High Space Velocity Research Articles

Ni catalyst supported on nitrogen-doped activated carbon for selective hydrogenation of acetylene with high concentration

Ethylene is a widely used industrial raw material. Due to petroleum resources shortage, direct hydrogenation of acetylene by calcium carbide to ethylene has attracted much attention. Accordingly, Ni/AC-N-x catalyst was synthesized by a simple impregnation method to investigate how the support's N-content influenced the catalyst performance. We studied the support-metal interactions by temperature-programmed reduction and X-ray photoelectron spectroscopy and the changes were studied. The acetylene conversion remained at ~96% for 10 h at 200 °C under a high gas hourly space velocity of 4000 mL·g−1·h−1 over Ni/AC-N-0.5 while over Ni/AC, the acetylene conversion decreased to 29% in 10 h.

Superior Ce–Nb–Ti oxide catalysts for selective catalytic reduction of NO with NH3

A series of Ce–Nb–Ti oxide catalysts synthesized by a single step sol-gel (SG) method were evaluated and characterized for selective catalytic reduction (SCR) of NO with ammonia. The experimental results showed that the doping of Nb could enhance the catalytic activity of CeO2/TiO2 significantly. Ce20Nb20Ti exhibited the best NH3–SCR activity and great resistance to high gas hourly space velocity (GHSV), H2O, and SO2. Its superior SCR performance could be ascribed to the better dispersion of Ce, more chemisorbed oxygen species and Ce3+/Ce4+ redox pairs on the catalyst surface, increased redox ability and surface acidity, and strong interaction among Ce, Nb, and Ti species after the introduction of Nb2O5. In situ DRIFT results revealed that the doping of Nb was conducive to the adsorption of NOx and NH3 species, and enhanced the reaction of pre-adsorbed NH3 species with NOx species and that of adsorbed NOx species with NH3 species.

Customized Ni–MgO–Al2O3 catalyst for carbon dioxide reforming of coke oven gas: Optimization of preparation method and co-precipitation pH

• A Ni–MgO–Al 2 O 3 catalyst was customized for CO 2 reforming of coke oven gas. • Co-precipitation was selected as the optimum catalyst preparation method. • The co-precipitated pH was optimized as 12.0. • Complex NiO species–support interactions were key for catalytic activity. • The optimized catalyst lowered the H 2 /CO ratio of coke oven gas from 7.00–1.66. Coke oven gas can be upgraded through catalytic reforming using carbon dioxide as a soft oxidant. Herein, a customized Ni–MgO–Al 2 O 3 catalyst for the carbon dioxide reforming of coke oven gas was developed by selecting an appropriate catalyst preparation method and further optimizing the key variable of the selected method. Catalysts were prepared by various methods including co-precipitation, incipient wetness impregnation, hydrothermal, co-impregnation, and sequential impregnation. Among them, the co-precipitated catalyst showed the best performance. As the key variable, the co-precipitated pH of the precursor solution was optimized as 12. The catalysts were characterized using X-ray diffraction, Brunauer–Emmet–Teller method, H 2 –temperature-programmed reduction, H 2 –chemisorption, and thermogravimetric analysis. The interaction between a complex NiO species and the support material showed a strong relationship with the number of Ni° active sites, which also has a direct connection with the activity of the catalyst. The developed catalyst successfully lowered the high H 2 /CO ratio of coke oven gas from 7.00–1.66 through the carbon dioxide reforming reaction. Moreover, the catalytic activity was maintained over 50 h and thermodynamic equilibrium was achieved, even at a high gas hourly space velocity of 600,000 h −1 .

