Diffuse Reflectance IR Fourier Transform Spectroscopy Research Articles

Strong CO2 Chemisorption in a Metal-Organic Framework with Proximate Zn-OH Groups.

A novel Zn benzotriazolate metal-organic framework (MOF), [Zn9(OAc)6(bbtm)6] (1, bbtm2- = bis(benzotriazolyl)methanone, OAc- = acetate), has been synthesized and structurally characterized using micro-crystal electron diffraction. The framework contains 12-connected nonanuclear Zn clusters with Zn-OAc groups separated by short intercluster Zn···Zn distances of 6.06 Å. Postsynthetic OAc-/OH- ligand exchange followed by thermal activation generates 1a-OH, which adsorbs CO2 at very low pressures (1.37 mmol/g at 2.5 mbar) and requires an unusually high desorption temperature (>160 °C). Diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations have been used to interrogate the CO2 binding mechanism in 1a-OH. The formation of unsymmetric bridging carbonate ligands within the Zn···Zn pockets accompanied by strong hydrogen bonding of the carbonate with a neighboring zinc aqua ligand explains the remarkably strong CO2 affinity of 1a-OH.

Electrospun ZnO/Pd Nanofibers as Extremely Sensitive Material for Hydrogen Detection in Oxygen Free Gas Phase.

The development of safety sensors is an urgent necessity for the successful use of hydrogen in real conditions, which may differ, in particular, by the oxygen content in the surrounding atmosphere. Palladium-modified zinc oxide shows the high sensitivity when detecting hydrogen in air; however, studies of the sensor properties and the operation mechanism of the ZnO/Pd sensor when reducing gases are detected in an oxygen deficient or inert atmosphere have not been effectuated. In this work, we synthesized the ZnO and ZnO/Pd nanofibers by electrospinning and for the first time determined their sensor properties in the detection of CO, NH3 and H2 in different oxygen backgrounds. The microstructure and composition of nanofibers were characterized by electron microscopy, X-ray diffraction, X-ray fluorescent spectroscopy, and X-ray photoelectron spectroscopy. The interaction with the gas phase was investigated in situ by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100–450 °C towards CO, NH3 and H2 in the N2/O2 gas mixtures containing 0.0005–20% O2. When detecting CO, a decrease in the oxygen concentration from 20 to 0.0005% in the gas phase does not lead to a significant change in the sensor response. At the same time, when detecting NH3 and especially H2, a decrease in oxygen concentration down to 0.0005% results in the dramatic increase in the sensor response of ZnO/Pd nanofibers. This result is discussed in terms of palladium hydride formation, modulation of the potential barrier at the ZnO/Pd interface, as well as changes in the concentration of donor defects and charge carriers in the ZnO matrix. Synthesized electrospun ZnO/Pd nanofibers are extremely promising materials for sensors for detecting hydrogen in an oxygen free atmosphere.

Gas Phase Modification of Silica Nanoparticles in a Fluidized Bed: Tailored Deposition of Aminopropylsiloxane.

Functionalized nanoparticles have various applications, for which grafting of a chemical moiety onto the surface to induce/improve certain properties is needed. When incorporated in polymeric matrices, for instance, the modified nanoparticles can alter the interfacial characteristics leading to improvements ofthe macroscopic properties of the nanocomposites. The extent of these improvements is highly dependent on the thickness, morphology and conformity of the grafted layer. However, the common liquid-phase modification methods provide limited control over these factors. A novel gas-phase modification process was utilized, with 3-aminopropyltriethoxysilane (APTES) as precursor, to chemically deposit amino-terminated organic layers on fumed silica nanoparticles in a fluidized bed. A self-limiting surface saturation was achieved when the reaction was done at 200 °C. With this self-limiting feature, we were able to graft multiple layers of aminopropylsiloxane (APS) onto the silica nanoparticles using water as the coreactant. The feasibility of this process was analyzed using thermogravimetric analysis (TGA), diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and elemental analysis (EA). By altering the number of APTES/water cycles, it was possible to control the thickness and conformity of the deposited aminopropylsiloxane layer. This novel approach allows to engineer the surface of nanoparticles, by introducing versatile functionalized layers in a controlled manner.

