Thermosetting mechanism study of silicon-containing polyarylacetylene via in situ FTIR and solid-state NMR spectroscopy

Silicon-containing polyarylacetylene (abbreviated as PSA) resins, which contain Si-(R₁, R₂) (R₁ and R₂ represent methyl or phenyl groups) and —CC—, exhibits high thermal stability upon curing up to 250 °C. The structure and thermosetting mechanism of PSA were characterized using FTIR, in situ FTIR, ¹³C and ²⁹Si CP-MAS spectroscopy, and thermogravimetric analysis. From the experimental results we can conclude that: (1) biphenyl and naphthalene rings are formed via a Diels–Alder reaction between the Ph—CC and CC groups at 210 °C, (2) the terminal alkyne mainly transforms into ethylenic bonds at 170 and 210 °C, and (3) an oxidation reaction occurs to give the oxide structure (Si—O—Si) and carbon dioxide at 250 °C. A new curing procedure has been proposed to maximize the T_d5 up to 635.2 °C on that basis. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47301.     

Metal dispersion dependent selectivities for syngas conversion on faujasite X hosted rhodium

Different metal dispersions of Na-faujasite X hosted rhodium were generated and characterized by transmission electron microscopy (TEM) and IR-spectroscopy. CO hydrogenation has been studied over these Rh/NaX-zeolite catalysts. The finer rhodium dispersion with a mean particle size of 1–2 nm shows a higher selectivity towards oxygenates (methanol, ethanol and dimethyl ether) as compared to the coarser dispersion (4–5 nm), where nearly exclusively methane and higher hydrocarbons are formed.     

In situ laser raman spectroscopy during sequential oxidizing and reducing conditions for a vanadium-phosphorous-oxide catalyst

A VPO catalyst prepared in an organic medium has been studied by in situ laser Raman spectroscopy for n-butane oxidation to maleic anhydride. Data could be obtained at low laser power and brief collection times. Raman characterization during continuous flow (steady-state) studies revealed that (VO)₂P₂O₇ was present. Sequential oxidizing (10% O₂ in N₂) and reducing (2% n-butane in N₂) conditions were explored at 350°C and 400°C. These cycling (unsteady-state) operations revealed that formation of V(5⁺) phases was enhanced during oxidizing conditions. The intensity of Raman bands due to (VO)₂P₂O₇ increased during reducing conditions.     

In situ IR,Raman, and UV-Vis DRS spectroscopy of supported vanadium oxide catalysts during methanol oxidation

The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated with the fully oxidized surface (V⁵⁺) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm⁻¹) and bridging V–O–V (∼900–940 cm⁻¹) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V⁵⁺ LMCT band (>25,000 cm⁻¹) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V⁵⁺) species to reduced phases (V³⁺/V⁴⁺) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V³⁺/V⁴⁺ phases in the d–d transition region (10,000–30,000 cm⁻¹), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V⁵⁺ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region (2800–3000 cm⁻¹) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed. This revised version was published online in August 2006 with corrections to the Cover Date.     

In situ Raman spectroscopy analysis of local structure in dry gel for zeolite beta synthesis

To investigate faster crystallization of zeolite beta by the dry-gel conversion method, the local structure of the dry gel before synthesis was quantitatively evaluated using in situ Raman spectroscopy during the drying process. The dry gel prepared from Si and Al sources, and tetraethylammonium hydroxide solution was crystallized after several hours by the dry-gel conversion method. The conformational change of TEA⁺ cations was observed during the drying process by the deconvolution of the spectrum, and the conformational change was larger than that during the synthesis process. The rate of conformational change was increased with the drying temperature, and the apparent activation energy was estimated to be 68.2 kJ/mol. The generation and transformation of double three-membered silicate rings (D3Rs) and 4-2 type secondary building units (SBUs), which are essential for the crystallization of zeolite beta, were observed during the drying process. The transformation from D3R to 4-2 SBU in the dry gel during drying process could be confirmed quantitatively by the difference of the time variation for the amounts of these silicate building units estimated by in situ observation.     

