Catalysts For Formic Acid Oxidation Research Articles

Screening of Pd-Co-Ni catalyst arrays by scanning electrochemical microscopy and characterization of the potential catalyst for formic acid oxidation

BackgroundAppropriate anodic catalyst is one of the key points in improving the performance of direct formic acid fuel cells (DFAFCs). This study utilizes the concept of a combination method to design and screen anodic catalysts for formic acid oxidation. MethodsThe tip generation-substrate collection (TG-SC) mode of scanning electrochemical microscopy (SECM) was employed to screen the Pd-Co-Ni ternary catalyst array for catalyzing formic acid oxidation. The potential catalyst was further characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Significant findingsThe composition ratios of the prepared Pd-Co-Ni ternary catalyst array matched the concentration ratios set by the precursor solution. SECM screening revealed that the optimal ratio for formic acid oxidation was the Pd2Co6Ni2 catalyst. The cyclic voltammograms and CO stripping voltammograms indicate that the Pd2Co6Ni2 electrode exhibits lower onset potentials for the oxidation of formic acid and its intermediates than the pure Pd electrode. Electrochemical impedance spectroscopy (EIS) confirms that the Pd2Co6Ni2 electrode possesses lower interfacial resistance than the Pd electrode, with its pseudo-inductive behavior at high potential aligning with the outcomes observed in cyclic voltammograms and CO stripping voltammograms.

Pd-M (M = Ni, Co) Bimetallic Catalysts with Tunable Composition for Highly Efficient Electrochemical Formic Acid Oxidation

Bimetallic Pd-based catalysts for formic acid oxidation (FAO) are one of the most promising anode materials for the next generation of direct formic acid fuel cells (DFAFC). It is imperative to develop a simple strategy for preparing efficient, stable, and clean nanoparticle catalysts. Herein, we prepared a series of Pd, PdNi, and PdCo nanoparticle catalysts using the nanoparticle beam composite deposition system, which revealed good catalytic activity and stability in the process of FAO. The incorporation of Ni or Co prevents the adsorption of active intermediates and the accumulation of toxic intermediates in the process of FAO. Therefore, more Pd active centers can be used to decompose formic acid directly by dehydrogenation. The results indicate that PdNi-2 (Pd0.9Ni0.1) and PdCo-3 (Pd0.89Co0.11) catalysts exhibit the optimal catalytic performance, with the mass activity of 1491.5 A g−1Pd and 1401.7 A g−1Pd, respectively, which is 2.1 and 2 times that of the pure Pd sample. By optimizing the rate of Pd to transition metal M (Ni, Co), a high-performance Pd-based catalyst was obtained through their synergistic effect, which provides a new approach for designing efficient anode catalysts for DFAFCs.

Iron- and Cobalt-Doped Palladium/Carbon Nanoparticles as Catalysts for Formic Acid Oxidation

Iron- and Cobalt-Doped Palladium/Carbon Nanoparticles as Catalysts for Formic Acid Oxidation

Enhanced stability of PdPtAu alloy catalyst for formic acid oxidation

Enhanced stability of PdPtAu alloy catalyst for formic acid oxidation

Facile synthesis of heterophase sponge-like Pd toward enhanced formic acid oxidation

The controllable preparation of noble metallic nanocrystals with a 3D architecture is of great interest for electrocatalysis applications due to their enhanced atomic utilization, facilitated mass transport, and other attractive properties. In this work, a facile, one-pot, and surfactant-free method of synthesizing heterophase sponge-like Pd (S-Pd) is developed. The prepared S-Pd has a four-fold higher peak current density and better durability than a commercial Pd/C catalyst for formic acid oxidation. It therefore has great potential as an anode catalyst for direct formic acid fuel cells.

Carbon-Supported Single-Atom Catalysts for Formic Acid Oxidation and Oxygen Reduction Reactions.

