Alumina-supported Nickel Catalysts Research Articles

Deactivation of Ni/\u03b3-Al2O3 Catalysts in CO Methanation: Effect of Zr, Mg, Ba and Ca Oxide Promoters

Catalyst deactivation is one of the major concerns in the production of substitute natural gas (SNG) via CO methanation. Catalysts in this application need to be active at low temperatures, resistant to polymeric carbon formation and stable at high temperatures and steam partial pressures. In the present work, a series of alumina-supported nickel catalysts promoted with Zr, Mg, Ba or Ca oxides were investigated. The catalysts were tested under low temperature CO methanation conditions in order to evaluate their resistance to carbon formation. The catalysts were also exposed to accelerated ageing conditions at high temperatures in order to study their thermal stability. The aged catalysts lost most of their activity mainly due to sintering of the support and the nickel crystallites. Apparently, none of these promoters had a satisfactory effect on the thermal resistance of the catalyst. Nevertheless, it was found that the presence of Zr can reduce the rate of polymeric carbon formation.

Hydrogen production on alumina-supported platinum catalysts

Hydrogen production is commercially performed by methane steam reforming over alumina-supported nickel catalysts, which deactivate by coke requiring improved catalysts. Aiming to find alternative catalysts for the reaction, the properties of Pt/Al2O3-MgO catalysts were studied in this work. It was found that small amounts of magnesium (Al/Mg (molar)=5) enter into alumina lattice and produce magnesium aluminate on the surface. For higher amounts (Al/Mg=2) only magnesium aluminate is produced. For even higher contents (Al/Mg=0.2) aluminum enters into magnesia lattice, producing magnesia and magnesium aluminate on the surface. The specific surface area, acidity and platinum dispersion changed with magnesium amount, the aluminum-richest catalyst (Al/Mg=5) showing the highest values. Consequently, the activity and selectivity of the catalysts change in methane steam reforming and in water gas shift reaction, different values of products yield and H2/CO being obtained. The sample with Al/Mg=5 is the most promising catalyst to produce hydrogen.

A comparative study of alumina-supported Ni catalysts prepared by photodeposition and impregnation methods on the catalytic ozonation of 2,4-dichlorophenoxyacetic acid

The heterogeneous catalytic ozonation on unsupported and supported oxides has been successfully tested for the removal of several refractory compounds in aqueous solution. In this work, alumina-supported nickel catalysts prepared by photodeposition and impregnation methods were compared in the catalytic ozonation of 2,4-dichlorophenoxyacetic acid (2,4-D). The catalysts were characterized by high-resolution electron microscopy and X-ray photoelectron spectroscopy. The photochemical decomposition of Ni acetylacetonate to produce Ni(OH)2, NiO, and traces of Ni° deposited on alumina was achieved in the presence of benzophenone as a sensitizer. A similar surface composition was found with the impregnated catalyst after its reduction with hydrogen at 500 °C and exposed to ambient air. Results indicated a higher initial activity and maleic acid (byproduct) concentration with the photodeposited catalyst (1 wt% Ni) compared to the impregnated catalyst (3 wt% Ni). These findings suggest the use of the photodeposition method as a simple and reliable procedure for the preparation of supported metal oxide/metal catalysts under mild operating conditions.

Reaction mechanism and kinetic modeling for the hydrodeoxygenation of triglycerides over alumina supported nickel catalyst

The present work provides a systematic study to delineate the reaction mechanism and develop a mechanistic kinetic model for the hydrodeoxygenation (HDO) of triglycerides (TG) over alumina supported nickel catalyst. The HDO of 1:2 molar mixtures of tripalmitin and tristearin was studied in a batch reactor over a wide range of process conditions. The results showed that TG instantaneously converted to respective fatty acids. The fatty acids further converted to the fatty aldehydes. The fatty aldehydes, then, rapidly converted to alkanes by two parallel reaction pathways. The decarbonylation of fatty aldehyde (RP-I) was the dominating route compared to the reduction of the fatty aldehyde to fatty alcohol followed by its dehydration and hydrogenation (RP-II). A mechanistic kinetic model was developed based on the observed reaction pathway to correlate the experimental results. The rate constants for the conversion of palmitic acid and stearic acid to alkanes were matched closely with each other thereby demonstrating that HDO is independent of fatty acid chain length. The developed kinetic model was further validated using experimental data at various hydrogen-to-nitrogen mole ratios in the gas phase. Furthermore, the rate constants obtained for various catalyst loadings were correlated by a linear equation with zero intercept.

