AVS 67 Session HC-Invited On Demand: Fundamental Discoveries in Heterogenous Catalysis Invited On Demand Session

Selective alkane conversion to high value fuels or chemicals, such as methanol or ethylene, has drawn increasing interest due to abundance of natural gas. Recently, we reported a combined temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) study of below room temperature (T ~ 120 K) activation of methane on the stoichiometric IrO₂(110) surface under ultrahigh vacuum (UHV) conditions. Subsequent work also showed that ethane to ethylene selectivity could be increased on IrO₂(110) by creating some proportion of hydrogenated bridging oxygen (O_br-H) sites, which limit the oxidative capacity of the surface. This past work suggests that substitution of O_br sites with inert species may be an avenue to control selectivity between full and partial oxidation. We have used DFT to explore the effects of Cl substitution of O_br. Chlorine is isoelectronic to O_br-H and selective Cl substitution of O_br has been demonstrated on RuO₂(110) by Over and co-workers. In this work, we apply DFT to explore selective conversion of ethane on Cl-doped IrO₂(110). We find that the Cl-doped IrO₂(110) is still facile in converting ethane to ethylene. In contrast, the ethylene selectivity step shows a strong effect due to Cl doping. Specifically, we find that H atom transfer from ethylene to the saturated O_br sites is unfavorable and therefore Cl doping will promote ethylene desorption over further reaction. Current efforts to develop a microkinetic model to explore ethylene selectivity under both TPRS and reaction conditions will be discussed.

The electrode-electrolyte interfacial reactivity and stability govern the efficiency and lifetime of electrochemical devices, especially under aggressive reaction conditions such as high temperature, high potential, and corrosive environments. Modulating the metal-oxygen bonding environment at the electrode surface offers an effective path towards enhancing the interfacial reactivity. However, the high interfacial reactivity can trigger undesired interfacial reactions that lead to local structural changes, phase segregation and dissolution, and ultimately the complete degradation of the pre-designed metal-oxygen bonding environment. The rational design of a highly active and stable electrocatalyst is largely hindered by this dilemma. Understanding how the electrocatalyst-electrolyte interface transforms under operating conditions can generate mechanistic insights into identifying the catalytically active motif and establishing methodologies to circumvent the dilemma by repairing the degraded structure. In this presentation, we report that operando synchrotron spectroscopic and microscopic analyses suggest that the interfacial degradation can be reversible between catalytic oxidation and reduction potentials. Such reversibility allows us to develop an intermittent reduction methodology to revivify the catalytic activity under operating conditions, enhancing catalyst durability.

The high thermochemical stability of CO₂ makes very difficult the catalytic conversion of the molecule into alcohols or other hydrocarbon compounds which can be used as fuels or the starting point for the generation of fine chemicals. Pure metals and bimetallic systems used for the CO₂ → CH₃OH conversion usually bind CO₂ too weakly and, thus, show low catalytic activity. Here, I will discuss a series of recent studies that illustrate the advantages of metal-oxide and metal-carbide interfaces when aiming at the conversion of CO₂ into methanol. CeO_x/Cu(111), Cu/CeO_x/TiO₂(110) and Au/CeO_x/TiO₂(110) exhibit an activity for the CO₂ → CH₃OH conversion that is 2-3 orders of magnitude higher than that of a benchmark Cu(111) catalyst. In the Cu-ceria and Au-ceria interfaces, the multifunctional combination of metal and oxide centers leads to complementary chemical properties that open active reaction pathways for methanol synthesis. Efficient catalysts are also generated after depositing Cu and Au on TiC(001). In these cases, strong-metal support interactions modify the electronic properties of the admetals and make them active for the binding of CO₂ and its subsequent transformation into CH₃OH at the metal-carbide interfaces.

Chirality detection of gas-phase molecules at low concentrations is challenging as the molecular number density is usually too low to perform conventional circular dichroism absorption experiments. In recent years, new spectroscopic methods have been developed to detect chirality in the gas phase. In particular, the angular distribution of photoelectrons after multiphoton laser ionization of chiral molecules using circularly polarized light is highly sensitive to the enantiomeric form of the ionized molecule (multiphoton photoelectron circular dichro-ism (MP-PECD)). In the talk, I will present the MP-PECD as an analytic tool for chirality detection of the bicyclic monoterpene fenchone desorbing from a Ag(111) crystal. We recorded velocity resolved kinetics of fenchone desorption on Ag(111) using pulsed molecular beams with ion imaging techniques. In addition, we measured temperature-programmed desorption spectra of the same system. Both experiments indicate weak physisorption of fenchone on Ag(111). We combined both experimental techniques with enantiomer-specific detection by recording MP-PECD of desorbing molecules using photoelectron imaging spectroscopy. We can clearly assign the enantiomeric form of the desorption product fenchone in sub-monolayer concentration. The experiment demonstrates the combination of MP-PECD with surface science experiments, paving the way for enantiomer-specific detection of surface reaction products on heterogeneous catalysts for asymmetric synthesis.