AVS 69 Session HC-ThP: Fundamental Discoveries in Heterogeneous Catalysis Poster Session

A single atom alloy (SAA) is made by substituting catalytically active dopant metal atoms into the topmost atomic layer of a relatively inert host metal. Commonly, Pd and Pt are used for hydrogenation reactions due to their virtually zero barrier for H₂ activation. Yet, these catalysts are easily coked by CO and their high activity often limits their selectivity. Industrially, these catalysts are often doped with a less active metal to prevent coking and enhance their selectivity. One of the most important steps in the refinement of alkene streams for the industrial-scale production of high-quality polymers is the selective hydrogenation of 1,3-butadiene. In the literature, Pd (111) shows higher selective control over hydrogenation of 1,3-butadiene to butene than Pt (111). Therefore, the presence of isolated Pd atoms on a Cu host surface can be a suitable SAA catalyst model for this particular reaction. In this study, we used reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) to investigate the adsorption of butadiene and butene, as well as the hydrogenation of butadiene on Cu (111) and a Pd/Cu (111) SAA. The TPD study shows butadiene binds strongly to the Cu (111) surface compared to 1-butene. The desorption peak in the 120-130 K temperature range indicates the production of 1-butene from the hydrogenation of butadiene on the Pd/Cu (111) SAA. Auger electron spectroscopy (AES) shows no accumulation of surface carbon demonstrating molecular desorption of butadiene from the surface under UHV. The observation of both in-plane (CH₂-wag at 908 cm^-1) and out-of-plane (C-H bend at 1023 cm^-1) modes with RAIRS at monolayer coverage indicates that butadiene is adsorbed neither parallel nor perpendicular to the surface. In addition, IR peaks at 1464 cm^-1 for (-CH₃) antisymmetric deformation and 1412 cm^-1 for (= CH₂) scissors modes are observed on the Pd/Cu (111) SAA, indicating 1-butene formation from butadiene hydrogenation. The ambient pressure RAIRS studies are currently underway to calculate the turnover frequency (TOF) of the above-mentioned catalyst for the hydrogenation of 1,3-butadiene.

Recent theoretical studies have proposed the creation of a database of experimentally determined barrier heights for model reactions on transition metal surfaces as a tool for the development of chemically accurate computational methods for heterogeneously catalyzed reactions¹. This approach has been effectively applied to the dissociative chemisorption of CH₄ on Pt and Ni surfaces using semi-empirical specific reaction parameter density functional theory (SRP-DFT)². Current work on SRP-DFT involves expanding its application to other surfaces, including Ru(0001). There is, however, substantial disagreement in the current literature as to the height of the barrier on Ru(0001), with experimentally determined values ranging from 0.38 to 0.85^3,4 eV. Our work applies supersonic molecular beam reactivity measurements to this system in order to provide an important experimental benchmark for additional SRD-DFT studies. By measuring reactivity across a wide range of translational energies, surface temperatures, and vibrational states, our data will also serve as a rigorous test of the theoretical predictions of any future first-principles methods. We will present the results from ongoing reactivity measurements of the dissociative chemisorption of CH₄ on Ru(0001) and their implications for the dynamics of the reaction.

(1)Mallikarjun Sharada, S.; Bligaard, T.; Luntz, A. C.; Kroes, G.-J.; Nørskov, J. K. SBH10: A Benchmark Database of Barrier Heights on Transition Metal Surfaces. J. Phys. Chem. C2017, 121 (36), 19807–19815. https://doi.org/10.1021/acs.jpcc.7b05677.

(2)Migliorini, D.; Chadwick, H.; Nattino, F.; Gutiérrez-González, A.; Dombrowski, E.; High, E. A.; Guo, H.; Utz, A. L.; Jackson, B.; Beck, R. D.; Kroes, G.-J. Surface Reaction Barriometry: Methane Dissociation on Flat and Stepped Transition-Metal Surfaces. J. Phys. Chem. Lett.2017, 8 (17), 4177–4182. https://doi.org/10.1021/acs.jpclett.7b01905.

(3)Larsen, J. H.; Holmblad, P. M.; Chorkendorff, I. Dissociative Sticking of CH4 on Ru(0001). 7.

(4)Mortensen, H.; Diekhöner, L.; Baurichter, A.; Luntz, A. C. CH4 Dissociation on Ru(0001): A View from Both Sides of the Barrier. The Journal of Chemical Physics2002, 116 (13), 5781–5794. https://doi.org/10.1063/1.1456509.

