Effects of Water and Ion-Exchanged Counterion on the FTIR Spectra of ZSM-5. II. (Cu+–CO)-ZSM-5: Coordination of Cu+–CO Complex by H2O and Changes in Skeletal T–O–T Vibrations

The 2157 cm^–1 (strong) and 2108 cm^–1 (very weak) (CO) IR bands due to Cu⁺–CO in ZSM-5 zeolite with ¹²C and ¹³C isotopes, respectively, are reversibly red-shifted by subsequent adsorption of H₂O at 293 K. On the contrary, the locally perturbed internal (T–O–T) asymmetric stretching framework vibration [ _as ^int (TOT)(Cu⁺–CO)=965 cm^–1] is reversibly blue-shifted. The courses of the band shifts revealed notable features. Charge transfers from water to Cu⁺ ions, changes in coordination spheres of Cu⁺(CO)(H₂O) _n aqua complexes and secondary (solvent-like) effects were considered to explain the results.     

Dehydrogenation of cyclohexanol on Cu–ZnO/SiO2 catalysts: The role of copper species

For the dehydrogenation of cyclohexanol a series of Cu–ZnO/SiO₂ catalysts with various Cu to ZnO molar ratios was prepared using the impregnation method, with the loading of copper fixed at 9.5 at.%. The catalysts were characterized by XPS, H₂–N₂O titration, BET, H₂-TPR, NH₃-TPD and XRD techniques. The results indicate that the addition of ZnO can improve the dispersion of copper species on reduced Cu–ZnO/SiO₂ (CZS) catalysts. Cu⁰ and Cu⁺ species were found on the reduced CZS catalysts surface, and the amount of Cu⁺ increased with the content of ZnO increasing. The addition of ZnO increased the acidity of the CZS catalysts. However, only Cu⁰ species can be found on the reduced Cu/SiO₂ (CS) catalyst surface. According to the reaction results, we found that the selectivity to phenol was related to the amount of Cu⁺ species, the Cu⁺ species should be the active sites for the production of phenol, the Cu⁰ is responsible for cyclohexanol dehydrogenation to cyclohexanone.     

A DRIFTS study of the morphology and surface adsorbate composition of an operating methanol synthesis catalyst

The nature of the species adsorbed on a Cu/ZnO/Al₂O₃ catalyst while it was producing methanol has been elucidated in this study using DRIFTS. The species are carbonates, formate, CO, oxygen atoms ( 2% of a monolayer) and methoxy on the Cu and methoxy on the ZnO. The frequencies observed for the C-O stretch on Cu, 2076, 2092, 2105 and 2132 cm^–1, have revealed the morphology of the copper component of the operating catalyst. The surface of the copper is predominantly the (111) face ( 65%) (the 2076 cm^–1 peak) with the (755) (the 2092 cm^–1 peak) and the (311) (the 2105 cm^–1 peak) faces occupying roughly 20% and 15%, respectively, of the copper area. The 2132 cm^–1 peak derives from CO adsorbed on Cu⁺ site on the copper which is 2% of a monolayer.     

The difference in the active sites for CO2 and CO hydrogenations on Cu/ZnO-based methanol synthesis catalysts

The effect of Zn in copper catalysts on the activities for both CO₂ and CO hydrogenations has been examined using a physical mixture of Cu/SiO₂+ZnO/SiO₂ and a Zn-containing Cu/SiO₂ catalyst or (Zn)Cu/SiO₂. Reduction of the physical mixture with H₂ at 573–723 K results in an increase in the yield of methanol produced by the CO₂ hydrogenation, while no such a promotion was observed for the CO hydrogenation, indicating that the active site is different for the CO₂ and CO hydrogenations. However, the methanol yield by CO hydrogenation is significantly increased by the oxidation treatment of the (Zn)Cu/SiO₂ catalyst. Thus it is concluded that the Cu–Zn site is active for the CO₂ hydrogenation as previously reported, while the Cu–O–Zn site is active for the CO hydrogenation.     

