Reversible carbonylation/decarbonylation of zeolite-entrapped tetrairidium clusters

Iridium carbonyl clusters in NaY zeolite have been prepared from adsorbed [Ir(CO)₂(acad)]. The infrared spectra and the yellow color of the sample are consistent with the formation of Ir₄(CO)₁₂] in the zeolite cages, presumably the product of reductive carbonylation of the mononuclear precursor. The iridium carbonyl cluster in the zeolite could be decarbonylated by treatment with flowing H₂ at 300 ° C and 1 atm and recarbonylated by treatment with CO at 40 °C and 1 atm. The carbonylation/decarbonylation process is reversible, provided that the temperature of the decarbonylation is low, which suggests that the decarbonylated clusters may be Ir₄. Treatment of the sample in H₂ at 425 ° C and 1 atm led to the formation of particles or iridium metal outside the zeolite pores.     

Suppressed hydrogen chemisorption of zeolite encaged metal clusters: discrimination between theoretical models on the basis of Ru/NaY

The effects of thermal treatment and zeolite proton concentration on the chemical state and metal particle size of zeolite Y supported ruthenium (3.0 wt%) have been investigated using H₂-TPR, H₂-TPD, TPMS, FTIR, TEM, and EXAFS. Heating in Ar of the precursor after ion exchange, [Ru(NH₃)₆]³⁺/NaY, up to 400°C leads to nearly 100% autoreduction of the ruthenium, as evidenced by H₂-TPR and TPMS. Heating in O₂ results in the formation of volatile ruthenium oxides. After autoreduction, the Ru clusters are extremely small, their coordination numbers, derived from EXAFS, are 0.6 for Ru/HY and 0.8 for Ru/NaY. Subsequent treatment at 500°C in flowing H₂ induces Ru agglomeration to particles which are about the size of the zeolite Y supercages, as indicated by TEM and EXAFS. The Ru-Ru distances are contracted compared to bulk Ru metal. Washing of autoreduced Ru/NaY with NaOH, thus removing the protons formed during autoreduction, results in Ru agglomeration to large particles (60-100 Å). Comparison of the hydrogen adsorption of Ru clusters with similar sizes of 10-15 Å reveals a marked interaction of the Ru clusters with zeolite protons. Increasing the H⁺/Ru ratio from 3 for Ru/NaY to 10 for Ru/HY, results in a suppression of hydrogen chemisorption per Ru atom by 75%. The conclusion that formation of metal-proton adducts affects the electronic structure of the Ru clusters, thus being one of the main causes of the lowering of the heat of hydrogen chemisorption, is supported by FTIR data of adsorbed CO. The most pronounced C-O vibration band in Ru/HY is located at 2099 cm^-1; this band is absent in Ru/NaY. Significant blue-shifting of the IR bands is in conformity with electron-deficiency of the Ru clusters in Ru/HY. The results confirm that adsorptive properties of zeolite encaged metal clusters can be "tuned" by other ions sharing the same cavities.     

Density Functional Studies on the Structure and Reverse Hydrogen Spillover in Au6 Cluster Supported on Zeolite

We have studied the structure of Au₆ cluster supported on acidic form of faujasite zeolite using density functional theory. In the gas phase Au₆ minimizes to a triangular structure while on zeolite support it has a three-dimensional structure with three apical centers bending towards the support. The hydrogenated clusters Au₆H/(2H)-FAU, Au₆H₂/H-FAU and Au₆H₃/FAU generated by stepwise reverse hydrogen spillover from bridging OH groups of zeolite are energetically preferred over the Au₆/(3H)-FAU structure. The calculated reverse hydrogen spillover energy per hydrogen atom for zeolite supported Au₆H, Au₆H₂ and Au₆H₃ clusters are ?67.6, ?65.6 and ?59.1 kJ/mol, respectively.     

Rhodium supported in basic zeolite Y: A selective and stable catalyst for CO hydrogenation

Catalysts prepared by a condensation reaction of Rh(CO)₂(acac) within the supercages of zeolite Y made basic by treatment with NaN₃ are active for CO hydrogenation and selective for low-molecular-weight olefins and methanol. High partial pressures of CO (or CO + H₂) stabilize the catalyst. The predominant species in the catalyst are suggested to be rhodium carbonyl clusters trapped in the zeolite cages.     

