Large Ni Particles Research Articles

Unveiling the Structure-Property Relationship of MgO-Supported Ni Ammonia Decomposition Catalysts from Bulk to Atomic Structure by In Situ/Operando Studies.

Ammonia is currently being studied intensively as a hydrogen carrier in the context of the energy transition. The endothermic decomposition reaction requires the use of suitable catalysts. In this study, transition metal Ni on MgO as a support is investigated with respect to its catalytic properties. The synthesis method and the type of activation process contribute significantly to the catalytic properties. Both methods, coprecipitation (CP) and wet impregnation (WI), lead to the formation of Mg1-xNixO solid solutions as catalyst precursors. X-ray absorption studies reveal that CP leads to a more homogeneous distribution of Ni2+ cations in the solid solution, which is advantageous for a homogeneous distribution of active Ni catalysts on the MgO support. Activation in hydrogen at 900 °C reduces nickel, which migrates to the support surface and forms metal nanoparticles between 6 nm (CP) and 9 nm (WI), as shown by ex situ STEM. Due to the homogeneously distributed Ni2+ cations in the solid solution structure, CP samples are more difficult to activate and require harsher conditions to reduce the Ni. The combination of in situ X-ray diffraction (XRD) and operando total scattering experiments allows a structure-property investigation of the bulk down to the atomic level during the catalytic reaction. Activation in H2 at 900 °C for 2 h leads to the formation of large Ni particles (20-30 nm) for the samples synthesized by the WI method, whereas Ni stays significantly smaller for the CP samples (10-20 nm). Sintering has a negative influence on the catalytic conversion of the WI samples, which is significantly lower compared to the conversion observed for the CP samples. Interestingly, metallic Ni redisperses during cooling and becomes invisible for conventional XRD but can still be detected by total scattering methods. The conditions of activation in NH3 at 650 °C are not suitable to form enough reduced Ni nanoparticles from the solid solution and are, therefore, not a suitable activation procedure. The activity steadily increases in the samples activated at 650 °C in NH3 (Group 1) compared to the samples activated at 650 °C in H2 and then reaches the best activity in the samples activated at 900 °C in H2. Only the combination of complementary in situ and ex situ characterization methods provides enough information to identify important structure-property relationships among these promising ammonia decomposition catalysts.

(Invited) On Degradation Mechanisms of Ni-YSZ Fuel Electrodes in Solid Oxide Cells

The solid oxide cell (SOC) is reversible. It has about equally good performance both in solid oxide fuel cell (SOFC) and in solid oxide electrolyzer cell (SOEC) mode. The classical Ni-YSZ cermet SOC fuel electrode has an excellent initial performance provided that it has a good structure in terms of particle size of both Ni and YSZ, and a suitable porosity with sufficient contact between Ni-Ni, YSZ-Ni, YSZ-YSZ particles, and in absence of certain impurities such as silica and sulfur. The essential entity of the Ni-YSZ electrode is the length of the three phase boundary (3pb) between the three phases of Ni electron conductor, YSZ oxide ion conductor, and H2-H2O gas, which have electronic contact to the main Ni electrode, ionic contact to the bulk YSZ electrolyte, and access to the the main gas atmosphere, respectively.Nano-particular Ni is an excellent electrocatalyst for the reduction of H2O to H2 + O2- and for the oxidation. It seems that it is generally accepted that at operation temperatures (above 650 °C and above) the initial nano-sized Ni particles, which are in electric contact with each other, will over time continue to sinter into larger and larger Ni particles until all the Ni has become one dense body or the growth of the Ni particles has been blocked by particles of another phase like YSZ. Thus, the relative high mobility of Ni continues to pose a problem towards the lifetime of a highly functional Ni-YSZ electrode.A most serious type of degradation of Ni-YSZ electrodes in solid oxide electrolysis cells (SOEC) seems to be the one coupled to Ni-migration, which is regarded as an important obstacle for the commercialization of SOEC. Post mortem scanning electron microscopy investigations of the degraded Ni-YSZ electrode reveal that a thin (up to 3 – 5 µm) zone of the original nano-structured active Ni-YSZ cathode has been significantly been depleted in Ni. This degradation is clearly driven by the electrochemical polarization of the Ni-YSZ electrode [1], but the mechanism is highly discussed. Several researchers have the hypothesis that the Ni depletion is a result of Ni migration in a gradient in Ni/YSZ interface energy, see e.g. [2,3]. However, a review of relevant literature points out that this hypothesis cannot explain several reported clear experimental results, whereas our hypothesis based on Ni+ ions from Ni particles, which have lost electrochemical contact, migrate as NiOH across the YSZ particles to new active 3pbs formed further away from the bulk electrolyte. This hypothesis may qualitatively explain all reported results from steam electrolysis cells and H2 fuel cells so far [1]. However, our hypothesis does not directly explain the similar migration of Ni away from the YSZ bulk electrolyte in case of CO2 electrolysis.Therefore, a revision of our hypothesis will be presented. The essential change is simply to propose that it is the Ni+-ion that migrates in the YSZ-surface layer as either NiOH in case of steam electrolysis, or as “Ni2CO3” in case of CO2 electrolysis. A monolayer of “Ni2CO3” on a YSZ surface is not assumed to be any kind of crystalline Ni2CO3, but rather a layer of adsorbed Ni+ and “CO3 1-“ in which CO3 2- has one of its oxygen atoms incorporated into the YSZ surface crystalline structure which is formed by reaction between CO2 from gas and an unsaturated surface oxygen with a “dangling” electron with similarity to CO2 reduction on ceria [4]. Furthermore, the possible role of SiO2 impurities in the loss of contact between Ni and YSZ at negatively polarized Ni will be presented as another revision of the hypothesis together with further new details.References M.B. Mogensen et al., Fuel Cells, (2021), 1–15; DOI: 10.1002/fuce.202100072, and references therein.M. Trini et al., Acta Materialia, 212 (2021) 116887; DOI: 10.1016/j.actamat.2021.116887.L. Rorato et al., J. Electrochem. Soc., 170 (2023) 034504, DOI: 10.1149/1945-7111/acc1a3, and references therein.E.M. Sala et al., Phys. Chem. Chem. Phys. (2023), DOI: 10.1039/d2cp05157e.

