Catalytic behaviour of hybrid LNT/SCR systems: Reactivity and in situ FTIR study

Continuous Unsteady-State De-NOx System via Tandem Water-Gas Shift, NH3 Synthesis, and NH3-SCR under Periodic Lean/Rich Conditions

The development of urea-free and platinum group metal (PGM)-free catalytic systems for automotive emission control is a challenging task. Herein, we report a new de-NOx system using cyclic feeds of rich and lean gas mixtures with PGM-free catalysts. Initial catalyst screening tests showed that Cu/CeO2 with 5 wt % Cu loading was the most suitable for the water-gas shift reaction (WGS, CO + H2O → CO2 + H2), followed by the selective NH3 synthesis by the NO + H2 reaction. The unsteady-state system under alternating feeds of rich (0.1% NO + 0.5% CO + 1% H2O) and lean (0.1% NO + 2% O2 + 1% H2O) gas mixtures over a mixture of Cu/CeO2 and Cr-exchanged mordenite (CrMOR) showed higher NOx conversion than the steady-state (0.1% NO + 0.35% CO + 0.6% O2 + 1% H2O) reaction between 200 and 500 °C. The de-NOx mechanism under periodical rich/lean conditions was studied by operando infrared (IR) experiments. In the rich period, the WGS reaction on the Cu/CeO2 catalyst yield H2, which reduces NO to NH3 on the Cu/CeO2 catalyst. NH3 is then captured by the Brønsted acid sites of CrMOR. In the subsequent lean period, the adsorbed NH3 acts as a reductant for the selective catalytic reduction of NOx catalyzed by the Cr sites of CrMOR. This study demonstrates a new urea-free and PGM-free catalytic system that can provide an alternative de-NOx technology for automotive catalysis under periodic rich/lean conditions.

Operando QEXAFS Study of Pt–Fe Ammonia Slip Catalysts During Realistic Driving Cycles

Bifunctional Fe–Pt ammonia slip catalysts were studied by operando quick-scanning extended X-ray absorption fine structure spectroscopy (QEXAFS) under conditions mimicking rapid temperature variations that occur in an automotive exhaust gas aftertreatment system during real driving. Two catalysts, Pt/Al2O3 and Fe-ZSM-5, were tested individually, as mixtures and in dual bed arrangements. Applying QEXAFS allowed to track changes of active metal state with high time resolution. It uncovered a strong dependence of the active metal state on reaction conditions and catalyst bed layout. For example, proximity to platinum stabilized iron species in their more active oxidized state and led to higher Fe-ZSM-5 activity. On the contrary, isolated iron species were more susceptible to overreduction by ammonia which led to deactivation and low selectivity. The use of transient conditions uncovered the influence of non-equilibrium phenomena on catalytic performance under industrially relevant conditions. Specifically, the effect of ammonia storage on the increase of activity was shown. This was also accompanied by elevated N2O production not observed during tests with gradual heating. Additionally, unusually high NOx selectivity was detected for Fe-ZSM-5 under these conditions. Lastly, tracking catalyst state under dynamic reaction conditions disclosed that Fe-ZSM-5 activity did not grow directly with temperature increase but rather depended on the oxidation state of Fe and surface concentration of ammonia.

Optimizing the distribution and proportion of various active sites for better NH3-SCR property over Cu/SSZ-13

The Cu/SSZ-13 catalyst with Si/Al ratio of 10 and Cu/Al ratio in the range of 0.005 to 0.5 was synthesized by ion exchange method. The effect of Cu/Al ratio on the performance, selectivity, and active center of the catalyst SCR was studied. The samples with too low Cu/Al ratio have low catalytic activity in the entire temperature range due to the small number of active sites. The samples with too high Cu/Al ratio have good performance in the low temperature range due to the large number of active sites. However, a large amount of copper that has not undergone ion exchange will generate CuO, which greatly enhances the oxidation of NH₃ at high temperatures, which in turn leads to a decrease in catalytic performance at high temperatures. XRD and BET experiments show that all catalyst samples have good CHA crystal structure and microporous structure, and the pore size is mostly about 2 nm. In situ DRIFTS analysis shows that when the Cu/Al ratio is low, the proportion of Z₂Cu in the active sites is higher, and the stability of the catalyst is enhanced. When the copper-aluminum ratio is high, the proportion of ZCuOH increases, and the low-temperature activity of the catalyst is higher.