Efficient defect engineering in Co-Mn binary oxides for low-temperature propane oxidation

Cobalt-manganese composite oxides have high-activity potential for catalytic oxidation of volatile organic compounds (VOCs). However, low defect content and poor oxygen mobility of the spinel structure make it difficult to efficiently activate CH bonds of propane at low-temperature. Herein, novel amorphous Co-Mn binary oxides with enhanced defect were fabricated by a facile self-redox approach. Highly defective amorphous CoMn binary oxides with low-temperature reducibility, weak MnO bond strength and good mobility of the surface lattice oxygen remarkably improved the catalytic activity for propane oxidation. Highly defective amorphous Co1Mn3Ox catalyst showed the best catalytic activity for propane mineralization and the T90 was 207℃ under high space velocity (18 000 mL·g−1 h−1). Moreover, in situ spectroscopy technologies were utilized to explore the structure-activity relationship and the degradation mechanism of propane oxidation under real reaction condition. Significantly, as an effective defect engineering strategy, amorphous binary oxides exhibit excellent activity and can provide some enlightenments for developing efficient VOCs catalysts.

Preparation of nickel-foam-supported Pd/NiO monolithic catalyst and construction of novel electric heating reactor for catalytic combustion of VOCs

Metal-based monolithic catalysts have excellent electrical and thermal conductivity, impact resistance, ductility, and mechanical properties. Thus, they have a broader application prospect than ceramic-based monolithic catalysts. In this work, high-dispersion Pd nanoparticle dispersion liquid was synthesized by using the solvothermal method. Moreover, nickel-foam-supported Pd/NiO (Pd/NiO/Ni foam) monolithic catalyst with good catalytic combustion performance was prepared successfully by simply spraying the dispersion liquid on the surface of Ni foam, which was oxidized at high temperature to grow NiO film. A novel electric heating catalytic combustion reactor with fast response and safety was constructed by nesting Pd/NiO/Ni foam with an insulated heating rod. Notably, even at the high space velocity of 50,000 h−1, the constructed reactor can completely degrade 2500 ppm of toluene at 230 ℃ and exhibits good catalytic activity for hydrocarbons and oxygen-containing and heteroatom-containing organic compounds.

Freezing-induced hierarchical porous manganese dioxide with excellent humidity tolerance for indoor formaldehyde removal

The K-Birnessite type MnO2 was recognized to be a promising material to remove indoor low-level formaldehyde at room temperature. However, moisture was considered to significantly affect the activity of MnO2 for working durably in the complex indoor environment. Here, we prepared a hierarchical porous δ-MnO2 with modest moisture adsorption–desorption content. The formaldehyde removal efficiency exceeds 90% for 200 h test at high gas-hourly space velocity (GHSV: 600 L·g−1·h−1) in both high and low humidity environments, indicating an excellent humidity tolerance. This work provides a meaningful long lifetime catalyst for using in an indoor environment.

A high propylene productivity over B2O3/SiO2@honeycomb cordierite catalyst for oxidative dehydrogenation of propane

Boron-based metal-free catalysts for oxidative dehydrogenation of propane (ODHP) have drawn great attention in both academia and industry due to their impressive activity and olefin selectivity. Herein, the SiO2 and B2O3 sequentially coated honeycomb cordierite catalyst is designed by a two-step wash-coat method with different B2O3 loadings (0.1%–10%) and calcination temperatures (600, 700, 800 °C). SiO2 obtained by TEOS hydrolysis acts as a media layer to bridge the cordierite substrate and boron oxide via abundant SiOH groups. The well-developed straight channels of honeycomb cordierite make it possible to carry out the reactor under high gas hourly space velocity (GHSV) and the thin wash-coated B2O3 layer can effectively facilitate the pore diffusion on the catalyst. The prepared B2O3/SiO2@HC monolithic catalyst exhibits good catalytic performance at low boron oxide loading and achieves excellent propylene selectivity (86.0%), olefin selectivity (97.6%, propylene and ethylene) and negligible CO2 (0.1%) at 16.9% propane conversion under high GHSV of 345,600 ml·(g B2O3)−1·h−1, leading to a high propylene space time yield of 15.7 g C3H6·(g B2O3)−1·h−1 by suppressing the overoxidation. The obtained results strongly indicate that the boron-based monolithic catalyst can be properly fabricated to warrant the high activity and high throughput with its high gas/surface ratio and straight channels.

