High Space Velocity Research Articles

A highly dispersed Pt/copper modified-MnO2 catalyst for the complete oxidation of volatile organic compounds: The effect of oxygen species on the catalytic mechanism

Manganese oxide (MnO2) exhibits excellent activity for volatile organic compound oxidation. However, it is currently unknown whether lattice oxygen or adsorbed oxygen is more conducive to the progress of the catalytic reaction. In this study, novel hollow highly dispersed Pt/Copper modified-MnO2 catalysts were fabricated. Cu2+ was stabilized into the δ-MnO2 cladding substituting original K+, which produced lattice defects and enhance the content of adsorbed oxygen. The 2.03 wt% Pt Cu0.050-MnO2 catalyst exhibited the highest catalytic activity and excellent stability for toluene and benzene oxidation, with T100 = 160 °C under high space velocity (36,000 mL g−1 h−1). The excellent performance of catalytic oxidation of VOCs is attributed to the abundant adsorbed oxygen content, excellent low-temperature reducibility and the synergistic catalytic effect between the Pt nanoparticles and Cu0.050-MnO2. This study provides a comprehensive understanding of the Langmuir–Hinshelwood (L-H) mechanism occurring on the catalysts.

Impact of Feedstock Properties on the Deactivation of a Vacuum Gas Oil Hydrocracking Catalyst

The aim of this study was to understand the impact of vacuum gas oil (VGO) properties on the deactivation rate of a hydrocracking catalyst (nickel–molybdenum sulfide dispersed on a carrier containing USY zeolite). For this purpose, two hydrotreated feeds of different densities, organic nitrogen (∼120–150 ppmw) and aromatic content, were hydrocracked under operating conditions that favor catalyst deactivation, that is, high temperature (T = 418 °C) and high space velocity (LHSV = 3 h–1). The catalyst performance was followed by measuring the VGO conversion (370 °C+ petroleum cut) and determining the apparent kinetic constants for the main hydrocracking reactions (cracking, hydrodenitrogenation, hydrodesulfurization, and aromatics hydrogenation). The experiments were stopped after different times on stream (either 6 or 30 days) in order to assess the evolution of the catalyst as a function of time. The spent catalysts, obtained from three different reactor locations, were characterized by elemental and textural analyses and by thermogravimetry to investigate the quantity and nature of the coke formed. Catalytic tests with different model compounds (toluene and n-heptane) were carried out to determine the residual activity of the hydrogenating and acid catalyst functions. It was found that, at the evaluated conditions, both the nature and the content of organic nitrogen and aromatics compounds of the feedstock have a determinant role in the deactivation rate. Organic nitrogen determines the ratio between available metal and acid sites. The aromatics generate coke precursors on the available acid sites. Both factors play a coupled role that promotes coke deposition on the catalyst surface, which leads to an increase in the deactivation rate on top of the end boiling point of the feed.

In-situ grown Co3O4 nanoparticles on wood-derived carbon with natural ordered pore structure for efficient removal of Hg0 from flue gas

Sorbent morphology is significant to Hg0 removal performance due to its serious impacts on the number and availability of adsorption sites and the mass transfer of the Hg0 removal process. Given this, a novel sorbent was obtained by in-situ growth of Co3O4 nanoparticles on channel walls of wood vessel which has regular mesoporous channels. The effects of synthetic conditions of hydrothermal temperature and NH4F concentration on sorbent morphology and Hg0 removal performance are investigated. Characterization results show that Co3O4 nanoparticles integrate a monolithic shell as synthesis temperature rises, while the nanoparticles grow radially with the increase of NH4F concentration. Radial growth of nanoparticles results in a higher crystallinity. The sorbent synthesized at hydrothermal temperature of 90 °C with the NH4F concentration of 0.05 M (denoted as T90C0.05) has uniform Co3O4 nanoparticle size, homogeneous dispersion and a larger specific surface area. Furthermore, T90C0.05 has a higher chemisorbed oxygen concentration and a higher Co3+/Co2+, which is of benefit to Hg0 removal process. Therefore, T90C0.05 has an excellent Hg0 removal efficiency of 97% with a high gas hourly space velocity of 180, 000 h−1 at 200 °C and the Co3O4 loading is only wt.5%. The effects of compounded flue gas components on the Hg0 removal performance were also studied. NO can weaken the inhibition effect of NH3 through the reaction between NO and NH3 forming labile [NH2NO]. In contrast, SO2 can not weaken the inhibition effect of NH3 due to the production coming from the reaction of SO2 and NH3 is stable at Hg0 removal temperature.

