Synthesis and characterization of catalysts produced from paper mill sludge: I. Determination of NOx removal capability

Characteristics and catalytic properties of a series of carbon-based catalysts (CBCs) produced from paper mill sludge were evaluated. The major processes involved in the production of the catalysts were chemical activation, impregnation, pyrolysis, and post pyrolysis rinsing. The porous structure, catalytic activity and thermostability of the catalysts were tailored during the production stage by introducing hetero-atoms (zinc chloride, and ferric nitrate) in the carbon structure. Characterization of the produced CBCs included determination of the surface area, pore size, and pore size distribution (PSD) from standard N₂-adsorption isotherm data. The extent of graphitization and the presence of metal crystals were identified from X-ray diffraction (XRD). The limit of the catalyst gasification was estimated from thermogravimetric analysis (TGA) conducted in an oxidized environment. The NO_x reduction capability of the produced catalysts was evaluated in the presence of carbon monoxide using a fixed bed reactor. The reaction temperature ranged from 300 to 500°C. It was shown that paper mill sludge is an excellent precursor for the production of CBCs with NO_x removal capability of 66–94%. The catalytic capability of the produced CBCs varied according to the method of production, catalyst surface properties (surface area, pore structure, PSD), metal composition and reaction temperature. The highest NO_x removal capacity was observed for the catalytic reactions carried out at 400°C. The mesoporous catalyst produced with a Zn:Fe molar ratio of 1:0.5 exhibited the maximum NO_x removal catalytic activity of 94%.     

Free‐Standing Holey Ni(OH)2 Nanosheets with Enhanced Activity for Water Oxidation

Electrochemical water oxidation is the key technology in water‐splitting reactions and rechargeable metal–air batteries, which is very attractive for renewable energy conversion and storage. Replacement of precious catalysts with cost‐effective and highly active alternatives is still a great challenge. Herein, based on theoretical predictions, holey structures are designed and fabricated on the free‐standing conventional 2D OER catalyst. By well‐controlled defects engineering, uniform tiny holes are created on the free‐standing Ni(OH)₂ nanosheets via a sol–gel method, with the embedded Zn components as the template for holes production. The whole preparation process is feasible and effective to make full use of the basal plane of 2D nanomaterials, which can provide higher surface area, richer defects, more grain boundaries, and edge sites, as well as greater distorted surfaces. Meanwhile, these holes developed inside the sheet structure can supply tremendous permeable channels for ions adsorption and transportation, enable a fast interfacial charge transfer and accelerate the reaction process. The as‐prepared 2D holey Ni(OH)₂ nanostructures exhibit excellent catalytic performance toward electrochemical water oxidation, with lower onset overpotentials and higher current densities compared with the pristine Ni(OH)₂ catalyst, suggesting the holey defects engineering is a promising strategy for efficient water‐splitting devices and rechargeable metal–air batteries.     

Hydrogen production of nickel–scandia-stabilized zirconia and copper/nickel–scandia-stabilized zirconia catalysts through steam methane reforming for solid oxide fuel cell operation

In this study, the catalytic activities of the steam methane reforming (SMR) reactions with two catalysts, including nickel–scandia-stabilized zirconia (Ni–SSZ) and copper/nickel–scandia-stabilized zirconia (Cu/Ni–SSZ), were examined and compared. The microstructure and crystallinity of the as-prepared catalysts were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Mass spectrometer was applied in the outlet streams, in order to simultaneously monitor the time-dependent kinetics in the reactor for an activity test and conversion examination. Finally, thermogravimetric analysis (TGA) and Raman spectrometer were implemented for further verification of carbon residuals on the catalysts. It was found that the incorporation of Cu on Ni–SSZ imposed significant constraints on the growth of nickel crystallites from NiO during the annealing process in reducing atmospheres. The methane conversion of Ni–SSZ and Cu/Ni–SSZ catalysts (annealed at 300 °C for 2 h) was 36.2 and 26.0%, respectively. However, the amount of carbon residuals on Cu/Ni–SSZ catalyst (300 °C for 2 h) was 18.6%, which is lower than that of the Ni–SSZ catalysts (33.2%) from TGA results. Further Raman experiments revealed that more graphite-like carbon residuals and less defects or amorphous carbons (I_G/I_D?=?2.0) were found in the case of Cu/Ni–SSZ catalysts (300 °C for 2 h). Among the catalysts in this study, the Cu/Ni–SSZ catalyst (300 °C for 2 h) is considered as a promising catalyst for SMR reaction, since it has a fair methane conversion, and characterized higher CO₂ selectivity and lower CO selectivity without compromising the hydrogen purity. More importantly, the least amount of carbon residuals was found in Cu/Ni–SSZ catalyst (300 °C for 2 h), which assured a better lifetime.     

