Pt–Ru catalysts are well known for their high activity towards the electro-oxidation of methanol [1
]. Nevertheless, drawbacks such as slow oxidation kinetics and methanol crossover make the efficiency of the Direct Methanol Fuel Cells (DMFC) still insufficient for practical applications [1
]. Therefore, further optimizations of the anode material and the membrane are necessary for the development and commercialization of DMFC. In this context, an attractive approach for the anode, which appears as a possible solution to reduce metal loading and increase the catalytic efficiency, is the use of novel carbonaceous materials as electrocatalyst supports [6
]. The nature of the support, as well as the interaction between the latter and the metal, has been demonstrated to be extremely important, given that it determines the physico-chemical properties of catalysts, such as dispersion, stability and morphology of metallic crystallites [10
]. In addition, characteristics of the support can also determine the electrochemical properties of catalysts by altering mass transport, active electrochemical area and metal nanoparticle stability during the cell operation [13
Among the numerous new carbon materials that can be found in the literature, carbon xerogels, cryogels and aerogels constitute an interesting alternative to carbon blacks [15
]. These materials are obtained either by supercritical drying or evaporative drying of organic gels, followed by pyrolysis. Their texture is fully controllable within a wide range of pore sizes and distribution via the synthesis process of the organic gel [14
]. The use of carbon gels as catalysts supports has been previously reported. Catalysts supported on carbon gels (aerogels, cryogels and xerogels) showed higher activities towards methanol oxidation and oxygen reduction, in comparison to catalysts supported on commercial carbon blacks, such as Vulcan [12
]. Vulcan XC-72R, with a surface area of ca.
, has been commonly used as a catalyst support, especially in DMFC anode catalyst preparation. However, an accessible and sufficiently large surface for maximum catalyst dispersion has been argued to be a necessary but insufficient condition for obtaining optimized carbon-supported catalysts. First of all, Vulcan has a preponderance of small pores that cannot be filled with polymer molecules. This portion inside the micropores has less or even no electrochemical activity due to the difficulty in reactant accessibility. Besides, the poor surface chemistry of this carbon material makes its impregnation with the metallic precursor difficult.
Some studies have dealt with different preparation methods of catalysts onto this kind of supports. Arbizzani et al.
developed PtRu catalysts, prepared by both chemical and electrochemical routes, on mesoporous cryo- and xerogel carbons [18
]. Their results were compared to those obtained with Vulcan-supported PtRu, resulting in almost double specific catalytic activity when Vulcan was substituted by the former carbons. Job et al.
reported the use of the ‘Strong Electrostatic Adsorption’ (SEA) method to prepare Pt/carbon xerogel catalysts, exhibiting high Pt dispersion at high metal content [15
]. Figueiredo et al.
prepared Pt catalysts supported on carbon xerogels by impregnation with H2
, studying the effect of different reduction protocols [12
Carbon xerogels have also been employed as catalyst supports in previous studies of our group [19
]. Carbon xerogels were used as support for Pt and PtRu nanoparticles, synthesized by an impregnation and reduction with sodium borohydride method. Catalysts performed higher activities than commercial catalysts Pt/C, ETEK and PtRu/C, ETEK, that are supported on Vulcan carbon black [19
]. In another paper, we reported the synthesis of two carbon xerogels of different textural properties, which were subsequently functionalized through several oxidation treatments. These carbon xerogels were used as supports in the preparation of several Pt catalysts which were characterized and tested for CO and methanol electro-oxidation, performing higher activities than Pt supported on Vulcan [20
]. In another work, PtRu catalysts were prepared using a highly mesoporous carbon xerogel submitted to different oxygen functionalization treatments: diluted and concentrated nitric acid as well as gas-phase 5% O2
oxidation. Catalysts with 20 wt% loading and equimolar Pt:Ru metallic phase were prepared using an impregnation procedure involving chemical reduction with formic acid. Catalysts supported on the carbon xerogel presented higher activities towards methanol oxidation than the catalyst supported on Vulcan prepared by the same procedure [21
]. In comparison to the commercial carbon black Vulcan, carbon xerogel doubles the SBET
value determined for Vulcan. Such features favour diffusion of reagents and products to and from active sites when using carbon xerogels as catalysts supports, instead of Vulcan, making catalysts more active. Although catalysts supported on carbon xerogels showed higher performances than when supported on Vulcan, not proper dispersion was achieved in these works for any of the methods assayed (impregnation and reduction with different reduction protocols), pointing out the need for further research in synthesis methods providing low crystallite size and high metallic dispersion.