Tungsten-Based Catalysts for Environmental Applications

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).

Aftertreatment Technologies for Diesel Engines: An Overview of the Combined Systems

The abatement of the pollutants deriving from diesel engines in the vehicle sector still represents an interesting scientific and technological challenge due to increasingly limiting regulations. Meeting the stringent limits of NOx and soot emissions requires a catalytic system with great complexity, size of units, and number of units, as well as increased fuel consumption. Thus, an after-treatment device for a diesel vehicle requires the use of an integrated catalyst technology for a reduction in the individual emissions of exhaust gas. The representative technologies devoted to the reduction of NOx under lean-burn operation conditions are selective catalytic reduction (SCR) and the lean NOx trap (LNT), while soot removal is mainly performed by filters (DPF). These devices are normally used in sequence, or a combination of them has been proposed to overcome the drawbacks of the individual devices. This review summarizes the current state of NOx and soot abatement strategies. The main focus of this review is on combined technologies for NOx removal (i.e., LNT–SCR) and for the simultaneous removal of NOx and soot, like SCR-on-Filter (SCRoF), in series LNT/DPF and SCR/DPF, and LNT/DPF and SCR/DPF hybrid systems.

An Overview on the Catalytic Materials Proposed for the Simultaneous Removal of NOx and Soot

Vehicular pollution has become a major problem in urban areas due to the exponential increase in the number of automobiles. Typical exhaust emissions, which include nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO), soot, and particulate matter (PM), doubtless have important negative effects on the environment and human health, including cardiovascular effects such as cardiac arrhythmias and heart attacks, and respiratory effects such as asthma attacks and bronchitis. The mitigation measures comprise either the use of clean alternative fuels or the use of innovative technologies. Several existing emission control technologies have proven effective at controlling emissions individually, such as selective catalytic reduction (SCR) and lean NOx trap (LNT) to reduce NOx and diesel particulate filter (DPF) specifically for PM abatement. These after-treatment devices are the most profitable means to reduce exhaust emissions to acceptable limits (EURO VI norms) with very little or no impact on the engine performances. Additionally, the relative lack of physical space in which to install emissions-control equipment is a key challenge for cars, especially those of small size. For this reason, to reduce both volume and cost of the after-treatment devices integrated catalytic systems (e.g., a sort of a "single brick") have been proposed, reducing both NOx and PM simultaneously. This review will summarize the currently reported materials for the simultaneous removal of NOx and soot, with particular attention to their nature, properties, and performances.

Investigation of the common intermediates over Fe-ZSM-5 in NH3-SCR reaction at low temperature by in situ DRIFTS

The surface species formed in the reaction of NO and NO₂ with pre-adsorbed NH₃ over a Fe-ZSM-5 catalyst (1.27 wt.% Fe, SiO₂/Al₂O₃ = 25) at low temperature (140°C) were studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Through using a background spectrum of NH₃-saturated Fe-ZSM-5, we clearly observed the formation of common intermediates resulting from the reaction of NO₂ or NO + O₂ with pre-adsorbed NH₃. This presents strong evidence that the oxidation of NO to form surface nitrates and nitrites is the key step for standard SCR at low temperature. In addition, the results suggest that in the SCR reaction at low temperature, the NH₄⁺ ions absorbed on Brønsted acid sites are less active than NH₃ adsorbed on Lewis acid sites related to Fe species.

