Extending the toolbox from mono-to bimetallic catalysts is key in realizing efficient chemical processes 1 . Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition 1-3 , often resulting in a compromise between the catalytic properties of the two metals 4-6 . Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single-crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell-thickness-dependent catalytic activity, indicating that not only the nature of the surface but also several subsurface layers play a crucial role in the catalytic performance, and rationalize this finding using density functional theory calculations. Our results open up an alternative avenue for the structural design of bimetallic catalysts.Synergy arises when two catalytically active metals are combined such that the catalytic performance exceeds that of the monometallic counterparts 1 . This makes bimetallics an interesting class of materials for catalysing a variety of chemical processes ranging from selective hydrogenation 1,5,7 to oxidation 2,4,8 and electrochemical reactions [9][10][11] . The main focus has been on alloyed nanoparticles, as they are easily accessible with standard catalyst preparation methods and allow facile tuning of their catalytic properties via the average metal composition. However, the arrangement in which the atoms are assembled is also crucial; gas induced and thermally induced metal redistribution can have a large impact on the catalytic performance 12,13 . Thanks to recent advances in material science, it is now possible to synthesize bimetallic nanoparticles with precisely defined atomic arrangements, such as single-atom alloys 14,15 , intermetallic structures 16 and core-shell materials 17,18 . Yet, only a limited number of studies systematically link the metal distribution to the performance of bimetallic catalysts 12,13 . In particular, the catalytic behaviour of core-shell nanoparticles is largely unexplored, despite successful demonstrations of core-shell catalysts in electrocatalysis, where changes in the electronic properties of the shell atoms induced by the underlying core resulted in enhanced catalytic performances [9][10][11]19 .Here, by employing colloid synthesis 18,20 , we prepared a welldefined Au-Pd model system with a precisely tunable atomic structure allowing a direct correlation between the metal distribution, to six atomic Pd-shell layers. Our study highlights the importance of tuning the atomic distribution in bimetallic catalysts, and lays a foundation for the rational design of bimetallic catalysts with optimal synergistic performances.

Bimetallic nanorods are important colloidal nanoparticles for optical applications, sensing, and light-enhanced catalysis due to their versatile plasmonic properties. However, tuning the plasmonic resonances is challenging as it requires a simultaneous control over the particle shape, shell thickness, and morphology. Here, we show that we have full control over these parameters by performing metal overgrowth on gold nanorods within a mesoporous silica shell, resulting in Au–Ag, Au–Pd, and Au–Pt core–shell nanorods with precisely tunable plasmonic properties. The metal shell thickness was regulated via the precursor concentration and reaction time in the metal overgrowth. Control over the shell morphology was achieved via a thermal annealing, enabling a transition from rough nonepitaxial to smooth epitaxial Pd shells while retaining the anisotropic rod shape. The core–shell synthesis was successfully scaled up from micro- to milligrams, by controlling the kinetics of the metal overgrowth via the pH. By carefully tuning the structure, we optimized the plasmonic properties of the bimetallic core–shell nanorods for surface-enhanced Raman spectroscopy. The Raman signal was the most strongly enhanced by the Au core–Ag shell nanorods, which we explain using finite-difference time-domain calculations.

<div>Extending the toolbox from mono- to bimetallic catalysts is key in realizing efficient chemical processes. Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition, often resulting in a compromise between the catalytic properties of the two metals. Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell thickness dependent catalytic activity, indicating that not only the nature of the surface but also several sub-surface layers play a crucial role in the catalytic performance, and rationalize this finding using density-functional-theory calculations. Our results open up a novel avenue for the structural design of bimetallic catalysts.</div><div><br></div>

<div>Extending the toolbox from mono- to bimetallic catalysts is key in realizing efficient chemical processes. Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition, often resulting in a compromise between the catalytic properties of the two metals. Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell thickness dependent catalytic activity, indicating that not only the nature of the surface but also several sub-surface layers play a crucial role in the catalytic performance, and rationalize this finding using density-functional-theory calculations. Our results open up a novel avenue for the structural design of bimetallic catalysts.</div><div><br></div>