Alumina-based Catalysts Research Articles

Construction of La/Al2O3 nanorod-like catalyst with rich basic sites: Enabling efficient conversion of COS-superior selectivity and theoretical insights

The introduction of surface basic sites to COS hydrolysis catalysts can improve H2O activation and COS adsorption, leading to enhanced catalytic performance. However, maximizing the concentration of basic sites remains a challenge due to the limited exposed surface area of these catalysts. Herein, a La/Al2O3 catalyst with abundant alkaline sites to catalyze the hydrolysis of COS was prepared by a simple ammonia-assisted hydrothermal method. This synthesized La/Al2O3 catalyst exhibits a thin rod-like structure formed by the random accumulation of nanoparticles. Lanthanum, one of the most abundant rare earth metals, is an ideal doping aid to further promote the COS-catalyzed hydrolysis performance of Al2O3. Characterization studies show that the proper lanthanum loading effectively enhances the formation of basic sites, leading to improved lattice oxygen reactivity and –OH adsorption capacity. As a result, the prepared La-loaded Al2O3 nanorod sample with abundant basic sites exhibits nearly 100 % COS conversion and total H2S yield at 160 °C. In addition, the reaction pathways and deactivation mechanisms for selective catalytic hydrolysis of COS on the La-doped Al2O3 catalyst were revealed by using in situ DRIFTS to evaluate COS adsorption and hydrolysis. Density functional theory calculations showed that La loading enhances the electron transfer between the La and Al atoms, improves the formation of basic sites, and enhances the adsorption of –OH. This study provides new insights into the design of efficient alumina-based catalysts for the catalytic hydrolysis of COS.

Synergistic Effect of Co and Ni Co-Existence on Catalytic Decomposition of Ammonia to Hydrogen—Effect of Catalytic Support and Mg-Al Oxide Matrix

Hydrotalcite-derived mixed metal oxides containing Co and Ni and containing these metals supported on MgO and Al2O3 were prepared and tested as catalysts for the decomposition of ammonia to hydrogen and nitrogen. The obtained samples were characterised in terms of chemical composition (ICP-OES), structure (XRD), textural parameters (low-temperature N2 adsorption–desorption, SEM), form and aggregation of transition-metal species (UV-Vis DRS), reducibility (H2-TPR) and surface acidity (NH3-TPD). The catalytic efficiency of the tested systems strongly depends on the support used. Generally, the alumina-based catalyst operated at lower temperatures compared to transition metals deposited on MgO. For both series of catalysts, a synergistic effect of the co-existence of cobalt and nickel on the catalytic efficiency was observed. The best catalytic results were obtained for hydrotalcite-derived catalysts; however, in the case of these catalysts, an increase in the Al/Mg ratio resulted in a further increase in catalytic activity in the decomposition of ammonia.

New Approach to Ultrafast Oxidative Desulfurization in the Presence of Sulfated Alumina-Based Catalysts

New Approach to Ultrafast Oxidative Desulfurization in the Presence of Sulfated Alumina-Based Catalysts

Importance of plasma discharge characteristics in plasma catalysis: Dry reforming of methane vs. ammonia synthesis

Plasma catalysis is a rapidly growing field, often employing a packed-bed dielectric barrier discharge plasma reactor. Such dielectric barrier discharges are complex, especially when a packing material (e.g., a catalyst) is introduced in the discharge volume. Catalysts are known to affect the plasma discharge, though the underlying mechanisms influencing the plasma physics are not fully understood. Moreover, the effect of the catalysts on the plasma discharge and its subsequent effect on the overall performance is often overlooked. In this work, we deliberately design and synthesize catalysts to affect the plasma discharge in different ways. These Ni or Co alumina-based catalysts are used in plasma-catalytic dry reforming of methane and ammonia synthesis. Our work shows that introducing a metal to the dielectric packing can affect the plasma discharge, and that the distribution of the metal is crucial in this regard. Further, the altered discharge can greatly influence the overall performance. In an atmospheric pressure dielectric barrier discharge reactor, this apparently more uniform plasma yields a significantly better performance for ammonia synthesis compared to the more conventional filamentary discharge, while it underperforms in dry reforming of methane. This study stresses the importance of analyzing the plasma discharge in plasma catalysis experiments. We hope this work encourages a more critical view on the plasma discharge characteristics when studying various catalysts in a plasma reactor.