Electrospun ZnO/Pd Nanofibers: CO Sensing and Humidity Effect.

Variable air humidity affects the characteristics of semiconductor metal oxides, which complicates the reliable and reproducible determination of CO content in ambient air by resistive gas sensors. In this work, we determined the sensor properties of electrospun ZnO and ZnO/Pd nanofibers in the detection of CO in dry and humid air, and investigated the sensing mechanism. The microstructure of the samples, palladium content, and oxidation state, type, and concentration of surface groups were characterized using complementary techniques: X-ray fluorescent spectroscopy, XRD, high-resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray (EDX) mapping, XPS, and FTIR spectroscopy. The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100–450 °C in the concentration range of 5–15 ppm CO in dry (RH25 = 0%) and humid (RH25 = 60%) air. It was found that under humid conditions, ZnO completely loses its sensitivity to CO, while ZnO/Pd retains a high sensor response. On the basis of in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) results, it was concluded that high sensor response of ZnO/Pd nanofibers in dry and humid air was due to the electronic sensitization effect, which was not influenced by humidity change.

Citrate-capped gold nanoparticles as a sensing probe for determination of cetyltrimethylammonium surfactant using FTIR spectroscopy and colorimetry

A novel, facile, and low-cost method was developed for determination of cetyltrimethylammonium (CTA+) cationic surfactant in water samples using diffuse reflectance Fourier transform IR (FTIR) spectroscopy and colorimetry. Cetyltrimethylammonium bromide was chosen as a model compound to demonstrate the optimization of the method for determination of CTA+ in water samples. The absorption peak at 3015.96 cm-1 (for CTA+) was enhanced when gold nanoparticles were used as a chemical sensor in diffuse reflectance FTIR spectroscopy, and this absorption peak was used for determination of CTA+. Alternatively, the color change from wine red (525 nm) to blue (740 nm) and the redshift of the localized surface plasmon resonance band in the visible region were used as a sensing probe for determination of CTA+. A linear calibration curve for determination in water samples was obtained in the range from 10 to 100 ng mL-1 with a limit of detection of 3 ng mL-1 by diffuse reflectance FTIR spectroscopy and in the range from 20 to 400 ng mL-1 with a limit of detection of 7 ng mL-1 by colorimetry. The advantageous features of the methods are their simplicity, rapidity, and sensitivity for the determination of CTA+ in water samples. Graphical abstract.

The Oxidation of Carbon Monoxide as an Integrated Part of the Coupled Alkane Oxidation Process: Gas-Phase Oxidation over Supported Metal-Complex Catalysts

Heterogeneous rhodium–copper chloride catalysts for gas-phase oxidation processes were prepared via the cold impregnation of γ-Al2O3 with aqueous RhCl3 and CuCl2 solutions. Heptafluorobutyric or pentafluorobenzoic acids were additionally deposited onto these catalysts to simulate the action of homogeneous rhodium–copper chloride catalytic systems in the coupled alkane–carbon monoxide oxidation reaction. The catalysts were studied in the reactions of carbon monoxide oxidation and coupled propane–CO oxidation with dioxygen by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR). The obtained data indicate the probable transfer of electrons between rhodium and copper compounds.

A Study of Ziegler-Natta Propylene Polymerization Catalysts by Spectroscopic Methods.

