Surface composition of Pd/Cu(111) single-atom alloys and its impact on selective non-oxidative butanol dehydrogenation

• Characterization of Pd/Cu(111) alloys at varying Pd content (0.01–0.55 ML) employing STM, IRAS, and HR-XPS. • Three Pd species identified: Pd single atoms, ensembles, and subsurface Pd. • Annealing drives Pd from edges to terraces, forming isolated atoms and destroying ensembles. • Isolated Pd atoms catalyze selective butanol dehydrogenation to butanal, whereas Pd ensembles promote butanol decomposition to CO. Single atom alloys (SAAs) offer a powerful strategy to achieve high catalytic selectivity while minimizing the use of expensive noble metals. Control over their selectivity requires a fundamental-level understanding of how it depends on the electronic structure and geometric arrangement of the active single atoms embedded into the host matrix as well as how these properties are affected by the alloy composition and the preparation conditions. Here, we present a comprehensive study of Pd/Cu(111) surfaces over a wide range of Pd coverages (0.01–0.55 ML), combining infrared reflection absorption spectroscopy (IRAS) employing CO as a highly sensitive probe for different adsorption sites, scanning tunneling microscopy (STM), synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Specifically, the correlation of the IRAS, STM and HR-XPS data, which were obtained as a function of changing preparation parameters (Pd loading and annealing temperature), allowed to identify three different forms of Pd: (i) single Pd atoms embedded into Cu, (ii) Pd ensembles consisting of several Pd atoms and residing on Cu surface as well as (iii) subsurface Pd formed as a result of Pd diffusion into Cu bulk. Immediately after deposition at 300 K, Pd nucleates preferentially at step edges of the Cu(111) crystal, forming isolated Pd atoms at coverages below 0.1 ML and larger Pd ensembles at higher Pd loadings. At coverages above 0.55 ML, 3D Pd clusters emerge near the step edges. Annealing to 550 K significantly alters this distribution: Pd diffuses to terrace sites, forming isolated atoms and leading to disintegration of Pd ensembles. Up to 0.3 ML, isolated Pd atoms dominate on terraces, while ensembles are absent. At coverages exceeding 0.3 ML, however, both IRAS and XPS reveal reformation of Pd ensembles. The structural information was linked to the catalytic performance in butanol dehydrogenation to butanal and decomposition to CO. TPD experiments correlated with the structural information on the surface composition suggest that isolated Pd atoms embedded in the Cu(111) terrace selectively catalyze butanal formation, while Pd ensembles promote extensive C–C and C–H bond scission, yielding CO. However, when Pd atoms are located at the step edges, both pathways occur, indicating that the low-coordinated environment enables not only H abstraction but also C–C bond cleavage in butanol. DFT calculations reveal similar electronic structures for isolated Pd atoms embedded into the (111) terraces and step edges, suggesting that reactivity differences arise primarily from the geometric effects. With this, the local geometric environment of the active metal emerges as highly important factor governing selectivity in alcohol dehydrogenation.