High Hydrogen Selectivity Research Articles

Stable Steam Reforming of Ethanol in a Two-Zone Fluidized-Bed Reactor

The oxidative steam reforming of ethanol in a two-zone fluidized-bed reactor (TZFBR) and in a conventional fluidized-bed reactor over a Ni/Al2O3 catalyst has been investigated. Catalyst deactivation has been studied for both contact modes. Coke generation has been verified by several techniques, and a stable performance was obtained in the TZFBR, where coke formation was counteracted with continuous catalyst regeneration. The effects of the main operating variables (steam/ethanol ratio S/E, oxygen/ethanol ratio O/E, temperature, and relative velocity with respect to the minimum fluidization velocity) have been studied. Both stable results and continuous operation without catalyst deactivation were achieved in the TZFBR in a wide range of S/E values, showing its viability for carrying out this process. High hydrogen selectivity with total conversion of ethanol was achieved even at low S/E and O/E ratios.

Synthesis and Microstructural Analysis of Homogeneously Dispersed Nickel Nanoparticles in Amorphous Silica

Silica containing uniformly dispersed Ni nanoparticles has been synthesized by a precursor solution method, involving heat treatment and reduction stages, and the transformation of the Si–Ni–O precursor into the composite powder has been investigated systematically to determine the formation mechanism of the nanoparticles. During heat treatment, nickel oxide nanoparticles are found to form simultaneously with the Si–O network, as decomposition of the alkoxy ligand and nucleation of NiO nanoparticles, both occur at around 300°C. The formation of ionically bound Si–Oδ−–Niδ+ units plays a key role in anchoring the NiO in the Si–O network and impeding further crystalline growth. Moreover, the formation of the Si–O2−–Ni2+ interactions in the composite of dispersed NiO in SiO retard the reduction of NiO, resulting in a characteristic microstructure consisting of highly dispersed metallic Ni nanoparticles within an Si–O matrix. This nickel–silica composite is expected to show enhanced reversible hydrogen adsorption, making it attractive for use in separation membranes with high hydrogen selectivity.

Synthesis and Characterization of Ni-doped Silica Membranes Prepared Using a Hybrid Sol–Gel/CVD Method

Abstract Improving the hydrogen permeation flux, while maintaining high hydrogen selectivity, is important for reducing membrane areas and enhancing membrane reactor (MR) performance. We have attempted to enhance the hydrogen permeation through the addition of nickel, which will improve both hydrogen adsorption and pore size control. The silica layer was deposited with high selectivity, using an extended counter-diffusion CVD method.

Investigation of catalyst performance and microstructured reactor configuration for syngas production by methane steam reforming

This work involves comparative investigation of two catalytic microchannel configurations for synthesis gas generation by methane steam reforming. In the first configuration, catalyst is in the form of a layer which is coated onto the microchannel wall, while the second one is defined by the packing of particulate catalyst into an empty microchannel of the same dimension. Comparison between these geometries is carried out for four different catalysts – Rh, Ru, Pd, Ni – all of which are supported on alumina, and involves testing of different reaction temperatures (873, 923, 973, 1023 K) and steam-to-carbon ratios (2.5, 3.0, 3.5) for each catalyst through a parametric approach. Compared with the packed version, wall-coated microchannel geometry leads to higher methane conversions and syngas production rates as well as lower H 2/CO ratios in the entire parameter and catalyst range investigated. Rh is the best catalyst in terms of activity and H 2 and CO throughputs, while Ni gives the highest hydrogen selectivity. The results indicate the possibility of reducing H 2/CO ratio in methane steam reforming by the use of reformers characterized by wall-coated microchannels.

