One-pot synthesis of Pd20-xAux nanoparticles embedded in nitrogen doped graphene as high-performance electrocatalyst toward methanol oxidation

Mixed Pd–Au bimetallic nanoparticles embedded nitrogen doped graphene composites (PdAu/NG180) are explored for efficient electrocatalytic oxidation of methanol. A simple hydrothermal one-pot polyol method, involving simultaneous reduction of both Pd and Au, is utilized for the synthesis of Pd_20-xAu_x/NG180 (x wt % = 0, 5, 10 and 15). This method is of multiple advantages such as inexpensiveness, reagent-free and environment-friendly being surfactant free. The morphology, crystal structure and chemical composition of NG180, Pd/NG180 and Pd_20-xAu_x/NG180 catalysts are analyzed by XRD, FESEM-EDX, TEM, XPS and Raman spectroscopy methods. Electrocatalytic activities of PdAu/NG180 nanocomposites toward methanol oxidation reaction (MOR) in alkaline media are investigated by cyclic voltammetry, chronoamperometry and CO stripping measurements. Pd_20-xAu_x/NG180 exhibited an increase in the electroactive surface area of Pd to twice by the coexistence of Au. In cyclic voltammetry studies, Pd₁₀Au₁₀/NG180 catalyst exhibits highest peak current density for MOR and is 1.5 times highly efficient compared to Pd₂₀/NG180 with an enhanced shift in the onset potential by 140 mV to lower overpotentials. Besides, Pd₁₀Au₁₀/NG180 catalyst exhibited enhanced electroactive surface area and long-time durability in comparison to Pd₂₀/NG180 catalyst. The steady state current density for MOR observed with Pd₁₀Au₁₀/NG180 at the end of 4000 s (98 mA mg⁻¹_Pd) is higher than those observed with all the other catalysts at the end of mere 1000 s alone (97, 61, and 32 mA mg⁻¹_Pd). The promising high electrocatalytic activity of Pd₁₀Au₁₀/NG180 is well corroborated from CO stripping experiments that the specific adsorption of CO onto Pd₁₀Au₁₀/NG180 (0.71 C m⁻²) is merely half to that observed onto Pd₂₀/NG180 (1.49 C m⁻²).     

Fast hydrogen evolution by formic acid decomposition over AuPd/TiO2-NC with enhanced stability

Well-dispersed AuPd nanoparticles were immobilized on TiO₂-NC supports derived from NH₂-MIL-125(Ti) and used as highly active, stable catalysts for hydrogen production from formic acid under mild conditions. The highest total turnover frequency, i.e., 3207 h⁻¹, for formic acid dehydrogenation was achieved with Au₂Pd₈/TiO₂-NC-800 as the catalyst at 60 °C; this is 1.4 times that achieved with Au₂Pd₈/TiO₂–C-800 under the same conditions. The excellent performance of the Au₂Pd₈/TiO₂-NC-800 catalyst originates from the high anatase TiO₂ content, pyridinic N and oxygen vacancies in the support, the small size and alloying effect of the AuPd nanoparticles, and the metal–support synergistic effect. Doping the support with N improves the catalyst stability because N prevents metal particle aggregation to some extent. These results provide guidelines for the future development and applications of catalysts based on TiO₂ and metal–organic-framework-derived carbon-based materials.     

Single Pd atom and Pd dimer embedded graphene catalyzed formic acid dehydrogenation: A first-principles study

The catalytic decomposition of formic acid was studied on a series of candidate catalysts using density functional theory calculations. The candidate catalysts were modelled by a single Pd atom embedded in mono- and divacancy in graphene (Pd_1m-G vs. Pd_1d-G), as well as a Pd dimer embedded in di-, tri-, and quadrivacancy in graphene (Pd_2d-G vs. Pd_2t-G vs. Pd_2q-G). These catalysts can effectively and selectively catalyze formic acid dehydrogenation into hydrogen. Pd_2d-G is the most favorable catalyst among the five models with the rate-determining step energy barrier of only 0.68 eV, which is comparable to one of the most active catalysts i.e., Pd(111). Pd_1d-G is comparatively less active, with the rate-determining step energy barrier of 0.90 eV. The rest of the three models, i.e., Pd_1m-G, Pd_2t-G and Pd_2q-G, have energy barriers of 1.26, 1.12 and 1.06 eV, respectively. The model catalysts studied in this work are promising for reducing usage of the precious and rare metal Pd compared with Pd bulk catalysts. Additionally, unlike Pd nanoparticle catalysts, the model catalysts in this work clarify the catalytic mechanism.     