In situ FTIR spectroscopic studies of electrooxidation of ethanol on Pd electrode in alkaline media

Electrooxidation of ethanol on a polycrystalline Pd disk electrode in alkaline media was studied by in situ Fourier transform infrared (FTIR) reflection spectroscopy. The emphasis was put on the quantitative determination of intermediates and products involved in the oxidation. It has revealed that most of ethanol was incompletely oxidized to acetate. The selectivity for ethanol oxidation to CO₂ (existing as CO₃²⁻ in alkaline media) was determined as low as 2.5% in the potential region where Pd electrode exhibited considerable electrocatalytic activity (−0.60 to 0.0 V vs. SCE). Nevertheless, the ability of Pd for breaking C-C bond in ethanol is still slightly better than that of Pt under the same conditions. Besides, a very weak band of adsorbed intermediate, bridge-bonded CO (CO_B) was identified on the Pd electrode for the first time, suggesting that CO₂ and CO₃²⁻ species may also be generated through CO pathway (i.e., indirect pathway).     

马来酸在离子液体[BMIM]BF4中的电化学还原和原位红外光谱研究

作者自行合成了离子液体[BMIM]BF4,用循环伏安法(CV)、计时电量法(CA)和电化学原位红外反射光谱(in situFTIR),从分子水平考察了离子液体中马来酸在玻碳(GC)电极上的电化学还原过程。结果表明,[BMIM]BF4中马来酸在GC电极上的还原为不可逆过程,测得扩散系数D=9.62×10-8cm2/s;in situFTIRS研究发现,马来酸在离子液体[BMIM]BF4和水溶液中的电还原生成丁二酸的机理不同。在[BMIM]BF4中马来酸还原发生在其中的一个羧基上,即马来酸首先获得一个电子生成阴离子自由基,随后可能获得一个电子生成二价阴离子,或者获得一个电子并在2个H+的作用下生成醛类物质和水。     

不同结构分子筛的甲醇制丙烯催化性能

在常压、空速为1.5 h^-1、反应温度为450℃条件下,考察了4种具有不同拓扑结构的分子筛(SAPO-34、ZSM-48、ZSM-5和beta)在甲醇转化制丙烯(MTP)反应中的催化性能,并对催化剂的积炭失活行为进行了研究。结果表明,从8元环到12元环,分子筛孔口尺寸越小,低碳烯烃(乙烯+丙烯)选择性越高,积炭失活速率也越快。孔道尺寸越大,丙烯/乙烯(P/E)比越高,但产物分布向C₄以上组分偏移,丙烯选择性降低。10元环分子筛具有较高的丙烯选择性,但催化剂的积炭失活速率随孔道体系的不同有很大差异。一维直通孔道的ZSM-48容易积炭失活,而具有三维交叉孔结构的ZSM-5表现出了优异的抗积炭失活性能。不同结构分子筛在MTP反应中催化性能的差异主要归因于分子筛的过渡态择形和产物择形作用的不同。     

Spatially resolved infrared spectroscopy: a novel technique for in situ study of spatial surface coverage during CO oxidation on supported catalysts

A spatially resolved infrared (IR) imaging technique to monitor the linear adsorbed CO coverage on supported catalyst surface combining an IR bandpass filter and an IR thermography camera has been developed. Images acquired during the CO adsorption/desorption and ignition indicate that the technique provides an excellent method to image the change of surface coverage with a spatial resolution. It is expected that the combination of infrared thermography with spatially resolved imaging of surface coverage will provide a deeper insight in the dynamics of spatio-temporal patterns on heterogeneous catalysts.     

In situ Raman spectroscopy studies of catalysts

In situ Raman spectroscopy is rapidly becoming a very popular catalyst characterization method because Raman cells are being designed that can combine in situ molecular characterization studies with simultaneous fundamental quantitative kinetic studies. The dynamic nature of catalyst surfaces requires that both sets of information be obtained for a complete fundamental understanding of catalytic phenomena under practical reaction conditions. Several examples are chosen to highlight the capabilities of in situ Raman spectroscopy to problems in heterogeneous catalysis: the structural determination of the number of terminal M=O bonds in surface metal oxide species that are present in supported metal oxide catalysts; structural transformations of the MoO₃/SiO₂ and MoO₃/TiO₂ supported metal oxide catalysts under various environmental conditions, which contrast the markedly different oxide–oxide interactions in these two catalytic systems; the location and relative reactivity of the different surface M–OCH₃ intermediates present during CH₃OH oxidation over V₂O₅/SiO₂ catalysts; the different types of atomic oxygen species present in metallic silver catalysts and their role during CH₃OH oxidation to H₂CO and C₂H₄ epoxidation to C₂H₄O; and information about the oxidized and reduced surface metal oxide species, isolated as well as polymerized species, present in supported metal oxide catalysts during reaction conditions. In summary, in situ Raman spectroscopy is a very powerful catalyst characterization technique because it can provide fundamental molecular‐level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