Spray deposition method for the synthesis of supported catalysts with superior metal dispersion

We introduce a novel ‘spray deposition’ method as a continuous, reproducible, versatile, simple, and scalable method for the preparation of highly-dispersed, uniformly-sized, and sintering-resistant supported metal nanoparticles on porous supports. Instant drying of aqueous aerosol droplets containing the metal precursor (e.g. palladium nitrate) and the support (e.g. ferrierite zeolite) in a commercial bench-scale spray drier proves the key for maximizing the metal dispersion. In contrast, the much lower rates of solvent removal characteristic of dry and wet impregnation methods using classical drying or even freeze drying led to broad particle size distributions. The spray-dried Pd/ferrierite catalyst exhibited high alkene selectivity in the gas-phase hydrogenation of propyne. These results demonstrate that spray drying is not only a highly suited unit operation to prepare catalyst supports but also functional catalysts in one step. The method offers attractive opportunities for large-scale catalyst manufacture and as well as for preparation of well-defined systems for academic studies.

Hydrogen selective thin palladium–copper composite membranes on alumina supports

Abstract Thin and defect-free Pd–Cu composite membranes with high hydrogen permeances and selectivities were prepared by electroless plating of palladium and copper on porous alumina supports with pore sizes of 5 and 100 nm coated with intermediate layers. The intermediate layers on the 100 nm supports were prepared by the deposition of boehmite sols of different particle sizes, and provided a graded, uniform substrate for the formation of defect-free, ultra-thin palladium composite layers. The dependence of hydrogen flux on pressure difference was studied to understand the dominant mechanism of hydrogen transport through a Pd–Cu composite membrane plated on an alumina support with a pore size of 5 nm. The order in hydrogen pressure was 0.98, and indicated that bulk diffusion through the Pd–Cu layer was fast and the overall process was limited by external mass-transfer or a surface process. Scanning electron microscopy (SEM) images of the Pd–Cu composite membrane showed a uniform substrate created after depositing one intermediate layer on top of the alumina support and a dense Pd–Cu composite layer with no visible defects. Cross-sectional views of the membrane showed that the Pd–Cu composite layer had a top layer thickness of 160 nm (0.16 μm), which is much thinner than previously reported.

Effect of phosphorus on the hydrodesulfurization and hydrodenitrogenation performance of presulfided NiMo/Al 2O 3 catalyst

Phosphorus promoted presulfided NiMo/Al 2O 3 catalysts were prepared by successive incipient wetness impregnation and characterized by XRD, N 2-physisorption, TPR and HRTEM. The effect of phosphorus on the hydrodesulfurization (HDS) hydrodenitrogenation (HDN) performance of the presulfided NiMo/Al 2O 3 catalyst was investigated. The results indicate that the main active phase is type II MoS 2 or Ni-Mo-S phase because of their weak interaction with the support. The incorporation of phosphorus in the catalyst enhances the dispersion of MoS 2 particles by increasing the stacking number and reducing the slab length. Phosphorus has no influence on the metal-sulfur bonding energies in the Ni-Mo-S phases and bulk MoS 2, but it increases the proportion of Ni-Mo-S phases in the metal sulfides. NiMoP-1.2 catalyst (containing 1.2% phosphorus) exhibits high hydrogenation (HYD) selectivity in the HDS of dibenzothiophene (DBT) and high HDN ratio of quinoline, which can be attributed to its high dispersion of the active phase. The HYD activity in HDS and HDN ratio on NiMoP-1.8 catalyst with excess phosphorus (1.8%) is declined due to the aggregation of MoS 2 particles and the decrease of the catalyst surface area.

Hydrogen production by oxidative steam reforming of methanol over Ce 1− xZn xO y catalysts prepared by combustion method

Oxidative steam reforming of methanol to produce hydrogen was studied over Ce 1− x Zn x O y catalysts with different Ce/Zn ratio prepared by combustion method. Ce 1− x Zn x O y catalysts showed better catalytic activity, higher hydrogen selectivity and lower CO selectivity than both pure CeO 2 and ZnO. When x ≤ 0.1, the catalysts by combustion formed Ce 1− x Zn x O y solid solution and the catalyst of Ce 0.92Zn 0.08O y showed the best activity. The addition of ZnO promoted the reduction of CeO 2 and significantly enhanced the adsorption of methanol, which may be responsible for the high activities of Ce 1− x Zn x O y .

