The effect of surface coverage on N2, NO and N2O formation over Pt(111).

The effect of surface coverage of species, θ, on the kinetic parameters of N2, NO and N2O formation in a system simulating ammonia oxidation over Pt(111) has been studied by using periodic density functional theory (DFT). The energy barriers for product formation decrease as θ increases, with the effect being more significant above 0.25 ML. The heat of surface reaction decreases as θ increases, making the dissociation of the products less favourable due to a weaker interaction of the intermediates with the surface. The effect of θ on the binding energy is stronger for N* than for either O* or NO*, but it is more apparent in the co-adsorption with O* and NO*. Similarly, the coverage of N* more strongly affects the activation energy of N2 and N2O desorption than does the coverage of O* for NO* and N2O formation.

In situ synchrotron XRD analysis of the kinetics of spodumene phase transitions.

The phase transition by thermal activation of natural α-spodumene was followed by in situ synchrotron XRD in the temperature range 896 to 940 °C. We observed both ß- and γ-spodumene as primary products in approximately equal proportions. The rate of the α-spodumene inversion is first order and highly sensitive to temperature (apparent activation energy ∼800 kJ mol-1). The γ-spodumene product is itself metastable, forming ß-spodumene, with the total product mass fraction ratio fγ/fß decreasing as the conversion of α-spodumene continues. We found the relationship between the product yields and the degree of conversion of α-spodumene to be the same at all temperatures in the range studied. A model incorporating first order kinetics of the α- and γ-phase inversions with invariant rate constant ratio describes the results accurately. Theoretical phonon analysis of the three phases indicates that the γ phase contains crystallographic instabilities, whilst the α and ß phases do not.

Effect of the Local Atomic Ordering on the Stability of ß-Spodumene.

This study focuses on the relative energetic stability of ß-spodumene configurations with different atomic ordering, evaluated using electronic structure methods based on static periodic density functional theory. We found that ß-spodumene configurations with a framework containing exclusively Al-O-Si linkages are energetically the most stable, consistent with the aluminum avoidance principle. A correlation between the interstitial sites occupied by lithium and the stability of the configuration was established: highly stable configurations contain greater proportions of lithium associated with the edges of AlO4 tetrahedrons. The identified low-energy configurations have a band gap of ∼4.8 eV, and similar electronic band structures and densities of states. Both the PBE and PBEsol functionals predict small differences in the relative stabilities of the different configurations of ß-spodumene. However, only PBEsol is able to reproduce the experimentally observed stability differences between α-spodumene and ß-spodumene. ß-Spodumene is the preferred polymorph at high temperatures, with the PBEsol inversion temperature from α- to ß-spodumene predicted to occur at 1070 K.

Site Isolation Leads to Stable Photocatalytic Reduction of CO2 over a Rhenium-Based Catalyst.

A porous organic polymer incorporating [(α-diimine)Re(CO)3Cl] moieties was produced and tested in the photocatalytic reduction of CO2, with NEt3 as a sacrificial donor. The catalyst generated both H2 and CO, although the Re moiety was not required for H2 generation. After an induction period, the Re-containing porous organic polymer produced CO at a stable rate, unless soluble [(bpy)Re(CO)3Cl] (bpy=2,2'-bipyridine) was added. This provides the strongest evidence to date that [(α-diimine)Re(CO)3Cl] catalysts for photocatalytic CO2 reduction decompose through a bimetallic pathway.

Hydrogen from formic acid through its selective disproportionation over sodium germanate--a non-transition-metal catalysis system.

A robust catalyst for the selective dehydrogenation of formic acid to liberate hydrogen gas has been designed computationally, and also successfully demonstrated experimentally. This is the first such catalyst not based on transition metals, and it exhibits very encouraging performance. It represents an important step towards the use of renewable formic acid as a hydrogen-storage and transport vector in fuel and energy applications.

Carbon Monoxide Oxidation on Ceria-Supported Nanoclusters.

