Pd Thickness Optimization on Silicate Sheets for Improving Catalytic Activity

Abstract

Formic acid, nonetheless, has become a favorite for energy production [3] because of its facile processing and high hydrogen reservoir capacity. [3b] A prevalent class of catalysts for the dehydrogenation of formic acid is based on Pd nanomaterials [4] that efficiently dehydrogenate formic acid to hydrogen and carbon dioxide. Although tremendous efforts have been dedicated to improving Pd-based catalysts, [5] such as the catalytic activities of the Pd's crystal planes, [6] some critical mechanistic aspects of Pd's catalytic dehydrogenation of formic acid remain unexplored. [7] Decreasing the size of Pd particles, that is, moving toward a single-atom catalyst, has been hinted to boost the Pd's catalytic activity. [8] However, no optimal Pd particle size has been identified for optimal catalytic formic acid dehydrogenation. [9] Moreover, the support material's role, if any, in determining Pd's optimal shape and size is to be established. Here, we demonstrate that plate-like Pd nanoparticles supported on the silicate must have at least six atom-layer-thick (≥1 nm) for an optimal catalytic property toward formic acid dehydrogenation that is considerably higher than single-atom Pd catalysts.To design plate-like Pd architecture, we use silicate sheets obtained via partial exfoliation of a commercially available layered silicate that possesses a richer surface silanol group (SiOH/SiO − ) compared with the other layered silicates. [10] The rationale behind our choice is that a richer hydroxyl-layered surface appears more efficient in immobilizing metals and grafting organic functional groups necessary to immobilize metals. [11] However, the exfoliation of silanol-rich layered silicates has been recognized as challenging due to several factors, including higher-layer charge density (stronger Coulombic interaction between layers). In this study, we utilize iminodiacetate (IDA) immobilized on the silicate surface as an exfoliating agent and Pd-adsorption assisting agent precursors, [12] which is subsequently eliminated when the plate-like Pd formation attaches to the silicate nanosheets' surface (Figure 1). Results and Discussion Materials Synthesis and CharacterizationLayered materials often show relatively slow diffusibility to reactants (ions and molecules) due to the absence of open pores in the interlayer space. [13] Therefore, the exfoliation of layered materials while immobilizing catalytically active sites is crucial Maximizing surface-to-body ratio demands ever smaller metallic palladium (Pd) nanoparticles for catalytic applications. The quest for miniaturization is now reaching the single-atom limit. However, if the supported Pd is below a critical size, the Pd hybridization with the supporting material can detrimentally reduce the labile electrons that facilitate the catalytic reactions. Thus, the smallest attainable size, i.e., single-atom Pd, may not offer the best efficiency. Here, it is demonstrated that Pd with at least six atomic layers (or thickness of ≈1 nm) on the silicate sheets, synthesized via ...

Feeble Single-Atom Pd Catalysts for H2 Production from Formic Acid

“…Our previous studies about Pd thickness optimization on silicate sheets show that such a particle size can exhibit excellent metallicity and catalytic activity. 10 The recovered SiO 2 −NH 2 /Pd SA from the first run, in which the Pd SA is converted into nanoparticles, exhibits a higher activity than that of the initial sample. Eventually, the catalytic activity stabilizes in the further runs under identical reaction conditions.…”

“…8,9 In our recent investigation, we demonstrated that the thickness of Pd plate-like nanoparticles on any given support should be investigated to find the optimal catalytic performance since the optimal Pd thickness to deliver maximum catalytic activity depends on the type of the support and its surface chemistry. 10 However, how a functional group can influence Pd's electronic structure in conjunction with the support material has rarely been investigated. Additionally, the interdependence of stability and catalytic activity is another point that requires careful investigation in the case of Pdsupported catalysts possessing a functionalized surface.…”

“…14−17 This latter interaction becomes more significant in determining Pd's electronic structure as the size shrinks as the hybridization with the support strengthens. 10,16 Single-atom catalysis is a recently developed concept that is being widely investigated for several catalytically active metals on various supports for various reactions. 18−22 It has been demonstrated that the support material plays a fundamental role in the catalytic activity of single-atom catalysts.…”

“…A vast fraction of investigations so far have also been focused on reactant/Pd interactions, that is, how potent incoming molecules vs how weak output molecules are. − However, the interplay between the support and Pd is undeservedly less investigated. − This latter interaction becomes more significant in determining Pd’s electronic structure as the size shrinks as the hybridization with the support strengthens. , …”

Atomically‐Dispersed Metal Heterocatalysts: A Practical Step Toward Sustainability

“…To address the effect of the numbers of layers on the activity of the catalyst, Doustkhah et al reported that silicate sheets with six atomic layers of palladium with approximately 1 nm thickness, showed higher activities in the selective dehydrogenation of formic acid for hydrogen production in water compared to isotropic palladium nanoparticles and a single-atom palladium catalyst. [64] Zenobi et al used tipenhanced Raman spectroscopy to visualize the active sites of a (sub)monolayer of palladium deposited on the surface of Au (111) with the aim of a better understanding of the reaction mechanism. [65] The results indicated that for the hydrogenation of chloronitrobenzenethiol, the reactive regions are not limited on the palladium regions, but extend up to ~20 nm into the gold regions as a result of hydrogen spillover.…”