First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO₂ oxidation. Vanadia/silica catalysts impregnated with Cs₂SO₄ were exposed to various SO₂/O₂/SO₃ atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V₂O₅–Cs₂SO₄ and V₂O₅–Cs₂S₂O₇ molten salts. The data indicate that (1) the V^V complex V^VO₂(SO₄)₂ ^3– (with characteristic bands at 1034 cm^–1 due to (V=O) and 940 cm^–1 due to sulfate) and Cs₂SO₄ dominate the catalyst surface after calcination; (2) upon admission of SO₃/O₂ the excess sulfate is converted to pyrosulfate and the V^V dimer (V^VO)₂O(SO₄)₄ ^4– (with characteristic bands at 1046 cm^–1 due to (V=O), 830 cm^–1 due to bridging S–O along S–O–V and 770 cm^–1 due to V–O–V) is formed and (3) admission of SO₂ causes reduction of V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Creation of the active site for methanol synthesis on a Cu/SiO2 catalyst

The effect of ZnO/SiO₂ in a physical mixture of Cu/SiO₂ and ZnO/SiO₂ on methanol synthesis from CO₂ and H₂ was studied to clarify the role of ZnO in Cu/ZnO-based catalysts. An active Cu/SiO₂ was prepared by the following procedure: the Cu/SiO₂ and ZnO/SiO₂ catalysts with a different SiO₂ particle size were mixed and reduced with H₂ at 523-723 K, and the Cu/SiO₂ was then separated from the mixture using a sieve. The methanol synthesis activity of the Cu/SiO₂ catalyst increased with the reduction temperature and was in fairly good agreement with that previously obtained for the physical mixture of Cu/SiO₂ and ZnO/SiO₂. These results indicated that the active site for methanol synthesis was created on the Cu/SiO₂ upon reduction of the physical mixture with H₂. It was also found that ZnO itself had no promotional effect on the methanol synthesis activity except for the role of ZnO to create the active site. The active site created on the Cu/SiO₂ catalyst was found not to promote the formation of formate from CO₂ and H₂ on the Cu surface based on in situ FT-IR measurements. A special formate species unstable at 523 K with an OCO asymmetric peak at ~1585 cm^-1 was considered to be adsorbed on the active site. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared studies of the interaction of carbon monoxide and dinitrogen with ferrisilicate MFI-type zeolites

Fourier transform IR spectroscopy of CO and N₂, adsorbed at liquid nitrogen temperature, was used to characterize an MFI-type H-FeZSM-5 ferrisilicate which was synthesized with a Si/Fe ratio of 50. Thermal treatment of this material at 773 and 973 K was performed in order to follow formation of extraframework species. On samples fired at 773 K Brønsted acid sites were present which gave an O-H stretching band at 3630 cm^–1. These hydroxyl groups formed adducts with both probe molecules, which were monitored by the corresponding bathochromic shift, . Corresponding values for adsorbed CO and N₂ were = –270 and = – 100, respectively. Increasing the firing temperature up to 973 K led to complete removal of iron from the zeolite framework, and consequent disappearance of Brønsted acidity. In this process, extraframework iron oxide species were formed which were also characterized by IR spectroscopy of the adsorbed probe molecules. Both CO and N₂ gave Lewis-type adducts with coordinatively unsaturated Fe³⁺ ions present in the extraframework material. A comparison is made with results of a previous study on the H-GaZSM-5 isomorph.     

Pulsed Cathodoluminescence and Vibrational Structure of Localized Electronic States in Alkali Silicate Glasses

The cathodoluminescence of localized electronic states (L intrinsic centers) in glasses of the composition (mol %) 22Me ₂O · 3CaO · 75SiO₂ and Me ₂O · 3SiO₂ (Me = Li, Na, K) is investigated upon excitation with a pulsed electron beam (180 keV, 700 A/cm², 2 ns). The luminescence spectra recorded in a pulsed periodic mode contain bands of L centers of two types whose occurrence reflects the formation of fragments with different degrees of atomic ordering in the microstructure of glasses. The line spectra with separations between the lines ₀ = 820 cm^-1 and ₁ = 520-640 cm^-1 are measured in a single-pulse mode. The effect revealed is attributed to the manifestation of the vibronic interactions during radiative relaxation of the triplet state of L centers. It is demonstrated that, according to the mechanism of cathodoluminescence, electronic excitations interact with local vibrations of nonbridging oxygen atoms and phonon modes of the glass network. The corresponding interactions are classified as vibronic(₀) and electron-phonon (₁) interactions.     

An EXAFS study of Cu/ZnO and Cu/ZnO-Al2O3 methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹⁺ species ( 60%). Reduced Cu/ZnO-Al₂O₃ catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.     