Photocatalytic degradation of formic acid with simultaneous production of hydrogen over Pt and Ru-loaded CdS/Al-HMS photocatalysts

The CdS/Al-HMS, Pt- and Ru-loaded CdS/Al-HMS photocatalysts were prepared by template, ion exchange and precipitation reaction, and were characterized by N₂ adsorption/desorption measurements, X-ray diffraction (XRD), UV-Visible diffuse reflectance spectra (UVDRS), X-ray fluorescence (XRF), transmission electron microscopy (TEM) and fluorescence emission spectra (FES). The result showed that CdS in the form of clusters was highly dispersed inside the channels of Al-HMS. The reaction activities of photocatalysts were examined by photocatalytic degradation of formic acid under visible light irradiation (λ ≥ 420 nm). The photocatalyst, CdS/Al-HMS loaded 0.34 wt.% Pt, showed the highest H₂ evolution at a rate of 7.63 mL h^− 1 with an apparent quantum yield of 2%.     

Characterization of Pd catalysts supported on USY zeolites with different SiO2/Al2O3 ratios for the hydrogenation of naphthalene in the presence of benzothiophene

A series of ultra-stable Y-type (USY) zeolites with different SiO₂/Al₂O₃ ratios in the range of 10–80 were used as supports for preparing Pd/USY at 2 wt% Pd loading. The FT-IR of hydroxyl groups of USY zeolites, the n-butylamine chemisorption and the temperature-programmed desorption were used in combination to characterize the zeolite acidity. TPR, H₂-TPD and chemisorption using H₂ were used to characterize the Pd reduction and dispersion. The hydrogenation of naphthalene was conducted at 200 °C in the presence of benzothiophene at different sulfur/metal ratios. The hydrogenation activity, selectivity, and the sulfur tolerance strongly depended on the SiO₂/Al₂O₃ ratio (thus the acidity) of the zeolites. The activity decreased with increasing SiO₂/Al₂O₃ in this range. The IR and n-butylamine TPD showed that both the amount and strength of Brönsted acidity decreased with the increase of the SiO₂/Al₂O₃ ratio. The good relationship between the acidity modification and catalytic performance suggests that the sulfur tolerance of Pd/USY zeolite might be due to the desired metal-support interaction, which resulted in larger amount of electron-deficient Pd. However, as shown in TGA and TPO-IR studies, the higher hydrogenation performance on more acidic zeolite also caused higher amount of carbonaceous species on the catalyst.     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

EXAFS characterization of supported metal-complex and metal-cluster catalysts made from organometallic precursors

Nearly uniform (nearly molecular) supported metals made from molecular organometallic precursors are ideally suited to characterization by EXAFS spectroscopy at the metal edge. Among the most thoroughly investigated mononuclear metal complexes on metal oxide and zeolite supports are MgO-supported rhenium subcarbonyls, approximately Re(CO)₃{OMg}₃ (where the braces denote groups terminating the bulk of the support). These were made, e.g., from [HRe(CO)⁵] and from [H₃Re₃(CO)₁₂]; the Re–O_surface distance determined by EXAFS spectroscopy is 2.15 ± 0.03 Å the support is a tridentate ligand. The Re–O_surface distances in related supported complexes of Groups 7 and 8 metals are all in the range of 2.1–2.2 Å, matching those in molecular analogues. HTa{OSi}₂, prepared from [Ta(CH₂C(CH₃)₃)₃(=CHCCH₃)₃] on SiO₂, catalyzes a new reaction, propane metathesis. Supported complexes made from [HRe(CO)⁵] catalyze alkene hydrogenation but not cyclopropane hydrogenolysis, whereas catalysts made from [H₃Re₃(CO)₁₂] catalyze both these reactions, and EXAFS data indicate neighboring Re centers on the latter (but not the former), which are implicated in the catalysis. EXAFS data similarly indicate supported Mo and W pair sites as catalysts. Supported metal clusters made by decarbonylation of metal carbonyl clusters, e.g., Ir₄/γ-Al₂O₃ and Ir₆/γ-Al₂O₃ or Rh₆/zeolite NaY, are indicated by EXAFS data to be tetrahedra and octahedra, respectively. Such clusters are the species detected by EXAFS spectroscopy at 298 K in the presence of propene and H₂ undergoing catalytic hydrogenation, and they are identified as the catalytically active species. The catalytic activities of the clusters for toluene hydrogenation and alkene hydrogenation are almost unaffected by changes in metal oxide support composition, but they depend on the cluster size, although the catalytic reaction is structure insensitive. Thus, supported metal clusters offer new catalytic properties.     