The commercialization of fuel cells, especially for direct formic acid fuel cells (DFAFCs) and proton-exchange membrane fuel cells (PEMFCs), is significantly restrained by the high cost, poor stability, and sluggish kinetics of platinum group metals (PGM) catalysts for both the anodic formic acid oxidation reaction (FAOR) and the cathodic oxygen reduction reaction (ORR). Currently, it has confronted with challenges, including exploring highly active, cost-effective, and stable catalysts to replace PGM for DFAFCs and PEMFCs. Recently, the increasing investigation has been focused on the single-atom catalysts (SACs) to enhance the catalytic performance owing to the maximum atom utilization and highly exposed active sites. The aim of this review is to present the recent research activities on carbon supported SACs. At the beginning of the review, metal-based SACs supported on different carbon supports, and the typical characterization methods are introduced. Subsequently, recent advances in metal-based SACs for FAOR and ORR catalysis are scientifically summarized. Particularly, some representative metal-based SACs for ORR activity are further exemplified with a deeper understanding of structure-activity relationships. Finally, the challenges and opportunities of SACs are prospected, such as the mechanism understanding and commercial applications.

Highly Active and Durable Pt-Doped Nanoporous Au-Cu Catalysts for Formic Acid Oxidation

A substantial work has been done in the field of electrocatalysis to address all known drawbacks of Pt-based catalysts and to ultimately improve their activity and durability in the formic acid oxidation (FAO) process. Among the entertained approaches, alloying of Pt with other metals has been key for achieving better performance. Bi-metallic catalysts such as Pt-Ru, Pt-Cu, Pt-Au, Bi decorated Pt, Pt-Pd and Pt-Ag were synthesized in order to achieve better activity by suppressing the dehydration pathway. Alternatively, some researchers focused on studying the application of Pd nanoparticles or Pd based nano-alloys due to palladium’s preference for supporting oxidative formic-acid dehydrogenation to form CO2 instead of CO. However, most of Pd based catalysts feature substantially poorer durability in comparison with Pt based ones. Therefore, in order to tune the electronic surface structure of feasible alloy catalysts and obtain better overall performance, some research groups resorted to the investigation of tri-metallic catalysts like Pd-Cu-Co, Pt-Au-Ru, Pt-Pd-Cu, and Pt-Au-Cu with tunable composition.Inspired by the recent developments of de-alloying in ternary systems, we report herein on the synthesis, characterization and testing of a nanoporous (np) Au-Cu-Pt alloy catalyst with ultralow-Pt loading. Our catalyst has been synthesized by a two-step process including Cu de-alloying from previously co-electrodeposited Au0.27Cu0.70Pt0.03 alloy. As-synthesized np Au-Cu-Pt catalysts have been assessed for overall surface area by Pb underpotential deposition (PbUPD), and for Pt surface content using HUPD paralleled with CO stripping work. Scanning electron microscopy has been employed to image the morphology of as-synthesized and catalytically tested np Au-Cu-Pt films. Energy dispersive spectroscopy has been applied to confirm the synthesized alloy's atomic composition before and after catalytic testing. FAO tests have been conducted by cyclic voltammetry (CV) and chronoamperometry (CA). The CV testing provides information about the mass activity of the alloy catalyst, its overall propensity to CO poisoning and passivation with cycling. The durability of np Au-Cu-Pt catalysts has been assessed by long-term experiments including CA at constant potential (CP) and harsh potential cycling.In the main body of this talk we discuss in detail activity tests showing excellent catalytic performance of prepared catalysts manifested by more than 200 times higher mass activity than commercial Pt/GC counterparts along with remarkable durability in both harsh CV cycling and constant potential (CP) long-term tests. An unique, no-passivation cycling behavior retained for up to either 60 harsh CV cycles or 12 hours of CP testing indicates that our np Au-Cu-Pt catalysts exhibit outstanding CO-poisoning tolerance as well as unseen to date resistance to anodic passivation. Results from the testing runs have been compared critically with best performing Pt and binary Pt-alloy counterpart catalysts. The optimal electronic structure, high surface-to-volume ratio, synergistic effects, ensemble effects, strain effects, and d-band center shift are deemed responsible for the outstanding catalytic properties of the np Au-Cu-Pt catalyst developed in this work.