In general, the Pt/carbon gel catalysts are classically obtained via deposition from the liquid phase; in most cases, impregnation of the support by H2
solutions is used and followed by various post-treatments, such as liquid phase reduction or drying followed by gas phase reduction under hydrogen [15
]. Nevertheless, it has been noticed that the presence of chloride ions during the deposition can have a negative effect on the later performance of the catalysts for methanol oxidation [15
]. In this paper a sulfite-complex based method is used for the first time for carbon xerogels, in order to avoid the use of chloride species. This preparation method presents an advantage over the straight reduction of chloride salts since no chloride ions are present during the deposition of the metals onto the support. Further, given that this method leads to small metallic particles, with a high dispersion, two thermal treatments at different temperatures were carried out, in order to slightly increase crystal and particle size, favoring catalytic activity towards methanol oxidation reaction (MOR).
3. Experimental Section
3.1. Carbon Xerogel Synthesis
CXG was synthesized as described in [27
] by the pyrolysis at 800 °C of an organic gel obtained by the polycondensation of resorcinol and formaldehyde in stoichiometric ratio (2 mol of formaldehyde per mol of resorcinol). The gelation and curing process took place at an initial pH of 6.0 and using sodium carbonate as catalyst (0.04 mol% with respect to total content of resorcinol + formaldehyde). Curing of the organic gel was carried out for 24 h at room temperature, 24 h at 50 °C and 120 h at 85 °C. Subsequently, remaining water was exchanged with acetone and the gel was dried under subcritical conditions before its pyrolysis. Pyrolysis took place at 800 °C under a nitrogen atmosphere for 3 h.
3.2. Catalysts Preparation
PtRu nanoparticles were deposited on the synthesized carbon xerogels by the sulfite complex method (a type of colloidal method) never reported before for carbon xerogels. A 20 wt.% nominal metal concentration on CXGs was chosen. Sulfite complexes of Pt and Ru, in appropriate amounts, were decomposed by hydrogen peroxide to form aqueous colloidal solutions of Pt-Ru oxides. These particles were adsorbed on CXGs. The amorphous oxides on CXGs were thus reduced in a hydrogen stream to form metallic particles. The reduction process was considered complete when no significant H2 consumption was detected in the outlet stream by using a thermal conductive detector (TCD). Two aliquots of this catalyst were further treated in hydrogen atmosphere at 200 °C and 400 °C for 1 h, with the aim of evaluating the effect of this thermal and reducing treatment in the features of the catalysts, mostly in terms of increased metallic crystal size. These catalysts were named PtRu/CXG-COL-TT200 and PtRu/CXG-COL-TT400, respectively.