Ag–Ni bimetallic SiBEA zeolite as an efficient catalyst of hydrodechlorination of 1,2-dichloroethane towards ethylene

Dealuminated form of BEA zeolite with Si/Al ratio of 1500 was used for the synthesis of Ag_2.0SiBEA, Ni_2.0SiBEA and Ag_2.0Ni_2.0SiBEA by two-step postsynthesis method. The calcination of zeolite samples led to the formation of well dispersed isolated mononuclear Ag(I) and Ag_n^δ + clusters and a pseudo-tetrahedral Ni(II), incorporated in BEA framework as evidenced by DR UV–vis investigations. The treatment of samples in flowing 10% H₂/Ar stream gave small (average 3.1 nm) and well dispersed metal nanoparticles. Reduced catalysts were investigated in 1,2-dichloroethane hydrodechlorination at atmospheric pressure, at low reaction temperature (523 K) with ~ 100% of selectivity to ethylene, desired product of the reaction.     

Photochemical reduction of nitrate to ammonia using layered hydrous titanate/cadmium sulphide nanocomposites

The photocatalytic activity of H₂Ti₄O₉/CdS nanocomposites incorporating CdS particles, less than 0·8 nm thick, in the interlayer of H₂Ti₄O₉ was evaluated for the reduction of NO₃⁻ with and without methanol. NO₃⁻ was photochemically reduced by bandgap illumination in the presence of H₂Ti₄O₉/CdS nanocomposites with and without methanol although unsupported CdS showed no noticeable photocatalytic activity for NO₃⁻ reduction. The oxidation of CdS in H₂Ti₄O₉/CdS to SO₄^2- was accompanied by NO₃⁻ reduction without methanol, whereas addition of methanol was useful to promote the NO₃⁻ reduction and depress the oxidation of CdS. The catalytic activity of H₂Ti₄O₉/CdS greatly increased with doping Pt particles in the interlayer.     

The Reduction and Oxidation of Co Species in CoZSM-5 Zeolites Studied by IR Spectroscopy

The process of reducing Co ions in zeolite CoZSM-5 by NO, H₂ and CO, as well as of oxidizing them by O₂ was studied by IR spectroscopy with CO as the probe molecule for Co²⁺ and NO as the probe for Co³⁺. Two zeolites of different Co status were used: in the first, Co²⁺ ions were localized mostly in cation exchange positions, and in the second most of the Co²⁺ occurred in the form of CoO and oxide-like clusters. IR studies evidenced that NO reduced some Co³⁺ to Co²⁺ already at room temperature, and also reduced some Co²⁺ to lower oxidation states (probably Co⁺) at 670 and 1070 K. Only Co²⁺ in exchange positions could be reduced with NO. The treatment of zeolite CoZSM-5 with H₂ or with CO at 670 and 1070 K made the reduction of Co³⁺ to Co²⁺ (the contribution of Co³⁺ decreased and the contribution of Co²⁺ increased). IR studies illustrated that the most electron-acceptor Co³⁺ (characterized by a high frequency Co³⁺–NO band) were transformed, by reduction, into the most electron-acceptor Co²⁺ (characterized by a high frequency Co²⁺–CO band). The treatment of zeolite CoZSM-5 with oxygen at 670–1070 K resulted in the oxidation of Co²⁺ to Co³⁺ (the contribution of Co²⁺ decreased while the contribution of Co³⁺ increased).     