Size-selected Pt­­_nGe_m clusters supported on alumina have been developed and are being investigated under ultra-high vacuum for the selective dehydrogenation of light alkanes. Surface analysis techniques such as temperature programmed desorption (TPD), x-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS), are used to characterize the clusters and probe binding sites and energies of small alkanes. TPD experiments have shown that the addition of Ge to Pt­_n limits catalyst deactivation by carbon deposition (coking) and sintering. The number of Pt atoms in the cluster affects the number of available binding sites for ethylene on the surface. We will present a study of butane and isobutane dehydrogenation and cracking Pt_n/alumina and Pt_n/Ge_m/alumina. In addition to the surface science experiments, we will present data from a MEMS microreactor device that allows reactants to be flowed over a sample of deposited clusters at pressures up to 1 atm and temperatures to 1000 K. Reactions on clusters will be compared to reactions on Pt nanoparticles made via chloroplatinate drop-casting and reduction.

In catalysis research, the material gap is one of several that the surface science community is trying to bridge, along with the pressure and temperature gaps. In the specific case of carboxylic acids on TiO₂, previous studies discovered that for acetic acid reacting with anatase nanoparticles at ambient pressure, an environment very much like real world catalytic processes, acetone could be produced. However, under ultrahigh vacuum (UHV) conditions, with a few monolayers of acetic acid on anatase(101) single crystals, acetone was not observed as a product. In contrast, formic acid reactions with either nanoparticles or single crystals showed no evidence of C-C bond formation. Here, we prepared a sample with layers of synthesized anatase nanoparticles with mostly (101) facets in an UHV system and compared the reactivity of acetic acid with that on a anatase(101) single crystal, using TPD (temperature programmed desorption) and RAIRS (reflection absorption infrared spectroscopy) to track the elementary steps, with the goal to bridge this material gap and fully understand the reaction mechanisms of carboxylic acids on metal oxides.

In this research ZnO at different Zinc acetate concentration from 0.1-0.4 M was synthesized through colloidal synthesis and applied to mineralize Rhodamine B dye under direct sunlight irradiation, showing that this is strongly related to the particle size and defects presence at the particle. This relationship is well correlated to the UV-Vis absorbance spectrum and photoluminescence (PL), showing up to 90 % of photocatalyst efficiency after 100 min for sample obtained at 0.3M, this behavior also is showed for those samples obtained at 0.1, 0.2 and 0.4 molar concentration. The XRD data analysis let the structure and crystallite average size (18- 27 nm) to be determined meanwhile the morphology and composition was obtained from HRSEM and EDS respectively. From the FTIR results the organic dye mineralization evolution was determined. Albeit the defects contribution was determined by PL.

It is well known that VOCs being recognized as major responsible for the increase in global air pollution. Catalytic combustion is an efficient technology for the abatement of VOC, which are oxidized over a catalyst at temperatures much lower than those of the thermal process. Specifically, gold supported catalysts on CeO₂ have shown a great performance in the oxidation of CO, methanol, toluene, etc. Besides, it is important to clarify the role of the support in such reaction. Ceria has the key property of high oxygen storage capacity which originates in its ability to rapidly switch from Ce to Ce oxidation states as the environment changes from reducing to oxidizing and vice versa. Its redox behavior is influenced by the substituent lattice groups that could be incorporated during different catalyst pretreatments and could affect the oxidation of VOC. This could be understood as the influence of oxygen vacancies and/or absorbed or coadsorbed H on the activation of oxygen molecules. The latter leads to the formation of superoxide and peroxide molecules on the surface, which could in principle be highly reactive towards oxidation of organic molecules.

In this context, we study, by IR spectroscopy and mass spectrometry, the interaction of O₂ with the modified CeO₂ based material, by creating vacancies following different reduction treatments. The possible role of the vacancies and/or presence of H atoms in the electron transfer from the surface to the oxygen molecule is discussed. Using AP-XPS we are able to prove that the surface/near surface of CeOx presents a charging effect which could be due to extra charge/electrons which then transfer to O₂ to form superoxide and peroxide species sequentially and presenting different thermal stabilities. Furthermore, the presence of different facets on the surface of the material could change the stability or amount of active species, thus comparison of results from polycrystalline ceria and CeO₂ nanocubes provide information about the structure-activity relationship for the rational design of catalytic materials.

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. Our undergraduate research lab uses ultrahigh vacuum temperature programmed desorption (UHV-TPD) to investigate the fundamental interactions between small alcohols on Au(111) and the reactivity of TiO₂/Au(111) inverse model catalysts on small alcohol redox behavior. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C₁-C₄) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The desorption energy of small primary alcohols was shown to trend linearly with increasing C₁-C₄ carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. These energy insights are particularly interesting when studying the redox behavior of small alcohols over TiO₂/Au(111).Depending on the surface preparation conditions, Au(111) supported TiO₂ nanoparticles react with small alcohols to form either reduced and oxidized products. The reactivity of the surface for ethanol oxidation was altered by controlling the oxidation state of TiO_x (x<2) and coverage of TiO₂. Low coverages of fully oxidized TiO₂ nanoparticles on Au(111) are active for the selective oxidation of ethanol to form acetaldehyde, but not all small alcohols behave similarly.