Catalytic Oxidation Of Ammonia Research Articles

Regulating the valence and size of the active center of Co/Al2O3 catalyst improved the performance and selectivity of NH3-SCO

To improve the activity of Co/Al2O3 catalysts in selective catalytic oxidation of ammonia (NH3-SCO), valence state and size of active centers of Al2O3-supported Co catalysts were adjusted by conducting H2 reduction pretreatment. The NH3-SCO activity of the adjusted 2Co/Al2O3 catalyst was substantially improved, outperforming other catalysts with higher Co-loading. Fresh Co/Al2O3 catalysts exhibited multitemperature reduction processes, enabling the control of the valence state of the Co-active centers by adjusting the reduction temperature. Changes in the state of the Co-active centers also led to differences in redox capacity of the catalysts, resulting in different reaction mechanisms for NH3-SCO. However, in situ diffuse reflectance infrared Fourier transform spectra revealed that an excessive O2 activation capacity caused overoxidation of NH3 to NO and NO2. The NH3-SCO activity of the 2Co/Al2O3 catalyst with low redox capacity was successfully increased while controlling and optimizing the N2 selectivity by modulating the active centers via H2 pretreatment, which is a universal method used for enhancing the redox properties of catalysts. Thus, this method has great potential for application in the design of inexpensive and highly active catalysts.

Engineering a PtCu Alloy to Improve N2 Selectivity of NH3-SCO over the Pt/SSZ-13 Catalyst.

Improving the N2 selectivity is always a great challenge for the selective catalytic oxidation of ammonia (NH3-SCO) over noble-metal-based (especially Pt) catalysts. In this work, Cu as an efficient promoter was introduced into the Pt/SSZ-13 catalyst to significantly improve the N2 selectivity of the NH3-SCO reaction. A PtCu alloy was formed in the PtCu/SSZ-13 catalyst, as confirmed by X-ray diffraction, transmission electron microscopy, energy dispersive spectrometry mapping, and X-ray absorption spectroscopy results. As indicated by the X-ray photoelectron spectroscopy analysis, the Pt species in the alloyed PtCu nanoparticle was mainly present in the electron-rich state on PtCu/SSZ-13, while the electron-deficient Cu and isolated Cu2+ species were both present on the surface of PtCu/SSZ-13. Due to such a unique alloyed structure with an altered oxidation state, the N2 selectivity of NH3-SCO on the PtCu/SSZ-13 catalyst was remarkably improved, while the NH3-SCO activity was kept comparable to that on Pt/SSZ-13. The reaction path was changed from the NH mechanism on Pt/SSZ-13 to both NH and internal selective catalytic reduction mechanisms on the PtCu/SSZ-13 catalyst, which was considered the main reason for the enhanced N2 selectivity. This work provides a new route to synthesize efficient alloy catalysts for optimizing the N2 selectivity of NH3-SCO for NH3 slip control in diesel exhaust purification.

Electrochemical ammonia oxidation catalyzed by ruthenium complexes: investigation of substituent effect of axial pyridine ligands

Abstract We have examined catalytic ammonia oxidation using ruthenium complexes bearing 2,2′-bipyridine-6,6′-dicarboxylate ligand and axial 3- and 4-substituted pyridine ligands under electrochemical conditions to study the substituent effect of the axial pyridine ligands. Ruthenium complex bearing 3,4-dibromopyridine ligands shows the highest catalytic activity to produce up to 145 equivalents of dinitrogen based on the catalyst.

Efficient Cu-Co Dual-Site promotes selective catalytic oxidation of ammonia over cobalt based catalysts

Selective catalytic oxidation of NH3 (NH3-SCO) into N2 and H2O is an efficient method to eliminate excessive NH3 emission from stock farming, agriculture, industrial sectors and NH3 slip in coal-fired power plants and diesel vehicles. However, it is a great challenge to develop the Co3O4 catalyst with high activity and high N2 selectivity at low temperature. Herein, a series of Cu doped Co3O4 catalysts is designed and synthesized, with discovering the role of Co-O bond strength by copper regulation in modulating oxygen vacancies and lattice oxygen to enhance the intrinsic activity and N2 selectivity of NH3-SCO. The Cu1Co2O4 catalyst (93 % NH3 conversion and 80 % N2 selectivity at 160 °C) achieves a higher catalytic performance compared with Co3O4 (78 % NH3 conversion and 50 % N2 selectivity at 160 °C). In addition, the N2 generation rate by normalizing over specific surface area for Cu1Co2O4 (13.1 μmol m-2h−1) is 1.7 times to that of Co3O4 (8.0 μmol m-2h−1), and the apparent activation energy of Cu1Co2O4 (36.7 kJ mol−1) is lower compared to that of Co3O4 (59.8 kJ mol−1). Reactive oxygen species and enhanced electron transfer promote the oxidation of NH3 to nitrate species, which can be further converted to N2 rather than N2O by remaining NH3 on the Cu site. The Cu-Co dual sites significantly contribute to the catalytic activity and N2 selectivity. This doping strategy is beneficial for the development of Co-O bond strength modulation in spinel catalyst and also provides a new idea for improving the NH3-SCO activity and N2 selectivity of Co-based spinel.