Oxidation of Formic Acid, Methanol, and Ethanol at Surface-Modified Pt/C Catalysts

Pt-Bi/C and Pt-Pb/C catalysts were prepared by surface decoration of a commercial Pt/C catalyst, and their catalytic activities for electrochemical oxidation of formic acid, methanol and ethanol were compared. The electrocatalytic activity of these catalysts towards formic acid oxidation has been studied by both cyclic voltammetry and chronoamperometry. It was found that after addition of Bi or Pb, the catalytic activity of the Pt/C catalyst for formic acid oxidation increased greatly at low potentials. Bi in particular was found to greatly enhance oxidation of formic acid through the direct pathway in which absorbed CO is not formed, while it strongly inhibited methanol oxidation. It was found that the currents at 0 V vs SCE for formic acid oxidation at the Pt-Bi/C and Pt-Pb/C catalysts were ~ 6 and ~ 2 times higher, respectively compared to the unmodified Pt/C catalyst. In addition, the Pt-Bi/C catalyst showed more stable activity relative to Pt-Pb/C. The Pt-Bi/C catalyst also showed slightly higher activity for ethanol oxidation at low potentials compared to the unmodified Pt/C.

Characterization of Formic Acid Oxidation at Surface-Modified Pt/C Catalysts

Over the last several decades, much attention has been given to low temperature fuel cells which provide energy based on electrochemical oxidation of fuels such as hydrogen and small organic molecules. Many researchers in the field of fuel cell technology have been focusing on the development of cost effective and durable catalysts. Among the small organic fuel molecules, electrochemical study of formic acid oxidation is being intensively investigated for two main reasons; formic acid can be used as a fuel in direct formic acid fuel cells and it can serve as a model reaction that can be used to understand more complex oxidation reactions, such as the oxidation of methanol and ethanol. Platinum is one of the most commonly used and effective catalysts for the oxidation of small organic molecules, despite the fact that it has significant disadvantages, such as high cost and loss of performance due to extreme susceptibility to poisoning by CO. To solve these problems several techniques have been applied, most of which are based on modifying the bulk and/or surface of Pt by a second metal. Heterogeneous bimetallic catalysts often show significantly enhanced catalytic performances because of their improved electronic and chemical properties that are different from their parent metals. Bimetallic catalysts provide opportunities to significantly improved catalytic selectivity, activity and durability. In order to improve the electrochemical oxidation of formic acid, addition of second metals such as Ru, Pb, Os, Sn, Cu, Pd, Rh, Bi has been widely applied. Among them, bismuth especially has received much attention as a Pt surface modifier. Various structures such as PtBi alloy, PtBi intermetallic, and surface modified systems have been reported. Co-deposited, carbon supported PtBi/C [1], under potential deposition [2] and irreversibly absorbed [3] Bi modified Pt surfaces were reported to be good catalysts for formic acid oxidation, in terms of onset potential, activity as well as stability and durability. The primary objective of this research is to obtain mechanistic information on the oxidation of formic acid, methanol and ethanol by comparing the effects of various modifying metals on voltammetric data at surface-modified Pt/C catalysts. For example, Bi was found to greatly enhance oxidation of formic acid through the direct pathway in which absorbed CO is not formed, while it strongly inhibits methanol oxidation. Data for Pb and Rh modified Pt will also be presented.

Effective PtAu nanowire network catalysts with ultralow Pt content for formic acid oxidation and methanol oxidation

Pt is the ideal anode catalyst in fuel cells. In this paper, in order to increase the utilization of Pt, the PtAu nanowire networks (NWNs) with ultralow content of Pt are fabricated by a simple silicon monoxide (SiO) reduction method without any capping agent. PtAu NWNs supported on carbon black with Pt content of 1 wt% (Pt0.05Au NWNs) are employed as catalysts for formic acid oxidation (FAO) and methanol oxidation reaction (MOR), whose mass activities are as high as 4998.9 and 2282.3 mA∙mgPt−1, respectively. The network structure facilitates the electron transfer and increases the stability of the catalysts. The PtAu composite experiences compressive lattice strain as confirmed by X-ray powder diffraction (XRD). The Pt0.05Au NWNs catalyst with low Pt content results in the largest strain variation compared with PtAu composited of other ratios, which may downshift the d-band center of Pt and lead to the higher electrocatalytic activity in oxidation reaction.