Periodic density functional theory is used to evaluate the minimum energy pathways of CO oxidation on cerium oxide-supported platinum and palladium nanoclusters (Pt/CeO2 and Pd/CeO2). For Pt/CeO2, the oxidation process involves the participation of lattice oxygen from CeO2 at the boundary sites of the cluster-ceria interface, which exhibits an exceptionally low energy barrier. Conversely, on Pd/CeO2, oxidation predominantly occurs through oxygen species bound to the Pd cluster. Experimental analysis using the temperature-programmed reduction of the oxidized Pd/CeO2 catalyst reveals a lower CO oxidation temperature compared to Pt/CeO2. This observation aligns with the anticipated decrease in the energy barrier for CO oxidation due to the oxygen coverage of the Pd cluster.

Tailoring and Identifying Brønsted Acid Sites on Metal Oxo-Clusters of Metal-Organic Frameworks for Catalytic Transformation.

Metal-organic frameworks (MOFs) with Brønsted acidity are an alternative solid acid catalyst for many important chemical and fuel processes. However, the nature of the Brønsted acidity on the MOF's metal cluster or center is underexplored. To design and optimize the acid strength and density in these MOFs, it is important to understand the origin of their acidity at the molecular level. In the present work, isoreticular MOFs, ZrNDI and HfNDI (NDI = N,N'-bis(5-isophthalate)naphthalenediimide), were prepared as a prototypical system to unravel and compare their Brønsted and Lewis acid sites through an array of spectroscopic, computational, and catalytic characterization techniques. With the aid of solid-state nuclear magnetic resonance and density functional calculations, Hf6 oxo-clusters on HfNDI are quantitatively proved to possess a higher density Brønsted acid site, while ZrNDI-based MOFs display stronger and higher-population Lewis acidity. HfNDI-based MOFs exhibit a superior catalytic performance in activating dihydroxyacetone (DHA) and converting DHA to ethyl lactate, with 71.1% selectivity at 54.7% conversion after 6 h. The turnover frequency of BAS-dominated Hf-MOF in DHA conversion is over 50 times higher than that of ZSM-5, a strong BAS-based zeolite. It is worth noting that HfNDI is reported for the first time in the literature, which is an alternative platform catalyst for biorefining and green chemistry. The present study furthermore highlights the uniqueness of Hf-based MOFs in this important biomass-to-chemical transformation.

Conformational and thermodynamic properties of gaseous levulinic acid.

Molecular modeling is used to determine low-energy conformational structures and thermodynamic properties of levulinic acid in the gas phase. Structure and IR vibrational frequencies are obtained using density functional and Møller-Plesset perturbation theories. Electronic energies are computed using G3//B3LYP and CBS-QB3 model chemistries. Computed anharmonic frequencies are consistent with reported experimental data. Population analysis shows a boat- and a chainlike structure to be most abundant at 298 K, with increasing proportions of two other conformers at higher temperatures. Population mean distribution values for thermodynamic quantities are derived. At 298 K and 1 atm, the enthalpy of formation, entropy, and heat capacity are -613.1 ± 1.0 kJ·mol(-1), 407.4 J·mol(-1)·K(-1), and 132.3 J·mol(-1)·K(-1), respectively.

A Review of Terminology Used to Describe Soot Formation and Evolution under Combustion and Pyrolytic Conditions.

This review presents a glossary and review of terminology used to describe the chemical and physical processes involved in soot formation and evolution and is intended to aid in communication within the field and across disciplines. There are large gaps in our understanding of soot formation and evolution and inconsistencies in the language used to describe the associated mechanisms. These inconsistencies lead to confusion within the field and hinder progress in addressing the gaps in our understanding. This review provides a list of definitions of terms and presents a description of their historical usage. It also addresses the inconsistencies in the use of terminology in order to dispel confusion and facilitate the advancement of our understanding of soot chemistry and particle characteristics. The intended audience includes senior and junior members of the soot, black carbon, brown carbon, and carbon black scientific communities, researchers new to the field, and scientists and engineers in associated fields with an interest in carbonaceous material production via high-temperature hydrocarbon chemistry.