Influence of space velocity and catalyst pretreatment on COx free hydrogen and carbon nanotubes production over CoMo/MgO catalyst

The influence of catalyst pretreatment and space velocity in methane decomposition into COx-free hydrogen and carbon nanotubes were investigated over CoMo/MgO catalyst. The reduction of catalyst before methane decomposition leads to a hydrogen production without significant formation of COx (concentration lower than 5 ppm after 25 min of reaction) suitable for its use in fuel cells. However, a high hydrogen space velocity in the pretreatment increased the rate of catalyst deactivation. The CO and CO2 formation rates showed a common trend for all conditions tested: there was a high initial rate and, after 2 min of reaction, there was a lower stabilized rate. The increase in methane space velocity increased the hydrogen formation rate and the degree of carbon nanotubes graphitization. However, it strongly decreases methane conversion, as expected. The use of low hydrogen space velocity, 0.25 h−1, in catalyst pretreatment and high methane space velocity, 8 h−1, in reaction step, provided the highest hydrogen yield and well-structured carbon nanotubes.

Honeycomb monolithic design to enhance the performance of Ni-based catalysts for dry reforming of methane

Supported Ni catalysts (4.5 wt%) using a Ce-Zr oxide (18/82 molar ratio and a ceria-rich surface) depicting advanced redox properties, were deposited by washcoating over cordierite honeycombs (230 and 400 cpsi). FIB-STEM unveiled nanostructure details otherwise undistinguishable by conventional techniques. The catalytic performance was evaluated in the dry reforming of methane at 700−900 °C, using a CH4:CO2 1:1 feedstock, and exploring high Weight Hourly Space Velocity (115−346 L g−1 h−1). The structured catalysts exhibited better performance than the corresponding powders, reaching values close to thermodynamic limits for both reactants conversion and H2/CO ratio, from 750 °C, and no deactivation was observed in prolonged experiments (24−48 h). This was related to both the high catalyst efficiency after being deposited with low loading on the cordierite and the intrinsic advantages of the monolithic reactor, like preventing from the kinetic control that operates in powdered samples under high WHSV or limiting the deactivation.

Comprehensive understanding of the superior performance of Sm-modified Fe2O3 catalysts with regard to NO conversion and H2O/SO2 resistance in the NH3-SCR reaction

Sm-doped Fe2O3 catalysts, with a homogeneous distribution of Sm in Fe2O3 nanoparticles, were synthesized using a citric acid-assisted sol-gel method. Kinetic studies show that the reaction rate for NOx reduction using the optimal catalyst (0.06 mol% doping of Sm in Fe2O3) was nearly 11 times higher than that for pure Fe2O3, when calculated based on specific surface area. Furthermore, the Fe0.94Sm0.06Ox catalyst maintains > 83% NOx conversion for 168 h at a high space velocity in the presence of SO2 and H2O at 250 °C. A substantial amount of surface-adsorbed oxygen was generated on the surface of Fe0.94Sm0.06Ox, which promoted NO oxidation and the subsequent fast reaction between NOx and NH3. The adsorption and activation of NH3 was also enhanced by Sm doping. In addition, Sm doping facilitated the decomposition of NH4HSO4 on the surface of Fe0.94Sm0.06Ox, resulting in its high activity and stability in the presence of SO2 + H2O.

Elucidation of Water Promoter Effect of Proton Conductor in WGS Reaction over Pt-Based Catalyst: An Operando DRIFTS Study

A conventional Pt/CeO2/Al2O3 catalyst physically mixed with an ionic conductor (Mo- or Eu-doped ZrO2) was tested at high space velocity (20,000 h−1 and 80 L h−1 gcat−1) under model conditions (only with CO and H2O) and industrial conditions, with a realistic feed. The promoted system with the ionic conductor physically mixed showed better catalytic activity associated with better water dissociation and mobility, considered as a rate-determining step. The water activation was assessed by operando diffuse reflectance infrared fourier transformed spectroscopy (DRIFTS) studies under reaction conditions and the Mo-containing ionic conductor exhibited the presence of both dissociated (3724 cm−1) and physisorbed (5239 cm−1) water on the Eu-doped ZrO2 solid solution, which supports the appearance of proton conductivity by Grotthuss mechanism. Moreover, the band at 3633 cm−1 ascribed to hydrated Mo oxide, which increases with the temperature, explains the increase of catalytic activity when the physical mixture was used in a water gas shift (WGS) reaction.

