Relationship between the formation of surface species and catalyst deactivation during the gas-phase photocatalytic oxidation of toluene

Various partial oxidation products were identified on the surface of TiO₂ and an 8% SiO₂–TiO₂ binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid.

The SiO₂–TiO₂ photocatalyst is more active and appears to deactivate slower than TiO₂. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO₂. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Brønsted acidity but when SiO₂ is cofumed with TiO₂, Brønsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it.     

Liquid Phase Oxidation of Toluene to Benzaldehyde with Molecular Oxygen over Copper‐Based Heterogeneous Catalysts

We conducted the liquid phase oxidation of toluene with molecular oxygen over heterogeneous catalysts of copper‐based binary metal oxides. Among the copper‐based binary metal oxides, iron‐copper binary oxide (Fe/Cu=0.3 atomic ratio) was found to be the best catalyst. In the presence of pyridine, overoxidation of benzaldehyde to benzoic acid was partially prevented. As a result, highly selective formation of benzaldehyde (86% selectivity) was observed after 2 h of reaction (7% conversion of toluene) at 463 K and 1.0 MPa of oxygen atmosphere in the presence of pyridine. These catalytic performances were similar or better than those in the gas phase oxidation of toluene at reaction temperatures higher than 473 K and under 0.5–2.5 MPa. It was suggested from competitive adsorption measurements that pyridine could reduce the adsorption of benzaldehyde. At a long reaction time of 4 h, the conversion increased to 25% and benzoic acid became the predominant reaction product (72% selectivity) in the absence of pyridine. The yield of benzoic acid was higher than that in the Snia‐Viscosa process, which requires corrosive halogen ions and acidic solvents in the homogeneous reaction media. The catalyst was easily recycled by simple filtration and reusable after washing and drying.     

Selective oxidation of benzaldehyde by molecular oxygen over molybdovanadophosphoric acid supported MCM-41

A series of molybdovanadophosphoric acid (MVPA) supported on mesoporous silica was synthesized by an incipient wetness impregnation method. The catalysts were characterized by nitrogen adsorption?Cdesorption, X-ray powder diffraction, Fourier-Transform Infra red spectroscopy (FT-IR), UV?CVis Diffused reflectance spectroscopy (UV?CVis DRS), Temperature programmed reduction (TPR) and ³¹P MAS Nuclear magnetic resonance(NMR) study. The characterization data reveals the incorporation of vanadium in phosphomolybdic acid and retention of intact Keggin ion on the support. The catalytic activities were evaluated for oxidation of benzaldehyde using molecular oxygen as oxidant as the new green reaction system. Among all the promoted catalysts, 50wt% molybdovanadophosphoric acid supported on MCM-41 exhibits highest catalytic activity in oxidation of benzaldehyde, giving 95% conversion. Other oxidants like H₂O₂ and tert-butyl hydrogen peroxide (TBHP) were also tested for benzaldehyde oxidation reaction.     

Initial reaction steps in photocatalytic oxidation of aromatics

Transient reaction at 273 and 300 K was used to study the initial steps in the photocatalytic oxidation (PCO) of benzene, toluene, p-xylene, mesitylene, benzyl alcohol, benzaldehyde, and m-cresol adsorbed on a thin film of TiO₂ catalyst. Adsorbed aromatics were oxidized by O₂ photocatalytically in the absence of gas-phase aromatics, and the compounds remaining on the surface were characterized by temperature-programmed oxidation and desorption (TPO, TPD). Benzene and methyl benzenes oxidize rapidly at 273 or 300 K to form adsorbed intermediates that are more strongly adsorbed and much less reactive than the original aromatic, which is relatively weakly adsorbed on TiO₂. The catalyst is expected to be covered with these intermediates during steady-state reaction. The rates of PCO of benzene and methyl benzenes to CO₂ are slow relative to complete oxidation of alcohols or chlorinated hydrocarbons. The intermediates do not appear to be alcohols or aldehydes formed by oxidation of a methyl group, nor do they correspond to addition of an hydroxyl to the aromatic ring. Benzyl alcohol oxidizes photocatalytically to benzaldehyde and then to CO₂ and H₂O during PCO, but adsorbed m-cresol does not photocatalytically oxidize.     