Pt Deposits on Bi/Pt NP Catalyst for Formic Acid Oxidation: Catalytic Enhancement and Longer Lifetime.

This work presents an improvement in the activity and catalytic lifetime of Pt deposits on Bi-modified Pt nanoparticles (Bi/Pt NP) toward formic acid oxidation (FAO). Using an irreversible adsorption method, Bi was deposited on Pt NP to form Bi/Pt NP and sequentially Pt was deposited on Bi/Pt NP to form Pt/Bi/Pt NP. Voltammetric studies of Pt NP, Bi/Pt NP, and Pt/Bi/Pt NPs supported that Pt deposits of Pt/Bi/Pt NPs provided quite a unique behavior: simultaneous surface oxidation of deposited Pt and Bi and significant resistance to the oxidative removal of Bi. Furthermore, combined spectroscopic investigations revealed that the concentration of the employed Pt precursor ion solution determined the amount of deposited Pt from ∼0.2 to ∼0.4 in coverage. The best Pt/Bi/Pt NP catalyst with a Pt coverage of ∼0.25 enhanced the dehydrogenation processes below ∼0.4 V by a factor of more than 2 and increased the FAO current at ∼0.8 V roughly by 15 times, referring to those of Bi/Pt NP. The lifetime measurement works revealed that after the 1000th voltammetric cycle to 0.4 V, the FAO currents of Pt/Bi/Pt NPs were 2 and 4 times higher than those of Bi/Pt NP and Pt NP, respectively. The Pt deposits on Bi/Pt NP were concluded to play two roles in FAO: the promotion of FAO processes to increase the activity and the retardation of Bi oxidative removal to maintain the activity much longer.

Platinum and Palladium Monolayer Electrocatalysts for Formic Acid Oxidation

The direct formic acid fuel cell holds great promise as a next generation portable power source. Here we report an experimental study on Pt and Pd monolayer (Pt* and Pd*) atop the close-packed facet of transition metal substrates (denoted Pt*/M or Pd*/M, where M = Pt, Pd, Ir, Au, Rh and Ru) model catalysts for formic acid oxidation (FAO) and demonstrate a promotional effect on Pt*/Au(111) and Pd*/Pt(111) surfaces. Contrary to findings on monometallic surfaces, in situ infrared reflection adsorption spectroscopy collected during formic acid oxidation at potentials anodic of 375 mV reveal that the poisoning reaction intermediate CO is not formed on the monolayer surfaces, suggesting a promotion of the direct mechanisms of FAO at the expense of the indirect mechanism through CO. Our density functional theory calculations on model surfaces with 1:1 stoichiometry for overlayer to support metal atoms for the thermochemistry of the elementary steps defined by adsorbed CO, OH, COOH, and HCOO on the (111) facet of Pt, PtML/Au, Pd, and PdML/Pt largely attribute the promotional effect observed for the monolayer catalysts to easier removal of CO brought about by easier activation of water, relative to the monometallic catalysts. This synergy between experiments and theory brings us closer to a fundamental understanding of FAO electrocatalysts under reaction conditions, paving the way to achieving more active electrocatalysts possibly with less Pt or Pd; a pre-requisite for the mass commercialization of this technology.

Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation

To meet the requirements of potential applications, it is of great importance to explore new catalysts for formic acid oxidation that have both ultra-high mass activity and CO resistance. Here, we successfully synthesize atomically dispersed Rh on N-doped carbon (SA-Rh/CN) and discover that SA-Rh/CN exhibits promising electrocatalytic properties for formic acid oxidation. The mass activity shows 28- and 67-fold enhancements compared with state-of-the-art Pd/C and Pt/C, respectively, despite the low activity of Rh/C. Interestingly, SA-Rh/CN exhibits greatly enhanced tolerance to CO poisoning, and Rh atoms in SA-Rh/CN resist sintering after long-term testing, resulting in excellent catalytic stability. Density functional theory calculations suggest that the formate route is more favourable on SA-Rh/CN. According to calculations, the high barrier to produce CO, together with the relatively unfavourable binding with CO, contribute to its CO tolerance.

