AVS 72 Session SS-WeM: Heterogeneous Catalysis

Heterogeneous catalysts based on bimetallic single atom alloys (SAA) play a significant role in numerous technical processes. The fundamental working principles of these systems, however, remain poorly understood, especially the aspects related to the nanoscopic nature of bimetallic particles and the associated structure-reactivity relationships. In this study, we developed well-defined SAA catalysts consisting of Pd atomically dispersed in Cu nanoparticles prepared under ultra-high vacuum conditions on model Al₂O₃/NiAl(110) support.¹ Employing a unique combination of surface sensitive techniques – scanning tunning microscopy (STM), infrared reflection absorption spectroscopy (IRAS), molecular beams – and density functional theory (DFT) calculations, we performed detailed structural characterization of these systems at the microscopic level. We demonstrate that Pd disperses atomically in Cu nanoparticles and becomes partly negatively charged. Importantly, these Pd/Cu nanostructured systems show an outstanding catalytic performance in selective dehydrogenation of butanol and exhibit 100 % selectivity toward butanal over a broad range of Pd loadings – the property that cannot be reproduced employing simplified single crystalline Pd/Cu(111) counterparts.

In the second part, the catalytic decomposition of 2-propanol to acetone over Co₃O₄(111) catalyst will be addressed with the specific focus on the role of water and its derivatives in the reaction mechanism and the overall catalytic activity.² Pre-treatment of this catalyst with water at elevated temperatures (573 K) was shown to result in a substantial increase in the acetone formation rate. Combined STM and IRAS studies revealed that this procedure leads to the formation of isolated hydroxyls involving a lattice oxygen atom, which play a key role in H abstraction, both from molecular 2-propanol and from the propoxy reaction intermediate, leading to efficient formation of the target product acetone. Our results suggest that the isolated O_sH group is substantially more efficient hydrogen acceptor than the adsorbate-free lattice oxygen O_s.

1. “Pd/Cu single atom alloys for selective alcohol dehydrogenation: from single crystalline to nanostructured model catalysts”, P. A. Fredersdorff et al, Angew. Chem. Int. Edit. (2026) 65, e21885
2. “Promoting role of isolated surface hydroxyls on selective dehydrogenation of 2-propanol to acetone over Co₃O₄ catalyst: a mechanistic study.”J. Smyczek et al, ACS Catalysis (2025) 15, 19268

The selective activation of methane under mild conditions remains one of the central challenges in heterogeneous catalysis. Metal–oxide interfaces, particularly those formed between transition metals and ceria (CeO₂), provide a unique platform to control both activity and selectivity by coupling metal reactivity with the redox flexibility of the oxide support.

In this contribution, we present recent advances in the understanding and design of metal–ceria catalysts for methane conversion, combining DFT+U simulations with operando and in situ characterization (AP-XPS, XRD, XAFS) and catalytic testing. We show that low metal loadings (Ni, Co, Pt, Pd) on ceria generate highly active interfacial sites, where electron transfer to Ce 4f states stabilizes oxidized metal species, enabling methane activation at unusually low temperatures and efficient methane dry reforming under mild conditions. These systems deviate from conventional linear scaling relationships, demonstrating that nanostructured metal–oxide interfaces can overcome intrinsic activity–selectivity limitations.[1]

Building on this concept, we highlight recent results on the direct conversion of methane to methanol under mild conditions. In particular, PdAu/CeO₂ catalysts enable selective methane oxidation using water as the sole oxidant, achieving high methanol selectivity (~80% at 500 K). The alloy–oxide interface plays a decisive role: isolated Pd sites at the PdAu–ceria interface promote methane activation and water dissociation, while Au suppresses over-oxidation pathways leading to methanol decomposition. This synergistic interplay effectively tunes Pd reactivity, balancing activity and selectivity.[2]

These findings establish a unifying picture in which metal–oxide and alloy–oxide interfaces act as tunable active sites that decouple elementary steps and break traditional scaling constraints. This provides a rational framework for designing catalysts capable of selectively converting methane into value-added chemicals under technologically relevant conditions.

This work builds on a close and highly fruitful collaboration with José A. Rodriguez and his team at Brookhaven National Laboratory.

References

[1] Lustemberg, P. G. et al. J. Phys. Chem. Lett. 11, 9131−9137 (2020)

[2] Fernández-Villanueva, E. et al. Angew. Chem. Int. Ed. 127, e202505716 (2025)

Recently, Supported Catalytically Active Liquid Metal Solutions (SCALMS) were shown to surpass conventional alkane dehydrogenation catalysts in both activity and stability, due to their liquid nature and the presence of catalytically active single atom sites. SCALMS consist of a liquid metal matrix (Ga) that dissolves a catalytically active transition metal (Pt, Rh, Pd, Ni). Binary SCALMS systems are the subject of extensive research on the fundamental level with the aim to reach optimal performance.

