Enhanced methane decomposition over transition metal-based tri-metallic catalysts for the production of COx free hydrogen

In this study, COx-free hydrogen production via methane decomposition was studied over Cu–Zn-promoted tri-metallic Ni–Co–Al catalysts. The catalysts have been prepared by the constant pH co-precipitation method, and the nominal Ni metal loading was fixed at 50 wt % along with other metals at 10 wt% each. The catalyst activity for methane decomposition reaction was examined in a reactor between 400 °C and 700 °C and at atmospheric pressure. Different techniques such as N₂-physisorption, X-ray diffraction, H₂-TPR SEM, TEM, ICP-MS, TGA, and Raman spectroscopy were applied to characterize the catalysts. The relation between the catalyst composition and their catalytic activity has been investigated. The controlled synthesis has resulted in a series of catalysts with a high surface area. Ni–Co–Cu–Zn–Al was the most active and productive catalyst. Various characterizations indicate that the promotional effects of Cu–Zn interaction were the critical factor in catalysts' activity and stability. Ni–Co–Cu–Zn catalyst gave the highest methane conversion of 85% at 700 °C. Zn addition improves the stability of the catalyst by retaining the active metal size during the decomposition reaction. The catalyst was active for 80 h of stability study. The rapid deactivation of the Ni–Co catalyst was due to the sintering of the catalyst at 650 °C. Moreover, carbon species accumulated during the methane decomposition reaction depend on the catalysts' composition. Zn promotes the growth of reasonably long and thin carbon nanotubes, whereas the diameter of carbon nanotubes on unpromoted catalysts was large.     

Hydrogen production by methanol steam reforming on a cordierite monolith reactor coated with Cu–Ni/LaZnAlO<Subscript>4</Subscript> and Cu–Ni/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

The performance of Cu–Ni/LaZnAlO₄ and Cu–Ni/γ-Al₂O₃ catalysts in the methanol reforming process in a monolith reactor in the temperature range of 200–350 °C, feed flow rate of WHSV = 20.8 h^?1 and atmospheric pressure has been investigated. In order to perform a more thorough investigation, surface area, morphology and crystalline structure of the synthetic catalysts have been studied using BET, FE-SEM, TPR, FT-IR, TEM, TGA and XRD analyses. The results have shown that Cu–Ni/LaZnAlO₄ catalyst synthesized by combustion reaction method under ultrasound irradiation has a very high efficiency and catalytic activity, low reduction temperature, high mechanical resistance and large pore sizes. The latter causes a higher percentage of active metal impregnation and better distribution on the support, greater resistance against sintering and maintenance of catalyst inertness at temperatures over 1000 °C, in comparison with conventional catalysts such as Cu–Ni/γ-Al₂O₃. This make its substitution for currently used catalysts affordable.     

Effect of the Ni/Cu ratio on the composition and catalytic properties of nickel-copper alloy in anisole hydrodeoxygenation

The activity of NiCu-SiO₂ catalysts with a metal content of 90% and different Ni/Cu ratios has been investigated in the hydrodeoxygenation of anisole, a model compound of bio-oil, at 280°C and 6 MPa. A homogeneous phase composition of the active component has been synthesized by the co-decomposition of nickel and copper nitrates followed by the introduction of SiO₂ as a stabilizer. The resulting catalysts have been characterized by temperature-programmed reduction, X-ray powder diffraction, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy combined with energy-dispersive microanalysis. The bulk and surface composition of active-component particles has been determined by XPS and X-ray diffraction. In all of the catalysts containing 15–85 wt % Ni, there are two types of solid solutions. One has a constant composition, Cu_0.95Ni_0.05, which is independent of the Ni/Cu ratio in the catalyst; in the other, the nickel stoichiometry increases with an increasing Ni content of the active component. A correlation has been established between the Ni/Cu ratio and the rate constants of the reaction examined and between the Ni/Cu ratio and the degree of hydrodeoxygenation for all samples. The most active catalyst is Ni85Cu5-SiO₂.     