Ultrathin Film PtxPd(1-x) Alloy Catalysts for Formic Acid Oxidation Synthesized by Surface Limited Redox Replacement of Underpotentially Deposited H Monolayer

This work emphasizes the development of a green synthetic approach for growing ultrathin film PtxPd(1-x) alloy catalysts for formic acid oxidation (FAO) by surface limited redox replacement of underpotentially deposited H sacrificial layer. Up to three-monolayers-thick PtxPd(1-x) films with different composition are generated on Au electrodes and characterized for composition and surface roughness using XPS and electrochemical methods, respectively. XPS results show close correlation between solution molar ratio and atomic composition, with slightly higher Pt fraction in the deposited films. The accordingly deposited Pt42Pd58 films demonstrated remarkable specific and mass activities of up to 35 mAcm−2 and 45 Amg−1 respectively, lasting for more than 1500 cycles in FAO tests. This performance, found to be better twice or more than that of pure Pt counterparts, renders the Pt42Pd58 films comparable with the frontrunner FAO catalysts. In addition, the best alloy catalyst establishes a nearly hysteresis-free FAO CV curve a lot earlier than its Pt counterpart and thus supports the direct FAO pathway for longer. Overall, the combination of high Pd activity and CO tolerance with the remarkable Pt stability results in highly active and durable FAO catalysts. Finally, this facile and cost-effective synthetic approach allows for scaling the catalyst production and is thus appropriate for foreseeable commercialization.

Heteroatom doped 3D graphene aerogel supported catalysts for formic acid and methanol oxidation

In this study, nitrogen (N) and boron (B) heteroatom doped graphene aerogel support materials have been employed for the dispersion of platinum (Pt) nanoparticles to improve their electrocatalytic activities for formic acid and methanol oxidation. Pt nanoparticles dispersed on the heteroatom doped graphene aerogel (GA) support materials by a microwave heating method. The as-prepared catalysts were characterized by a variety of means such as SEM, EDS, ICP-MS, TEM, XRD, BET and XPS. The electrocatalytic activities, stability and impedance of the synthesized catalysts were investigated for formic acid and methanol oxidation using electrochemical measurements. The 3D graphene aerogels have higher capacitive currents than the Vulcan XC-72 in the double layer region. The results of electrochemical chronoamperometry tests reveal that Pt/BGA shows the best stability for methanol oxidation and also exhibited superior electrocatalytic activity towards the oxidation of methanol in cyclic voltammetry. In addition to, heteroatom doped GA supported catalysts higher activity compared to the Vulcan XC-72 supported catalyst for formic acid oxidation.

Ultralow Pt loading nanoporous Au-Cu-Pt thin film as highly active and durable catalyst for formic acid oxidation

Ultralow Pt loading tri-metallic nanoporous (np) Au-Cu-Pt catalysts for formic acid oxidation (FAO) have been synthesized by electrodeposition of Au0.27Cu0.70Pt0.03 alloy thin film, followed by dealloying for oxidative Cu removal. Underpotential deposition and CO stripping are used for measuring the electrochemical active surface (overall and Pt fraction) of synthesized catalysts and SEM is performed for characterization of their morphology. Also, EDS and inductive coupled plasma are used for compositional analysis. Potential cycling FAO tests show excellent performance of these catalysts manifested by up to 250(mgPt) and 17(mgPt+Au) times higher mass activity (at 0.0V/MSE) than commercial Pt/C(40%) nanoparticulate counterparts. This is accompanied by remarkable durability in both cycling and constant potential tests. A no-hysteresis voltammetry for 55 cycles of synthesized np Au-Cu-Pt catalysts demonstrates unseen to date resistance to anodic passivation along with outstanding CO-poisoning tolerance. A combination of electronic, ensemble, and synergistic effects is believed to enable catalysts’ outstanding performance.

