Catalytic reduction of NO2 in nanocrystalline NaY zeolite

Gas-Phase Nitrous Acid (HONO) Is Controlled by Surface Interactions of Adsorbed Nitrite (NO2-) on Common Indoor Material Surfaces

Nitrous acid (HONO) is a household pollutant exhibiting adverse health effects and a major source of indoor OH radicals under a variety of lighting conditions. The present study focuses on gas-phase HONO and condensed-phase nitrite and nitrate formation on indoor surface thin films following heterogeneous hydrolysis of NO₂, in the presence and absence of light, and nitrate (NO₃^-) photochemistry. These thin films are composed of common building materials including zeolite, kaolinite, painted walls, and cement. Gas-phase HONO is measured using an incoherent broadband cavity-enhanced ultraviolet absorption spectrometer (IBBCEAS), whereby condensed-phase products, adsorbed nitrite and nitrate, are quantified using ion chromatography. All of the surface materials used in this study can store nitrogen oxides as nitrate, but only thin films of zeolite and cement can act as condensed-phase nitrite reservoirs. For both the photo-enhanced heterogeneous hydrolysis of NO₂ and nitrate photochemistry, the amount of HONO produced depends on the material surface. For zeolite and cement, little HONO is produced, whereas HONO is the major product from kaolinite and painted wall surfaces. An important result of this study is that surface interactions of adsorbed nitrite are key to HONO formation, and the stronger the interaction of nitrite with the surface, the less gas-phase HONO produced.

Adsorption of NO, NO2 and H2O in divalent cation faujasite type zeolites: a density functional theory screening approach

Emissions of diesel exhaust gas in confined work environments are a major health and safety concern, because of exposition to nitrogen oxides (NO_x). Removal of these pollutants from exhaust gas calls for engineering of an optimum sorbent for the selective trapping of NO and NO₂ in the presence of water. To this end, periodic density functional theory calculations along with a recent dispersion correction scheme, namely the Tkatchenko-Scheffler scheme coupled with iterative Hirshfeld partitioning TS/HI, were performed to investigate the interactions between NO, NO₂, H₂O and a series of divalent cation (Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Pd²⁺, and Pt²⁺) faujasites. This enabled the identification of the optimum zeolites to selectively capture NO_x in the presence of H₂O, with respect to two important criteria, such as thermodynamic affinity and regeneration. Our results revealed that Pt²⁺ and Pd²⁺ containing faujasites are the best candidates for effective capture of both NO and NO₂ molecules, which paves the way towards the use of these sorbents to address this challenging application.

Quantitative determination of the Cu species, acid sites and NH3-SCR mechanism on Cu-SSZ-13 and H-SSZ-13 at low temperatures

The NH₃-SCR mechanism and the number of acid sites and various Cu species on Cu-SSZ-13 and H-SSZ-13 were quantitatively determined.

Recent advances in supported molecular sieve catalysts with wide temperature range for selective catalytic reduction of NOX with C3H6

NO_X is a major atmospheric pollutant that is emanated by motor vehicles, thermal power plants, and industrial boilers. Therefore, the removal of NO_X is a research hotspot in the exhaust gas treatment field. Numerous methods have been used to eliminate NO_X: the selective catalytic reduction of NO_X using C₃H₆ as the reducing agent (C₃H₆-SCR) is an effective method to remove NO_X. The key issue in NO_X removal in C₃H₆-SCR is to obtain catalysts with low-temperature activity and wide operating temperatures. Till date, different supported wide-temperature-active molecular sieve catalysts have been prepared and used in C₃H₆-SCR reactions. Studies have shown that the catalytic performance of supported catalysts is related not only to the active component but also to the structural and textural parameters of the molecular sieve supports. This review summarizes the structural and textural characteristics, catalytic properties, and catalytic mechanism of molecular sieve catalysts with different pore structures for C₃H₆-SCR reactions. The design strategies of supported molecular sieve catalysts are suggested. The goal of this review is to highlight (1) the structural and textural characteristics and low-temperature catalytic performance of different supported molecular sieve catalysts; (2) the relationship between wide-temperature window and loaded active components, as well as carriers of the supported molecular sieve catalysts; and (3) design strategies and development prospects of supported molecular sieve catalysts with low-temperature activity and wide-temperature operating range for C₃H₆-SCR reactions.

NO_X is a major atmospheric pollutant that is emanated by motor vehicles, thermal power plants, and industrial boilers.

Direct Decomposition of NO into N2 and O2 over Cu /ZSM-5 Containing Ce and Zr as Promoter

The Zr-Cu-Ce/ZSM-5 catalysts prepared by the simultaneous ion-exchange showed high activity for NO decomposition in the presence of O2, the highest conversion of NO was up to 75%, indicating that the Ce and Zr addition can enhance the activity of catalysts. A new highly active site, which facilitates oxygen mobilization and desorption, could be formed in the sample due to the Ce and Zr addition. In addition, the Zr addition can significantly improve the thermal stability of the catalyst.

Adsorption and thermal reaction of DMMP in nanocrystalline NaY

In this study, FTIR spectroscopy and solid-state magic angle spinning (MAS) NMR were used to investigate the adsorption and thermal reaction of the nerve gas simulant dimethyl methylphosphonate (DMMP) in nanocrystalline NaY with a crystal size of approximately 30 nm. DMMP adsorbs molecularly in nanocrystalline NaY at 25 degrees C. Gas-phase products of the reaction of DMMP and oxygen in nanocrystalline NaY at 200 degrees C were monitored by FTIR spectroscopy and determined to be carbon dioxide (major product), formaldehyde, and dimethyl ether. In the presence of water, the thermal reaction of DMMP in nanocrystalline NaY at 200 degrees C yielded methanol (major product), carbon dioxide, and dimethyl ether. When the thermal reaction of DMMP in nanocrystalline NaY at 200 degrees C was conducted in the presence of water and oxygen, the predominant products were methanol and carbon dioxide. Hydroxyl sites located on the external zeolite surface were consumed during the DMMP thermal reactions as monitored by FTIR spectroscopy and were therefore determined to be the active sites in this reaction. 31P solid-state MAS NMR experiments were used to identify the surface-bound phosphorus complexes. The reactivity per gram of zeolite was comparable to other recently studied metal oxides such as MgO, Al2O3, and TiO2, and was found to have comparable, if not higher reactivity. Future improvements in reactivity may be achieved by incorporating a reactive transition metal ion or metal oxide nanocluster into the nanocrystalline NaY to enhance reaction rates and to achieve complete reaction of DMMP.