A thermodynamic model of the surface hydroxylation of γ-Al2O3.

Rational design of γ-alumina-based catalysts relies on an extensive understanding of the distribution of hydroxyl groups on the surface of γ-alumina and their physicochemical properties, which remain unclear and challenging to determine experimentally due to the structural complexity. In this work, by means of DFT and thermodynamic calculations, various hydroxylation modes of γ-alumina (110) and (100) surfaces at different OH coverages were evaluated, based on which a thermodynamic model to reflect the relationship between temperature and the surface structure was established and the stable hydroxylation modes under experimental conditions were predicted. This enables us to identify the experimentally measured IR spectra. The effect of hydroxyl coverages on the surface Lewis acidity was then analyzed, showing that the presence of hydroxyl groups could promote the Lewis acidity of neighboring Al sites. This work provides fundamental insights into the molecular level understanding of the surface properties of γ-alumina and benefits the rational design of alumina-based catalysts.

Enhanced catalytic performance with adjustable morphology and surface properties of alumina for propane dehydrogenation: Promote the catalytically active chromium sites, dispersion and anti-coking

To examine the effect of the morphology of alumina on the catalytic performance during non-oxidative dehydrogenation of propane, a series of Al2O3 supports with specific morphologies including sheet-like, rod, and nucleoshell were synthesized using a template-free hydrothermal route at different temperatures. The activity results show that catalysts with special morphology have better catalytic performance than the conventional alumina-based catalysts. Cr-AlNC catalyst with nucleoshell morphology cause a significant growth of Cr6+ species compared to other catalysts. The reduction of this species results in the creation of the most reactive Cr sites that are coordinatively unsaturated in the PDH reaction. This reduction also prevents the transformation of Cr species into α-Cr2O3. Cr-AlNC catalyst show anti-coking ability and great resistance against deactivation due to their unique morphology and more importantly, the strong metal-support interaction in addition to the low deactivation rate and coke deposition of 0.1 h−1 and 1.5 %, respectively. However, the corresponding values for the reference catalyst are 0.44 h−1 and 6.1 %, respectively.

Ni alumina-based catalyst for sorption enhanced reforming - Effect of calcination temperature

Effect of calcination temperature (350 °C – 850 °C) on the physicochemical properties as well as catalytic performance of Ni-based catalyst for the hydrogen production via steam methane reforming (SMR) and sorption enhanced reforming (SER) has been investigated. Catalyst calcined at highest temperature (850 °C) shows formation of NiAl2O4 confirmed by XRD. Consequently, a much higher reduction temperature (875 °C) is required to reduce this Ni aluminate spinel to an active Ni catalyst. Catalyst calcined at highest temperature (850 °C) showed much higher CH4 conversion and H2 production in SMR compared to the catalysts calcined at lower temperatures. In addition, higher CH4 conversion and H2 production was observed for the 15%Ni/alumina_850 catalyst after aging (long exposure to steam and high temperature) compared to commercial reforming catalyst. Finally, a much better stability is observed for the 15%Ni/alumina_850 catalyst compared to the commercial reforming catalyst after 100 SER/regeneration cycles under relevant reaction conditions. The formation of NiAl2O4 during high temperature calcination plays a vital role in the robustness and stability of the Ni-based catalysts and can be useful synthesis procedure for increasing the catalyst lifetime.

Mechanism Study of Organic Sulfur Hydrogenation over Pt- and Pd-Loaded Alumina-Based Catalysts.

The hydrogenation of organic sulfur (CS2) present in industrial off-gases to produce sulfur-free hydrocarbons and H2S can be achieved by using noble-metal catalysts. However, there has been a lack of comprehensive investigation into the underlying reaction mechanisms associated with this process. In this study, we have conducted an in-depth examination of the activity and selectivity of Pt- and Pd-loaded alumina-based catalysts, revealing significant disparities between them. Notably, Pd/Al2O3 catalysts exhibit an enhanced performance at low temperatures. Furthermore, we have observed that CS2 displays a higher propensity for conversion to methane when employing Pt/Al2O3 catalysts, while Pd/Al2O3 catalysts demonstrate a greater tendency for coke deposition. By combining experimental observations with theoretical calculations, we revealed that the capability of H2 spillover along with the adsorption capacity of CS2, play pivotal roles in determining the observed differences. Moreover, the key intermediate species involved in the methanation and coke pathways were identified. The intermediate CH2S* is found to be crucial in the methanation pathway, while the intermediate CSH* is identified as significant in the coke pathway.