Recent Progress in the Application of Transition‐Metal Containing MFI topologies for NH3‐SCR‐DeNOx and NH3 oxidation

AbstractTransition metal‐containing MFI‐based catalysts are widely investigated in the selective catalytic reduction of NOx with ammonia (NH3‐SCR‐DeNOx), and the selective catalytic oxidation of ammonia (NH3‐SCO) into nitrogen and water vapor. While MFI‐based catalysts are less intensively studied than smaller pore zeolites (i. e., chabazite, CHA) they are still used commercially for these processes and are of great interest for future study in particular to better understand structure‐activity relationships. Hierarchically porous MFI catalysts (containing both micropores and mesopores) often show enhanced catalytic properties compared to conventional (microporous) materials in both NH3‐SCR‐DeNOx and NH3‐SCO. Thus, a critical overview of the current understanding of the salient physico‐chemical properties that influence the performance of these catalysts is examined. Furthermore, strategies for the development of ZSM‐5 based catalysts with enhanced catalytic lifetime, supported by the investigations of reaction mechanisms are reviewed and discussed.

Insight into hydroxyl groups in anchoring Ir single–atoms on vacancy–deficient rutile TiO2 supports for selective catalytic oxidation of ammonia

High–performance catalysts are extremely required for controlling NH3 emission via selective catalytic oxidation (SCO), and the anchoring structural feature of active sites is a key prerequisite for developing them. This study confirms the importance of hydroxyl groups on vacancy–deficient reducible oxides as active groups. On the one hand, spontaneous atomic dispersion of active metal Ir is promoted by the abundant terminal hydroxyl groups. On the other hand, Ir cations anchor on the TiO2 surface through exchange with H+ in Ti–OH groups, and thus occupy the Brönsted acid sites. The adsorption strength of NH3 is another key factor affecting the reaction rate–determining step, namely NH3 dehydrogenation, which occurs at a faster rate in the coordinated L–NH3 rather than the ionic B–NH4+. Meanwhile, the coordinated L–NH3 significantly avoids the competitive adsorption of water vapor in the NH3–SCO reaction by reducing the number of hydrogen bonding. The TOF of preferred 0.8Ir/TiO2 sample is significantly higher than 0.2Ir/TiO2 sample, although Ir is almost always atomic dispersed. Finally, NH3 conversion is 85% in a wet circumstance (5% H2O) at 240 °C (GHSV = 85 000 h–1), with a N2 selectivity of up to 65% on 0.8Ir/TiO2 sample.

Zeolite-like ion-exchanged Cu-attapulgite catalysts for promoted selective oxidation of ammonia

Zeolite-like ion-exchanged Cu-attapulgite catalysts have been developed for selective catalytic oxidation of ammonia.

Selective catalytic oxidation of ammonia to nitrogen on mixed and dual-layer monolithic catalysts

In this paper, the effects of mixed and dual-layer monolithic catalysts on NH3 conversion efficiency and by-product formation performance were studied under different conditions, in order to find a coating structure with both high activity and high N2 selectivity. The results show that in the absence of NO, the dual-layer structure of Cu-SSZ-13 as the top layer and Ag/Al2O3 as the bottom layer reduces the NH3 conversion efficiency, and the NH3 conversion efficiency further decreases with the increase of the catalyst loading on the top layer, and this trend is further strengthened at higher space velocity. However, the dual-layer catalyst significantly reduces the formation of NOx byproducts. At 600 ℃, NO decreases from 305 ppm in single-layer to 51, 40 and 35 ppm respectively, NO2 and N2O are hardly produced on the dual-layer structure catalyst. In the presence of NO, the dual-layer catalyst is conducive to improving the conversion efficiency of NH3 and NO, and the conversion efficiency of NH3 and NO will be further improved with the increase of Cu-SSZ-13 catalyst loading. In addition, the results of catalytic oxidation performance of mixed structure and dual-layer catalyst under different air conditions show that the mixed structure catalyst has a higher NH3 conversion efficiency than the dual-layer catalyst in the absence of NO, but the by-product generation amount of the mixed structure catalyst is slightly higher than that of the dual-layer catalyst. In the presence of NO, the dual-layer catalyst showed a higher NO conversion efficiency than the mixed catalyst.