A robust electrocatalytic activity and stability of Pd electrocatalyst derived from carbon coating

Palladium (Pd) as an efficient anodic catalyst has been extensively investigated in direct formic acid fuel cells (DFAFCs); while, Pd catalyst is electrochemically unstable in acidic electrolyte resulting in low stability retarding the widespread application of DFAFCs. In this study, a new method is invented to prevent the Pd nanoparticles from rapid dissolution by carbon layer originated from the carbonization of glucose. Ascribing to the presence of carbon layer, Pd electrocatalyst demonstrates much higher stability in comparison with Pd electrocatalyst without carbon layer in the course of stability tests. Robust electrocatalytic activities toward formic acid and methanol/ethanol oxidation are observed for carbon-stabilized Pd electrocatalyst resulted from the higher content of metallic Pd atoms coming from the carbonization process, in which Pd (II) species are further reduced. Moreover, the fuel cell performance of carbon-stabilized Pd electrocatalyst reaches 90 mW cm_Pd⁻² measured with 1 M formic acid; while, power density of bare Pd electrocatalyst is only 74 mW cm_Pd⁻². This work highlights that carbon layer carbonized from glucose improves not only the stability of Pd electrocatalyst, but also the electrocatalytic activity.     

Bimetallic PdAu catalysts for formic acid dehydrogenation

A series of monometallic and bimetallic palladium gold catalyst were prepared and studied for the formic acid dehydrogenation reaction. Different Pd/Au compositions were employed (Pd_xAu_100-x, where x = 25; 50 and 75) and their impact on alloy structure, particle size and dispersion was evaluated. Active phase composition and reaction parameters such as temperature, formic acid concentration or formate/formic acid ratio were adjusted to obtain active and selective catalyst for hydrogen production. An important particle size effect was observed and related to Pd/Au composition for all bimetallic catalysts.     

Pd/C nanocatalyst with high turnover frequency for hydrogen generation from the formic acid–formate mixtures

Pd/C nanocatalyst with high turnover frequency (TOF) for hydrogen generation from the formic acid (FA)–sodium formate (SF) mixtures was prepared via an ex situ reduction of PdCl₂ used formate in the presence of citric acid. The morphology and property of the Pd/C catalyst before and after decomposition of FA–SF mixture were characterised using transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, X-ray diffractometer and Fourier transform infrared spectrometer. Over this Pd/C catalyst, a TOF of 228.3 h⁻¹ was observed for a FA–SF mixture with a FA/SF ratio of 1:9. The observed TOF was the highest ever reported for heterogeneous Pd/C catalysts. The deactivation of the Pd/C catalyst was attributed to desorption of citric acid, reduction of Pd^II content and adsorption of CO. Washing and drying could partially recover the activity of the Pd/C catalyst.     

A “special” anhydrous system for the preparation of alloyed Pd1Ce0.5 nanonetworks catalyst supported on carbon nanotubes with high electrochemical oxidation activity for formic acid

Herein, Pd₁Ce_0.5 alloy nanonetworks (ANNs) on multi-walled carbon nanotubes (MWCNTs) supported bimetallic catalyst (referred to Pd₁Ce_0.5/MWCNTs-D) was prepared in deep eutectic solvents (DESs). The Pd₁Ce_0.5/MWCNTs-D catalyst shows remarkable catalytic performance toward formic acid oxidation (FAO) (1968.5 mA mg_Pd^?1) and better CO anti-poisoning capability compare with Pd/MWCNTs-D, Pd/MWCNTs-W (prepared in water) and commercial Pd/C catalysts. The excellent network structure and synergistic effect are the main reasons for the improvement of electrochemical activity of Pd₁Ce_0.5/MWCNTs-D catalyst. This study provides a new method for preparation of high performance Pd-based electrocatalysts for direct formic acid fuel cell (DFAFC) applications.     