Pyrolysis of model plastic component polypropylene to hydrogen over mesoporous alumina catalysts: Effect of pluronic structure directing agents

Catalytic pyrolysis of the large and complex polymeric structure of plastic to gaseous fuel has been investigated in this work. A two-stage pyrolytic system is reported for cracking model waste plastic, polypropylene, over mesoporous alumina catalyst to produce H2 and CH4. The experiments have been conducted in the temperature range of 750–900 °C and various catalyst-to-plastic ratios under atmospheric pressure. The alumina-based catalysts were synthesized via the one-pot evaporation-induced self-assembly (EISA) method using four different structure-directing agents (SDA) namely PEG-1000, PEG-4000, P123, and F127 to understand the effect on catalytic properties and performance evaluation. Results revealed that the structured directing agents improve the catalyst's surface properties and affect the product yield. All synthesized catalysts were characterized for crystallinity, topography, and surface properties using X-ray diffraction analysis (XRD), Scanning Electron Microscope (SEM), Brunauer–Emmett–Teller (BET), Pyridine-Transmission Fourier transform infrared (Py-FTIR) spectroscopy, and atomic force microscopy (AFM) analysis. A significant effect of SDA was observed on the surface area of the synthesized mesoporous alumina (SMA) catalyst. The decreasing order in terms of the surface area of the SMA is SMAF127 >SMAP40 >SMAP123 >SMAP10> Alumina (without SDA). Approximately, a tenfold increase in surface area was observed with SMAF127 (mesoporous alumina) compared to non-mesoporous alumina. A maximum yield of hydrogen∼40.79 mmol/gram polypropylene was obtained with a catalyst-plastic (C/P) ratio of 1:2.5 at a temperature of 900 °C using SMAF127 owing to its high surface area, and other physiochemical properties.

Determination of platinum valence state in alumina based catalysts by X-Ray fluorescence spectrometry

A novel method for the estimation of the platinum valence state in alumina-based catalysts using the intensities of L- and M-series lines of X-ray fluorescence spectrum is proposed. The measurements of metallic platinum (Pt0) and potassium tetrachloroplatinate K2[PtCl4] (PtII) were carried out using wavelength-dispersive spectrometer. We considered the ratios of the most intense groups of closely spaced platinum X-ray fluorescence spectrum L-series lines (Lα2/Lα1, Lβ2/Lβ1, Lβ4,6/Lβ1), as well as the broadening of PtMα1,2 line as parameters influenced by the chemical bond. After decomposition using a pseudo-Voigt function, the ratio of the Lα1 and Lα2 peaks amplitudes for K2[PtCl4] was 0.118, for Pt0–0.162; the difference in the ratio of peak amplitudes in the group of Lβ lines is much smaller. For the broadening of Mα1,2 line, the largest intensity difference (25 rel.%) between Pt0 and K2[PtCl4] was observed at 2.044 keV. Both studied analytical parameters for set of alumina-based catalysts were closer to values for Pt0 than to values of K2[PtCl4] that indicates that platinum in the studied catalysts is in a state close to metallic, probably with a slight shift towards the ionic state.

Low temperature destruction of gas-phase per- and polyfluoroalkyl substances using an alumina-based catalyst

