 3 reference. It could be explained by the redox reaction of Fe 2+ with Ag + , leading to the formation of Fe 3+ [50] and the presence of iron in both oxidation states simultaneously. In the case of AgFeMOR, the free Fe 2+ present in the solution undergoes a redox reaction with Ag + MOR, and consequently, the resulting Ag 0 diffuses from the mordenite channels, agglomerating into nanoparticles in the mesopores and on the surface of crystals.…”

“…The order of deposition of the metal components in the preparation of bimetallic catalysts has been shown to strongly influence the redox interaction of metals, contributing to the formation of sites of interest for HC-SCR of NO x [49,50]. Based on these premises, the goal of this work was to study the formation of Fe-Ag species deposited on mordenite and their physicochemical properties, focusing on a two steps ion-exchange procedure with a different order of insertion.…”

Selective Catalytic Reduction of Nitric Oxide with Propylene over Fe/Beta Catalysts Under Lean-Burn Conditions

“…Up to now, the removal of NO x in the exhaust of diesel engine under lean-burn conditions is still a significant challenge. Selective catalytic reduction of NO x by hydrocarbons (HC-SCR) is a promising technology for reducing NO x , which can achieve the elimination of both hydrocarbons and nitric oxides in pollution source at the same time [3]. Transition metal (e.g., Cu, Fe) modified zeolites exhibit remarkable catalytic activity in the HC-SCR process, many studies have been conducted on zeolite structure, metals nature and preparation methods of zeolite catalysts [4][5][6].…”