Low-temperature Catalytic Performance Research Articles

Engineering Surface Properties of CuO/Ce0.6Zr0.4O2 Catalysts for Efficient Low-Temperature Toluene Oxidation

The development of superior low-temperature catalytic performance and inexpensive catalysts for the removal of volatile organic compounds (VOCs) is crucial for their industrial application. Herein, CuO/Ce0.6Zr0.4O2 catalysts calcinated at different temperatures (Cu/CZ-X, X represented calcination temperature) were prepared and used to eliminate toluene. It can be found that Cu/CZ-550 presented the highest low-temperature catalytic activity, with the lowest temperature (220 °C) 50% conversion of toluene, the highest normalized reaction rate (3.1 × 10−5 mol·g−1·s−1 at 180 °C) and the lowest apparent activation energy value (86.3 ± 4.7 kJ·mol−1). Systematically, the surface properties analysis results showed that the optimum redox property, abundant oxygen vacancies, and plentiful surface Ce3+ species over Cu/CZ-550 were associated with the strong interaction between Cu and support could significantly favor the adsorption and activation of toluene, thus resulting in its superior catalytic performance.

Mechanistic and performance insights into low-temperature NH3-SCR based on Ce-modified Mn-Ti catalysts

In this study, Mn-Ti supported Ce catalysts (derived from manganese tailings) were synthesized by coprecipitation method. The denitration performance and SO2 resistance of the catalysts were investigated. The results showed that introducing Ce oxides into Mn-Ti catalyst improved the low temperature catalytic performance, with almost 100% NO conversion at 225 °C. Additionally, the N2 selectivity was improved due to the decrease in lattice oxygen. The modified catalyst also presented excellent SO2 resistance, with around 80% NO conversion after 10 h of SO2 injection. The modified catalyst had higher ratios of Mn4+ and Mn3+ species, stronger surface acidity, and more redox properties than the Mn-Ti catalyst, which improved NH3-SCR performance. After SO2 poisoning, the surface Lewis acidity decreased and the surface Brønsted acidity became stronger, indicating that the loss of Lewis acidity might cause the decrease of NO conversion. Moreover, the nitrate species on Mn-Ti catalyst were less than those on Mn-Ce-Ti catalyst in the presence of SO2, suggesting that doping Ce into Mn-Ti catalyst retained more oxidation performance after being poisoned by SO2.

Low-temperature catalytic performance of toluene oxidation over Cu-Mn oxide catalysts derived from LDH precursor

A series of CuMn mixed oxides catalysts (CMO-T) were prepared by calcination of CuMn layered double hydroxides (CuMn-LDH) for catalytic oxidation of toluene. The obtained CMO-400 catalyst exhibited a high catalytic activity with T50 and T90 of 210 and 231 °C (WHSV = 30000 mL·g−1·h−1). Meanwhile, the CMO-400 exhibited high stability and durability even at 20 vol% water vapor. The effects of calcination temperature on the texture and structural properties were investigated systematically, which proved the formation of copper-manganese solid solution with abundant multiple-phase interfaces. In addition, the strong interaction between Cu and Mn could lead to more surface adsorbed oxygen due to oxidation reduction cycle of Mn3++Cu2+=Mn4++Cu+. Accordingly, for CMO-500 catalyst, the formation of spinel Cu1.5Mn1.5O4 phase prevented good dispersion of the manganese-oxide phases and weakened the reducibility and oxygen mobility. In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) revealed that the intermediates accumulated continuously were difficult to be oxidized rapidly without the replenishment gaseous oxygen. Moreover, the complete transformation of the intermediate benzoate may be the key rate-controlling step in the reaction. The Facile method of preparing mixed-metal solid solution from LDH precursor system could be widely developed for the design of transition metals in catalytic oxidation of toluene.

