Gas-phase Oxidation Processes Research Articles

Molecular signatures and formation mechanisms of water-soluble chromophores in particulate matter from Karachi in Pakistan

Excitation-emission matrix (EEM) fluorescence spectroscopy is a widely-used method for characterizing the chemical components of brown carbon (BrC). However, the molecular basics and formation mechanisms of chromophores, which are decomposed by parallel factor (PARAFAC) analysis, are not yet fully understood. In this study, we characterized the water-soluble organic carbon (WSOC) in aerosols collected from Karachi, Pakistan, using EEM spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We identified three PARAFAC components, including two humic-like components (C1 and C2) and one phenolic-like species (C3). We determined the molecular families associated with each component by performing Spearman correlation analysis between FT-ICR MS peaks and PARAFAC component intensities. We found that the C1 and C2 components were associated with nitrogen-enriched compounds, where C2 with the longest emission wavelength exhibited a higher level of aromaticity, N content, and oxygenation than C1. The C3 associated formulas have fewer nitrogen-containing species, a lower unsaturation degree, and a lower oxidation state. An oxidation pathway was identified as an important process in the formation of C1 and C2 components at the molecular level, particularly for the assigned CHON compounds associated with the gas-phase oxidation process, despite their diverse precursor types. Numerous C2 formulas were found in the “potential BrC” region and overlapped with the BrC-associated formulas. It can be inferred that the compounds that fluoresce C2 contributed considerably to the light absorption of BrC. These findings are essential for future studies utilizing the EEM-PARAFAC method to explore the sources, processes, and compositions of atmospheric BrC.

Field Detection of Highly Oxygenated Organic Molecules in Shanghai by Chemical Ionization-Orbitrap.

Secondary organic aerosol, formed through atmospheric oxidation processes, plays an important role in affecting climate and human health. In this study, we conducted a comprehensive campaign in the megacity of Shanghai during the 2019 International Import Expo (EXPO), with the first deployment of a chemical ionization─Orbitrap mass spectrometer for ambient measurements. With the ultrahigh mass resolving power of the Orbitrap mass analyzer (up to 140,000 Th/Th) and capability in dealing with massive spectral data sets by positive matrix factorization, we were able to identify the major gas-phase oxidation processes leading to the formation of oxygenated organic molecules (OOM) in Shanghai. Nine main factors from three independent sub-range analysis were identified. More than 90% of OOM are of anthropogenic origin and >60% are nitrogen-containing molecules, mainly dominated by the RO2 + NO and/or NO3 chemistry. The emission control during the EXPO showed that even though the restriction was effectual in significantly lowering the primary pollutants (20-70% decrease), the secondary oxidation products responded less effectively (14% decrease), or even increased (50 to >200%) due to the enhancement of ozone and the lowered condensation sink, indicating the importance of a stricter multi-pollutant coordinated strategy in primary and secondary pollution mitigation.

Formation and evolution of secondary particulate matter during heavy haze pollution episodes in winter in a severe cold climate region of Northeast China.

The formation and evolution of sulfate (SO42-) and nitrate (NO3-) secondary contaminants under different stages of pollution episodes and different meteorological and emission conditions were compared, based on the simultaneous observation of fine particulate matter (PM2.5) and its chemical components in four heavy haze pollution episodes at 14 sampling sites in a severe cold climate region of Northeast China in winter from 2017 to 2019. The results yielded two main findings. (1) Nitrate formation during the day was mainly due to the combination of high emissions and high relative humidity (RH, 50-90%), high temperature (T, 0 to 5°C), high atmospheric oxidizability (ozone (O3) and nitrous acid (HONO) concentrations), and high ammonia (NH3) concentrations. Nitrate was formed by a gas-phase homogeneous reaction of the hydroxyl radical (OH·) with nitrogen dioxide (NO2), sulfur dioxide (SO2), and ammonia (NH3). (2) The main differences in SO42- formation between Northeast China and other regions were that the gas-phase oxidation process played an important role. This was mainly a result of the promotion of the gas-phase oxidation of SO42- due to the high oxidizing ability and the suppression of the aqueous reaction due to the low Ts in winter and low-sulfur coal emissions. Sulfate formation mostly occurred through an aqueous phase reaction in winter, but the highest yield and the fastest production capacity were produced by the gas-phase reaction.

