Diffuse Reflectance IR Spectroscopy Research Articles

Reactivity of Platinum Hydrides in the Selective Hydrogenation of Acetic Acid on Pt–ReOx/TiO2 Catalysts

The interaction between hydrogen and subnanometer deposited Pt–ReOx particles active in dehydrogenation of carboxylic acids is investigated via in situ diffuse reflectance IR spectroscopy. Absorption bands of platinum hydrides are detected in the range of 2025–2043 cm−1, and their high reactivity with respect to adsorbed acetic acid is revealed. The absorption band of platinum hydride shifts toward higher frequencies and becomes more intense, due to the effect of adjacent acetates on the electron state of platinum. It is established that in a hydrogen medium the intensity of platinum hydride bands sharply increases after the adsorption of acetic acid and then gradually decreases owing to the reaction of the hydrides with surface acetates.

Features of the Mechanism of the Dimethyl Ether to Light Olefins Conversion over MgZSM-5/Al2O3: Study by Vibrational Spectroscopy Experimental and Theoretical Methods

Features of the mechanism of dimethyl ether (DME) conversion to olefins over ZSM-5 zeolite catalysts were studied using experimental and theoretical methods of vibrational spectroscopy. A catalytic activity comparison of the catalysts Mg-HZSM-5 without a binder and Mg-HZSM-5/Al2O3 containing 1% Mg (by weight) and 33% Al2O3 (by weight) as a binder in the DME conversion was carried out. Using high-temperature diffuse reflectance IR (DRIR) spectroscopy in situ combined with quantum-chemical simulations in the temperature range of 25–450 °C in a stream of dry Ar and DME, the bands corresponding to OH bonds, including BAS and H3O+, were interpreted. Depending on the temperature and the presence of magnesium and a binder in the catalyst composition, the intensity and position of these bands maxima vary greatly. The intermediates of the catalytic DME conversion were discovered and identified. At low temperatures (below 200 °C), in a DME stream, methoxy groups (CH3O-Al-), ketene (CH2 =C=O), and a carbocation (CH3 +) were formed on the surface of the catalysts. As the temperature rises (above 300° C), the bands from ketene completely disappear in the spectra of catalysts, and bands from oxonium cations or ylide particles appear, leading the process of DME conversion by the oxonium-ylide mechanism. In the presence of H3O+, the conversion of DME on the zeolite catalyst surface was more effective; however, selectivity for olefins was lower.

Comparative study on redox properties in CO oxidation of layered oxides LnFe0.5Sb1.5O6 (Ln = La, Ce, Pr) synthesized by different methods

The impact of the synthesis method and morphology of the LaFe0.5Sb1.5O6 samples on their catalytic activity in CO oxidation was studied in detail. The samples obtained by solid-phase reaction and co-precipitation showed higher catalytic activity. The features of the morphology, which cause the lowering of catalytic activity, were revealed for the samples synthesized in molten salts with and without preliminary microwave-hydrothermal treatment. A comparative study of all synthesized rosiaites LnFe0.5Sb1.5O6, Ln = La–Pr, LaFe0.5-xGaxSb1.5O6 and also pyrochlore Pr2(Fe,Sb)O7 using an in situ diffuse reflection IR spectroscopy method was undertaken to clarify the mechanism of CO oxidation. It was revealed that in the course of the reaction the intermediate complexes of metal carbonyls are formed on the surface. At the same time, inactive carbonyls, which exist on the of Ce- and Pr-containing samples surface before the reaction mixture is fed, were found. This feature gives rise to a decrease in their catalytic activity. A reaction mechanism occurring via intermediate carbonyl complexes on the active centers - [Sb3+ - VO- - Fe3+] – is proposed. The CO oxidation on completely gallium-substituted samples indicates the existing of an additional channel for proceeding of redox process, which is not associated with Fe3+. It is suggested that the nature of this channel is possibly associated with a rosiaite layered structure, which creates a unique environment conducive to complex adsorption phenomena.

Синтез наночастиц палладия на поверхности оксида церия(IV) под воздействием ультрафиолетового излучения и их диагностика

Проведен синтез наночастиц палладия с использованием комплексного оксалата палладия в качестве прекурсора в водной среде методом УФ-облучения. Образцы исследованы различными лабораторными методиками, такими как просвечивающая электронная микроскопия, рентгенофлуоресцентный анализ, рентгеновская порошковая дифракция, XAFS-спектроскопия и ИКспектроскопия диффузного отражения с использованием зондирующих молекул CO. Микроизображения, полученные с помощью просвечивающей электронной микроскопии, не позволили отличить наночастицы Pd от материала подложки, но показали отсутствие влияния УФ-излучения на размеры наночастиц CeO2. Данные рентгенофлуоресцентного анализа показали наличие в материале атомов церия и палладия. Дифрактограммы свидетельствуют о присутствии фаз диоксида церия и металлического палладия, также анализ спектров XAFS за K-краем палладия показал наличие в системе фазы PdO. Приблизительный размер наночастиц палладия оценен по ИК-спектрам после адсорбции CO, он составил не более 2 нм, что существенно меньше среднего размера наночастиц Pd, полученных аналогичным способом без подложки CeO2(1.5–9.5 нм).

