AVS 68 Session HC+AS+SS-TuM: Energetic Processes and Tailored Surfaces in Heterogeneous Catalysis

Understanding the adsorption and reactivity of carboxylic acids on anatase TiO₂ is of great interest in catalysis for the potential synthesis of selective carbon-coupling reaction products. A comparative investigation of the adsorption and reaction of acetic acid on single-crystal planar and faceted nanoparticle anatase TiO₂(101) surfaces was performed using a combination of infrared spectroscopy, temperature-programmed reaction spectroscopy and scanning tunneling microscopy. Acetic acid adsorbs as bidentate acetate and monodentate acetic acid dependent on the adsorption temperature as determined by infrared spectroscopy and scanning tunnelling microscopy measurements on single-crystal TiO₂ under ultra-high vacuum conditions. The adsorption of bidentate acetate has a saturation coverage of 0.5 ML. However, high-density phases containing a mixture of bidentate acetate and monodentate acetic acid and solely monodentate acetic acid occur at low temperature conditions. The sole presence of bidentate acetate is observed at elevated temperature due to the facile conversion of monodentate acetic acid to bidentate acetate and desorption of monodentate acetic acid which precludes investigating the reactivity of high-density phases under vacuum conditions. The presence of monodentate acetic acid persists to notably higher temperatures on the faceted nanoparticles than single-crystal TiO₂. The reaction of bidentate acetate produces ketene and water as determined by temperature-programmed reaction spectroscopy on single-crystal TiO₂. Notably, there is an absence of carbon-carbon coupled products, such as acetone, from the sole reaction of bidentate acetate. The selectivity of bidentate acetate is independent of coverage up to the saturation coverage. Similarly, the reaction of acetic acid on faceted nanoparticle TiO₂ produces predominantly ketene and water as determined by temperature-programmed reaction spectroscopy under low vacuum conditions. This study provides mechanistic insight into the reactivity and stability of surface-bound intermediates that are present for the reaction of acetic acid on anatase TiO₂ catalysts.

Understanding the reactivity of various crystal faces in different polymorphs of TiO₂ in photoreaction is important for many photocatalytic applications. Here, we monitored and studied the anatase-rutile phase transition (ART) processes of individual micro-sized TiO₂ crystals from the pure anatase phase to the mixed-phase, then to the rutile phase. High-quality micro-sized anatase TiO₂ crystals with a large percentage of (001) facets were hydrothermally synthesized. Micro-Raman spectroscopy mapping and scanning electron microscope (SEM) images were obtained at different annealing stages to correlate the crystal structure transformation with the morphology change. The ART processes of individual anatase particles are distinctive and depend on the various defects which serve as rutile nucleation sites. Two types of transition pathways are observed. In one type of growth pathway, rutile nucleation was formed at a corner of an anatase crystal. The rutile phase then gradually propagated over the rest part of the microparticle. The phase concentration calculated from Raman spectra revealed that the ART transition follows the first-order reaction mechanism. In the other type of growth pathway, multiple rutile nucleation sites formed simultaneously on different edges and corners of the microcrystal. The rutile phase then spreads over the whole crystal from these nucleation sites forming multi-grain crystals.

Single-atom catalysis is a growing area of research due to the potential to qualitatively transform the activity and selectivity of supported metal catalysts. However, the fundamental surface science studies of their unique properties have not been widely studied. Here we investigate the reactivity of formic acid on several model Rh catalysts, including single Rh adatoms, Rh incorporated into the support oxide lattice, and metallic Rh clusters on Fe₃O₄(001) under ultrahigh vacuum conditions. The single Rh adatoms are found to be the most active towards CO₂ production, followed by incorporated Rh and then by Rh nanoclusters. While the reaction of formic acid with Fe₃O₄(001) primarily results in CO desorption at ~570 K, the addition of small amounts of single adatom Rh (~0.01 Rh per Fe₃O₄(001) unit cell (u.c.)) results in a dramatic shift towards CO₂ production at lower temperatures starting at 540 K and dropping to ~470 K at larger HCOOH coverages. The incorporated Rh system also yields CO₂ product, but it takes ~15-20 times as much Rh to achieve the same activity as the Rh adatoms. Rh nanoclusters exhibit reactivity similar to metallic Rh with low-temperature CO₂ and H₂ desorption at ~310 K while still producing substantial amounts of CO₂ at 450-500 K, not observed on pure Fe₃O₄(001). These differences demonstrate that different forms of Rh supported on Fe₃O₄(001) affect the reactivity of the catalytic surface beyond the local stabilized transition metal site. The stabilized adatoms turn over multiple adsorbates to products demonstrating the importance of spillover from the support oxide. Furthermore, the single Rh adatoms and incorporated Rh exhibit an intermediate activity distinct from metallic Rh nanoparticles or the Fe₃O₄(001) substrate, demonstrating the additional degree of reactive tuning provided by single-atom catalysts. The similarity in reactive behavior between the Rh adatom and mixed Rh oxide systems hints toward a common mechanism between the systems. The much larger reactivity of the Rh adatom system suggests that under reaction conditions, a small amount of the mixed Rh oxide may be dynamically converted to Rh adatom and that the Rh adatom is responsible for the lower temperature CO₂ activity.

