AVS 71 Session SS-TuA: Heterogeneous Catalysis I

Interstellar molecular clouds, the regions where new stars and planetary systems form, are home to surprisingly complex chemistry. In spite of the very low temperatures and pressures characterizing these clouds more, than 330 different molecules have so far been detected. Nanoscale interstellar dust grains and polycyclic aromatic hydrocarbons are expected to play a dominant role as catalysts for the low temperature reactions resulting in the formation of these molecules. Their catalytic effect is not only ascribed to a lowering of reaction barriers, but also to their role in dissipating the energy released in the reaction. In some cases, the “catalysts” are even seen to increase the reaction barriers, while still enabling the reaction to proceed by providing energy dissipation pathways. The last 20 years have seen major advancements in our understanding of interstellar reactions, specifically with regards to simple molecules, however, the degree of chemical complexity attainable via such reactions is still under exploration. Recently it was shown that the simplest amino acid, glycine, can form under interstellar conditions. In this case a non-diffusive reaction mechanism was proposed. A more detailed quantum dynamical understanding of low temperature solid state radical-radical reactions could provide the answer to the question of whether the molecular building blocks of life – amino acids, DNA bases, sugars and fatty acids – can form in interstellar space, even before the formation of stars and planets. To answer this question, we recreate interstellar conditions in the laboratory and employ the full toolbox of surface science to study heterogenous catalytic reactions on interstellar dust grain analogue surfaces. As an example scanning tunneling microscopy measurements allows us to directly image low temperature ice cluster formation, as well as low temperature reaction products with single molecule detection efficiency.

The ability to obtain velocity distributions of molecules desorbing from surfaces with both high temporal precision and angular resolution provide newfound insight into both the kinetics and the dynamics of the CO oxidation reaction and subsurface emergence.

I will discuss our observations of CO oxidation by co-adsorbed and absorbed oxygen on Rh(111) and how the velocity distribution shifts in comparison to the thermally-dominated desorption pathways found for surface-adsorbed oxygen. In addition, the role of systematic defects will be covered for both the oxidation reaction and surface oxidation. I will discuss these observations and their potential impacts in oxidation reactions in heterogeneously catalyzed reactions over transition metal surfaces.

The high temperature dehydrogenation, cracking, and coking chemistry of n-butane and isobutane, catalyzed by sub-nanometer Pt_n/alumina and Pt_nGe_m/alumina catalysts will be presented.The mechanisms are explored by temperature programmed desorption (TPD) experiments with size-selected clusters deposited on alumina supports, and detailed DFT calculations.The calculations probe cluster geometric and electronic structures, including the effects of both Ge and carbon addition, and examine binding and activation of the C4 alkanes and alkenes.N-Butane is observed to dehydrogenate efficiently on Pt catalysts with and without Ge.For pure Pt_n, there is some coking initially, but the coking decays over time and the dehydrogenation activity increases slightly, i.e., coking is self-limiting and does not deactivate Pt_n for n-butane dehydrogenation.With Ge present, there is essentially no coking for n-butane, even in the initial reaction.In contrast, isobutane on pure Pt_n/alumina catalysts simply cokes with hydrogen evolved, with no significant alkene or diene products.Ge addition to the Pt_n does suppress coking for isobutane, resulting in C₄H₈ product formation, but only for the the Pt₇-based catalyst (Pt₇Ge₂/alumina).In addition, we find that both 2- and isobutene coke badly on Pt_n/alumina, deactivating the catalysts, and that coking is suppressed by Ge addition.This work was supported by the Air Force Office of Scientific Research (AFOSR FA9550-19-1-0261).

Nb is the highest temperature elemental superconductor; however, its application in particle accelerators and quantum computers is limited by growth of native surface Nb oxides. Au capping layers have been shown to prevent deleterious Nb oxide growth but Au morphology, kinetics, and degree of passivation at various coverages on Nb have not been fully investigated. This work characterizes the physical deposition and oxygen contamination of sub-ML to 10 ML Au coverages on Nb(100). We analyze the physical features and chemical states of the surface using Scanning Tunneling Microscopy, X-ray and UV Photoelectron Spectroscopy, and Auger Electron Spectroscopy. Preliminary results show that a post-deposition anneal as low as 350 C causes Au island formation at Sub-ML to 1 ML coverages, substantially exposing Nb to oxidation. Thus, understanding the effects of temperature and coverage on Au formation will be critical for revealing the optimal method to passivate Nb.

Transient heating can be a powerful approach to control the selectivity of catalytic reaction networks, especially for endothermic reactions where undesirable species can be formed under steady state conditions. Mechanocatalysis and pulsed Joule heating are two approaches where surface temperature changes of 500-1000 K can be achieved on millisecond timescales. This talk will discuss how a combination of experiments and computational simulations have been used to understand the reaction conditions that are accessible with these unconventional heating methods. Examples will include the use of computational modeling to probe temperature inhomogeneities in realistic models of carbon fiber supports during pulsed Joule heating and the use of single impact experiments and simulations to quantify heat delivery and chemical reactivity during mechanochemical depolymerization.

