Mechanistic study of the low temperature activity of transition metal exchanged zeolite SCR catalysts

Understanding deNOx mechanisms in transition metal exchanged zeolites

Transition-metal-containing zeolites have wide-ranging applications in several catalytic processes including the selective catalytic reduction (SCR) of NOx species. To understand how transition metal ions (TMIs) can effect NOx reduction chemistry, both structural and mechanistic aspects at the atomic level are needed. In this review, we discuss the coordination chemistry of TMIs and their mobility within the zeolite framework, the reactivity of active sites, and the mechanisms and intermediates in the NH3-SCR reaction. We emphasise the key relationship between TMI coordination and structure and mechanism and discuss approaches to enhancing catalytic activity via structural modifications.

Study on the effect of wet scrubbing technique on emissions in a dual fuel engine operating with diesel and hydrogen

Reducing emissions from internal combustion (IC) engines is a crucial goal, encompassing nitrogen oxide (NO), hydrocarbon (HC), carbon monoxide (CO), and smoke. To enhance both performance and emissions, contemporary IC engines have turned to alternative gases such as hydrogen (H2) and exhaust after-treatment systems. A promising method to effectively decrease exhaust emissions entails the application of the scrubber technique as an exhaust gas after-treatment. This study's objective is to explore two avenues for curtailing exhaust emissions. The first involves substituting traditional fuels in IC engines with hydrogen gas (H2) at a flow rate of 6 LPM. The second entails integrating a liquid chemical solution into the scrubber technique. Notably, the utilization of KMnO4 solutions exhibits an appreciable reduction in NO and CO emissions compared to solutions containing NaOH. The experimental process included two aspects: investigating hydrogen fuel (H2) as an alternative fuel for IC engines and incorporating a scrubber technique using both KMnO4 and NaOH solutions. These experiments were conducted using a single-cylinder engine with a power output of 5.2 kW, cooled by water. The engine underwent tests under various load conditions, spanning from minimal to maximal loads. The findings revealed that employing KMnO4 solutions within the scrubber technique led to reductions of 25% and 40% in NO and CO emissions, respectively, in contrast to the utilization of NaOH solutions. Similarly, introduction hydrogen gas also has a significant effect on emission reduction.

Study of 4A and 5A zeolite as a catalyst material in a catalytic converter for NO emission reduction in a CI engine

The major contribution to atmospheric air pollution is from the heavy vehicular emission. At present, it is rising at an alarming rate. These automotive pollutions can be reduced to a great extent by the exhaust gas after-treatment methods. Among these, catalytic converter (CC) is the major source to reduce regulatory emission in the internal combustion (IC) engine. Most catalytic materials work in some specific temperature ranges, and they are also costly. In this study, the zeolite 4A (ZSM 4A) and zeolite 5A (ZSM 5A) powder were converted into a solid mold and tested as a catalytic material in the converter. The experimental readings were taken with the fabricated CC at the exhaust with various loads (0, 4, 8, 12, and 16 kg) in the single-cylinder Kirloskar 5.2 kW diesel engine. Waste plastics were pyrolyzed into oil and blended with diesel in the 50:50 ratio of diesel plastic blend (DPB) for this study. Nitrogen oxide (NO) and hydrocarbon (HC) were reduced by 18% and 22% respectively for ZSM 5A and 12% and 16% respectively for ZSM 4A.

In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR

The influence of the zeolite framework structure on the formation of N2O during ammonia-SCR of NOx was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA).

IR-Spectroscopic Study of Complex Formation of Nitrogen Oxides (NO, N2O) with Cationic Forms of Zeolites and the Reactivity of Adsorbed Species in CO and CH4 Oxidation

The formation of complexes and disproportionation of nitrogen oxides (NO, N2O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N2O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν10-2 and the combination bands of the stretching vibrations with other vibrational modes for N2O complexes with cationic sites in zeolites (ν30-1 + ν10-1, ν10-1 + δ0-2) are more significantly influenced by the nature of the zeolite. The presence of several IR bands in the region of 2400-2600 cm-1 (the ν10-1 + δ0-2 transitions) for different zeolite types was explained by the availability of different localization sites for cations in these zeolites. The frequencies in this region also depend on the nature of the cation (its charge and radius). The data can be explained by the specific geometry of the N2O complex formed, presumably two-point adsorption of N2O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH4 on the samples with preliminarily adsorbed N2O at 20-180 °C does not result in any oxidation of these molecules. NO+ and N2O3 species formed by disproportionation of NO are capable of oxidizing CO and CH4 molecules to CO2, whereas NOx is reduced simultaneously to N2 or N2O. The peculiarities in the behavior of cationic forms of different zeolites with respect to adsorbed nitrogen oxides determined by different density and localization of cations have been established.

MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) for Low-Temperature NH3-SCR and In Situ DRIFTS Study Reaction Mechanism

Monometallic and bimetallic MOF-74-M (M = Mn, Co, Ni, Zn, MnCo, MnNi, and MnZn) catalysts were prepared by the solvothermal method for NH₃-SCR. XRD, BET, SEM, and EDS-mapping tests indicate the successful synthesis of the MOF-74-M catalyst with uniform distribution of metal elements and large specific surface area, and the morphology is almost hexagonal. Adding Mn element to a single-metal catalyst can enhance activity, which is mainly because of the existence of various valence states of Mn so that it has excellent redox properties; the catalytic activity of water and sulfur resistance tests showed that the catalytic activity of MOF-74-M increases after adding a proper amount of SO₂, mainly because of the increase in acidic sites. In situ DRIFTS results indicate that the low-temperature range of MOF-74-MnCo and MOF-74-Mn is dominated by the E-R mechanism and the high-temperature range is dominated by the L-H mechanism. The entire temperature range of MOF-74-Zn is dominated by the L-H mechanism.

Thermodynamics of Water-Cationic Species-Framework Guest-Host Interactions within Transition Metal Ion-Exchanged Mordenite Relevant to Selective Anaerobic Oxidation of Methane to Methanol

Low-temperature anaerobic methane conversion to methanol (MTM) using copper ion-exchanged mordenite (Cu-MOR) as the catalyst and water as the sole source of oxygen is promising for sustainable utilization of methane. Integrating in situ calorimetric, spectroscopic, and structural methodologies, we report a systematic study on energetics of water-cationic species-framework guest-host interactions as a function of water loading for several mordenites relevant to low-temperature MTM. Notably, the near-zero coverage hydration enthalpy on Cu-MOR is -133.1 ± 6.0 kJ/mol water, which is related to Cu-MOR regeneration using water as oxidant. The copper oxo sites are thermally stable up to 915 °C and remain chemically intact as an oxygen source after complete hydration and dehydration. This study underscores the importance of manipulating the oxidation state and coordination chemistry of transition metal guest species in zeolites by fine-tuning the partial pressure of water as a strategy for rational design, synthesis, and modification of catalysts.

DeNOx Abatement over Sonically Prepared Iron-Substituted Y, USY and MFI Zeolite Catalysts in Lean Exhaust Gas Conditions

Iron-substituted MFI, Y and USY zeolites prepared by two preparation routes-classical ion exchange and the ultrasound modified ion-exchange method-were characterised by micro-Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet (UV)/visible diffuse reflectance spectroscopy (UV/Vis DRS). Ultrasound irradiation, a new technique for the preparation of the metal salt suspension before incorporation to the zeolite structure, was employed. An experimental study of selective catalytic reduction (SCR) of NO with NH₃ on both iron-substituted reference zeolite catalysts and those prepared through the application of ultrasound conducted during an ion-exchange process is presented. The prepared zeolite catalysts show high activity and selectivity in SCR deNO_x abatement. The MFI-based iron catalysts, especially those prepared via the sonochemical method, revealed superior activity in the deNO_x process, with almost 100% selectivity towards N₂. The hydrothermal stability test confirmed high stability and activity of MFI-based catalysts in water-rich conditions during the deNO_x reaction at 450 °C.

