Density functional theory (DFT) and DRIFTS investigations of the formation and adsorption of enolic species on the Ag/Al2O3 surface

Molecular structure and vibrational frequencies of the novel surface enolic species intermediate on Ag/Al2O3 have been investigated by means of density functional theory (DFT) calculations and in situ infrared spectroscopy. The geometrical structures and vibrational frequencies were obtained at the B3P86 levels of DFT and compared with the corresponding experimental values. Theoretical calculations show that the calculated IR spectra are in good agreement with the experimental spectroscopic results. In addition, the adsorption energy of enolic species on the Ag/Al2O3 catalyst surface was also evaluated. The reaction mechanism from C2H5OH to enolic species on Ag/Al2O3 catalyst was proposed.     

Conformational analysis of sulfate species on Ag/Al2O3 by means of theoretical and experimental vibration spectra

The formation and configuration of sulfate species on Ag/Al2O3 were studied by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. The comparison between theoretical and experimental vibration spectra enable us to draw the following conclusions: Bidentate sulfate species rather than tridentate sulfate species are the predominant surface species on Ag/Al2O3. Moreover, both bidentate and tridentate sulfate species may coexist on the catalyst surface at a lower coverage. The accumulation of surface sulfate species could well explain the blue shift of the sulfate species in IR spectra. In addition, the in situ DRIFTS could distinguish between the sulfate species that linked to Al site and Ag site, which was well supported by temperature-programmed desorption (TPD) results.     

Conformational analysis and comparison between theoretical and experimental vibration spectra for isocyanate species on Ag/Al2O3 catalyst

Density functional theory (DFT) calculations were performed to study the structure and vibrational frequencies of the intermediate for the isocyanate (NCO) surface species on Ag/Al(2)O(3) catalyst using the GAUSSIAN 98 suite of programs. Thermal decomposition of adsorbed CH(3)NO(2) species leads to the formation of two types of NCO species (NCO on Ag and NCO on Al(2)O(3)) above 298K. In order to investigate the reaction mechanism, we have designed the calculated models (a-g) for the NCO intermediate. Through the analysis of the vibration mode and the vibration frequency of the calculated models (a-g), we conclude that there is an excellent agreement between the calculated vibration spectrum of the model (e) containing the -AgNCO group or model (g) containing the -AlNCO group and the experimental vibration spectrum.     

低温等离子体协同Ag/Al2O3选择性催化丙烯还原氮氧化物反应的原位红外光谱研究

 研究了稀燃条件下低温等离子体(NTP)协同丙烯在Ag/Al2O3催化剂上选择性催化还原NOx反应,通过原位漫反射傅里叶变换红外光谱(DRIFTS)对NTP协同前后反应气中NO和C3H6的吸附以及丙烯选择性催化还原反应进行了表征. 结果表明,丙烯的活化是Ag/Al2O3上选择性催化还原反应的关键步骤. NTP活化反应气体后, Ag/Al2O3表面-NCO、R-NO2和有机酸根等物种的数量大幅度增加,并且其催化还原NOx的低温(＜350 ℃)活性也显著提高. 在NTP协同前后,选择性催化还原过程可能存在两条反应路径,较低温度下主要是-NCO 生成N2, 而较高温度下则是-CN向N2转化.     

NO2 adsorption on BaO/Al2O3: the nature of nitrate species

Temperature programmed desorption, infrared spectroscopy, and (15)N solid state NMR spectroscopy were used to characterize the nature of the nitrate species formed on Al(2)O(3) and BaO/Al(2)O(3) NO(x) storage/reduction materials. Two distinctly different nitrate species were found: surface nitrates that are associated with a monolayer BaO on the alumina support, and a bulk-like nitrate that forms on this thin BaO layer. The surface nitrates desorb as NO(2) at lower temperatures than do the bulk-like nitrates, which decompose as NO+O(2) at higher temperatures. The amount of NO(x) stored in the monolayer nitrate is proportional to the surface area of the catalyst, while that in the bulk nitrate increases with BaO coverage.     