Bimetallic Co–Ni/TiO<Subscript>2</Subscript> catalysts for selective hydrogenation of cinnamaldehyde

Bimetallic Co–Ni catalysts in the composition range Co_(1?x)Ni_x with x?=?0.0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1.0, with total metal loading of 15% w/w and supported on TiO₂-P25, have been prepared by chemical reduction of the metal acetates by glucose in aqueous alkaline medium and characterized by XRD, TEM, TPR, XPS and H₂-TPD techniques. Selective hydrogenation of cinnamaldhyde (CAL) to hydrocinnamaldehyde (HCAL), cinnamyl alcohol (COL) and hydrocinnamyl alcohol (HCOL) has been investigated at 20 bar pressure, in the temperature range 120–140 °C. Co/Ni crystallite sizes in the range 6.0?±?1 nm are observed by TEM. TPR and XPS results indicate the formation of nanoscale Co–Ni alloys, which tend to weaken M–H bond strength, as revealed by H₂-TPD measurements. Ni/TiO₂ displays very high conversion of CAL (86.9%) with high selectivity (78.7%) towards HCAL formation at 140 °C. Co/TiO₂, on the other hand, exhibits relatively lower CAL conversion (55%) and higher selectivity (61.3%) for COL formation at the same temperature. However, bi-metallic Co–Ni catalysts in the composition range x?=?0.3–0.6 display very high conversion (>?98%) due to alloy formation and weakening of M–H bonds. Bimetallic Co_0.7Ni_0.3 catalyst displays high conversion of CAL (98.1%) and high selectivity (82.9%) towards HCOL. Overall CAL hydrogenation activity at 140 °C, when expressed as TOF, displays a maximum value at the composition Co_0.5Ni_0.5. Activity and selectivity patterns have been rationalized based on the reaction pathways observed on the catalysts and the influence of Co–Ni alloy formation and M–H bond strength. Thus, a synergetic effect, originating from an appropriate composition of base metal catalysts and reaction conditions, could result in hydrogenation activity comparable with noble metal based catalysts.     

Revealing the Influence of Silver in Ni–Ag Catalysts on the Selectivity of Higher Olefin Synthesis from Stearic Acid

Results on the conversion of stearic acid to olefins over Ni–Ag/γ-Al₂O₃ catalysts are presented. XANES and EXAFS experiments in situ and DFT calculations were applied to reveal the structure of active sites therein. It is shown that the introduction of Ag to Ni catalysts leads to an increase in the olefin yield. After a reduction in hydrogen (350°C, 3 h) alumina-supported nanoparticles of nickel sulfides and metallic Ag are formed. The role of metal hydrides formed during the reaction is extensively discussed.     

Selective hydrogenation of phenylacetylene on Ni and Ni-Pd catalysts modified with heteropoly compounds of the Keggin type

It is established that unmodified Ni catalysts and Ni catalysts modified with Mo- and W-heteropoly compounds (HPC) of the Keggin type (6 wt %) along with catalyst containing 6% K₄SiW₁₂O₄₀/Al₂O₃ appear to be active in the reaction of phenylacetylene (PA) hydrogenation. At low temperatures (100?C150°C), the selectivity of the process strongly depends on the nature of the modifier or second active metal (Pd). It is demonstrated that in the presence of 6% Ni-0.015% Pd/Al₂O₃ modified by HPC K₄SiMo₆W₆O₄₀, the conversion of PA at 100°C was 87% at a styrene: ethylbenzene ratio of 1: 1. The acidity of HPC is found to influence the side reactions of alkylation and condensation. Transmission electron microscopy demonstrates that Ni in modified HPC 6% Ni/Al₂O₃ is present in the form of the particles below 2 nm in size, and these particles of Ni become larger when affected by the reaction medium during PA hydrogenation.     

CO oxidation catalyzed by Ag/SBA-15 catalysts: Influence of the hydrothermal treatment

Four types of SBA-15 were prepared with different times and temperatures of treatment in order to obtain a range of micropore sizes. CO oxidation was used as a probe reaction in order to evaluate the nature of the active species when SBA-15s were doped with ca 10% Ag deposited from an AgNO₃ solution and calcined or reduced at 350 °C. The texture (TEM, nitrogen physisorption), structure (XRD) and reducibility (TPR) of the various catalysts (Ag/SBA-15) were studied and compared to those of a catalyst prepared by deposition of silver on fumed silica as a reference. These catalysts differ initially by the nature of silica and by pore sizes. In CO oxidation, pre-reduced catalysts are more active than pre-oxidised ones. This has to do with two phenomena, i.e. sintering, which produces large inactive silver particles, and formation of active silver species in the form of small Ag₂O particles.     

