Monodisperse AgPd Alloy Nanoparticles and Their Superior Catalysis for the Dehydrogenation of Formic Acid

Abstract

Formic acid (FA, HCOOH) is a common small organic acid with a melting point of 8.4 8C and boiling point of 100.8 8C. It can undergo a dehydrogenation reaction, HCOOH!H 2 + CO 2 , releasing H 2 that will be important for hydrogen-based energy applications. [1] Traditionally, the dehydrogenation of FA is catalyzed by metal complexes dissolved in an organic solvent and the catalysis is enhanced by adding an additive, such as sodium formate or amine adducts. [2] To make more practical catalyst for the dehydrogenation reaction of FA, heterogeneous catalysts based on metal nanoparticles (NPs) have been developed. These catalysts are generally more stable but much less active than the homogeneous ones. [3] Recently, bimetallic NP catalysts were found to be more active than their single component counterparts for the dehydrogenation of FA. [4] For example, AgPd NPs supported on cerium oxide or AuPd NPs immobilized in a metal-organic framework showed an enhanced FA dehydrogenation catalysis with the initial turnover frequency (TOF) reaching 210 h À1 or 192 h À1 at 90 8C, respectively. [5] However, the high rate of hydrogen generation observed from these heterogeneous catalysts could only be achieved when an additive was present and the reaction was maintained at temperatures close to 100 8C. [6] Under these "harsh" conditions, HCOOH was also subject to an undesired dehydration reaction, HCOOH!H 2 O + CO. [7] Interestingly, Ag/Pd core/shell NPs were found to be promising in catalyzing the dehydrogenation of FA in an aqueous FA solution at lower temperatures (up to 50 8C) without any additive. [8] But their initial TOFs were in the range of 125-252 h À1 at temperatures between 25-50 8C.Considering the limitation seen from the previous syntheses in controlling the NP size and composition, we decided to re-evaluate the binary alloy NPs on their catalysis for the dehydrogenation of FA. Our very recent report showed that monodisperse 4 nm AuPd NPs were more active in catalyzing the dehydrogenation of FA in water at 50 8C without using any additive and their initial TOF reached 230 h À1 . [9] Encouraged by this result, we further improved our solution phase synthesis and produced monodisperse 2.2 nm AgPd NPs with the desired composition controls. We found that these monodisperse 2.2 nm AgPd alloy NPs were a highly active heterogeneous catalyst for the dehydrogenation of FA. In water without any additive, the Ag 42 Pd 58 NPs showed the highest catalytic activity among all AgPd NPs tested with their initial TOF reaching 382 h À1 at 50 8C and apparent activation energy at 22 AE 1 kJ mol À1 . These are the best values ever reported by a heterogeneous catalyst for the dehydrogenation of FA in aqueous solution. It demonstrates the great potential of binary alloy NPs as a more practical catalyst for the dehydrogenation of FA and hydrogen generation.The 2.2 nm AgPd alloy NPs were synthesized by coreduction of silver(I) acetate, Ag(Ac), and palladium(II) acetylacetonate, Pd(acac) 2 , in oleylamine (OAm), oleic acid (OA) and 1-octadecen...

MnOx-Promoted PdAg Alloy Nanoparticles for the Additive-Free Dehydrogenation of Formic Acid at Room Temperature

“…46 The influence of the type of the support material was also investigated by synthetically replacing N-SiO 2 with some of the ubiquitous support materials used in catalysis such as Influence of MnO x Nanoparticles on Poisoning Resistivity of PdAg Alloy Nanoparticles. The enhancement of Pd activity in FA dehydrogenation through Ag incorporation has already been reported for Ag@Pd 18 and PdAg 23 NPs, in which the activity increase has been attributed to a synergic effect. 47 In order to shed some light on the effect of MnO x NPs, we conducted linear sweep voltammetry (LSV), CO stripping voltammetry, and in situ FTIR analyses.…”

“…It is worth mentioning that quenching of the SPR bands due to alloying was also observed for oleylamine-stabilized PdAg alloy NPs. 23 The compositional analyses of the PdAg-MnO x /N-SiO 2 catalyst were also performed by detailed HAADF-STEM measurements. …”

Hydrogen generation from formic acid decomposition at room temperature using a NiAuPd alloy nanocatalyst

“…Xu and co-works have employed AuePd/ED-MIL-101 as a catalyst for the dehydrogenation of FA, and high activity was observed at 363 K in the presence of sodium formate [25]. Most recently, Sun and co-works have confirmed that AgPd alloy NPs supported on Ketjen carbon exhibited highly active for this reaction at 323 K [30]. However, those solid catalysts mainly composed of noble metal(s) are not suitable for large-scale practical application because of their scarcity and high cost [26].…”

Modification of Pd for formic acid decomposition by support grafted functional groups

“…3,6 In terms of heterogeneous catalysts, Pd is widely reported as the most active monometallic system at close to ambient conditions, and tailoring of nanoparticles' electronic properties through alloy or core-shell formation has shown dramatic promotional effects on catalytic activity. 4,7 Addition of Ag to Pd, either as the core of a core-shell particle or in an alloy has resulted in approximately a fiveand tenfold increase over Pd particles, respectively. 4,7 Tedsree et al 8 demonstrated a linear correlation between the activity of metal-core Pd-shell particles and the work function of the core metal, indicating the reaction is highly dependent on surface electron density.…”

“…4,7 Addition of Ag to Pd, either as the core of a core-shell particle or in an alloy has resulted in approximately a fiveand tenfold increase over Pd particles, respectively. 4,7 Tedsree et al 8 demonstrated a linear correlation between the activity of metal-core Pd-shell particles and the work function of the core metal, indicating the reaction is highly dependent on surface electron density. The mechanisms of the two decomposition reactions have been observed to occur through different surface bound intermediates, which have been reported to be favored on different catalytic sites.…”