Year-round modeling of sulfate aerosol over Asia through updates of aqueous-phase oxidation and gas-phase reactions with stabilized Criegee intermediates

Sulfate aerosol (SO42−) is a main component of particulate matter in Asian countries. Accurate numerical modeling is important for appropriate policy-making aimed at reducing SO42− concentrations. The modeling performance for SO42− is superior among aerosol components, however, current models underestimate SO42− concentrations during winter over Asia. Previous studies have proposed a heterogeneous process for winter haze events, but these kinds of studies are limited spatially and temporally because they cover only mainland China in winter. Underestimation has also been reported in other regions where the aerosol concentration is much lower than in China. In this study, the aqueous- and gas-phase oxidation processes in the current modeling were reconsidered, and their roles were evaluated over Asia using a year-round simulation. The existing aqueous-phase oxidation of O2 with Fe and Mn as catalysts was refined, and oxidation with NO2 was added due to the neutralized atmosphere over Asia. For gas-phase oxidation, three stabilized Criegee intermediates (SCIs; formaldehyde oxide (CH2OO), acetaldehyde oxide (CH3CHOO), and propionaldehyde oxide ((CH3)2COO)) were introduced. Considering the uncertainty of the reaction of CH2OO with water, the upper and lower limits of the rate constant were applied. The updated oxidation processes led to an increase in the modeled SO42− concentration. The model performance over Asia in winter was effectively improved, and the updates did not degrade the model performance in other seasons. The improvements in biases were approximately 3% during winter, whereas the deterioration in biases were within 1% in spring to summer and 2% in autumn, when the lower limit of the CH2OO rate constant with water was used. The role of SCIs depended strongly on the rate constants of the reaction of CH2OO with water. The simulated concentration of SCIs agreed well with the estimated levels, and the lower limit of the rate constant of CH2OO with water fitted within the estimated SCIs concentration levels. SCIs may play an important role in SO42− production over Asia in winter, especially the downwind region of China. The approach taken in this study has the potential to improve modeling performance in other regions as well.

High Molecular Diversity of Organic Nitrogen in Urban Snow in North China.

Snow serves as a vital scavenging mechanism to gas-phase and particle-phase organic nitrogen substances in the atmosphere, providing a significant link between land-atmosphere flux of nitrogen in the surface-earth system. Here, we used optical instruments (UV-vis and excitation-emission matrix fluorescence) and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) to elucidate the molecular composition and potential precursors of snow samples collected simultaneously at four megacities in North China. The elemental O/N ratio (≥3), together with the preference in the negative ionization mode, indicates that the one and two nitrogen atom-containing organics (CHON1 and CHON2) in snow were largely in the oxidized form (as organic nitrates, -ONO2). This study assumed that scavenging of particle-phase and gas-phase organic nitrates might be significant sources of CHON in precipitation. A gas-phase oxidation process and a particle-phase hydrolysis process, at a molecular level, were used to trace the potential precursors of CHON. Results show that more than half of the snow CHON molecules may be related to the oxidized and hydrolyzed processes of atmospheric organics. Potential formation processes of atmospheric organics on a molecular level provide a new concept to better understand the sources and scavenging mechanisms of organic nitrogen species in the atmosphere.

Quantum-State Controlled Reaction Channels in Chemi-ionization Processes: Radiative (Optical-Physical) and Exchange (Oxidative-Chemical) Mechanisms.

