Effect of Ceria on the Storage and Regeneration Behavior of a Model Lean NO x Trap Catalyst

Dielectric Properties of Materials Used for Microwave-Based NOx Gas Dosimeters

Nitrogen oxides (NOx), primarily generated from combustion processes, pose significant health and environmental risks. To improve the coordination of measures against excessive NOx emissions, it is necessary to effectively monitor ambient NOx concentrations, which requires the development of precise and cost-efficient detection methods. This study focuses on developing a microwave- or radio frequency (RF)-based gas dosimeter for NOx detection and addresses the optimization of the dosimeter design by examining the dielectric properties of LTCC-based (Low-Temperature Co-fired Ceramics) sensor substrates and barium-based NOx storage materials. The measurements taken utilizing the Microwave Cavity Perturbation (MCP) method revealed that these materials exhibit more pronounced changes in dielectric losses when storing NOx at elevated temperatures. Consequently, operating such a dosimeter at high temperatures (above 300 °C) is recommended to maximize the sensor signal. To evaluate their high-temperature applicability, LTCC substrates were analyzed by measuring their dielectric losses at temperatures up to 600 °C. In terms of NOx storage materials, coating barium on high-surface-area alumina resolved issues related to limited NOx adsorption in pure barium carbonate powders. Additionally, the adsorption of both NO and NO2 was enabled by the application of a platinum catalyst. The change in dielectric losses, which provides the main signal for an RF-based gas dosimeter, only depends on the stored amount of NOx and not on the specific type of nitrogen oxide. Although the change in dielectric losses increases with the temperature, the maximum storage capacity of the material decreases significantly. In addition, at temperatures above 350 °C, NOx is mostly weakly bound, so it will desorb in the absence of NOx. Therefore, in the future development of a reliable RF-based NOx dosimeter, the trade-off between the sensor signal strength and adsorption behavior must be addressed.

Advancements in low-temperature NH3-SCR of NOx using Ba-based catalysts: a critical review of preparation, mechanisms, and challenges

Presently, selective catalytic reduction (SCR), with either carbon monoxide, urea, hydrocarbons, hydrogen, or ammonia as a reductant, has become a nitrogen oxide (NOx) removal technology (NOx conversion) of many catalytic companies and diesel engine exhaust gas. Although, there exists a serious threat of low-temperature limitations. So far, certain scientists have shown that barium-based (Ba-based) catalysts have the potential to be highly effective at SCR of NOx at low temperatures when ammonia is used as the reducing agent. The process of NOx storage and reduction which alternate SCR is known as the Lean NOx trap. Herein, we give the condensed advancements and production of the catalysts that involve BaO in low-temperature NH3-SCR of NOx, the advantages of BaO catalysts compared to the recently hot electrocatalysis, the stability of BaO catalyst materials, and the condensed advancements and production of the catalysts that involve BaO in low-temperature NH3-SCR of NOx. These catalysts are viewed in the light of their preparation method, particulate, and posture in mixed oxides. Also, the characteristic features of Ba-based catalysts are carefully considered and briefed under the following areas: preparation method and precursor, crystallinity, calcination temperature, morphology, acid sites, the specific surface area for reaction, redox property, and activation energy of catalysts. More to these are the discussions on Eley-Rideal [E-R] and Langmuir-Hinshelwood [L-H] mechanisms, the H2O/SO2 and O2 permissiveness, and the NH3-SCR reaction mechanism over Ba-based catalysts highlighting their possible effects. Finally, we proposed the prospect and the likely future research plan for the low-temperature NH3-SCR of NOx.

On the Suitability of NOx-Storage-Catalysts for Hydrogen Internal Combustion Engines and a Radio Frequency-Based NOx Loading Monitoring

Hydrogen combustion engines can contribute to CO2-free mobility. However, they produce NOx emissions, albeit only to an extremely small extent when operated very leanly. One approach to reduce these emissions even further is to use exhaust gas aftertreatment systems like NOx storage catalysts (NSC). So far, they have mainly been used in diesel or gasoline applications. This contribution shows that under conditions such as those prevailing in hydrogen engines, the NSC can achieve not only a higher storage capacity for nitrogen oxides (NOx) but also a higher conversion. To ensure permanently high conversion rates, the amount of stored NOx has to be monitored permanently to prevent NOx breakthroughs. Conventional NOx sensors may not be accurate enough due to the very low NOx emissions. The functionality of the radio frequency (RF) sensor, which enables a direct determination of the NOx loading, is demonstrated for operation under hydrogen conditions. Furthermore, the influence of rich exhaust gas on the RF signal, which is relevant for a correct NOx loading determination during regeneration, is analyzed.