ABSTRACT Per- and polyfluoroalkyl substances (PFAS) pose a major health and environmental problem. Methods are needed to ensure that PFAS are not released into the environment during their use or disposal. Alumina-based catalysts have been used for the abatement of small perfluorocarbons, e.g. tetrafluoromethane and perfluoropropane, emitted during the silicon etching process. Here, an alumina-based catalyst was tested to determine if these catalysts may facilitate the destruction of gas-phase PFAS. The catalyst was challenged with two nonionic surfactants with eight fluorinated carbons, 8:2 fluorotelomer alcohol and N-Ethyl-N-(2-hydroxyethyl)perfluorooctylsulfonamide. The catalyst helped decrease the temperatures needed for the destruction of the parent PFAS relative to a thermal-only treatment. Temperatures of 200°C were sufficient to destroy the parent PFAS using the catalyst, although a significant number of fluorinated products of incomplete destruction (PIDs) were observed. The PIDs were no longer observed by about 500°C with catalyst treatment. Alumina-based catalysts are a promising PFAS pollution control technology that could eliminate both perfluorocarbons and longer chain PFAS from gas streams. Implications: The release of per- and polyfluoroalkyl substances (PFAS) into the atmosphere can cause problems for human health and the environment. It is critical to reduce and eliminate PFAS emissions from potential sources, such as manufacturers, destruction technologies, and fluoropolymer processing and application sites. Here, an alumina-based catalyst was used to eliminate the emissions of two gas-phase PFAS with eight fully fluorinated carbons. No PFAS were observed in the emissions when the catalyst was at 500°C, lowering the energy requirements for PFAS destruction. This shows that alumina-based catalysts are a promising area for research for PFAS pollution controls and the elimination of PFAS emissions into the atmosphere.

Role of the supports during phosphorus poisoning of diesel oxidation catalysts

Phosphorus (P) poisoning is one of the main factors accounting for the deactivation of diesel oxidation catalysts (DOC) apart from sulfur poisoning and sintering of the Pt active sites. This study compares the impact of P with loading up to 2.4 wt% on the catalytic performance of monometallic and bimetallic Pt-Pd catalysts using alumina and high silica BEA zeolites as the supports. P poisoning caused deactivation for CO, C3H6, C3H8 and NO oxidation; however, the degree of the impact of P in terms of temperatures at which 50% of the component is converted (T50) depends not only on the types of the active phase (Pt and Pt-Pd) but also on the types of supports (alumina and BEA zeolite). The influence of P impregnation on the textural properties of the materials is more significant for zeolite than alumina-based catalysts, which is in line with the activity measurements. A weak interaction between P and high silica zeolite resulted in the formation of a prominent fraction of P2O5 in the P-Pt/BEA, whereas a strong binding between P and alumina accounted for a dominant fraction of phosphate in the P-Pt/Al2O3 as revealed by XPS and NMR measurements. Phosphorus compounds partially covered the available surface of the active sites and this lowered the catalytic activity. For alumina-based catalysts, P mainly reacted with the support and only deactivated a part of the active noble metals. Whereas, for zeolite-based catalysts, P existed mainly in the form of phosphorus oxides that significantly blocked the catalyst surface and thereby deactivated more of the available active sites than that on alumina-based materials, which is consistent with the CO chemisorption data.

Alumina-supported cobalt catalysts in the hydrogenation of CO2 at atmospheric pressure

20 wt.% cobalt catalysts supported on pure and 5 wt.% silica-containing alumina have been prepared and characterized by X-Ray Diffraction, IR and DR-UV-vis-NIR spectroscopies and Field-Emission Scanning Electron Microscopy (FE-SEM). The presence of a cobalt-containing surface spinel phase Co3-xAlxO4 and, for the silica-containing sample, of a segregated Co3O4 phase is evident. These catalysts have been tested in CO2 hydrogenation at atmospheric pressure in steady state conditions in the temperature range 523–773 K. Both catalysts are active in CO2 hydrogenation to methane (methanation) and to CO (reverse Water Gas Shift, rWGS). CO2 hydrogenation activity is higher on freshly pre-reduced silica-free Co/Al2O3, while selectivity to methane is slightly higher on the silica-containing sample. Spent catalysts contain clustered or amorphous cobalt metal centers as active sites for methanation. The silica-containing catalyst shows slow deactivation in CO2 hydrogenation upon 13 h experiments, with quite stable or even slightly increasing rWGS activity and decreasing CH4 selectivity. This confirms previous data suggesting that, over cobalt catalysts, sites for methanation are metal centers prone to deactivation by carbon deposition. However, in contrast with what happens with unsupported and silica-supported cobalt catalysts, where deactivation is very fast, over these alumina-based catalysts carbon deposition and deactivation occur much more slowly. Sites for rWGS are unreduced cobalt centers which do not undergo such a deactivation phenomenon.

