Catalysts For Selective Catalytic Oxidation Research Articles

Catalytic performance and mechanism of PtxVW catalyst for selective catalytic oxidation of high-concentration NH3

It is foreseeable that a high-concentration NH3 may exist in exhaust gas of NH3-fuel engines, which may result in some negative damages to ecological environment. Herein, a series of PtxVW catalysts with extremely low Pt doping amounts were synthesized with the aim to remove high-concentration NH3 efficiently via selective catalytic oxidation (SCO) method. The results showed that, Pt0.04VW catalyst (0.04 wt% Pt) achieved a complete NH3 conversion at 250 ºC. Both catalytic activity and N2 selectivity of Pt0.04VW catalyst were much superior over Pt/Al2O3 catalyst (1 wt% Pt) in a wide temperature range of 250–450 ºC. XPS, Raman and H2-TPR results indicated that Pt existed in the lattice of VOx and formed Pt-V bimetallic oxides. The interaction between Pt and V played a key role in the NH3-SCO reactions. DFT calculation demonstrated that dual electron transfer pathways between Pt and V atoms in PtxVW catalysts were beneficial for NH3 coordination on Lewis acid sites, then enhancing NH3-SCO reactions. In-situ DRIFTS and NH3-TPD product analysis suggested that NH3-SCO reactions on Pt0.04VW catalyst surface abide by internal selective catalytic reduction (i-SCR) and hydrazine mechanisms. Moreover, coordinated NH3 species on Lewis acid sites, bidentate nitrate species were the main intermediates in NH3-SCO reactions over Pt0.04VW catalyst.

Temperature-driven dynamic evolution process of Ag species on silver-loaded Zr0.2Sn0.8O2 catalyst for selective catalytic oxidation of ammonia

Silver-based catalysts are extensively utilized in the selective catalytic oxidation of ammonia (NH3-SCO). However, enhancing low-temperature activity often causes a decline in N2 selectivity, presenting a substantial challenge in balancing activity and N2 selectivity. In this work, we introduced zirconium into the SnO2 support, resulting in a significant enhancement of low-temperature activity by 200 °C for the Ag/Zr0.2Sn0.8O2 catalyst; yet, this also led to the formation of N2O at low temperatures and NOx at high temperatures. Characterizations such as XPS and in situ DRIFTS revealed that the dynamic changes of silver species driven by temperature had a profound impact on the reaction mechanism and guided the formation of different products. Below 200 °C, the predominance of Ag+ directed the imide mechanism, with the –HNO species being the key intermediate leading to the production of N2O. Above 300 °C, Ag0 became dominant, favoring the internal selective catalytic reduction (i-SCR) mechanism and the production of NOx. This work provides novel insights into improving N2 selectivity of Ag-based catalysts and contributes to a deeper understanding of the reaction mechanisms.

Rational Design of Cobalt Phthalocyanine (CoPc)‐Anchored TiO2 Nanorods for High‐Efficiency Selective Catalytic Oxidation

AbstractQuinclorac is important precursor for pharmaceutical, agricultural, and synthetic chemistry. The state‐of‐art synthesis of quinclorac via condensation, chlorination and oxidative hydrolysis often uses homogeneous catalysts and strong acid oxidant agents to promote the catalytic oxidation, which requires huge manpower input for the late‐stage purification process and is usually environmentally unfriendly. In this work, we successfully fabricated a stable cobalt phthalocyanine (CoPc) Co‐based composite (CoPc/TiO2) by anchoring CoPc on the surface of TiO2 nanorods for the selective oxidation of 3,7‐dichloro‐8‐dichloro methyl quinoline (3,7‐D‐8‐DMQ) into quinclorac. More impressively, CoPc/TiO2‐2.5 %‐Mn‐Br exhibits a high selectivity of 91.9 % for the catalytic oxidation of 3,7‐D‐8‐DMQ to quinclorac in acetic acid, with a quinclorac yield of 86.4 %, which is approximately 2.43 times higher than that of pristine CoPc‐Mn‐Br. The obtained heterogeneous catalytic system shows good reusability. Detailed mechanistic investigations reveal that the system works through a free radical mechanism via the formation of Co2+/Co3+ redox cycles. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the designs of catalysts for selective catalytic oxidation.