Acrylic Acid Production by Gas-Phase Dehydration of Lactic Acid over K+-Exchanged ZSM-5: Reaction Variable Effects, Kinetics, and New Evidence for Cooperative Acid–Base Bifunctional Catalysis

This study investigates the effects of reaction variables (including space velocity, temperature, and partial pressures of LA and H2O) and acidic and basic additives on the gas-phase dehydration of lactic acid (LA) for acrylic acid (AA) production, using an aluminum-rich K+-exchanged ZSM-5 zeolite (molar SiO2/Al2O3 = 27, K/Al = 1.0, K/Na = 8) identified among many high-silica zeolites as the most efficient catalyst in our prior work (Yan et al. ACS Catal. 2017, 7, 538−550). Under widely varied reaction conditions (340–380 °C, feed GHSV: 9,500–38,000 h–1, 1.8–11.7 kPa LA, 52.0–73.0 kPa H2O), the catalyst offers not only high selectivity for AA (>75 mol %) but also long-term catalytic stability. Rate equations and their associated kinetic parameters for LA consumption and AA production are obtained by measuring the kinetically relevant catalytic rates under high feed space velocity (GHSV ≥ 57,000 h–1). Acidic (CO2, acetic acid, and trifluoroacetic acid) and basic (NH3) additives are added, respectively, to the reaction feed to investigate their effects on performance of the working catalyst. While addition of either CO2 or acetic acid shows no effect on the catalyst performance, the addition of either NH3 or trifluoroacetic acid causes significant decline in both the catalyst activity and selectivity for AA production, due to poisoning of the surface acidic sites by NH3 or basic sites by trifluoroacetic acid. These data well highlight the excellent catalytic performance of the investigated catalyst and offer a new piece of evidence in support of the cooperative acid–base bifunctional catalysis for the LTA reaction.

Extended Model for Filtration in Gasoline Particulate Filters under Practical Driving Conditions.

In order to reliably predict the particle number filtration of gasoline particulate filters (GPF) under practical driving conditions, an extension to established filtration models is developed. For the validation of this approach and in order to close a gap of available measurement data at high space velocity in the literature, the particle-size-resolved fresh filtration efficiency of seven different cordierite filters is determined experimentally. Moreover, the experiments on a dynamic engine test bench focus on the impact of the pore-size distribution and the filter wall thickness under steady-state as well as transient, cold-start conditions. In order to model all trends observed, a new correlation for the particle collection due to inertial deposition is proposed and embedded in a heterogeneous multiscale model framework for a GPF. The presented approach can predict all trends observed in the measurements, including a stabilization of the filtration efficiency with increasing space velocities above a certain level. A comparison of several modeling approaches reveals the partly different behaviors at varying space velocities for the here presented model as well as for established filtration models.

Understanding structure-activity relationships in highly active La promoted Ni catalysts for CO2 methanation

Ni-based catalysts are selective in the hydrogenation of CO2 to CH4 but their activity and stability need improvement. Herein, we propose a hydrotalcite-derived high loaded Ni-Al2O3 catalyst promoted by La. The effect of La on the catalyst properties is investigated and compared with that of Y and Ce. The NiOx crystallite size and basic properties (rather than the nickel reducibility) as well as the catalytic activity depend on the rare-earth element. The La-catalyst achieves a more relevant activity enhancement at low temperature and high space velocity (480 L g−1 h−1, CO2/H2/N2 = 1/4/1 v/v), high CH4 productivity (101 LCH4 gNi−1 h−1) and stability, even under undiluted feeds. In situ DRIFTS and the characterization of spent catalysts confirm that this enhanced performance is related to the combination of dissociative and associative CO2 activation on more reduced, highly dispersed and stable Ni nanoparticles and basic sites in the La2O3-Al2O3 matrix, respectively.

