AVS 68 Session HC-ThP: Heterogenous Catalysis Poster Session

Bimetallic nanoparticles are important catalysts in heterogeneous catalysis; due to the bimetallic synergistic effect, they show higher catalytic performance than monometallic nanoparticles. Pt, a widely used catalyst, is known to alloy well with 3d transition metals (TM, e.g., Ni, Fe, Cu, and Co). In Pt-3d TM bimetallic systems, surface restructuring occurs in a variety of environments. Under oxidation conditions, Co atoms in Pt-Co nanoparticles emerged onto the surface, forming a CoO layer. Moreover, the interface between Pt and CoO enhanced catalytic activity for the Pt-Co bimetallic nanoparticle. This catalytic improvement could be explained by the strong metal-support interaction (SMSI) at the Pt-3d TM oxide interface, from the surface restructuring.

Effusive molecular beam experiments were used to measure CH₄ dissociative sticking coefficients, S(T_g, T_s ;J) on a Rh(111) crystal, for which the impinging gas temperature, T_g, and surface temperature, T_s, could be independently varied, along with the angle of incidence, J, of the impinging gas. The 500 – 900 K temperature range explored is relevant to heterogeneous catalytic processes such as methane partial oxidation. A dynamically biased precursor mediated microcanonical trapping (PMMT) model of dissociative chemisorption was used to analyze the experimental results. Modelling indicates that the enhanced reactivity of step sites is not promptly poisoned on Rh(111) but rather contributes substantially to the reactivity even as high coverages of carbon accumulate at the surface. Threshold energies for dissociative chemisorption on the terraces and steps sites were optimally modeled as 74.3 kJ/mol and 36.7 kJ/mol. Translations parallel to the surface and rotations were treated as spectator degrees of freedoms. The efficacy of vibrational energy to promote reactivity relative to normal translational energy was h_n=0.55 and one surface oscillator participated in energy exchange within the collisionally formed precursor complexes. A two-channel Arrhenius model restricted to only the thermal dissociative sticking coefficient measured along the direction of surface normal, S_n(T=T_g=T_s), yielded apparent activation energies of 70.6 and 25.5 kJ/mol which could be attributed to terrace and step sites, respectively. PMMT modeling of the step site reactivity on Rh(111) could be extrapolated to replicate the thermal dissociative sticking coefficient of the “defect dominated” Rh film surfaces measured by Ehrlich at temperatures in the 250 -350 K range where much of the elevated kinetic isotope effect (9 to 15) could be attributed to quantum mechanical tunneling through the reactive barrier.

Adsorption properties and surface chemistry of acrolein and its hydrogenation products propanal, 1-propanol, and 2-propenol on Cu(111) and Pd-Cu(111) single atom alloy surfaces were studied by polarization-dependent reflection absorption infrared spectroscopy (PD-RAIRS) and temperature-programmed desorption (TPD). The experimental RAIR spectra were obtained by adsorbing multilayers of each molecule at 85 K and then annealing the surface up to 200 K to desorb the multilayer and produce the most stable monolayer structure on both surfaces. Each molecule adsorbs weakly to the surface and desorbs without reaction at temperatures below 200 K. Several TPD spectra were collected following adsorption at 85 K using similar coverages as those in the RAIRS experiments. Compared to acrolein and propanal, the two alcohols, 2-propenol and 1-propanol, have notably higher desorption temperatures and display strong hydrogen bonding in the multilayers as revealed by a broadened and redshifted O-H stretch. For acrolein, the out-of-plane bending modes are more intense than the C=O stretch at submonolayer coverage, indicating that the molecular plane is mainly parallel to the surface. In contrast, the opposite intensity trend was observed for multilayer acrolein, suggesting that the C=O bonds are tilted away from the surface. For 1-propanol, annealing the surface to 180 K disrupts the hydrogen bonding to produce unusually narrow peaks. This indicates that 1-propanol forms a highly ordered monolayer and adsorbs as a single conformer at low coverage. For 2-propenol, hydrogen bonding in the multilayer correlates with the observation of the C=C stretch at 1647 cm^-1, which is invisible for the monolayer. This suggests that the C=C bond is parallel to the surface for monolayer coverages of 2-propenol. Similarly, for propanal, the C=O stretch peak at 1735 cm^-1 compared to those at 1671 and 1695 cm^-1 is very weak for the low coverage. Still, it becomes the most prominent peak for the multilayer, indicating a change in molecular orientation. These results provided further insights into previous studies [1,2] on hydrogenation pathways of acrolein on the Ag(111) and Pd-Ag(111) SAA surface and into the challenges of selectively increasing the yield of the unsaturated alcohol.