Study of Fe- and Ni-Containing Lignins by Diffuse Reflectance IR Spectroscopy and X-ray Diffraction

Study of Fe- and Ni-Containing Lignins by Diffuse Reflectance IR Spectroscopy and X-ray Diffraction

Dynamics of Lignin Destruction in Pine Wood under the Action of Ozone

The destruction of lignin in pinewood during ozonization is studied via diffuse reflection IR spectroscopy (DRIFT) and Raman spectroscopy. The DRIFT spectra are deconvoluted into Lorentz components. Samples of dioxanlignin obtained from wood are analyzed by means of UV spectroscopy (second derivatives). Based on the DRIFT, Raman, and UV spectra, specific rates of ozone (Qr) consumption corresponding to the oxidation of different structures on the wood surface are determined. In the range of Qr ≤ 1.5 mmol/g, the predominant process is the breakdown of aromatic rings. In the range of 1.5–3.0 mmol/g, hydroxyaromatic compounds formed at the first stage of ozonization are oxidized. At Qr higher than 2.0 mmol/g, aliphatic structures of lignin are also oxidized by ozone, along with carbonyl and carboxylic oxidation products occurring on the surface. These lignin oxidation products block to some extent the accessibility of lignin for ozone.

Synthesis, Crystal Structure, and Molecular Structure of Maleate Tris(Benzohydrazide) Cobalt(II) Tetrahydrate [Co(L1)3](Mal) · 4H2O and Maleate Tris(Phenylacethydrazide) Nikel(II) Monohydrate [Ni(L2)3](Mal) · H2O

Coordination compounds [Co(L1)3](Mal) · 4H2O (I) and [Ni(L2)3](Mal) · H2O (II) (L1 is benzohydrazide, L2 is phenylacethydrazide, and H2Mal is maleic acid) have been synthesized and characterized by IR spectroscopy and diffuse reflectance spectroscopy. Crystal structures of I and II have been determined by single-crystal X-ray diffraction. Structural units of crystals I and II are complex cations [Co(L1)3]2+ and [Ni(L2)3]2+, respectively, maleate-ions Mal2–, and crystallized water molecules bound by a branched system of hydrogen bonds. Metal atoms at the complex cations of crystals I and II have octahedral coordination (3O + 3N) formed by three bidentate chelate ligands Ln (n = 1, 2).

Highly Selective Anaerobic Oxidation of Alcohols Over Fe‐doped SrTiO3 Under Visible Light

AbstractPhotocatalytic oxidation of alcohols with high selectivity is a promising approach for the synthesis of organic compounds under mild conditions and for solar energy conversion. In this work, we report on the highly selective anaerobic photooxidation of alcohols to carbonyl compounds with coupled H2 production over Pt‐loaded Fe‐doped SrTiO3 under visible light. Representatively, an optimized apparent quantum efficiency of 13.2 % at 420 nm was obtained for benzyl alcohol oxidation. X‐ray absorption fine structure and in situ diffuse reflectance IR spectroscopy revealed that the surface oxygen vacancies and the fine‐tuned valence band edge position induced by Fe doping not only contributed to the activation of α‐C−H bonds in alcohols, but also avoided the over‐oxidation of the obtained carbonyl compounds.

Zr- and La-containing Catalysts in Synthesis of Methanol and Dimethyl Ether

Zirconium oxide samples synthesized and modified with Al, Si, W oxides were studied as dehydrating components of bifunctional catalysts for synthesis of dimethyl ether. ZrO2 modified with Al and Si oxides is inactive in the dehydration of methanol, but its dehydrating activity becomes comparable with the activity of γ-Al2O3 upon introduction of WO3. According to the data on methanol adsorption at 46°C, the adsorption capacity of ZrO2 modified with SiO2 is comparable with the capacity of γ-Al2O3, and that of ZrO2 modified with WO3 is four times lower. The difference in the catalytic and adsorption behavior of the samples is accounted for by the data of in situ high-temperature diffuse reflection IR spectroscopy. A high content of weak Bronsted acid centers is observed on the ZrO2 surface, with the interaction of these with methanol resulting in the methoxylation of the catalyst surface. When W oxide is deposited onto zirconium oxide, the surface has strong Bronsted centers involved in the reaction of methanol dehydration. A coprecipitation from solution of nitrate salts of Cu, Zn, La with Na carbonate, followed by the procedures of filtration, washing, and thermal treatment, yielded a catalyst for synthesis of methanol, the activity of which is close to that of the industrial catalyst.

