Alumina-supported Catalysts Research Articles

Management of γ-Alumina with High-Efficient {111} External Surfaces for HDS Reactions

A series of γ-alumina samples with different exposure ratio of {111} facet were synthesized by an efficient hydrothermal method via adjusting the pH value of the gel precursor. The nanorod alumina supported catalyst with the highest exposure of {111} facet exhibited the best hydrodesulfurization (HDS) activities of both thiophene and dibenzothiophene (DBT). Characterization of the sulfided NiMo/Al2O3 catalyst with preferential exposure of {111} facet showed that the MoS2 nano slabs were inclined to distribute in the direction along the edges of alumina nanocrystal in reduced stacking layers. The selective exposure of {111} facet played a decisive role in obtaining alumina-supported HDS catalysts with improved intrinsic activity. This work helps to better understand the relationship between catalytic properties and varied support surfaces, which demonstrate a proper design of the catalyst support morphology on the facet-level.

Conversion of ethane to ethylene and hydrogen by utilizing carbon dioxide: Screening catalysts

Owing to the necessity of carbon dioxide conversion and exploring new routes for ethylene and hydrogen production, carbon-dioxide-assisted dehydrogenation over alumina-supported catalysts is evaluated in the present contribution. In this regard, the experimental results of a wide range of catalysts at 700 °C and the GHSV of 3600 Lreactants/kgcatalyst.hr are presented and compared. The utilized catalysts showed activity toward the conversion of ethane. However, a few of them showed good selectivity for the production of either ethylene or hydrogen. The catalysts made up of the oxides of cobalt and molybdenum showed very good conversion, selectivity, and yield for ethylene production. Investigating the effect of time on steam on the catalyst performance indicated that these catalysts would be suitable choices in the course of ethylene production. 58% conversion of ethane and 21.9% ethylene yield are the achievements of utilizing the molybdenum-cobalt oxide catalyst. By utilizing rhenium-platinum nickel-potassium catalysts, 100% ethane conversion in tandem with more than 260.0% hydrogen yield was also obtained.

Effect of Silica, Activated Carbon, and Alumina Supports on NiMo Catalysts for Residue Upgrading

The effect of different supports such as silica (SBA-15), activated carbon (AC), and mesoporous alumina (Al2O3) on catalytic activities of hydrotreating nickel molybdenum (NiMo) catalysts was demonstrated for upgrading vacuum residue. Nitrogen adsorption-desorption analysis showed that SBA-15 and the AC-supported NiMo catalyst possessed a very high surface area compared to the alumina-supported catalyst. However, NiMo/Al2O3 catalyst possesses a higher pore diameter and pore volume with an appropriate surface area. X-ray diffraction (XRD) analysis showed that active metals were dispersed in the catalytic supports. Transmission electron microscopy (TEM) analysis revealed the presence of type II active MoS2 sites in the NiMo/Al2O3 catalyst, which showed weak metal-support interactions having a high intrinsic activity. Catalyst activities such as hydrodesulfurization (HDS), hydrodemetallization (HDM) and asphaltene conversion (HDAs), and hydrocracking conversions of a vacuum residue were evaluated. The highest hydrotreating and hydrocracking conversions were observed with the NiMo catalyst supported on mesoporous alumina. The results also supported that the catalyst that has a large pore diameter, high pore volume, and better active metals dispersion is highly desirable for the upgrading of a vacuum residue.

Evidence of Octahedral Co–Mo–S Sites in Hydrodesulfurization Catalysts as Determined by Resonant Inelastic X-ray Scattering and X-ray Absorption Spectroscopy

