Greatly changed performance of a metal Pd catalyst by a rather easily formed and removed species–PdHx

Abstract

Pd catalysts, which are often prepared by reducing Pd salts with NaBH₄, have been intensely studied in HCOOH dehydrogenation (FAD) for H₂ production. FAD can also be used to hydrogenate or reduce various chemicals by using its in situ generated hydrogen, which exhibits superiority over the direct use of H₂ gas. Herein, by using FAD to reduce Cr(VI) as a room-temperature probe reaction, we show that some Pd atoms in the catalyst form PdH_x under normal conditions during both catalyst preparation and catalytic reaction (NaBH₄ or HCOOH as a hydrogen donor). PdH_x greatly hinders Cr(VI) reduction and, interestingly, boosts H₂ generation. That is to say, PdH_x can be negative or positive species, depending on the focused product. The reason for this phenomenon was also analyzed. Heat treatment for only 10 min can destroy PdH_x species and thus, improve the activity for Cr(VI) reduction by up to ∼3 times, which is effective in each catalyst reuse. These findings are critical for controlling the design and functioning of Pd catalyst.