DFT analysis of the reaction paths of formaldehyde decomposition on silver.

Periodic density functional theory is used to study the dehydrogenation of formaldehyde (CH(2)O) on the Ag(111) surface and in the presence of adsorbed oxygen or hydroxyl species. Thermodynamic and kinetic parameters of elementary surface reactions have been determined. The dehydrogenation of CH(2)O on clean Ag(111) is thermodynamically and kinetically unfavorable. In particular, the activation energy for the first C-H bond scission of adsorbed CH(2)O (25.8 kcal mol(-1)) greatly exceeds the desorption energy for molecular CH(2)O (2.5 kcal mol(-1)). Surface oxygen promotes the destruction of CH(2)O through the formation of CH(2)O(2), which readily decomposes to CHO(2) and then in turn to CO(2) and adsorbed hydrogen. Analysis of site selectivity shows that CH(2)O(2), CHO(2), and CHO are strongly bound to the surface through the bridge sites, whereas CO and CO(2) are weakly adsorbed with no strong preference for a particular surface site. Dissociation of CO and CO(2) on the Ag(111) surface is highly activated and therefore unfavorable with respect to their molecular desorption.

Computational study of the reaction SH + O2.

The reaction of SH + O2 has been characterized using multireference methods, with geometries and vibrational frequencies determined at the CASSCF/cc-pVTZ level and single-point energies calculated at the MRCI/aug-cc-pV(Q+d)Z level. The dominant product channels are found to be SO + OH and HSO + O. Whereas the formation of SO + OH has a barrier of approximately 81 kJ mol-1, it is energetically more favorable than the formation of HSO + O, which is barrierless beyond the endothermicity of approximately 89 kJ mol-1 at 0 K. Thus, the reaction SH + O2 --> SO + OH is 2 orders of magnitude faster than the reaction SH + O2 --> HSO + O at room temperature, revealing that the atmospheric oxidation of SH leads directly to the formation of SO + OH with the rate coefficient of approximately 1.0 x 10(-2) cm3 mol-1 s-1. At temperatures above 1000 K, however, the rates of the two channels become comparable. This may be attributed to the entropy effects leading to the higher pre-exponential factor for the channel (forming HSO + O) via a more loose transition state than that (forming SO + OH) entailing a four-centered transition state. Whereas the hydrogen abstraction reaction producing S + HO2 is found to proceed on the quartet surface, the substantial barrier of approximately 165 kJ mol-1 means that it occurs as a minor product channel. Finally, the formation of possible products SO2 + H is prohibited due to the lack of a transition state for the direct sulfur insertion.

Theoretical study of reactions in the multiple well H2/S2 system.

The potential energy surface of the H(2)/S(2) system has been characterized at the full valence MRCI+Davidson/aug-cc-pV(Q+d)Z level of theory using geometries optimized at the MRCI/aug-cc-pVTZ level. The analysis includes channels occurring entirely on either the singlet or the triplet surface as well as those involving an intersystem crossing. RRKM-based multiple well calculations allow the prediction of rate constants in the temperature range of 300-2000 K between 0.1 and 10 bar. Of the SH recombined on the singlet surface, the stabilization of the rovibrationally excited adduct HSSH is at the low-pressure limit at 1 bar, but it has a rate comparable to that forming another major set of products H(2)S + S (via an intersystem crossing) at temperatures below 1000 K; at higher temperatures, HSS + H becomes the dominant product. For the reaction H(2)S + S, the presence of an intersystem crossing allows the formation of the singlet excited adduct H(2)SS, most of which rearranges and stabilizes as HSSH under atmospheric conditions. At high temperatures, the majority of excited HSSH dissociates to SH + SH and HSS + H. Compared to reported shock tube measurements of the reaction H(2)S + S, most of the S atom consumption can be described by the triplet abstraction route H(2)S + S --> SH + SH, especially at high temperatures, but inclusion of the singlet insertion channel provides a better description of the experimental data. The reaction HSS + H was found to proceed predominantly on the singlet surface without a chemical barrier. The formation of the major product channel SH + SH is very fast at room temperature (approximately 4 x 10(15) cm(3) mol(-1) s(-1)). While the formation of H(2)S + S or S(2) + H(2) via an isomerization or an intersystem crossing, respectively, are minor product channels, their rates are significantly higher than those of the corresponding direct triplet channels, except at elevated temperatures. Finally, due to the relatively shallow nature of its well, the stabilization of H(2)SS is negligible under conditions of likely interest.