Ni0+ particle size-dependent pyrolytic reforming of toluene over Ni/Al2O3 for the co-production of hydrogen and CNTs

Pyrolytic reforming of toluene over Ni-based catalysts loaded on nano-sized and micro-sized γ-Al2O3 is investigated in a fixed bed reactor under the temperature from 500 °C to 800 °C. The physicochemical properties of γ-Al2O3 and Ni/Al2O3 catalysts were characterized by HRTEM, XPS, XRD and H2-pulse chemisorption. The metal Ni loaded on micro-sized γ-Al2O3 gave the steady toluene conversion of more than 80% and graphite yield of 87.3% after 1 h of reforming reaction. The rapid decrease of H2 yield from 5.2 mmol/g-toluene to 0.4 mmol/g-toluene over Ni/nano-Al2O3 was obtained during the 1 h of reforming reaction at 500 °C. The particle size of metal Ni over nano-sized and micro-sized γ-Al2O3 were centered at 7.8 nm and 28.8 nm, respectively. The metastable NiCx crystallites were observed over the spent Ni/nano-Al2O3 catalyst, while only metal Ni were obtained over the spent Ni/micro-Al2O3 catalyst. Ni0+ particle size-dependent crystalline phase was evidenced by MD simulations, where incorporation of more carbon atoms was achieved on small Ni particles. The energy barrier for –CH2 dehydrogenation over Ni32C, Ni4C and Ni3C were estimated to be 90.2, 105.1 and 116.0 kJ/mol through the DFT simulation, respectively. It can be concluded that the crystalline Ni phase was maintained over Ni/micro-Al2O3 with large Ni particle size, prolonging the catalytic endurance and promoting the CNTs (Carbon Nanotubes) growth without the deposition of unexpected amorphous carbon.