Optimization and lumped kinetic model study of coal-based aerospace kerosene hydrogenation process

The effects of various factors on the hydrodegumming (HDG) of oil during the deep hydrogenation of coal-based aerospace kerosenee (CBAK) were studied by single factor experiment and response surface methodology (RSM) analysis on a fixed-bed hydrogenation unit with the existent gum content as the observation value. The results show that in order to achieve better HDG effect of CBAK, it must be carried out at high reaction temperature (T), high reaction pressure (P) and low liquid hourly space velocity (LHSV). The influence of various factors on the gum content of hydrogenated oil was analyzed by using the Design Expert software. The influence degree order was as follows: LHSV > P > T. At the same time, the process conditions of deep hydrogenation were optimized by simulation: P = 11.91 MPa, LHSV = 1.60 h−1, V(H2)/V(Oil) = 800, T = 619.84 K, under which the expected gum content was 0.26 mg (100 mL)−1. On this basis, according to the speed of CBAK-HDG reactivity, the existent gum in the raw material were divided into three lumps, which are high reactivity, medium reactivity, and slow reactivity, then the three lumped kinetic model of CBAK-HDG was established. The kinetic parameters of the model were calculated by Levenberg–Marquardt method, and the model was verified by the measured data. The experimental results show that the average relative error of the model is 8.28%, the predicted value is reliable and the extrapolation is quite good. The calculation results of the model parameters correspond to the results of RSM, which is consistent with the hydrogenation reaction law. The results of this study are expected to provide some theoretical basis and reference for the further study of coal-based space kerosene hydrofining process and the optimization of operation conditions of industrial units.

Reaction mechanism of propane oxidation over Co3O4 nanorods as rivals of platinum catalysts

Co3O4 nanorods and Pt/Al2O3 catalysts were synthesized and tested for catalytic oxidation of propane. In comparison to Pt/Al2O3, the catalytic results showed that Co3O4 nanorods were more active for propane oxidation under relatively high space velocity. Kinetic data suggested that the extraction of hydrogen from C-H bonds in propane can be considered as the reaction of kinetic relevance for both Co3O4 nanorods and Pt/Al2O3 catalysts. On both catalysts, propane could be first transformed into C3H7O* species. The intermediates further dissociate and are oxidized into carboxylates, with acetate and/or formate species as the primary products detectable by Fourier-transform infrared spectroscopy (FTIR). These species are later oxidized into CO2 and H2O as final products. However, the specific oxidation pathways might be different over these two catalysts. Over Co3O4 nanorods, propane oxidation appears to take place via the Langmuir-Hinshelwood mechanism. The rate-determining step is the C–H bond activation by the abstraction of H from adsorbed propane on two neighboring oxygen atoms. In contrast, on Pt/Al2O3 catalysts, the propane oxidation reaction follows an Eley-Rideal mechanism, and the rate-determining step is the activation of gaseous C3H8 on a neighboring oxygen atom and a vacant site.

Morphology Dependent Catalytic Activity of Mn3O4 for Complete Oxidation of Toluene and Carbon Monoxide

Manganese oxide catalysts having pure hausmanite (Mn3O4) phase but different morphologies were synthesized using three different facile methods namely precipitation method (MPM), reduction method (MRM) and solution combustion method (MSM). In spite of having the same phase, their catalytic activities showed strong dependence on their morphologies. Mn3O4 synthesized using the reduction method had a 3D framework of interconnected hexagonal crystallites. Consequently, it showed the best activity among the synthesized and commercial Mn3O4 catalysts both for carbon monoxide and toluene oxidation. This superior activity of MRM was correlated to its uniform pores and ability to desorb oxygen adspecies easily due to its morphology. The catalytic oxidation activity of this catalyst was not significantly affected at high space velocities. Thermal stability of the best performing catalyst was demonstrated through its stable cyclic performance, both for carbon monoxide and toluene oxidation.

Co3O4 Nanosheets Preferentially Growing (220) Facet with a Large Amount of Surface Chemisorbed Oxygen for Efficient Oxidation of Elemental Mercury from Flue Gas.