References:

[1] Muir, M., et al., J Phys. Chem. C (2020) 124 (44), 24271

[2] Muir, M., et al., Phys. Chem. Chem. Phys. (2020) 22 (43), 25011

The persistently growing global demand for energy, transportation, and production will inevitably further environmental deterioration by means of hazardous emissions. Minimizing environmental damage thus requires the development of technologies that can mitigate the pollution associated with such anthropogenic sources. The selective catalytic reduction (SCR) of nitrogen oxides (NO_x) is one of the major breakthrough technologies in the abatement of pollutants from vehicular and industrial emissions, and Ag/Al₂O₃ has been widely studied as an ideal catalyst for SCR systems due to its selectivity and tolerance to poisoning by SO₂ and water vapor. However, SCR using Ag/Al₂O₃ exhibits poor efficacy at temperatures below ~250 °C due to diminished activity of the reductant species. A promising means of overcoming these deficiencies is in the synergy between nonthermal plasma (NTP) and Ag/Al₂O₃ catalysts. Despite the potential of plasma-assisted SCR for pollution abatement as shown in the literature, the chemistry underlying the NTP/catalyst couple must be resolved to optimize the process for commercial use.

In order to further explore these synergies, optical emission spectroscopy (OES) is used to characterize a NTP (N₂/O₂/Ar) and Ag/Al₂O₃ couple on the basis of energy partitioning, densities, and formation/decay kinetics of relevant excited states in the plasma bulk. An inverse proportionality between NO^* density and vibrational temperature (T_v) is observed for NTP coupled with Al₂O₃ surfaces (0-10 wt% loadings in Ag). At high input powers (175-200 W), NO^* T_v’s increase only marginally in the presence of 2, 5, and 10 wt% loadings, whereas a marked increase is observed in the presence of 0 wt% Ag/Al₂O₃. Even so, NO^* densities are comparably diminished (relative to the density observed at 25 W) for all systems studied. The catalytic role of Ag species in abating NO^* is further explored by the decay kinetics of NO^*. While the initial NO^* emission intensity decays with time for all NTP’s coupled with substrate, the decay rate constants were greatest for bulk NTP coupled with 5 and 10 wt% Ag/Al₂O₃. Differing formation and decay behaviors of the excited states are observed with successive plasma treatments, depending on the degree of silver loading. Further investigations, such as plasma pre-treatment of the catalytic surfaces and varying surface analyses provide additional insights into the interconnected plasma-surface system. The combination of these data provide a unique insight into the mechanistic behavior and the role of the system conditions on plasma-enhanced SCR.

The abundance and low cost of silicon presents the ideal candidate of silicon anodes as an energy-dense, sustainable alternative to graphite anodes in lithium-ion batteries with a reported theoretical capacity of 4200 mAh/g. However, challenges such as significant volume expansion, rapid pulverization, relatively poor conductivity, and unstable sei layers formed with the use of silicon anodes hinders the practical applications. Micro silicon and nanosilicon demonstrate promise tackling these challenges while utilizing the unique characteristic of silicon due to a larger surface area allowing for greater charge intercalation with the anode material and electrolyte without causing significant volume expansion. We have characterized micro- and nano-silicon with particle size ranges of 50 μm, 20-30 nm, and 150-200 nm through Fourier-Transform Infrared Spectroscopy (FTIR) to evaluate the presence of impurities in our samples and X-Ray Diffraction Spectroscopy (XRD) to determine the crystal structure of our samples. For electrochemical characterization, we utilized cyclic voltammetry (CV) to computationally identify the theoretical capacitance of different micro and nano-silicon-based inks (combinations of carbon black and silicon) deposited on nickel foam electrode material. Likewise we utilized Galvanostatic Charge Discharge (GCD) to identify performance characteristics of the material in battery type conditions identifying energy density, power density, and Coulombic efficiency. Our FTIR analysis indicated nano-silicon of particle sizes ranging from 20-30 nm was more prone to oxygen-related impurity formation due to a strong peak at 1071 cm-1 that corresponds to the asymmetric vibration of Si-O bonds. Through our computational analysis, we found theoretical specific capacitance values of 23.59 F g-1.of nanosilicon 01 (150-200 nm), 22.04 F g-1 of nanosilicon 02 (20-30 nm) , and 23.3 F g-1 of Microsilicon. Through testing optimal parameters of various silicon particle sizes we seek to identify the most efficient size of nano-silicon and evaluate its efficiency in an actual model lithium-ion battery utilizing COMSOL software models.