天然苯甲醛光氧化制备天然苯甲酸的研究

以空气为氧化剂,在光照射状态下,光催化氧化天然苯甲醛制备天然苯甲酸,重点考查了温度和时间对苯甲酸收率的影响。在自制的1L光氧化反应器中,反应温度75℃,反应时间12h,天然苯甲醛转化率达到50%以上,苯甲酸选择性超过98%。     

The influence of the hydrogen origin at the surface of Mo suboxide during the deoxygenation of carboxylic acid

This work investigates the reactivity of a molybdenum suboxide in the deoxygenation of benzoic acid in the presence of hydrogen or propylene. Major differences are highlighted with Mo₈O₂₃ as the active phase. When the reaction is performed in the presence of hydrogen as the reductant, benzoic acid is converted to benzaldehyde, toluene and benzene. If molecular hydrogen is replaced by propylene as a weaker reductant, the only product formed during the reaction is benzaldehyde. It is demonstrated that, in absence of molecular hydrogen in the reaction mixture, a large amount of benzoic acid remains irreversibly adsorbed on the catalyst surface. The comparison of the two series of catalytic tests allows to somehow discriminate how the nature and the mobility of the hydrogen atoms involved in the reaction dictate the behaviour of the catalyst.     

锰卟啉-醋酸钴复合催化体系对甲苯氧气氧化的催化作用

研究了在无溶剂体系中,对氯四苯基锰卟啉［T(p-Cl)PPMnCl］和醋酸钴［Co(OAc)2］复合催化下,空气氧化甲苯制苯甲醛、苯甲醇和苯甲酸的新工艺。实验发现,T(p-Cl)PPMnCl/Co(OAc)2为复合催化剂时比单独使用T(p-Cl)PPMnCl 或Co(OAc)2时有更高的甲苯转化率和苯甲醛、苯甲醇、苯甲酸的收率,表现出明显的复合催化作用。研究表明,反应温度、反应时间和催化剂比例对T(p-Cl)PPMnCl/Co(OAc)2的复合催化效果有影响。     

The Dynamic Roles of Interstitial and Surface Defects on Oxidation and Reduction Reactions on Titania

Defect sites in titania have a substantial effect on many thermal and photochemical reactions. Three common types of defects are titanium interstitials, oxygen vacancies and oxygen adatoms—all of which can react with organic molecules adsorbed on the surface. We look broadly at thermal reduction and photochemical oxidation reactions of oxygenates. In particular, we focus on the reductive coupling of benzaldehyde, the photo-oxidation of butyrophenone, the photostability of benzoic acid, and the photo-oxidative coupling of methanol to methyl formate. Methods used include temperature programmed reaction spectroscopy, scanning tunneling microscopy, and density functional theory.     

微波辐射合成苯甲酸

以苯甲醛为原料,KMnO4为氧化剂,在微波辐射下相转移催化合成了苯甲酸。讨论了微波功率、辐射时间、催化剂等条件对反应的影响,实验结果表明,反应的最佳条件为：苯甲醛0．04mol,微波功率750W,辐射时间10min,四丁基溴化铵1．0g,碳酸钠4．0g,高锰酸钾42g,产率可达77.9％。     

MnOx/WO3/TiO2低温选择性催化还原NOx机理的原位红外研究

浸渍法制备15% MnO_x/5% WO₃/TiO₂低温脱硝催化剂,利用原位傅里叶变换红外（in situ FT-IR）设计包括多种吸附反应以及不同预处理方式的微观暂态试验与微观稳态试验,研究其NH₃-SCR脱硝反应机理,并推测反应路径。结果表明,催化剂的NH₃-SCR反应主要以Eley-Rideal机理方式进行,仅在一定温度条件下可以看到Langmuir-Hinshclwood反应路径。催化剂表面Lewis酸位的NH₃吸附是还原剂的主要来源,Brønsted酸位吸附的NH₄⁺随温度上升参与反应的比例略有提高。NH₃的吸附活化是整个反应的控制步骤,吸附态NH₃更易与NO₂发生反应,NO与催化剂表面的相互作用明显弱于NO₂。NO会在催化剂表面氧化活性中心形成大量双齿配位型硝酸盐,阻碍NH₃的吸附和活化,O₂存在条件下促进NH₃-SCR反应进行,阻止NO在催化剂表面形成双齿硝酸盐。NO与NH₃在催化剂表面存在吸附竞争,NO的吸附作用强于NH₃,温度达到100℃后吸附的NH₃方可大量活化并与NO_x发生进一步反应。     

Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523–573 K. The catalyst consisted of 0.37 monolayer of VO_x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm⁻¹) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO₂ and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction.     