Efficient and Stable Ni/SBA-15 Catalyst for Dry Reforming of Methane: Effect of Citric Acid Concentration

Citric acid, one of the representative chelate compounds, has been widely used as an additive to achieve the highly dispersed metal-supported catalysts. This study aimed to investigate the effect of citric acid concentration on the preparation of the highly dispersed Ni catalysts on mesoporous silica (SBA-15) for the dry reforming of methane. A series of Ni/SBA-15 catalysts with citric acid were prepared using the acid-assisted incipient wetness impregnation method, and the Ni/SBA-15 catalyst as a reference was synthesized via the impregnation method. First of all, the citric acid addition during the catalyst synthesis step regardless of its concentration resulted in highly dispersed Ni particles of ~4–7 nm in size in Ni/SBA-15 catalysts, which had a superior and stable catalytic performance in the dry reforming of methane (93% of CO2 conversion and 86% of CH4 conversion). In addition, the amount of coke formation was much lower in a series of Ni/SBA-15 catalysts with citric acid (~2–5 mgcoke gcat−1 h−1) compared to pristine Ni/SBA-15 catalysts (~22 mgcoke gcat−1 h−1). However, when the concentration of citric acid became higher, the more free NiO species that formed on the SBA-15 support, leading to large Ni particles after the stability test. The addition of citric acid is a very clear strategy for making highly dispersed catalysts, but its concentration needs to be carefully controlled.

Regulating the states of Ni species by controlling the silanols of MCM-41 support to promote the hydrogenation of maleic anhydride

Nickel (Ni) supported on MCM-41 zeolite as hydrogenation catalysts have been widely applied in various reactions, however, the synergism between Ni and the support remains obscure. To prepare more efficient catalysts and reveal the metal-support interaction, MCM-41 zeolite was modified with ammonium hexafluorosilicate (AHFS) before Ni loading. Results show that fluoride ions could selectively convert the vicinal silanols into isolated silanols on MCM-41 surface, which weakens the interactions between Ni precursors and the SiO2 supports. Over the modified MCM-41 zeolites, the attenuated interaction results in the agglomeration of NiO particles that is more easily to be reduced, rather than the formation of nickel phyllosilicate which is extraordinarily difficult towards reduction. The easily-reduced Ni particles provided more adsorption sites for CC bond instead of CO bond, and the adsorption strength was also regulated. Therefore, the AHFS modified catalyst demonstrated a notable enhancement in hydrogenation of maleic anhydride (MA) into succinic anhydride (SA), presenting 98.4 % MA conversion and 99.8 % SA selectivity at 90 °C under 3.0 MPa H2. Moreover, the optimum catalyst was recycled for 5 runs to analyze the deactivation mechanism, it was found out that the loss of activities stems from the leaching of large Ni particles. Overall, our work shed light on the metal-support interactions for Ni/MCM-41 catalyst, determining the vicinal hydroxyl groups were responsible for the state of Ni species, which is helpful for the rational design of other hydrogenation catalysts.

Effect of Metal Dopant on the Performance of Ni@CeMeO2 Embedded Catalysts (Me = Gd, Sm and Zr) for Dry Reforming of Methane

Biogas upgrading by a catalytic process has been studied in order to obtain syngas using renewable source of methane. This work evaluates the influence of metal dopant (Gd, Sm, and Zr) on the CeO2 structure for the dry reforming of methane over Ni nanoparticle embedded catalysts. The doping with Zr improved the thermal stability of the catalyst, leading to the formation of small Ni nanoparticles, while Ni metal sintering was observed for Ni@CeO2, Ni@CeGdO2, and Ni@SmO2, according to in situ XRD under reduction conditions. The ceria reducibility was affected by the dopant nature, for which the addition of Zr caused distortions in the ceria lattice, promoting the diffusion of oxygen bulk to surface. The doping with Gd and Sm created oxygen vacancies by charge compensation, and the saturation of oxygen vacancies in the fresh samples decreased the degree of Ce reduction, according to TPR results. The larger Ni particles and poor redox behavior for Ni@CeGdO2 and Ni@CeSmO2 were responsible for the high carbon formation on these catalysts during the DRM reaction. The Ni@CeZrO2 catalyst did not present coke formation because of smaller Ni crystallite size and higher ceria reducibility. Therefore, the control of Ni particle size and the high oxygen mobility in the Ni@CeZrO2 catalyst inhibits carbon deposition and enhances the mechanism of carbon removal, promoting the catalyst stability.

