AVS 68 Session SS+AS+TF-MoM: Dynamics and Mechanisms at Surfaces and Interfaces

Grazing-incidence fast-projectile diffraction, measured experimentally for the first time in 2007^1,2, has been proposed both as a complement and an alternative to thermal-energy projectile scattering, which explains the interest that this technique has received in recent years, especially in the case of atomic projectiles (GIFAD)^3,4. Grazing-incidence fact-molecule diffraction (GIFMD), on the other hand, has received much less attention (mostly theoretical^5,6), despite the fact that the H₂ molecule:(i) is as easy to generate as atomic H (a widely used projectilein GIFAD experiments); (ii) is lighter than He (another widelyused projectile in GIFAD), which would further reduce surface-phononsinelastic processes; and (iii) can reveal aspects of thesurface landscape that may be relevant in other contexts dueto the internal degrees of freedom (DOFs) and, in the case ofthe ionic surfaces, to the interaction of its quadrupole momentwith the electric field created by the ionic crystal, which is verysensitive to the surface details.Here, we present a theoretical study of grazing-incidence fast-molecule diffraction of H₂ from KCl(001) usinga six-dimensional density functional theory based potential energy surface and a time-dependentwavepacket propagation method. The analysis of the computed diffraction patterns as a function ofthe molecular alignment, and their comparison with the available experimental data, where the initialdistribution of rotational states in the molecule is not known, reveals a puzzling stereo dynamics effectof the diffracted projectiles: Diffracted molecules aligned perpendicular, or quasi perpendicular,to the surface reproduce rather well the experimental diffraction pattern, whereas those moleculealigned parallel to or titled with respect to the surface do not behave as in the experiments. Theseresults call for more detailed investigations of the molecular beam generation process.

1- P. Rousseau, et al., Phys. Rev. Lett., 98, 016104 (2007).

2- A. Schuller et al., Phys. Rev. Lett., 98, 016103 (2007).

3- H. Winter and A. Schuller, Prog. Surf. Sci., 86, 169 (2011).

4- M. Debiossac, P. Pan and P. Roncin, Phys. Chem. Chem. Phys., 23, 7615 (2021).

5- M. del Cueto et al., Phys. Chem. Chem. Phys., 19, 16317 (2017).

6- A. S. Muzas et al., Phys. Rev. B, 96, 205432 (2017).

Hydrogenation of carbonyl compounds is an important step in many applications in heterogeneous catalysis. This class of reactions is, however, experimentally highly challenging as it requires the activation of a normally very stable C=O bond. There is an ongoing discussion on an alternative mechanism of C=O bond hydrogenation, which involves keto-enol tautomerization as a first step. In this mechanism, a H atom transfers to oxygen in an intramolecular process to produce a C-O(H) single bond, leaving behind a C=C double bond and forming the enol species. Several theoretical studies predict a significantly lower activation barrier for hydrogenation of the C=C bond in enol as compared to the direct hydrogenation of the C=O bond in ketone for different classes of mono- and dicarbonyl compounds.

In this contribution, we present a mechanistic study on atomistic-level mechanisms of enol formation and stabilization via lateral interactions with co-adsorbed surface species over catalytically active metal surfaces (Pt and Pd).^{1, 2} We employ a broad range of carbonyl compounds including acetophenone, acetylpyridine, butanal and ethyl pyruvate, whose adsorption and reactivity behavior were investigated using a combination of infrared reflection absorption spectroscopy (IRAS), scanning tunneling microscopy (STM) and molecular beam techniques. We found that enols can be efficiently formed in different types of carbonyl-containing molecules, however, they require stabilization on the surface via lateral interaction, e.g. by establishing hydrogen bonding between the –OH group of an enol and a carbonyl group of the neighboring adsorbate. Stabilization of formed enols via lateral interactions with the adjustment molecules results in formation of different types of oligomers, including one of more enol molecules. The efficiency of enol formation was found to strongly depend on the chemical structure of the adsorbates and can be affected by e.g. the insertion of the functional groups, such as phenyl or pyridine groups. Also the presence of subsurface hydrogen in Pd was shown to strongly enhance keto-enol tautomerisation in some of the investigated carbonyls. Finally, we provide the first experimental evidence for a low-temperature hydrogenation pathway of carbonyl compounds, which occurs in ketone-enol dimers of acetophenone formed on Pt.³ In this process, stabilization of enol species via lateral interactions with a neighboring carbonyl is crucial for enabling the target hydrogenation pathway.

1. Attia, S. et al Angew. Chem. Int. Edit. 2018,57, 16659.
2. Attia, S.et al ACS Catal. 2019,9, 6882.
3. Attia, S. et al, J. Phys. Chem. C 2019,123 (48), 29271.

In heterogeneous catalysis, the reactivity of for example oxidation reactions is often enhanced by transition group metal surfaces as catalysts. After exposure, the oxygen molecules readily dissociate into oxygen atoms on the surface forming characteristic surface reconstruction patterns. However, not only the formation of surface oxygen (O_surf) structures but also of subsurface oxygen (O_sub) phases is possible, especially when aggressive oxidation agents such as NO₂ or atomic oxygen are used as oxygen source. The O/Rh(111) has been adapted as a benchmark system for O_sub formation in the past. In temperature programmed desorption (TPD) experiments, O_sub emerges as a narrow desorption feature around 800 K, while O_surf forms a subsequent broad desorption feature over several 100 K. Although extensive research has been done on the formed reconstructions of O_surf, few is known about the microscopic details of O_sub formation.