Hybrid Technology for DeNOxing by LNT-SCR System for Efficient Diesel Emission Control: Influence of Operation Parameters in H2O + CO2 Atmosphere

The behavior and operation parameters were analyzed for the hybrid LNT-SCR (Lean NOx-Trap–Selective Catalytic Reduction) system with advanced catalyst formulations. Pt-Ba-K/Al2O3 was used as an NSR (NOx Storage and Reduction) or LNT catalyst effective in NOx and soot simultaneous removal whereas Cu-SAPO-34 with 2 wt.% of copper inside the structure was the small pore zeolite employed as the SCR catalyst. Under alternating and cyclic wet conditions, feeding volumetric concentrations of 1000 ppm of NO, 3% of O2, 1.5% of water, 0.3% of CO2, and H2 as a reductant, the NOx-conversion values were above 95% and a complete mineralization to nitrogen was registered using θ ≤ 3 (20 s of regeneration) and a hydrogen content between 10,000 and 2000 ppm in the whole temperature range tested. An excess of hydrogen fed (above 1% v/v) during the rich phase is unnecessary. In addition, in the low temperature range below 250 °C, the effect is more noticeable due to the further ammonia production and its possible slip. These results open the way to the scale up of the coupled catalytic technologies for its use in real conditions while controlling the influence of the operation map.

Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification

Ceria/spinel-based lean NO_x trap compositions with and without barium were modified with MnO_x via incipient wetness impregnation. The effect of the MnO_x layer on the aged materials (850 °C) as to the NO_x storage and release properties was investigated via NO_x adsorption (500 ppm NO/5% O₂/balance N₂) carried out at 300 °C in a dual-bed with a 1% Pt/Al₂O₃ catalyst placed upstream of the samples to generate sufficient amounts of NO₂ required for efficient NO_x storage. Subsequent temperature programmed desorption (TPD) experiments were carried out under N₂ from 300 °C to 700 °C. The addition of MnO_x to the barium free composition led to a slightly reduced NO_x storage capacity but all of the ad-NO_x species were released from this material at significantly lower temperatures (ΔT ≈ 100 °C). The formation of a MnO_x layer between ceria/spinel and barium had a remarkable effect on ageing stability as the formation of BaAl₂O₄ was suppressed in favour of BaMnO₃. The presence of this phase resulted in an increased NO_x storage capacity and lower desorption temperatures. Furthermore, NO_x adsorption experiments carried out in absence of the Pt-catalyst also revealed an unexpected high NO_x storage ability at low NO₂/NO ratios, which could make this composition suitable for various lean NO_x trap catalysts (LNT) related applications.

Influence of the nature and environment of manganese in Mn-BEA zeolites on NO conversion in selective catalytic reduction with ammonia

Manganese-containing BEA zeolites, Mn_xSiBEA (x = 1-4 wt%) and Mn_(I.E.)AlBEA, were prepared by a two-step post-synthesis method and a conventional wet ion-exchange, respectively, and applied as catalysts in the selective catalytic reduction of NO with ammonia (NH₃-SCR). The physicochemical analysis of zeolite properties by high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and X-ray emission spectroscopy (XES) uncovered that the coordination, geometry and oxidation state of Mn species are strongly related to the preparation method. Additionally, the study of catalyst acidity by FTIR spectroscopy with CO and pyridine probe molecules provided important insight into the number and type of acidic centres present on the catalyst surface. The catalytic results revealed that NO conversion depended on the state and content of Mn. The zeolites obtained by the two-step post-synthesis method and with a low Mn content were very active in the medium temperature range (NO conversion ∼100%) with simultaneous high selectivity to N₂ due to the presence of isolated, framework Mn(iii) and Mn(ii) species. The N₂O formation was especially high in the case of catalysts containing extra-framework polynuclear Mn species and negligible in the case of Mn_(I.E.)AlBEA containing predominantly isolated, extra-framework Mn(ii) species.

Dual resistance to alkali metals and SO2: vanadium and cerium supported on sulfated zirconia as an efficient catalyst for NH3-SCR

Vanadium and cerium supported on sulfated zirconia is an efficient SCR catalyst with dual resistance toward both potassium and SO₂ poisoning.