Boosting Bifunctional Oxygen Electrocatalysis with 3D Graphene Aerogel‐Supported Ni/MnO Particles

Electrocatalysts for oxygen‐reduction and oxygen‐evolution reactions (ORR and OER) are crucial for metal–air batteries, where more costly Pt‐ and Ir/Ru‐based materials are the benchmark catalysts for ORR and OER, respectively. Herein, for the first time Ni is combined with MnO species, and a 3D porous graphene aerogel‐supported Ni/MnO (Ni–MnO/rGO aerogel) bifunctional catalyst is prepared via a facile and scalable hydrogel route. The synthetic strategy depends on the formation of a graphene oxide (GO) crosslinked poly(vinyl alcohol) hydrogel that allows for the efficient capture of highly active Ni/MnO particles after pyrolysis. Remarkably, the resulting Ni–MnO/rGO aerogels exhibit superior bifunctional catalytic performance for both ORR and OER in an alkaline electrolyte, which can compete with the previously reported bifunctional electrocatalysts. The MnO mainly contributes to the high activity for the ORR, while metallic Ni is responsible for the excellent OER activity. Moreover, such bifunctional catalyst can endow the homemade Zn–air battery with better power density, specific capacity, and cycling stability than mixed Pt/C + RuO₂ catalysts, demonstrating its potential feasibility in practical application of rechargeable metal–air batteries.     

Synthesis and investigation of bimetallic Ni-Co/Al2O3 nanocatalysts using the polyol process

ABSTRACT

Ni-Co/Al₂O₃ catalysts with different Ni:Co ratios by weight were prepared using a simple polyol process. The activities of the catalysts were evaluated for the catalytic partial oxidation of methane (CPOM) in the temperature range of 600–800°C. Numerous techniques such as x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, inductively coupled plasma-mass spectroscopy (ICP-MS), thermogravimetric analysis (TGA), high-resolution transmission electron microscopy analysis (HRTEM), scanning electron microscopy analysis (SEM-EDS) and temperature-programmed oxidation (TPO) were applied to characterize fresh and spent catalysts. The XRD analysis confirmed that the loaded particles were metals and showed possible bimetallic nano-alloy Ni-Co formation for Ni- and Co-containing catalysts. The highest metal dispersion was 15.7% for the Ni_2.8Co_2.6/Al₂O₃ catalyst. The catalytic test results showed no correlation between metal dispersion and the metal particle size, and the activity decreased in the order of Ni_7.7/Al₂O₃ > Ni_2.8Co_2.6/Al₂O₃ ≈ Ni_3.8Co_1.5/Al₂O₃ > Ni_2.0Co_3.8/Al₂O₃ >> Co_6.8/Al₂O₃ under a flow rate of 157,500 L kg^?1 h^?1 with CH₄/O₂ = 2 (using air as an oxidant) at 800°C. The obtained results also showed that when the actual atomic Ni/Co ratio was 1.07 in the Al₂O₃-supported catalyst, the dispersion of the active sites appeared to be promoted by Co addition, and the catalytic activity was stable over a reaction time of 10 h. Among all the tested catalysts, the Ni_2.8Co_2.6/Al₂O₃ catalyst exhibited acceptable activity (75%) without coking.     

