Quenching-Induced Defect-Rich Platinum/Metal Oxide Catalysts Promote Catalytic Oxidation

Enhancing oxygen activation through defect engineering is an effective strategy for boosting catalytic oxidation performance. Herein, we demonstrate that quenching is an effective strategy for preparing defect-rich Pt/metal oxide catalysts with superior catalytic oxidation activity. As a proof of concept, quenching of α-Fe₂O₃ in aqueous Pt(NO₃)₂ solution yielded a catalyst containing Pt single atoms and clusters over defect-rich α-Fe₂O₃ (Pt/Fe₂O₃-Q), which possessed state-of-the-art activity for toluene oxidation. Structural and spectroscopic analyses established that the quenching process created abundant lattice defects and lattice dislocations in the α-Fe₂O₃ support, and stronger electronic interactions between Pt species and Fe₂O₃ promote the generation of higher oxidation Pt species to modulate the adsorption/desorption behavior of reactants. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) characterization studies and density functional theory (DFT) calculations determined that molecular oxygen and Fe₂O₃ lattice oxygen were both activated on the Pt/Fe₂O₃-Q catalyst. Pt/CoMn₂O₄, Pt/MnO₂, and Pt/LaFeO₃ catalysts synthesized by the quenching method also offered superior catalytic activity for toluene oxidation. Results encourage the wider use of quenching for the preparation of highly active oxidation catalysts.

Promotional catalytic activity and reaction mechanism of Ag-modified Ce0.6Zr0.4O2 catalyst for catalytic oxidation of ammonia

Ce_1-xZr_xO₂ composite oxides (molar, x = 0-1.0, interval of 0.2) were prepared by a cetyltrimethylammonium bromide-assisted precipitation method. The enhancement of silver-species modification and catalytic mechanism of adsorption-transformation-desorption process were investigated over the Ag-impregnated catalysts for low-temperature selective catalytic oxidation of ammonia (NH₃-SCO). The optimal 5 wt.% Ag/Ce_0.6Zr_0.4O₂ catalyst presented good NH₃-SCO performance with >90% NH₃ conversion at temperature (T) ≥ 250°C and 89% N₂ selectivity. Despite the irregular block shape and underdeveloped specific surface area (∼60 m²/g), the naked and Ag-modified Ce_0.6Zr_0.4O₂ solid solution still obtained highly dispersed distribution of surface elements analyzed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) (mapping), N₂ adsorption-desorption test and X-ray diffraction (XRD). H₂ temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that Ag-modification enhanced the mobility and activation of oxygen-species leading to a promotion on CeO₂ reducibility and synergistic Ag⁰/Ag⁺ and Ce⁴⁺/Ce³⁺ redox cycles. Besides, Ag⁺/Ag₂O clusters could facilitate the formation of surface oxygen vacancies that was beneficial to the adsorption and activation of ammonia. NH₃-temperature programmed desorption (NH₃-TPD) showed more adsorption-desorption capacity to ammonia were provided by physical, weak- and medium-strong acid sites. Diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments revealed the activation of ammonia might be the control step of NH₃-SCO procedure, during which NH₃ dehydrogenation derived from NH_x-species and also internal selective catalytic reduction (i-SCR) reactions were proposed.

A Comparative Mini-Review on Transition Metal Oxides Applied for the Selective Catalytic Ammonia Oxidation (NH3-SCO)

The selective catalytic oxidation of NH₃ (NH₃-SCO) into N₂ and H₂O is an efficient technology for NH₃ abatement in diesel vehicles. However, the catalysts dedicated to NH₃-SCO are still under development. One of the groups of such catalysts constituted transition metal-based catalysts, including hydrotalcite-derived mixed metal oxides. This class of materials is characterized by tailored composition, homogenously dispersed mixed metal oxides, exhibiting high specific surface area and thermal stability. Thus, firstly, we give a short introduction to the structure and composition of hydrotalcite-like materials and their applications in NH₃-SCO. Secondly, an overview of other transition metal-based catalysts reported in the literature is given, following a comparison of both groups. The challenges in NH₃-SCO applications are provided, while the reaction mechanisms are discussed for particular systems.

Promoting the effects of CuSO4 on N2 selectivity in selective catalytic oxidation of ammonia over Pt/TiO2 catalysts

The NH3-SCO reaction mechanism over a Pt/TiO2 catalyst was transferred into the i-SCR mechanism after adding CuSO4, and thus the formation of N2 was promoted.

Multipollutant Control (MPC) of Flue Gas from Stationary Sources Using SCR Technology: A Critical Review

The emission of gaseous pollutants from the combustion of fossil fuels is believed to be one of the most serious environmental challenges in the 21st century. Given the increasing demands of multipollutant control (MPC) via adsorption or catalysis technologies, such as NO_x, volatile organic compounds (VOCs), heavy metals (Hg etc.), and ammonia, and considering investment costs and site space, the use of existing equipment, especially the selective catalytic reduction (SCR) system to convert pollutants into harmless or readily adsorbed substances, is one of the most practical approaches. Consequently, many efforts have been directed at achieving the simultaneous elimination of multipollutants in a SCR convertor, and this method has been widely used to mitigate the stationary emission of NO_x. However, the development of active, selective, stable, and multifunctional catalysts/adsorbents suitable for large-scale commercialization remains challenging. Herein, we summarize recent works on the applications of SCR in MPC, describing the approaches of (i) SCR + VOCs oxidation, (ii) SCR + heavy metal control, and (iii) SCR + NH₃ reduction to reveal that the efficiency of simultaneous elimination depends on catalyst composition and flue gas parameters. Furthermore, the synergistic promotional/inhibitory effects between SCR and VOCs/ammonia/heavy metal oxidations are shown to be the key to the feasibility of the reactions.

