Chemical dynamics and heterogeneous catalysis
Professor George C Schatz, Northwestern University, USA
CO2 reduction to syngas and N2 reduction to ammonia are processes of fundamental interest to energy science. In this talk, theory is used to provide new directions to this research in two areas. The first part of the talk discusses plasma enhanced dry reforming, and photocatalytic conversion of N2 to ammonia. Dry reforming is a process wherein CH4 and CO2 react to give syngas and/or liquid fuels. Dry reforming is normally done under high temperature and pressure conditions, with a Ni catalyst, however it has recently been discovered that if a plasma is also present near the catalyst, then it is possible to get this reaction to go under modest conditions close to room temperature and atmospheric pressure. The role of the plasma in this process is poorly understood. The talk focuses on the use of electronic structure studies and Born-Oppenheimer molecular dynamics to describe gas-surface reactions that arise from plasma species. The plasma is known to fragment the reacting gases, especially CH4 into CH3 + H and CH2 + 2H, so a highlight of this work concerns reaction of atomic hydrogen and CH2 with adsorbed CO2 and CO to give CO, water, formaldehyde, methanol and other products. The results include comparisons with experiments from the Koel group at Princeton, and with other groups, and it is found that hot-atom and Eley-Rideal mechanisms play an important role. The second part of this talk considers the reaction mechanism underlying recent work from the Kanatzides group at Northwestern in which it was discovered that iron-sulphur clusters present in gel materials can participate in the photocatalytic conversion of N2 to ammonia under ambient conditions. The theoretical studies use broken symmetry density functional theory to reveal a mechanism for this process that is related to what happens with the nitrogenase enzyme, but with important differences that arise from photon-induced delivery of electrons to the iron-sulphur clusters.
Towards a chemically accurate description of reactions of molecules with metal surfaces
Professor Geert-Jan Kroes , Leiden University, The Netherlands
Heterogeneously catalyzed processes are of large importance: the production of the majority of chemicals involves catalysis at some stage. Heterogeneously catalyzed processes consist of several elementary reactions. Accurately calculating their rates requires the availability of accurate barriers for the rate controlling steps. Unfortunately, currently no first principles methods can be relied upon to deliver the required accuracy. To solve this problem, in 2009 a novel implementation of the specific reaction parameter approach to density functional theory (SRP-DFT) was formulated. This allowed reproducing experiments for H2 reacting on copper surfaces, and to determine barrier heights for H2-Cu systems, with chemical accuracy. The original procedure used was not extendable to reactions of molecules heavier than H2 with surfaces, because the metal surface was treated as static. This problem has now been solved through a combination of SRP-DFT with Ab Initio Molecular Dynamics (AIMD). This method was applied to the dissociative chemisorption of methane on a Ni surface, a rate-limiting step in the steam reforming reaction. Experiments on CHD3 + Ni(111) were reproduced with chemical accuracy, and a value of the reaction barrier height was derived that is claimed to be chemically accurate. Even better results were obtained for CHD3 + Pt(111), suggesting that the SRP density functional for methane interacting with Ni(111) is transferable to methane interacting with other group X metal surfaces. Even more interestingly for applications to catalysis, the SRP functional derived for methane reacting with Ni(111) also gives a very accurate description of molecular beam sticking experiments on CHD3 + Pt(211).
Quantum dynamics studies of the Cl + CH4 reaction
Professor Dong Hui Zhang, Dalian Institute of Chemical Physics, China
The Cl+CH4→HCl+CH3 reaction has been the subject of extensive experimental and theoretical investigations due to its crucial role in the Cl/O3 destruction chain mechanism in the stratosphere, and has also become a prototype for studying mode specificity and bond selectivity in polyatomic reactions with a late barrier. Earlier quantum dynamics studies on a high-quality potential energy surface (PES) constructed by Czako and Bowman (CB) revealed that there is a distinctive peak in the total reaction probabilities for the total angular momentum J=0 at collision energy of about 3 kcal/mol, which was inferred to be related to a dynamics resonance in the reaction. In this talk, Professor Zhang will present a quantum dynamics study of the reaction on a new PES by using the reduced dimensionality model of Palma and Clary by restricting the non-reacting CH3 group in a C3V symmetry. The calculated total reaction probabilities for the total angular momentum J=0 on the new PES, which is of a quantitative level of accuracy, exhibit a clear peak structure as on the CB PES. Detailed dynamics analysis uncovered that the peak structure does not originate from a dynamics resonance, it is a very reaction probability oscillation associated with the heavy-light-heavy (HLH) nature of the reaction. State-to-state quantum dynamics calculations revealed that the HLH oscillation in the reaction has important influence to product rotation distributions, and also leave a clear peak in the backward scattering direction which can be detected by a cross-molecular beam experiment.
Quantum interferences in inelastic molecular collisions
Professor Millard H Alexander, University of Maryland, USA
In the textbook experiment of Young, quantum interference between two distinct trajectories leads to oscillations in the observed scattering of an atomic beam through two slits. Similar interferences patterns can be observed as oscillations in the intensities of scattering of a molecular beam from a given initial rotational state into various final states. This quantum interference structure becomes especially rich when the initial and final states are coupled by two electronic potential energy surfaces. This field will be reviewed with particular attention to work done in collaboration with Professor Clary.