Synthesis and characterization of catalysts produced from paper mill sludge. I. Determination Of NOx removal capability

Characteristics and catalytic properties of a series of carbon-based catalysts (CBCs) produced from paper mill sludge were evaluated. The major processes involved in the production of the catalysts were chemical activation, impregnation, pyrolysis, and post pyrolysis rinsing. The porous structure, catalytic activity and thermostability of the catalysts were tailored during the production stage by introducing hetero-atoms (zinc chloride, and ferric nitrate) in the carbon structure. Characterization of the produced CBCs included determination of the surface area, pore size, and pore size distribution (PSD) from standard N₂-adsorption isotherm data. The extent of graphitization and the presence of metal crystals were identified from X-ray diffraction (XRD). The limit of the catalyst gasification was estimated from thermogravimetric analysis (TGA) conducted in an oxidized environment. The NO_x reduction capability of the produced catalysts was evaluated in the presence of carbon monoxide using a fixed bed reactor. The reaction temperature ranged from 300 to 500°C. It was shown that paper mill sludge is an excellent precursor for the production of CBCs with NO_x removal capability of 66–94%. The catalytic capability of the produced CBCs varied according to the method of production, catalyst surface properties (surface area, pore structure, PSD), metal composition and reaction temperature. The highest NO_x removal capacity was observed for the catalytic reactions carried out at 400°C. The mesoporous catalyst produced with a Zn:Fe molar ratio of 1:0.5 exhibited the maximum NO_x removal catalytic activity of 94%.     

In-situ synthesis and characterizations of Bi2(O,S)3/Zn(O,S) composites for visible light hexavalent chromium reduction

Heterojunction construction with low band gap materials is an effective way of utilizing UV light active materials under visible light irradiation. Here, we report the synthesis of Bi₂(O,S)₃/Zn(O,S) heterostructure using simple solvothermal method without surfactant. The catalysts were investigated with different characterization techniques. All the composite catalysts showed high light absorption capacity in the whole visible light spectrum. The catalytic activity of the catalysts was evaluated by Cr(VI) reduction. While pure Zn(O,S) catalyst showed no significant Cr(VI) reduction, higher photocatalytic activity than individual components were exhibited after heterojunction construction with Bi₂(O,S)₃. 20-BiZnOS catalyst with Bi/Zn molar percentage of 20% showed the best photocatalytic activity among the composites with 99.5% Cr(VI) reduction within 12 min under visible light irradiation. Heterojunction formation between Bi₂(O,S)₃ and Zn(O,S) nanoparticle, and selective adsorption of Cr(VI) and desorption of Cr(III) on the surface of 20-BiZnOS composite catalyst were ascribed to the enhanced photocatalytic activity of the composite catalyst.     

Features of obtaining an effective catalyst for synthesis of carbon nanostructured materials

Abstract

In the present paper, the features of obtaining a metal oxide Fe–Co/Al₂O₃ catalyst for the synthesis of carbon nanostructured materials through thermal decomposition are considered. It was experimentally proved that the temperature and duration of the stage of thermal decomposition of the initial components solution significantly affect the catalyst activity in the synthesis of carbon nanostructured materials by the CVD method. It was found that controlling the mode parameters of the thermal decomposition of the initial components solution of the Fe–Co/Al₂O₃ catalyst, one can not only obtain a catalyst with the required characteristics but also directionally synthesize carbon nanostructured materials over the resulting catalytic systems. During the experiments, rational modes for the implementation of the method for producing catalysts were determined, allowing to form a metal oxide system with a specific surface area of 108?m²/g, the use of which in the synthesis of carbon nanostructured materials leads to the formation of multi-walled carbon nanotubes with an external diameter of 30?nm.     

Effect of palladium precursor and preparation method on the catalytic performance of Pd/SiO2 catalysts for benzene hydrogenation

Supported Pd catalysts on silica were prepared by different synthesis methods using Pd(Ac)₂ and PdCl₂ as salts precursors. The obtained materials were characterized by X-Ray Diffraction (XRD), H₂ chemisorption, and temperature programmed desorption of hydrogen (H₂-TPD). The catalytic performances of these catalysts have been evaluated in the hydrogenation of benzene. The obtained results show that metal dispersion and catalytic activity are strongly dependent on the salts precursor and the method of preparation of the catalyst. The catalysts prepared by hydrazine reduction exhibit higher activity in benzene hydrogenation than that by the polyol reduction method. Moreover, the catalyst prepared with palladium acetate showed higher catalytic activity than those prepared with palladium chloride. The superior catalytic performance of this catalyst in the hydrogenation of benzene was ascribed to a significantly better dispersion of Pd particles on the silica support.     