Effect of the preparation method on activity of Cu-ZSM-5 nanocatalyst for the selective reduction of NO by NH3

The selective catalytic reduction of NO with ammonia (NH₃-SCR) was studied over Cu-ZSM-5 nanocatalysts which were prepared by several methods, including conventional ion-exchange (IE), conventional impregnation (IM), ultrasound-enhanced impregnation (UIM), and conventional deposition-precipitation (DP) using NaOH and homogeneous deposition-precipitation (HDP) using urea. The nanocatalysts were subsequently characterized by Fourier transform infrared spectroscopy, temperature-programmed reduction with hydrogen (H₂-TPR), ammonia temperature-programmed desorption (NH₃-TPD), X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscope, and Brunauer-Emmett-Teller. The catalytic activity of the Cu-ZSM-5 nanocatalysts for NO removal decreased in the following order: Cu-ZSM-5 (HDP) > Cu-ZSM-5 (UIM) > Cu-ZSM-5 (IM) > Cu-ZSM-5 (IE) > Cu-ZSM-5 (DP). The effect of various preparation methods for Cu-ZSM-5 on the activity of NO conversion was compared and catalytic experiments revealed a strong correlation between the acid sites' strength and easily reducible isolated Cu²⁺ with NO conversion. Catalyst (HDP) (which showed excellent deNO activity) mainly contains easy reducibility of isolated Cu²⁺ ions, more strong acid sites, and higher specific surface area when compared with other catalysts.

A comparative study of metal oxide and sulfate catalysts for selective catalytic reduction of NO with NH3

The properties and characteristics of metal oxide and sulfate catalysts with different active elements for selective catalytic reduction of NO with NH₃ were investigated. Cerium-based oxide catalyst showed the widest temperature window for NO _x removal and manganese-based oxide catalyst exhibited the best catalytic performance at low temperature. For all the catalysts, the SCR activities at low temperature were directly related with the redox abilities of catalysts. The existence of sulfate groups inhibited the redox abilities of active species for sulfate catalysts compared with the metal oxide catalysts. The catalytic activities of CeWTi-S and MnWTi-S were seriously decreased in contrast to CeWTi-N and MnWTi-N. The temperature window of CuWTi-S was shifted toward higher temperature comparing with CuWTi-N. The FeWTi-N and FeWTi-S catalysts both showed high NO _x conversion in the temperature range between 300°C and 400°C and N₂O concentrations for iron-based samples were least among the same kind of catalysts. The abundance of acid sites and weak stability of surface sulfate groups for iron- and copper-based sulfate catalysts might be the main reasons accounting for the better NO _x conversion in the medium-temperature range.

Novel proposition on mechanism aspects over Fe–Mn/ZSM-5 catalyst for NH3-SCR of NOx at low temperature: rate and direction of multifunctional electron-transfer-bridge and in situ DRIFTs analysis

Most outstanding activities achieved over 15Fe–Mn/ZSM-5 catalyst on account of synergistic effects between RMETB and DMETB.

SSZ-13 Crystallization by Particle Attachment and Deterministic Pathways to Crystal Size Control

Many synthetic and natural crystalline materials are either known or postulated to grow via nonclassical pathways involving the initial self-assembly of precursors that serve as putative growth units for crystallization. Elucidating the pathway(s) by which precursors attach to crystal surfaces and structurally rearrange (postattachment) to incorporate into the underlying crystalline lattice is an active and expanding area of research comprising many unanswered fundamental questions. Here, we examine the crystallization of SSZ-13, which is an aluminosilicate zeolite that possesses exceptional physicochemical properties for applications in separations and catalysis (e.g., methanol upgrading to chemicals and the environmental remediation of NO(x)). We show that SSZ-13 grows by two concerted mechanisms: nonclassical growth involving the attachment of amorphous aluminosilicate particles to crystal surfaces and classical layer-by-layer growth via the incorporation of molecules to advancing steps on the crystal surface. A facile, commercially viable method of tailoring SSZ-13 crystal size and morphology is introduced wherein growth modifiers are used to mediate precursor aggregation and attachment to crystal surfaces. We demonstrate that small quantities of polymers can be used to tune crystal size over 3 orders of magnitude (0.1-20 μm), alter crystal shape, and introduce mesoporosity. Given the ubiquitous presence of amorphous precursors in a wide variety of microporous crystals, insight of the SSZ-13 growth mechanism may prove to be broadly applicable to other materials. Moreover, the ability to selectively tailor the physical properties of SSZ-13 crystals through molecular design offers new routes to optimize their performance in a wide range of commercial applications.