Mechanistic insights into hydrogen production from formic acid catalyzed by Pd@N-doped graphene: The role of the nitrogen dopant

The catalytic decomposition of formic acid (HCOOH) is a crucial process for hydrogen production technologies. Herein, periodic density functional theory (DFT) calculations were employed to explore the effect of N-doping on the decomposition of formic acid. We designed a series of single Pd-atoms deposited in the single vacancy of N-doped graphene sheets, namely Pd-DGr, Pd–N1Gr, Pd–N2Gr, and Pd–N3Gr, as the proposed catalysts. Our findings show that H₂ production from HCOOH dehydrogenation on these surfaces proceeds via the formate (HCOO) pathway (Path-I) rather than the carboxylate (COOH) pathway (Path-II). Furthermore, the Pd–N3Gr catalyst shows the greatest catalytic reactivity toward HCOOH dehydrogenation via Path-I, requiring an activation energy (E_a) of 0.38 eV.On the other hand, the undesirable dehydration of HCOOH to carbon monoxide (CO) through COOH (Path-IIIA) or formyl (HCO) (Path-IIIB) intermediates is unlikely to occur on Pd–N3Gr due to a large activation energy. We found that the active species on the catalyst surface increased with N-doping concentration. Additionally, microkinetic simulations of the HCOOH decomposition on these surfaces confirmed the high activity and selectivity of the Pd–N3Gr catalyst toward HCOOH dehydrogenation (Path-I). These calculated results highlight that the Pd–N3Gr catalyst is a promising candidate for the formic acid decomposition reaction to yield hydrogen.     

Synthesis of ultrafine low loading Pd–Cu alloy catalysts supported on graphene with excellent electrocatalytic performance for formic acid oxidation

Here, surfactant free composite catalysts (Pd–Cu/rGO) with Pd–Cu alloy nanoparticles uniformly distributed on graphene sheets are successfully prepared via a facile hydrothermal approach. Compared with pure Pd/rGO catalyst, the introduction of copper could dramatically enhance the performance of the catalyst in the electrocatalytic formic acid oxidation (FAO) due to the strain effect and the ligand effect. With the optimized atomic ratio of 3:1 between palladium and copper, the alloy nanoparticle shows the smallest size of 2.12 nm, thus endowing the composite catalyst with highest catalytic efficiency. With Pd load as low as 14.5%, a maximum mass current density of 1580 mA mg_Pd⁻¹, and residual current of 69.93 mA mg_Pd⁻¹ at 3000 s was achieved with our Pd₃Cu₁/rGO catalyst in the electrocatalytic FAO process.     

Pd nanoparticles immobilized on aniline-functionalized MXene as an effective catalyst for hydrogen production from formic acid

A simple wet chemical method was used to prepare two-dimensional transition metal carbides (MXene); PDA-MXene was prepared by alkalization of p-phenylenediamine (PDA) on MXene. And further, Pd metal nanoparticles (NPs) were conveniently loaded on the surface to catalyze the dehydrogenation of formic acid. The as-prepared Pd/PDA-MXene catalyst for the formic acid dehydrogenation was characterized by XRD, IR, TEM, and XPS. Pd-NPs with a size of about 4 nm were formed upon the PDA-MXene support surface and were well dispersed. The Pd/PDA-MXene exhibited good catalytic activity in the formic acid decomposition process without any additives, and the turnover frequency value at 50 °C was 924.4 h⁻¹, which is comparable to most of the reported noble metal catalysts under similar conditions. It is worth mentioning that the prepared catalyst maintained good catalytic activity in five consecutive catalytic cycles of the formic acid dehydrogenation experiment.     

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

Formic acid has been widely regarded as a safe and sustainable hydrogen storage material. Despite tremendous efforts, developing low-noble-metal-loading material with high activity for the dehydrogenation of formic acid remains a great challenge. Here, carbon supported highly homogeneous trimetallic NiAuPd alloy nanoparticles are prepared and employed as catalyst for the selective dehydrogenation of formic acid. Unexpectedly, at Ni molar contents as high as 40%, the resultant Ni_0.40Au_0.15Pd_0.45/C exhibits high activity and 100% hydrogen selectivity for hydrogen generation from formic acid aqueous solution without any additives even at 298 K. Such a low-noble-metal-loading catalyst with high activity may greatly encourage the practical application of formic acid as a hydrogen storage material.     