In the present work, WO₃/Ag₂S heterojunction was fabricated to achieve an improved photoelectrochemical (PEC) water splitting performance. To prepare the working electrodes,a two step method was adopted which includes, a thin film of WO₃ deposited using DC sputtering and a well-separated Ag₂S nanorods fabricated by glancing angle deposition,. The PEC response was studied for bare WO₃, Ag₂S, and WO₃/Ag₂S heterojunction. The as-prepared WO₃/Ag₂S heterojunction samples revealed higher absorption as well as higher photocurrent density of 2.40 mA/cm² (at 1V Ag/AgCl) as compared to bare WO₃ thin film (0.34 mA/cm²). The enhancement in the photocurrent density of WO₃/Ag₂S electrodes could be ascribed to the formation of the type-II heterojunction between WO₃ and Ag₂S which effectively separates and transfers the charge carriers at the interface. In addition, increased trapping of light due to vertically tilted Ag₂S nanorods structures results in an effective absorption of light. Furthermore, electrochemical impedance spectra measurements showed that WO₃/Ag₂S samples have lower charge transfer resistance at the semiconductor electrolyte interface with high flat band potential. The present work provides a deeper insight on the role of the interface formed between WO₃ and Ag₂S for the photoelectrochemical water splitting response.

TiO₂ is a widely used photocatalyst. Its ability to oxidize organic contaminants makes it useful, for example, in air and water purification systems and as a thin-film coating for self-cleaning surfaces. As a result of titanium dioxide’s practical applications and its potential use in photocatalytic water spitting, it has been the subject of a tremendous amount of research. We have investigated the photon-stimulated reactions of small molecules, such as O₂, CO, and acetone, on rutile TiO₂(110). Experiments on clean, well-characterized single crystal surfaces, including azimuth- and angle-resolved measurements of photo-desorption products, provide key insights into the photochemical reactions of interest. This talk will highlight examples of this approach, including the photochemistry of chemisorbed O₂, the photo-oxidation of CO, and the photon-stimulated reactions of acetone. For the photo-oxidation of CO, the results show that a bridging O-O-C-O intermediate ejects CO₂ perpendicular to the bridging oxygen rows, which is consistent with density functional theory calculations. However, in contrast to earlier suggestions, the reaction kinetics show that the oxidation is a multi-step process. For acetone, the azimuth-resolved measurements of the methyl fragments revealed a second reaction channel that probably involves an enolate intermediate on the surface.

Understanding how the local environment of a “single-atom” catalyst affects stability and reactivity remains a significant challenge. Fe₂O₃ is the most common iron-oxide support material utilized for SAC, but little is known about how metal adatoms bind at its surfaces. In this talk, I will compare and contrast the behavior of Pt, Rh,^1,2 and Ir atoms on the flat, well-ordered (1×1) termination of Fe₂O₃(1-102).^3,4 Using a combination of scanning probe microscopy and spectroscopic data, as well as theoretical calculations, I will demonstrate significant differences between the adsorption site and thermal stability of the metals, as well as differences in their interaction with water and carbon monoxide.

¹F. Kraushofer et al., Single Atom Catalysts: Surface Reduction State Determines Stabilization and Incorporation of Rh on α-Fe₂O₃(1-102) (Adv. Mater. Interfaces 8/2021). Advanced Materials Interfaces8, 2170045 (2021).

²F. Kraushofer et al., Single Rh Adatoms Stabilized on α-Fe₂O₃(11̅02) by Coadsorbed Water. ACS Energy Letters7, 375-380 (2022).

³F. Kraushofer et al., Atomic-Scale Structure of the Hematite α-Fe₂O₃(1-102) “R-Cut” Surface. J. Phys. Chem. C122, 1657-1669 (2018).

⁴ G. Franceschi et al., A Model System for Photocatalysis: Ti-Doped α-Fe₂O₃(1-102) Single-Crystalline Films. Chem. Mater.32, 3753-3764 (2020).