A combined MAS nuclear magnetic resonance spectroscopy,in situ FT infrared spectroscopy and catalytic study of the conversion of allyl alcohol over zeolite catalysts

A combined study of allyl alcohol conversion over zeolite catalysts using catalytic measurements in a flow microreactor, in situ FTIR and MAS NMR spectroscopy is reported. Rate constants for the conversion in the flow reactor and the static in situ reactor used in the FTIR studies are in broad agreement, emphasising the viability of the experimental approach. In the flow microreactor allyl alcohol conversion over the zeolite catalyst is shown to form diallyl ether, hydrocarbons and acrolein. The in situ study successfully models the formation of diallyl ether and hydrocarbon as initial reaction products, but unfortunately acrolein is found to be rapidly converted to hydrocarbons under the condition used in the in situ cells. The studies are combined to provide a model for the reaction which involves two parallel pathways for the formation of the hydrocarbons and acrolein.     

In situ surface Raman spectroscopy studies on the interaction between oxygen and ethanol on electrolytic silver catalyst

In situ Raman spectroscopy is employed to investigate the oxidation of ethanol on the electrolytic silver catalyst under catalytic conditions. Over the temperature range of 300–873 K, the configuration of the surface intermediates is detected. The ethoxide species, acetate species, adsorbed acetaldehyde and surface hydroxide exist on the silver surface. The mechanism for the oxidation of ethanol on the silver surface under industrial conditions is discussed and compared with that obtained in ultrahigh vacuum systems.     

In situ ATR-FTIR spectroscopy on Ni-P alloy electrodes

In situ ATR-FTIR spectroscopy has been extended to the study of CO and pyridine adsorptions at crystalline and amorphous Ni-P alloy film electrodes, with an emphasis on the alloying effect of P on their adsorption configurations on Ni sites. The Ni-P films were prepared with initial seeding of a catalytic Pd layer on Si, followed by chemical deposition. Transition from bridge-bonded CO (CO_B) dominant to linearly bonded CO (CO_L) dominant adsorption was found with increasing P content, together with a blue-shift in the CO_L vibrational frequency and an attenuation of the Stark tuning rate of CO_L. As for Py on Ni-P electrodes, essentially the N-end-on adsorption was revealed, in contrast to the edge-tilted adsorption on Ni electrode. Modification in the adsorption configurations as compared to that on Ni electrode is ascribed mainly to the site-blocking effect with alloying P, rather than to partial electron transfer between Ni and P.     

In situ FTIR study on reaction pathways in Ni(CO)4/CH3OK catalytic system for low-temperature methanol synthesis in a liquid medium

An in situ infrared spectroscopic study was conducted to elucidate the reaction pathways for low-temperature methanol synthesis in a catalytic system composed of Ni(CO)₄ and CH₃OK (denoted as Ni(CO)₄/CH₃OK). The reaction was conducted in a liquid medium at 313–333 K with an initial pressure of 3.0 MPa. When CH₃OK was added to Ni(CO)₄ solution at 293 K, different carbonylnickelates, [Ni₅(CO)₁₂]²⁻, [Ni₆(CO)₁₂]²⁻ and [Ni(CO)₃(COOCH₃)]⁻, were immediately formed from Ni(CO)₄. The species and the composition of the carbonylnickel complexes varied with temperature. The variations in concentrations of methanol (MeOH) and methyl formate (MF) during the run, which were determined from their IR absorptions, indicated a pattern characteristic of consecutive reactions with MF as an intermediate. Thus, it was shown that methanol was produced through the carbonylation of MeOH to MF and the subsequent hydrogenation of MF to MeOH. Stable hydridocarbonylnickel anions, [HNi(CO)₃]⁻ and/or [HNi₂(CO)₆]⁻, were observed together with a small amount of Ni(CO)₄ throughout the methanol synthesis. Since Ni(CO)₄ alone showed no activity for the hydrogenation of MF, the hydridocarbonylnickel anions generated in the presence of CH₃OK must be responsible for the reaction. The dual role of CH₃OK in the catalytic system was stated.     