Ziegler–Natta polymerization catalysts were characterized by a complex of surface- and bulk-sensitive methods (DRIFTS, XPS, ESR, and XAS = XANES + EXAFS). A diffuse-reflectance Fourier-transform IR spectroscopy (DRIFTS) study showed the presence of strong Lewis acid sites in different concentrations and absence of strong basic sites in the polymerization catalysts. X-ray photoelectron spectroscopy (XPS), electron-spin resonance (ESR), and (X-ray absorption near-edge structure (XANES) analysis revealed the presence of Ti4+, Ti3+, Ti2+, and Ti1+ species in the surface layers and in the bulk of catalysts. The samples under study differ drastically in terms of the number of ESR-visible paramagnetic sites. The EXAFS study shows the presence of a Cl atom as a nearest neighbor of the absorbing Ti atom.

Regeneration of LOHC dehydrogenation catalysts: In-situ IR spectroscopy on single crystals, model catalysts, and real catalysts from UHV to near ambient pressure

The Liquid Organic Hydrogen Carrier (LOHC) concept offers an efficient route to store hydrogen using organic compounds that are reversibly hydrogenated and dehydrogenated. One important challenge towards application of the LOHC technology at a larger scale is to minimize degradation of Pt-based dehydrogenation catalysts during long-term operation. Herein, we investigate the regeneration of Pt/alumina catalysts poisoned by LOHC degradation. We combine ultrahigh vacuum (UHV) studies on Pt(111), investigations on well-defined Pt/Al2O3 model catalysts, and near-ambient pressure (NAP) measurements on real core⿿shell Pt/Al2O3 catalyst pellets. The catalysts were purposely poisoned by reaction with the LOHC perhydro-dibenzyltoluene (H18-MSH) and with dicyclohexylmethane (DCHM) as a simpler model compound. We focus on oxidative regeneration under conditions that may be applied in real dehydrogenation reactors. The degree of poisoning and regeneration under oxidative reaction conditions was quantified using CO as a probe molecule and measured by infrared reflection-absorption spectroscopy (IRAS) and diffuse reflectance Fourier transform IR spectroscopy (DRIFTS) for planar model systems and real catalysts, respectively. We find that regeneration strongly depends on the composition of the catalyst surface. While the clean surface of a poisoned Pt(111) single crystal is fully restored upon thermal treatment in oxygen up to 700K, contaminated Pt/Al2O3 model catalyst and core⿿shell pellet were only partially restored under the applied reaction conditions. Whereas partial regeneration on facet-like sites on supported catalysts is more facile than on Pt(111), carbonaceous deposits adsorbed at low-coordinated defect sites impede full regeneration of the Pt/Al2O3 catalysts.

Preparation of bimetallic gold catalysts by redox reaction on oxide-supported metals for green chemistry applications

The major problem in successful preparation of bimetallic supported catalysts is the formation of an extended contact area between the monometallic phases. The selective deposition of Au on the surface of nanoparticles of a primary oxide-supported metal has been performed by a redox method that is based on the reduction of the second metal (M2) ions with hydrogen adsorbed on the surface of first metal (M1) or with M1 itself. The gold containing bimetallic catalysts with different atomic ratios and metal combinations Au–M1 (M1=Pd, Pt, Rh, Ru, and Au–RuOx) deposited on oxide supports (TiO2, SiO2 or θ-Al2O3) have been prepared by this method with special emphasis on the preparation of low-loaded gold containing bimetallic catalysts. The samples were characterized by X-ray diffraction analysis (XRD), scanning transmission electron microscopy (STEM), temperature programmed reduction (TPR) analysis, CO adsorption, X-ray photoelectron spectroscopy (XPS), Diffuse reflectance Fourier-transform IR spectroscopy of adsorbed CO (DRIFTS-CO). By varying the conditions of the preparation procedure, the direct contact area was obtained either between two metal phases (Au/M1) or between metal and a metal oxide phase (Au/M1Ox) that changed the catalytic properties of primary supported M1. The prepared bimetallic catalysts exhibited the high activity in various reactions: from the Cl-VOC oxidative degradation to the up-to-date “green” reactions aimed at the synthesis of fine chemicals from ethanol and glycerol.