Remarkable N2-selectivity enhancement of NH3-SCR over HPMo modified MnCo-BTC@SiO2 catalyst

In this work, the phosphomolybdate (HPMo) modification strategy was applied to improve the N2 selectivity of MnCo-BTC@SiO2 catalyst for the selective catalytic reduction of NOx, and further, the mechanism of HPMo modification on enhanced catalytic performance was explored. Among MnCo-BTC@SiO2-x catalysts with different HPMo concentrations, MnCo-BTC@SiO2-0.75 catalyst exhibited not only the highest NH3-SCR performance (∼95% at 200-300°C) but also the best N2 selectivity (exceed 80% at 100-300°C) due to the appropriate redox capacity, greater surface acidity. X-ray photoelectron spectrometer (XPS) and temperature programmed reduction of H2 (H2-TPR) results showed that the modification with HPMo reduced the oxidation-reduction performance of the catalyst due to electron transfer from Mo5+ to Mn4+/Mn3+ and prevent the excessive oxidation of ammonia adsorption species. NH3 temperature-programmed desorption of (NH3-TPD) results showed that the modification with HPMo could significantly improve the surface acidity and NH3 adsorption, which enhance the catalytic activity and N2 selectivity. In-situ diffused reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) revealed that modification with HPMo increased significantly the amount of adsorbed NH3 species on the Bronsted acid site and CB/CL, it suppressed the production of N2O by inhibiting the production of NH species, the deep dehydrogenation of ammonia adsorption species. This study provided a simple design strategy for the catalyst to improve the low-temperature catalytic performance and N2 selectivity.

CO2 hydrogenation over cubic yttrium oxide support: Effect of metal type

CO2 methanation is an effective strategy for making full use of waste gases and converting them into valuable chemicals. Nowadays, the key challenge is the design of efficient catalysts to enhance low-temperature catalytic performance. The present work studies the effect of metal type on the catalytic performance of yttrium oxide-supported catalysts for CO2 methanation reaction. Ru/Y2O3, Ni/Y2O3, and Co/Y2O3 catalysts were prepared by wetness incipient impregnation method, characterized using N2-adsorption/desorption isotherm, XRD, FTIR, and H2-TPR and TGA techniques to evaluate the surface, crystal phase, and thermal stability. The catalytic test was conducted with the use of a fixed-bed reactor under atmospheric pressure. The temperature of catalytic performance was 350 °C with a supply of H2: CO2 molar ratio of 4 and a total flow rate of 200 mL/min. The main products of the reaction were CH4, and traces of carbon monoxide were present among the products. The methane yield reached 64.67%, 60.03%, and 50.82% over Ru/Y2O3, Ni/Y2O3, and Co/Y2O3 catalysts, respectively.

The study on Cu-ZK-5 catalyst of copper content regulation and anti-propylene poisoning mechanism in NH3-SCR reaction

Cu-ZK-5 catalysts have been attempted to utilize as NH3-SCR because they have small pores and eight-membered ring structures. In this work, Cu-ZK-5 catalysts with different copper contents were investigated. It was found that the Cu7.55-ZK-5 catalyst showed excellent activity and resistance to propylene poisoning for NH3-SCR. When the copper content is too low, the number of active copper species is less, and the activity is poor. When the copper content is too high, CuOx species will be generated to destroy the structure of the molecular sieve and affect the activity, which manifested that the optimal copper loading of Cu-ZK-5 catalyst possesses more Cu2+ species improving the activity for NH3-SCR. Furthermore, the presence of propylene on Cu7.55-ZK-5 catalyst had little impact on the low-temperature catalytic performance and exhibited excellent propylene resistance at medium and high temperatures. The influence on the low-temperature catalytic performance of the Cu-ZK-5 catalyst is mainly since C3H6 has competitive adsorption with NO on the surface of Cu-ZK-5, which may be the main reason for slightly reducing the NOx conversion of Cu-ZK-5 catalyst.

Rational Regulation of Reducibility and Acid Site on Mn-Fe-BTC to Achieve High Low-Temperature Catalytic Denitration Performance.