Fundamental understanding of the active sites in CoMo/Al2O3 catalysts is crucial to improve the production of clean transportation fuels by hydrodesulfurization (HDS). In this work, the coordination state and number of active sites (“Co–Mo–S”) in sulfided CoMo/Al2O3 catalysts prepared with and without chelating additives were determined using in situ 1s2p resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS). Based on the evaluation of thiophene HDS activity as a function of the Co/Mo ratio, it was determined that only for Co/Mo < 0.1, cobalt is exclusively in interaction with the MoS2 phase, making these compositions ideal for identifying the nature of the active cobalt sites for hydrogenolysis. These cobalt promoter atoms were in centrosymmetric coordination with the MoS2 phase, with six sulfur ligands coordinated to each cobalt center. The octahedral coordination of cobalt in Co–Mo–S in alumina-supported HDS catalysts contrasts the tetrahedral cobalt coordination found in Co-promoted MoS2 model systems supported on gold or carbon. We attribute the difference to more disordered MoS2 structures in technical alumina-supported HDS catalysts. The number of Co–Mo–S sites depended strongly on the catalyst preparation method. Chelating additives, such as citric acid and nitrilotriacetic acid, increased the number of Co–Mo–S sites without significantly altering their intrinsic activity. These results demonstrate that the state-of-the-art structural characterization tools, such as RIXS, identify the structure of active sites in actual catalysts and that these structures may be different from model systems. Combining such characterization tools with guided catalyst design can provide an insight into the structure of amorphous, nanostructured materials, which aid in the development of improved heterogeneous catalysts. KEYWORDS:Hydrodesulfurization Co−Mo−S RIXS active site XAS

CO2 hydrogenation to liquid hydrocarbons via modified Fischer–Tropsch over alumina-supported cobalt catalysts: Effect of operating temperature, pressure and potassium loading

The effect of promoting 15%Co/Al2O3 catalyst with potassium on CO2 hydrogenation to longer-chain hydrocarbons was investigated. The catalysts used in this study were synthesized using an incipient wetness impregnation of the support with nitrate solutions. All catalysts were supported on γ-alumina and promoted with potassium (0 – 8%). The synthesized catalysts were characterized by XRD, BET, XPS, TPR and CO2-TPD analyses. The catalysts were evaluated for CO2 hydrogenation using a fixed-bed tube reactor. The CO2 conversion was found to increase with both the reaction temperature and pressure. The TPR data revealed that potassium limited the reduction of the catalyst, decreased the selectivity to methane and increased the selectivity to C2+ hydrocarbons. The maximum C2+ yield of 10.2%, with CO2 conversion of 42.3%, was obtained when 6 wt.% of potassium was added to the catalyst. It is believed that during the CO2 hydrogenation process over the catalysts that were promoted with potassium, CO2 is first converted to CO via reverse–water–gas–shift reaction, followed by subsequent hydrogenation of CO to hydrocarbons.

Synergistic effects of Pt-embedded, MIL-53-derived catalysts (Pt@Al2O3) and NaBH4 for water-mediated hydrogenolysis of biomass-derived furfural to 1,5-pentanediol at near-ambient temperature

We demonstrate an effective and selective conversion of biomass-derived furfural (FAL) to 1,5-pentanediol (1,5-PD) with high yields through a water-mediated hydrogenolysis process under mild reaction conditions (45 °C, aqueous media). A novel alumina-supported platinum catalyst (Pt@Al2O3) with high-loading and uniform distribution of Pt nanoparticles is prepared through in situ synthesis of Pt-embedded metal–organic frameworks (i.e., MIL-53(Al)-NH2). As a typical example, a high yield of 75.2% 1,5-PD can be achieved from FAL conversion. A possible reaction mechanism is proposed based on the experimental and computational findings, including XPS analysis, kinetic studies, acidity measurements, and density functional theory (DFT) calculations. The high effectiveness of the proposed system is attributed to (1) the strong metal support interaction (SMSI) between Pt and penta-coordination aluminum, and (2)the synergistic effects of Brønsted acidic alumina support and the presence of sodium metaborate (NaBO2). Sodium borohydride (NaBH4) acts as both a hydrogen donor and a precursor for NaBO2, which results in an exclusive FAL to 1,5-PD (i.e., no 1,2-pentanediol) by regulating the water-mediated hydrogenolysis pathway as revealed by experiments and DFT calculations. The reaction strategy proposed in this study has also manifested remarkable versatility for a wide range of furan derivatives.