Ambivalent effect of Ni loading on gas sensing performance in SnO2 based gas sensor

The gas sensing characteristics of pure and 0.4–2.0at% Ni-loaded SnO2 nanoparticles have been measured in dry and humid atmospheres. Approximately the same response to 50ppm CO, response/recovery kinetics, and resistance in air regardless of wide range of humidity variation from dry to 80% r.h. have been accomplished by loading 1.0 and 2.0at% Ni to SnO2. The role of Ni related surface species in the decrease of humidity dependence of gas sensing characteristics has been elucidated by diffuse-reflectance Fourier transform IR spectroscopy. The work function values determined from the transient of sensor resistance and contact potential difference revealed that Ni loading to SnO2 determines the appearance of surface electron acceptors responsible for a significant upward energy bands bending even in N2 atmosphere (>0.5eV), and, ultimately, explains the significant increase of the sensors baseline resistance and the decrease of the sensor signals. In this way, the origins of the ambivalent effect of Ni loading are clarified and the way towards a rational optimization of the sensor performance opened.

Spectroscopic Detection of Active Species on Catalytic Surfaces: Steady-State versus Transient Method

Discrimination between active and spectator species is an important and demanding task in catalysis research. A comparative study of the Pd-catalyzed CO hydrogenation using in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) in steady-state and dynamic (transient) experiments shows that the information on surface species differs significantly depending on the type of experiment. In order to discriminate between active species and spectator species not involved in the surface reactions, DRIFTS was combined with a transient technique, modulation excitation spectroscopy (MES). This approach allows the detection of surface species responding to a specific periodic external stimulus, i.e.achieved by concentration modulation, and thereby offers excellent potential to unveil features of the surface processes, which are not accessible by steady-state experiments. However, the example of CO hydrogenation shows that the perturbation imposed to the system has to be chosen properly to benefit from the transient technique. Modulation of the CO concentration did not provide deeper insight into the reaction mechanism, whereas periodic changes of the hydrogen concentration provided valuable information concerning the active surface species and the reaction pathway. The study revealed that only a small fraction (about 4%) of CO molecules adsorbed on specific Pd sites reacted with hydrogen, while the majority of adsorbed CO was inactive. The inactive CO molecules overwhelmingly contributed to the spectra measured under steady-state conditions.

State of active components on the surface of the PdCl2-CuCl2/γ-Al2O3 catalyst for the low-temperature oxidation of carbon monoxide

The state of the active constituents of the freshly prepared PdCl2-CuCl2/γ-Al2O3 catalyst for the low-temperature oxidation of the carbon monoxide by molecular oxygen was studied by X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). It was shown that copper in the form of a crystalline phase of Cu2Cl(OH)3 with the structure of the mineral paratacamite and palladium chloride in an amorphous state occurred on the surface of γ-Al2O3. According to XAS data, the local environment of palladium consisted of four chlorine atoms, which formed a flat square with an increased distance between palladium and one of the chlorine atoms. The evolution of the local environments of copper and palladium upon a transition from the initial salts to the impregnating solutions and chlorides on the surface of γ-Al2O3 was considered. The role of γ-Al2O3 in the formation of the Cu2Cl(OH)3 phase was discussed. It was found by the DRIFTS method that linear (2114 cm−1) and bridging (1990 and 1928 cm−1) forms of coordinated carbon monoxide were formed upon the adsorption of CO on the catalyst surface. The formation of CO2 upon the interaction of coordinated CO with atmospheric oxygen was detected. Active sites including copper and palladium were absent from the surface of the freshly prepared catalyst.

