NOx

About: NOx is a research topic. Over the lifetime, 26367 publications have been published within this topic receiving 496555 citations.

Dimers in α-pinene secondary organic aerosol: effect of hydroxyl radical, ozone, relative humidity and aerosol acidity

Abstract: . The formation of secondary organic aerosol (SOA) from both ozonolysis and hydroxyl radical (OH)-initiated oxidation of α-pinene under conditions of high nitric oxide (NO) concentrations with varying relative humidity (RH) and aerosol acidity was investigated in the University of North Carolina dual outdoor smog chamber facility. SOA formation from ozonolysis of α-pinene was enhanced relative to that from OH-initiated oxidation in the presence of initially high-NO conditions. However, no effect of RH on SOA mass was evident. Ozone (O3)-initiated oxidation of α-pinene in the presence of ammonium sulfate (AS) seed coated with organic aerosol from OH-initiated oxidation of α-pinene showed reduced nucleation compared to ozonolysis in the presence of pure AS seed aerosol. The chemical composition of α-pinene SOA was investigated by ultra-performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS), with a focus on the formation of carboxylic acids and high-molecular weight dimers. A total of eight carboxylic acids and four dimers were identified, constituting between 8 and 12% of the total α-pinene SOA mass. OH-initiated oxidation of α-pinene in the presence of nitrogen oxides (NOx) resulted in the formation of highly oxidized carboxylic acids, such as 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) and diaterpenylic acid acetate (DTAA). The formation of dimers was observed only in SOA produced from the ozonolysis of α-pinene in the absence of NOx, with increased concentrations by a factor of two at higher RH (50–90%) relative to lower RH (30–50%). The increased formation of dimers correlates with an observed increase in new particle formation at higher RH due to nucleation. Increased aerosol acidity was found to have a negligible effect on the formation of the dimers. SOA mass yield did not influence the chemical composition of SOA formed from α-pinene ozonolysis with respect to carboxylic acids and dimers. The results support the formation of the high-molecular weight dimers through gas-phase reactions of the stabilized Criegee Intermediate (sCI) formed from the ozonolysis of α-pinene. The high molecular weight and polar nature of dimers formed in the gas phase may explain increased particle number concentration as a result of homogenous nucleation. Since three of these dimers (i.e. pinyl-diaterpenyl dimer (MW 358), pinyl-diaterebyl dimer (MW 344) and pinonyl-pinyl dimer (MW 368)) have been observed in both laboratory-generated and ambient fine organic aerosol samples, we conclude that the dimers observed in this study can be used as tracers for the O3-initiated oxidation of α-pinene, and are therefore indicative of enhanced anthropogenic activities, and that the high molecular weight and low volatility dimers result in homogenous nucleation under laboratory conditions, increasing the particle number concentration.

Catalytic decomposition/regeneration of Pt/Ba(NO3)2 catalysts: NOx storage and reduction

Abstract: The introduction of lean-burn engine technology has prompted the development of NOx storage-reduction (NSR) catalysts, which are currently based on a Pt/Ba(NO3)2/Al2O3 system. In this work a series of powdered catalysts based on this system were prepared and their reactivities examined with a pulsed-flow reactor, which was used to carry out temperature programmed desorption (TPD) experiments and to simulate the NSR reaction itself. TPD experiments reveal that Ba(NO3)2 is catalytically decomposed by the presence of Pt, at a significantly lower temperature than otherwise (by >200 K), with the extent of the decomposition being dependent on the amount of Pt loaded. The products formed from the decomposition depend on the oxidation state of the Pt; firstly N2 and N2O are evolved (formed from cracking of the NO produced via the Ba(NO3)2 decomposition), but as the adsorbed oxygen resulting from their formation builds up, NO and O2 appear and N2 and N2O are lowered. NOx storage has been shown to occur even at room temperature following Ba(NO3)2 decomposition from a 0.5 wt.% Pt/Al2O3 catalyst impregnated with 10 wt.% Ba(NO3)2. These experiments can be carried out in a reversible way. Simulations of the NSR reaction, using H2 and CO as reductants, indicate a remarkable difference between the two; CO appears to facilitate Ba(NO3)2 decomposition, but not NOx reduction, whereas H2 enables both to take place, with excellent conversion to N2. The difference between the two reductants in these experiments reflects the difference in binding between the two adsorbates and consequent Pt surface poisoning by CO.

Fourier transform IR study of NOx adsorption on a CuZSM-5 DeNOx catalyst

Abstract: The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO2, NH3, H2O). NO2 adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu2+NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N2O3 → N2O4 → NO2 → NO3. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO2, N2, O2, and are only slightly affected by H2O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NOx into highly active surface nitrates.