Yolk-Shell Nanocapsule Catalysts as Nanoreactors with Various Shell Structures and Their Diffusion Effect on the CO2 Reforming of Methane.

Well-geometric-confined yolk-shell catalysts can act as nanoreactors that are of benefit for the antisintering of metals and resistance to coke formation in high-temperature reactions such as the CO2 reforming of methane. Notwithstanding the credible advances of core/yolk-shell catalysts, the enlarged shell diffusion effects that occur under high space velocity can deactivate the catalysts and hence pose a hurdle for the potential application of these types of catalysts. Here, we demonstrated the importance of the shell thickness and porosity of small-sized Ni@SiO2 nanoreactor catalysts, which can vary the diffusional paths/rates of the diffusants that directly affect the catalytic activity. The nanoreactor with an ∼4.5 nm shell thickness and rich pores performed the best in tolerating the shell diffusion effects, and importantly, no catalytic deactivation was observed. We further proposed a shell diffusion effect scheme by modifying the Weisz-Prater and blocker model and found that the "gas wall/hard blocker" formed on the openings of the shell pores can cause reversible/irreversible interruption of the shell mass transfer and thus temporarily/permanently deactivate the nanoreactor catalysts. This work highlights the shell diffusion effects, apart from the metal sintering and coke formation, as an important factor that are ascribed to the deactivation of a nanoreactor catalyst.

CO2 effect on catalytic abatement of VOC emissions over Cu-Co binary oxide films

The control of Cu loading in Co3O4 structure was used as noble-free thin-film catalysts for efficient short-chain VOC (volatile organic compound) emissions treatment. All catalysts were capable to decompose C2H2 and C3H6 at high gas hourly space velocity. Slight-promoted Co3O4 catalyst with Cu element resulted in the highest catalytic activity owing to the improved active surface oxygen amount and the high dispersion of Cu. High amount of Cu-loading in the cobalt oxide exhibited a more enhancement of the reaction stability on-stream due to high density of the active phase at the surface. The CO2 effect was investigated. Using DFT (density function theory) calculations, Cu exhibited an active site for C3H6 and C2H2 adsorption, and CO2 could react as an inhibitor in the adsorption process. Such strategy of tailoring the activity and stability-on-stream through the control of Cu-promoting oxide catalysts could establish a promising way to improve the catalytic performance towards gas emissions control.

Ion exchange resin derived magnetic activated carbon as recyclable and regenerable adsorbent for removal of mercury from flue gases

Regenerable ion exchange resin derived magnetic activated carbon (Fe/AC) was prepared to remove mercury from flue gases at 120–180 °C under a high space velocity of 240000 h−1, in this study. The Fe/AC showed higher mercury removal efficiency than those of either magnetic activated carbon without activation or commercial coconut-activated carbon. The optimum temperature for removal of mercury from flue gases was determined to be 120 °C and the existence of SO2 in flue gases promoted the mercury removal efficiency of Fe/AC. Mercury compounds of HgS (metacinnabar), HgO, HgS (cinnabar) and HgSO4 were found over spent adsorbents. The HgO was generated through the reaction of mercury with lattice oxygen and chemisorbed oxygen, while the formation of HgS was due to the reaction between mercury, FeS and sulfur over the adsorbent. The lattice oxygen could oxidize SO2 to SO3, which further reacted with mercury to form HgSO4. The Fe/AC presented excellent regeneration and reuse ability, the mercury removal efficiency of which showed a great increase even after 5 runs of regeneration. The crystalline structure of Fe/AC changed remarkably after regeneration and the FeS over Fe/AC was converted to Fe3O4. The above excellent properties suggest that Fe/AC might be a promising adsorbent to remove mercury from flue gases.