Ultrafast synthesis of uniform 4–5 atoms-thin layered tremella-like Pd nanostructure with extremely large electrochemically active surface area for formic acid oxidation

Herein, a uniform 4–5 atoms-thin layered tremella-like Pd nanostructure with extremely large electrochemically active surface area is ultrafast-synthesized in 1 min by a one-pot surfactant-free route at room temperature, delivering 4.2 times higher peak current density and much faster electron transfer than commercial Pd/C toward formic acid oxidation thus becoming the highest catalytic activity over the reported Pd-based nanosheet catalysts in terms of mass activity and peak potential. The enhancement mechanism of this unique architecture indicates that ultrathin layered and interconnected “clean” structure offers modified Pd electronic properties, extremely large active surface area and high conductivity for fast electrode kinetics. This work has universal significance to provide a facile and fast method to synthesize a unique nanostructure for highly active electrocatalysts, while holding great promise for an active surfactant-free Pd catalyst for formic acid oxidation.

Surfactant-free room temperature synthesis of PdxPty/C assisted by ultra-sonication as highly active and stable catalysts for formic acid oxidation

Abstract Formic acid oxidation is usually catalyzed on PdPt bimetallic catalysts, which are synthesized by co-reduction of noble metal precursors in the presence of high molecular capping agents. In this work, surfactant-free PdxPty/C catalysts are synthesized by H2 reduction in ethylene glycol assisted with ultrasonication vibration at room temperature. Nanoparticle agglomeration in the course of preparation has been sufficiently curbed by strong mechanical ultrasonication instead of traditionally-employed surfactants. As a result, “clean” surfactant-free PdxPty/C catalysts necessitate only simple washing before collection. The catalysts are characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The compositionally optimized Pd100Pt1/C catalyst registers a mass activity of 3171 A g−1 (Pt + Pd) for formic acid oxidation in 0.5 M H2SO4+0.5 M HCOOH, which lists one of the best results reported so far and surpasses that of a commercial Pd/C by 5.6 times. Stability of the catalysts is investigated by cyclic voltammetric as well as chronoamperometric evaluations. This work offers a convenient and environmentally benign room-temperature route to synthesize highly active and stable catalysts for formic acid oxidation.

Effective immobilization of nanoscale Pd on a carbon hybrid for enhanced electrocatalytic performances: stabilization mechanism investigations.

The grand challenge inhibiting the use of electrocatalysts is the degradation of active species which results in poor durability and long-term performances. Studying the origin of active metal particle stabilization mechanisms by using supports and the immobilization-induced changes of active particles is of significant importance. This study describes the preparation of Pd nanoparticles supported by carbon hybrid NPG-CN, revealing that the mass and specific activities (1987 A g-1 Pd and 28.7 A m-2) of this catalyst for formic acid oxidation significantly exceed those of commercial Pd/C, and excellent stability and enhanced CO-poisoning tolerance properties are obtained. The origin of this behavior is probed by surface analytical techniques and identical-location transmission electron microscopy (IL-TEM), and the enhanced activity of Pd/NPG-CN is ascribed to the electronic effect of the substrate, the high content of surface metallic Pd0, and the reduced extent of active Pd leaching and physical ripening during the FOA process compared with commercial Pd/C. In addition, theoretical calculations demonstrate that NPG-CN can efficiently trap Pd atoms, which accumulate and form Pd clusters at trapping (nucleation) sites.

Facile fabrication of stable PdCu clusters uniformly decorated on graphene as an efficient electrocatalyst for formic acid oxidation

Small-sized nanocrystals uniformly distributed on suitable supports are promising as active electrocatalysts due to their unique electronic structure and high accessible surface area. Herein, we report the synthesis of graphene supported PdCu nanoclusters with a uniform size of 2.3 nm via a facile one-pot solvothermal method. This nanohybrid exhibits much higher inherent catalytic activity and stability for formic acid oxidation in comparison to graphene supported Pd clusters and commercial Pd/C. The PdCu nanoclusters can maintain their original size and dispersity on graphene even after 200 CV cycles. This work offers an efficient low Pd-loading catalyst for formic acid oxidation, and the synthetic approach is of significance to preparing uniform metal-based clusters decorated on supports with superior catalytic properties for fuel cells and sensors.