A comparative study of Ni/Al_2O_3–SiC foam catalysts and powder catalysts for the liquid-phase hydrogenation of benzaldehyde

In this study, Al_2O_3-washcoated SiC(Al_2O_3–SiC) foams and Al_2O_3 powder were employed as the supports of a Ni catalyst for the liquid-phase hydrogenation of benzaldehyde. A series of Ni/Al_2O_3–SiC foam catalysts and Ni/Al_2O_3 powder catalysts with a Ni loading from 10 wt% to 37 wt% of the weight of Al_2O_3 were first prepared by a deposition–precipitation(DP) method. The catalytic activity and recyclability of both kinds of catalysts were then compared. Although it had a smaller accessible surface area with the reactant, the foam catalyst with a Ni loading of 16 wt% exhibited a slightly higher conversion of benzaldehyde after 6 h(of 99.3%) in comparison with the Ni/Al_2O_3 catalyst with identical Ni loading(conversion of 97.5%). When the Ni loading increased from 16 wt% to 37 wt%, the reaction rate obtained with the foam catalyst increased significantly from 0.108 to 0.204 mol L~(-1)h~(-1), whereas the reaction rate obtained with the powder catalyst increased from 0.106 to 0.123 mol L~(-1)h~(-1). Furthermore, the specific activity(moles of benzaldehyde consumed by 1 g min~(-1)of Ni) of the foam catalyst with a Ni loading above 30 wt% was superior to that of the powder catalyst because of its smaller Ni-particle size and higher mass-transfer rate. The foam catalyst displayed a high recyclability as a function of run times owing to the strong interaction between the Ni component and the Al_2O_3 coating. The conversion of benzaldehyde over the foam catalyst remained almost unchanged after being used 8 times. In comparison, a drop of 43% in the conversion of benzaldehyde with the powder catalyst was observed after being used 7 times due to the leaching of the Ni component.     

The Effect of Precipitants on MnOx-Ni/TiO2 SCR Catalysts Prepared by Titanium Slag as Raw Material

Mn-Ni/TiO₂ catalysts were prepared using a joint precipitation method with acid-dissolved titanium slag as raw material for selective catalytic reduction of nitrogen oxide. The joint precipitation method was accomplished with different precipitants, such as sodium hydroxide, carbamide, ammonia, or hydrogen peroxide. The deNOx activities of the catalysts prepared by different precipitants were investigated with SCR activity reactor within temperatures of 90–350°C. It is evident that the Mn-Ni/TiO₂ catalyst with carbamide-ammonia-hydrogen peroxide as precipitant has superior catalytic activity, which the NO conversion can reach 90% at 120°C, with a wider temperature window (120–300°C) for the NH₃-SCR reaction. H₂-TPR results showed that the reduction potential of MnOx species on Mn-Ni/TiO₂ catalysts is increased compared to that of Mn/TiO₂ catalysts. Oxygen mobility is enhanced by interaction between Ni and Mn atoms due to use of carbamide-ammonia peroxide as precipitant. XPS results suggest that the presence of MnO₂ is the major phase in nickel-doped Mn/TiO₂ catalysts. Our NH₃-TPD results illustrated that the catalysts have a lot of acid-active sites, which lead to attracting more ammonia to participate in the catalytic reaction.     

Dye degradation over the multivalent charge- and solid solution-type n-MoS2/p-WO3 based diode catalyst under dark condition with a self-supporting charge carrier transfer mechanism

Pollution of water by synthetic dyes is of great concern because of the large production and usage of dyestuffs throughout the world. For dye elimination purpose, the p-n heterojunction Sb-doped MW-x composite catalysts prepared with different Mo:W precursor molar contents were synthesized via one-pot hydrothermal method. The selection in metal precursors is intended to design a composite catalyst with multiple valency in each phase. The structural, morphological, electrochemical property, and optical band gap of the composite catalyst were studied. The catalytic performance of Sb-MW composite catalysts was studied for the degradation of MB without light irradiation. It was observed that Sb-MW-4 showed superior catalytic performance to completely degrade 20 ppm MB dye solution with 20 mg catalyst in 14 min under dark condition. The composite catalyst was composed of n-(Mo,W)(O,S)₂ oxysulfide in a disordered MoS₂ framework and p-(W,Mo)(S,O)₃ sulfo-oxide in a WO₃ structure or was abbreviatedly named as n-MoS₂/p-WO₃ based catalyst. The degradation reaction for MB dye had an apparent rate constant of 3.5 × 10⁻¹ min⁻¹. This catalyst also showed excellent stability and reusability for degrading MB dye pollutant. The degradation mechanism under dark condition is proposed.     