Catalytic hydrogenation of quinoline over recyclable palladium nanoparticles supported on tannin grafted collagen fibers

Black wattle tannin (BT), a typical natural plant polyphenol, was grafted onto collagen fiber (CF) to act as the stabilizer and carrier of Pd nanoparticles, and as a result, a recyclable heterogeneous Pd catalyst (Pd-BT-CF) was synthesized. The main physicochemical properties of the Pd-BT-CF catalyst were characterized by Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). It was found that the catalyst was in an ordered fibrous state, on which Pd nanoparticles with diameter of 4 nm were highly dispersed. Subsequently, the Pd-BT-CF catalyst was employed for the hydrogenation of quinoline. The influences of reaction time, temperature, H 2 pressure and solvent on the catalytic hydrogenation of quinoline were systematically investigated. The experimental results suggested that the Pd-BT-CF catalyst exhibited high catalytic activity and selectivity for quinoline hydrogenation. The average turnover frequency (TOF) of Pd-BT-CF was as high as 165.3 mol mol −1 h −1, and the selectivity to 1,2,3,4-tetrahydroquinoline was 100% when the reaction was carried out at 60 °C and 2.0 MPa for 1.0 h. Moreover, the activity and selectivity of Pd-BT-CF catalyst were not significantly reduced after being reused 6 times, exhibiting a satisfied reusability. Further XPS, Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and TEM analyses confirmed that the Pd species were stabilized by the hydroxyls of BT-CF, which prevented the aggregation and leakage of Pd during the reaction.

Thermodynamic analysis of hydrogen generation via oxidative steam reforming of glycerol

A thermodynamic analysis of the oxidative steam reforming of glycerol (OSRG) for hydrogen production has been carried out with Aspen plus TM. The reaction was investigated at ambient pressure within the carbon-to-oxygen (C/O) ratio of 0.5–3.0, steam-to-carbon (S/C) ratio of 0.5–8.0 and temperature of 400–850 °C. Higher C/O and S/C ratios favor the production of hydrogen from glycerol. The highest hydrogen selectivity is obtained at 600–700 °C. To predict the potential technical obstacles in the glycerol reforming process, the OSRG process was compared with oxidative steam reforming of ethanol (OSRE) in terms of hydrogen production, autothermal condition and carbon deposition. The selectivity to hydrogen via OSRG is lower than that via OSRE under identical conditions. To achieve autothermal reforming, higher S/C and C/O ratios are required for reforming of glycerol than for ethanol due to the higher oxygen content in a glycerol molecule. From the viewpoint of thermodynamics, the glycerol reforming is more resistant to the carbon deposition. On the basis of the thermodynamic analysis and the preliminary experimental study, suggestions were proposed to guide the development of the glycerol reforming technique.

Synthesis of SnO2 hollow microspheres from ultrasonic atomization and their role in hydrogen sensing

Abstract Nanostructured SnO2 hollow microspheres were synthesized using ultrasonic atomization technique. It is interesting that hollow microspheres could be prepared from ultrasonic atomization technique without any aid of template and surfactant. X-ray powder diffraction (XRD) confirmed the material to be SnO2 having tetragonal structure. Average crystallite size calculated from X-ray diffractogram using Scherer's equation was found to be 8.45 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the microscopic study of fine powder particles. Powder consists of hollow microspheres of average diameter of 0.58 μm as well as nanoparticles of average diameter of 6 nm. The sensors fabricated from such powder show high hydrogen (1000 ppm) response (S = 2379) under the optimized experimental conditions. Sensor performance merits, such as, high hydrogen response, high hydrogen selectivity, short response time (2 s) and quick recovery time (15 s) may be due to both nanocrystallites and hollow microspheres associated in SnO2 sensing material. The dramatic change in gas response was explained by the rapid diffusion of the target gas through the nano-porous structure of SnO2 hollow microspheres.