Synthesis of SrFexTi1-xO3-δ nanocubes with tunable oxygen vacancies for selective and efficient photocatalytic NO oxidation

Oxygen vacancies of metal oxides play critical roles in tunning activity and selectivity for many photocatalysis mediated reactions, yet the mechanism of NO oxidation on defect enriched photocatalyst surface is seldomly discussed. Herein, we provide detailed insight into the relationship between oxygen vacancy manipulation by extrinsic Fe3+ substitution in SrTiO3 host lattice and the photocatalytic performance of NO abatement. In particular, the hydrothermal synthesized SrFexTi1-xO3-δ nanocubes (denoted as SFTO-hyd sample) rather than the impregnated-post annealing sample, enabled oxygen vacancy formation, and promoted O2 adsorption and superoxide anion radicals (O2−) formation. The SFTO-hyd (x = 5%) sample showed remarkably higher NO removal activity and selectivity under Xe lamp (λ > 420 nm), in comparison with the pristine SrTiO3, P25 and impregnation-doped SFTO sample, underlining the important roles played by coexisted Fe3+ sites and oxygen vacancies. The in situ diffuse reflectance IR spectroscopy (DRIFTS) mechanically revealed that SrTiO3 provided Lewis acidic sites for NO dark adsorption and photoreaction with nitrates as final products; the substitutional Fe3+ sites provided more active sites for NO adsorption and photoreaction with enhanced number of radicals. This study deepens the understanding of photocatalytic NO abatement on defective surface, and may also provide a simple and cost effective strategy for synthesizing efficient and selective photocatalysts for environmental remediation.

Hydrothermal treatment for regeneration of activated carbon loaded with organic micropollutants

Hydrothermal treatment (HT) at 200 °C and 240 °C for 4 and 16 h was studied for the regeneration of granular activated carbon (AC) loaded with a range of organic micropollutants having a broad range of physico-chemical properties. Carbamazepine, diazinon, diclofenac, estrone, iohexol, metoprolol and sulfamethoxazole were fully converted. Limits were seen for the conversion of caffeine, ibuprofen and perfluorooctanesulfonate (PFOS). However, the degree of degradation was enhanced for the latter compounds in the adsorbed state as compared to experiments in aqueous solution. The methodology was tested in five loading and regeneration cycles for selected compounds with no change of the degradation potential and of the AC properties. In particular, the surface properties of the AC did not deteriorate upon HT as determined by the specific surface area (from BET isotherms), the point of zero charge, and the surface functional groups (from diffuse reflectance IR spectroscopy). As the total concentration of the loaded pollutants was minimized by HT, this method could be considered as a new low temperature regeneration technology for spent AC.

Theoretical study on Pb2VO2F5: large birefringence derived from optical anisotropies of VO2F4 groups

The Pb2VO2F5 crystal was synthesized by hydrothermal method. The crystal structure consists of tetranuclear [Pb4O6F22] and twinborn V2O4F6 units, which share the O and F atoms forming a three-dimensional framework. The birefringence of the compound was investigated using the first principles. The calculations results show that the compound has a large birefringence of 0.336 at 1064 nm. Researches indicate that its large birefringence originates mainly from the distorted VO2F4 groups based on the real-space atom-cutting method and structure–property relationships analysis. Furthermore, the IR spectrum, thermal behaviors, and UV–Vis–IR diffuse reflectance spectroscopy of the compound are also presented.

Control of the properties of catalysts for methane aromatization by synthesizing ZSM-5 zeolites with different crystallite sizes

MFI zeolite materials (ZSM-5) with crystal sizes in the range from 0.10 to 1.70 μm have been synthesized. Acidic and surface properties, phase and morphological composition of the prepared zeolites have been studied by the IR sprectroscopy, nitrogen porosimetry, XRD, and scanning electron microscopy. Increasing crystal size was shown to decrease the general acidity of the zeolite. Synthesized zeolites served as supports for molybden-containing catalysts for methane aromatization prepared by using the solid phase synthesis approach. Diffuse reflectance IR spectroscopy, thermoprogrammed desoption of ammonia and 27Al NMR spectroscopy were used to characterize the physicochemical properties of the catalysts. An increase in the crystallite size of the zeolite favors a decrease in the acidity of the catalysts and inhibits the formation of alumina molybdate during the catalyst preparation. As a result, a tendency to coke formation is suppressed and the performance of the catalysts in methane aromatization improved: methane conversion and aromatic hydrocarbon yield increase.