Size-Dependent Strong Metal–Support Interactions of Rutile TiO2-Supported Ni Catalysts for Hydrodeoxygenation of m-Cresol

A series of rutile TiO2-supported Ni catalysts with varying Ni sizes were prepared and reduced at 650 °C to explore the effect of Ni size on the strong metal–support interactions (SMSI) and its consequences on the hydrodeoxygenation (HDO) of m-cresol at 350 °C and atmospheric pressure. When the Ni size increases from 4 to 29.1 nm, the SMSI becomes stronger, e.g., the thickness of the TiOx overlayer and the coverage extent of TiOx on the Ni particle surface increase. Direct deoxygenation to toluene is the dominant pathway on Ni/TiO2 catalysts with varying Ni loadings, with almost no CH4 being formed. These results indicate that the TiOx overlayer significantly alters the property of Ni. That is, the C-C hydrogenolysis activity on bare Ni is completely inhibited due to SMSI, while the deoxygenation activity is improved at the Ni-TiOx interfacial perimeter sites. Meanwhile, the turnover frequency of HDO on small Ni particles of 4 nm is > 2 times higher than that on large Ni particles of 29.1 nm, indicating that the small Ni particle with moderate SMSI appears to be optimal for the direct deoxygenation of m-cresol to toluene. The results suggest HDO activity may be enhanced by tuning the metal particle size and SMSI degree.

Developing silicalite-1 encapsulated Ni nanoparticles as sintering-/coking-resistant catalysts for dry reforming of methane

The stability of catalysts in dry reforming of methane (DRM) is a known issue. In this paper an encapsulation strategy has been employed to improve the stability compared with conventional impregnation methods. Herein, nickel nanoparticles encapsulated in silicalite-1 were prepared using a range of methods including post treatment, direct hydrothermal and seed-directed methods to investigate the effect of synthesis protocol on the properties of catalysts, such as degree of encapsulation and Ni dispersion, and anti-coking/-sintering performance in DRM. The Ni@SiO2-S1 catalysts obtained by the seed-directed synthesis presented the full encapsulation of Ni NPs by the zeolite framework with small particle sizes (∼2.9 nm) and strong metal-support interaction, which could sterically hinder the migration/aggregation of Ni NPs and carbon deposition. Therefore, Ni@SiO2-S1 showed stable CO2/CH4 conversions of 80% and 73%, respectively, with negligible metal sintering and coking deposition (∼0.5 wt%) over 28 h, which outperformed the other catalysts prepared. In contrast, the catalysts developed by the post-treatment and ethylenediamine-protected hydrothermal methods showed the co-existence of Ni phase on the internal and external surfaces, i.e. incomplete encapsulation, with large Ni particles, contributing to Ni sintering and coking. The correlation of the synthesis-structure-performance in this study sheds light on the design of coking-/sintering-resistant encapsulated catalysts for DRM.

Controlling carbon formation over Ni/CeO2 catalyst for dry reforming of CH4 by tuning Ni crystallite size and oxygen vacancies of the support

This work investigates the effect of Ni crystallite size and oxygen vacancies of the support on the formation of carbon over Ni/CeO2 catalysts for dry reforming of methane at 1073 K. A large crystallite size variation is achieved by using different Ni loading (5 and 10 wt%) and calcination temperatures (673, 873, 1073 and 1473 K). In situ XRD and XANES experiments reveal that the increase in calcination temperature increases the Ni crystallite size, whereas the amount of oxygen vacancies decreases. The amount of carbon formed during DRM increases as Ni crystallite size increases, achieving a maximum at around 20−30 nm and then, it continuously decreases. However, carbon deposition is negligeable below 10 nm and above 100 nm. For the catalysts with very large Ni crystallite sizes, the CH4 dissociation rate is likely so low that carbon species formed reacts and carbon accumulation does not take place. However, the oxygen vacancies of ceria do not contribute to the carbon removal from the Ni surface due to the low metal-support interface on these large Ni particles.

Carbon resistance of xNi/HTASAO5 catalyst for the production of H2 via CO2 reforming of methane

xNi/HTASAO5 catalysts (x = 2.5, 3.3, 4.4, 5.8, 8.2) were prepared for CO2 reforming of methane. No crystalline nickel species formed on the catalysts with lower nickel content (≤4.4%), and large Ni0 crystallite formed on 5.8% (10 nm) and 8.2 wt%Ni/HTASAO5 (17 nm), whereas the surface concentration of Ce3+ decreased with Ni loading. The initial conversion of CH4 increased from 29.5% to 46.9% with Ni loading. The xNi/HTASAO5 (x ≤ 4.4%) performed stably in the reaction due to the presence of dispersed Ni species and high surface Ce3+ content without coke formation, however, 5.8% and 8.2 wt%Ni/HTASAO5 exhibited decreased activity with time on stream, because of the formation of large Ni particles with lower surface Ce3+, leading to carbon accumulation. Thus, CH4 conversion stabilized at about 43% and no carbon formed on 4.4 wt%Ni/HTASAO5 with optimum Ni loading.

