Pt-Ru-based electrocatalyst

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

Some Pt/Ru-based trimetallic electrocatalysts, such as Pt/Ru/Mo, give enhanced catalytic activity leading to a power density, in an elementary single DMFC, at least twice that of Pt/Ru catalyst. 

Pd nanoparticles supported on PANI-NFs are efficient semi-heterogeneous catalysts for Suzuki coupling between aryl chlorides and phenylboronic acid, the homocoupling of deactivated aryl chlorides, and for phenol formation from aryl halides and potassium hydroxide in water and air [493], PANl-NF-supported FeCl3 as an efficient and reusable heterogeneous catalyst for the acylation of alcohols and amines with acetic acid has been presented [494]. Vanadate-doped PANI-NFs and PANI-NTs have proven to be excellent catalysts for selective oxidation of arylalkylsulfides to sulfoxides under nuld conditions [412]. Heterogeneous Mo catalysts for the efficient epoxidation of olefins with ferf-butylhydroperoxide were successfully synthesized using sea urchin-Uke PANI hollow microspheres, constructed with oriented PANI-NF arrays, as support [495]. Pt- and Ru-based electrocatalyst PANI-NFs—PSSA—Ru—Pt, synthesized by the electrodeposition of Pt and Ru particles into the nanofibrous network of PANI-PSSA, exhibited an excellent electrocatalytic performance for methanol oxidation [496]. A Pt electrode modified by PANI-NFs made the electrocatalytic oxidation reaction of methanol more complete [497]. Synthesis of a nanoelectrocatalyst based on PANI-NF-supported... 

Some work was carried to modify Pt-Ru catalysts by adding a third or even a fourth constituent. These Pt-Ru-based ternary or quaternary electrocatalysts such as Pt-Ru-Os, Pt-Ru-W, Pt-Ru-Ni, Pt-Ru-Mo, Pt-Ru-Pb, Pt-Ru02-Ir02, and Pt-Ru-Os-Ir showed improved catalytic activities than Pt-Ru binary catalysts [61, 80-85]. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

In conclusion, the computational study of ternary Pt-Ru-X alloys suggests that future strategies toward more active electrocatalysts for the oxidation of methanol should be based on a modification of the CO adsorption energy of Pt (ligand effect), rather than on the enhancement of the oxophilic properties of alloy components (enhanced bifunctional effect). 

Pt-based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMSC). In direct-methanol fuel cells (DMFCs), aqueous methanol is electro-oxidized to produce COj and electrical current. To achieve enhanced DMFC performance, it is important to develop electrocatalysts with higher activity for methanol oxidation. Pt-based catalysts are currently favored for methanol electro-oxidation. In particular, Pt-Ru catalysts, which gave the best results, seem to be very promising catalysts for this application. Indeed, since Pt activates the C-H bounds of methanol (producing a Pt-CO and other surface species which induces platinum poisoning), an oxophilic metal, such as Ru, associated to platinum activates water to accelerate oxidation of surface-adsorbed CO to... 

Adzic and coworkers proposed a radically new approach in electrocatalysis and catalysis that can alleviate both problems. It is based on a catalyst consisting of only a submonolayer Pt deposited on carbon-supported Ru nanoparticles. The Pt submonolayer on Ru (PtRu2o) electrocatalyst demonstrated higher CO tolerance than commercial catalysts in rotating disk experiments. Tests of the long-term stabihty of the fuel cells detected no loss in perform-... 

Pt has the highest adsorption of methanol on its surface, but its catalytic properties are low due to the formation of poison species (most notably CO) that can be oxidized only after the Pt is covered with OH. Platinum-based bimetallic electrocatalysts, such as Pt-Ru alloys and Ru-decorated Pt materials, are the most active ones. The bi-functional mechanism is to a large extent operative in these catalysts. Most commercial Pt-Ru catalysts are based on 1 1 Pt-Ru alloy. While the alloys typically show enhanced activity in comparison with pure Pt, there is significant Pt loading in the bulk of the alloy in which catalysis does not proceed because the sites are inaccessible for methanol adsorption hence, the need for reducing the Pt content. 

Case Study 3 Surface-doped Pt/Ru/Co Carbon Based on the above-mentioned DFT calculations performed by Norskov [168] we have prepared trimetallic electrocatalysts having PtRu/C surface-doped with Co(0) in order to produce highly active but at the same time CO tolerant electrocatalysts. For example, Pt/Ru/Fe/C, Pt/Ru/Ni/C, and Pt/Ru/Co/C systems were manufactured with the metal ratios being 45 45 10 a/o and a total metal loading of 20 wt.% on Vulcan XC 72. The resulting catalysts were compared with the industrial PtsoRuso standard catalyst under identical conditions. Full characterization was done via a combination of TEM, XRD, XPS, andXAS measurements, further BET, and electrochemical tests [171]. 

Recently it has been reported that Ru addition to PtCo/C results in catalysts with improved tolerance to methanol under typical ORR reaction conditions i.e. potentials more positive than 0.7 V NHE and O2 saturated acid electrolyte. This is because under these reaction conditions, upper oxide Ru species are stable and hinder methanol adsorption. Another interesting alternative is the Ru-based chalcogenides. In particular Ru Sey-based electrocatalysts have received a great deal of attention because of their high tolerance to methanol, even if their performance as electrocatalysts for the ORR is inferior to Pt/C by =40%. ... 