The mechanism of the water dissociation and dehydrogenation of glycerol on Au (111) and PdAu alloy catalyst surfaces

Hydrogen is an energy carrier found from renewable sources such as biomass, geothermal, solar, or wind. Water splitting and dehydrogenation of glycerol is a sustainable process of H₂ production from renewables because water is abundant, and the glycerol is formed from the biomass-derived compounds. However, finding a suitable and best catalyst for these processes is challenging. Thus, this paper proposed a theoretical study to find the mechanism of the dissociation of water and dehydrogenation of glycerol using Au metal and PdAu alloy catalysts using the density functional theory (DFT) method. Four PdAu alloys have been constructed with different atomic compositions ranging from 1 to 3 of Pd metal to Au metal. The result showed strong adsorption on the Pd₁Au₃ catalyst surface, and the water splitting is best on the Pd₃Au₁ catalyst surface. Simultaneously, the glycerol adsorption on catalyst surfaces is tested before proceeding for the complete dehydrogenation mechanism of glycerol. Strong adsorption was found at the Pd₁Au₃ catalyst compared to other catalyst surfaces on the glycerol adsorption. The dehydrogenation mechanism was found toward a downhill reaction and removed eight hydrogens from the glycerol compared to Au metal, referring to easy dehydrogenation of glycerol using the alloy PdAu. The final species that adsorbed on the Pd₁Au₃ surface is the carbon monoxide will be turned later into carbon dioxide.     

An effective low Pd-loading catalyst for hydrogen generation from formic acid

As an interesting hydrogen carrier, formic acid is bio-renewable, non-toxic and available in the liquid state at room temperature. The development of active and low-cost catalyst is of significance for hydrogen generation from formic acid. In this study, both a relatively cheap metal (Ag) and a functional support (nitrogen modified reduced graphene oxide, N-rGO) were applied to prepare Pd catalyst. It was found that the Ag atoms facilitated the formation of Pd-rich surface in the preparation strategy, in which the reductive N-rGO and a two-step feeding process of metal precursors played important roles. In addition, Ag additive was found to benefit catalyst stability. Most interestingly, the obtained low Pd-loading Pd₁Ag₆/N-rGO catalyst showed a specific Pd loading turnover frequency of 171 mol Pd^?1 h^?1 and a specific metal cost turnover frequency of 64.2 $^?1 h^?1, which were predominant among currently available Pd-based catalysts towards formic acid decomposition without any additive under room temperature.     

Formic acid dehydrogenation over PtRuBiOx/C catalyst for generation of CO-free hydrogen in a continuous-flow reactor

Liquid-phase formic acid dehydrogenation using a solid carbon supported PtRuBiO_x catalyst offers a promising and convenient method to produce CO-free hydrogen. In this study, the regenerability of the catalyst and the kinetics of formic acid dehydrogenation were investigated in a continuous-flow reactor. The kinetic experiments were carried out at temperatures between 300 and 333 K and formic acid concentrations ranging from 1.3 to 8.0 mol/L. It was found that an Arrhenius temperature dependence of the kinetic constant could represent the kinetics of formic acid dehydrogenation over the catalyst. The kinetics had first-order dependence for HCOO⁻ and half-order with respect to HCOOH under the investigated conditions. The average apparent activation energy was determined to be about 38.1 kJ/mol, which is close to the previous value (37.3 kJ/mol) obtained in a batch reactor. To gain more insight into the formic acid dehydrogenation over the catalyst, two possible mechanisms with adsorption of HCOOH or HCOO⁻ were proposed based on the experimental results and available information in literature. Two kinetic expressions were derived from the proposed reaction mechanisms. The corresponding kinetic parameters were estimated and further correlated with the apparent activation energies obtained at different formic acid concentrations.     

PdBi alloy nanoparticle-enhanced catalytic activity toward formic acid oxidation

Improved performance and reduced costs are crucial to develop catalysts for direct formic acid fuel cells. In this study, PdBi alloy nanoparticles were synthesized using a facile seed-mediated growth method. The as-synthesized Pd₁Bi₁ alloy nanoparticles exhibited a large electrochemical surface area (46.3 m² g⁻¹) and a high mass activity (1.44 A mg⁻¹), which was 1.19- and 4.8-fold higher than that of commercial Pd/C catalysts, respectively. The PdBi alloy nanoparticle is a promising catalyst for direct formic acid fuel cells.     