Cobalt phthalocyanine (CoPc) anchored on Ti3C2 MXene nanosheets for highly efficient selective catalytic oxidation.

Quinclorac is an important precursor for pharmaceutical, agricultural, and synthetic chemistry. The state-of-the-art synthesis of quinclorac via condensation, chlorination and oxidative hydrolysis often uses homogeneous catalysts and strong acid oxidant agents to promote the catalytic oxidation, which requires huge manpower input for the late-stage purification process and is usually environmentally unfriendly. In this work, we successfully fabricated a stable cobalt phthalocyanine (CoPc) Co-based composite (CoPc/Ti3C2) by anchoring CoPc on the surface of Ti3C2 nanosheets for the selective oxidation of 3,7-dichloro-8-dichloro methyl quinoline (3,7-D-8-DMQ) into quinclorac. More impressively, CoPc/Ti3C2-4.5%-Mn-Br exhibits a high selectivity of 91.8% for the catalytic oxidation of 3,7-D-8-DMQ to quinclorac in acetic acid, with a quinclorac yield of 87.5%, which is approximately 2.46 times higher than that of pristine CoPc-Mn-Br. The obtained heterogeneous catalytic system shows good reusability. Detailed mechanistic investigations reveal that the system works through the free radical mechanism via the formation of Co2+/Co3+ redox cycles. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the design of catalysts for selective catalytic oxidation.

Copper loading effect on active species formation over Cu/SSZ-13 catalysts for selective catalytic oxidation of high-concentration NH3

Cu/SSZ-13 catalysts not only exhibit superior SCR performance but also possess excellent selective catalytic oxidation (SCO) ability to remove NH3 escaped from engine exhaust gas fueled by NH3. To elucidate the effect of Cu loading on Cu/SSZ-13 catalysts, a series of Cu/SSZ-13 catalysts were prepared through wet impregnation method to remove high-concentration NH3 (5000 ppm). The SCO activity test results showed that when Cu loading increased from 1% to 10%, there was an obvious improvement effect on NH3 conversion performance at low temperature. Cu10/SSZ-13 catalyst reached a high NH3 conversion of 96% at 250 °C, and N2 selectivity was higher than 86% in the whole temperature range. However, when further increasing Cu loading from 10% to 20%, there was no distinctive difference in NH3 conversion and N2 selectivity. The characterization results indicated that there were abundant active species, adsorbed oxygen species, and acid sites on the surface of Cu10/SSZ-13 catalyst. The [Cu(OH)]+ and isolated Cu2+ species in zeolite channels of Cu10/SSZ-13 catalyst rather than CuO species on the surface played a key role in NH3-SCO reactions. Since 20% Cu loading would result in a significant decrease in isolated Cu2+ species, it resulted in both [Cu(OH)]+ species in zeolite channels and superfluous Cu species on zeolite surface being main active species in NH3-SCO reactions. In-situ DRIFTS experiments suggested that NH3-SCO reactions over Cu/SSZ-13 catalysts followed the internal selective catalytic reduction (i-SCR) mechanism.

Remarkable N2 selectivity enhancement of NH3-SCO reactions over V0.5/Pt0.04/TiO2 catalyst through Cu-Er co-modification.

Pt-V bimetallic catalysts maybe promising substitutes to precious metal catalysts for selective catalytic oxidation (SCO) of NH3. But it remains a major challenge for Pt-V bimetallic catalysts to pursue high NH3 conversion rate and N2 selectivity simultaneously. In this work, both Cu and Er were adopted to modify V0.5/Pt0.04/TiO2 catalyst (denoted as V/PT), and the influences of Cu and Er doping amounts on NH3-SCO performance of V/PT catalysts were investigated systematically. The results indicated that the co-modification of Cu and Er imposed little influence on NH3 conversion efficiency, but significantly boosted N2 selectivity. Compared with other Cu-Er-modified V/PT catalysts, CEV/PT-4 catalyst exhibited outstanding NH3-SCO performance, which attained completely 100% NH3 conversion efficiency and > 90% N2 selectivity in the temperature range of 225-450°C. It was significantly superior to the NH3-SCO performance of most previously reported catalysts. The characterization results indicated that the adequate doping amounts of Cu and Er resulted in an obvious enhancement on redox property and surface acidity of CEV/PT-4 catalyst. It also led to abundant Pt0 and surface chemisorbed oxygen species on catalyst surface, which facilitated the oxidation of NH3 to NOx and enhanced i-SCR reactions. In situ DRIFTS results showed that -NH2 species on the surface of CEV/PT-4 catalyst could actively react with nitrate species to generate N2 and H2O.