Comparative Study of Strategies for Enhancing the Performance of Co3O4/Al2O3 Catalysts for Lean Methane Combustion

Spinel-type cobalt oxide is a highly active catalyst for oxidation reactions owing to its remarkable redox properties, although it generally exhibits poor mechanical, textural and structural properties. Supporting this material on a porous alumina can significantly improve these characteristics. However, the strong cobalt–alumina interaction leads to the formation of inactive cobalt aluminate, which limits the activity of the resulting catalysts. In this work, three different strategies for enhancing the performance of alumina-supported catalysts are examined: (i) surface protection of the alumina with magnesia prior to the deposition of the cobalt precursor, with the objective of minimizing the cobalt–alumina interaction; (ii) coprecipitation of cobalt along with nickel, with the aim of improving the redox properties of the deposited cobalt and (iii) surface protection of alumina with ceria, to provide both a barrier effect, minimizing the cobalt–alumina interaction, and a redox promoting effect on the deposited cobalt. Among the examined strategies, the addition of ceria (20 wt % Ce) prior to the deposition of cobalt resulted in being highly efficient. This sample was characterized by a notable abundance of both Co3+ and oxygen lattice species, derived from the partial inhibition of cobalt aluminate formation and the insertion of Ce4+ cations into the spinel lattice.

The Application of Attenuated Total Reflection Infrared Spectroscopy to Investigate the Liquid Phase Hydrogenation of Benzaldehyde Over an Alumina-Supported Palladium Catalyst

The hydrogenation of benzaldehyde in cyclohexane over a 5 wt% Pd/Al2O3 catalyst at 313 K is firstly investigated at ambient pressure in a stirred batch reactor. The formation of benzyl alcohol is a facile process and a small mass imbalance is indirectly attributed to the formation of benzene as a by-product. No hydrogenolysis reaction to form toluene is observed. Secondly, examination of this reaction system by attenuated total reflection infrared (ATR-IR) spectroscopy enables the chemistry at the liquid/solid interface to be probed. Specifically, the ν(C=O) modes of solvated and adsorbed benzaldehyde are evident at 1712 and 1691 cm−1 respectively, providing information on how the reagent is partitioning within the reaction medium. Spectral acquisition on initiation of hydrogenation then enables the benzaldehyde → benzyl alcohol transition to be tracked. The additional presence of a broad CO stretching band of chemisorbed carbon monoxide (1852–1929 cm−1) is attributed to the hydrogen-assisted decarbonylation pathway that forms the benzene by-product.

On the nature of active sites in alumina-supported zinc propane dehydrogenation catalysts

Zinc-based materials are currently considered as an alternative for toxic chromium and costly platinum catalysts for propane dehydrogenation. So far however insufficient attention has been devoted to these materials in the literature. In this work we have revealed the nature of active sites in zinc-alumina (γ-alumina) catalysts. We have investigated zinc-alumina catalysts prepared by two different methods: traditional incipient wetness impregnation and zinc chemical vapor deposition (CVD) on alumina. The catalysts were characterized by infrared spectroscopy of adsorbed CO and H2 molecules, TPR measurements and tested in propane dehydrogenation. It has been shown that high-temperature interaction of zinc vapor with alumina results in the formation of surface Zn2+ cations which are active in hydrogen dissociation and propane conversion. The amount of zinc that can be introduced by this procedure depends on the alumina dehydroxylation degree. It has been found that molecular hydrogen heterolytically dissociates on Zn2+ surface cations resulting in the formation of surface zinc hydride species. The Zn2+ sites on the surface of alumina are heterogeneous and are characterized by different band positions in the IR spectra of adsorbed CO and H2 molecules. The Zn2+ sites are subjected to reduction under the reaction conditions.

Transformation of alumina-supported Pt-Au alloyed nanoparticles into core-shell Pt@Au structures during high-temperature treatment

A series of alumina-supported bimetallic AuPt catalysts was prepared by decomposition of a double complex salt containing simultaneously gold and platinum. The decomposition was performed in both the reductive and oxidative atmospheres. Monometallic Au/γ-Al2O3 and Pt/γ-Al2O3 catalysts were used as reference samples. Physicochemical characterization of the catalysts has revealed that despite the preparation route and total metal loading, the alloyed nanoparticles are being formed with close values of the average particle size and similar particle size distribution, while the composition and the structure of the particles differ significantly. Thermal aging of the catalysts at 600 and 800 °C leads to redistribution of metals in the alloyed particles and formation of core-shell Pt@Au nanostructures, which results in significant deactivation of the catalysts concerning a model reaction of CO oxidation. The formation of core-shell particles in the case of high-loaded samples was found to prevent the agglomeration process at elevated temperatures.