In situ high temperature FTIR studies of NOx reduction with propylene over Cu/ZSM-5 catalysts

High temperature in situ FTIR has been used to investigate the surface species present on Cu/ZSM-5 during the reduction of NO_x with propylene in a lean environment. Parallels have been observed between adsorbed surface species and catalytic activity for this reaction. Species detected at low temperatures are not representative of those detected at high temperatures where the catalyst is active. An oxidized nitrogen-containing species has been observed at 2580 cm^–1 on Cu during reaction conditions (400°C). In contrast, at low temperatures, where the catalyst is less active, coke and Cu⁺-CO predominated. The effects of Cu weight loading, C/NO ratio, reaction temperature, and catalyst deactivation by steaming have been investigated with IR.     

NOx adsorption on MnO2/NaY composite: an in situ FTIR and EPR study

The NO, NO/O₂, and NO/O₂/H₂O adsorption on MnO₂/NaY (5 and 15 wt.% MnO₂) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O₂ on NaY and MnO₂/NaY proceeds via oxidation of NO forming NO₂⁻ and NO₃⁻ species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NO_x species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO₂. Dimerisation of NO₂ to N₂O₄ and following disproportionation of the latter causes the formation of NO⁺ and NO₃⁻ species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn²⁺ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O₂ adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO₂ dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn⁴⁺ is reduced to Mn²⁺ as evidenced by in situ EPR measurements.     

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     

In situ FTIR studies on the electrochemical hydrodechlorination of 3,4,5,6-tetrachloropicolinic acid on Ag cathode

The electrochemical hydrodechlorination reaction from starting material 3,4,5,6-tetrachloropicolinic acid (3,4,5,6-TCP) to the end product 3,6-dichloropicolinic acid (3,6-DCP) was investigated by cyclic voltammetry and in situ Fourier transform infrared spectroscopy (in situ FTIR). Compared with copper and glassy carbon, Ag cathode showed a high electrocatalytic activity for the irreversible reduction process of 3,4,5,6-TCP in NaOH aqueous solution. In situ FTIR results suggested that electrochemical hydrodechlorination took place in the 4- or 5-position of 3,4,5,6-TCP on Ag cathode after receiving an electron to get mixed trichloropicolinic acid free radical, which could receive another electron and give 3,5,6-trichloropicolinic acid (3,5,6-TCP) and 3,4,6-trichloropicolinic acid (3,4,6-TCP) at the potential more positive than −1000 mV afterwards. Finally, 3,5,6-TCP and 3,4,6-TCP were further dechlorinated to produce 3,6-dichloropicolinic acid (3,6-DCP) at the potential more negative than −1000 mV. Further studies of preparative electrolysis experiments by constant current electrolysis were carried out. The results were in good agreement with those from in situ FTIR investigations.     

In situ FTIR experimental results in the silylation of low-k films with hexamethyldisilazane dissolved in supercritical carbon dioxide

In situ Fourier Transform Infrared Spectroscopy measurements were performed using an innovative equipment to study the surface modification reaction between a functionalized porous MSQ-film and hexamethyldisilazane (HMDS) dissolved in CO₂ at supercritical conditions (scCO₂). scCO₂ was used in the heterogeneous reaction due to enhancing properties, ideal for porous materials. Different infrared signatures, from the gas and solid phases, were observed and identified, implying gas–gas and solid–gas phase reactions. Among the different component signatures observed in the gas phase, carbonic acid was observed as a possible silylating gas phase nucleophilic component, while in the solid phase the predominant reaction mechanism proceeded by forming SiOSi bonds and Trimethylaminosilane (as gas phase product).     

Studies on electrochemical hydrodebromination mechanism of 2,5-dibromobenzoic acid on Ag electrode by in situ FTIR spectroscopy

Cyclic voltammetry and in situ FTIR were employed to study the electrochemical hydrodebromination (EHB) mechanism of 2,5-dibromobenzoic acid (2,5-DBBA) in NaOH solution. Compared with titanium and graphite electrodes, silver electrode exhibited a high electrocatalytic activity for the hydrodebromination reaction of 2,5-DBBA. On the basis of in situ FTIR data, EHB reaction of 2,5-DBBA on Ag cathode might be represented as a sequence of electron additions and bromine expulsions. Firstly, from potential at approximately −1100 mV, 2,5-DBBA received an electron to form 2,5-DBBA radical anion, which lost a bromine ion in the 2-position to form 3-bromobenzoic acid (3-BBA) free radical. Then the free radical received a proton to give 3-BBA. Finally, 3-BBA further took off another bromine ion to produce benzoic acid free radical and the end product benzoic acid was obtained by receiving another electron and a proton with the potential shifting to more negative values.