Microwave-Assisted Synthesis of Perovskite SrSnO3 Nanocrystals in Ionic Liquids for Photocatalytic Applications

Nanosized SrSnO3 photocatalysts have been successfully synthesized by microwave synthesis in various ionic liquids (ILs) followed by a heat treatment process to optimize the materials' crystallinity. The influence of the ILs with various cations such as 1-butyl-3-methylimidazolium ([C4mim]+), 6-bis(3-methylimidazolium-1-yl)hexane ([C6(mim)2]2+), butylpyridinium ([C4Py]+), and tetradecyltrihexylphosphonium ([P66614]+) and bis(trifluoromethanesulfonyl)amide ([Tf2N]-) as the anion on the structure, crystallization, and morphology of the products was investigated. The samples were characterized by X-ray diffraction (XRD), thermogravimetry (TG), scanning electron microscopy (SEM), surface area analysis by gas adsorption, X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-vis spectroscopy, and Raman and IR spectroscopy. According to structure characterization by XRD and Raman spectroscopy all samples crystallized phase-pure in the orthorhombic GdFeO3 perovskite structure type. SEM reveals that, on the basis of the IL, the obtained SrSnO3 nanoparticles exhibit different morphologies and sizes. Rod-shaped particles are formed in [C4mim][Tf2N], [C6(mim)2][Tf2N]2, and [P66614][Tf2N]. However, the particle dimensions and size distribution vary depending on the IL and range from quite thin and long needlelike particles with a narrow size distribution obtained in [P66614][Tf2N] to relatively larger particles with a broader size distribution obtained in [C6(mim)2][Tf2N]2. In contrast, in [C4Py][Tf2N] nanospheres with a diameter of about 50 nm form. For these particles the highest photocatalytic activity was observed. Our investigations indicate that the improved photocatalytic activity of this material results from the synergistic effect of the relatively large surface area associated with nanosize and an appropriate energy band structure.

Synthesis and characterization of mixed alkali borophosphate with a new 1D chain: Li3Cs2BP4O14

A new anhydrous mixed alkali metals borophosphate, Li3Cs2BP4O14, has been synthesized by the solid-state reaction. Single crystal X-ray diffraction analysis reveals that the title compound crystallized in a tetragonal geometry with space group of P42/mbc (No. 135) and cell parameters of a=10.3413(5)Å, b=10.3413(5)Å, c=12.042(2)Å. Its one-dimensional (1D) chain structure is constructed by fundamental building unit (FBU) of [B(P2O7)2]5− which contains P–O–P connections with B:P=1:4. Thermal analysis, UV–Vis-near-IR diffuse reflectance spectroscopy, IR spectroscopy and theoretical calculations have been well performed on this new borophosphate compound.

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.

Zeolite ZSM-5 containing copper ions: The effect of the copper salt anion and NH4OH/Cu2+ ratio on the state of the copper ions and on the reactivity of the zeolite in DeNO x

Isotherms of copper cation sorption by H-ZSM-5 zeolite from aqueous and aqueous ammonia solutions of copper acetate, chloride, nitrate, and sulfate are considered in terms of Langmuir’s monomolecular adsorption model. Using UV-Vis diffuse reflectance spectroscopy, IR spectroscopy, and temperatureprogrammed reduction with hydrogen and carbon monoxide, it has been demonstrated that the electronic state of the copper ions is determined by the ion exchange and heat treatment conditions. The state of the copper ions has an effect on the redox properties and reactivity of the Cu-ZSM-5 catalysts in the selective catalytic reduction (SCR) of NO with propane and in N2O decomposition. The amount of Cu2+ that is sorbed by zeolite H-ZSM-5 from aqueous solution and is stabilized as isolated Cu2+ cations in cationexchange sites of the zeolite depends largely on the copper salt anion. The quantity of Cu(II) cations sorbed from aqueous solutions of copper salts of strong acids is smaller than the quantity of the same cations sorbed from the copper acetate solution. When copper chloride or sulfate is used, the zeolite is modified by the chloride or sulfate anion. Because of the presence of these anions, the redox properties and nitrogen oxides removal (DeNOx) efficiency of the Cu-ZSM-5 catalysts prepared using the copper salts of strong acids are worse than the same characteristics of the sample prepared using the copper acetate solution. The addition of ammonia to the aqueous solutions of copper salts diminishes the copper salt anion effect on the amount of Cu(II) sorbed from these solutions and hampers the nonspecific sorption of anions on the zeolite surface. As a consequence, the redox and DeNOx properties of Cu-ZSM-5 depend considerably on the NH4OH/Cu2+ ratio in the solution used in ion exchange. The aqueous ammonia solutions of the copper salts with NH4OH/Cu2+ = 6–10 stabilize, in the Cu-ZSM-5 structure, Cu2+ ions bonded with extraframework oxygen, which are more active in DeNOx than isolated Cu2+ ions (which form at NH4OH/Cu2+ = 30) or nanosized CuO particles (which form at NH4OH/Cu2+ = 3).