Bioethanol steam reforming on Co/ITQ-18 catalyst: Effect of the crystalline structure of the delaminated zeolite ITQ-18

ITQ-18 delaminated zeolite has been used as Co support to synthesize steam reforming catalysts. The results obtained in the steam reforming of bioethanol indicate that a highly active and selective Co-based catalyst can be prepared using this delaminated zeolite as support. The high hydrogen selectivity exhibited by this catalyst, together with its good stability and low CH4 and CO production, can be related to the high surface area and singular crystalline structure of this delaminated zeolite formed by array of “cups” distributed along of sheets. This particular crystalline structure seems to supply good positions for the stabilization of the Co metal particles improving their dispersion and preventing their sintering during reaction stages.

Highly oriented, neutral and cation-free AlPO4 LTA: from a seed crystal monolayer to a molecular sieve membrane

An oriented, neutral and cation-free AlPO(4) LTA molecular sieve membrane with high hydrogen selectivity was prepared on porous α-Al(2)O(3) supports through secondary growth of a highly oriented AlPO(4) LTA monolayer.

Ionic liquid [BMIM]PF6 as a medium for the selective hydrogenation of 1,4-diacetoxybut-2-yne on the Pd-containing catalysts

A possibility of using a ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6), as a reaction medium in the liquid-phase hydrogenation of 1,4-diacetoxybut-2-yne was examined. Two types of catalysts were studied: Pd(10%)/C and the palladium-containing catalytic system based on the biopolymer chitosan supported on silica gel (Pd(1%)/chitosan/SiO2). The data obtained indicate high selectivity of hydrogenation of 1,4-diacetoxybut-2-yne to cis-1,4-diacetoxybut-2-ene under selected conditions.

Thermo Analytic Investigation of Hydrogen Effusion Behavior – Sensor Evaluation and Calibration

AbstractThe well established carrier gas analysis (CGA) method was used to test different hydrogen detectors comprising a thermal conductivity detector (TCD) and a metal oxide semiconducting (MOx) sensor. The MOx sensor provides high hydrogen sensitivity and selectivity, whereas the TCD exhibits a much shorter response time and a linear hydrogen concentration dependency. Therefore, the TCD was used for quantitative hydrogen concentration measurements above 50 µmol/mol. The respective calibration was made using N2/H2 gas mixtures. Furthermore, the hydrogen content and degassing behaviour of titanium hydride (TiH2‐x) was studied. This material turned out to be a potential candidate for a solid sample calibration. Vacuum hot extraction (VHE) coupled with a mass spectrometer (MS) was then calibrated with TiH2‐x as transfer standard. The calibration was applied for the evaluation of the hydrogen content of austenitic steel samples (1.4301) and the comparison of CGA‐TCD and VHE‐MS.

The effect of the precursor structure on the catalytic properties of the nickel—chromium catalysts of hydrogenation reactions

The influence of the Ni2+/Cr3+ ratio in the precursor compound on the formation of the catalyst structure and its transformation upon the thermal treatment and reductive activation in hydrogen was studied. The precursors with the cation ratio Ni2+/Cr3+ = (2.3–3)/1 represent a homogeneous system of the stichtite-type structure. The treatment of the precursors at T ∼400 °C in an inert atmosphere forms a nanosized phase of the NiO-type structure with the lattice parameter a = 4.186±0.005 A. At 600 °C the lattice parameter of this phase decreases to the tabulated value (a = 4.177±0.005 A). The phase of nickel chromite of the cubic spinel structure with the lattice parameter a = 8.320±0.005 A is also observed. Hydrogen activation of the catalyst preheated at 300 °C in an inert gas leads to the formation of Ni0 crystallites with a size of ∼5.5 nm and a specific surface area of ∼7.0 m2 g−1. This catalyst exhibits high activity and selectivity in benzene hydrogenation and preferential CO hydrogenation in the presence of CO2. The catalysts with the ratio Ni2+/Cr3+ = 1/(2−3) containing nickel and chromium hydroxocarbonates as precursors are less active in the hydrogenation of benzene to cyclohexane.