Most recently, Pt-based electrocatalysts with novel nanostructures such as nanowire, nanotube, hollow, core-shell, and nanodendrite structures have been investigated [71-74, 101]. One-dimensional ternary PtRuM (M = Ni, Co, and W) nanowire catalysts were synthesized, and these catalysts outperform Pt-Ru commercial catalyst and have a low noble-metal content due to the incorporation of an Earth-abundant element [101]. 

One should note that poisoning of PEMFC anode catalysts by CO is also a severe problem as CO is found to some extent in most H2 gas supplies, as H2 is usually produced by steam reforming of CH4 (and CO is a by-product). It has been reported that a CO content as low as 10 ppm in H2 fuel will result in the poisoning of Pt electrocatalysts [74], As shown in Eqs. 17.8 and 17.9, the formation of OHads by water oxidation at the Pt surface is necessary for the oxidative removal of adsorbed CO. However, the formation of Pt-OH only occurs appreciably above 0.8 V vs. RHE [75]. This factor is considered to be the origin of the high overpotentials for the MOR and COOR and, often, a second metal that can provide oxide species at low potentials is added to Pt electrocatalysts to reduce such overpotentials. For example, Pt-based alloys containing elements such as Ru, Mo, W, and Sn have been used in attempts to speed up the electrocatalysis of methanol [70,76,77]. The Pt-Ru alloy (1 1 atomic ratio) is the most active binary catalyst and is most frequently used as the anode catalyst in DMFCs [78]. Ru is more easily oxidised than Pt and is able to form Ru-oxide adsorbates at 0.2 V vs. RHE, thereby promoting the oxidation of CO to CO2, as summarised in Eqs. 17.11-17.13 ... 

Commercial application of H2 is generally based on the gas obtained by hydrocarbons reforming and therefore, it is necessary to dispose of an anode electrocatalyst able to tolerate a limited amoimt of CO, either rmder steady state operation or under transient conditions of high CO content. In addition, CO2 adversely affects the catalyst performance, because a reverse gas-shift reaction yielding CO could take place depending on the fuel composition [42]. It seems that Pt-Ru better tolerates CO2 than Pt [43]. The preferred electrocatalysts for HOR in the presence of CO include Pt and several Pt alloys or Pt mixtures with other noble or non-noble metals [44]. The most used include ruthenium, molybdenum and tin. 

The present chapter summarized the fundamental aspects and recent advances in electrocatalysts for the oxidation reactions of H2/CO, methanol, and ethanol occurring at fuel cell anodes emphases were placed on the state-of-the-art Pt-Ru- and Pt-Sn-based catalytic systems. Pt-based catalysts are still considered to be the most viable for the anodic reactions in acidic media. The major drawback however, is the price and limited reserves of Pt. To lower the Pt loading, the core-shell strucmre comprising Pt shells is more beneficial than the alloy structure, since all the Pt atoms on the nanoparticle surfaces can participate in the reactions (and those in cores do not) particularly, the Pt submonolayer/monolayer approach would be an ultimate measure to minimize the Pt content [30-35]. The architectures in nanoscale also have a significant effect on the reactivity and durability [54, 94] and thus should be explored continuously in the future. As for the ethanol oxidation, Rh addition is shown to enhance the selectivity towards C-C bond splitting [70, 71] however, Rh is even more expensive than Pt, and thus less expensive constituents replacing Rh are necessary to be found. 

In alkaline media, Ni-based supports were also explored in conjunetion with PtRu and PtRuMo electrocatalysts [200-202]. Pt Ru compositions between 1.1 1 and 2.1 1 atomic ratio supported on Ni were found to yield the lowest faradaic resistances for the oxidation of 1 M ethanol in 1 M NaOH, determined by eleetrochemical impedance spectroscopy [200]. It was speculated that the role of Ni support extends beyond purely mechanical passive interaction with the catalyst, and Ni could contribute to the electrocatalytic activity by its surface and electronic properties as an oxophilic element. Further studies are required in this area. 

Pt-based electrocatalysts in fuel cells are severely poisoned by the CO intermediate, which is strongly adsorbed on the Pt surface and considerably reduces its electroactivity. It has been widely accepted that the addition of Ru to Pt-based catalysts can promote CO removal by the so-called bifunctional mechanism [160, 161]. However, recent theoretical and experimental studies suggest that Mo-Pt combinations could be not only much cheaper but even more efficient catalysts for CO oxidation than Ru-Pt [162, 163]. To explore more effective metal promoters for CO removal, a fundamental understanding of the adsorption of CO on the metal promoter is very important. UVPS is a powerful surface analysis tool in adsorption studies. It can tell whether the adsorbate species on a solid surface is associative or dissociative, because the contributions of the two species to the valence electronic stracture should be quite different. 

Due to the faeile poisoning effect of CO on Pt, many Pt-based binary alloys, such as Pt-Ru, Pt-Os, Pt-Sn, Pt-W, Pt-Mo, and so on, have been investigated as electrocatalysts for the methanol oxidation reaction (MOR) on flic DMFC anode. Among them, the Pt-Ru alloy has been found to be the most active binary alloy catalyst, and is commonly used in state-of-the-art DMFCs [32]. 

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