CO Preferential Oxidation in a Microchannel Reactor Using a Ru-Cs/Al2O3 Catalyst: Experimentation and CFD Modelling

This work presents an experimental and modelling evaluation of the preferential oxidation of CO (CO PROX) from a H2-rich gas stream typically produced from fossil fuels and ultimately intended for hydrogen fuel cell applications. A microchannel reactor containing a washcoated 8.5 wt.% Ru/Al2O3 catalyst was used to preferentially oxidise CO to form CO2 in a gas stream containing (by vol.%): 1.4% CO, 10% CO2, 18% N2, 68.6% H2, and 2% added O2. CO concentrations in the product gas were as low as 42 ppm (99.7% CO conversion) at reaction temperatures in the range 120–140 °C and space velocities in the range 65.2–97.8 NL gcat−1 h−1. For these conditions, less than 4% of the H2 feed was consumed via its oxidation and reverse water-gas shift. Furthermore, a computational fluid dynamic (CFD) model describing the microchannel reactor for CO PROX was developed. With kinetic parameter estimation and goodness of fit calculations, it was determined that the model described the reactor with a confidence interval far greater than 95%. In the temperature range 100–200 °C, the model yielded CO PROX reaction rate profiles, with associated mass transport properties, within the axial dimension of the microchannels––not quantifiable during the experimental investigation. This work demonstrates that microchannel reactor technology, supporting an active catalyst for CO PROX, is well suited for CO abatement in a H2-rich gas stream at moderate reaction temperatures and high space velocities.

Enhancement effects of Er modification on comprehensive performance of FeMn/TiO2 catalysts for selective reduction of NO with NH3 at low temperature

Erbium (Er) has been used as a novel dopant to modify FeMn/TiO2 catalyst for the selective catalytic reduction of NO with NH3. The optimized ErxFeMn/TiO2 catalyst exhibited excellent catalytic performance at low temperature under a high space velocity of 30,000 h−1. The temperature window for NO conversion efficiency higher than 80% ranged from 100° to 240°C. The catalysts were characterized using XRD, BET, H2-TPR, NH3-TPD, XPS, TGA and In-situ DRIFT. The results showed that Er modification on FeMn/TiO2 catalysts increased the specific surface area, dispersion, reducibility, Mn4+ concentration, and especially the concentration of Lewis acid sites and surface chemisorbed labile oxygen, resulting in an excellent catalytic performance at low temperature. Additionally, Er modification also restrained the formation of sulfate and/or bisulfate salts, which improved the resistance to SO2 and H2O of the catalyst.

Synthesis gas production from carbon dioxide reforming of methane over Ni-MgO catalyst: Combined effects of titration rate during co-precipitation and CeO2 addition

The Ni-MgO (NM) and Ni-MgO-CeO2 (NMC) catalysts prepared by co-precipitation at different titration rates have been applied to carbon dioxide reforming of methane (CDR). The effects of titration rates on the catalytic properties and reaction behaviors of the catalyst were varied significantly depending on the CeO2 addition. The titration rate mainly influenced the physical properties, such as Ni crystallite size and Ni dispersion, in NM catalysts, but for the NMC catalysts, chemical properties, such as oxygen storage capacity, also were affected. Regarding the change of titration rates, the NM catalysts exhibited the difference of activity, but NMC catalysts showed the difference of activity as well as stability. As a result, the NMC catalyst prepared at fast titration rate achieved the highest catalytic CDR performance at 800 °C and a high gas hourly space velocity of 720,000 mL∙g−1∙L−1.

Synthesis of Highly Porous Cu2O Catalysts for Efficient Ozone Decomposition

At present, it is urgent to synthesize highly active ozone decomposition catalysts to cope with the ever-increasing ozone concentration in the atmosphere. In this study, a highly porous Cu2O catalyst was prepared by using combined surfactants of triblock copolymer P123 and n-butanol through a simple solution reduction method by ascorbic acid. Transmittance electron microscopy, X-ray diffraction, and N2 adsorption–desorption characterizations verify the highly porous structure with a relatively high surface area of 79.5 m2·g−1 and a small crystallite size of 2.7 nm. The highly porous Cu2O shows 90% ozone conversion activity in harsh conditions, such as a high space velocity of 980,000 cm3·g−1·h−1, or a high relative humidity of 90% etc., which is not only attributable to the high surface area but also to the high concentration of surface oxygen vacancy. The results show the promising prospect of the easily synthesized, highly porous Cu2O for effective ozone decomposition applications.