EFFECT OF THE NATURE OF SYNTHETIC SUPPORTS ON THE GEOMETRY OF SURFACE COPPER(II) COMPLEXES

Synthetic supports and coper(II) complexes anchored on themby an incipient wetness impregnation with subsequent drying at 100 °C have been characterized by IR and UV-vis diffuse reflectance spectroscopy. As can be seen, the synthetic zeolites demonstrate IR bands corresponding to both water molecule vibrations (stretching and deformation) and stretching vibrations of the crystal framework. The 6 main absorption bands in the IR spectrum of SiO2are characteristic of stretching and deformation vibrations of OH groups, water molecules, and the bonds in Si–O–Si, Si–O, and Si–OН fragments. Absorption bands in the IR spectrum of Al2O3correspond to stretching vibrations of OH groups bound to aluminum being in its tetrahedral (AlIV) and octahedral (AlVI) configurations. As appears from the UV-vis diffuse re-flectance spectra, on all supports under study, i.e. synthetic zeolites (NaX, NaA, KA), alumo-gel, and silica gel (KSMG grade), copper(II) is situated in the mixed-ligand (H2O, Cl-) octa-hedral surroundings. A wavelength corresponding to d-d transition in the Cu(II) ion anchored to the supports under study decreases in the order CuCl2/SiO2(KSMG) (820 nm) >CuCl2/NaA (733 nm) >CuCl2/Al2O3 (720 nm) ≈CuCl2/NaХ (716 nm) >CuCl2/KA (690 nm). Thus, in addi-tion to the geometry of the surface copper(II) complexes, it has been found that the role of the support is in the short-wave shifting of the band corresponding to d-d transition in the Cu(II) ion showing a distortion in the octahedral symmetry of Cu(II) environment.

Pd nanoparticles formation inside porous polymeric scaffolds followed by in situ XANES/SAXS

Simultaneous time-resolved SAXS and XANES techniques were employed to follow in situ the formation of Pd nanoparticles from palladium acetate precursor in two porous polymeric supports: polystyrene (PS) and poly(4-vinyl-pyridine) (P4VP). In this study we have investigated the effect of the use of different reducing agents (H2 and CO) from the gas phase. These results, in conjunction with data obtained by diffuse reflectance IR (DRIFT) spectroscopy and TEM measurements, allowed us to unravel the different roles played by gaseous H2 and CO in the formation of the Pd nanoparticles for both PS and P4VP hosting scaffolds.

Surface reactions of dimethyl ether on γ-Al2O3

The surface reactions of dimethyl ether (DME) on industrial alumina (γ-Al2O3) were studied by chromatographic analysis of the products at the outlet of the flow reactor and (independently) by diffuse reflectance IR spectroscopy. The major products of the reactions at 250°С were found to be methanol formed in the reaction of DME with hydroxyl groups (the 3720 and 3674 cm–1 bands in the diffuse reflectance spectrum) and various methoxy groups (the 1121, 1070, 695, and 670 cm–1 bands in the differential spectra). The presence of molecularly adsorbed methanol was confirmed by experiments with methanol fed in a high-temperature IR cell. The interaction of the resulting methanol molecule with the hydroxyl group led to the formation of a water molecule in the gas phase and a methoxy group on the oxide surface. Strong adsorption of molecular DME was revealed, which was favored by an increase in the temperature of the preliminary calcination of oxide from 250 to 450–500°С; treatment of alumina with water vapor after its preliminary contact with DME led to a recovery of the hydroxyl coating and a replacement of molecularly adsorbed DME with hydroxyl. The thermal effect recorded in a flow reactor was positive during the adsorption of DME and negative during the desorption of weakly bonded DME. Schemes of formation of methoxy groups in the interaction of DME and methanol with surface hydroxyls were suggested.