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

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

Effect of the Ni/Al Ratio on the Performance of NiAl2O4 Spinel-Based Catalysts for Supercritical Methylcyclohexane Catalytic Cracking

Supercritical methylcyclohexane cracking of NiAl2O4 spinel-based catalysts with varying Ni/Al deficiencies was investigated. Thus, catalysts with Ni content of 10–50 wt.% were prepared by typical co-precipitation methods. The calcined, reduced, and spent catalysts were characterized by X-ray diffraction, O2 temperature-programmed oxidation, NH3 temperature-programmed desorption, N2 physisorption, O2 chemisorption, scanning and transmission electron microscopy, and X-ray fluorescence. The performance and physicochemical properties of the reference stoichiometric Ni3Al7 catalyst differed significantly from those of the other catalysts. Indeed, the Ni-deficient Ni1Al9 catalyst led to the formation of large Ni particles (diameter: 20 nm) and abundant strong acid sites, without spinel structure formation, owing to the excess Al. These acted with sufficient environment and structure to form the coke precursor nickel carbide, resulting in a pressure drop within 17 min. On the other hand, the additional NiO linked to the NiAl2O4 spinel structure of the Al-deficient Ni5Al5 catalyst formed small crystals (10 nm), owing to the excess Ni, and displayed improved Ni dispersion. Thus, dehydrogenation proceeded effectively, thereby improving the resistance to coke formation. This catalytic behavior further demonstrated the remarkable activity and stability of this catalyst under mild conditions (450 °C and 4 Mpa).

Investigating hydrogenation and decarbonylation in vapor-phase furfural hydrotreating over Ni/SiO2 catalysts: Propylene production

Furfural can be mass-produced from lignocellulose biomass and can be a platform chemical for producing valuable chemicals. In this study, we examine Ni/SiO2 catalysts for the conversion of furfural under a hydrogen atmosphere. The reactivity and the product selectivity are governed by the reaction temperature and the Ni particle size. A catalyst pretreatment by including calcination prior to hydrogen reduction leads to Ni/SiO2–CR with large Ni particles (∼ 15 nm) and a high selectivity to furfuryl alcohol (FA) at below 200 °C. The Ni/SiO2-R is hydrogen-pretreated without a prior calcination and it contains small Ni particles (∼ 5 nm) and exhibits relatively high selectivity to furan. The turnover frequency (TOF) of furfural conversion is 239 and 408 h−1, respectively, on Ni/SiO2-CR and Ni/SiO2-R at 175 °C, when the former shows 100% selectivity to FA and the latter exhibits a selectivity of around 38% and 62%, to FA and furan, respectively. Moreover, the furan can be reacted to produce propylene and CO by the Ni catalysts at above 200 °C and Ni/SiO2-R exhibits a higher activity than Ni/SiO2-CR. The results suggest that the furfural hydrotreating reaction over Ni catalysts is structure sensitive and a proper design of catalyst and operating temperature can provide a full furfural utilization, either to FA (a biofuel), or to furan and propylene (as chemical feedstock).

Structure sensitivity and its effect on methane turnover and carbon co-product selectivity in thermocatalytic decomposition of methane over supported Ni catalysts

Thermocatalytic decomposition of methane (TCD) is a promising approach for producing CO2-free hydrogen and solid carbon co-product. In this study, a series of Al2O3- and MgAl2O4-based Ni catalysts, prepared with varying synthesis and pretreatment methods, were evaluated for methane TCD performance at 650 °C and characterized before and after reaction to elucidate activity-structure relationships. We found that methane TCD turnover increases with Ni particle size. Further, large Ni particle sizes (i.e., >20 nm) are selective toward the formation of carbon nanotubes (CNTs), while small Ni particle sizes (i.e., <10 nm) are selective toward the formation of graphitic carbon layers. The formation of graphitic carbon layers block access to Ni active sites, thus rendering the catalyst inactive more quickly than when CNTs are produced. Additionally, the catalyst deactivation observed with time-on-stream is due to the fragmentation of Ni particles into smaller Ni particles followed by their encapsulation with graphitic carbon layers.