Theoretical study of hydrogen abstraction and sulfur insertion in the reaction H2S + S.

The reaction of H2S + S has been characterized at the multireference configuration interaction level with the geometries optimized using the aug-cc-pVTZ basis set and the single-point energy calculated using the aug-cc-pV(Q+d)Z basis set. As in the analogous reaction of H2 + S, the presence of an intersystem crossing enables products (SH + SH) to be formed on the singlet surface through S insertion, which bypasses the triplet barrier (19.1 kJ mol-1 relative to SH + SH) of the H abstraction route. This provides theoretical evidence for SH + SH formation without barrier beyond endothermicity at sufficiently low temperatures. The H abstraction route, however, is expected to be competitive at higher temperatures due to a much higher Arrhenius pre-exponential factor (6.9 x 10(14) cm3 mol-1 s-1 derived from TST calculation) than that of S insertion channel (3.7 x 10(13) cm3 mol-1 s-1, derived by a least-squares fit to the measurements). With a slightly higher transition-state barrier than that of the H abstraction channel, the production of S2 + H2 is less favored due to proceeding via intersystem crossing and insertion. While the formation of HSS + H is energetically unfavorable relative to SH + SH, recombination channels producing H2SS or the more stable HSSH are expected to occur under collisional stabilization conditions at high pressures.

Deportment and management of metals produced during combustion of CCA-treated timbers.

Experiments were conducted to study CCA-treated wood combustion over a range of temperature and oxygen concentrations with a view to understanding the factors affecting energy and metals recovery from waste treated timber. CCA-treated wood was burned in a furnace at temperatures from 400 to 940 degrees C and oxygen concentrations between 5 and 21%. The ash and condensed volatiles were digested for total concentrations of metals and subjected to leaching tests to determine the stabilized concentrations of metals. Arsenic volatilisation increased with increasing furnace temperature whereas the copper and chromium reported mainly to the ash product. The effect of oxygen concentration was weak although it appeared that more arsenic volatilises at higher oxygen concentrations. However, a larger proportion of the arsenic in the ash generated at lower oxygen concentrations is solubilised during leaching tests, with the result that the concentration of stabilized arsenic in the ash is relatively unaffected by oxygen concentration.

Reaction of hydrogen with Ag(111): binding states, minimum energy paths, and kinetics.

The interaction of atomic and molecular hydrogen with the Ag(111) surface is studied using periodic density functional total-energy calculations. This paper focuses on the site preference for adsorption, ordered structures, and energy barriers for H diffusion and H recombination. Chemisorbed H atoms are unstable with respect to the H(2) molecule in all adsorption sites below monolayer coverage. The three-hollow sites are energetically the most favorable for H chemisorption. The binding energy of H to the surface decreases slightly up to one monolayer, suggesting a small repulsive H-H interaction on nonadjacent sites. Subsurface and vacancy sites are energetically less favorable for H adsorption than on-top sites. Recombination of chemisorbed H atoms leads to the formation of gas-phase H(2) with no molecular chemisorbed state. Recombination is an exothermic process and occurs on the bridge site with a pronounced energy barrier. This energy barrier is significantly higher than that inferred from experimental temperature-programmed desorption (TPD) studies. However, there is significant permeability of H atoms through the recombination energy barrier at low temperatures, thus increasing the rate constant for H(2) desorption due to quantum tunneling effects, and improving the agreement between experiment and theory.

Molecular dynamics study of Acid-catalyzed hydrolysis of dimethyl ether in aqueous solution.