Selective Catalytic Oxidation of Ammonia over Cr-Ce Mixed Oxide Catalysts in the Presence of Sulfur Dioxide

Selective Catalytic Oxidation of Ammonia over Cr-Ce Mixed Oxide Catalysts in the Presence of Sulfur Dioxide

Catalytic oxidation of ammonia: A pre-occupied-anchoring-site strategy for enlarging Ag nanoparticles at low Ag loading and achieving enhanced activity and selectivity on Ag-CuOx/Al2O3 catalyst

The Ag nanoparticles (AgNPs) in Ag/Al2O3 catalysts play a crucial role in the selective catalytic oxidation of NH3 (NH3-SCO). To enhance NH3-SCO activity, Cu, which has stronger anchoring ability than Ag, is introduced onto Al2O3, reducing available anchoring sites for Ag. As Ag cannot displace anchored Cu species, Ag species agglomerate into larger AgNPs even with low Ag loading. Consequently, these enlarged AgNPs become more active centers for NH3-SCO. The optimal Ag:Cu molar ratio is confirmed as 2:3. This 'pre-occupied-anchoring-site’ strategy decreases Ag loading to 1/5 of the original, reducing catalyst costs while maintaining activity. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) studies reveal that NH3-SCO on 2Ag1.8Cu/Al (weight ratio) catalyst follows the hydrazine mechanism below 200 °C, coexisting with the imide mechanism from 200–250 °C, and solely the imide mechanism beyond 250 °C. This strategy is applicable to various transition metals, including Mn, Co, Ni, and Fe, promoting cost-effective AgNPs formation.

Regulating Electron Metal-Support Interaction to Suppress N2O Formation in the Selective Catalytic Oxidation of Ammonia.

N2O is a common byproduct in the selective catalytic oxidation of ammonia, and its generation often needs to be inhibited due to its strong greenhouse effect. In this paper, using Ag/ZSO-Y as a model catalyst, the N2O selectivity was reduced by 30% through modulation of the electron metal-support interaction. The results demonstrate that the work function of the support can be regulated by the content of the doping element. As the Zr content increases in SnO2, the work function of the support decreases. Moreover, there is a positive correlation between the charge transfer amount and the work function of the support. A series of in situ DRIFTS and density functional theory calculations revealed that the -NO and -N reactions are the primary pathways for N2O formation. By adjustment of the work function of the support through varying the Zr doping level, the electronic structure of Ag NPs was further tuned, resulting in an increased reaction energy barrier for -NO and -N reactions, effectively suppressing N2O formation.

Review of the Catalytic System, Synthetic Process, and Support Morphological Research in Selective Catalytic Oxidation of Ammonia

Review of the Catalytic System, Synthetic Process, and Support Morphological Research in Selective Catalytic Oxidation of Ammonia

Tuning the Interaction between Platinum Single Atoms and Ceria by Zirconia Doping for Efficient Catalytic Ammonia Oxidation.

Aiming at the development of an efficient NH3 oxidation catalyst to eliminate the harmful NH3 slip from the stationary flue gas denitrification system and diesel exhaust aftertreatment system, a facile ZrO2 doping strategy was proposed to construct Pt1/CexZr1-xO2 catalysts with a tunable Pt-CeO2 interaction strength and Pt-O-Ce coordination environment. According to the results of systematic characterizations, Pt species supported on CexZr1-xO2 were mainly in the form of single atoms when x ≥ 0.7, and the strength of the Pt-CeO2 interaction and the coordination number of Pt-O-Ce bond (CNPt-O-Ce) on Pt1/CexZr1-xO2 showed a volcanic change as a function of the ZrO2 doping amount. It was proposed that the balance between the reasonable concentration of oxygen defects and limited surface Zr-Ox species well accounted for the strongest Pt-CeO2 interaction and the highest CNPt-O-Ce on Pt/Ce0.9Zr0.1O2. It was observed that the Pt/Ce0.9Zr0.1O2 catalyst exhibited much higher NH3 oxidation activity than other Pt/CexZr1-xO2 catalysts. The mechanism study revealed that the Pt1 species with the stronger Pt-CeO2 interaction and higher CNPt-O-Ce within Pt/Ce0.9Zr0.1O2 could better activate NH3 adsorbed on Lewis acid sites to react with O2 thus resulting in superior NH3 oxidation activity. This work provides a new approach for designing highly efficient Pt/CeO2 based catalysts for low-temperature NH3 oxidation.