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N-doped Carbon Catalysts

Ni,N-doped carbon catalysts have shown promising catalytic performance for CO₂ electroreduction (CO₂R) to CO; this activity has often been attributed to the presence of nitrogen-coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO₂ reduction (CO₂R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile-derived Ni,N-doped carbon electrocatalysts (Ni-PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO₂R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X-ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square-planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.     

Effect of the order of Ni and Ce addition in SBA-15 on the activity in dry reforming of methane

Dry reforming of methane has been carried out on SBA-15 catalysts containing 5 wt% Ni and 6 wt% Ce. The effect of the order of Ni and Ce impregnation on the catalytic activity has been studied. Both metals were added using the “two-solvent” method that favors metal dispersion inside the pores. Characterizations by XRD (low and high angles), N₂ sorption, SEM and TEM of the materials after metal addition and calcination indicate good preservation of the porosities and high NiO and CeO₂ dispersion inside the porous channels. Reduction was carried out before the catalytic tests and followed by TPR measurements. The most active reduced catalyst was the Ni–Ce/SBA-15 sample prepared by impregnating cerium first, then nickel. All catalysts were highly active and selective towards H₂ and CO at atmospheric pressure. Full CH₄ conversion was obtained below 650 °C. The higher performances compared to those reported in the literature for mesoporous silica with supported Ni and Ce catalysts are discussed.     

Study of catalytic hydrodeoxygenation performance for the Ni/KIT-6 catalysts

Ni/KIT-6 catalysts loaded with different amounts of metallic Ni were prepared by impregnation method. The prepared catalysts and their precursors were investigated through wide- and low-angle XRD, TEM, BET, H₂-TPR, and H₂-TPD analyzes. The catalytic hydrodeoxygenation performance of the catalysts was evaluated using ethyl acetate as a model bio oil compound. Results indicate that the catalytic hydrodeoxygenation performance of the prepared catalysts was directly related to hydrogen storage properties, hydrogen desorption properties, dispersion of the active component Ni, and so on. The ethyl acetate conversion and ethane selectivity of 25?wt% Ni/KIT-6 catalyst were 100 and 96.8%, respectively, at 300?°C, which shows the best performance. The hydrodeoxygenation activity of ethyl acetate was higher than that of methyl acetate and isopropyl acetate because of the effect of molecular polarity and size. And, this reaction is a structure sensitive reaction.     

MoP/Al2O3 as a novel catalyst for sulfur‐resistant methanation

We reported γ‐alumina supported molybdenum phosphide (MoP) catalysts as a novel catalyst for sulfur‐resistant methanation reaction. The precursors of the catalyst were prepared by impregnation method and the effect of reduction temperatures (550 °C, 600 °C, 650 °C) of the precursors for sulfur‐resistant methanation was examined. The results indicated catalyst obtained by lower reduction temperature delivered better sulfur‐resistant methanation performance. Meanwhile, the influence of H₂/CO ratios and H₂S content was also investigated. The results indicated that high H₂/CO ratio and low H₂S content was favorable for methanation of MoP catalysts. The catalysts were characterized by N₂ adsorption–desorption, XRD, XPS and TEM. The results confirmed that the MoP phase was formed on all the catalysts and the physicochemical properties of the samples influenced the performance for sulfur‐resistant methanation.     

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO₂ electroreduction (CO₂R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni?N bonding and correlating CO₂ reduction (CO₂R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO₂R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.     

Surface properties of Ni-Pt/SiO2 catalysts for N2O decomposition and reduction by H2

The surface properties of bimetallic Ni-Pt/SiO2 catalysts with variable Ni/Ni + Pt atomic ratio (0.75, 0.50, and 0.25) were studied using N2O decomposition and N2O reduction by hydrogen reactions as probes. Catalysts were prepared by incipient wetness impregnation of the silica support with aqueous solutions of the metal precursors to a total metal loading of 2 wt %. For both model reactions, Pt/SiO2 catalyst was substantially more active than Ni/SiO2 catalyst. Mean particle size by TEM was about the same (in the range 6-8 nm) for all catalysts and truly bimetallic particles (more than 95%) were evidenced by EDS in the Ni-Pt/SiO2 catalysts. CO adsorption on the bimetallic catalysts showed differences in the linear CO absorption band as a function of the Ni/Pt atomic ratio. Bimetallic Ni-Pt/SiO2 catalysts showed, for the N2O decomposition, a catalytic behavior that points out an ensemble-size sensitive behavior for Ni-rich compositions. For the N2O + H2 reaction, the bimetallic catalysts were very active at low temperature. The following activity order at 300 K was observed: Ni75Pt25 > Ni25Pt75 approximately Ni50Pt50 > Pt. TOF values for these catalysts increased 2-5 times compared to the most active reference catalyst (Pt/SiO2). The enhancement of the activity in the Ni75Pt25 bimetallic catalysts is explained in terms of the presence of mixed Ni-Pt ensembles.     