Oxygen vacancies can capture and activate gaseous oxygen, forming surface chemisorbed oxygen, which plays an important role in the Hg0 oxidation process. Fine control of oxygen vacancies is necessary and a major challenge in this field. A novel method for facet control combined with morphology control was used to synthesize Co3O4 nanosheets preferentially growing (220) facet to give more oxygen vacancies. X-ray photoelectron spectroscopy (XPS) results show that the (220) facet has a higher Co3+/Co2+ ratio, leading to more oxygen vacancies via the Co3+ reduction process. Density functional theory (DFT) calculations confirm that the (220) facet has a lower oxygen vacancy formation energy. Furthermore, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results suggest that Co3O4 nanosheets yield more defects during the synthesis process. These results are the reasons for the greater number of oxygen vacancies in Co3O4 nanosheets, which is confirmed by electron energy loss spectroscopy (EELS), Raman spectroscopy, and photoluminescence (PL) spectroscopy. Therefore, Co3O4 nanosheets show excellent Hg0 removal efficiency over a wide temperature range of 100-350 °C at a high gas hourly space velocity (GHSV) of 180 000 h-1. Additionally, the catalytic efficiency of Co3O4 nanosheets is still greater than 83%, even after 80 h of testing, and it recovers to its original level after 2 h of in situ thermal treatment at 500 °C.

Mechanism by which three-dimensional ordered mesoporous CeO2 promotes the activity of VOx-MnOx/CeO2 catalysts for NOx abatement: Atomic-scale insight into the catalytic cycle

Submonolayer (below one monolayer coverage) vanadium oxide (VOx)-supported catalysts exhibit high activity, which can be attributed to the nature of the active surface VOx species and electronic effects. Here, a catalyst architecture of submonolayer dimeric VOx species and MnOx supported on three-dimensional (3-D) ordered mesoporous CeO2 was successfully synthesized (VOx-MnOx/kit-CeO2) to enhance the selective catalytic reduction (SCR) of NOx with ammonia activity at low temperatures (<300 °C). The VOx-MnOx/kit-CeO2 catalyst performed excellent NH3-SCR activity at 250 °C under a high gas hourly space velocity of 160000 h−1. The larger surface area and 3-D mesoporous channels of the VOx-MnOx/kit-CeO2 catalyst endowed the material with a significantly strong NH3 adsorption capacity, surface reducibility and surface oxygen activity, which were crucial to determine the low-temperature NH3-SCR performance. The highly dispersed submonolayer dimeric VOx sites over the large surface area (>100 m2/g) and 3-D mesoporous channels of VOx-MnOx/kit-CeO2 produced abundant Brønsted and Lewis acid sites, subsequently enhancing the acid cycle of NH3-SCR. The high ratio of Ce3+/(Ce3++Ce4+) and Mn4+/(Mn3++Mn2+) and easy electron transfer sped up the NH3-SCR redox cycle. Ammonia species on both Brønsted and Lewis acid sites rapidly reacted with bridging nitrite species to yield N2 and H2O.

Mesoporous silica “plated” copper hydroxides/oxides heterostructures as superior regenerable sorbents for low temperature H2S removal

Metal oxides exhibit highest adsorption capacities towards H2S but are challenged by lack of regenerability due to sulfide layer formation during adsorption, sulfate phase formation during regeneration, sintering, spalling, and sublimation. Protection strategies such as supporting the metal oxide may mitigate such problems but comes at the expense of the regenerable capacity. Herein, we present a methodology for the synthesis of heterostructures comprised of CuO encapsulated by an ultra-thin mesoporous SiO2 matrix, starting from nanoparticles of Cu(OH)2 as the intermediate seeds of CuO and by using a surfactant based method for the silica encapsulation. Upon calcination, a complex pore structure arises due to the removal of the surfactant and the shrinkage of the intermediate seeding nanoparticles (Cu(OH)2 to CuO). The sorbents achieved high regenerable sulfur capacity, up to 10 mmol/gCuO due to the high CuO content, and combination of large interconnected macro/meso-cavities (~40–50 nm). Those attributes confer a regenerable H2S adsorption capacity that is unprecedented in the literature to the best of our knowledge especially considering the relatively low testing temperature and high space velocity (150 °C, 300,000 h−1). The methodology presented herein can be extended to other gas–solid reactive sorption-based systems as in carbon capture.