A Thermodynamic Investigation of Ni on Thin-Film Titanates (ATiO3)

Thin, ~1-nm films of CaTiO3, SrTiO3, and BaTiO3 were deposited onto MgAl2O4 by Atomic Layer Deposition (ALD) and then studied as catalyst supports for ~5 wt % of Ni that was added to the perovskite thin films by Atomic Layer Deposition. Scanning Transmission Electron Microscopy demonstrated that both the Ni and the perovskites uniformly covered the surface of the support following oxidation at 1073 K, even after redox cycling, but large Ni particles formed following a reduction at 1073 K. When compared to Ni/MgAl2O4, the perovskite-containing catalysts required significantly higher temperatures for Ni reduction. Equilibrium constants for Ni oxidation, as determined from Coulometric Titration, indicated that the oxidation of Ni shifted to lower PO2 on the perovskite-containing materials. Based on Ni equilibrium constants, Ni interactions are strongest with CaTiO3, followed by SrTiO3 and BaTiO3. The shift in the equilibrium constant was shown to cause reversible deactivation of the Ni/CaTiO3/MgAl2O4 catalyst for CO2 reforming of CH4 at high CO2 pressures, due to the oxidation of the Ni.

Size Effects of Ni Particles on the Cleavage of C–H and C–C Bonds toward Hydrogen Production from Cellulose

Hydrogen production from cellulose via aqueous-phase reforming (APR) has aroused increasing attention, but its efficiency remains challenging. Layered double oxide (LDO)-supported Ni particle has been reported as an efficient catalyst for H2 production from cellulose in our previous work: basic sites on LDO play an important role in H2 production by activating O–H bonds, which facilitates the cleavage of C–H and C–C bonds on Ni particles. The interfacial Niδ+ sites further increase H2 yield by promoting the water–gas shift (WGS) reaction. Here, this work modulates the size of Ni particles by tailoring the Ni content in Ni-containing layered double hydroxides (LDHs), and the effects of Ni particle size on the H2 production have been investigated. The dependence of C–H and C–C cleavage on the size of Ni particle has been revealed. It has been found that a smaller Ni particle possesses lower electron density, thus significantly accelerating the C–H cleavage after the rupture of O–H bonds. But the C–C cleavage is inhibited due to lack of electron-rich adjacent Ni0 sites on smaller Ni particles, resulting in severe coke formation. Increasing the Ni particle size favors the C–C cleavage. As a result, the H2 yield demonstrates a “volcanic” dependence on the Ni particle size. Also, this work further confirms the role of interfacial Niδ+ sites in facilitating CO removal by WGS reaction and suppressing the methanation of CO and/or CO2 especially on larger Ni particles.

Lanthanum-Modified MCF-Derived Nickel Phyllosilicate Catalyst for Enhanced CO2 Methanation: A Comprehensive Study.

For the traditional preparation method, it is challenging to fabricate a supported nickel catalyst with fine size at high loading. In this work, a group of La-modified mesostructured cellular foam (MCF)-derived nickel phyllosilicates was designed and synthetized by a hydrothermal method followed by an impregnation-modification of La2O3, whose Ni contents varied from 25.3 to 32.2 wt %. Both the special property of phyllosilicate and the addition of a La2O3 modifier played significant roles in achieving high Ni dispersion and excellent catalytic performance. The formed nickel phyllosilicate was beneficial to obtain small Ni nanoparticles (<5 nm) due to its strong metal-support interaction and high specific surface area; the addition of the La2O3 modifier could further reduce the Ni particle size and decrease the reduction difficulty of the fabricated samples. On the contrary, a large Ni particle size of 13.0 nm was observed on the impregnated Ni/MCF (N/M-Im) catalyst with a Ni content of 31.7 wt %. As a result, the nickel phyllosilicate catalyst showed higher catalytic activity than the impregnated one, and the La modifier could further improve the catalytic activity especially at low temperature (<400 °C). Among all catalysts, the modified phyllosilicate catalyst N/M-P-32-5L with 180 °C-32 h-hydrothermal treatment and La2O3 content of 5 wt % was the best owing to its small-sized Ni particles, high H2 and CO2 chemisorption capacity, large turnover frequency (TOF) value, and low activation energy of 69.83 kJ mol-1. In addition, the intermediates of formate and CO were detected through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis. In a 100 h-lifetime test under harsh conditions and 600 °C-steam treatment, N/M-P-32-5L showed both high sintering resistance of Ni particles and high thermal stability without the collapse of pores as well as decrease of catalytic activity, which was attributed to the special physical and chemical properties of MCF-derived nickel phyllosilicate, strong metal-support interaction over the catalyst, and the promotion of the La2O3 modifier.