Selective catalytic reduction with ammonia is the mainstream technology of flue gas denitration (de-NOx). The reducibility and acid site are two important factors affecting the de-NOx performance, and effective regulation between them is the key to obtain a highly efficient de-NOx catalyst. Herein, a series of Mn-Fe-BTC with different ratios of Mn and Fe are synthesized, among which 2Mn-1Fe-BTC with 2:1 molar ratio of Mn and Fe has excellent low-temperature (LT) de-NOx performance (above 90% NO conversion between 60 and 270 °C) and good tolerance to H2O and SO2 poisoning (88% NO conversion at 150 °C with 100 ppm of SO2 and/or 6% H2O). It is revealed that the reducibility properties and acid sites of Mn-Fe-BTC can be flexibly tuned by the ratio of Mn and Fe. The difference in electronegativity between Fe and Mn leads to the redistribution of valence electrons, which enables the controllable reducibility of Mn-Fe-BTC. Furthermore, different amounts of Mn and Fe lead to different electron transport, which determines the type and number of acid sites. The synergistic effect of Mn and Fe endows Mn-Fe-BTC with enhanced surface molecular adsorption capacity and enables the catalyst to selectively chemisorb NH3 and NO at different active sites. This research provides guidance for the flexible regulation of reducibility and acid site of LT de-NOx catalyst.

Low-temperature selective catalytic reduction of NOx with NH3 over in-situ grown MnOx-Fe2O3/TiO2/Ti monolithic catalyst

The design and synthesis of highly efficient and low-temperature NOx reduction catalysts with long-term stability and good water resistance still remain a challenge in the field of gas catalysis. In this work, we demonstrated the integration of MnOx-Fe2O3/TiO2/Ti monolithic catalysts on flexible Ti mesh through plasma electrolytic oxidation technology, associated with hydrothermal reaction, and chemical bath deposition methods. The in-situ grown TiO2 nanosheet supports hold a very strong substrate adhesion with Ti mesh, and provide ideal nucleation sites for the following deposition of high density of MnOx-Fe2O3 active components. The brush-like nanostructure morphology of active MnOx-Fe2O3 catalysts enables the complete adsorption and reaction of gas molecules on the catalyst surface. The selective catalytic reduction of NOx with NH3 tests indicate that MnOx-Fe2O3/TiO2/Ti monolithic catalysts exhibit excellent low-temperature catalytic performance with 100% of NOx conversion at 137 °C, good stability and water resistance, showing great potential and broad application prospects in comparison with powder catalysts and honeycomb ceramic catalysts. The strategy of depositing multi-components active catalysts on in-situ grown ultra-thin TiO2 nanosheet supports on flexible Ti mesh paves a solid way toward the practical applications of novel low-temperature selective catalytic reduction catalysts in diverse fields.

Ultrathin NiZrAl layered double hydroxide nanosheets derived catalyst for enhanced CO2 methanation

It is challenging to fabricate supported Ni catalysts with more active sites to improve their low-temperature catalytic performance in CO2 methanation. Herein, firstly, ultrathin (∼4 nm) NiZrAl layered double hydroxide (LDH) nanosheets were synthesized by co-precipitation method, followed by aqueous miscible organic solvent treatment (AMOST), and applied as catalyst precursors for CO2 methanation. After H2 reduction, the surface area of Ni particles in NiZrAl-LDH-AMO-R was significantly higher than that of NiZrAl-LDH-R without AMOST. As a result, the NiZrAl-LDH-AMO-R showed higher catalytic activity than NiZrAl-LDH-R owing to its higher H2 and CO2 chemisorption capacity and lower activation energy. During the 100 h-lifetime test, NiZrAl-LDH-AMO-R maintained a steady CO2 conversion of about 92.3%. Moreover, NiZrAl-LDH-AMO-R maintained its catalytic activity after a 600 °C-hydrothermal treatment, suggesting its high stability. In situ DRIFTS results reveal that CO2 methanation on both NiZrAl-LDH-R and NiZrAl-LDH-AMO-R followed the HCOO∗ route. Interestingly, more active sites obtained after AMOST strongly promoted the generation and decomposition of HCOO∗, and thus significantly improved the activity of NiZrAl-LDH-AMO-R at low temperatures.