The interaction of oxygen with the surfaces of catalytically active transition metals has attracted much interest because of the relevance to heterogeneous catalysis. Recently, we have shown that oxygen coverages in excess of 1 ML are achievable using gas-phase atomic oxygen (AO) to dose the metal surfaces. This talk will discuss some recent results comparing the uptake of AO and O2 on Ag(111) to Rh(111). On Rh(111), subsurface oxygen readily forms from exposure to AO and the surface phases are dependent on the exposure temperature. We also discovered that subsurface O emerges at defects and alongside surface phase transitions. Finally, the uptake of oxygen on Ag(111) is discussed; unlike Rh(111), where little surface reconstruction occurs, Ag(111) undergoes several phase transformations as the oxygen coverage is increased. These results using AO demonstrate that UHV compatible dosing can prepare the same surfaces resulting high pressure O2 exposures, allowing for quantitative and structural analysis of the oxidized surfaces.

Integrating carbon capture and conversion is a promising direction towards a decarbonized future for industries that are heavily dependent on carbon-based raw materials. Progress has been made in achieving the electrocatalytic reduction of carbon dioxide to carbon monoxide from CO₂ captured in a monoethanolamine solution on silver catalysts. In this work, we use density functional theory calculations to test a hypothesis that the coverage of alkali cation controls the relative binding strength of proposed species of reactants, byproducts, and products, and hence the reaction product selectivity. Our results support the hypothesis that the alkali cation affects the relative binding strength of species for CO₂ reduction and hydrogen evolution, with increased coverage favoring CO₂ reduction. This discovery provides a useful computational descriptor to better understand the process conditions that can control product selectivity in heterogeneous catalysis.

Biofuels can be used to reduce global dependence on fossil fuels while contributing to a carbon neutral cycle. Biobutanol has low volatility and multiple transportation options which make it an attractive alternative fuel. Understanding the fundamental thermal catalysis processes of butanol over heterogeneous model catalysts can aid in the design of more efficient catalysts. Butanol isomers give rise to products including isobutyraldehyde, 2-butanone, butyraldehyde, isobutene, and butene, all of which have applications ranging from gasoline additives to bioplastics. For the partial oxidation of butanol isomers, TiO₂/Au(111) inverse model catalysts are promising due to their ability to catalyze redox reactions of C₁ – C₃ alcohols. Titania coverage effects were not reported for methanol or 2-propanol, but lower TiO₂ coverages in the presence of excess oxygen enhance selectivity of the partial oxidation of ethanol. To better understand how butanol breaks down in heterogeneous catalytic processes, temperature programmed desorption (TPD) is used to investigate its reaction. In this study, the reactivity of butanol isomers, specifically 1-butanol, 2-butanol, and isobutanol, on TiO₂/Au(111) was investigated. TPD was used to detect products and atomic force microscopy (AFM) highlighted the morphology of the surface. At low coverages of TiO₂, only 2-butanol showed expected oxidation reactivity, while, 1-BuOH exhibited low reactivity and formed reduced products, and isobutanol produced the recombinative product. At higher coverages of TiO₂/Au(111), 2-butanol formed both oxidized and reduced products, 1-butanol only formed reduced products, isobutanol produced oxidation, reduced, and recombinative products. The selectivity of the reaction was not altered during successive desorption experiments, indicating that the model catalyst was stable without reoxidation between experiments. AFM images show that the Au(111) crystal has ~0.13 ML and 0.27 ML of TiO₂ with predominantly 1D wire-like nanoparticles. Higher coverages of TiO₂ result in more particles distributed across the surface indicating that the reactivity was influenced by butanol proximity to TiO₂ nanoparticle rather than differences in size or shape.