Solvothermal hydrodeoxygenation of hydroxymethylfurfural derived from biomass towards added value chemicals on Ni/TiO2 catalysts

Lignocellulosic biomass is one of the most abundant renewable source of commodity chemicals like hydroxymethylfurfural. Solvothermal hydrodeoxygenation of hydroxymethylfurfural towards value added chemicals was performed with a series of nickel based catalysts supported on different titania materials, ie. hydrothermally-prepared high aspect ratio and commercial TiO2 with different morphologies and crystallographic structures. The kind of titania strongly influenced the properties of the supported nickel nanoparticles, which allowed to tune the reaction selectivity towards specific products. Rutile-containing titania forced stronger Ni-Ti interaction, enhanced hydrogen adsorption, and formed larger Ni particles which resulted in high activity. Hydrothermally-prepared materials allowed to obtain selectively 2,5-bishydroxymethyltetrahydrofuran (biopolymer precursor) due a relatively small surface acidity and large Ni particles. By contrast, large surface area anatase with small nickel particles and small surface acidity allowed to selectively obtain 2,5-dimethylfuran (biofuel additive).

Synthesis Chemistry and Properties of Ni Catalysts Fabricated on SiC@Al2O3 Core-Shell Microstructure for Methane Steam Reforming

Heat and mass transport properties of heterogeneous catalysts have significant effects on their overall performance in many industrial chemical reaction processes. In this work, a new catalyst micro-architecture consisting of a highly thermally conductive SiC core with a high-surface-area metal-oxide shell is prepared through a charge-interaction-induced heterogeneous hydrothermal construction of SiC@NiAl-LDH core-shell microstructures. Calcination and reduction of the SiC@NiAl-LDH core-shell results in the formation of Ni nanoparticles (NPs) dispersed on SiC@Al2O3, referred to as Ni/SiC@Al2O3 core-shell catalyst. The Ni/SiC@Al2O3 exhibit petal-like shell morphology consisting of a number of Al2O3 platelets with their planes oriented perpendicular to the surface, which is beneficial for improved mass transfer. For an extended period of methane-stream-reforming reaction, the Ni/SiC@Al2O3 core-shell structure remained stable without any significant degradation at the core/shell interface. However, the catalyst suffered from coking and sintering likely associated with the relatively large Ni particle sizes and the low Al2O3 content. The synthesis procedure and chemistry for construction of supported Ni catalyst on the core-shell microstructure of the highly thermal conductive SiC core, and the morphology-controlled metal-oxide shell, could provide new opportunities for various catalytic reaction processes that require high heat flux and enhanced mass transport.

Syngas production by bi-reforming methane on an Ni-K-promoted catalyst using hydrotalcites and filamentous carbon as a support material.

Steam reforming of methane (SRM) and dry reforming of methane (DRM) are frequently used in the production of syngas; however, the bi-reforming of methane (BRM) is an interesting and alternative process. In this study, BRM was studied over MgO, a layered double hydroxide (LDH) phase that was destroyed between 600 °C and 900 °C during the reaction. It showed good sorption capacity for CO2 at relatively low temperatures (<500 °C), with CO2 adsorption being a pre-requisite for its catalytic conversion. Among the tested materials, the potassium-promoted LDH showed the highest activity, achieving a maximum CO2 conversion of 75%. The results suggest that at high temperature, the electronic structure of the used materials influences the destabilization of the feed in the order of methane, water and carbon dioxide. K promotes the catalytic activity, compensates the presence of large Ni particle sizes originating from the high metal loading, and favors the formation of Mg–Al-spinel. K is known to be an electronic promoter that releases electrons, which flow to the active metal. This electron flow induces instability on the molecule to be converted, and most probably, also induces size variations on the respective active nickel metal. The influence of the operating conditions in the range of 300 °C to 900 °C on the conversion of the reactants and product distribution was studied. Accordingly, it was concluded that it is only possible to obtain molar ratios of hydrogen-to-carbon monoxide close to two at high temperatures, a pre-requisite for the synthesis of methanol.