Tuning reactivity in trimetallic dual-atom alloys: molecular-like electronic states and ensemble effects

Abstract

Single-atom alloys (SAAs) have drawn significant attention in recent years due to their excellent catalytic properties. Controlling the geometry and electronic structure of this type of localized catalytic active site is of fundamental and technological importance. Dual-atom alloys (DAAs) consisting of a heterometallic dimer embedded in the surface layer of a metal host would bring increased tunability and a larger active site, as compared to SAAs. Here, we use computational studies to show that DAAs allow tuning of the active site electronic structure and reactivity. Interestingly, combining two SAAs into a dual-atom site can result in molecular-like hybridization by virtue of the free-atom-like electronic d states exhibited by many SAAs. DAAs can inherit the weak d-d interaction between dopants and hosts from the constituent SAAs, but exhibit new electronic and reactive properties due to dopant-dopant interactions in the DAA. We identify many heterometallic DAAs that we predict to be more stable than either the constituent SAAs or homometallic dual-atom sites of each dopant. We also show how both electronic and ensemble effects can modify the strength of CO adsorption. Because of the molecular-like interactions that can occur, DAAs require a different approach for tuning chemical properties compared to what is used for previous classes of alloys. This work provides insights into the unique catalytic properties of DAAs, and opens up new possibilities for tailoring localized and well-defined catalytic active sites for optimal reaction pathways.

Fig. 1. (a) H₂ molecular orbitals result from combining two H 1s orbitals. (b) Molecular-like states may result from combining free-atom-like d states of two different dopants in a metal host. (c–i) pDOS of d states and wavefunctions of Ir₁Cu (c and d), Ir₁Ti₁Cu (e–g), and Ti₁Cu (h and i). (c) and (h) show narrowed d states of Ir and Ti atoms in their SAA form in a Cu host. (e) shows shifted and broadened d states and two new small peaks, all due to Ir–Ti hybridization in the Ir₁Ti₁Cu DAA. (d) and (i) are d_xz orbitals of Ir at −0.8 eV and of Ti at 0.4 eV. The combination of (d) and (i) result in a d–d π orbital (f) at −1.7 eV and a d–d π* orbital (g) at 1.1 eV.

Fig. 2. Screening of homodimer and heterodimer formation energies for M₁Pd₁Ag (a), M₁Pd₁Au (b), M₁Pt₁Ag (c) and M₁Ti₁Cu (d). The dopants in light blue areas are predicted to only form heterodimers; those in the white area are predicted to give predominantly heterodimers with some homodimers; those in the light pink area are predicted to form few or no heterodimers. Some dopants are omitted due to their high homodimer formation energies, resulting in values far below the diagonals. Complete results are listed in Table S1.

Fig. 3. (a) Top views of CO on a top adsorption site of M in SAA and DAAs, and on a bridge site between M and M'. (b) M₁Pd₁Ag, M₁Pd₁Au, and M₁Ti₁Cu trimetallic alloys from the white and light blue areas in Fig. 2 were divided into three groups based on the CO adsorption energies in Table S2. The alloys in the dark yellow area have strong electronic effects while the alloys in the blue area have strong ensemble effects. The alloys in the gray area show weaker electronic and ensemble effects. (c) CO adsorption energy on top of the dopant in SAAs (E_ads(SAA)) vs. in DAAs (E_ads(DAA,M)). The labels correspond to the alloys in (b). The ensemble-effect cases are close to the diagonal, indicating little difference in the top-site adsorption energies. The strong electronic-effect cases (|(E_ads(DAA,M)) − E_ads(SAA)| > 0.14 eV; outside dashed lines) exhibit a large deviation from the diagonal, indicating large changes in reactivity. (d) Trend for CO top-site adsorption energy difference and shift in d-band center. All strong effect cases listed in (b) are shown. Some weak-effect cases are omitted. The inverse correlation agrees with the d-band model. (e) CO adsorption energies on different sites. For these three ensemble-effect cases, while there is a only small difference between E_ads(SAA) and E_ads(DAA,M), the E_ads(DAA,Bri) is stronger.