Catalytic reduction of nitrate in water over alumina-supported nickel catalyst toward purification of polluted groundwater

Pollution of groundwater with NO3– is a serious problem in the world. While catalytic reduction of NO3– over Pd-bimetallic catalysts including Cu-Pd and Sn-Pd is a promising method for purification of the groundwater, the use of precious metal is a major obstacle for practical applications. In the present study, we applied Ni/Al2O3 for the catalytic reduction of NO3– and compared the catalytic performance with that of unsupported Ni catalyst. The reaction rate over 5 wt.% Ni/Al2O3 was about 5 times higher than that of the unsupported Ni catalyst, based on unit weight of catalyst. While the unsupported Ni catalyst was completely deactivated in low partial pressure of H2 (=0.75 atm) and high concentration of NO3– (=800 ppm), Ni/Al2O3 was still active even under less reductive conditions ([NO3–]0 = 800 ppm and P(H2) =0.5 atm). The unsupported Ni catalyst had the Ni0 particles formed by the reduction of NiO with H2 at 310–420 °C. On the other hand, 5 wt.% Ni/Al2O3 possessed the Ni0 particles formed from NiAl2O4 on Al2O3 by the reduction with H2 above 450 °C. It is plausible that those Ni0 particles had different properties, giving different catalytic properties. The Ni loadings for Ni/Al2O3 had a significant impact on the catalytic properties. The reaction orders with respect to both NO3– and H2 were 0.8 for 5 wt.% Ni/Al2O3, while those were 0 and –0.2, respectively, for 10 wt.% Ni/Al2O3. On 10 wt.% Ni/Al2O3, there were two kinds of the Ni° particles, which were formed by low (310-420 °C) and high (above 450 °C) temperature H2 reductions. Unlike the Pd-bimetallic catalysts, the reduction of NO3– over Ni/Al2O3 did not proceed through NO2–.

Synthesis of Dimethyl Ether via CO2 Hydrogenation: Effect of the Drying Technique of Alumina on Properties and Performance of Alumina-Supported Copper Catalysts.

Thermal treatment during catalyst preparation is one of the important factors affecting the characteristics and performance of a catalyst. To improve the catalytic performance of an alumina-supported copper catalyst prepared by an impregnation method for dimethyl ether (DME) synthesis from CO2, the effects of the use of hot air and infrared drying as well as calcination at 600 and 900 °C to prepare alumina supports were investigated. Infrared drying could shorten the required drying time by 75% when compared with hot air drying. Infrared drying could also help maintain the pore size and pore volume of the supports, leading to their larger surface areas. Different drying techniques were additionally noted to result in different sizes and shapes of the pores as well as to different copper distributions and intensities of acid sites of the catalyst. An increase in the calcination temperature resulted in a decrease in the surface area of the supports because of particle aggregation. The drying technique exhibited a more significant effect than calcination temperature on the space-time yield of DME. A catalyst utilizing the support prepared by infrared drying and then calcined at 600 °C exhibited the highest yield of DME (40.9 gDME kgcat–1 h–1) at a reaction temperature of 300 °C. Stability of the optimal catalyst, when monitored over a 24 h period, was noted to be excellent.

Supported palladium nanoclusters: morphological modification towards enhancement of catalytic performance using surfactant-assisted metal deposition

In the present study, alumina-supported palladium catalysts were prepared by modified electroless deposition (ED) method in presence of surfactants and their performance was significantly improved compared to that prepared in absence of surfactants. Depending on the type and concentration of surfactant, loading and morphology of deposited palladium varied. Both anionic (sodium dodecyl sulphate) and non-ionic (Tween 20) surfactants were observed to be the most effective in dispersing the metals in the precursor solution. Average metal cluster sizes obtained for catalysts prepared by wetness impregnation, modified electroless deposition and surfactant-assisted deposition method were 11.89, 4.6 and 1.18 nm, respectively. The conversion of butane and selectivity to butene was observed to be function of size of deposited Pd cluster. The conversion and selectivity towards butenes increased with the decreasing particle size. The SDS surfactant-assisted prepared catalyst, having the lowest metal cluster size (1.18 nm), showed the highest activity (33% conversion at 600 °C) and over 99% selectivity for butenes.

Role of the Various Surface Sites and Species in CO Hydrogenation Over Alumina-supported Co-Pd Catalysts

Role of the Various Surface Sites and Species in CO Hydrogenation Over Alumina-supported Co-Pd Catalysts

Dry reforming of methane over alumina-supported rhodium catalysts at low temperatures under visible and near-infrared light

An alumina-supported metallic Rh catalyst (Rh/Al2O3) was found to be an effective catalyst in photo-assisted dry reforming of methane at around 473 K under visible and near-infrared light.