ConspectusMost chemical processes are triggered by electron or charge transfer phenomena (CT). An important class of processes involving CT are chemi-ionization reactions. Such processes are very common in nature, involving neutral species in ground or excited electronic states with sufficient energy (X*) to yield ionic products, and are considered as the primary initial step in flames. They are characterized by pronounced electronic rearrangements that take place within the collisional complex (X···M)* formed by approaching reagents, as shown by the following scheme, where M is an atomic or molecular target: X* + M → (X···M)* → [(X+···M) ↔ (X···M+)]e− (X···M)+ + e– → final ions.Despite their important role in fundamental and applied research, combustion, plasmas, and astrochemistry, a unifying description of these basic processes is still lacking. This Account describes a new general theoretical methodology that demonstrates, for the first time, that chemi-ionization reactions are prototypes of gas phase oxidation processes occurring via two different microscopic mechanisms whose relative importance varies with collision energy, Ec, and separation distance, R. These mechanisms are illustrated for simple collisions involving Ne*(3P2,0) and noble gases (Ng). In thermal and hyperthermal collisions probing interactions at intermediate and short R, the transition state [(Ne···Ng)+]e− is a molecular species described as a molecular ion core with an orbiting Rydberg electron in which the neon reagent behaves as a halogen atom (i.e., F) with high electron affinity promoting chemical oxidation. Conversely, subthermal collisions favor a different reaction mechanism: Ng chemi-ionization proceeds through another transition state [Ne*······Ng], a weakly bound diatomic-lengthened complex where Ne* reagent, behaving as a Na atom, loses its metastability and stimulates an electron ejection from M by a concerted emission–absorption of a “virtual” photon. This is a physical radiative mechanism promoting an effective photoionization. In the thermal regime of Ec, there is a competition between these two mechanisms. The proposed method overcomes previous approaches for the following reasons: (1) it is consistent with all assumptions invoked in previous theoretical descriptions dating back to 1970; (2) it provides a simple and general description able to reproduce the main experimental results from our and other laboratories during last 40 years; (3) it demonstrates that the two “exchange” and “radiative” mechanisms are simultaneously present with relative weights that change with Ec (this viewpoint highlights the fact that the “canonical” chemical oxidation process, dominant at high Ec, changes its nature in the subthermal regime to a direct photoionization process; therefore, it clarifies differences between the cold chemistry of terrestrial and interstellar environments and the energetic one of combustion and flames); (4) the proposed method explicitly accounts for the influence of the degree of valence orbital alignment on the selective role of each reaction channel as a function of Ec and also permits a description of the collision complex, a rotating adduct, in terms of different Hund’s cases of angular momentum couplings that are specific for each reaction channel; (5) finally, the method can be extended to reaction mechanisms of redox, acid–base, and other important condensed phase reactions.

Technical Note: Characterization of Corrosion Films Formed on Austenitic Stainless Steel in Supercritical CO2 Containing H2O and O2

Future technologies require structural alloys resistant to corrosion in supercritical CO2 (sCO2) fluids containing impurities such as H2O and O2. Traditional pipeline steels are potentially unsuitable for these environments and more corrosion resistant alloys such as stainless steels might be required. Little is known about the corrosion products formed on, and hence the processes which control corrosion of, stainless steels in impure sCO2 environments. In this study, austenitic stainless steel 347H (UNS S34709) was exposed to sCO2 containing H2O and O2 at 8 MPa and 50°C or 250°C, and separately to the aqueous phase in equilibrium with the sCO2 at 50°C, to simulate conditions expected in sCO2-based power cycles and carbon capture and storage pipelines. Only thin (<20 nm) surface films formed after 500 h resulting in small mass changes and corrosion rates <10−4 mm/y, suggesting the material resists significant degradation in these environments. X-ray photoelectron spectroscopy and transmission electron microscopy were used to characterize the corrosion films in detail. Exposure to the aqueous phase resulted in a thin (<5 nm) Cr-oxide and/or -hydroxide passive film, while exposure to sCO2 phases resulted in a multilayer Fe-rich oxide structure characteristic of a gas-phase oxidation process.