Here, we present an operando NAPXPS study of the dehydrogenation of simple alkanes on gallium-platinum SCALMS (Supported Catalytically Active Liquid Metal Solutions) catalyst systems. As an industrially relevant test reaction, the dehydrogenation reaction of propane was used. In the study we compare the activity of oxidized GaPt SCALMS and liquid / metallic GaPt SCALMS surfaces, using Near-Ambient Pressure X-ray Photoelectron Spectroscopy. From the study, we conclude that the oxide shows a lower activity as compared to the metallic liquid GaPt SCALMS model systems. Further, the gallium matrix reduces under reaction conditions and coke residues disappear.

Our study thus underscores the practical value of NAPXPS in tracking surface changes and electronic states during catalytic reactions. By exploring these differences, the study sheds light on the role of oxides on the catalytic performance. These findings contribute essential insights for optimizing catalyst design and improving effectiveness, offering a pragmatic approach to catalytic enhancement.

Acknowledgement:

The research is funded by the DFG through SFB 1452 (Project number: 419654270).

Selective oxidation reactions, such as olefin epoxidation and alkane oxidation, are large-scale industrial processes and are key to the more efficient conversion of natural gas into value-added products. Given the ongoing challenge of optimizing industrial chemical processes, there is a great need for highly selective catalysts. We investigated Au, which is known to be exceptionally selective for many oxidation reactions, but whose inability to efficiently activate O₂ limits reaction rates.

Density functional theory (DFT) predicted that small amounts of Rh guest atoms could overcome the low intrinsic activity of Au toward oxygen activation while preserving its high selectivity. We prepared RhAu single-atom alloys (SAAs), in which Rh atoms remain isolated in the Au surface, and probed their reactivity using a combination of temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). At comparable O₂ pressures and sample temperature, the RhAu systems show a significantly higher amount of Au-associated oxygen than clean Au, providing strong evidence that O₂ is activated at Rh sites and subsequently spills over onto the Au surface, which is the initial step for selective oxidation on Au. We further studied how the coordination environment of the guest atoms influences reactivity by comparing the (111) and (110) facets. The latter forms a (1×2) reconstruction that gives rise to undercoordinated surface sites. Using AP-XPS, we investigated the stability of RhAu SAAs at different temperatures and during exposure to CO and O₂. Together, these findings show that RhAu SAAs can overcome the low oxygen-activation activity of pure Au while retaining the potential for highly selective oxidation chemistry.

Benzene (C₆H₆) and toluene (C₇H₈) are irreplaceable chemical feedstocks used to produce pharmaceuticals, polymers, and materials. These aromatics are currently produced primarily through petroleum cracking and reforming processes, which require high energy input, harsh operating conditions, and lack full selectivity. As hydrocarbon feedstocks increasingly shift from oil to shale gas, alternative catalytic routes for the selective synthesis of aromatics are of significant interest. Acetylene (C₂H₂) cyclotrimerization is one promising pathway because it uniquely produces C₆H₆ with 100% selectivity on Ag(111). However, this reaction requires more than one monolayer (ML) of C₂H₂ on Ag(111) to initiate C₆H₆ formation, necessitating high reactor pressures that limit its practical applicability.

To understand and overcome this limitation, C₂H₂ cyclotrimerization on Ag(111) was investigated using Temperature-Programmed Desorption (TPD), 12 K Scanning Tunneling Microscopy (STM), and Density Functional Theory (DFT). Coadsorption experiments with isotopically labeled benzene (C₆D₆) reveal that C₆D₆ promotes acetylene conversion to 20% at coverages of ≥0.6 ML. STM and DFT show that attractive C₂H₂-C₆D₆ interactions and increased molecular packing stabilize the key C₄ transition state involved in the rate-limiting step, making C-C coupling competitive with C₂H₂ desorption. In addition, coadsorption of C₂H₂ and propyne (C₃H₄) was examined to explore heterocoupling pathways for toluene (C₇H₈) formation. While coupling occurs at ≥1 ML total coverage with higher C₃H₄:C₂H₂ ratios, reaching a maximum at 1.5:1, C₆H₆ remains the dominant product (95%), with C₇H₈ forming only in minor quantities (5%). Guided by mechanistic insights from DFT, we further demonstrate that introducing chromium (Cr) dopant atoms into Ag(111) to form a Cr single-atom alloy dramatically alters the free energy landscape of C₂H₂ cyclotrimerization. This enables C₆H₆ formation at C₂H₂ coverages below 0.1 ML while maintaining the 100% selectivity of Ag(111) at Cr coverages ≤2%.

Together, these findings provide mechanistic insight into product-promoted C₂H₂ cyclotrimerization and C₂H₂-C₃H₄ coupling on Ag(111). They also demonstrate that CrAg(111) single-atom alloys can enable selective aromatic formation, offering a potential pathway for upgrading shale-gas-derived C_2,3 hydrocarbons into high-value aromatics without requiring high reactor pressures.