Hydration-Effect-Promoting Ni–Fe Oxyhydroxide Catalysts for Neutral Water Oxidation

Oxygen evolution reaction (OER) catalysts that function efficiently in pH-neutral electrolyte are of interest for biohybrid fuel and chemical production. The low concentration of reactant in neutral electrolyte mandates that OER catalysts provide both the water adsorption and dissociation steps. Here it is shown, using density functional theory simulations, that the addition of hydrated metal cations into a Ni–Fe framework contributes water adsorption functionality proximate to the active sites. Hydration-effect-promoting (HEP) metal cations such as Mg²⁺ and hydration-effect-limiting Ba²⁺ into Ni–Fe frameworks using a room-temperature sol–gel process are incorporated. The Ni–Fe–Mg catalysts exhibit an overpotential of 310 mV at 10 mA cm⁻² in pH-neutral electrolytes and thus outperform iridium oxide (IrO₂) electrocatalyst by a margin of 40 mV. The catalysts are stable over 900 h of continuous operation. Experimental studies and computational simulations reveal that HEP catalysts favor the molecular adsorption of water and its dissociation in pH-neutral electrolyte, indicating a strategy to enhance OER catalytic activity.     

Microwave treatment and fluorine addition on Co-B catalyst to improve the hydrogen production rate

In this paper, microwave heating treatment process and fluorine addition over Co-B-F catalyst was applied to produce hydrogen via the hydrolysis of NaBH₄. The effects of microwave heating treatment time, microwave heating treatment power, microwave inert gases and temperature on the catalyst were studied. X-ray absorption spectrometer, scanning electron microscopy coupled to energy-dispersive spectroscopy, nitrogen adsorption analyzer and infrared spectrometer were performed for the chemical characterization of the catalysts. It was found that Co-B-F and microwave-treated Co-B-F catalysts exhibited excellent catalytic activity to produce hydrogen. The rates of the maximum hydrogen production for untreated and microwave-treated Co-B-F catalysts are 1868 and 3400?mL/g/min, respectively.     

The chemical heterogeneity of active surface of solid catalysts

The chemical adsorption behaviors of three adsorbates on the Ni-W/F-Al₂O₃-SiO₂ and the supported Ni catalysts have been investigated. On the basis of fractal geometry, the roughness fractal dimension (D _r) and the chemisorption fractal dimension (D _c) of the catalysts are estimated. We use the difference between D _c and D _r to characterize the chemical heterogeneity of active sites on the catalytic surface. It is found that the chemical heterogeneity of the acid sites of the Ni-W/F-Al₂O₃-SiO₂ catalyst is very large, while it decrease with increasing of impregnating operation times in preparation of the supported Ni catalyst. After 4 times impregnation, the chemical heterogeneity of its active surface for the supported Ni catalyst is eliminated.     

Synthesis of binary and triple carbon nanotubes over Ni/Cu/Al2O3 catalyst by chemical vapor deposition

Novel binary and triple carbon nanotubes (CNTs) with one common catalytic particle encapsulated have been synthesized using Ni/Cu/Al₂O₃ catalyst, which was produced by a sol-gel method. But when using Ni/Al₂O₃ as catalyst, a mass of common CNTs, that is, one CNT with one catalytic particle encapsulated, was obtained. The results showed that copper-element doping to the Ni/Al₂O₃ catalyst played a key role in the synthesis of CNTs, signifying a novel approach to modify the Ni/Al₂O₃ catalyst. Based on the transmission electron microscopy observations, a simple growth mechanism was developed to describe the growth of the binary or triple CNTs, which could be well explained by a diffusion segregation process.     