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.

Study on the Mechanism of Cold and Negative Temperature Coefficient in Natural Process of Gasoline Hydrocarbon

In this paper, the stoichiometric mechanism of gas phase oxidation process of gasoline hydrocarbons was studied through using theoretical stoichiometry. The reason of the phenomenon of cold flame and negative temperature coefficient in the reaction of hydrocarbon molecules before the flame was explained from the molecular level. During the gas phase oxidation process, the alkoxy radical RO· reacts with hydroxyl ·OH to form a relatively stable intermediate such as aldehyde (or ketone) and H2O molecules, and the free radical chain reaction process.The temperature of the reaction process is very low, while the release of a large number of heat, the formation of aldehydes (or ketones) from the excited state back to the ground state when the emission of about 400nm wavelength of light blue fluorescence.

Thermally Activated Gas-Phase Chemical Reactions in a Single-Wafer Wet Clean Process

A conventional high-dose ion-implanted photoresist strip and clean process flow consists of using a dry remote plasma ashing tool to remove the crust layer and underlying photoresist formed during the implant, followed by sulfuric-peroxide mixture (SPM) and ammonium hydroxide-hydrogen peroxide (SC1) chemistries applied on a wet clean tool to remove the remaining bulk photoresist and implant residues. However, film loss control and residue removal are increasingly difficult to maintain for sub-10 nm devices with conventional approaches. We investigate an alternative sequence that replaces the remote plasma strip with a thermally activated, atmospheric gas-phase oxidation process and replaces the SPM chemistry with a de-ionized water/ozone (DIO3) wet clean. The alternative sequence is entirely processed in a modified single-wafer wet clean chamber. For the oxidation process, a dry O3/O2 gas mixture decomposes and reacts directly with the ion-implanted photoresist wafer that is heated above 300°C from the backside. 9,000 Å/min strip rates are measured on 193 nm photoresist, and 650 Å/min are measured for the implanted crust removal. Based on both X-ray photoelectron spectroscopy (XPS) analysis of the reacted surface as well as the composition of the gas phase products by residual gas analysis (RGA), the crust layer and bulk photoresist are removed by oxidation mechanisms similar to remote oxygen plasma strip processes. Above 350°C, it is found that O2 gas itself reacts with the photoresist but to a lesser extent than the O3/O2 mixture. Reactivity to silicon is lower than a typical remote oxygen plasma based on ~1 Å silicon loss measured on blanket films. This in-situ thermally activated oxidation reaction allows for subsequent wet processing without any queue time delay. The integrated process completely removes 5E15-1E16 at/cm2 high-dose implanted 193 nm photoresist with no residue on patterned substrates. Fig 1: In-situ thermally activated oxidation reaction in a single-wafer wet clean chamber process sequence consisting of (a) a dry ozone-oxygen gas environment, (b) ozone-oxygen decomposition and reaction on a heated wafer surface, (c) ozonated de-ionized water rinse in an N2 environment, (d) ammonium hydroxide-hydrogen peroxide clean in an N2 environment, and (e) a de-ionized water rinse and spin dry. Figure 1

Thermally Activated Gas-Phase Chemical Reactions in a Single-Wafer Wet Clean Process

We investigate an alternative approach to conventional high dose ion-implanted photoresist strip and clean that replaces a remote plasma strip with a thermally activated, atmospheric gas-phase oxidation process and replaces sulfuric-peroxide mixture (SPM) chemistry with a de-ionized water/ozone (DIO3) chemistry. The gas phase oxidation process is first characterized in a mini-test chamber. Then the alternative dry and wet clean sequence is entirely processed in a modified single-wafer wet clean chamber. The atmospheric dry O3/O2 gas mixture decomposes and reacts with the ion-implanted photoresist wafer that is heated above 300 °C from the backside. 9,000 Å/min and 600 Å/min removal rates are measured on 193 nm photoresist and high dose implanted crust, respectively. Silicon reactivity from ellipsometry measurements is lower than a typical remote oxygen plasma based on ~1 Å silicon loss. The integrated process completely removes 5E15-1E16 at/cm2 high dose implanted 193 nm photoresist with no residue remaining on patterned substrates.