Hydrogen storage in Pd–Ni doped defective carbon nanotubes through the formation of CH x ( x = 1, 2)

Hydrogen storage by chemisorption on multiwalled carbon nanotubes (MWCNTs) was studied. Pristine MWCNTs could only store 0.1 wt.% of hydrogen at 573 K and ambient pressure, however, oxidation treatment to produce defects and subsequent loading with a Pd–Ni catalyst significantly increased the hydrogen storage capacity up to 6.6 wt.%. The hydrogen desorption temperature was above 500 K and an in situ diffuse reflectance IR Fourier-transform spectroscopy study indicated that the hydrogen was stored in the form of CH x ( x = 1, 2) species. The study indicated that the most appropriate hydrogen chemisorption temperature was 550 K. For comparison, oxidized unloaded MWCNTs, oxidized MWCNTs separately loaded with either Pd or Ni, unoxidized fresh MWCNTs loaded with Pd–Ni, and activated carbon loaded with Pd–Ni were studied. The results showed that the defects and Pd–Ni catalyst were two essential factors for the high chemisorption of hydrogen on carbon nanotubes.

Effect of Mn and Fe on the Reactivity of Sulfated Zirconia toward H2 and n-Butane: A Diffuse Reflectance IR Spectroscopic Investigation

Sulfated zirconia (SZ) and sulfated zirconia promoted with 2 wt % manganese (MnSZ) or iron (FeSZ), all active in n-butane isomerization, were investigated using diffuse reflectance Fourier transform IR spectroscopy (DRIFTS). By adsorption of H(2) at 77 K or of n-butane at room temperature, it was found that the promoters neither enhance the Lewis nor the Brønsted acid strength. SZ and promoted SZ do not exhibit higher acid strength than zeolites. In a batch experiment using 70 hPa of H(2), SZ did not react at 473 K. Reaction of H(2) with MnSZ produced water (band at 5242 cm(-1)) and a decrease in the sulfate groups (multiple bands). Heating of SZ in 10 hPa n-butane to 573 K caused total reduction of sulfate to H(2)S (2583, 2570 cm(-1)) and partial and total oxidation of butane to olefinic species (3062 cm(-1)), CO(2), and water. MnSZ and FeSZ reacted with n-butane already at 373 K; products of skeletal isomerization (methyne CH vibration at 2910 cm(-1)) were detected and sulfate groups were consumed. Rather than increasing the acidity, the promoters enhance the oxidation potential of sulfate and facilitate alkane activation via oxidative dehydrogenation.

Modification of acid supports by solid-state redox reaction: Part I. Preparation and characterization

The solid-state redox reaction of a number of metals in contact with ZSM-5 and Y zeolites, γ-alumina, and silica was investigated by temperature-programmed hydrogen evolution (TPHE), diffuse reflectance Fourier transform IR spectroscopy (DRIFT), X-ray absorption near-edge structure spectroscopy (XANES), and X-ray powder diffraction (XRD). Among all studied metals, gallium, zinc, manganese, and iron were found to undergo a solid-state redox reaction. On zeolites, Brønsted acid sites and extraframework species and on γ-alumina all kinds of hydroxyl species are involved, whereas on silica silanol groups undergo reaction. Magnesium, on the other hand, reacted with the water adsorbed on the supports to magnesium oxide evolving hydrogen. No reaction was observed for the systems with nickel, aluminum, and tin. The solid-state redox reaction was found to be a function of the melting point provided that the metal is base enough.

Controlled deposition of chromium hexacarbonyl on silica surfaces in a fluidised bed reactor

Cr(CO) 6/silica samples were prepared in a fluidised bed reactor by vapor phase adsorption of zerovalent chromium hexacarbonyl under N 2 flow. Two different preparation methods were used; a vaporisation method without decarbonylation step and a pulse method in which a sample was decarbonylated after vaporisation, followed by repeated vaporisation–decarbonylation cycles. The chromium content can be controlled and weakly adsorbed chromium species can also be removed from the silica surface using the pulse method. The chromium carbonyl/silica interactions were characterised using diffuse reflectance Fourier transform IR spectroscopy (DRIFT). 29Si cross-polarisation magic-angle spinning (CP MAS) NMR studies indicated that chromium hexacarbonyl interacts with single silanols of the silica surface even though the interaction is either physisorption or weak chemisorption. Samples were also characterised by temperature programmed techniques. The preoxidation temperature influences to the reduction of the samples.

