Elsevier

Carbon

Volume 42, Issue 15, 2004, Pages 3217-3227
Carbon

Synthesis of transition metal-doped carbon xerogels by solubilization of metal salts in resorcinol–formaldehyde aqueous solution

https://doi.org/10.1016/j.carbon.2004.08.013Get rights and content

Abstract

The pore texture of carbon materials obtained from evaporative drying and pyrolysis of resorcinol–formaldehyde aqueous gels is controlled by the initial pH of the precursors solution. In order to produce transition metal-containing carbons with tailored texture, various metallic salts were dissolved in the precursors solution. When necessary, a complexing agent (HEDTA or DTPA) was added to render the metal ions soluble. Ni, Fe and Pd loaded carbon xerogels were synthesized and their pore texture was studied after evaporative drying and after pyrolysis. The carbon texture was also studied with regard to the nature of the metal and the amount of complexing agent. The solubilization of transition metal salts in the resorcinol–formaldehyde aqueous solution does not prevent the texture regulation, even though this texture control is influenced: the limits of the pH interval leading to micro–mesoporous carbon materials can slightly differ when a metal salt and/or a complexing agent are added. The pH range shift depends mainly on the amount and nature of the complexing agent, but also slightly on the nature of the metal ion. Nevertheless, the metal particles obtained are rather big (diameter > 15 nm). For catalytic applications, the metal dispersion must be enhanced, especially in the case of expensive metals.

Introduction

Porous carbon materials prepared by sol–gel process from hydroxylated benzene and aldehyde in a solvent have been extensively studied for the past 10 years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] and used in electrochemical applications [7], [8], [9]. This type of material is usually synthesized by polycondensation of an hydroxylated benzene (phenol, catechol, resorcinol, hydroquinone or phloroglucinol …) with an aldehyde (formaldehyde, furfural …) in a solvent. Drying and pyrolysis of the gels obtained lead to various carbon materials whose texture depends on the nature of the precursors, the gelation conditions and the drying method.

The most common precursors are resorcinol and formaldehyde and the polymer is usually synthesized using water as solvent and Na2CO3 as catalyst [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Though the most commonly used drying method is the CO2 hypercritical drying process, still very porous carbon materials can be produced by simple evaporative drying and pyrolysis of resorcinol–formaldehyde aqueous gels whose synthesis variables are accurately chosen. Moreover, the pore texture can be easily tailored [17], [18], [19]. Indeed, the operating variable that mainly controls the pore texture of the carbons is the initial pH of the resorcinol–formaldehyde solution which can be fixed at will by addition of any base provided that no secondary reaction occurs. No pre-treatment is needed before the drying process, and the gels can be dried by vacuum drying or under gas flow. The carbon material obtained can be totally non-porous, or display very high specific surface area and pore volume depending on the pH chosen, all the other synthesis variables being fixed. For example, under well defined synthesis conditions [19], the carbons are non-porous after evaporative drying and pyrolysis under nitrogen flow at 800 °C when the pH is higher than 6.25 (±0.05). Under the same synthesis conditions, and for pH ranging from 6.25 to 5.50, the materials are micro–mesoporous and possess a good mechanical resistance. The specific surface area remains almost constant (about 600 m2/g) and the total void volume increases when the pH decreases (from 0.40 cm3/g to 1.40 cm3/g). This corresponds to the decrease of the voids between the carbon particles that constitute the mesopores volume. When pH is lower than 5.50, the material becomes rather crumbly because of the development of macropores, and loses its mechanical strength.

The texture control of the carbon materials obtained by this chemical method is very interesting for the synthesis of carbon supported metal catalysts. Carbons commonly used as catalysts supports are usually active charcoals produced from natural sources. Their pore texture and composition depend then strongly on the raw material chosen (wood, nutshells, pits …). Moreover, active charcoals are mainly microporous, whereas carbon xerogels can contain mesopores. The presence of mesopores should enhance mass transfer and allow to minimize diffusion limitations in catalysts synthesized by sol–gel process.

Previous attempts to incorporate metals in carbon aerogels, prepared by hypercritical drying, have shown that the material texture is modified by the addition of metal salts in the precursors solution [14], [15], [16]. As metal salts often possess acido-basic properties, their addition should modify the pH, which can fall beyond the limits of the pH range leading to high pore volumes and specific surface areas. The pH must then be fixed within the adequate range at the beginning of the polymer synthesis. The present work shows that the material texture is still controllable when a metal is incorporated into the gel by dissolution of a salt in the precursor solution. In order to synthesize carbon supported metal catalysts, Pd, Fe or Ni salts were incorporated to the resorcinol–formaldehyde aqueous solution. When necessary, a complexing agent was added to render the metal cation soluble and to keep the metal dissolved during gelation. The texture of the carbon materials obtained after evaporative drying and pyrolysis at 800 °C under nitrogen flow was studied with regard to the pH chosen. Two other samples series were synthesized with the same initial pH in order to study the influence of the complexing agent and of the metal salt on the carbon texture: one containing no metal but various complexing agent amounts and another containing the same complexing agent amounts but various metal salts. Texture analysis shows that both complexing agent and metal salt have an influence on the carbon texture.

Section snippets

Synthesis of carbon supported metals

Aqueous metal salts solutions were first prepared. These solutions were used as a solvent for the polycondensation of resorcinol with formaldehyde. Acetate was chosen as counter-ion because its degradation during pyrolysis treatment leaves no ashes inside the carbon material and mainly produces water and carbon oxides.

Some salts can be dissolved easily and kept dissolved during the gel synthesis (nickel acetate for example). But in many cases, the addition of a complexing agent proved to be

Texture analysis

Fig. 1a shows the nitrogen adsorption–desorption isotherms of nickel loaded resorcinol–formaldehyde gels after drying (series Ni). Fig. 1b shows isotherms related to the same series of samples after pyrolysis at 800 °C under nitrogen flow.

After drying, as pH increases from 6.00 to 7.00, the Ni loaded samples evolve from micro–macroporous material (combination of type I and type II isotherms) to micro–mesoporous material (combination of type I and type IV isotherms). One can assume by

Discussion

According to literature, the resorcinol–formaldehyde polymerisation mechanism includes two steps: (i) formation of resorcinol anions by hydrogen abstraction (enhanced by OH) and formaldehyde addition to obtain hydroxymethyl derivatives (mono, di- and tri-substituted methylolphenols); (ii) condensation of the hydroxymethyl derivatives and clusters growth (catalysed by H+). These two distinct steps explain the pH dependence of the final texture. This mechanism is questionable in acidic

Conclusion

The pore texture obtained after evaporative drying and pyrolysis of resorcinol–formaldehyde aqueous gels is controllable by the pH of the aqueous solution [19]. This texture is still adjustable when a metal (and a complexing agent if necessary) is introduced in the gel. Ni, Fe and Pd loaded carbons with controlled texture were synthesized by solubilization of metal acetates in resorcinol–formaldehyde solutions. HEDTA and DTPA were used to stabilize iron or palladium under oxidized state during

Acknowledgments

The authors thank the Belgian Fonds National de la Recherche Scientifique, the Région Wallonne––Direction Générale des Technologies, de la Recherche et de l’Energie––, the Ministre de la Communauté française––Direction de la Recherche scientifique and the Fonds de Bay for their financial support.

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