Evaluation and Application of Dual-Reactor Chamber for Studying Atmospheric Oxidation Processes and Mechanisms

A new smog chamber with dual reactors was designed and constructed to study atmospheric oxidation processes that may form ozone or secondary organic aerosols (SOAs). The chamber consists of two 5 m(3) fluorinated ethylene propylene (FEP) Teflon-film reactors housed in a thermally isolated enclosure, in which the temperature can be well controlled in the range of -10 to 40 degrees C. The influence of the light source on the gas-phase oxidation mechanism of propene was investigated. The results showed that multiple ultraviolet (UV) light sources were better than traditional narrow-band black-lamp light sources. Preliminary experiments on propene and m-xylene photo-oxidation processes were performed. The results showed that the dual-reactor chamber can simulate the gas-phase oxidation processes that form ozone or SOAs, and can be used to determine the effects of various species by comparing experiments performed using different initial concentrations. The SOA yield data from m-xylene photo-oxidation under different NOx conditions were in good agreement with those from previous studies. This proves that the chamber can simulate gas-to-particle conversion processes. The dual reactors have the advantage of enabling experiments to be performed with only one key parameter being changed. This will help us to further understand the role of key factors in complex atmospheric pollution processes.

Deposition of SnO2 on the Anatase TiO2 {105} Facets with High Photocatalytic Performance

AbstractAnatase TiO2 as a promising photocatalyst has been widely employed in the decontamination treatment of polluted water, air purification and water splitting. Coupling TiO2 with other semiconductor materials could further enhance the photocatalytic activity. Here, we successfully synthesized the SnO2/TiO2 catalyst by depositing SnO2 particles on the anatase TiO2 {105} facets through a gas phase oxidation process. The SnO2/TiO2 catalyst shows higher photocatalytic activity for decomposition of MB than that of the pure TiO2 catalyst. The enhanced photocatalytic activity can be attributed to the efficient charge separation since TiO2 and SnO2 catalyst have staggered energy level.

Enhancing photocatalytic activity of Sn doped TiO2 dominated with {1 0 5} facets

Electronic structures of semiconductor photocatalysts control the light absorbance, charge-carrier separation and mobility, activation energy and consequently, photocatalytic activity. Ion doping has been widely used to modify the electronic structure of a semiconductor photocatalyst. Here, we successfully synthesized Sn doped single crystalline anatase TiO2 particles dominated with {105} facets by a gas phase oxidation process, and their structure and composition were thoroughly analyzed by XRD/TEM/FESEM/XPS. The photoluminescence emission spectra measurements reveal that the small amount of doped Sn in TiO2 could suppress the recombination of photogenerated electron–hole pairs. Thus, the Sn doped TiO2 shows a significantly enhanced photocatalytic hydrogen evolution performance, with its hydrogen generation rate being 4.5 times higher than that of pure TiO2. First-principle simulation results suggest the doped Sn at the edge exhibit higher adsorption energy toward H, which could promote the H2 generation from the splitting of water.