Rational design of active sites in alumina-based catalysts to optimize antibonding-orbital occupancy for tetrafluoromethane decomposition

Tetrafluoromethane (CF4) is a potent greenhouse gas with high stability, thus its effective decomposition is crucial for mitigating its environmental impact.

Effect of Basic Promoters on Porous Supported Alumina Catalysts for Acetins Production

A facile strategy for the design of porous supports was obtained by modifying the sol-gel method followed by the wet impregnation technique. In this respect, herein, the acidity of the γ-Al2O3 phase was modulated by adding basic MgO, La2O3 or ZnO promoters to form binary supported catalysts. The Ni and Co dispersion on the supports associated with their tunable acidity and morphologies resulted in highly porous supported alumina-based catalysts. The physicochemical properties of the solids were comprehensively investigated by XRD, textural properties, Raman and FTIR spectroscopy, SEM-EDS, TEM, EPR and XPS analyses. The catalytic performances in the esterification of glycerol in the presence of acetic acid (EG) for the acetins production were evaluated. The highly dispersed NiO and Co3O4 active species on binary porous supports produced synergistic effects appearing to be the reason for the activity of the solids in the EG reaction. Under the optimized reaction conditions, NiCo/MgO-Al2O3 was found to be a robust solid with superior catalytic performance and improved stability in four reaction cycles with 65.0% of glycerol conversion with an exclusive selectivity of 53% for triacetin. The presence of Co2+/Co3+ and Ni2+ strongly interacting with the spinel γ-Al2O3 and MgAl2O4 phases, the latter having a large number of lattice oxygen species, was considered another active component besides those of Ni and Co in the esterification of glycerol.

Mechanochemical synthesis of alumina-based catalysts enriched with vanadia and lanthana for selective catalytic reduction of nitrogen oxides.

Novel alumina-based materials enriched with vanadia and lanthana were successfully synthesized via in situ modification using a mechanochemical method, and were applied in ammonia-induced selective catalytic reduction of nitrogen oxides (SCR process). The synthesis was optimized in terms of the ball milling time (3 or 5h), vanadium content (0.5, 1 or 2 wt% in the final product), and lanthanum content (0.5 or 1 wt% in the final product). Vanadium (V) oxide was immobilized on an alumina support to provide catalytic activity, while lanthana was introduced to increase the affinity of nitrogen oxides and create more active adsorption sites. Mechanochemical synthesis successfully produced mesoporous materials with a large specific surface area of 279-337m2/g and a wide electrokinetic potential range from 60 to (-40)mV. Catalytic tests showed that the incorporation of vanadia resulted in a very large improvement in catalytic performance compared with pristine alumina, increasing its efficiency from 14 to 63% at 400°C. The best SCR performance, a 75% nitrogen oxide conversion rate at a temperature of 450°C, was obtained for alumina enriched with 2 and 0.5wt% of vanadium and lanthanum, respectively, which may be consideredas a promising result.

Advantageous Role of N-doping on K@Al in COS/CS2 Hydrolysis: Diminished Oxygen Mobility and Rich basic sites

Basic sites are important to promote carbonyl sulfide (COS) and carbon disulfide (CS2) hydrolysis activity while improving the oxidation resistance of hydrolysis catalysts is a challenge. Here, we report an N-doped alumina-based catalyst (N-K@Al), where the complete hydrolysis removal of COS and CS2 by the N-doped catalyst was extended up to 15 h at 160 °C, and the turnover frequency (TOF) value of the catalyst was increased from 0.85 × 10-2 s−1 to 1.16 × 10-2 s−1. The results showed that the strong interaction between nitrogen and potassium decreased the reactive oxygen density and activated –OH, weakening the oxygen mobility. In addition, the beneficial effects of N-doped included the addition of moderately and weakly basic sites by pyridine N. The optimized N-K@Al showed good catalytic performance in the hydrolysis of COS and CS2 removal with lower reaction temperature and higher total hydrogen sulfide yield. Density functional theory calculations revealed that N-K@Al reduced the Gibbs free energy of COS and CS2 hydrolysis to 0.21 eV and 0.33 eV, respectively, contributing to the enhanced catalytic activity. The present study presents new strategies for developing efficient antioxidant organosulfur hydrolyzers and other practical industries.