Microkinetic study of NO oxidation, standard and fast NH3-SCR on CeWOx at low temperatures

A comprehensive microkinetic model was established to describe NO oxidation, standard SCR and fast SCR on CeWOx at low temperatures for the first time, providing an in-depth understanding of the NH3-SCR mechanism. The model divided the NH3 adsorption sites into two groups, including sites that could prohibit NO adsorption and that could not affect NO adsorption. Different rate-limiting steps during NO oxidation, standard and fast SCR were identified, and different reaction orders toward NO, NH3 and NO2 were obtained in different reactions. The mechanisms of standard and fast SCR were quite similar. Both the Eley-Rideal and Langmuir-Hinshelwood mechanisms were present, and the reaction between nitrites and adsorbed NH3 made the main contribution to the NOx conversion. The apparent activation energy was affected by the reaction between nitrites and adsorbed NH3 and the reaction between NO and NH4NO3, and the activation energy of the latter reaction was higher. The activation energy of fast SCR was higher than that of standard SCR due to a greater contribution by the reaction between NO and NH4NO3, but larger amounts of nitrites and NH4NO3 caused higher activity for fast SCR than standard SCR. The sites for NO adsorption to produce nitrites were low in number but highly active, which might be a reason why CeWOx is highly active for standard SCR under high gas hourly space velocity, and increasing the sites for NO adsorption might be an effective way to accelerate NH3-SCR. This work provides guiding information for the development of new and effective NH3-SCR catalysts.

Steam reforming of clean biogas over Rh and Ru open-cell metallic foam structured catalysts

The valorisation of clean biogas (CH4/CO2 = 60/40 v/v) by steam reforming over metal open-cell foam-based structured catalysts was investigated. NiCrAl foams were coated by RuMgAl or RhMgAl hydrotalcite-type compounds through electrodeposition to obtain, after calcination, a thin and stable catalytic film of oxides. The active sites for the reforming reactions are highly dispersed Rh or Ru nanoparticles stabilized by a strong metal support interaction, large Ni particles segregated from the support during reduction and reaction, and Rh/Ni bimetallic particles formed by the interaction of the two formers. Rh-based catalysts show superior activity and stability with the time on stream than Ru catalysts, and a low carbon deposition, which is mainly ascribable to the presence of large Ni particles. In comparison to a pelletized catalyst, the structured catalysts are allowed to operate at high space velocities and low Steam to CH4 ratio, increasing the biogas valorisation and thus the productivity.

CeMn/TiO2 catalysts prepared by different methods for enhanced low-temperature NH3-SCR catalytic performance

Ce(1.0)Mn/TiO2 catalysts with the same Ce/Mn molar ratio were prepared by impregnation method and an in situ deposition method. Compared with the Ce(1.0)Mn/TiO2-IP catalyst prepared by impregnation, the Ce(1.0)Mn/TiO2-SP catalyst prepared by in-situ deposition showed better catalytic performance in a wide temperature window (150–300 °C) under high gas hourly space velocities ranging from 10,500 to 27,000 h−1. And Ce(1.0)Mn/TiO2-SP catalyst by in situ deposition method has a better sulfur resistance than Ce(1.0)Mn/TiO2-IP catalyst. Various characterization methods were used to explore the difference of the catalysts prepared by the two methods. The Ce(1.0)Mn/TiO2-SP catalyst exhibited a higher surface area, better thermal stability, a higher concentration of acidic sites, and less agglomeration than the Ce(1.0)Mn/TiO2-IP catalyst. Moreover, the in-situ deposition method resulted in an enhanced electron mobility effect that originated from MnOx and CeOx, thereby enhancing the low-temperature DeNOx efficiency.

Vapor-phase hydrogenation of levulinic acid to γ-valerolactone over Cu-Ni alloy catalysts

Vapor-phase hydrogenation of levulinic acid (LA) to ɤ-valerolactone (GVL) was investigated over supported-type Cu-Ni/Al2O3 catalysts in H2 flow at 250 °C. Ni-rich Cu-Ni/Al2O3 catalysts, typically 6 wt.% Cu and 14 wt.% Ni, achieved high LA conversion with high stability and high GVL selectivity. XRD analyses of the catalysts clarified that Cu-Ni alloy nanoparticles were produced on the alumina support by forming a solid solution of CuO-NiO. The Cu-Ni/Al2O3 catalyst showed the highest GVL productivity of 11.0 kg kgcat−1 h−1 with a selectivity of 98.6 %, although the catalyst was gradually deactivated with time on stream under high space velocity conditions. In the characterization of the used catalysts, the catalyst deactivation would be caused by the sintering of active Cu-Ni alloy nanoparticles, which could be induced by the cycle of the oxidation with H2O and the reduction with H2.

