Why does Pd-catalyzed electrochemical hydrodechlorination proceed much slower than hydrodechlorination using hydrogen gas?

The rate of Pd-catalyzed electrochemical hydrodechlorination (HDC-E) towards chlorinated organic pollutants (COPs) is much slower than that of the hydrodechlorination carried out using hydrogen gas (HDC-H2). Aiming to unravel the rate bottleneck of the HDC-E, we carried out a comparative study of the HDC-E and HDC-H2 using Pd nanoparticles supported on activated carbon (PdNPs/C) and a Pd-modified nickel foam (Pd/Ni foam) as the catalytic materials, and 4-chlorophenol as the representative COP. When PdNPs/C catalyst was employed, it was found that the HDC-H2 rate was 25 times greater than that of the HDC-E, and this was attributed mainly to the higher dispersity of the catalyst, and the H2 diffusion effect in the HDC-H2. Moreover, when Pd/Ni foam was employed, the rate of the HDC-H2 was only 2 times higher than that of the HDC-E, which in this case was attributed to the higher concentration of adsorbed hydrogen in the former. These results suggest that rather than improving the intrinsic activity of the Pd catalyst, improving the dispersion of the catalyst and enhancing the mass transfer of the reaction substrate are currently more promising routes to greatly improve the rate of the HDC-E. [Display omitted]