AVS 66 Session HC-ThP: Fundamental Discoveries in Heterogeneous Catalysis Poster Session

Dry reforming of methane (DRM) has been proposed as an alternative to coal or natural gas production of synthesis gas, and the process has been optimized with various metal catalysts to enhance catalytic activity and reduce surface carbon contamination. In this work, boron was paired with platinum, a highly active DRM catalyst, on a silica support material, and surface characterization techniques were used to determine how the presence of boron affected the metal catalyst and the catalytic performance. X-ray photoelectron spectroscopy (XPS) revealed the existence of trigonal (BO₃ structures) and tetrahedral (BO₄ structures) B-containing surface species, which agreed with solid-state ¹¹B NMR results. Density functional theory (DFT) calculations were performed to determine the thermodynamically stable configuration of the B/SiO₂ species, as well as the most favorable species for platinum adsorption. Tetrahedrally coordinated boron was found to be the most favorable species for platinum adsorption. A butane dehydrogenation reaction was utilized to produce fully deactivated Pt/B/SiO₂ catalysts samples for further analysis. After complete catalyst deactivation, ¹¹B NMR combined with theoretically-predicted isotropic chemical shifts calculated by Amsterdam Density Functional (ADF) revealed that, in addition to BO₃ and BO₄ fragments, a boron-carbon surface species had been formed during butane dehydrogenation, but this was only observed on catalysts containing platinum. From this observation, it has been concluded that platinum causes carbon contamination during this reaction, but it is mitigated by the presence of boron.

The surface species formed following the adsorption and hydrogenation of acetylene (C₂H₂) on Pd(111) in the temperature range of 90-450 K are identified and characterized. The techniques used to detect the species are reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Acetylene was adsorbed at low temperature and then heated to 300 K where it converts to ethylidyne (CCH₃), which is identified by its δ(CH₃) bending mode at 1327 cm^-1. The reaction mechanism for this conversion is quite complex; in the literature there are discrepancies in the identification of the intermediates with both vinyl (CHCH₂) and vinylidene (CCH₂) having been proposed. The experimental data available to date favors the formation of vinylidene as an intermediate in the conversion of acetylene to ethylidyne (CCH₃). Evidence for vinylidene is seen in the appearance of its bending mode (δ(HCH)) at 1425 cm^-1. The possible mechanism of ethylidyne (CCH₃) formation is through isomerization of acetylene to vinylidene (CCH₂) followed by hydrogenation of vinylidene. In the temperature range of 400 to 450 K, C₂H₂ decomposes and forms a C_xH_y species as verified by TPD spectra showing an H₂ peak at 440 K following C₂H₂ adsorption at low temperature. The low energy electron diffraction (LEED) pattern of acetylene adsorbed at 95 K gives rise to a (√3×√3)R30^∘ pattern. The experimental result is also supported by quantum mechanical computational calculations based on density functional theory.

Alumina supported silver catalysts have been used for decades for the direct oxidative conversion of ethylene to ethylene oxide. Ethylene oxide is used in many cleaning and sterilizing products; however, its major industrial application is in the production of ethylene glycol used in the generation of antifreeze, polyesters, liquid coolants and solvents. Global production of ethylene oxide is estimated to grow to 28 million metric tons in 2019 at an estimated market value of over $48 billion (1, 2).

Due to the huge size of this market, incremental improvements in the efficiency and longevity of the catalyst can be very beneficial. Many companies continuously explore ways in which to improve the various aspects of the catalyst system – the physical and chemical properties of the catalyst, the preparation technology, and the reaction conditions.

In this investigation, three surface sensitive (outermost ≤ 5 nm) techniques, X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), and Auger Electron Spectroscopy (AES) were used to study the changes that occur between fresh and aged Cs-promoted, alumina-supported Ag catalyst samples. XPS provided insights into the overall surface concentrations of the elements present as well as chemical state changes that occurred during aging. TOF-SIMS was used to detect and spatially locate elements at concentrations often too low to detect with XPS or AES, and AES high magnification elemental mapping and small area spectroscopy were used to study the localized elemental changes that occurred between fresh and aged catalyst samples. Information gained from these experiments provides valuable insights on the physical and chemical transformations taking place during aging of the catalyst. This information can then be used to suggest further modifications to improve the performance of the catalyst system.

References

1. PR Newswire, Global Ethylene Oxide Market Report 2018 - Forecast to 2023: The Growing Demand for PET Bottles from the Packaging Industry, Nov 26, 2018

2. Business Wire, Global Ethylene Oxide Market Report By Application, April 24, 2018

As a zero-emission, eco-friendly fuel, hydrogen gas can be generated via electrochemical (EC) water splitting. Achieving high-efficiency water splitting requires the use of a catalyst to minimize the overpotential to drive the hydrogen evolution reaction (HER). Noble metals such as platinum (Pt) can provide an excellent catalytic activity for HER but are too expensive and scarce for broad applications. Therefore, the development of active HER catalysts made from low-cost materials constitutes a crucial challenge in the utilization of hydrogen energy.

Earth-abundant transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), have been discovered recently, which exhibit good activity and stability for electrocatalytic reactions. In order to fully explore the untapped potential of MoS₂, the synthesis recipe for MoS₂ needs to be optimized. Such an optimization process needs scientists to search through a combinatorically large space of experimental parameters, which will be time-consuming and costly if using conventional trial-and-error approaches.

In this report, MoS₂ HER catalytic activity optimization is performed by examining different combinations of synthesis parameters during the hydrothermal process. To investigate the structure-activity relationship, scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy and various electrochemical characterizations have been conducted. A strong correlation between hydrothermal conditions and HER performance matrix has been observed. In order to accelerate the search for the best synthesis recipe, machine-learning (ML) techniques have been introduced to help identify the optimal parameter combinations for producing MoS₂. The hydrothermal parameters with the corresponding onset potentials and Tafel slopes are adopted as prior knowledge and are incorporated into the Bayesian Optimization model. The model will be able to guide the wet chemical synthesis of MoS₂and yield the most effective HER catalyst eventually.

Metal oxide nanoparticles supported on Au(111) are active catalysts for the oxidation of small alcohols to form industrially significant products and intermediates such as aldehydes. In a systematic study to better understand the adsorption behavior of these small alcohols, coverage studies of methanol, ethanol, and 1-propanol have been conducted on Au(111) using ultrahigh vacuum temperature-programmed desorption (TPD). These three alcohols molecularly adsorb to distinct terrace, step edge, kink, and multilayer sites, for which desorption energies are calculated by Redhead’s peak maximum method. The use of complete analysis as a more accurate method for the calculation of desorption energies is also explored for the case of methanol. TPD experiments and subsequent analysis uniquely allow for the investigation of both adsorbate-surface interactions and adsorbate-adsorbate intermolecular interactions. This provides a more robust understanding of surface phenomena and affords valuable data for modelling and predicting the adsorption behavior of small alcohols on Au(111)-based catalysts. It is shown that, for all adsorption sites, desorption energy is directly proportional to carbon chain length. However, the rate at which desorption energy increases varies distinctly between adsorption sites. This study indicates that the role of intermolecular interactions in the adsorption behavior of small alcohols varies between adsorption sites, and can be predicted for other small alcohols.