The uptake of SO2 on synthetic sea salt and some of its components

The uptake of sulfur dioxide (SO2) on synthetic sea salt (SSS) and its components, NaCl and MgCl2•6H2O, was studied at 298 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. Significant uptake on dried salts was not observed, placing upper limits on the uptake coefficients, γ, of < 1 × 10−4 for NaCl, < 5 × 10−4 for MgCl2•6H2O, and < 8 × 10−5 for SSS. However, SSS and MgCl2•6H2O that had not been dried before use showed significant uptake of SO2. The magnitude of the uptake depended strongly on the exposure time and the amount of water desorbing. Initially, the measured uptake coefficients for SO2 on SSS were as high as 0.09, but they rapidly decreased below 10−2 with a t−1/2 dependence as expected for approach to the equilibrium saturation concentration in an aqueous solution. The decreasing uptake coefficient slowly approaches zero over hours, consistent with reactions in a water layer with species such as CaCO3. The products of the reaction were shown by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) to include low solubility metal sulfites. These studies show that uptake of SO2 on sea salt particles, even below their deliquescence/effluorescence points, can be treated as if it is into an aqueous salt solution.

Catalytic activity of carbonyl precursor based W/Al 2O 3 and CoW/Al 2O 3 catalysts in hydrodesulfurization of thiophene

Zerovalent carbonyl precursor based W/Al 2O 3 and CoW/Al 2O 3 catalysts were prepared by controlled and gradual gas phase adsorption in a fluidized bed reactor. The preparation process was followed by diffuse reflectance Fourier transform IR spectroscopy (DRIFT) and temperature programmed desorption (TPD). The catalytic activity of the catalysts was tested in thiophene hydrodesulfurization (HDS), and the reduction behavior was determined by temperature programmed reduction (TPR). Promising activities were achieved, especially with the bimetallic tungsten cobalt catalyst. TPR profiles were in agreement with the observed activity trends. The activity and reduction measurements suggest that preparation method applied together with zerovalent precursors, leads to adsorbed surface species that are more easily activated by sulfur or reduced than traditional tungsten oxide catalysts.

Characterization of Alkali-Doped Ni/SiO2 Catalysts

The effects of the alkali doping on the reducibility and morphological properties together with distribution and crystallography of phases of a Ni/SiO2 catalyst have been investigated by N2 adsorption measurements, diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), CO adsorption, DRIFTS determinations, temperature-programmed reduction (TPR), X-ray diffraction line broadening (XRDLB), H2 chemisorption, and TEM observations. Alkaline dopants produce an increase in the weakly adsorbed fraction of hydrogen and important changes in the adsorption modes of CO. Lithium and potassium modify the distribution of Ni across the support, inducing sintering phenomena, while sodium keeps nickel particle dispersion.

The use of flow micro-calorimetry and FTIR techniques for characterising filler/organic acid interactions

Reaction of filler grade magnesium hydroxide (Martinswerke Magnifin H 10) with oleic acid (cis 9-octadecenoic acid) and stearic acid (octadecanoic acid) has been investigated using flow micro-calorimetry (FMC) and solution adsorption isotherm methods (both from heptane). Diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), sedimentation volume measurements and X-ray diffraction were also carried out on the isolated and dried adsorption isotherm treated samples. Stearic acid adsorbed to a greater extent (24 mg g-1) than oleic acid (12 mg g-1 ) and this is likely to correspond to vertical adsorption of both acids based on measured filler surface area and adsorbate molecular area. It is considered that stearic acid forms a layer more than one molecule in thickness and was shown by X-ray diffraction, in the dry state at least, to exhibit structural order. Oleic acid adsorbs as a single molecular layer with any additional (i.e. excess) acid present probably reacting fully with the magnesium ion, thus forming the double salt which then dissolves in the heptane. Magnesium stearate is not soluble in heptane. FTIR analysis of the supernatant liquors for the oleic acid treated filler showed no free acid, only magnesium oleate.