Electrokinetic recovery of Cd, Cr, As, Ni, Zn and Mn from waste printed circuit boards: Effect of assisting agents

The printed circuit boards (PCBs) contains large number of heavy metal such as Cd, Cr, As, Ni, Zn and Mn. In this study, the use of electrokinetic (EK) treatment with different assisting agents has been investigated to recover the heavy metals from waste PCBs, and the effectiveness of different assisting agents (HNO₃, HCl, citric acid) was evaluated. The PCBs were first pre-treated by supercritical water oxidation (SCWO) process, then subjected to EK process. The heavy metal speciation, migration and recovery efficiency in the presence of different assisting agents during EK process were discussed. The mass loss of Cd, Cr, As and Zn during the SCWO process was negligible, but approximately 52% of Ni and 56% of Mn were lost in such a process. Experimental results showed that different assisting agents have significant effect on the behavior and recovery efficiency of different heavy metals. HCl was highly efficient for the recovery of Cd in waste PCBs due to the low pH and the stable complexation of Cl⁻. Citric acid was highly efficient for the recovery of Cr, Zn and Mn. HNO₃ was low efficient for recovery of most heavy metals except for Ni.     

Atomically Dispersed Fe/N4 and Ni/N4 Sites on Separate-Sides of Porous Carbon Nanosheets with Janus Structure for Selective Oxygen Electrocatalysis

Dual single atoms catalysts have promising application in bifunctional electrocatalysis due to their synergistic effect. However, how to balance the competition between rate-limiting steps (RDSs) of reversible oxygen reduction and oxygen evolution reaction (OER) and fully expose the active centers by reasonable structure design remain enormous challenges. Herein, Fe/N₄ and Ni/N₄ sites separated on different sides of the carbon nanosheets with Janus structure (FeNi_jns/NC) is synthesized by layer-by-layer assembly method. Experiments and calculations reveal that the side of Fe/N₄ is beneficial to oxygen reduction reaction (ORR) and the Ni/N₄ side is preferred to OER. Such Janus structure can take full advantage of two separate-sides of carbon nanosheets and balance the competition of RDSs during ORR and OER. FeNi_jns/NC possesses superior ORR and OER activity with ORR half-wave potential of 0.92 V and OER overpotential of 440 mV at J = 10 mA cm⁻². Benefiting from the excellent bifunctional activities, FeNi_jns/NC assembled aqueous Zn–air battery (ZAB) demonstrates better maximum power density, and long-term stability (140 h) than Pt/C+RuO₂ catalyst. It also reveals superior flexibility and stability in solid-state ZAB. This work brings a novel perspective for rational design and understanding of the catalytic mechanisms of dual single atom catalysts.     

Effect of Co-Mo catalyst preparation and CH4/H2 flow on carbon nanotube synthesis

Abstract

Supported Co-Mo catalysts with a given ratio of metals were prepared from polyoxomolybdate Mo₁₂O₂₈(μ₂-OH)₁₂{Со(H₂O)₃}₄ using impregnation and combustion methods. Effects of the type of catalyst and the ratio and flow of methane and hydrogen gases on the structure of carbon nanotubes (CNTs) synthesized by catalytic chemical vapor deposition (CCVD) method were studied using transmission electron microscopy and Raman spectroscopy. The catalyst prepared by combustion method yielded mainly individualized CNTs, while the CNTs were highly entangled or bundled when impregnation method was used. In both cases, addition of hydrogen to methane led to reduction of the CNT yield. The samples synthesized using two different catalysts and the same CH₄/H₂ ratio and flow of gases were tested in electrochemical capacitors. A higher specific surface area of the CNTs grown over impregnation-prepared catalyst caused a better performance at scan rates from 2 to 1000?mV/s.     

Comparison of Structure and Conductivity of Multiwall Carbon Nanotubes Obtained over Ni and Ni/Fe Catalysts

Abstract

Multiwall carbon nanotubes (MWNT) were produced by pyrolysis of acetonitrile (CH₃CN) on metallic particles of Ni and Ni/Fe at 850°C. The special program for statistical treatment of electron micrograph images was developed. Research of diameter distribution of MWNT grown over different catalysts was carried out. Two kinds of carbon nanotubes with different diameter and microstructure are formed on Ni catalyst. The MWNT with smaller diameter and cylindrical packing of layers were found to have the higher conductivity.