Ag／Al2O3选择性催化丙烯还原氮氧化物表面反应机理的原位红外光谱研究

 在富氧条件下Ag／Al2O3对以丙烯为还原剂选择性催化还原NOx的反应有很高的催化活性．本实验成功地利用原位红外光谱分析手段,在真实的催化反应条件下,探讨了丙烯选择性还原NOx的反应机理,证实了催化剂表面反应中间体Al－NCO和Ag－NCO虽在真空中稳定,但在实际反应条件下极为活泼．催化剂表面上R－ONO和R－NO2分解成NCO是整个反应的速度控制步骤．这一结论与在真空系统条件下对其反应机理的研究结果一致,证明了在这一反应体系中两种研究方法的相关性．同时本实验还从反应机理上探讨了催化剂的水蒸气中毒现象．水蒸气的存在阻碍了催化剂表面R－ONO和R－NO2的生成,并进一步阻碍了反应的速度控 制步骤,即R－ONO和R－NO2向Al－NCO和Ag－NCO的转化,但这是一种完全可逆的暂时中毒现象．结合相应的催化剂活性评价结果对表面反应机理进行了讨论．     

Roles of surface nitrogen oxides in propene activation and NO reduction on Ag/Al2O3

The activation of propene in selective catalytic reduction (SCR) of NO on 4% Ag/Al₂O₃ has been studied by in situ infrared (IR) spectroscopy. Distinctive propene activation products were detected in the SCR of NO, depending on the nature of surface oxygen and nitrogen oxide species on Ag/Al₂O₃. C₃H₆ was oxidized to acetate species in an O₂ + C₃H₆ atmosphere on Ag/Al₂O₃ above 573 K. The addition of NO to the C₃H₆ + O₂ feed gas suppressed the formation of acetate species but increased the proportion of acrylate species. Acrylate species were further confirmed to be formed preferentially from C₃H₆ oxidation without the O₂ atmosphere on Ag/Al₂O₃ or nitrate-adsorbed Ag/Al₂O₃. On the other hand, adsorption of NO led to the formation of nitrito species on Ag/Al₂O₃, but the nitrito was barely oxidized to nitrate species unless there was an O₂ atmosphere at 473–673 K. Thus, the oxidation of propene to acetate species, or the formation of nitrate from nitrito, is attributed to two competitive electrophilic reactions. The formation of nitrate from nitrito species decreased electrophilic oxygen species that oxidized propene to acetate. Nevertheless, the first dehydrogenation of propene to form acrylate species on nitrate-adsorbed Ag/Al₂O₃ is a nucleophilic reaction, as it is on Ag/Al₂O₃. Furthermore, there was no decrease in reaction activity for formation of acrylate species on nitrate-adsorbed Ag/Al₂O₃ compared to Ag/Al₂O₃. This led to the total reaction occurring easily through the propene nucleophilic oxidation branch because the presence of the adsorbed nitrogen oxides changed selectively the formation rates of the surface reductants. IR spectra data further demonstrate that acrylate and acetate species, as the surface reductants, reacted with nitrate to generate isocyanate intermediates in the SCR of NO. The effect of structures of different reductants on NO reduction is discussed.     

原位漫反射红外光谱法研究SO2对Ag/Al2O3选择性催化丙烯还原NOx反应的影响

 以Ag/Al2O3为催化剂,采用原位漫反射傅里叶变换红外光谱法研究了SO2对C3H6选择性还原NOx反应的影响. 结果表明, SO2在催化剂表面转化为硫酸盐,并且随着硫酸盐累积量的增加,其主要红外特征吸收峰由低波数向高波数漂移. 高浓度表面硫酸盐的存在不仅抑制了催化剂表面硝酸盐的生成,而且抑制了硝酸盐与表面烯醇式物种(RCH=CH-O-)或乙酸盐物种进一步反应,生成活泼的反应中间体异氰酸酯(-NCO), 这是导致Ag/Al2O3催化剂上C3H6选择性还原NOx活性降低的主要原因.     