Ordered mesoporous TiO2 framework confined CeSn catalyst exhibiting excellent high activity for selective catalytic reduction of NO with NH3 at low temperature

Synthesis of the cerium-based catalyst with excellent activity, selectivity, and toxicity resistance is one of the difficult problems for the selective reduction of NO by NH3 (NH3-SCR) at low temperature. Herein, we synthesize a novel and rational framework-confined ordered mesoporous CeSnOx/TiO2-E catalyst. Through a classical evaporation-induced self-assembly (EISA) strategy, the active components participate in the formation process of ordered mesoporous, which confines the catalytic active species into the interior of the overall structure. Via an array of microscopic characterizations and spectroscopic analysis, we have discovered that the presence of the ordered mesoporous structure makes the catalyst have a prominent confined effect on the framework. The high dispersion of nanoparticles in the system is realized here. Meanwhile, the cerium-based with Sn-doped catalyst system achieves the change of electronic structure and increases the charge exchange frequency, which enables the catalyst to acquire outstanding redox ability and abundant acidic sites. In addition, the ordered mesoporous structure of the catalyst guided the active components to produce a synergistic effect, and the uniform pore structure and metal doping enhanced the anti-poisoning ability. This strategy not only improves the low-temperature catalytic performance of the catalyst but also protects the nanoparticles from poisoning, which is beneficial to the long-time stability of the NH3-SCR reaction.

Abundant Oxygen Vacancies Induced by the Mechanochemical Process Boost the Low-Temperature Catalytic Performance of MnO2 in NH3-SCR

Manganese oxides (MnOx) have attracted particular attention in the selective catalytic reduction of NOx with NH3 (NH3-SCR) because of their excellent low-temperature activity. Herein, we prepared a highly efficient MnO2 (MnO2-M) catalyst through a facile ball milling-assisted redox strategy. MnO2-M shows a 90% NOx conversion in a wide operating temperature window of 75–200 °C under a gas hourly space velocity of 40,000 h−1, which is much more active than the MnO2 catalyst prepared by the redox method without the ball-milling process. Moreover, MnO2-M exhibits better H2O and SO2 resistance. The enhanced catalytic properties of MnO2-M originated from the high surface area, abundant oxygen vacancies, more acid sites, and higher Mn4+ content induced by the ball-milling process. In situ DRIFTS studies probed the reaction intermediates, and the SCR reaction was deduced to proceed via the typical Eley–Rideal mechanism. This work provides a facile method to enhance the catalytic performance of Mn-based catalysts for low-temperature denitrification and deep insights into the NH3-SCR reaction process.

Highly efficient Mn-Fe bimetallic oxides for simultaneous oxidation of NO and toluene: Performance and mechanism

A series of PEG modified Mn-Fe bimetallic oxides with various of Mn/Fe molar ratio were synthesized by co-precipitation method to investigate the performance and mechanism of simultaneous oxidation of NO and toluene. The Mn2Fe1 catalyst with Mn/Fe molar ratio of 2:1 obtained good low-temperature catalytic oxidation performance with 87 % NO oxidation efficiency at 230 °C and 100 % CO2 selectivity for toluene at 200 °C. The excellent catalytic performance benefited from the synergistic interaction between Mn and Fe, which increased the specific surface area, enhanced the dispersion and low-temperature reducibility, and produced abundant adsorbed oxygen as well as Mn3+ and Fe3+. The generated oxygen vacancies promoted the activation of oxygen and the adsorption-oxidation of NO and toluene. The activity results showed that NO had promoting effect on toluene removal in the presence of O2, while toluene could inhibit NO removal within 200 °C. However, the inhibitory effect of toluene on NO disappeared above 200 °C. Additionally, NO showed negative effect on toluene removal within 140 °C in the absence of O2 for Mn2Fe1 catalyst, but this negative effect of NO on toluene disappeared with the increase of temperature. Toluene oxidation took the superiority during the simultaneous oxidation of NO and toluene. It was also found that O2 significantly promoted the NO and toluene deep oxidation, besides, NO also played a positive role in the deep oxidation of toluene to a certain extent due to the formation of strong oxidizing NO2. While the presence of toluene showed negative effect on NO deep oxidation, especially in the absence of O2. In addition, SO2 acted a negative role for NO and toluene removal due to the accumulation of sulfate on catalyst surface. Interestingly, NO was inhibited first, followed by toluene. Furthermore, in situ DRIFTS was performed to study the intermediates in the oxidation of NO and toluene, and a possible reaction mechanism for simultaneous oxidation of NO and toluene over Mn2Fe1 was proposed.