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.

The enhancement effect of Cu modification on N2 selectivity of V0.5/Pt0.04/TiO2 catalyst for selective catalytic oxidation of NH3

BackgroundCurrently selective catalytic oxidation (SCO) is considered as an effective approach to remove slipped ammonia from exhaust gas of NH3-fuel engines. But it is still challenging to achieve high NH3 conversion and N2 selectivity simultaneously. MethodsCu was introduced to modify V0.5/Pt0.04/TiO2 catalysts and the effects of Cu addition step and amounts on NH3 removal performance of the composite catalysts were investigated. Significant findingsCompared to V0.5/Pt0.04/TiO2 catalyst, Cu0.5V0.5/Pt0.04/TiO2 catalyst exhibited a similar NH3 conversion rate but a much better N2 selectivity. When reaction temperature was higher than 225 °C, a complete NH3 removal could be achieved for Cu0.5V0.5/Pt0.04/TiO2 catalyst, while N2 selectivity was higher than 80 % in a wide temperature range of 225–375 °C. The introduction of Cu resulted in the formation of CuVOx species with strong catalytic oxidation activity due to double redox couples of V5+/V4+ and Cu2+/Cu1+. There was a synergistic effect between CuVOx species and Pt species, which significantly facilitated i-SCR reactions, thus improving N2 selectivity. As main intermediates, -NH2 species actively participated in NH3-SCO reactions over Cu0.5V0.5/Pt0.04/TiO2 catalyst.

High throughput screening of noble Metal-Free High-Entropy alloys catalysts for selective catalytic oxidation of NH3

The high price and scarcity of Pt require the development of alternative low-cost catalysts for selective catalytic oxidation of ammonia (NH3-SCO). This work uses first-principles-based high-throughput calculations to evaluate noble metal-free high-entropy alloys (NMF-HEAs) as potential NH3-SCO candidate catalysts to replace Pt. Firstly, by employing empirical rules to determine the formation probability of single-phase solid solution, 350 NMF-HEAs are found to be single-phase solid solution among the vast configuration space (about 104) consisting of 20 non-noble metals. Among these, NMF-HEAs containing Cu and Zn elements are identified as promising catalysts based on the reactivity and selectivity rules of the NH3-SCO process. Further analysis of elemental composition and distribution on configurations with N-atom adsorption energies reveals that the co-existence of Cu and Zn elements and the symmetric distribution of surface atoms is conducive to tuning N-binding ability in the optimal region. Finally, full reaction paths of NH3-SCO processes on the optimal CuZnNiMoCo HEA are calculated and confirm their activity comparable to that of Pt catalysts and better selectivity. This study gains insight into the effects of the composition and surface configuration of HEAs on the NH3-SCO process and provides a theoretical framework for designing and optimizing efficient catalysts of the NH3-SCO process.

Sulfur-resistance properties of WS2-added Pt/TiO2 catalysts for selective catalytic oxidation

An representative method to reduce CO and NH3 emissions is selective catalytic oxidation (SCO). Pt-based catalysts with relatively high resistance to SO2 compared with other materials are widely used for this purpose. However, the sulfuric acid generated from the oxidation of SO2 in the temperature range of 200–300 °C coats the surface of the catalyst, resulting in its deactivation. In this study, we added WS2 to Pt/TiO2 catalyst to synthesize a catalyst that is not easily poisoned by SO2 and shows improved oxidation performance. We confirmed that the coexistence of WS2 and WO3 formed during calcination conferred the catalysts with resistance to SO2. Loading of the catalysts with 3.6 wt% WS2 improved their Pt dispersion and increased the Pt0/Pttotal ratio owing to the formation of Pt0. Our findings suggest a direct method for the efficient preparation of oxidation catalysts with good applicability to the steelmaking industry.

