Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction

The importance of maintaining a balanced nitrogen cycle and rectifying the accumulation of nitrate in water streams creates a need for technologies that can convert nitrate species. Heterogeneous electrocatalytic nitrate...

Efficiency of Phosphotungstic Acid Modified Mn-Based Catalysts to Promote Activity and N2 Formation for Selective Catalytic Reduction of NO with Ammonia

In this work, the modified Mn-based NH3-SCR (NH3 low-temperature selective catalytic reduction) catalysts with excellent NO conversion and N2 selectivity be designed. N2 yield was hardly more than 75 % over MnOx/TiO2 for NH3-SCR reaction, whereas the NH3-SCR performance has been significantly improved by using 50 wt.% HPW (H3PW12O40)-MnOx/TiO2. 100 % NO conversion and more than 95 % N2 yield was obtained in wide operating temperature window (150–400°C), suggesting that the addition of HPW could effectively improve the NO reduction conversion. After that, the catalysts were further characterized by XRD, H2-TPR, XPS and in situ DRIFT. DRIFT analysis implied that the introduction of HPW significantly improve the capacity of NH4 + species adsorbed on Brønsted acid sites accompanied with inhibiting the formation and consumption of nitrite species. It proved that the non-selective catalytic reduction reaction over HPW-MnOx/TiO2 catalysts are restrained. HPW could accelerate the formation and consumption of NH4 + species adsorbed on Brønsted acid sites with deactivation of nitrate species. In addition, NH3(ad) could be hardly oxidized to NH species and then reacted with nitrate species (L-H mechanism) and gaseous NO (E-R mechanism). More importantly, the oxidation of NH3 was also suppressed, which plays a dominate role to form N2O above 300°C. Besides, the deactivation of potassium poisoning on the SCR activity significantly weakened for modified samples compared to parent catalyst.

The Δ¹⁷O and δ¹⁸O values of simultaneously collected atmospheric nitrates from anthropogenic sources – Implications for polluted air masses

There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NOx) emitted from anthropogenic sources into nitrates (NO3−), two of them being that NO3− contributes to acidification and eutrophication of terrestrial and aquatic ecosystems, and particulate nitrate may play a role in climate dynamics. For these reasons, oxygen isotope ratios (δ18O, Δ17O) have been applied to infer the chemical pathways leading to the observed distribution of wet (w-NO3−), particulate (p-NO3−), and the sum of p-NO3− and gaseous HNO3, while the gaseous form (HNO3) has never been separately characterized for 17O. Previous research studies have investigated w-NO3−, p-NO3− or p-NO3− + HNO3 from non-polluted or polluted air masses, and inferred seasonal changes in the dominance of oxidation pathways to account for higher δ18O and Δ17O values in winter relative to summer. However, none of the polluted air studies collected samples specific to targeted emission sources. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO3 and p-NO3−, and co-collect w-NO3−, downwind from five different anthropogenic sources. Overall, the w- and p-NO3 δ18O and Δ17O values show expected differences between cold and warm seasons, but only the Δ17O values of HNO3 follow this pattern. The HNO3 δ18O ranges are distinct from the w- and p-NO3− patterns. Interestingly, the Δ17O differences between p-NO3− and HNO3 shifts from positive during cold sampling periods to negative during warm periods. The summer pattern may be due to the presence of nitrates derived from NOx that has not yet reached isotopic equilibrium with O3 and subsequent differences in dry deposition rates, while the larger proportion of p-NO3− formed via the N2O5 pathway can explain the fall-winter pattern. Very low p-NO3− Δ17O values observed during warm months may be due to this non-equilibrated NOx, though contribution from RO2 oxidation remains a possibility. Our results show that the isotopic signals of HNO3, w-NO3− and p-NO3− are not interchangeable and that their differences can further our understanding of NOx oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NOx to refine our understanding of nitrate worldwide and to develop effective emission reduction strategies.

The effect of soot on ammonium nitrate species and NO 2 selective catalytic reduction over Cu–zeolite catalyst-coated particulate filter

A selective catalytic reduction (SCR)-coated particulate filter was evaluated by means of dynamic tests performed using NH 3 , NO 2 , O 2 and H 2 O. The reactions were examined both prior to and after soot removal in order to study the effect of soot on ammonium nitrate formation and decomposition, ammonia storage and NO 2 SCR. A slightly larger ammonia storage capacity was observed when soot was present in the sample, which indicated that small amounts of ammonia can adsorb on the soot. Feeding of NO 2 and NH 3 in the presence of O 2 and H 2 O at low temperature (150, 175 and 200°C) leads to a large formation of ammonium nitrate species and during the subsequent temperature ramp using H 2 O and argon, a production of nitrous oxides was observed. The N 2 O formation is often related to ammonium nitrate decomposition, and our results showed that the N 2 O formation was clearly decreased by the presence of soot. We therefore propose that in the presence of soot, there are fewer ammonium nitrate species on the surface due to the interactions with the soot. Indeed, we do observe CO 2 production during the reaction conditions also at 150°C, which shows that there is a reaction with these species and soot. In addition, the conversion of NO x due to NO 2 SCR was significantly enhanced in the presence of soot; we attribute this to the smaller amount of ammonium nitrate species present in the experiments where soot is available since it is well known that ammonium nitrate formation is a major problem at low temperature due to the blocking of the catalytic sites. Further, a scanning electron microscopy analysis of the soot particles shows that they are about 30–40 nm and are therefore too large to enter the pores of the zeolites. There are likely Cu x O y or other copper species available on the outside of the zeolite crystallites, which could have been enhanced due to the hydrothermal treatment at 850°C of the SCR-coated filter prior to the soot loading. We therefore propose that soot is interacting with the ammonium nitrate species on the Cu x O y or other copper species on the surface of the zeolite particles, which reduces the ammonium nitrate blocking of the catalyst and thereby results in higher NO 2 SCR activity.