Hexairidium carbonyl clusters in the micropores of faujasite zeolites: evidence from transmission electron microscopy

[Ir₆(CO)₁₆] was formed in the pores of zeolite NaY by adsorption of [Ir(CO)₂(acac)] followed by treatment in CO + H₂. [Ir₆(CO)₁₅]²⁻ in zeolite NaX was prepared similarly. Each sample was characterized by high‐resolution transmission electron microscopy. The images indicate the presence of the iridium clusters in the zeolite micropores, with almost no scattering centers indicating iridium outside these pores. The supported [Ir₆(CO)₁₆] and [Ir₆(CO)₁₅]²⁻, which have previously been characterized by infrared and extended X‐ray absorption fine structure spectroscopies, are among the most uniform and structurally best defined supported metal clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Construction of NiPx/MoS2/NiS/CdS composite to promote photocatalytic H2 production from glucose solution

Constructing multi-component photocatalyst is an efficient method to achieve high photocatalytic efficiency. In this work, CdS nanorods modified with NiS nanoparticles are first prepared to improve the photocatalytic performance, as no H₂ generates on single NiS or CdS catalyst from glucose solution. MoS₂ and NiP_x, as the cocatalysts for H₂ production, are loaded on the surface of NiS/CdS composite. With step-by-step solvothermal synthesis, four components (CdS, NiS, Mo₂S, and NiP_x) are fully combined in the NiP_x/MoS₂/NiS/CdS nanorods, generating many intimate contact interfaces. Moreover the optimized NiP_x/MoS₂/NiS/CdS performs a significantly increased photocatalytic activity, with H₂ production rate at 297 µmol h^–1 g^–1. The synergistic effects of heterostructure (NiS/CdS) and cocatalysts (MoS₂ and NiP_x) are the main reasons in enhancing photocatalytic performance, which facilitate the separation of charge carriers and prolong their lifetimes. This work provides an effective strategy to design photocatalysts with multiple components and fast charge separation for highly efficient H₂ production.     

Structure,stability and permeation properties of NaA zeolite membranes for H2O/H2 and CH3OH/H2 separations

NaA zeolite membranes were synthesised in the secondary growth hydrothermal method based on the seeding of the inner surface of a ceramic α-alumina tube. The impacts of crystallisation time and zeolite precursor concentration (in H₂O) were investigated. The structure and stability of the prepared NaA zeolite membranes were also investigated with operating temperatures, times and pressures. The results indicate that the optimal synthesis gel molar composition was 3Na₂O: 2SiO₂: Al₂O₃: 200H₂O. This led to cubic-shaped NaA zeolite which showed good stability. The optimal NaA zeolite membrane had H₂O and CH₃OH fluxes of 2.77 and 0.19 kg/m²h, with H₂O/H₂ and CH₃OH/H₂ separation factors of ∞ and 0.09 at a temperature of 30 °C. The NaA zeolite membrane had high thermal stability, but poor separation performance at high temperature (240 °C). The results suggested that the H₂ permeation flux is significantly influenced by preferential adsorption of vapour in the NaA zeolite membrane.     

Adsorption of H2, CO2, CH4, CO,N2 and H2O in Activated Carbon and Zeolite for Hydrogen Production

Abstract

The design of a layered pressure swing adsorption unit to treat a specified off-gas stream is based on the properties of the adsorbent materials. In this work we provide adsorption equilibrium and kinetics of the pure gases in a SMR off-gas: H₂O, CO₂, CH₄, CO, N₂, and H₂ on two different adsorbents: activated carbon and zeolite. Data were measured gravimetrically at 303–343 K and 0–7 bar. Water adsorption was only measured in the activated carbon at 303 K and kinetics was evaluated by measuring a breakthrough curve with high relative humidity.     

Carbon Monoxide Hydrogenation on Cobalt/Zeolite Catalysts

The synthesis of hydrocarbons from catalytic hydrogenation of CO/H₂ was investigated over Co/zeolite catalysts at 1 atm, 493–553 K, H₂/CO = 2, and GHSV = 1200. Various zeolites, such as NaA, NaX, NaY, KL and NaMordenite, were used as the supports. The catalysts were prepared by impregnation and were characterized by H₂/CO chemisorption and temperature-programmed reduction (TPR). Based on TPD measurements, the CO/H₂ adsorption ratio can be used as an index for the extent of metal-zeolite interaction. The stronger the metal-zeolite interaction is, the higher the Co/H₂ adsorption ratio on metal is. The activity and selectivity of cobalt supported in zeolites were affected by complex factors such as framework structure, Si/Al ratio, and the complementary cations. The activity of the catalyst is in the order: Co/KL > Co/NaX > Co/NaY > Co/NaMordenite > Co/NaA. All of the Co/zeolite catalysts had a very high selectivity to C₂–C₄ olefins, which would decrease with increasing reaction temperature. Cobalt oxide supported in zeolite was difficult to reduce. Increasing the reduction temperature could increase the reducibility of cobalt and resulted in the increase of activity.     