Construction of oxygen vacancies in δ-MnO2 for promoting low-temperature toluene oxidation

Oxygen vacancy engineering is a proven method to promote catalytic performance. In this paper, a series of alkaline earth metal-doped layered MnO2 catalysts were prepared to modulate the oxygen vacancies in MnO2 and applied to catalytic oxidation of toluene at low-temperature. Various characterizations and DFT calculations show that the alkaline earth metals enter the MnO2 lattice, leading to the distortion of the lattice and the formation of oxygen vacancies, which eventually lead to the activation of the lattice oxygen. Ba-doped MnO2 shows the highest content of oxygen vacancies and the highest lattice oxygen activity, thus exhibiting the best low-temperature catalytic performance for toluene (T90 = 182 °C). This work provides an innovative approach for the construction of oxygen vacancies on high-performance catalysts for toluene oxidation.

Preparation of a transition metal-supported TiO2 catalyst and the catalytic oxidative degradation of toluene

The synthesis of supported transition metal oxides with satisfactory low-temperature catalytic performance and application potential is still a challenge for the complete degradation of VOCs. Ni-Cu-loaded TiO2 catalysts were prepared by an impregnation method. The lattice defect and oxygen vacancy concentrations of the prepared materials could be tuned by controlling the Ni/Cu molar ratio and the mass proportion of the TiO2 support. 5Ni-Cu/TiO2 exhibited the best catalytic activity and showed superior catalytic stability during complete toluene oxidation. The obtained catalysts were fully characterized by XRD, FT-IR, TEM, N2 adsorption analysis, XPS, H2-TPR, O2-TPD, TPD analysis, and XPS. The 5Ni-Cu/TiO2 catalyst had improved pore sizes and specific surface area, had more oxygen vacancies on its surface, had increased concentrations of adsorbed and lattice oxygen species, and had good redox properties; these characteristics were favorable for low-temperature catalytic performance and catalytic stability. Cyclic transformations of metal valence states between Cu+ and Cu and between Ni and Ni2+ were observed in the catalytic system. A potential catalytic reaction process based on the M-vK mechanism was proposed. This work serves as a reference for the application of supported transition metal catalysts for the degradation of VOCs.

Insights into the structure-performance relationship of CuOx-CeO2 catalysts for preferential oxidation of CO: Investigation on thermally induced copper migration process

A series of CuOx-CeO2 catalysts is designed by coating composite copper-ceria precursor with different thickness of CeO2, followed by thermal treatment at 500 and 700 °C respectively to investigate the outward migration phenomenon of Cu species induced by high temperature thermal treatment. For the CuOx-CeO2 catalysts calcined at 500 °C, the coating of CeO2 covers the active surface Cu entities and degrades the low temperature catalytic performance for preferential oxidation of CO (CO-PROX). However, thermal treatment at 700 °C induces the migration of large amounts of Cu entities from bulk to the surface of CeO2 nanoparticles and recovers the low temperature catalytic performance. In addition, 700 °C thermal treatment of CuOx-CeO2 catalyst without CeO2 coating causes excessive outward migration of Cu species and severe CuO agglomeration, which decreases the catalytic performance at low reaction temperature, but broadens the high temperature operation window on account of the formation of inverse CeO2-CuO structure. This work provides deeper insight into the regime of thermally induced Cu migration phenomenon, and illuminated the structure-performance relationship of copper-ceria based catalysts for CO-PROX.

Enhanced Low-temperature Catalytic Decomposition Performance of Naphthalene over Hydrothermal Carbon-based Nanomaterials with a Hierarchical Heterostructure

Enhanced Low-temperature Catalytic Decomposition Performance of Naphthalene over Hydrothermal Carbon-based Nanomaterials with a Hierarchical Heterostructure

Spotlight on Cu/SAPO-34 with high hydrothermal stability induced by a small amount of SSZ-39