The acid-catalyzed hydrolysis of dimethyl ether (DME) to methanol was examined using ab initio density functional metadynamics simulations. Diffusion of the acid proton from the aqueous medium, leading to the formation of a protonated DME, is 12.3 kcal mol(-1) activated and 9.3 kcal mol(-1) endothermic, indicating a greater affinity of the acid proton to water than to the ether group. Subsequent scission of the protonated ether bond is found to be 30.7 kcal mol(-1) activated, leading to the formation of a solvated methyl-carbocation, which is thermodynamically unstable. The methyl-carbocation reacts readily to form methanol and regenerate the acid proton.

Local site selectivity and conformational structures in the glycosidic bond scission of cellobiose.

Car-Parrinello molecular dynamics combined with metadynamics simulations were used to study the acid-catalyzed hydrolysis of cellobiose (CB) in aqueous solution. The hydrolysis was studied in two steps. Step 1 involves the proton transfer from solvent to CB and dissociation of the glycosidic bond to ß-glucose and oxacarbenium ion species. Step 2 involves the formation of α-glucose from oxacarbenium and regeneration of the acid proton species. Step 1 is endothermic, while Step 2 is exothermic. The overall activation free energy of CB hydrolysis is 32.5 kcal mol(-1), and the overall reaction free energy is -5.9 kcal mol(-l), consistent with available experimental data. We observe that a stepwise mechanism generally described in the literature for Step 1 is not significantly favored relative to a concerted ß-1,4' linkage dissociation process.

Density functional study of the reaction of carbon surface oxides: the behavior of ketones.

The reactions of a ketone surface oxide group have been studied on two forms of the zigzag edge and the armchair edge of a model char using density functional theory at the B3LYP/6-31G(d) level of theory. Rearrangement and surface migration reactions were found to occur much more rapidly than desorption reactions on both the zigzag and armchair edges. A number of desorption pathways characterized here go some way toward explaining the experimentally observed broad activation energy profile for CO desorption. Three separate desorption processes were characterized; on the zigzag surface two were found with activation energies of 275 and 367 kJ mol(-1), while on the armchair surface one was found with an activation energy of 296 kJ mol(-1). The activation energies for these processes were found to be insensitive to increasing the size of the char fragment. On a larger char fragment, however, an extra desorption process was found to be possible, with an activation energy of 160 kJ mol(-1).

Role of the direct reaction H2S + SO2 in the homogeneous Claus reaction.

Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H(2)S, SO(2), and S(2)O such as might occur in the front-end furnace of the Claus process. The direct reaction between H(2)S and SO(2) occurs via a 5-centered transition state with an initial barrier of approximately 135 kJ mol(-1) and an overall barrier of approximately 153 kJ mol(-1) to produce S(2)O and H(2)O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by approximately 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H(2)S and S(2)O can occur via two possible pathways; the analogous reaction to H(2)S + SO(2) has an initial barrier of approximately 117 kJ mol(-1) and an overall barrier of approximately 126 kJ mol(-1) producing S(3) and H(2)O, and a pathway with a 6-centred transition state has a barrier of approximately 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.

Gas-phase interaction of H2S with O2: A kinetic and quantum chemistry study of the potential energy surface.

Quantum chemical calculations were carried out to study the interaction of hydrogen sulfide with molecular oxygen in the gas phase. The basic mechanism, the rates of reaction, and the potential energy surface were calculated. Isomers and transition states that connect the reactants with intermediates and products of reaction were identified using the G2 method and B3LYP/6-311+G(3df,2p) functional. Hydrogen abstraction to form HO2 + SH is the dominant product channel and proceeds through a loose transition state well-described at the level of calculation employed. The temperature dependence of the rate coefficient in the range 300-3000 K has been determined on the basis of the ab initio potential energy surface and with variational transition-state theory. The reaction is 169.5 kJ mol(-1) endothermic at 0 K with a rate constant given by 2.77 x 10(5) T(2.76) exp(-19 222/T) cm3 mol(-1) s(-1) and should proceed slowly under atmospheric thermal conditions, but it offers a route to the initiation of H2S combustion at relatively low temperatures.