Mass transfer studies at iron felts

Mass transfer has been studied at flow-through iron felts using the reduction of ferricyanide or copper cementation on iron as test reactions. Empirical correlations between a modified Sherwood number and the Reynolds number are proposed. Comparisons of the mass-transfer performance of iron felts with other three-dimensional structures are made.List of symbols a ₃ specific surface area per unit felt volume (m^–1) - A empty cross-section of the reactor (m²) - C concentration (mol m^–3) - C ₀ inlet concentration (mol m^–3) - d _h hydraulic diameter (m) - e fibre thickness (m) - E electrode potential (V) - D diffusion coefficient (m²s^–1) - F Faraday constant (A s mol^–1) - i current density (A m^–2) - I total current (A) - I _L limiting current (A) - J _m mass transfer j-factor=(k/v)Sc ^2/3 - K mass transfer coefficient (m s^–1) - l fibre width (m) - L electrode thickness (m) - Re Reynolds number - vd _h/ - Re modified Reynolds number - vl/ - Sc Schmidt number = /D - Sh modified sherwood number = ka _e l ²/D - t time (s) - T Temperature (K) - superficial liquid flow velocity (m s^–1) Greek characters void fraction - dynamic viscosity (kg m^–1 s^–1) - kinematic viscosity (m² s^–1) - ₃ charge number of the electrode reaction - iron density (kg m^—) - _a apparent density of the felt (kg m^–3) - _m residence time of the reservoir (s)     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

Study on the Mechanism of CO Formation in Reverse Water Gas Shift Reaction Over Cu/SiO2 Catalyst by Pulse Reaction, TPD and TPR

The mechanism of reverse water gas shift reaction over Cu catalyst was studied by pulse reaction with QMS monitoring, temperature programmed desorption (TPD) and temperature programmed reduction (TPR) of Cu/SiO₂ catalyst. The reduced and/or oxidized copper offered low catalytic activity for the dissociation of CO₂ to CO in the pulse reaction study with 1 ml volume of He/CO₂, but the rate of CO formation was significantly enhanced with H₂ participating in the reaction. The TPD spectra of CO₂ obtained by feeding H₂/CO₂ over copper at 773 K provided strong evidence of the formation of formate at high temperature. The formate derived from the association of H₂ and CO₂ is proposed to be the key intermediate for CO production. The formate dissociation mechanism is the major reaction route for CO production.     

Evidence for the migration of ZnOx in a Cu/ZnO methanol synthesis catalyst

The behavior and role of ZnO in Cu/ZnO catalysts for the hydrogenations of CO and CO₂ were studied using XRD, TEM coupled with EDX, TPD and FT-IR. As the reduction temperature increased, the specific activity for the hydrogenation of CO₂ increased, whereas the activity for the hydrogenation of CO decreased. The EDX and XRD results definitely showed that ZnO _x (x = 0–1) moieties migrate onto the Cu surface and dissolve into the Cu particle forming a Cu-Zn alloy when the Cu/ZnO catalysts were reduced at high temperatures above 600 K. The content of Zn dissolved in the Cu particles increased with reduction temperature and reached 18% at a reduction temperature of 723 K. The CO-TPD and FT-IR results suggested the presence of Cu⁺ sites formed in the vicinity of ZnO _x on the Cu surface, where the Cu⁺ species were regarded as an active catalytic component for methanol synthesis.     