Bi-functional two-dimensional cobalt silicate catalyst for selective catalytic oxidation of ammonia

We have newly synthesized the zeolitic cobalt silicate catalysts via a simple hydrothermal treatment of borosilicate MWW precursor with Co precursor solution. As the hydrothermal temperature increased, a structural transformation occurred, shifting from three-dimensional (3D) to two-dimensional (2D) delaminated MWW layers (DML), and further to 2D amorphous phyllosilicate, accompanied by a continuous increase in Co content. The synthesized bi-functional 2D cobalt silicate facilitates the selective catalytic oxidation of NH3 (NH3-SCO), a process that requires both surface acidity and redox characteristics. In particular, Co-DML-160 prepared by hydrothermal treatment at 160 °C exhibited the coexistence of Co3O4 and framework Co species resulting in a strong redox ability and high Lewis acidity, respectively, due to their synergetic effect.

Computational design and experimental validation of monometallic-doped SnO2 catalysts for selective catalytic oxidation of ammonia

Due to the intricate structure of catalysts, traditional catalyst design relies on iterative trial-and-error experiments. We have systematically established a catalyst design strategy to evaluate the performance of NH3-SCO reactions from the perspective of DFT calculations. Specifically, we used the catalyst formation energy as a stability descriptor and the adsorption energies of NH3, NH2, and O p-band center as performance descriptors, we identified the four most promising catalysts among 45 kinds of doped SnO2 catalysts. Subsequently, experimental validation was performed to demonstrate the outstanding consistency between the DFT-driven descriptors and the stability and catalytic performance of the catalyst. Particularly, the Ce doping resulted in a 175 °C reduction in the T90 compared to the SnO2 catalyst. It is noteworthy that Ce doping promotes the cycling between oxygen vacancies and lattice oxygen, which contributes primarily to the enhancement of O2 activation capability and, consequently, the improvement in catalytic activity.

Synergistic catalytic wet oxidation of ammonia over carbon-supported low-loading Ru/Cu bimetallic catalyst

The manner of ammonia-containing wastewater in industrial production has always been an urgent issue that needs to be solved. Accordingly, a carbon-supported low-loading Ru/Cu bimetallic catalyst with high activity and hydrothermal stability was produced through facile disposal for catalytic wet air oxidation (CWAO). In the preparation process, Ru and Cu ions were loaded onto micro mesoporous carbon prepared through the calcination of potassium citrate by impregnation. Various characteristic techniques were utilized to characterize their structures and morphology, and results show that the calcination temperature with micro-mesopores’ characteristics could regulate the morphology of carbon. The results of catalytic experiments demonstrated that the obtained catalyst showed excellent catalytic performance in the process of CWAO. The Ru/Cu@C-700 catalyst achieved 99.6 % conversion of ammonia water at 800 ppm and 99.1 % selectivity to nitrogen at 200 °C and 2 MPa O2 in 4 h. The ammonia nitrogen content in the treated water was less than 15 ppm, meeting the standard for direct discharge. Moreover, no apparent deactivation phenomenon was observed after five cycles, indicating that the prepared catalyst can effectively prevent the leaching of ruthenium and copper nanoparticles. The low loading (less than 1 %) of the Ru/Cu bimetallic catalyst can significantly reduce costs and can synergistically enhance conversion and selectivity, providing an effective strategy for studying the CWAO of ammonia.