Ag2S/Al2O3纳米复合物的水热合成及表征

采用水热技术结合煅烧的方法,成功制备了Ag2S/Al2O3纳米复合物。 通过SEM、EDX、XRD等测试技术对产品进行了表征,证明得到的产品是Ag2S/Al2O3纳米复合物;无孔Al2O3和多孔Al2O3作反应模板得到的复合物的形态和分布不同。     

Characterization and stereochemistry of alkyl 2-chloro-3-formylacrylates: Experimental NMR and theoretical DFT studies

The synthesis of alkyl 2-chloro-3-formylacrylates 1 from alkyl pyruvates was carried out using Vilsmeier reaction. The reaction is highly stereoselective and leads to a 95:5 mixture of Z and E stereoisomers. Both stereoisomers were characterized by ¹H and ¹³C NMR. In CDCl₃ solution, the NMR data, particularly the NOEDIFF experiments, show that the major species formed is the Z stereoisomer. Heating compound 1 in THF at reflux afforded the cyclic product 2 in 90 % yield. The interpretation of the experimental data were further supported by DFT/B3LYP calculations.     

Accurate theoretical IR and Raman spectrum of Al2O2 and Al2O3 molecules

The accurate harmonic vibration frequencies together with the infrared (IR) and Raman intensities of the most stable conformers of Al₂O₂ and Al₂O₃ molecules have been calculated by the density functional theory (DFT) method with B3LYP exchange–correlation potential and using a set of the augmented correlated consistent basis sets up to quintuple order. The anharmonic vibration frequencies of the non-linear Al₂O₂ molecule have also been calculated. The obtained equilibrium geometrical parameters, harmonic and anharmonic vibration frequencies along with the IR and Raman intensities good converge to their limits with increasing the size of the used basis set. A comparison of the calculated harmonic and anharmonic vibrational frequencies with the available experimental ones points out that the small differences between the calculated harmonic and experimental frequencies can be further substantially reduced when calculations of the anharmonic vibrational frequencies will be available for all types of molecular geometries.     

Bactericidal mechanism of Ag/Al2O3 against Escherichia coli

The bactericidal process of Ag/Al2O3 to Escherichia coli has been investigated to clarify the bactericidal mechanism. In SEM images, the configuration of E. coli cells contacting with the catalyst surface was quite different from that contacting with AgNO3 solution, which indicated that the Ag+ eluted from the catalyst did not play an important role in the bactericidal process. The bactericidal experiments strongly confirmed the contribution of multiform reactive oxygen species (ROS) (super oxide dismutase (SOD) and catalase as the scavengers for O2*- and H2O2, respectively) to bactericidal effect on the catalyst surface. Furthermore, the surface modification of Ag/Al2O3 by ultraviolet and formaldehyde influenced the bactericidal effect obviously, which not only confirmed the bactericidal mechanism of catalytic oxidation but also provided evidence for the synergistic effect between Ag and Al2O3 on the catalyst surface.     

Ag/Al2O3催化乙醇和甲醚选择性还原NOx反应机理的原位红外光谱研究

 尽管C2H5OH和CH3OCH3是同分异构体,但在以Ag/Al2O3为催化剂选择性催化还原NOx时,还原剂C2H5OH比CH3OCH3具有更高的活性. 原位漫反射红外光谱研究表明, C2H5OH和CH3OCH3在还原NOx反应过程中分别遵循不同的反应机理: 在Ag/Al2O3催化C2H5OH还原剂选择性还原NOx的反应过程中,烯醇式物种(RCH=CH-O-)和NO-3物种是主要的反应中间体,二者相互反应性能很强,可以生成反应关键中间体异氰酸酯(-NCO)表面吸附物种,因此NOx的去除率很高; 而在Ag/Al2O3催化CH3OCH3还原剂选择性还原NOx的反应过程中,甲酸盐(HCOO-)物种和NO-3物种是主要的反应中间体,二者之间反应生成-NCO 的活性较弱,因而NOx的去除率较低.     

氢气对Ag/Al2O3和Cu/Al2O3催化剂选择性催化C3H6还原Nox反应的影响

 添加H2对Ag/Al2O3和Cu/Al2O3催化剂选择性催化C3H6还原NOx反应具有不同的影响. 原位漫反射红外光谱分析表明,在Ag/Al2O3催化剂上, H2的存在促进了C3H6部分氧化产物烯醇式物种(RCH=CH-O-)和乙酸盐等的形成,烯醇式物种和硝酸盐为主要反应中间体,二者间的相互反应性能很强,能形成高浓度的反应关键中间体异氰酸酯(-NCO)表面吸附物种,因此NOx的去除活性提高; 而在Cu/Al2O3催化剂上, H2的存在并没有促进C3H6部分氧化产物的形成,而且抑制了硝酸盐的形成,进而抑制了C3H6部分氧化产物与硝酸盐反应形成表面-NCO 物种,导致NOx的去除活性降低.     

