The quest for an efficient process to convert methane efficiently to fuels and high value-added chemicals such as olefins and aromatics is motivated by their increasing demands and recently discovered large reserves and resources of methane. Direct conversion to these chemicals can be realized either oxidatively via oxidative coupling of methane (OCM) or nonoxidatively via methane dehydroaromatization (MDA), which have been under intensive investigation for decades. While industrial applications are still limited by their low yield (selectivity) and stability issues, innovations in new catalysts and concepts are needed. The newly emerging strategy using iron single sites to catalyze methane conversion to olefins, aromatics, and hydrogen (MTOAH) attracted much attention when it was reported. Because the challenge lies in controlled dehydrogenation of the highly stable CH4 and selective C–C coupling, we focus mainly on the fundamentals of C–H activation and analyze the reaction pathways toward selective rou...

Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects

How does a chemical reaction proceed at ultralow temperatures? Can simple quantum mechanical rules such as quantum statistics, single partial-wave scattering, and quantum threshold laws provide a clear understanding of the molecular reactivity under a vanishing collision energy? Starting with an optically trapped near-quantum-degenerate gas of polar 40K87Rb molecules prepared in their absolute ground state, we report experimental evidence for exothermic atom-exchange chemical reactions. When these fermionic molecules were prepared in a single quantum state at a temperature of a few hundred nanokelvin, we observed p-wave-dominated quantum threshold collisions arising from tunneling through an angular momentum barrier followed by a short-range chemical reaction with a probability near unity. When these molecules were prepared in two different internal states or when molecules and atoms were brought together, the reaction rates were enhanced by a factor of 10 to 100 as a result of s-wave scattering, which does not have a centrifugal barrier. The measured rates agree with predicted universal loss rates related to the two-body van der Waals length.

https://science.sciencemag.org/content/sci/327/5967/853.full.pdf

Quantum-State Controlled Chemical Reactions of Ultracold Potassium-Rubidium Molecules

Molecular Reaction Dynamics: Frontmatter

Extending our previous studies for the H2+OH reaction in five mathematical dimensions (5D) [J. Chem. Phys. 99, 5615 (1993); 100, 2697 (1994)], we present in this paper a full‐dimensional (6D) dynamics study for the title reaction. The 6D treatment uses the time‐dependent wave‐packet approach and employs discrete variable representations for three radial coordinates and coupled angular momentum basis functions for three angular coordinates. The present 6D study employs an energy projection method to extract reaction probabilities for a whole range of energies from a single wave‐packet propagation, while previous studies produced only energy‐averaged reaction probability from a single wave‐packet propagation. The application of the energy‐projection method allows us to efficiently map out the energy dependence of the reaction probability on a fine grid which revealed surprisingly sharp resonancelike features at low collision energies on the Schatz–Elgersma potential surface. Our calculation shows that the potential‐averaged 5D treatment can produce reaction probabilities essentially indistinguishable from the full‐dimensional result. We also report initial state‐selected reaction cross sections and rate constants which are in good agreement with our previous calculations. The effect of OH vibration on H2+OH reaction is examined in the present study and our calculation shows that the OH vibration can enhance the rate constant by about a factor of 1.7 in good agreement with the experimental estimate of about 1.5.

Full‐dimensional time‐dependent treatment for diatom–diatom reactions: The H2+OH reaction

This review discusses recent quantum scattering calculations on bimolecular chemical reactions in the gas phase. This theory provides detailed and accurate predictions on the dynamics and kinetics of reactions containing three atoms. In addition, the method can now be applied to reactions involving polyatomic molecules. Results obtained with both time-independent and time-dependent quantum dynamical methods are described. The review emphasises the recent development in time-dependent wave packet theories and the applications of reduced dimensionality approaches for treating polyatomic reactions. Calculations on over 40 different reactions are described.

Quantum scattering calculations on chemical reactions.