Heterogenized Ru(II) phenanthroline complex for chemoselective hydrogenation of diketones under biphasic aqueous medium

The chemoselective hydrogenation of acetylacetone to 4-hydroxypentan-2-one over immobilized ruthenium phenanthroline metal complexes in amino functionalized MCM-41 in biphasic aqueous reaction medium was investigated. The catalyst was characterized by XRD, TEM, surface analysis, FT-IR and UV–vis to understand the morphology, complex geometry, and XPS such that the oxidation state of the metal complex inside the MCM-41 framework could be understood. The use of water as a solvent, not only gives high activity and selectivity for hydrogenation of acetylacetone, but also gives a path for an environmentally safer process. The optimizations of ligand, metal to ligand ratio, the choice of solvent and other reaction parameters were studied in detail. The heterogeneous catalytic system gave a higher degree of chemoselectivity (99%) towards 4-hydroxypentan-2-one as compared to homogeneous catalyst when hydrogenation was carried out using water as a solvent. The immobilized ruthenium–phenanthroline complex was easily separated and reused.

Effect of Mg and Ca addition on coke deposition over Cu–Ni/SiO 2 catalysts for ethanol steam reforming

The production of hydrogen from ethanol steam reforming with Cu–Ni catalysts supported on MgO- and CaO-modified silica has been studied. Two promoting effects have been found: reduction of the metallic Cu–Ni particles size and strengthening of the metal–support interaction. Moreover, Mg- and Ca-promoted catalysts favour the formation of defective carbon, which is more reactive and thermodynamically easier to be removed during the ethanol steam reforming process. Consequently, higher hydrogen production and lower coke formation are achieved when Cu–Ni catalysts are supported on Mg- or Ca-modified silica in comparison to unmodified Cu–Ni/SiO 2 catalyst. The highest hydrogen selectivity (84.8 mol%) is reached with a Cu–Ni/Mg–SiO 2 catalyst containing 10 wt% Mg, while the incorporation of 10 wt% Ca into Cu–Ni/SiO 2 catalyst reduces considerably the amount of coke deposited from 58.4 to 26.3 wt%, after 3 h of time on stream.

Molecular‐Sieve Membrane with Hydrogen Permselectivity: ZIF‐22 in LTA Topology Prepared with 3‐Aminopropyltriethoxysilane as Covalent Linker

High hydrogen selectivity and thermal stability are displayed by a membrane of zeolitic imidazolate ZIF-22 grown on porous ceramic supports by using 3-aminopropyltriethoxysilane (APTES) as covalent linker (see picture). At 323 K, H2/CO2 selectivity of 7.2 and H2 permeance of 1.6×10−7 mol m−2 s−1 Pa−1 were achieved.

Palladium Catalysts Synthesized by Atomic Layer Deposition for Methanol Decomposition

Atomic layer deposition (ALD) palladium films were deposited at 200 °C on various ALD metal oxide surfaces using sequential exposures to Pd(II) hexafluoroacetylacetonate (Pd(hfac)2) and formalin. In situ quartz crystal microbalance measurements as well as ex situ measurements performed on planar substrates revealed that the Pd growth begins with a relatively slow nucleation process and accelerates once an adequate amount of Pd has deposited on the surface. Furthermore, the Pd nucleation is faster on ALD ZnO surfaces compared to ALD Al2O3 surfaces. ALD was utilized to synthesize highly dispersed, uniform Pd nanoparticles (1 to 2 nm in diameter) on ALD ZnO and Al2O3 coated mesoporous silica gel, and the catalytic performances of these samples were compared in the methanol decomposition reaction. The ALD Pd−Al2O3 showed high activity and hydrogen selectivity at relatively low temperatures while the ALD Pd−ZnO showed very low activity as well as quick deactivation. In situ extended X-ray absorption fine struc...