Facile homogeneous precipitation method to prepare MnO2 with high performance in catalytic oxidation of ethyl acetate

Altering the urea decomposition rate in the homogeneous precipitation progress enabled the formation of various morphologies of α-MnO 2 . The decrease in crystalline size led to an increase in the surface area and dominance of the (2 1 1) crystal plane. The abundant surface oxygen species and high reducibility related to unsaturated manganese played critical roles in the decomposition of ethyl acetate. • A facile method driven by a synchronous reaction of urea hydrolysis and potassium permanganate reduction was proposed. • Pure α-MnO 2 with various morphologies was obtained by tuning reaction parameters. • Oxygen species on the (2 1 1) crystal plane were found to be most active in oxidation. • The optimal α-MnO 2 catalyst exhibited high and stable catalytic performance. A facile homogeneous precipitation method driven by synchronous reactions of urea hydrolysis and potassium permanganate reduction is proposed for preparation of MnO 2 catalysts. Pure α-MnO 2 nanostructures with different morphologies, including nanowires, nanorods and nanoparticles, were obtained by simply tuning the precipitation conditions. The evolution of materials derived from different preparation conditions was investigated via X-ray diffraction, transmission electron microscopy, N 2 adsorption, X-ray photoelectron spectroscopy, chemisorption and DFT calculations. The precipitation temperature and time significantly influenced the physiochemical properties of as-prepared catalysts. With increasing precipitation temperature and time, the degree of crystallinity, BET surface area and amount of surface-adsorbed oxygen of α-MnO 2 exhibited a substantial increase. The main exposed surface facets varied from (2 0 0), (3 1 0) to (2 1 1) as temperature and time increased. The best precipitation temperature and time were 90 °C and 24 h respectively. The optimal α-MnO 2 catalyst demonstrated 100% conversion of 1000 ppm ethyl acetate under the high space velocity of 78,000 h −1 during a 113-h test at 190 °C, outperforming other manganese oxide catalysts and many typical noble metal catalysts in terms of activity and stability. Experimental results and DFT calculations were consistent and indicated that surface oxygen species originating from the (2 1 1) surface of α-MnO 2 were most active. This study provides important insights for manipulating the morphology of MnO 2 by a facile method and remarkably promoting the performance for ethyl acetate VOC elimination.

Magnesium Hydrogen Phosphate: An Efficient Catalyst for Acrylic Acid Production from Biorenewable Lactic Acid.

A series of Magnesium hydrogen phosphate (MgHP) catalysts with different magnesium to phosphorous (Mg/P) mole ratios at varying calcination temperatures has been synthesised, bearing in mind the effectiveness as well as the stability of MgHP to catalyse acrylic acid (AA) production from biorenewable lactic acid (LA), a synthetic process applicable to biomass conversion. The physicochemical properties of the MgHP catalysts have been thoroughly characterised and the formation of Mg(NH₄)PO₄, MgHPO₄ and Mg₂P₂O7 with different structural and acidic properties have been reported. The high catalytic performance of MgHP catalysts with high AA yields (100% conversion and 85% selectivity) at high space velocities (WHSVLA = 3.13 h-1) have been achieved at 360 °C. NH₃-Temperature programmed desorption (TPD) and pyridine FTIR have shown that the effectiveness of a catalyst is accounted for not primarily by the actual strength of acidic sites, but is due to the presence of Lewis acidic sites compared to Bronsted sites.