Synthesis of Supported Ultrafine Non‐noble Subnanometer‐Scale Metal Particles Derived from Metal–Organic Frameworks as Highly Efficient Heterogeneous Catalysts

The properties of supported non‐noble metal particles with a size of less than 1 nm are unknown because their synthesis is a challenge. A strategy has now been created to immobilize ultrafine non‐noble metal particles on supports using metal–organic frameworks (MOFs) as metal precursors. Ni/SiO₂ and Co/SiO₂ catalysts were synthesized with an average metal particle size of 0.9 nm. The metal nanoparticles were immobilized uniformly on the support with a metal loading of about 20 wt %. Interestingly, the ultrafine non‐noble metal particles exhibited very high activity for liquid‐phase hydrogenation of benzene to cyclohexane even at 80 °C, while Ni/SiO₂ with larger Ni particles fabricated by a conventional method was not active under the same conditions.     

Catalytic Activity of the Oxide Catalysts Based on Ni<Subscript>0.75</Subscript>Co<Subscript>2.25</Subscript>O<Subscript>4</Subscript> Modified with Cesium Cations in a Reaction of N<Subscript>2</Subscript>O Decomposition

The Ni_0.75Co_2.25O₄ catalysts were prepared by a coprecipitation method and modified with cesium cations by impregnation with a solution of cesium nitrate or cesium nitrate with citric acid and ethylene glycol additives (the Pechini method). The catalysts obtained were investigated by X-ray diffraction analysis, the BET method, X-ray photoelectron spectroscopy, temperature-programmed reduction, and the temperatureprogrammed desorption of oxygen. The activity of the samples in a reaction of nitrous oxide decomposition was determined at temperatures of 200–300°C, in particular, in the presence of oxygen and water in the reaction mixture. It was found that the use of the Pechini method for supporting Cs makes it possible to obtain a more active catalyst, as compared with that prepared by impregnation with cesium nitrate, at the same cesium content (~2%) of the samples.     

Effect of preparation method on structural characteristics and propane steam reforming performance of Ni–Al2O3 catalysts

Propane steam reforming was studied over Ni–Al₂O₃ catalysts that were prepared by a conventional impregnation (IM) method and a one-step sol–gel (SG) technique. Both Ni–Al₂O₃ catalysts showed similar initial activity. However, IM-Ni–Al₂O₃ deactivated severely with time-on-stream of propane steam reforming. The catalyst prepared using a SG technique demonstrated stable catalytic performance. The two catalysts also showed major differences in product distribution, with SG catalyst giving much higher yields of hydrogen. Catalysts were characterized with temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), temperature-programmed oxidation (TPO), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. It was revealed that, with sol–gel preparation, highly dispersed small Ni crystallites are formed with a strong interaction with the support. This is shown to be important for coke suppression and catalyst stability.     

An Effect of a Support Nature and Active Phase Morphology on Catalytic Properties of Ni-Containing Catalysts in Hydrogenation of Biphenyl

Ni/Sup catalysts were prepared, where SBA-15, γ-Al₂O₃, SiO₂ were used as supports (Sup). The synthesized catalysts were investigated by the methods of low-temperature nitrogen adsorption, temperatureprogrammed reduction (TPR), and high-resolution transmission electron microscopy. The catalytic properties of the prepared catalysts were tested in liquid phase hydrogenation of biphenyl under conditions of a flow installation at temperatures of 60–100°C, pressure of 4 MPa, volumetric feed rate of 4–10 h^–1 and H₂: feed ratio of 1500 nM.. A 1 wt % solution of biphenyl in heptane,, as a model mixture, was used. It has been established that the activity of nickel hydrogenation catalysts depends on the nickel content and the type of support. The activity of supported nickel catalysts decreases in the series Ni-12/SBA-15 > Ni-12/SiO₂ >> Ni-12/Al₂O₃. The kinetic characteristics of the biphenyl hydrogenation reaction were determined: the rate constants and activation energy for the hydrogenation of the first and second aromatic rings of the substrate molecule.     