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalyst (Cu4/Al2O3) by means of Density Functional Theory

The adsorption and activation of carbon dioxide over copper cluster (Cu4) and copper doped on the alumina support (Cu4/Al2O3) catalytic systems have been investigated by using density functional theory and climbing image nudged elastic band. The adsorption energies, geometrical configurations, and electronic properties are analysed. The results show the strong chemical interaction between the copper cluster and the alumina support. Both the Cu4 cluster and Cu4/Al2O3 systems have a high adsorption ability for CO2, and the adsorption process is of chemical nature. The role of the alumina support in the adsorption and activation of CO2 has been addressed. The calculated results show that the “synergistic effect” between Al2O3 and Cu4 is the key factor in the activation of CO2.

Unraveling the Active Site and Mechanism for C–S Bond Activation in Alumina-Supported Pt Catalysts: Ab Initio Insights into Catalytic Desulfurization

Understanding the active sites of metal/oxide catalysts is of great significance in heterogeneous catalysis. In this work, we performed periodic density functional theory calculations to study the ...

Structure and Catalytic Behavior of Alumina Supported Bimetallic Au-Rh Nanoparticles in the Reduction of NO by CO

Alumina-supported bimetallic AuRh catalysts, as well as monometallic reference catalysts, were examined with regard to their structural and catalytic properties in the reduction of NO by CO. Depending on the molar ratio of Au:Rh, the nanoparticles prepared by borohydride co-reduction of corresponding metal salt solutions had a size of 3.5–6.7 nm. The particles consisted of well-dispersed noble metal atoms with some enrichment of Rh in their surface region. NO conversion of AuRh/Al2O3 shifted to lower temperatures with increasing Rh content, reaching highest activity and highest N2 selectivity for the monometallic Rh/Al2O3 catalyst. This behavior is attributed to the enhanced adsorption of CO on the bimetallic catalyst resulting in unfavorable cationic Rh clusters Rh+-(CO)2. Doping with ceria of AuRh/Al2O3 and Rh/Al2O3 catalysts increased the surface population of metallic Rh sites, which are considered most active for the reduction of NO by CO and enhancement of the formation of intermediate isocyanate (-NCO) surface species and their reaction with NO to form N2 and CO2.

Cooperative Catalysis of Methane Oxidation through Modulating the Stabilization of PdO and Electronic Properties over Ti-Doped Alumina-Supported Palladium Catalysts.

Poor low-temperature catalytic activity and durability are the main drawbacks of palladium-based catalysts for methane combustion. Herein, stable and active PdO particles are constructed by incorporating Ti into an alumina support, which makes the catalysts exhibit satisfactory methane combustion activity. The results of comprehensive characterization reveal that an appropriate amount of Ti doping induces the optimization of electron transfer and distribution, thus contributing to the construction and stabilization of active PdO lattices. The reactive oxygen mobility is improved and the optimal PdO/Pd0 combination is achieved, thanks to the amplified PdO–support interaction. In addition, the acid–base properties are regulated and Brønsted acid sites are generated by virtue of the adjustment of electronic properties, which facilitate stabilization of PdO as well. Hence, the Ti-containing catalyst exhibits superior activity for methane oxidation at low temperatures. Notably, the activity and cyclic performance of the catalyst can be further enhanced when undergoing long-term and isothermal heat treatment under the reactant stream and methane, and it demonstrates a high performance with 90% CH4 conversion at 340 °C.

Structure-sensitive hydro-conversion of oleic acid to aviation-fuel-range-alkanes over alumina-supported nickel catalyst

A highly active and stable γ-Al2O3-supported nickel nanoparticles catalyst (prepared by atomic layer deposition, ALD) was employed for the hydro-conversion of oleic acid to aviation-fuel-range-alkanes. The activity evaluation and characterization results suggested that this process is a structure-sensitive one for which step sites are more active than terrace sites. Ni(ALD)/Al2O3 catalyst enriched in Ni step sites thus presented high deoxygenation and hydrogenolysis activity. Additionally, Ni(ALD)/Al2O3 catalyst displayed high stability with only 6.5% drop in conversion after 10 h run owing to the development of strong metal-support interactions. After regeneration, the catalytic activity was recovered completely.