Characteristics and performance of the Co–CeO2 catalyst as a function of the promoter (La, Pr, and Zr) used in high temperature water gas shift reaction

The catalysts used to facilitate the water gas shift reaction (WGSR) are generally harmful to the environment. Therefore, catalysts that have high activity and stability in WGSR and do not pollute the environment need to be fabricated. Herein, three promoters (La, Pr, and Zr) are added into Co–CeO2 (CoCe) catalyst to improve catalytic performance in a high temperature WGSR to produce high-purity hydrogen from waste-derived synthesis gas. Various techniques are employed to confirm the changes in the properties that affect the catalytic performance. The catalytic reaction is performed at a high gas hourly space velocity to screen the performance of the promoted CoCe catalysts. The CoCeZr catalyst shows the highest CO conversion (XCO = 88% at 450 °C) due to its high Co dispersion and oxygen vacancy resulting from the addition of Zr to the CoCe catalyst; thus, it is most suitable for use in high temperature WGSR.

Metal Organic Framework (MOF)/Wood Derived Multi-cylinders High-Power 3D Reactor.

3D monolithic reactor has shown great promise for varied heterogeneous catalysis reactions including water treatment, energy generation and storage, and clean fuel production. As a natural porous material, macroporous wood is regarded as an excellent support for inorganic catalyst due to its abundant polar functional groups and channels. On the other hand, a metal organic framework (MOF) has been widely used as heterogeneous catalyst due to its high specific surface area and large amount of microporosities. Combining macroporous wood and a microporous MOF is expected to produce a high-performance 3D reactor and is demonstrated here for Fischer-Tropsch synthesis. The carbonized MOF/wood reactor retains the original cellular structure with over 180 000 channels/cm2. When being decorated with hexagonal-shaped core-shell Co@C nanoparticles aggregates derived from Co-MOF, the MOF/wood reactor resembles a multi-cylinders reactor for Fischer-Tropsch synthesis. Because of the unique combination of macro- and microporous hierarchical structure, the 3D MOF/wood reactor demonstrates exceptional performance under high gas hourly space velocity (81.2% CO conversion and 48.5% C5+ selectivity at 50 L·h-1·gcat-1 GHSV). This validates that MOF/wood can serve as a multi-cylinders and high-power reactor for catalytic reactions, which is expected to be applicable for environmental and energy applications.

Comparison of doped ZSM-5 and ferrierite catalysts in the dehydration of bioethanol to ethylene in a flow reactor

This study describes the performance of ten zeolite catalysts, doped with various elements, in the dehydration of bioethanol. As acidic catalysts are known as profitable for this reaction, ZSM-5 and ferrierite (FER) based catalysts were selected to use. The synthesized catalysts were characterized by different analytical methods (XRF, XRD, DRIFT, N2-physisorption and TPD-NH3). A high space velocity (15 h−1) and low temperature (220 °C) were used to study the differences on catalyst activity. The Ti-deZSM-5 was the most active catalyst with 48% conversion. Subsequent the screening tests were carried out in a flow micro-reactor at different operating conditions (220–250–280 °C and WHSV 7–11–15 h−1). Ti-deZSM-5 showed the highest bioethanol conversion (96%) and selectivity to ethylene (88%) at 280 °C and WHSV of 7 h−1 while in the case of FER it resulted in very small or no selectivity to ethylene. The results show that catalysts from the ZSM-5 group are more suitable for the dehydration of bioethanol to ethylene than FER catalysts.

Highly Efficient CO2 to CO Transformation over Cu‐Based Catalyst Derived from a CuMgAl‐Layered Double Hydroxide (LDH)

AbstractSupported Cu‐based catalysts are promising catalysts for RWGS reactions. Herein, CuMgAl‐LDHs (LDHs: Layered double hydroxides) were used as the precursors to obtain supported Cu with good dispersion and high stability. The reduction of CuMgAl‐LDH at 400 °C led to the formation of 0.3CuMgAl‐LDH‐400, with Cu nanoparticles well dispersed on MgO/Al2O3, which exhibited superior activity and high stability for RWGS. 33.5 % of CO2 was converted, with selectivity of almost 100 % to CO at 350 °C under a high space velocity, among the highest for RWGS ever reported for Cu‐based catalysts, especially at a low reaction temperature. In‐situ DRIFTS and CO2‐TPD revealed that its superior performance is attributed to high Cu dispersion which is beneficial for the dissociation of H2, as well as the presence of variety of basic sites for adsorption towards CO2 to facilitate the formation of formates. This study demonstrates the great opportunity of LDHs in the preparations of supported metal‐based catalysts.