Scattering data, measured for rare gas-rare gas systems under high angular and energy resolution conditions, have been used to probe the reliability of a recently proposed interaction potential function, which involves only one additional parameter with respect to the venerable Lennard-Jones (LJ) model and is hence called Improved Lennard-Jones (ILJ). The ILJ potential eliminates most of the inadequacies at short- and long-range of the LJ model. Further reliability tests have been performed by comparing calculated vibrational spacings with experimental values and calculated interaction energies at short-range with those obtained from the inversion of gaseous transport properties. The analysis, extended also to systems involving ions, suggests that the ILJ potential model can be used to estimate the behavior of unknown systems and can help to assess the different role of the leading interaction components. Moreover, due to its simple formulation, the physically reliable ILJ model appears to be particularly useful for molecular dynamics simulations of both neutral and ionic systems.

Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations.

This review describes advances which have occurred during the past decade in chemical reaction dynamics using crossed molecular beams. After a brief historical introduction, advances in the generation and state selection of beams of reactants, and in the schemes for product detection, which are at the basis of our increased ability to measure state-averaged and state-resolved reactive differential cross sections in crossed beam experiments, are described. The novel couplings of laser and synchrotron radiation to these experiments are noted. A selection of case studies are considered in some detail, to exemplify recent improvements in our understanding of gas-phase neutral reaction dynamics. The examples include prototype reactions involving three-atom and four-atom systems, and the results are discussed in the light of the most recent, synergistic, theoretical developments for treating both the potential energy surfaces and the reaction dynamics. Progress made in studies of reactions of molecular radicals and of chemically important atoms, such as carbon, nitrogen, and oxygen, with polyatomic molecules, as made possible by recent developments in the classic crossed beam technique with mass spectrometric detection, is emphasized. Some complementary techniques that recently have contributed to our understanding of chemical reactivity are also described briefly.

https://iopscience.iop.org/article/10.1088/0034-4885/63/3/203/pdf

Chemical reaction dynamics with molecular beams

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center‐of‐mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty‐two percent of the available energy is released as product translational energy.

Crossed beam studies of four‐atom reactions: The dynamics of OH+CO

The reactions of (I) Li+HF→LiF+H and (II) Li+HCl→LiCl+H have been studied by the crossed molecular beams method. Angular distributions [N(Θ)] of product molecules have been measured at 4 collision energies (Ec) ranging from about 2 to 9 kcal/mole and time‐of‐flight (TOF) measurements of product velocity distributions were made at approximately Ec=3 and 9 kcal/mole for both reactions (I) and (II). The combined N(Θ) and TOF results were used to generate contour maps of lithium–halide product flux in angle and recoil velocity in the center‐of‐mass (c.m.) frame. For reaction (I) at Ec=3 kcal/mole the c.m. angular distribution [T(ϑ)] shows evidence of complex formation with near forward–backward symmetry; slightly favored backward peaking is observed. The shape of this T(ϑ) indicates there is significant parallel or antiparallel spatial orientation of initial and final orbital angular momentum L and L′, even though with H departing L′ must be rather small and L?J′, where J′ is the final rotational angular mome...

/pdf/study-of-the-reaction-dynamics-of-li-hf-hcl-by-the-crossed-4cn9ekeyyk.pdf

Study of the reaction dynamics of Li+HF, HCl by the crossed molecular beams method

Dynamical studies of elementary gas-phase bimolecular reactions have progressed significantly during the last few years owing to advancements in molecular beam and laser techniques as well as in theoretical methodologies In this article we give a brief overview of the recent progress in the field of reaction dynamics and then survey recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method using mass-spectrometric detection Emphasis is on three-atom (A + BC) and four-atom (AB + CD) reactions for which the interplay between experiment and theory is the strongest and the most detailed Reactive differential cross-sections for the three-atom Cl + H2 and four-atom OH + H2 and OH + CO reactions are presented and compared with the results of quasiclassical and quantum-mechanical scattering calculations on ab initio potential-energy surfaces in an effort to assess the status of theory versus experiment The reaction dynamics of electronically excited atoms are discussed too; the effect of electronic excitation on the reaction dynamics of atomic oxygen is examined using the reaction O(3P, 1D)+ H2S as an example

Piergiorgio Casavecchia

Papers

Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations.

Chemical reaction dynamics with molecular beams

Crossed beam studies of four‐atom reactions: The dynamics of OH+CO

Study of the reaction dynamics of Li+HF, HCl by the crossed molecular beams method

Reactive scattering of atoms and radicals