Hydrogen generation from formic acid decomposition on Pd–Cu nanoalloys

Formic acid (HCOOH) as liquid hydrogen storage material is limited due to the lack of high activity and selectivity catalyst. Recently, Pd-based nanoclusters show its remarkable performance in the HCOOH decomposition process. In this study, HCOOH decomposition process on Pd, Cu and three Pd–Cu nanoclusters are investigated by density functional theory (DFT) calculations. After the analysis of reaction mechanism on these nanoclusters, it is found that the dehydration process is preferable on Pd₅₅, and the dehydrogenation process occurs on Cu_55. Alloying promotes the increase of selectivity of H₂ generation for monometallic Pd system and decreases the activation energy of rate-limiting step for monometallic Cu system. Among these clusters, Pd₄₃Cu₁₂ is of the highest activity for H₂ production from HCOOH decomposition. In addition, the effect of pre-adsorbed H₂O molecule during the whole reaction is discussed. Among all elementary reactions, the COOH1 dehydrogenation is deeply impacted except Pd₅₄Cu₁ in the presence of pre-adsorbed H₂O molecule, which is explained by the elongation of O–H band and the less charge transfer from H to O atom. Our results would shed new light on the design of Pd–Cu nanoalloys for hydrogen generation from HCOOH decomposition.     

Enhanced formic acid dehydrogenation by the synergistic alloying effect of PdCo catalysts supported on graphitic carbon nitride

The catalytic ability of graphitic carbon nitride (g-C₃N₄)-supported composition-controlled PdCo catalysts towards H₂ generation from the formic acid dehydrogenation reaction was assessed in this study and a noticeable composition dependence was evidenced. It was seen that the alloying effect combined with the nitrogen functionalities present on g-C₃N₄ assisted the formation of small and well-distributed nanoparticles. This fact, combined with the electronic promotion of Pd species via charge transfer from Co and basic features of the support, resulted in enhanced catalytic activities compared to that displayed by the counterpart Pd/g-C₃N₄, reaching a TOF value of 1193 h⁻¹ for the most active catalyst among investigated (PdCo/g-C₃N₄ (1/0.7)). Furthermore, the present catalytic system showed high selectivity towards formic acid dehydrogenation, suppressing the generation of undesired CO via formic acid dehydration, which makes it a suitable candidate for practical application in fuel cells.     

Ultrafine palladium nanoparticles anchored on NH2-functionalized reduced graphene oxide as efficient catalyst towards formic acid dehydrogenation

The development of active and stable catalyst is of significance for hydrogen generation from formic acid. Herein, a novel palladium catalyst with ultrafine metallic nanoparticles anchored on NH₂-functionalized reduced graphene oxide (NH₂-rGO) was synthesized by a facile wet chemical reduction process using sodium borohydride as the reducing agent. The TEM and XPS characterization results confirmed the successful functionalization of rGO with 3-aminopropyltriethoxysilane (APTES), which plays a very important role in evenly dispersing ultrafine Pd nanoparticles with a small average size of about 2.3 nm. As a result, the as-prepared Pd/NH₂-rGO catalyst exhibited excellent activity with a high initial turnover frequency of 767 h⁻¹ and 100% hydrogen selectivity, which was predominant among the currently available pure Pd catalysts towards formic acid dehydrogenation under room temperature.     

Amine-functionalized sepiolite: Toward highly efficient palladium nanocatalyst for dehydrogenation of additive-free formic acid

Formic acid has recently come to be considered as a promising material for chemical storage of hydrogen. To date, highly efficient and low-cost heterogeneous catalysts for formic acid dehydrogenation without additive under mild conditions are still limited. In this work, we employed amine-functionalized sepiolite, a typical natural clay mineral with fibrous structure, as support to synthesize a nanocatalyst by the simple impregnation-reduction method. The amine groups on amine functionalized sepiolite support are shown to significantly reduce the average size of Pd nanoparticles (from 7.60 to 2.80 nm) as well as provide a synergistic effect with the Pd nanoparticles, which boosts the rate of additive-free formic acid dehydrogenation (initial turnover frequency (TOF) values of 5587 h⁻¹ at 60 °C, equal to or even better than the most active heterogeneous catalysts). Compared to current catalyst systems, this catalyst can lead to a brighter prospect of cost-efficient heterogeneous catalysts for formic acid dehydrogenation.