Intrinsic photocatalytic oxidation of the dye adsorbed on TiO2 photocatalysts by diffuse reflectance infrared Fourier transform spectroscopy

Photocatalytic oxidation of aqueous pollutants by semiconductor photocatalysts was found efficient. The overall process by which the heterogeneous photocatalysis proceeds includes: the sequence of the adsorption of reactants, surface reaction and the desorption of final products. As a result, factors such as, the presence of oxygen concentration, pH values of the aqueous solution, pore properties for photocatalyst particles all determine the rate of photodegradation. This study is to concentrate on the photocatalytic mechanisms of the intrinsic reaction occurred on the photocatalysts and the adsorbed dye in air atmosphere. Additionally, the photocatalytic activities of adsorbed dyes prepared under different pH values were also examined and the solid-state results were compared with the aqueous systems at the same pH conditions. Dark adsorption experiments at different pH conditions showed that the saturation amount of dyes adsorbed on the catalysts differs significantly. However, the solid-state photodegradation rates of adsorbed dyes on TiO₂ at various pH values only showed slightly different, which is opposite to the results obtained from the aqueous systems. This evidence reveals that the external and internal mass transport processes are rate-controlling steps that restricted the photodegradation reaction of aqueous dyes at different pH conditions. Furthermore, this investigation supports a proposed direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process and the adsorbed dye will then be attacked by the excited hole–electron pairs and hydroxyl radicals from TiO₂ surface to produce final products.     

Electro-oxidation mechanisms of methanol and formic acid on Pt-Ru alloy surfaces

Voltammetry combined with single-potential alteration infrared spectroscopy (SPAIRS) were used to study the extent of adsorbed CO produced at Pt, Ru and Pt-Ru alloy electrodes during methanol and formic acid oxidation in acidic supporting electrolyte. The addition of even small atomic fractions of Ru to Pt surfaces caused a decrease in the quasi-steady-state level of CO on the surface for both reactions. This result is consistent with the bifunctional mechanism proposed previously: Ru sites nucleate oxygen containing species at ≈0.2-0.3 V lower potential than on the pure Pt surface; the adsorption of methanol occurs on Pt ensembles producing adsorbed CO; in the case of formic acid, adsorption is equally facile at Pt-Pt, Pt-Ru and Ru-Ru sites, with dehydration producing adsorbed CO; the further electro-oxidation of CO is catalyzed by oxygen-containing species nucleated onto nearby by Ru atoms. The improved efficiency of the alloy surfaces for oxidation of adsorbed CO at low potential shifts the rate limiting step to the adsorption step, which results in very low coverages of the surfaces by adsorbed CO.     

The behaviour of high polar organic solvents at platinum electrode—III. the study of adsorption and electrode reactions of N,N-dimethylformamide

The adsorption of N,N-dimethylformamide and oxidation of adsorbed species at platinized platinum electrode in 0.5 M aq H₂SO₄ were studied by potentiodynamic, potentiostatic and radiometric techniques. It was shown, that adsorption occurs with decomposition of DMF molecule. Below 0.15 V the adsorbed species are desorbed from the electrode surface.The kinetics of adsorption and oxidation of DMF are described by Langmuire type relation. The mechanism of adsorption of DMF as well as oxidation of adsorbed products has been considered.     

Mechanism of di(methyl)ether (DME) electrooxidation at platinum electrodes in acid medium

The electrooxidation of DME was studied at a bulk platinum electrode. It was shown that the DME adsorption was a slow step in the overall oxidation reaction. The DME adsorption is potential dependent in the hydrogen region of platinum and independent in the double layer region. From low potential scan rate voltammetry and DME stripping experiments, it was shown that the DME oxidation mechanism occurred via several reaction paths. At low potentials, DME oxidation leads to the existence of a positive current plateau. “In situ” Infrared Reflectance Spectroscopy experiments were carried out to identify the intermediate and reaction products of DME adsorption and oxidation at different potentials. CO_L (linearly bonded CO), CO_B (bridge bonded CO), adsorbed COOH species and CO₂ were detected. From these electrochemical and spectro-electrochemical results, it was proposed that some adsorbed DME was hydrolysed and directly oxidized to CO₂ or HCOOH species and some partially blocked platinum sites at the surface forming Pt–CHO and/or Pt–CO. Then, as soon as platinum becomes able to activate water, a bifunctionnal mechanism occurs to form either HCOOH or CO₂ again following two different reaction paths.     