Ni/Al2O3 catalysts for CO methanation: Effect of Al2O3 supports calcined at different temperatures

The correlation between phase structures and surface acidity of Al₂O₃ supports calcined at different temperatures and the catalytic performance of Ni/Al₂O₃ catalysts in the production of synthetic natural gas (SNG) via CO methanation was systematically investigated. A series of 10 wt% NiO/Al₂O₃ catalysts were prepared by the conventional impregnation method, and the phase structures and surface acidity of Al₂O₃ supports were adjusted by calcining the commercial γ-Al₂O₃ at different temperatures (600–1200 °C). CO methanation reaction was carried out in the temperature range of 300–600 °C at different weight hourly space velocities (WHSV = 30000 and 120000 mL·g^?1·h^?1) and pressures (0.1 and 3.0 MPa). It was found that high calcination temperature not only led to the growth in Ni particle size, but also weakened the interaction between Ni nanoparticles and Al₂O₃ supports due to the rapid decrease of the specific surface area and acidity of Al₂O₃ supports. Interestingly, Ni catalysts supported on Al₂O₃ calcined at 1200 °C (Ni/Al₂O₃-1200) exhibited the best catalytic activity for CO methanation under different reaction conditions. Lifetime reaction tests also indicated that Ni/Al₂O₃-1200 was the most active and stable catalyst compared with the other three catalysts, whose supports were calcined at lower temperatures (600, 800 and 1000 °C). These findings would therefore be helpful to develop Ni/Al₂O₃ methanation catalyst for SNG production.     

Ordered Mesoporous NiCeAl Containing Catalysts for Hydrogenolysis of Sorbitol to Glycols

Cellulose-derived sorbitol is emerging as a feasible and renewable feedstock for the production of value-added chemicals. Highly active and stable catalyst is essential for sorbitol hydrogenolysis. Ordered mesoporous M–xNiyCeAl catalysts with different loadings of nickel and cerium species were successfully synthesized via one-pot evaporation-induced self-assembly strategy (EISA) and their catalytic performance were tested in the hydrogenolysis of sorbitol. The physical chemical properties for the catalysts were characterized by XRD, N₂ physisorption, H₂-TPR, H₂ impulse chemisorption, ICP and TEM techniques. The results showed that the ordered mesopores with uniform pore sizes can be obtained and the Ni nanoparticles around 6 nm in size were homogeneously dispersed in the mesopore channels. A little amount of cerium species introduced would be beneficial to their textural properties resulting in higher Ni dispersion, metal area and smaller size of Ni nanoparticles. The M–10Ni2CeAl catalyst with Ni and Ce loading of 10.9 and 6.3 wt % shows better catalytic performance than other catalysts, and the yield of 1,2-PG and EG can reach 56.9% at 493 K and 6 MPa pressure for 8 h after repeating reactions for 12 times without obvious deterioration of physical and chemical properties. Ordered mesoporous M–NiCeAl catalysts are active and stable in sorbitol hydrogenolysis.     

PtM/C (M = Ni,Cu, or Ag) electrocatalysts: effects of alloying components on morphology and electrochemically active surface areas

The microstructures of Pt/C and PtM/C (M?=?Ni, Cu, or Ag) electrocatalysts were studied using X-ray diffraction and transmission electron microscopy (TEM). The electrochemically active surface areas of the prepared materials were estimated by cyclic voltammetry in 1 M H₂SO₄. The materials, with metal contents ranging from 30 to 35 wt.%, were synthesized by chemically reducing the metal precursors in water–ethylene glycol solutions. The actual composition of the bimetallic nanoparticles corresponds to a theoretical (1:1) composition for the PtAg/C catalysts, whereas in the PtNi/C and PtCu/C materials, a portion of the alloying component exists in an oxide form. Decreasing the average metallic crystallite sizes from 3.5 to 1.6 nm does not increase the electrochemically active surface area. This apparent contradiction is because a majority of the PtNi and PtCu nanoparticles consist of 2–4 disordered crystallites. In addition, a portion of the PtNi or PtCu nanoparticle surface is covered by nickel or copper oxides, respectively. PtAg nanoparticles, which have a smaller size relative to other bimetallic particles according to the TEM data, are characterized by an intense platinum surface segregation. The agglomeration processes are lowest for the PtAg nanoparticles.