Synthesis Gas Production via the Biogas Reforming Reaction Over Ni/MgO–Al2O3 and Ni/CaO–Al2O3 Catalysts

The energetic utilization of biogas, a gas mixture consisting mainly of CH4 and CO2 via the reforming or the dry reforming of methane reaction is of enormous interest as it converts these two greenhouse gases into synthesis gas (H2/CO mixtures). Nickel based catalysts have been extensively studied for both reactions, as they are highly active, but they suffer from fast deactivation by coking that can even lead to reactor blocking. It is thus desirable to learn more about their coking behavior, and their structural and catalytic stability. In this work, un-promoted and promoted with 6.0 wt% MgO or CaO alumina supported nickel catalysts (8.0 wt% Ni) were studied for the biogas reforming reaction. Supported nickel catalysts were synthesized following the wet impregnation method. The as synthesized Ni/Al2O3, Ni/MgO–Al2O3, Ni/CaO–Al2O3 samples were characterized by various techniques such as XRD, SEM, ICP and BET. Catalytic testing experiments were performed in a fixed-bed reactor at temperatures ranging from 500 to 850 °C and a feed gas mixture with a molar CH4/CO2 ratio of 1.5 simulating an ideal model biogas. It was concluded that the Ni/MgO–Al2O3 and Ni/CaO–Al2O3 catalysts exhibit higher values for XCH4, XCO2, YH2 compared to the ones of the Ni/Al catalyst for temperature ranging between 550 and 750 °C, while the opposite is evidenced for T > 750 °C. It was also evidenced that the presence of magnesium or calcium oxide in the support ensures a quite stable H2/CO molar ratio approaching to unity (ideal for the produced syngas) even for low reaction temperatures.

Hydrogenation of Liquid Styrene by Alumina Supported Nickel Catalysts: Comparison between Classical and Non-Classical Methods

Almina supported Ni catalysts (Ni/Al2O3) with different Ni weight percentages (wt%) were prepared via classical and non-classical methods. All samples were prepared via impregnation technique. The samples prepared via non-classical methods were reduced using KBH4 as the reducing agent. The catalysts were tested for the hydrogenation of styrene in liquid phase. Optimum activation conditions for the hydrogenation reaction were found to be 633 K for 2 hours. Comparison of the catalytic reactivity for all catalysts at these activation conditions showed that catalysts prepared via classical methods exhibited better activity. Furthermore the 7.6wt% Ni-Al2O3/C showed enhanced activity when compared to the 5.9wt% and 13.8wt% Ni-Al2O3/C catalyst. This phenomenon is mainly attributed to the type of Ni active sites available on the catalyst. The surface properties of the catalysts investigated via H2- temperature programmed reduction (H2-TPR), H2-chemisorption and H2-temperature programmed desorption (H2-TPD) confirm this.

A study on the kinetics of syngas production from glycerol over alumina-supported samarium–nickel catalyst

The current paper reports on the kinetics of syngas production from glycerol pyrolysis over the alumina-supported nickel catalyst that was promoted with samarium, a rare earth element. The catalysts were synthesized via wet-impregnation method and its physicochemical properties were subsequently characterized. Reaction studies were performed in a 10 mm-ID stainless steel fixed bed reactor with reaction temperatures maintained at 973, 1023 and 1073 K, respectively, employing weight-hourly-space-velocity of 4.5 × 104 ml g−1 h−1. The textural property examination showed that BET specific surface area was 2.09 m2 g−1 for the unpromoted catalyst while the samarium promoted catalyst has 2.68 m2 g−1. Interestingly, the results were supported by the FESEM images which showed that the promoted catalyst has smaller particle size compared to the unpromoted catalyst. Furthermore, the NH3- and CO2-TPD analyses proved that the strong and weak acid-basic sites were present. During glycerol pyrolysis, the syngas was produced directly from the glycerol decomposition. This has created H2:CO ratios that were always lower than 2.0, which is suitable for Fischer-Tropsch synthesis. The activation energy based on power law modeling for the unpromoted catalyst was 35.8 kJ mol−1 and 23.4 kJ mol−1 for Sm-promoted catalyst with reaction order 1.20 and 1.10, respectively. Experimental data were also fitted to the Langmuir–Hinshelwood model. Upon subjected to both statistical and thermodynamics consistency criteria, it can be conclusively proved that single-site mechanisms with associative adsorption of glycerol best describe the glycerol pyrolysis over both unpromoted and Sm promoted catalyst in the current work, with regression coefficient values of more than 0.9.