Engineering the Nucleophilic Active Oxygen Species in CuTiOx for Efficient Low-Temperature Propene Combustion

Industrialization has resulted in the rapid increase of volatile organic compound (VOC) emissions, which have caused serious issues to human health and the environment. In this study, an extensive Cu incorporating TiO₂ induced nucleophilic oxygen structure was constructed in the CuTiO_x catalyst, which exhibited superior low-temperature catalytic activity for C₃H₆ combustion. Thorough structural, surface characterization and density functional theory (DFT) calculations revealed that the Cu-O-Ti hybridization induced nucleophilic oxygen initiates C₃H₆ combustion by abstracting the C-H bond. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that incorporated copper species acted as the major adsorbent site for the propene molecule. In combination of the DRIFTS and DFT results, the promotion effect of the nucleophilic O on the C-H bond abstraction and CO₂ formation pathway was proposed. The surface doping induced nucleophilic oxygen as strong Brønsted basic sites for low-temperature propene combustion exemplified an efficient strategy for rational design of next-generation environmental catalysts.

Elucidating the Nature of the Cu(I) Active Site in CuO/TiO2 for Excellent Low-Temperature CO Oxidation

Stabilized Cu⁺ species have been widely considered as catalytic active sites in composite copper catalysts for catalytic reactions with industrial importance. However, few examples comprehensively explicated the origin of stabilized Cu⁺ in a low-cost and widely investigated CuO/TiO₂ system. In this study, mass producible CuO/TiO₂ catalysts with interface-stabilized Cu⁺ were prepared, which showed excellent low-temperature CO oxidation activity. A thorough characterization and theoretical calculations proved that the strong charge-transfer effect and Ti-O-Cu hybridization in Ti-doped CuO(111) at the CuO/TiO₂ interface contributed to the formation and stabilization of Cu⁺ species. The CO molecule adsorbed on Cu⁺ and reacted directly with Ti doping-promoted active lattice oxygen via a Mars-van Krevelen mechanism, leading to the enhanced low-temperature activity.

Low temperature benzene oxidation over copper–silver catalyst: roles of copper oxide and silver on cerium–zirconium mixed oxide

The synergetic effects of the non-PGM catalytic components enabled complete benzene oxidation at low temperature, below 200 °C.

High-Performance Ag-Cu Nanoalloy Catalyst for the Selective Catalytic Oxidation of Ammonia

High-performance Ag-Cu alloy nanoparticles (NPs) were successfully synthesized by a solventless mix-bake-wash method and tested for NH₃-SCO. The prepared Ag₂Cu₁ catalyst with a perfect Ag-Cu alloy structure exhibited better T₁₀₀ (200 °C, the temperature at which 100% NH₃ conversion was obtained), higher reaction rates, and lower E_a compared to that with ordinary bimetallic Ag-Cu (AgCuO_x). The characterization data revealed much smaller Ag-Cu alloy nanoparticles of the Ag₂Cu₁ catalyst and more Ag/Cu metallic species on the surface, which can increase the amount of chemisorbed surface oxygen (O_β) and enhance NH₃ adsorption and activation in the low-temperature range, therefore leading to a much higher NH₃-SCO activity. Kinetic studies and density functional theory calculations indicated that Cu decoration at Ag by Ag-Cu alloying could enhance the adsorption/activation of NH₃ and O₂. It has been found that O₂ was more easily transformed from the adsorption state to the transition state than NH₃, which enhanced the performance of NH₃ oxidation. In addition, the Ag₂Cu₁ catalyst exhibited excellent durability because of the stabilization of Ag sites by the Ag-Cu alloy structure.

Boosting the low-temperature activity and sulfur tolerance of CeZr2Ox catalysts by antimony addition for the selective catalytic reduction of NO with ammonia

In this paper, a series of Sb modified CeZr₂O_x mixed oxides (Sb_yCZ) were synthesized by citrate method for the selective catalytic reduction of NO with ammonia (NH₃-SCR). Experimental results exhibited that the Sb addition could bring a great improvement of SCR activity at 200-360 °C owing to the enhancement in surface area, redox ability and surface acidity. More importantly, the sulfur tolerance of the catalyst with proper Sb loading contents was dramatically improved. For instance, above 85% deNO_x efficiency was retained over Sb_0.5CZ catalyst after 24 h reaction in the presence of 100 ppm SO₂ and 5 vol.% H₂O. As for pure CeZr₂O_x and the catalysts with low Sb loading contents, the serious accumulation of ammonium sulfates resulted in the deactivation after SO₂ exposure. However, with excessive Sb addition, more labile oxygen readily reacted with SO₂ and the redox cycle was then disrupted, leading to the decrease of SCR activity. With an appropriate Sb loading contents, the sulfate species preferentially formed around Sb cations could restrain the further consumption of oxygen species in Ce-O-Ce or Ce-O-Zr mode by SO₂ via a space confinement effect. Thus, a certain amount of labile oxygen was preserved to drive the SCR reaction, thereby enhancing the sulfur tolerance of the catalyst.

Study of synergistic effect between CuO and CeO2 over CuO@CeO2 core–shell nanocomposites for NH3-SCO

CuO@CeO₂ core–shell nanocomposites were fabricated and applied in NH₃-SCO. Synergistic effect of CuO–CeO₂ promotes the formation of the Cu–O–Ce structure, which is beneficial to N₂ selectivity.

Mn3O4 doped with highly dispersed Zr species: a new non-noble metal oxide with enhanced activity for three-way catalysis

Herein, we demonstrate the influence of zirconium species on promoting the oxygen storage capacity and three-way catalytic properties of zirconium-manganese oxide catalysts.