Contributions of Washcoat Components in Different Configurations to the NOX and Oxygen Storage Performance of LNT Catalysts

In addition to SCR systems, lean NOX traps (LNTs) are also used for exhaust aftertreatment of lean burn internal combustion engines to sustainably reduce NOX emissions. Modern LNTs consist of different functional compounds to maximize the performance during NOX storage and regeneration. Based on the material analysis of a serial production LNT, PGM loaded BaO, Al2O3, MgAl2O4, and CeO2 were identified as the main base materials. In this paper, the NOX storage capacity (NSC) of these compounds is investigated both as single catalysts and as physical mixtures to identify possible synergistic effects. Therefore, commercially available support materials were loaded with Platinum and tested in granular form under realistic conditions. To optimize the performance by reducing the diffusion pathways for NOX molecules during storage, PGM, BaO, and Ceria were combined in a composite by the incipient wetness impregnation of alumina. As a result, the temperature dependent NSC of the commercial LNT could be reached with the Pt/Rh/Ba10Ce25/Al2O3 infiltration composite, while reducing the oxygen storage capacity by about 45%. Without the additional Rhodium coating, the low-temperature NSC was insufficient, highlighting the important contribution of this precious metal to the overall performance of LNTs.

The role of platinum on the NOx storage and desorption behavior of ceria: an online FT-IR study combined with in situ Raman and UV-vis spectroscopy

The role of platinum on the room temperature NOx storage mechanism and the NOx desorption behavior of ceria was investigated by combining online FT-IR gas-phase analysis with in situ Raman and UV-vis spectroscopy. The type of pretreatment, leading to the presence of different platinum states (Pt0, and mixed Pt0/Pt2+), is shown to have a major effect on the NOx storage and desorption properties. Upon loading of ceria with platinum (1 wt%), NOx storage capacities decrease except for reductively pretreated Pt/CeO2, enabling new reaction pathways via activation of gas-phase oxygen. In the absence of oxygen, NO is reduced by metallic platinum leading to N2O and N2 formation. In situ Raman spectra provide mechanistic information, by monitoring changes in ceria surface and subsurface oxygen, as well as PtOx during NOx storage. In the presence of gas-phase oxygen, NOx storage is related to the consumption of (sub)surface oxygen and PtOx, and proposed to involve NO2 or [NO + O2] intermediates reacting with surface oxygen. The NOx desorption behavior is shown to be strongly related to the stored NOx species. Oxidative pretreatment of ceria resulted in the largest amount of stored nitrates, consistent with NOx being mostly desorbed at elevated temperatures, i.e., within 300-500 °C. Reductive pretreatment and/or addition of platinum significantly increased the fraction of stored nitrite, thereby shifting the main NOx desorption temperature to values <300 °C. Storage and subsequent desorption of NOx in PtOx/CeO2 was associated with PtOx reduction and reoxidation, as monitored by in situ UV-vis and Raman spectra. Through detailed analysis we were able to elucidate the influence of platinum on NOx storage/desorption and demonstrate the participation of different platinum states in room temperature NOx storage, with each platinum state opening a distinct new reaction pathway.

A Gas Sensor Based on a Single SnO Micro-Disk

In this study, individual nanofabricated SnO micro-disks, previously shown to exhibit exceptional sensitivity to NO_x, are investigated to further our understanding of gas sensing mechanisms. The SnO disks presenting different areas and thickness were isolated and electrically connected to metallic electrodes aided by a Dual Beam Microscope (SEM/FIB). While single micro-disk devices were found to exhibit short response and recovery times and low power consumption, large interconnected arrays of micro-disks exhibit much higher sensitivity and selectivity. The source of these differences is discussed based on the gas/solid interaction and transport mechanisms, which showed that thickness plays a major role during the gas sensing of single-devices. The calculated Debye length of the SnO disk in presence of NO₂ is reported for the first time.

Influence of Co or Ce addition on the NOx storage and sulfur-resistance performance of the lean-burn NOx trap catalyst Pt/K/TiO2-ZrO2

The Pt/K/TiO(2)-ZrO(2) catalysts promoted by Co or Ce were prepared by successive impregnation or mechanically mixing method. The influence of Co or Ce addition on the NOx storage and sulfur-resistance performance of the catalyst was investigated carefully. The techniques of XRD, FT-IR, in-situ DRIFTS, H(2)-TPR and XPS were employed for catalyst characterization. The Co or Ce addition can greatly improve the NOx storage capacity of Pt/K/TiO(2)-ZrO(2) due to the enhanced oxidation ability and the release of more K sites. Ce addition induces higher K/Ti atomic ratio and larger NOx storage capacity as compared with Co addition. After sulfation and regeneration, the promoted catalysts shows more or less decreased NSC than Pt/K/TiO(2)-ZrO(2) due to the formation of more sulfates, especially for the Co-promoted catalysts, which possess better oxidation ability and facilitate the formation of large sulfates. The effect of Ce addition on Pt/K/TiO(2)-ZrO(2) largely depends on the addition mode. The high oxidation ability and the high K/Ti ratio of the mechanically prepared Ce-promoted catalyst make it still possess considerable NOx storage capacity (NSC) of 142 micromol/g after sulfation and regeneration. With the decrease of sulfur content in fuels, the Co- and Ce-promoted catalysts possessing large NOx storage capacity, will be applicable to the purification of lean-burn NOx.