富氧条件下Ag/Al2O3-Cu/Al2O3组合催化剂催化乙醇选择性还原NOx

 在富氧条件下,以乙醇为还原剂,考察了不同金属负载量的Ag/Al2O3和Cu/Al2O3催化剂选择性还原NOx的活性. 结果表明,4%Ag/Al2O3和10%Cu/Al2O3分别为各自体系中的最佳催化剂. 4%Ag/Al2O3具有很高的选择性催化还原NOx的活性,但同时会产生一定量的副产物CO. 10%Cu/Al2O3选择性催化还原NOx的活性不高,但却具有很好的消除CO的活性. 将4%Ag/Al2O3和10%Cu/Al2O3以不同方式组合,其评价结果表明,4%Ag/Al2O3-10%Cu/Al2O3组合催化剂具有与单纯4%Ag/Al2O3相似的NOx脱除活性,同时能够完全消除反应过程中产生的CO,并且能大大降低尾气中乙醇和乙醛的浓度.     

FTIR studies of CO adsorption on Rh-Ge/Al2O3 catalysts prepared by surface redox reactions

A series of bimetallic Al2O3-supported Rh-Ge catalysts was prepared by surface redox reactions under controlled hydrogen atmosphere. The surface properties of these catalysts were probed via in-situ FTIR spectroscopic studies of adsorbed CO and were compared to those of monometallic Rh catalysts that had undergone similar treatments. The results indicate that Ge addition results in the formation and stabilization of smaller rhodium ensembles at the expense of larger Rh0 surfaces. A charge-transfer mechanism from Ge to Rh is also inferred by the IR results for the high Ge loading samples. Air exposure of the catalysts leads to an irreversible segregation of the two metals and formation of large Rh crystallites.     

Al2O3负载Ag催化剂的杀菌作用

 考察了空气中Al2O3负载的Ag催化剂(Ag/Al2O3和AgCl/Al2O3)对大肠杆菌(E.coli)的杀灭作用. 结果表明,负载Ag的催化剂的杀菌效果明显好于Al2O3. 在空气中,催化剂上溶出的Ag+对细菌的重金属毒性可以忽略. Ag/Al2O3, AgCl/Al2O3和Al2O3在有氧和无氧条件下的杀菌效果表明, Ag促进了催化剂表面分子态氧向活性氧转化,活性氧通过氧化作用破坏E.coli 的细胞膜,从而提高了催化剂的杀菌能力. 扫描电镜结果证实在催化剂表面E.coli细胞的破坏是从细胞膜开始的.     

Laser-induced luminescence of model Fe/Al2O3 and Cr/Al2O3 catalysts

The laser-induced luminescence of Cr³⁺ impurity ions in model Fe/Al₂O₃ and Cr/Al₂O₃ catalysts with different calcination temperatures was studied. It was found that an additional luminescence band at 770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octahedrally coordinated Cr³⁺ ions with Fe³⁺ impurity ions. In the θ-Al₂O₃ phase with a concentration of Cr³⁺ ions higher than 0.1 wt %, the interaction of the Cr³⁺-Cr³⁺ ion pairs in the immediate surroundings resulted in the appearance of N _θ lines due to the splitting of R _θ lines. The differences of these lines from the N _α lines of α-Al₂O₃ were related to the individuality of the crystal lattice of the θ phase and the coordination of Cr³⁺ impurity ions in the immediate surroundings, which is different from that in the α phase. Based on the laser-induced luminescence spectroscopic data, it was found that regions with a local Cr³⁺ concentration higher than the average Cr³⁺ concentration in the bulk of a catalyst by one order of magnitude were formed in the α-Al₂O₃-Fe₂O₃ system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at 1220°C, as a result of the diffusion of chromium and iron ions.     

Experimental investigation for stability and surface properties of TiO2 and Al2O3 water-based nanofluids

Journal of Thermal Analysis and Calorimetry - Nanofluids have gained recent attention because of their potential applications in diverse engineering fields like enhancing thermal transport,...