Modulating multi-active sites in CeO2-modified MnOx/ZSM-5 catalyst for selective catalytic oxidation of diethylamine

An efficient MnOx/CeO2/ZSM-5 catalyst was developed for selective oxidation of diethylamine (DEA) with high N2 selectivity in a wide temperature window (ΔT = 140 °C), owing to the existence of oxidation and selective catalytic reduction (SCR) active sites. MnOx species prefer to locate on the surface of CeO2 compared to ZSM-5, and the presence of CeO2 is beneficial to form more Mn4+ species, the active sites for oxidation reaction. The mechanistic investigation suggests that internal SCR of NOx generated from the over-oxidized DEA is the crucial pathway for N2 generation. The high SCR activity of the MnOx/CeO2/ZSM-5 catalyst in oxidation conditions is mainly attributed to the synergistic catalysis of Mn4+ species and acid sites of ZSM-5, the active sites for SCR reaction. Thus, it has been clearly clarified that addition of CeO2 and its addition sequence could modulate the surface active sites of MnOx/ZSM-5 catalyst for the selective catalytic oxidation of DEA.

Synthesis of TixSn1-xO2 mixed metal oxide for copper catalysts as high-efficiency NH3 selective catalytic oxidation

A series of y Cu/TixSn1-xO2 catalysts for selective catalytic oxidation (NH3-SCO) were synthesized using TixSn1-xO2 as catalyst support and CuO as active species. Compared with pure TiO2 support, the strong metal-support interaction between Cu active species and TixSn1-xO2 was significantly promoted by the Sn-doping, thereby improving the catalytic activity and N2 selectivity of the catalyst, which meets the demand of diesel vehicles to completely convert NH3. The results of XRD and HRTEM showed that the particle size of 20 Cu/TiSnO2 sample is much more highly dispersed. The results of NH3-TPD and in-situ DRIFTS indicated that the copper species in 20 Cu/TixSn1-xO2 can provide more Lewis acid sites than 20 Cu/TiO2, which is conducive to the adsorption and reaction of NH3. Meanwhile, XPS results proved that adsorbed oxygen on the catalyst surface is the main active oxygen species. Finally, in-situ DRIFTs and kinetic measurements found that the NH3-SCO reaction over 20 Cu/TiSnO2 and 20 Cu/TiO2 follow the i-SCR and imide (–NH) mechanisms, respectively. This catalyst will provide solutions for the future design and application of ammonia slip from mobile and stationary sources.

Rational ensemble design of alloy catalysts for selective ammonia oxidation based on machine learning

High-throughput computation and machine learning studies are conducted for the rational design of ensembles of alloy catalysts for selective catalytic oxidation of NH3 (NH3-SCO).

Toward Selective Oxidation of Contaminants in Aqueous Systems.

The presence of diverse pollutants in water has been threating human health and aquatic ecosystems on a global scale. For more than a century, chemical oxidation using strongly oxidizing species was one of the most effective technologies to destruct pollutants and to ensure a safe and clean water supply. However, the removal of increasing amount of pollutants with higher structural complexity, especially the emerging micropollutants with trace concentrations in the complicated water matrix, requires excessive dosage of oxidant and/or energy input, resulting in a low cost-effectiveness and possible secondary pollution. Consequently, it is of practical significance but scientifically challenging to achieve selective oxidation of pollutants of interest for water decontamination. Currently, there are a variety of examples concerning selective oxidation of pollutants in aqueous systems. However, a systematic understanding of the relationship between the origin of selectivity and its applicable water treatment scenarios, as well as the rational design of catalyst for selective catalytic oxidation, is still lacking. In this critical review, we summarize the state-of-the-art selective oxidation strategies in water decontamination and probe the origins of selectivity, that is, the selectivity resulting from the reactivity of either oxidants or target pollutants, the selectivity arising from the accessibility of pollutants to oxidants via adsorption and size exclusion, as well as the selectivity due to the interfacial electron transfer process and enzymatic oxidation. Finally, the challenges and perspectives are briefly outlined to stimulate future discussion and interest on selective oxidation for water decontamination, particularly toward application in real scenarios.