Benzene hydrogenation over alumina-supported nickel nanoparticles prepared by polyol method

The reactivity of alumina-supported nickel catalysts can be improved by storing hydrogen in catalysts. This illustrates that a smooth correlation exists between the amount of stored hydrogen in catalysts and the catalytic activity.

Influence of structural and textural parameters of carbon nanofibers on their capacitive behavior

Herringbone, platelet, and tubular carbon nanofibers (CNFs) were synthesized by catalytic chemical vapor deposition using methane, propane, and ethylene as carbon precursors. Alumina-supported nickel and iron catalysts were used for the syntheses. The resultant CNFs were characterized by scanning electron microscopy, transmission electron microscopy, and nitrogen sorption at 77 K. The performance of a CNF-based supercapacitor working in 6 mol L−1 KOH was analyzed using cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy techniques. The Brunauer–Emmett–Teller (BET) surface area of the CNFs ranged between 150 and 296 m2 g−1. An increase in the CNF diameter was accompanied by a decrease in the BET surface area. A comparison of the porous textures and the structure types of the CNFs demonstrated that the performance of the CNF-based supercapacitor is enhanced primarily by the exposed edges of the graphitic layers on the CNF surface, followed by the specific surface area. Among the studied CNFs, the highest capacitance value, 26 F g−1 at 0.2 A g−1, was obtained for the platelet-type CNFs. Tubular CNFs exhibited the lowest capacitance value, which increased from 4 to 33 F g−1 at 0.2 A g−1 upon air treatment at 450 °C. The presence of exposed graphitic edges on the air-treated CNT surface and an increase in the specific surface area are considered to be responsible for the enhancement of the capacitor performance.

Promotion effect of rare-earth elements on the catalytic decomposition of ammonia over Ni/Al2O3 catalyst

Ammonia decomposition has attracted much attention as an efficient method for the on-site generation of hydrogen. In this study, alumina-supported nickel catalysts (Ni/Al2O3) modified by rare-earth elements were prepared by the impregnation method and their catalytic activity for the ammonia decomposition was investigated. The addition of rare-earth elements promoted the decomposition reaction over catalysts and the La-modified catalyst achieved the highest ammonia conversion in this work. For the modified catalysts, the adsorbed hydrogen, which is known to be an inhibitive species for the ammonia decomposition, desorbed at lower temperature compared to the unmodified one. Therefore, the effective alleviation of hydrogen inhibition would be responsible for the activity enhancement for the modified catalysts. The reaction kinetics study also supported this proposed mechanism. For the La-modified catalyst, the optimal pretreatment condition was investigated to enhance the catalytic activity. The catalyst calcined at 400°C followed by reduction at 600°C exhibited the highest ammonia conversion of 94% at 550°C.

DRIFTS study of a commercial Ni/γ-Al2O3 CO methanation catalyst

Diffuse reflectance infrared spectroscopy (DRIFTS) coupled to mass spectrometry (MS) is combined with the concentration modulation approach and phase sensitive detection (PSD) to follow CO methanation on a commercial alumina supported nickel catalyst. The modulation approach helps improving the sensitivity of DRIFTS under reaction conditions for adsorbed species that may be involved in the relevant reaction steps. Beside resident adsorbed CO species populating the surface of the catalyst during reaction at 200 and 300°C, carbonyl species are identified at 200°C that seem to indicate the sites where CO dissociation takes place. Such species are associated with sub-carbonyls or CO adsorbed on low coordinated Ni atoms. The product distribution observed during modulation of 12CH4 concentration in a 13CO/D2 feed at 300°C suggests that atomic C and H produced by CO and H2 dissociation on Ni during methanation and CH species may recombine to afford the methane product. However, the experiments do not help resolving whether adsorbed CO can be directly hydrogenated thus leaving open an important mechanistic issue.

Effect of metal–support interactions in Ni/Al2O3 catalysts with low metal loading for methane dry reforming