We investigate cyclic hydrocarbons — cyclohexane, cyclohexene, and benzene — on the magnetite Fe₃O₄(001) surface by means of temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and x-ray photoelectron spectroscopy (XPS). Through a detailed analysis of the TPD profiles, we uncover distinct interaction mechanisms between these molecules and the Fe₃O₄(001) surface, shedding light on the role of molecular structure and surface chemistry. Despite the structural similarities, the adsorption energies and desorption orders and hence the interaction with the surface are different for each molecule. Furthermore, the desorption behavior differs from other surfaces previously investigated in the literature.^1,2 The results provide a deeper understanding of the substrate-surface interactions, with implications for catalytic applications, such as hydrocarbon upgrading, and the design of oxide-supported catalysts for energy and chemical industries.

[1] Smith, R. S., & Kay, B. D. (2018).Desorption kinetics of benzene and cyclohexane from a graphene surface. J. Phys. Chem. B, 122(2), 587-594.

[2] Chen, L., Zhang, S., Persaud, R. R., Smith, R. S., Kay, B. D., Dixon, D., & Dohnalek, Z. (2019). Understanding the binding of aromatic hydrocarbons on rutile TiO₂(110). J. Phys. Chem. C, 123(27), 16766-16777.

Electronic metal-support interactions (EMSI) are often cited as an origin of enhanced selectivity and efficiency of oxide-supported metal nanoparticle catalysts. While it is understood that tuning the oxide support defect density and metal nanoparticle size impacts the EMSI, it is unclear how such structural modifications attenuate the electronic landscape of the catalyst at the atomic scale. In this work, rutile TiO₂(110) was used as a strong Lewis acid oxide support to determine the spatially resolved electronic consequences of Cu nano-particle size on EMSI. Using a combination of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), the TiO₂(110) (1×1), (1×2), (1×3), and (2×2) surface reconstructions were characterized to reveal the relationship between the coordination number of Ti and the observed local density of states (LDOS). Following this characterization, sub-monolayer quantities of Cu were deposited on TiO₂(110) and annealed to either 100 °C, 300 °C, or 500 °C to promote Cu diffusion and aggregation. STS taken atop the Cu particles showed the emergence of electronic states within the bandgap of TiO₂(110). The intensity and position of these electronic states were strongly dependent on the size of the Cu particle. STS collected at the Cu/TiO₂(110) interface showed unique LDOS when compared to the LDOS of the Cu particle and the TiO₂(110), suggesting an attenuation of the electronic structure at the Cu/TiO₂(110) interface. This attenuation of the LDOS extended beyond the immediate Cu/TiO₂(110) interface, with the distance of attenuation related to the size of the Cu particle. These results highlight the real-space, heterogeneous electronic landscape of oxide-supported metal nanoparticle systems which may have significant implications for overall reactivity and selectivity.

Ceria has recently returned into the focus of research thanks to the possibility to reversibly form and redisperse supported, catalytically active Pt clusters by controlling its morphology and redox state. In the present work, we systematically synthesize CeO₂(111) thin films to tune these parameters independently and investigate their influence on size-selected Pt₂₀ cluster dimensionality and sintering behavior. First, we present recipes for atomically flat islands and closed CeO₂(111) films with a thickness up to 18 monolayers (ML), grown on Rh(111), and characterize them by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and low-energy electron diffraction (LEED). Remarkably, XRD and LEED show an epitaxially grown, crystalline, and relaxed film with cube-on-cube alignment. Bulk or exclusive surface reduction is achieved by ultra-high vacuum (UHV) annealing or room temperature (RT) CH₃OH dosing and annealing cycles, respectively. The methanol procedure forms oxygen vacancies only in the surface, without reducing the deeper layers of the film or introducing roughening. When deposited on a fully oxidized (Figure 1 (a,d)) versus a surface-reduced (Figure 1 (b,e)) support, Pt₂₀ clusters show a strikingly different temperature-dependent dimensionality and sintering behavior. From STM images, we extract detailed cluster height distributions and coverages (Figure 1 (c,f)) and find that Ostwald ripening already sets in around 600 K on both, oxidized and reduced supports, without any indication for cluster diffusion and coalescence. Notably, at these temperatures, we obtain flat 2D clusters on the reduced support and 3D clusters on the oxidized support, where the atom detachment during sintering leads to the intermediate formation of Pt²⁺ species, in line with the redispersed single atoms at step edges observed in the literature [1-3].

References

[1] M. Farnesi Camellone, F. Dvořák, M. Vorokhta, A. Tovt, I. Khalakhan, V. Johánek, T. Skála, I. Matolínová, S. Fabris, J. Mysliveček, Adatom and Nanoparticle Dynamics on Single-Atom Catalyst Substrates, ACS Catal. 12 (2022) 4859–4871.

[2] F. Dvořák, M.F. Camellone, A. Tovt, N.D. Tran, F.R. Negreiros, M. Vorokhta, T. Skála, I. Matolínová, J. Mysliveček, V. Matolín, S. Fabris, Creating single-atom Pt-ceria catalysts by surface step decoration, Nat. Commun. 7 (2016).

[3] F. Maurer, J. Jelic, J. Wang, A. Gänzler, P. Dolcet, C. Wöll, Y. Wang, F. Studt, M. Casapu, J.D. Grunwaldt, Tracking the formation, fate and consequence for catalytic activity of Pt single sites on CeO₂, Nat. Catal. 3 (2020) 824–833.