Neighboring Pd single atoms surpass isolated single atoms for selective hydrodehalogenation catalysis

Abstract

Single atom catalysts have been found to exhibit superior selectivity over nanoparticulate catalysts for catalytic reactions such as hydrogenation due to their single-site nature. However, improved selectively is often accompanied by loss of activity and slow kinetics. Here we demonstrate that neighboring Pd single atom catalysts retain the high selectivity merit of sparsely isolated single atom catalysts, while the cooperative interactions between neighboring atoms greatly enhance the activity for hydrogenation of carbon-halogen bonds. Experimental results and computational calculations suggest that neighboring Pd atoms work in synergy to lower the energy of key meta-stable reactions steps, i.e., initial water desorption and final hydrogenated product desorption. The placement of neighboring Pd atoms also contribute to nearly exclusive hydrogenation of carbon-chlorine bond without altering any other bonds in organohalogens. The promising hydrogenation performance achieved by neighboring single atoms sheds light on a new approach for manipulating the activity and selectivity of single atom catalysts that are increasingly studied in multiple applications.

Fig. 1. Structural characterizations of Pd/SiC.

a–d High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of 0.5%-Pd/SiC, 1.0%-Pd/SiC, 5.6%-Pd/SiC, and Pd_nano/SiC (the inset shows the crystal structure of Pd nanoparticle). e Pd K-edge EXAFS spectra in R space for Pd/SiC and references. f–g Pd K-edge normalized XANES spectra and first derivative curves of Pd/SiC and references. h Compositions of isolated Pd atom, neighboring Pd atom, and metallic Pd nanoparticle.

Fig. 2. Catalytic performance of Pd/SiC in the hydrodehalogenation of organohalides.

a Hydrodehalogenation kinetics of 4-CP by Pd/SiC. b Conversion ratio and selectivity of 4-CP at 5 min by 5.6%-Pd/SiC over five consecutive cycles. c TOF per Pd atom basis calculated with the initial zero-order rate constant of 4-CP hydrogenation. d Selectivity of hydrodehalogenation. e–g Kinetic plots of the organohalide hydrogenation and product generation. The Cl⁻ concentrations in f and g were divided by a factor of two (i.e., each 2,4-D molecule contains two Cl atoms) or three (i.e., each TCP molecule contains three Cl atoms) to visualize the comparisons. Experimental conditions: catalyst (0.5 g L⁻¹), H₂ (1 atm), room temperature (20 °C). Error bars represent standard deviations from triplicate experiments.

Fig. 3. DFT-calculated minimum energy pathway of 4-CP hydrogenation.

a Hydrodehalogenation on i-Pd₁, n-Pd₁, and Pd_nano. All steps are exothermic except for H₂O, phenol, and Cl⁻ desorption (not calculated here). b Alternative 4-CP hydrogenation pathways on n-Pd₁ (i.e., C-O bond cleavage and benzene ring hydrogenation). c Alternative 4-CP hydrogenation pathways on Pd_nano. The blue, black, red, tan, and white spheres in geometrical models are Pd, C, O, Si, and H atoms, respectively. The solid and dashed lines represent minimum energy path and other reaction pathways, respectively.