Nickel catalysts prepared by a variety of different methods are commonly used for reforming reactions such as methane dry reforming. Two preparation methods, controlled adsorption and dry impregnation, are implemented to explore the effect of preparation method on the formation of active sites on alumina supported nickel catalysts. By varying only the preparation method, comparison of catalysts that differ primarily in metal–support interactions, strong metal–support interaction (controlled adsorption) and weak metal–support interactions (dry impregnation), are obtained. For controlled adsorption, optimal synthesis conditions are identified using point of zero charge measurements, pH-precipitation experiments, and adsorption isotherms. Using these conditions, a catalyst with a higher dispersion and strong metal–support interactions is prepared. Physicochemical characterization by N2 physisorption, H2 chemisorption, temperature programmed reduction (TPR), transmission electron microscopy (TEM), and environmental TEM (ETEM) shows that the types of nickel sites formed strongly depend on the synthesis method. Methane dry reforming reactivity studies show stable catalytic performance for at least 9h and provide additional insight into the types of active centers present. After reductive pretreatment, the nickel catalyst prepared by dry impregnation is found to primarily have nickel present as a surface NiAl2O4. In contrast, the active centers for the nickel catalyst prepared by controlled adsorption consist of nickel particles that are encapsulated by a nickel aluminate layer with 1–2nm in thickness. Combustion analysis and XPS of spent catalysts reveal different amounts and nature of carbonaceous deposits as a function of the synthesis method.

Impact and Detailed Action of Sulfur in Syngas on Methane Synthesis on Ni/γ-Al2O3 Catalyst

Stability and deactivation phenomena are of utmost importance for metal nanocatalysts from both fundamental and industrial points of view. The presence of small amounts of sulfur at ppm and ppb levels in the synthesis gas produced from fossil and renewable sources (e.g., biomass, coal) is a major reason for deactivation of nickel catalysts for carbon monoxide hydrogenation. This paper addresses reaction pathways and deactivation mechanisms of alumina-supported nickel catalysts for methane synthesis from pure syngas and syngas containing small amounts of sulfur. A combination of SSITKA and operando FTIR is indicative of both reversible molecular and irreversible dissociative carbon monoxide adsorption on nickel nanoparticles under the reaction conditions. Methanation reaction involves irreversible carbon monoxide adsorption, dissociation, and hydrogenation on nanoparticle steps and edges. Hydrogenation of adsorbed carbon species leading to methane seems to be the reaction kinetically relevant step. Molecular forms of carbon monoxide reversibly adsorbed on nickel terraces are likely not to be involved in carbon monoxide hydrogenation. The results suggest a competition between sulfur and carbon monoxide for nickel surface sites. During methanation, sulfur preferentially adsorbs on the sites of reversible molecular carbon monoxide adsorption, whereas the low-coordinated nickel sites responsible for carbon monoxide dissociation and hydrogenation are affected to a lesser extent by sulfur poisoning. The active sites of carbon monoxide hydrogenation are poisoned much more rapidly by sulfur, when the catalyst has been exposed to small amounts of H2S in the absence of methanation.

Glycerol conversion in the presence of carbon dioxide on alumina supported nickel catalyst

The glycerol conversion in the presence of carbon dioxide was investigated on a Ni/γ-Al2O3 catalyst. The catalyst was characterized by XRD, TPR, BET, XPS, H2-TPD, DRIFTS and Raman. A catalyst preparation method using ethylene glycol and a reduction passivation procedure provided an active catalyst. For the different catalytic runs in a batch reactor, a higher formation of hydrocarbons, alcohols, ketones, and furans was observed when the mass of catalyst was increased. For temperatures below 200°C, both glycerol and CO2 were converted, suggesting a potential route for glycerol valorization and CO2 sequestration, with the consequent production of oxirane, alcohols, ketones and carboxylic acids. For higher temperatures, there was the formation of a wide range of low yield products, with higher yields to ketones, alcohols and esters.

Effect of Interfacial NiAl<sub>2</sub>O<sub>4</sub> Layer and Calcination Temperature over Anodic Alumina Supported Ni Catalysts during Steam Reforming of Methane

A metal-monolithic anodic alumina supported nickel catalyst with an interfacial NiAl2O4 layer was prepared to investigate its performance in the steam reforming of methane (SRM). Compared with commercial catalysts, this catalyst showed excellent SRM durability at 700°C. This is believed to result from the presence of the interfacial NiAl2O4 layer, which could anchor the top metallic Ni particles, and thus effectively suppress oxidation and sintering of Ni. The nickel reduced from the interfacial NiAl2O4 layer was found to be vital for SRM durability. By varying the preparation conditions of calcination temperature and impregnation sequence, a catalyst with an optimum interfacial NiAl2O4 layer was obtained. The catalyst morphologies in this study are also examined.

The application of inelastic neutron scattering to investigate the steam reforming of methane over an alumina-supported nickel catalyst

An alumina-supported nickel catalyst, previously used in methane reforming experiments employing CO2 as the oxidant, is applied here in the steam reforming variant of the process. Micro-reactor experiments are used to discern an operational window compatible with sample cells designed for inelastic neutron scattering (INS) experiments. INS spectra are recorded after 6h reaction of a 1:1 mixture of CH4 and H2O at 898K. Weak INS spectra are observed, indicating minimal hydrogen retention by the catalyst in this operational regime. Post-reaction, the catalyst is further characterised by powder X-ray diffraction, transmission electron microscopy and Raman scattering. In a comparable fashion to that seen for the ‘dry’ reforming experiments, the catalyst retains substantial quantities of carbon in the form of filamentous coke. The role for hydrogen incorporation by the catalyst is briefly considered.