3.3. Physico-Chemical Characterization
The textural and morphological features of the different carbon supports and catalysts prepared were determined by means of nitrogen physisorption at −196 °C (Micromeritics ASAP 2020). Textural properties such as specific surface area, pore volume and pore size distribution were calculated from each corresponding nitrogen adsorption-desorption isotherms applying the Brunauer-Emmet-Teller (BET) equation, Barrett-Joyner-Halenda (BJH) and t-plot methods. Thermogravimetric complete oxidation in air of both the carbon support and PtRu catalysts was used to determine the total amount of metal deposited, in a Setaram Setsys evolution thermogravimetric analyzer at atmospheric pressure, with a temperature program from room temperature to 950 °C with a constant rate of 5 °C min−1. X-ray fluorescence (XRF) measurements were also used to determine the Pt:Ru atomic ratio, by using a Bruker AXS S4 Explorer spectrometer. Catalysts were as well characterized by X-Ray Diffraction (XRD), using a Bruker AXS D8 Advance diffractometer, with a θ-θ configuration and using Cu-Kα radiation. Crystallite sizes were calculated from the Scherrer’s equation on the (220) peak for platinum. X-ray photoelectron spectrometry (XPS) analysis were performed using a ESCAPlus Omicron spectrometer equipped with a Mg (1253.6 eV) anode, 150 W (15 mA, 10 kV) power, over an area of sample of 1.75 × 2.75 mm. C 1s (280–295 eV), O 1s (526–540 eV) and Pt 4f (65–84 eV) signals were obtained at 0.1 eV step, 0.5 s dwell and 20 eV pass energy. Spectra were deconvoluted using CasaXPS software. Particle sizes were evaluated from TEM images obtained in a JEOL 2100F microscope operated with an accelerating voltage of 200 kV and equipped with a field emission electron gun providing a point resolution of 0.19 nm. The standard procedure involved dispersing 3 mg of the sample in ethanol in an ultrasonic bath for 15 min. The sample was then placed in a Cu carbon grid where the liquid phase was evaporated.
3.4. Electrochemical Experiments
A cell with a three-electrode assembly and an AUTOLAB potentiostat-galvanostat were used to carry out the electrochemical characterization. The counter electrode consisted on a pyrolytic graphite rod, while the reference electrode was a reversible hydrogen electrode (RHE). Therefore, all potentials in the text are referred to the latter. The working electrode consisted of a pyrolytic graphite disk (7 mm) with a thin layer of the electrocatalyst under study deposited onto it. For the preparation of this layer, an aqueous suspension consisting of 3.6 mg of PtRu/CXG catalyst was obtained by ultrasonically dispersing it in Nafion solution 10% w/w (Sigma-Aldrich, St. Louis, MO, USA) (14.7 μL) and a mixture of ultrapure water (240 μL) (Millipore) and ethanol (240 μL) (Merck). Subsequently an aliquot of 40 μL of the dispersed suspension was deposited on top of the graphite disk and dried under inert atmosphere prior its use.
Polarization curves were performed to study the electro-oxidation of methanol, in a 2 M CH3OH + 0.5 M H2SO4 solution, at scan rate of 20 mV·s−1, between 0.05 and 0.8 V vs. RHE. Chronoamperometries were performed at 0.60 V vs. RHE in a 2 M CH3OH + 0.5 M H2SO4 solution, in order to evaluate the evolution of the electrocatalytic activity with time of the prepared catalysts in the electro-oxidation of methanol. All the experiments were carried out at room temperature, and current was normalized with respect to each catalyst metal amount (A/g PtRu).
PtRu nanoparticles were deposited on a highly mesoporous carbon xerogel for the first time by a sulfite complex method. Thermal treatments at 200 °C and 400 °C in H2 for 1 h were carried out, in order to increase the crystal size. This sulfite complex method led to catalysts with low crystal sizes (from 1.6 to 2.0 nm). Thermal treatment proved to be effective increasing the catalysts crystal size and the extent of metallic phase reduction.
It was observed, by means of XRF and XPS, that Pt segregated towards the surface of the metallic crystallites deposited on the carbon xerogel.
A certain extent of pore blockage was observed upon the loading of the active phase, but catalysts still maintained the initial mesopore-enriched structure of the carbon xerogel.
Methanol electro-oxidation was found to be dependent mainly on the crystal size and the extent of reduced metals (Pt0 and Ru0) on the composition of the catalyst. The most active catalysts were those treated at 400 °C, PtRu/CXG-COL-TT400, with the highest crystal size and the highest amount of reduced metals. The high segregation extent of Pt towards the surface of the particles/crystallites deposited, on the surface of the carbon xerogel, may have resulted in an optimal combination of Pt and Ru atoms enhancing the progress of the different controlling steps of methanol electro-oxidation mechanism at room temperature; starting from methanol dehydrogenation and completing the oxidation of the intermediate COads species by means of nearby OHads on Ru sites.