PdO/Pd⁰/TiO₂ Nanocatalysts Engineered by Flame Spray Pyrolysis: Study of the Synergy of PdO/Pd⁰ on H₂ Production by HCOOH Dehydrogenation and the Deactivation Mechanism

Abstract

Palladium-based catalysts are among the most efficient for H2 production via HCOOH (FA) dehydrogenation at near-ambient pressure and temperature. Herein, we show that [PdO/Pd0/TiO2] nanocatalysts bearing a tetragonal PdO nanophase can be optimized for enhanced FA dehydrogenation via engineering of the [PdO:Pd0] ratio on the TiO2 support. We have developed a sequential-deposition flame spray pyrolysis (SD-FSP) technique for deposition of Pd on TiO2 at a high [PdO:Pd0] ratio up to 75%. In addition, we have synthesized low-[PdO:Pd0]-ratio catalysts using an oxygen-lean FSP protocol. The SD-FSP-made [PdO/Pd0/TiO2] nanocatalysts with a high [PdO:Pd0] ratio >70% can achieve a high H2 gas production rate of 534 mmol/g of Pd/min that supersedes by >300% the efficiency of [Pd0/TiO2] nanocatalysts with low PdO content. The thermodynamic basis of the role of [PdO:Pd0] was investigated by an Arrhenius study, which reveals that the activation energy barrier E a is decreasing significantly, i.e., up to 50%, upon an increase of the [PdO:Pd0] ratio. A reduction of surficial PdO toward Pd0, by in situ generated H2, exerts a strong inhibitory effect on the catalyst. Overall, the present data indicate that both [i] maximization of the [PdO:Pd0] ratio and [ii] minimization of PdO reduction during H2 production are key prerequisites for enhanced FA dehydrogenation by TiO2-supported/Pd catalysts.

Double-Nozzle Flame Spray Pyrolysis as a Potent Technology to Engineer Noble Metal-TiO2 Nanophotocatalysts for Efficient H2 Production

“…The peak~534 eV corresponds to the metal-O bond, which is resolved only in Pd/TiO 2. As analyzed in detail previously [25], this XPS feature is indicative of the strong metal-support interactions between Pd and TiO 2 [25]. It is worth mentioning that in the case of Pt/TiO 2 and Ag/TiO 2 , no XPS signal of Pt or Ag was detected due to low concentration and the fine dispersion of noble metal.…”

“…The full XRD data for all synthesized materials are provided in Figures S1 and S2 in Supplementary Materials. [25]. Rietveld analysis, shows that, in all cases, the anatase/rutile phase ratio did not change, i.e., it was A: R~90:10.…”

“…The Flame Spray Pyrolysis set-up used, has been described in detail in our previous works [25,32,33]. Herein, two different FSP protocols (One-Nozzle, Double-Nozzle) were optimized and implemented, ( Figure 1) to obtain the desired NM 0 /TiO 2 nano-hybrids:…”

“…In a similar work, the catalytic efficiency of FSP-made Cu-Pt 0 /TiO 2 composites, in H 2 production from methanol photo-steam-reforming [24] was examined. Recently, our group engineered PdO/Pd 0 /TiO 2 nano-heterostructures, for formic acid dehydrogenation, using a novel Sequential-Deposition FSP method [25]. This work demonstrated that the sequence of metal-deposition on the TiO 2 , i.e., concurrent formation/deposition of {Pd 0 NP/TiO 2 } vs. sequential depiction of Pd on pre-formed TiO 2 particles made a significant difference in their catalytic H 2 production form HCOOH [25].…”

“…So far, however, the potential of DN-FSP to produce TiO 2 -based photocatalysts has not been studied. Despite this, from all the cited FSP works [24][25][26][27], it becomes obvious that the use of DN-FSP methodology allows production of particles with better controlled properties. This is due to the versatility of the method, i.e., DN-FSP offers a configuration where the parameters of the two Nozzles can be optimized independently [30].…”