Mass transport to reticulated vitreous carbon rotating cylinder electrodes

Mass transport to rotating cylinder electrodes (radius 0.5 cm and height 1.2 cm) fabricated from reticulated vitreous carbon (RVCRCE) was investigated using linear sweep voltammetry in a 0.5 m Na₂SO₄ + 1 mm CUSO₄ electrolyte at pH 2. At a fixed cupric ion concentration the limiting current was found to be dependent upon velocity to the power 0.55 to 0.71 depending upon the porosity grade of the carbon foam. The product of mass transport coefficient and specific electrode area, km A _e, was found to be approximately 0.51 s^–1 at 157 rad s^–1 (corresponding to 1500 rpm) for the 100 ppi material. The experimental data are compared to the predicted performance of a hydrodynamically smooth rotating disc electrode (RDE) and rotating cylinder electrode (RCS).Nomenclature A electrode area (cm²) - A _e active electrode area per unit volume (cm^–1) - C _B bulk copper concentration (mol cm^–3) - c ₀ concentration at t = 0 (mol cm^–3) - c _t concentration at time t (mol cm^–3) - D diffusion coefficient (cm²s^–1) - F Faraday constant (96 485 A s mol^–1) - h height of rotating cylinder electrode (cm) - I _L limiting current (A) - I _L,RDE limiting current at an RDE (A) - I _L,RCE limiting current at an RCE (A) - I _L,RVC limiting current at a rotating RVCRCE (A) - km mass transport coefficient (cm s^–1) - r radius of RCE (cm) - U electrolyte velocity (cm s^–1) - V reactor volume (cm ³) - V _e overall volume of electrode (cm ³) - x characteristic length (cm) - z number of electrons Greek symbols ratio of limiting current at an RVCRCE relative to an RDE of same diameter - ratio of limiting current at an RVCRCE relative to an RCE of same overall volume - thickness of the diffusion layer (cm) - electrolyte viscosity (cm²s^–1) - rotation speed (rads^–1 Dimensionless groups Re = U _/ Reynolds number - Sc = /D Schmidt number - Sh = k _m/D Sherwood number     

Methanol synthesis over Cu/SiO2 catalysts

The rates of CO and CO/CO₂ hydrogenation at 4.2 MPa and 523 K are reported for a series of Cu/SiO₂ catalysts containing 2 to 88 wt.% Cu. These catalysts were prepared on a variety of silica sources using several different Cu deposition techniques. In CO/CO₂ hydrogenation, the rate of methanol formation is proportional to the exposed Cu surface area of the reduced catalyst precursor, as determined by N₂O frontal chromatography. The observed rate, 4.2×10^–3 mole CH₃OH/Cu site-sec, is within a factor of three of the rates reported by others over Cu/ZnO and Cu/ZnO/Al₂O₃ catalysts under comparable conditions. These results suggest that the ZnO component is only a moderate promoter in methanol synthesis. Hydrogenation of CO over these catalysts also gives methanol with high selectivity, but the synthesis rate is not proportional to the Cu surface area. This implies that another type of site, either alone or in cooperation with Cu, is involved in the synthesis of methanol from CO.     

Support and additive effects in the synthesis of methanol over copper catalysts

Catalysts containing copper and ZnO in various combinations have been prepared, the copper surface areas have been measured by nitrous oxide frontal chromatography, and the activities in the reaction of CO/CO₂/H₂ and CO/H₂ mixtures to methanol have been determined at 250°C and 10 bar pressure. The results show that there is a strong synergy between copper and ZnO with the area specific rate of Cu/ZnO catalysts being about one order of magnitude larger than that of a Cu/SiO₂ catalyst. The synergy between copper and ZnO is observed both in the presence and absence of carbon dioxide. It is also observed that physical mixtures of Cu/SiO₂ and ZnO/SiO₂ catalysts are significantly more active than either of the components alone. The results are discussed in terms of possible interactions between copper and ZnO in the most active catalysts.     

Natural convection mass transfer at horizontal screens

The free convection mass transfer behaviour of horizontal screens has been investigated experimentally using an electrochemical technique involving the measurement of the limiting currents for the cathodic deposition of copper from acidified copper sulphate solutions. Screen diameter and copper sulphate concentration have been varied to provide a range ofSc.Gr from 22×10⁸ to 26×10¹⁰. Under these conditions, the data for a single screen are correlated by the equation:Sh=0.375(Sc.Gr)^0.305 Results have been compared with previous work on free convection at horizontal solid surfaces where mass transfer coefficients are somewhat lower.Mass transfer coefficients have been measured also for arrays of closely spaced parallel horizontal screens. The mass transfer coefficient was found to decrease with the number of screens forming the array.Symbols and units A area of mass transfer surface, cm² - C _b bulk concentration of ionic species, mol cm^–3 - D diffusivity, cm²s^–1 - F Faraday number, 96494 C g [equiv^–1] - Z number of electrons involved in the reaction - I _L limiting current, A - K mass transfer coefficient, cm s^–1 - Sh Sherwood number, dK/D - Sc Schmidt number,/D or/D - Gr Grashof numbergd ³/ ² _s - solution dynamic viscosity, g cm s^–1 - solution kinematic viscosity, cm² s^–1 - solution density, g cm^–3 - density difference between bulk solution and electrode/solution interface, g cm^–3 - _s solution density at electrode/solution interface, g cm^–3 - d screen diameter, cm - g gravitational acceleration, cm s^–2 On leave of absence, Chemical Engineering Department, Alexandria University, Alexandria, Egypt.     