Studies of the preparation method of ceria-promoted nickel catalyst for carbon dioxide reforming of methane

The CO2 reforming reactions of methane were carried out using an alumina-supported nickel catalyst. In order to reduce the coke deposition, the preparation methods of the ceria-promoted nickel catalysts were investigated. The physical mixing of CeO2 realized a 50% reduction in the deposited coke per 1mol of produced CO compared to the catalyst without CeO2. Meanwhile, the catalyst prepared by the impregnation of nickel aluminate with a cerium nitrate solution followed by reduction under a hydrogen stream showed the lowest coke deposition activity. Particularly, the use of alumina prepared by the alkoxide method gave a 5.8% deposited coke produced by the catalyst without CeO2. This type of nickel catalyst showed a high reforming activity without deactivation and a low coke deposition activity.

Co-precipitated Ni–Mg–Al catalysts for hydrogen production by supercritical water gasification of glucose

Supercritical water gasification (SCWG) is a promising process for hydrogen production from biomass. In this study, a series of Ni–Mg–Al catalysts with different Mg/Al molar ratios has been synthesized by a co-precipitation method for hydrogen production by SCWG of glucose. Effects of Mg addition on the catalytic activity, hydrothermal stability and anti-carbon performance of alumina supported nickel catalyst were investigated. The highly dispersed nickel catalysts prepared by co-precipitation could greatly enhance the gasification efficiency of glucose in supercritical water. Among the tested Ni–Mg–Al catalysts, NiMg0.6Al1.9 showed the highest catalytic activity with the hydrogen yield of 11.77 mmol/g (912% as that of non-catalytic test). NiMg0.6Al1.9 also showed the best hydrothermal stability probably due to the formation of MgAl2O4. Mg could efficiently improve the anti-carbon ability of Ni–Al catalyst by inhibiting the formation of graphite carbon. It is also confirmed that MgO supported nickel catalyst is not suitable for SCWG process owing to the difficulty on nickel oxides reduction in the precursors and the phase change of MgO to Mg(OH)2 under the hydrothermal condition.

The application of inelastic neutron scattering to investigate the ‘dry’ reforming of methane over an alumina-supported nickel catalyst operating under conditions where filamentous carbon formation is prevalent

The use of CO2 in reforming methane to produce the industrial feedstock syngas is an economically and environmentally attractive reaction. An alumina-supported nickel catalyst active for this reaction additionally forms filamentous carbon. The catalyst is investigated by inelastic neutron scattering as well as elemental analysis, temperature-programmed oxidation, temperature-programmed hydrogenation, X-ray diffraction, transmission electron microscopy and Raman scattering. Isotopic substitution experiments, using 13CO2 for 12CO2, show the oxidant to contribute to the carbon retention evident with this sample. At steady-state operation, a carbon mass balance of 95% is observed. A kinetic scheme is proposed to account for the trends observed.

Partial oxidation of methane to syngas on bulk NiAl2O4 catalyst. Comparison with alumina supported nickel, platinum and rhodium catalysts

In this work the catalytic performance of NiAl2O4 catalyst in the partial oxidation of methane (CH4(1)/O2(0.5)/N2(8.5) and 4800mLCH4g−1h−1) was investigated. This bulk catalyst resulted more active (70% conversion at 625°C) than alumina supported nickel catalysts prepared by conventional impregnation (11%Ni/Al2O3-30% conversion at 625°C and 14%Ni/Al2O3-65% conversion at 625°C) and a commercial platinum catalyst (1%Pt/Al2O3-30% conversion at 625°C). Further, the behaviour of NiAl2O4 was comparable to that of a commercial rhodium catalyst (1%Rh/Al2O3-75% conversion at 625°C). Fresh and spent catalysts were characterised by N2-physisorption, H2-TPR, UV-vis-DRS, WDXRF, XRD, TGA-MS, TEM and Raman spectroscopy. After catalyst reduction CH4 conversion over bulk NiAl2O4 was associated with the formation of large amounts of metallic nickel crystallites with a relatively small size. Although significant coking was noticed, conversion and selectivity to H2/CO were not significantly affected after prolonged time on stream.