Role of the Cu content and Ce activating effect on catalytic performance of Cu-Mg-Al and Ce/Cu-Mg-Al oxides in ammonia selective catalytic oxidation

Copper-containing mixed metal oxides were studied as catalysts for selective catalytic oxidation of ammonia. The performed research shows a high correlation between Cu amount and catalyst efficiency, especially dinitrogen selectivity. Copper species, mainly in the form of highly dispersed CuO oxides, were found to be responsible for high N2 selectivity, which decreased with increasing Cu loading and the formation of CuO bulk-like species. The addition of cerium seemed to have an effect on material’s catalytic activity and N2 selectivity. The increase in selectivity towards N2 of Ce-impregnated materials was attributed to the formation of copper-cerium redox couples on the material’s surface. The catalytic tests performed under different NH3/O2 molar ratios revealed that the developed materials exhibit high catalytic activity in a wide range of ammonia concentrations and oxygen contents.

A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes.

A copper-containing polyoxometalate-based metal-organic framework (POMOF), CuI12Cl2(trz)8[HPW12O40] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

Features of Nb2O5 as a metal oxide support of Pt and Pd catalysts for selective catalytic oxidation of NH3 with high N2 selectivity

Selective catalytic oxidation of ammonia (NH3-SCO) to N2 and H2O was examined as one of the most efficient solutions for NH3 pollution because NH3 has potentially harmful effects on human health and the environment. The acidic metal oxide Nb2O5 that possesses both Brønsted and Lewis acid sites was chosen as the support of Pt and Pd catalysts for SCO of 50 ppm NH3. Both Pt/Nb2O5 and Pd/Nb2O5 showed extremely high levels of N2 selectivity (98% and 100%, respectively) at 100% NH3 conversion. The role of the acid sites of Pt/Nb2O5 and Pd/Nb2O5 in NH3-SCO was investigated by the NaOH treatment to neutralize the Brønsted acid sites. The selectivity to N2 decreased by the NaOH treatment of Pt/Nb2O5 and Pd/Nb2O5. These results suggested that the Brønsted acid sites were essentially important for improving N2 selectivity. This study provides a new concept for the design of a catalyst by using the acid sites of acidic metal oxide to improve the N2 selectivity of NH3-SCO.

MnOx-CuOx cordierite catalyst for selective catalytic oxidation of the NO at low temperature.

Low-value solid waste cordierite honeycomb ceramics were used as carrier of SCO denitration catalyst, and the active component was supported by the impregnation method to improve the performance of the catalyst. Firstly, the effect of calcination conditions on the denitration performance of the Mn-loaded cordierite catalyst was studied for the cordierite-loaded active component MnOX. Secondly, the preferred catalyst was reloaded with another active component to further improve its denitration performance; the bimetal ratios were affected by the denitration performance, which was, finally, characterized by XRD, XPS, and SEM. The result shows the following: (1) Mn-loaded cordierite prepared at 450°C for 3h has a good denitration effect; (2) the MnOX-CuOX/CR catalyst is superior to MnOX-FeOX/CR, MnOX-CoOX/CR, and MnOX-CeOX/CR; (3) the MnO2 crystal form in the single metal-supported catalyst plays a major role, and Cu2Mn3O8 in the bimetallic catalyst affects the performance and activity of the catalyst. Graphical abstract.

Nature of Cu active sites in zeolite-based catalysts for selective catalytic oxidation of methane

Though copper-containing zeolites (Cu–zeolites) have shown great potential in the direct conversion of methane to methanol under low temperature, the yields of methanol and the catalytic efficiency remain far behind the necessary level for industrialization. The recent progress in Cu–zeolites catalysts for the direct conversion of methane to methanol by selective oxidation, especially in the investigation on the nature of Cu active species presented in zeolites, was summarized in this mini-review.