磷钼钒酸催化甲基丙烯醛氧化反应机制及过程研究

围绕甲基丙烯醛选择性氧化制甲基丙烯酸的反应过程,利用原位红外分析装置研究了甲基丙烯醛在磷钼钒酸催化剂上的原位吸附和反应过程,发现了甲基丙烯醛在杂多酸催化剂桥氧键吸附产生的中间结构,并提出了该催化过程的反应机理。进一步获得了不同温度下反应过程的中间状态,发现钒氧物种由一级结构迁移至二级结构能有效提高催化性能。优化了催化剂预还原温度,250℃下还原3 h后催化剂活性最高。     

活性炭表面模拟烟气与元素汞间均/异相氧化反应特性

通过固定床吸附实验,在130℃温度下模拟研究燃煤烟气组分在椰壳活性炭(CS-AC)表面吸附气态元素汞(Hg⁰)过程中的作用与影响,揭示模拟烟气组分、Hg⁰以及活性炭表面三者间的均相、异相氧化反应特性。实验研究了CS-AC在N₂和模拟烟气气氛下对Hg⁰的吸附,模拟烟气组分与Hg⁰间的均相氧化反应,以及吸附模拟烟气组分后CS-AC在N₂气氛下对Hg⁰的吸附。研究表明在N₂气氛下CS-AC对Hg⁰不具备物理吸附的能力,而在模拟烟气组分下CS-AC对Hg⁰具有较强的吸附能力,初始吸附效率达80%。仅在模拟烟气均相反应作用下,大约只有14%的Hg⁰被氧化为Hg²⁺。经过预先跟常规模拟烟气吸附反应,CS-AC表面具备一定氧化性化学元素基团（如NO_x）后,能够在N₂气氛下对Hg⁰进行化学吸附,初始吸附效率达67%左右。可以认为模拟烟气和Hg⁰两者间的均相作用不是促进CS-AC在模拟烟气组分下对Hg⁰具有较强的吸附能力的主因。活性炭吸附Hg⁰的过程中,活性炭表面在烟气组分氧化Hg⁰的过程中起到了积极的促进作用。无论在N₂气氛下,还是在模拟烟气下,CS-AC吸附Hg⁰是气氛中氧化性组分、Hg⁰和活性炭表面三者间的异相化学氧化吸附反应过程。     

煤表面对氧分子物理吸附的微观机理

应用量子化学的理论和方法研究煤这种非晶体物质的物理吸附机理,从微观上揭示其吸附机理和吸附过程,定量研究煤对氧分子发生物理吸附过程中吸附量和放出热量的关系,从而揭示煤炭氧化自燃初始反应的本质.从煤表面对多个氧分子吸附优化后的平衡几何构型可以看出,煤表面吸附5个以下的氧分子时,1个氧分子在煤表面含氮的侧链吸附,其余的氧分子被苯环所吸附.煤表面吸附6个氧分子以上时,苯环对氧分子的吸附减弱,被吸附的氧分子偏向侧链端,在煤表面侧链部位吸附了大量的氧分子,这也证明了在煤的氧化自燃过程中侧链首先被氧化的结论的正确性.煤表面对氧分子的吸附量与吸附能呈线性关系.     

Keggin型配合物[（CH2）5NH2]5BW12O40的合成及其催化氧化合成苯甲酸的研究

以钨酸钠、硼酸为原料合成母体酸,以六氢吡啶为有机配体合成了有机／无机杂化电荷转移配合物[（CH2）5NH2]5BW12O40。通过红外光谱、X-射线衍射和热重分析袁征,确认所合成的化合物中多酸阴离子仍保留Keggin结构。将新合成的[（CH2）5NH2]5BW12O40杂多酸哌啶盐应用到苯甲醛氧化合成苯甲酸反应中,考察了催化剂用量、氧化责4H2O2（质量分数为30％）的用量、反应温度、反应时间等对苯甲酸收率的影响。确定最佳工艺条件为：n（催化剂）：n（苯甲醛）=1．880×10-3：1;n（H2O2）：n（苯甲醛）=4．760：1;反应温度80℃;反应时间4h,此条件下苯甲酸的收率可达到80%。     

Aerobic oxidation of alcohols over Au/TiO2: An insight on the promotion effect of water on the catalytic activity of Au/TiO2

The unique and significant promotion effect of water has been evidenced by the selective oxidation of benzyl alcohol to benzaldehyde over Au/TiO₂ catalysts. Water has dual promotional functions in the reaction system: to help form unique microdroplets in a multiphase reaction system and to assist the oxygen adsorption and activation. The conversion of benzyl alcohol at a molar ratio of water to solvent (p-xylene) of 7 is 7 times higher than in the absence of water. The present work has highlighted the potential of Au/TiO₂ catalysts in aerobic oxidation of alcohols in the unique multiphase reaction system with water as promoting solvent.