Effect of extrusion-spheronization granulation and manganese loading on catalytic ozonation of petrochemical wastewater.

The petrochemical secondary effluent (PSE) is typical refractory wastewater derived from the petrochemical industries, which requires advanced treatment due to the strict environmental protection policies. Catalytic ozonation is one of the most widely used advanced oxidation technologies in wastewater treatment because of its high mineralization rate, in which the alumina-based catalyst usually plays an important role. Extrusion-spheronization is a promising technique for the preparation of alumina spheres because the synthesized alumina particles have high sphericity, high specific surface aera and narrow particle size distribution. In this paper, two kinds of alumina-based catalysts (catalyst A: manganese nitrate added after alumina granulation and catalyst B: manganese nitrate added into alumina powder before granulation) were prepared by the extrusion-spheronization method and used for PSE treatment by catalytic ozonation. The prepared alumina samples were characterized by Brunauer-Emmett-Teller (BET) method, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the wastewater samples were analyzed for Total organic carbon (TOC), UV254 and fluorescence spectroscopy. Results showed that manganese was uniformly distributed in both catalysts, and the specific surface area of two catalysts was 318.36 m2/g and 354.95 m2/g, respectively. Catalytic ozonation experiments were repeated nine times with each catalyst under the same conditions. The TOC removal rates for catalysts A and B in the first run were 48.88% and 49.06%, respectively, then it dropped to 28.05% for catalyst A but remained 47.81% for catalyst B after using for nine times. This implied that the long-term performance of catalyst B would be more stable than catalyst A. Similar result were found in three-dimensional fluorescence analysis. UV254 results indicated that the removal efficiency of aromatic and unsaturated substances by catalyst B was higher than catalyst A. A possible explanation is that the active component manganese oxide formed a catalyst skeleton in catalyst B, which makes it hard to dissolve. Effect of extrusion-spheronization granulation and manganese loading on advanced oxidant treatment of petrochemical wastewater.

Experimental Comparison of Hydrogen Peroxide Catalysts for a Hydrogen -Decane Bipropellant Combustor

This paper presents an experimental comparison of the catalytic activity of different alumina-based catalysts for highly concentrated hydrogen peroxide (85% w.) decomposition. Three pellet-shaped catalysts with manganese oxide or platinum active phases were considered. Their performances were investigated by measuring the temperature increase in the catalytic bed after several injections of of hydrogen peroxide in a constant-volume chamber. This setup evidenced a loss of reactivity after several injections, which was due to water poisoning. Pellets were, then, tested in a small-scale catalytic chamber plugged into a combustor. Hydrogen peroxide was injected at various mass flow rates (from 4.5 to ) through the catalytic chamber. The temperature evolution at the exhaust was recorded via a thin thermocouple (1 mm) and used to compare the activity of the different catalysts. The specific surface area, the active phase amount, and the platinum dispersion were measured after the tests. Catalysts experienced a decrease of those values, evidencing the ageing of the catalytic material. Preliminary combustion tests were performed for all catalysts in a hydrogen bipropellant combustor. Results were compared to evaluate the influence of the efficiency of the catalytic system on combustion performance.

Low-temperature hydrolysis of carbonyl sulfide in blast furnace gas using Al2O3-based catalysts with high oxidation resistance

Water vapor, O2 and various sulfides in blast furnace gases affect the carbonyl sulfide (COS) hydrolysis reaction. In this study, we investigated the hydrolysis and deactivation mechanisms of a series of catalysts that were synthesized by depositing K2O and MoO3 on industrial alumina-based catalysts. Alumina nanosheets were deposited with potassium oxide, in which MoO3 nanoparticles were incorporated resulting in needle-like structures. The modified catalysts had substantially enhanced COS hydrolysis activity at low temperatures. The introduction of K2O and MoO3 significantly increased the number of weakly basic sites on the catalysts and reduced the lattice oxygen content. The catalysts exhibited high oxidation resistance and hydrolysis properties. According to the poisoning mechanism, the deposition of sulfate species tended to reduce weakly basic sites. High-valent Mo easily excited the adsorbed H2O on the catalysts, producing –OH groups that facilitated the COS hydrolysis reaction. This modification facilitates the further investigation of catalysts capable of promoting COS hydrolysis and their application in the manufacturing industry.