Thermochemical Properties and Bond Dissociation Energies of C3–C5 Cycloalkyl Hydroperoxides and Peroxy Radicals: Cycloalkyl Radical + 3O2 Reaction Thermochemistry

Cyclic aliphatic hydrocarbons are major components in modern fuels; they can be present in the reactants, and they can be formed during the gas-phase oxidation processes. In combustion and thermal oxidation processes, these cyclics will form radicals that react with (3)O(2) to form peroxy radicals. In this study, density functional theory and higher level ab initio calculations are used to calculate thermochemical properties and bond dissociation energies of 3-5-membered cycloalkanes, corresponding hydroperoxides, hydroperoxycycloalkyl radicals, and cycloalkyl radicals that occur in these reaction systems. Geometries, vibration frequencies, and thermochemical properties, ΔH(f 298)°, are calculated with the B3LYP/6-31 g(d,p), B3LYP/6-31 g(2d,2p), composite CBS-QB3, and G3MP2B3 methods. Standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Group additivity contributions are developed, and application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities, S°(T) and C(p)°(T) (5 K ≤ T ≤ 5000), are determined using geometric parameters and frequencies from the B3LYP/6-31 g(d,p) calculations.

Synergy of Homogeneous and Heterogeneous Chemistry Probed by In Situ Spatially Resolved Measurements of Temperature and Composition

The interaction between heterogeneous and homogeneous chemistries is a crucial issue for high-temperature catalytic processes. In particular, the assessment of the main routes that control the selectivity to the desired products is essential for the design and safe operation of reaction units. This assessment is particularly true for the ultrafast conversion of hydrocarbons in short-contact-time (SCT) reactors that play a pivotal role in the effort to cope with the worldwide growing demand for more efficient exploitation of energy andmaterial resources. Examples are the catalytically assisted combustion for gas turbines with ultralow emissions, the catalytic partial oxidation (CPO) of hydrocarbons to H2 or CO/H2 mixtures (i.e., syngas), and the oxidative dehydrogenation (ODH) of light alkanes to olefins. As a common feature, these processes operate in autothermal and compact reactors, with noble-metal catalysts (palladium, rhodium, and platinum). An enormous energy intensity is peculiar to the SCT autothermal conversion of hydrocarbons over noble metals. Strongly exothermic and endothermic reactions proceed on the catalyst surface at extremely high rates. As a consequence, sharp gradients of temperature (up to 200 8Cmm ) and concentration are established within the small reactor volumes. Temperatures ranging from 250 to 1100 8C are generally experienced within a few millimeters. Such a level of severity—in terms of power density and extent of temperature and concentration gradients—is typical of flames and gas-phase oxidation processes in general. For a catalytic process, however, these conditions represent a thoroughly unconventional kinetic regime. To best grasp the intensity and the speed of the involved phenomena, we need to think of SCT conversion of light alkanes as the catalytic equivalent of a flame. This analogy can clearly depict the complexity of the process and emphasize the related scientific issues: To what extent does a catalytic process “stick” to the catalyst surface at these very high temperatures, where the adsorption of species is thermodynamically unfavored? Can the gas-phase activation of C H bonds (e.g., the formation and propagation of radicals) cooperate or compete with the catalytic process? In this respect, it is largely accepted that in the case of CH4 CPO over rhodium the gas-phase paths are negligible at atmospheric pressure and the catalytic route dominates. Conversely, the SCT-ODH of short alkanes over platinum proceeds mainly in the gas phase, thus giving rise to the production of olefins and other hydrocarbon species. On the basis of these examples, we could conclude that either catalytic or gas-phase chemistry governs the SCT conversion of hydrocarbons, depending only on the stability of the C H bond in the gas phase. Herein, by using novel techniques for collecting spatially resolved temperature and concentration profiles, we show that the partial oxidation of short-chain alkanes over rhodium breaks the paradigmatic compartments of heterogeneous processes and gas-phase processes, revealing the real complexity of these “flame-like” processes. Specifically, we examine the reaction of C3H8 CPO (C3H8+ 3/2O2!3CO+ 4H2) as a case study, and we apply novel techniques for collecting spatially resolved gas-phase and solid temperature and concentration profiles within a rhodium-coated honeycomb monolith to monitor the evolution of a propane/air mixture fed at high flow rate. The temperature and the composition of the reacting system were monitored from the inlet reactor section where the mixture was fed to the outlet section of the catalytic unit where the syngas stream was delivered. The results are reported in Figure 1 and Figure 2 as spatially resolved profiles of temperature and molar fraction of reactants and products. In the first 5 mm of the honeycomb, a sharp drop of O2 and C3H8 concentration was observed, accompanied by the formation of total oxidation products (CO2 and H2O) and partial oxidation products (H2 and CO). Correspondingly, a hot spot formed on the catalyst surface (980 8C, measured by the pyrometer) and a steep rise was observed in the gas temperature (up to 945 8C, measured by the thermocouple). In line with the occurrence of the endothermic steam reforming reaction, the evolution of H2O showed a maximum. Moreover, the temperature of the solid surface and in the gas phase decreased toward the exit of the honeycomb. Qualitatively, this behavior is what our and other research groups have observed also in the case of a CH4CPO experiment on rhodium, and can be fully explained by the catalytic production of syngas on rhodium. Thus, integral measurements (i.e., measurements of temperature and composition collected exclusively at the reactor outlet) would have suggested the unique existence of a heterogeneous process. A purely catalytic process was, for instance, [*] Dr. A. Donazzi, D. Livio, Dr. M. Maestri, Prof. A. Beretta, Prof. G. Groppi, Prof. E. Tronconi, Prof. P. Forzatti Laboratory of Catalysis and Catalytic Processes Dipartimento di Energia, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milano (Italy) Fax: (+39)0223993284 E-mail: alessandra.beretta@polimi.it Homepage: http://www.lccp.polimi.it