Hydrogen adsorption on M-ZSM-12 zeolite clusters (M = K,Na and Li): a density functional theory study

The molecular adsorption of hydrogen has been studied theoretically via DFT on additional framework with alkali metal atoms (K, Na and Li) in ZSM-12 zeolite. A 14T channel zeolite cluster model was used. Lewis acidity of alkali metals decreases with increasing atomic radius of alkali metal and H₂ adsorption. Adsorption enthalpy values were computed to be ?7.4 and ?5.1 kJ/mol on Li- and Na-ZSM-12 clusters, respectively. Hydrogen adsorption enthalpy values for Li- and Na-cases are meaningfully larger than the liquefaction enthalpy of hydrogen molecule. This designates that Li- and Na-ZSM-12 zeolites are potential cryoadsorbent materials for hydrogen storage.     

In situ carburization of metallic molybdenum during catalytic reactions of carbon-containing gases

Supported molybdenum clusters were prepared by sublimation of Mo(CO)₆ onto dehy-droxylated alumina followed by decomposition in flowing dihydrogen at 970 K. These alumina-supported molybdenum clusters were found by XAFS to transform into Mo₂C if heated in a 20% methane/H₂ mixture at 950 K. For the hydrogenolysis ofn-butane at 510 K and CO-H₂ reactions at 570 K, both at atmospheric pressure, molybdenum and carburized molybdenum showed similar, but different for each reaction, turnover rates. The product distribution was the same for each reaction on Mo and Mo₂C. In both reactions, in situ XAFS data for fresh and used catalysts indicated that Mo clusters progressively transformed into Mo₂C under the reaction conditions     

Facile Bulk Synthesis of Homogeneous and Transparent Nanocrystals Hybrids via In Situ Transformation of Ionomers into CdS Quantum-Dot-Polymer

We systemically report on a method for fabricating bulk CdS QD-polymer hybrids nanocomposites with tunable control photoluminescence (PL) by using as-prepared ionomers. Firstly, we rationally designed to synthesize well-dispersed PMMA/Cd(AA)₂ (cadmium acrylate) ionomers containing different concentrations of Cd²⁺ ion clusters via free-radical polymerization. And then, we introduced H₂S gas into the ionomer solutions to obtain transparent PMMA/CdS QD-polymers in situ. Subsequently, we employed PMMA/CdS QD-polymers to further produce claviform CdS/PMMA hybrid nanocomposites via in situ bulk polymerization. The properties of as-prepared QD-polymers and their hybrid nanocomposites were thoroughly investigated by Ultraviolet–visible (UV-vis), transmission electron microscope (TEM), Fourier transform infrared spectra (FT-IR), photoluminescence (PL), thermogravimetric analyses (TGA) measurements. We have found that these QD-polymers exhibit uniform dispersity, good optical property and an obvious advantage on thermal stability, along with facilely producing bulk nanocrystal/polymer hybrid nanocomposites with a large scale.     

Synthesis and preliminary gas permeation studies of a tubular NaA zeolite membrane (NZM)

Different from traditional seeded method, NaA zeolite membranes (NZMs) were prepared by in situ synthesis onto the inner side of porous α-alumina tubular supports in a hydrothermal synthesis reactor. The influences of pretreatment of porous tubular support, temperature, time, and synthetic cycle for the synthesis of the zeolite membranes were investigated. The operating conditions were optimized. Characterization of the membranes by scanning electron microscopy and X-ray diffraction showed that the crystalline materials on the inner surface of the porous α-alumina tubes were NaA-type zeolite. Single- and binary-gas permeation tests were conducted. Single-component permeabilities of hydrogen and nitrogen through the NZM changed slightly when the transmembrane pressure difference varied from 80 to 420?kPa. Its selectivity for H₂ relative to N₂ was about 5.3, which was greater than that of the Knudsen diffusion. The separation factors of binary gases H₂/N₂ and H₂/CO₂ at 473?K were 3.9 and 5.7, respectively, again exceeding the Knudsen diffusion level. The separation of binary gases suggests that the NaA-type zeolite membranes on α-alumina substrate were defect free and able to provide molecular sieving. The results demonstrate that the unseeded synthetic method presented in this work is successful and reliable.