The synergy between Cu and ZnO in methanol synthesis catalysts

The hydrogenation of CO₂ over physically-mixed Cu/SiO₂ and ZnO/SiO₂ was carried out to clarify the synergetic effect between Cu and ZnO in Cu/ZnO methanol synthesis catalysts. The activity of the physical mixtures significantly increased with increasing reduction temperature in the range of 573–723 K. TEM-EDX results definitely showed that ZnO_x moieties migrated from ZnO/SiO₂ particles onto the surface of Cu particles when the physical mixtures were reduced at high temperatures above 573 K. Upon the migration of the ZnO_x species, the oxygen coverage on the surface of Cu, measured after the hydrogenation of CO₂, increased with the reduction temperature. The results clearly showed that the synergetic effect of ZnO in the physical mixtures can be ascribed to the creation of active sites such as Cu⁺ which the ZnO_x moieties stabilize on the Cu surface. Further, XRD results showed that the migrated ZnO_x species partly dissolved into the Cu particles to form a Cu—Zn alloy.     

On the Issue of the Active Site and the Role of ZnO in Cu/ZnO Methanol Synthesis Catalysts

The problem concerning the active site and the role of ZnO in Cu/ZnO-based methanol synthesis catalysts can be consistently explained based on the literature results by distinguishing CO₂ and CO hydrogenations. Although only metallic copper has some activities for methanol synthesis by the hydrogenation of CO₂, Cu-Zn alloying in Cu particles is responsible for the major promotional role of ZnO in industrial Cu/ZnO-based catalysts. The morphology effect reported in the literature will probably appear for the system of highly dispersed Cu particles supported on ZnO. As for the hydrogenation of CO, Cu⁺ species or Cu-O-Zn sites are the active sites for methanol synthesis. The spillover effect of the Cu-ZnO system is not significant compared to the effect of ZnO on the creation of the Cu-O-Zn site.     

Effect of gas evolution on mass transfer in an electrochemical reactor

Experiments were conducted to study the effect of gas bubbles generated at platinum microelectrodes, on mass transfer at a series of copper strip segmented electrodes strategically located on both sides of microelectrodes in a vertical parallel-plate reactor. Mass transfer was measured in the absence and presence of gas bubbles, without and with superimposed liquid flow. Mass transfer results were compared, wherever possible, with available correlations for similar conditions, and found to be in good agreement. Mass transfer was observed to depend on whether one or all copper strip electrodes were switched on, due to dissipation of the concentration boundary layer in the interelectrode gaps. Experimental data show that mass transfer was significantly enhanced in the vicinity of gas generating microelectrodes, when there was forced flow of electrolyte. The increase in mass transfer coefficient was as much as fivefold. Since similar enhancement did not occur with quiescent liquid, the enhanced mass transfer was probably caused by a complex interplay of gas bubbles and forced flow.List of symbols A electrode area (cm²) - a constant in the correlation (k = aRe ^m , cm s^–1) - C _{R, bulk} concentration of the reactant in the bulk (mol^–1 dm^–3) - D diffusion coefficient (cm² s^–1) - d _h hydraulic diameter of the reactor (cm) - F Faraday constant - Gr Grashof number =gL ³/² (dimensionless) - g gravitational acceleration (cm s^–2) - i _g gas current density (A cm^–2) - i _L mass transfer limiting current density (A cm^–2) - k mass transfer coefficient (cm s^–1) - L characteristic length (cm) - m exponent in correlations - n number of electrons involved in overall electrode reaction, dimensionless - Re Reynolds number =Ud _h^–1 (dimensionless) - Sc Schmidt number = D ^–1 (dimensionless) - Sh Sherwood number =kLD ^–1 (dimensionless) - U mean bulk velocity (cm s^–1) - x distance (cm) - _N equivalent Nernst diffusion layer thickness (cm) - kinematic viscosity (cm² s^–1) - density difference = (_L – ), (g cm^–3) - _L density of the liquid (g cm^–3) - average density of the two-phase mixture (g cm^–3) - void fraction (volumetric gas flow/gas and liquid flow)