Oláh György Doctoral School (BME)

Department of Physical Chemistry and Material Science

Supervisor: Krisztina László

Metal Doped Carbon Gels

Introducing the research area

Carbon aerogels are constructed from covalently bonded nanoparticles. Owing to their special properties (extremely low density, high surface area and porosity, unusual combination of high electrical and low thermal conductivity, good IR absorption) the application potential of these materials is very wide. Their morphology and pore size distribution are tunable via the preparation, and they can be made in different shapes. The principal application fields are: supercapacitor electrodes (104 F/g, 77 F/cm³), adsorbents (approximately 50% porosity), membranes (tunable pore size), catalysts/catalyst supports and solar cells.

My aim was to prepare and characterize carbon gels containing transition metals (copper and titanium) that can be used as membranes (copper) for gas separation or for photocatalytic applications (TiO₂).

Brief introduction of the research place

The scientific activity of the Surface Chemistry Group of the Department of Physical Chemistry and Materials Science focuses on

• preparation, characterization, modification of nanoporous systems;

• investigation of solid/liquid interface phenomena;

• application of rigid and flexible gels in sorption processes;

• surface characterization by gas and liquid adsorption.

History and context of the research

The first organic precursor of carbon aerogels was obtained from the  Na₂CO₃ catalyzed sol-gel reaction of  resorcinol and formaldehyde in aqueous medium [1]. The three-dimensional hydrogel matrix resulting from this polycondensation was transformed into a solid carbon skeleton (carbon gel) (Figure 1).
Organic aerogels are brittle and harder than their inorganic counterparts. The extremely high thermal resistance (six times greater than glass wool) makes them potential insulators. Their outstanding features are the high surface area and pore structure. They can be obtained in various forms, from microspheres to binder-free monoliths (tens of centimeters).

Tuning of the pore structure and the surface area can be achieved by changing the initial conditions (stoichiometry, catalyst, concentration, solvent, temperature) or by the drying process (simple heat treatment, freeze-drying, supercritical extraction) and finally by the carbonization parameters (duration, atmosphere, temperature).
Metal doping may enhance the performance of the carbon gels. The synthesis route of RF gels has the advantage that metals can be introduced at several different stages. With appropriate transition metals, specific properties, such as membrane selectivity or catalytic activity, can be developed and/or improved.

Aim of the research

1. Study of the morphological differences caused by the different drying methods:
   Supercritical CO₂ (scCO₂) drying is the most widely used process to obtain mesoporous RF gels, but no systematic studies of the effect of the conditions were reported previously. The supercritical extraction referred in the literature means an extraction step with liquid CO₂, and CO₂ is transformed to  the supercritical state just before the depressurizing step. As the extraction capacity of scCO₂ is generally better than that of liqCO₂, my intention was to study the parameters of the real supercritical conditions.
   
   

2. Preparation of copper and titania containing carbon gels:
   Copper containing carbon gel can be used for separating CO/CO₂ gas mixtures, while TiO₂ is a well-known photocatalyst in the decomposition of aromatic compounds.
   I intended to study the possibility of doping with copper(II) and titania(IV) salts and their effect on the structure of the carbon gels. The metals were introduced into the systems at three different stages:
   

• during polymerization (1st step)

• impregnation of the dry polymer aerogel (2nd step)

• impregnation of the carbon gel (3rd step).

Methods

Synthesis

The metal-free RF hydrogel was prepared [2] in accordance with the recipe of Lin [3]. The one-week gelation process was performed at 85 °C. For doping, copper-acetate [4] and titanium-isopropoxide [5] were used as metal sources. Prior to drying, the water in the pores of the hydrogels was exchanged for acetone (or t-butanol, in the case of freeze-drying).
The effects of the concentration and the anion of the impregnating solutions on the metal uptake were studied independently on microporous carbons.

Drying

To obtain dry polymer gels from the hydrogel, three methods were employed:

• Heat-treatment [2] in nitrogen atmosphere, resulting in xerogels.

• Freeze-drying [2] at -45 °C to prepare cryogels.

• Aerogels were prepared with drying by CO₂ extraction. Samples were obtained either by the method described in the literature [2] or using true supercritical conditions and continuous CO₂ flow throughout the whole procedure. In the latter case the pressure, temperature and the duration of the drying were studied systematically.

Carbonization
The dried polymer gels were converted to carbon gels in a rotary quartz-reactor, in a one-hour long heat treatment at 900 °C, under nitrogen atmosphere.

Characterization

• Dynamic light scattering (DLS) measurements were made in the sol–gel reaction bath, using an ALV goniometer (ALV, Langen, Germany). A HeNe laser (632.8 nm) was used as the light-source, with an ALV 5000E multi-tau correlator. To avoid multiple scattering, the samples were placed in 1 or 2 mm diameter capillaries. Measurements were performed in the temperature range 30–50 °C.

• Surface morphological investigations were made on a JEOL 5500 (JEOL, Japan) scanning electronmicroscope (SEM). The polymer samples were coated with Au/Pd to prevent charging. Elemental analysis of the samples was performed by the EDS function of the apparatus, in high vacuum mode. The average metal concentration was calculated from at least six measurements.

• Small and wide angle X-Ray scattering (SAXS/WAXS) measurements were made on the BM2 small angle camera in the European Synchrotron Radiation Facility (ESRF), Grenoble, France. An indirect illumination CCD detector was used and the powdered samples were placed in glass capillaries of diameter 1.5 mm.

• The specific surface area and pore volume of the samples were determined from nitrogen adsorption/desorption measurements performed at -196 °C with an Autosorb-1 (Quantachrome, USA) computer controlled volumetric gas adsorption apparatus.

• The photocatalytic activity of the titania-doped samples was tested with phenol-decomposition measurements.
  

Results

Effect of drying on the morphology of the carbon gels

The effects of drying method on the carbon gel morphology were studied. SEM images (Figure 2) illustrate differences arising during drying. These persist after carbonization, yielding carbon xero-, cryo- and aerogels with significantly different morphologies.