Fig. 1. Design of Pd and Pd-Ag nanocatalysts, conceptual approaches, and research targets. Abbreviations of different molecules were denoted in the parenthesis.

Fig. 2. Characterizations of the developed key catalysts. (a) PXRD of the key catalysts. (b) The zoomed patterns at 2θ = 37°-41° showed the shift of the diffraction lines. (c-l) TEM images of the key catalysts, accompanied with the particle size distributions (insets). (j,k) Elemental color mapping images of different PdAg catalysts. The core-level Pd 3d (l,m) and Ag 3d (n) XPS spectra of the key catalysts. The deconvolution results were shown as the shadowed peaks. Dashed lines indicate the shifts of the binding energies that strongly hints the electronic transfer from Ag to Pd at the increasing particle sizes. (o) Bader charge analysis of small and large PdAg alloys. Large PdAg ensembles shows a higher average charge transfer from Ag to Pd than the small analogues (0.104 vs. 0.097 e).

Fig. 3. Catalytic performance of the developed catalysts in butadiene hydrogenation. (a) The conversion and butene selectivity as a function of bed temperature on monometallic and bimetallic catalysts. (b) The product distributions at near complete conversion at different H2:BD ratios. (c) Major products on small (s) and large (l) Pd and Pd-Ag ensembles. (d) Long-term stability performance of PdAg76.7. Reaction conditions: Wcat = 30 mg, Ftotal = 75 mL min-1, GHSV = 150000 mL gcat-1 h-1, P = 1 bar, a, H2:BD = 10; (d) H2:BD = 50 and T = 338 K. (e,f) Comparison on the catalytic performances of different catalysts in BD hydrogenation at approaching full conversion; yields of total butenes and 1-butene selectivity (e), heatmap of bed temperatures (f), catalyst codes (g).

Fig. 4. Reaction kinetics on different PdAd nanoalloys. The product distribution as a function of contact time in butadiene (a,b) and 1-butene (d) hydrogenation. (c) The conversion of 1-butene as a function of bed temperature. Reaction conditions: P = 1 bar, (a,b,d) GHSV was between 40000-1440000 cm3 gcat-1 h-1 by varying both the catalyst weight and total gas flow rate, H2:BD(1B) = 20, T = 313 K for (a,b) and 303 K for (d). (c) Wcat = 10 mg, H2:1B = 10, Ftotal = 75 mL min-1, GHSV = 450000 mL gcat-1 h-1. (e) The global activation energies, and partial reaction orders of H2 (f) and butadiene (g). Chemisorption properties: BD-TPD (h) and 1-butene TPD (i) profiles. (j) The relative desorption amount between BD and 1B (ABD:A1B) and the differences between their peak desorption temperatures (TBD − T1B). (k) Schematic illustrations on the different reaction paths on small and large Pd-Ag alloys.

Fig. 5. Structure-performance correlations. (a) The formation zones of different butadiene hydrogenation products on Pd and PdAg ensembles of varying particle sizes and Pd valent states. Correlations between Sbutenes (S1B) and average particle size of Pd or PdAg ensembles (b) and average valent state of Pd (c). (d) Critical selectivity descriptors in Pd and PdAg systems. Zone 1 for butane, zone 2 for 1-butene, and zone 3 for 2-butenes.

Fig. 6. DFT simulations of BD hydrogenation reaction coordinate on different model catalysts. (a,d) The energy profiles of each elementary steps. (b,e) Comparison on the energy barriers of three transient states. (c,f) The simplified reaction cycles on small PdAg ensembles (PdAg-s). (a-c) BD→trans-2B→butane path; (d-f) the BD→1B→butane path.