In the here presented work, velocity map imaging (VMI) was applied to the O/Rh(111) system. We combined TPD and VMI to investigate recombinatively desorbing O_sub from Rh(111). This allows a precise assignment of high-resolution velocity distributions of desorption products to certain TPD peaks. We observe a hyperthermal velocity distribution for recombinatively desorbing oxygen from subsurface as well as from surface states. These results provide valuable benchmark data, on which theoretical models describing subsurface oxygen dynamics can be developed and tested.

Alloy formation enables the enhancement of material properties from electrical and thermal conductivity, to magnetism, chemical reactivity, and mechanical strength and ductility. For example, Ti alloys are lightweight, corrosion resistant, have low Young’s modulus, and possess tunable strength and ductility at high temperatures. Their corrosion resistance and low Young’s modulus make them suitable for biomedical implants, while their light weight, tunable strength, and high working temperatures facilitate use in high-temperature applications. Traditional alloys often contain a principal metal comprising most of the alloy composition, with additional functionality (e.g., oxidation resistance) induced by minority components. In contrast, high-entropy alloys possess multiple principal components, and have recently attracted significant attention for their enhanced tunability and sometimes unexpected physical properties.

Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we have probed the initial oxidation of TiZrHfNb_0.3 refractory high-entropy alloys (RHEAs). Sputter-cleaning the as-cast alloy in ultrahigh vacuum removes adventitious carbon and native oxides, revealing a metallic alloy containing metal carbide species through its bulk. Subsequent vacuum annealing from room temperature (RT) to 100° C enriches the near-surface carbide content. This near-surface carbide enrichment continues with increasing temperature, accompanied by the formation of surface hydrocarbon species. Meanwhile, the relative compositions of Ti, Zr, Hf, and Nb are stable across the same temperature regime in vacuum. Despite their thermal stability, freshly sputter-cleaned, metallic alloy surfaces exposed to 1 mTorr of O₂ gas become enriched by a near-surface layer of Hf- and Zr- oxides. At the same time, the carbide component is suppressed, and a metal-oxide interface, containing Ti- and Zr- oxides, appears within the XPS probing depth (~8 nm). Subsequent RT oxygen exposure at higher O₂gas pressures induces comparatively minor changes in the surface oxide layer composition.

These results reveal the formation and nature of a thin protective oxide layer at the TiZrHfNb_0.3 RHEA surface in response to mild oxygen pressures. We present these results in terms of the O₂ pressure/temperature parameter space and discuss implications for the TiZrHfNb_0.3 RHEA behavior.

Future-proofing materials against degradation and failure means designing alloy systems with corrosion resistance built-in, and this is especially important for alloys in extreme environments. Ni-based superalloys are alloyed with Cr, Mo, and W which help form protective layers that are highly corrosion resistant mostly due to chromia (Cr2O3) formation. On polycrystalline alloy surfaces a wide range of crystallographic orientations coexist and are defined by the individual grains. For these often highly stepped or kinked surfaces, a complex surface faceting results from the tendency to minimize the surface free energy leading to an alloy with variable and complex surface topographies. Prior research indicates that different oxide species will nucleate along specific orientations resulting in oxide layer heterogeneity, which can introduce points of failure in the protective layer. Our work focuses on how these differences in surface crystallographic orientation can result in changes in the nucleation and growth of NiO and chromia, whose growth is kinetically controlled under our oxidation conditions.

PdAg and PtAg bimetallic catalysts are used in many important industrial applications. Therefore, an atomic scale understanding of these catalysts is important for their further development. In this study, the initial stages of submonolayer growth of Pd and Pt islands on Ag(111) at room temperature were investigated using scanning tunneling microscopy (STM). Although Pd (1.7 J m^-2) and Pt (2.2 J m^-2) have higher surface free energies than Ag (1.1 J m^-2) and a similar lattice mismatch (PdAg = 4.8% and PtAg = 4.2%), Pd and Pt show different behavior after deposition on Ag(111) at room temperature. Hexagonal Pd islands are formed on Ag(111) regardless of the coverage. In contrast, Pt shows a high density of small clusters and larger islands indicating less mobility for Pt than Pd on Ag(111). Due to Pd atom place exchange with Ag atoms, Pd-rich brims were observed at the ascending Ag step edges. But, Pt-rich brims were not observed. Because of the absence of Pt-rich brims , removal of Ag atoms created bays at the step edges. Surface Ag atoms migrate to cover both Pd and Pt islands, even at room temperature, creating vacancy pits on the Ag(111) surface. In addition to large vacancy pits, small mobile vacancy pits were observed on Pt/Ag(111). Pd and Pt islands show different moire structures on Ag(111) even though they have almost same lattice mismatch. Migrated Ag atoms nucleate near the center of Pd islands to grow the second layer, whereas, migrated Ag atoms nucleate at the corner of the Pt islands.