Anatase TiO2 Crystals with Exposed High‐Index Facets

Put your best face forward: The performance of TiO2 anatase crystals in energy and environmental applications is normally correlated with the TiO2 crystal facets exposed, and increasing the percentage of highly reactive surfaces is extremely important. A new gas-phase oxidation process using TiCl4 as precursor now yields anatase TiO2 single crystals with primarily high-index {105} facets, which can cleave water photocatalytically. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Birch Reduction of Benzene in a Low‐Temperature Plasma

Selective and specific dihydrogenation of benzene and other arenes has been observed in a low-temperature helium plasma. A surface Birch reduction mechanism has been proposed in which benzene molecules adsorbed on the discharge surface capture low-energy surface-adsorbed electrons and subsequently undergo protonation (see picture). Gas-phase oxidation processes accompany the reduction reaction.

Comparative evolved gas analyses on thermal degradation of thiourea by coupled TG-FTIR and TG/DTA-MS instruments

Identification and monitoring of gaseous species released during thermal decomposition of pure thiourea, (NH2)2C=S in argon, helium and air atmosphere have been carried out by both online coupled TG-FTIR and simultaneous TG/DTA-MS apparatuses manufactured by TA Instruments (USA). In both inert atmospheres and air between 182 and 240°C the main gaseous products of thiourea are ammonia (NH3) and carbon disulfide (CS2), whilst in flowing air sulphur dioxide (SO2) and carbonyl sulphide (COS) as gas phase oxidation products of CS2, and in addition hydrogen cyanide (HCN) also occur, which are detected by both FTIR spectroscopic and mass spectrometric EGA methods. Some evolution of isothiocyanic acid (HNCS) and cyanamide (NH2CN) vapours have also observed mainly by EGA-FTIR, and largely depending on the experimental conditions. HNCS is hardly identified by mass spectrometry. Any evolution of H2S has not been detected at any stage of thiourea degradation by either of the two methods. The exothermic heat effect of gas phase oxidation process of CS2 partially compensates the endothermicity of the corresponding degradation step producing CS2.

Enthalpies of Formation, Bond Dissociation Energies, and Molecular Structures of the n-Aldehydes (Acetaldehyde, Propanal, Butanal, Pentanal, Hexanal, and Heptanal) and Their Radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.