The stable structure is key to improving the performance of the selective catalytic reduction of nitrogen oxide (NOx) with ammonia (NH3-SCR) on Cu/zeolite catalysts after the high-temperature hydrothermal aging. In this work, the SSZ-39 seed with a stable texture property was introduced into the precursor of SAPO-34 to fabricate the zeolite with a hybrid microstructure of SAPO-34 + SSZ-39 (simplified as SA + SZ). The Si distributions of the original Al-rich SAPO-34 zeolite (simplified as SA) were optimized by the hybrid microstructure of SA + SZ. The results of solid nuclear magnetic resonance (NMR) and ammonia (NH3) adsorption indicate more Si(1OAl) structures with strong acidity on Cu/(SA + SZ) catalyst rather than on Cu/SA. The NH3-SCR performance within the reaction region of 150–550 °C is enhanced on the former. In addition, the Si(nOAl) (n = 2–4) structures of the original SAPO-34 zeolite are retained on Cu/(SA + SZ), which optimizes the low-temperature catalytic performance of Cu/SSZ-39. Furthermore, the SSZ-39 seed induces Cu/(SA + SZ) to form higher content of stable Si(0OAl) structures than Cu/SA. After hydrothermal aging, more isolated copper species can be formed on SA + SZ hybrid zeolite with advanced structural stability compared with Cu/SA, which better activates NOx on the former. And the loss of acid sites is also weakened on Cu/(SA + SZ) than on Cu/SA. These improvements on Cu/(SA + SZ) compensates for its hydrothermal deactivation to some degree. Hence, SA + SZ hybrid zeolite can be the new-generation candidate for NH3-SCR catalyst by introducing the second zeolite SSZ-39 in the synthesis process of SAPO-34. And the catalytic performance of Cu/(SA + SZ) is superior to Cu/SA.

Effect of K-Modified Blue Coke-Based Activated Carbon on Low Temperature Catalytic Performance of Supported Mn-Ce/Activated Carbon.

To clarify the K modified effects over activated carbon (AC) supported Mn–Ce oxide catalysts, several Mn–Ce/AC and xK–Mn–Ce/AC mixed oxide catalysts prepared via an impregnation method supported on AC were investigated for low-temperature selective catalytic reduction (SCR) of NO with NH3 in the simulated sintering flue gas. The Mn–Ce/AC catalyst with a K loading of 8% showed the highest catalytic activity, corresponding to 92.1% NO conversion and 92.5% N2 selectivity at 225 °C with a space velocity of 12,000 h–1. Furthermore, the 0.08K–Mn–Ce/AC catalyst exhibited better resistance to SO2 and H2O than Mn–Ce/AC, which could convert 72.3% and 74.1% of NO at the presence of 5% SO2 and H2O, respectively. After K modification, the relative ratios of Mn4+/Mnn+ as well as Ce3+/Cen+ and surface adsorbed oxygen increased. Additionally, the reduction performance of the catalyst was improved obviously, and both acid strength and quantity of acid sites increased significantly after the K species were introduced in Mn–Ce/AC. Especially, the NO adsorption capacity of the catalyst was enhanced, which remarkably promoted the denitration efficiency and SO2 resistance. The SCR of NO with NH3 on K–Mn–Ce/AC catalysts followed the L-H mechanism.

Enhanced performance of Pt-based diesel oxidation catalyst via defective MnOx: The role of Pt/MnOx interface

To strengthen the interface effect of Pt with MnOx, manganese oxides with different defective concentration were obtained via removing oxygen under reduction treatment. The excellent performance on the Pt-based oxidation catalyst with optimum reduction temperature is 250 °C, and it's the light-off temperature of CO, C3H6 and NO are 170 °C, 180 °C and 185 °C, respectively, and the highest NO maximum conversion of 76%. The defect on MnOx could induce the adsorption of Pt2+ to produce the higher platinum dispersion and more active oxygen species. The abundant oxygen vacancies on manganese manganite would transfer to the subsurface, reducing the Mn valence and affecting the electronic effect of Pt with MnOx. Meanwhile, the existence of interfacial structure induces the shrinkage of platinum cell with the reduction of lattice parameter, because of the doping of Mn ion. The strong interaction of Pt with MnOx could promote the reduction of manganese oxides at the lower temperature. The interface of Pt-MnOx could accelerate the oxygen mobility and facilitate the formation of more bridged nitrates and chelating nitrates. Due to the interfacial structure of Pt with MnOx, the low temperature catalytic performance and high NO conversion of catalyst are remarkably improved.

Low-Temperature Catalytic Performance of Toluene Purification in Air Over Cu-Mn Oxide Catalysts Derived from Ldh Precursor

Low-Temperature Catalytic Performance of Toluene Purification in Air Over Cu-Mn Oxide Catalysts Derived from Ldh Precursor