Reasonable design of Sm-modified Cu-based catalyst for NH3-SCO: Role of the amide intermediates

The selective catalytic oxidation of ammonia (NH3-SCO) is currently the most effective method used to eliminate NH3. However, one of the major challenges for NH3-SCO is the development of catalyst capable of completely converting NH3 into harmless N2 and water vapor. In this paper, a high N2 selective catalyst prepared by the sol-gel method using TiO2 as the support, Cu as the active species, and Sm as the auxiliary agent is presented. Compared to traditional Cu/TiO2 catalyst, the 4SmCu/TiO2 catalyst has higher catalytic activity and N2 selectivity at low temperatures. The NH3 conversion can reach 100%, and the selectivity of N2 can be maintained at 100% at 275 °C. The excellent catalytic activity is attributed to the highly dispersed active species, abundant Lewis acid sites (LASs), and the generation of large amounts of surface-adsorbed oxygen. In addition, doping Sm species will cause TiO2 lattice distortion, and at the same time load the distorted TiO2 with more active material. Moreover, in-situ DRIFTS analysis suggests that both the 4SmCu/TiO2 and Cu/TiO2 catalysts follow the "internal" selective catalytic reduction (iSCR) mechanism during NH3-SCO reactions. The 4SmCu/TiO2 catalyst generates more amides (-NH2), which reduces the non-selective oxidation of the catalysts and promotes the formation of N2. This provides a new idea and method for the selective catalytic oxidation of NH3 to N2 and water vapor using Cu-based catalysts.

Catalytic Ammonia Oxidation Using Ammonia Solution under Electrochemical Conditions: Investigation on Axial Ligand of Ruthenium Catalysts

Abstract We have investigated catalytic ammonia oxidation using ruthenium complexes as catalysts under electrochemical conditions. Cyclic voltammetry and bulk electrolysis with an ammonia solution in MeCN are conducted in the presence of a catalytic amount of ruthenium complexes bearing a 2,2′-bipyridine-6,6′-dicarboxylate ligand with various 6-substituted isoquinolines and phthalazine as axial ligands. As a result, the ruthenium complex bearing phthalazines shows the highest catalytic activity at low applied potentials, where up to 319 equivalents of dinitrogen per catalyst are generated.

A promising catalyst for catalytic oxidation of chlorobenzene and slipped ammonia in SCR exhaust gas: Investigating the simultaneous removal mechanism

As the emission limits for NOx become further stringent, development of a synergistic control technology to prevent emissions of slipped ammonia (NH3) and chlorinated organics from steel production and waste incineration is both a prospect and a challenge for the environmental issues. In this work, the designed ruthenium-based catalyst (RuNb/ST) showed the excellent chlorobenzene (CB)/NH3 synergistic removal performance and N2/CO2 selectivity above 300 ℃. The activity of NH3 and CB were suppressed at relatively low temperature due to the competitive adsorption of CB and NH3. Whereas, there was a significant improvement in the N2/CO2 selectivity at relatively high temperature. The removal mechanism was studied by experiments, in which the accumulation of chlorine species, the role of NO2 and the oxidation of CB were considerable. In contrast to the carrier, the vast majority of the chloride ions adsorbed onto the active sites after dissociating from CB could be efficiently removed in form of Cl2 via the Deacon Reaction catalyzed by RuO2. CB was predominantly converted into CO2 and H2O with a fraction persisting as organics. Partial of organics were attacked by Cl ions to generate organic chlorine-containing by-products. The presence of NO2 molecule was found to promote the cleavage of the benzene ring within CB, and expedited the carbon migration towards the ultimate product of CO2, resulting in a notable reduction of byproducts.

Unraveling the Lattice O Assisted Internal Selective Catalytic Reduction Mechanism on High N2 Selectivity of CuOx/PtCu Catalysts in NH3-SCO

Supported Pt catalysts suffer from low reaction selectivity (<65% at 100% conversion) in the selective catalytic oxidation of ammonia (NH3-SCO) because of their intrinsic over-oxidation tendencies. Herein, a strategy based on facilitating byproduct NOx reduction to N2 is demonstrated at CuOx/PtCu catalysts, aiming to tackle the low selectivity challenge. This PtCu alloy sample with a CuOx-rich surface displays an N2 selectivity of 95%, while its turnover frequencies are the highest among reported supported Pt-based catalysts. The enhanced reaction activities and N2 selectivities are primarily attributed to the Pt-Cuδ+ dual-site synergies at the CuOx/PtCu interface, in which dual sites exhibit distinct NH3 adsorption, activation, and dehydrogenation behaviors. The Olat assisted internal selective catalytic reduction mechanism at CuOx/PtCu with N2O22– as the central intermediates is proposed. Density functional theory calculation and temperature-programmed surface reaction measurements show that the N–O bond in N2O22– tends to break simultaneously with the assistance of Ov in CuOx followed by the easy transition from N2O22– to N2, which serve as the origin of the high selectivity.