Application of microwave heating in the preparation of functionalized activated carbons

As follow from the data presented in Table 1 the starting peat-based activated carbon is characterised by very high C^daf content (94.3 wt%), relatively high amount of mineral matter (7.5 wt%) and small admixture of non-carbon organic components, mainly oxygen. As seen, each of the modifications carried out caused significant changes in the chemical composition of starting material. These changes are most evident in case of AU and AN samples. Sample prepared by reaction with urea followed by thermal treatment at 800 °C (AU) shows the highest nitrogen and simultaneously the lowest oxygen content among the investigated carbon materials. In turn, in structure of AN sample (prepared by means of nitric acid oxidation followed by heat treatment at 600 °C), the lowest carbon content and the highest contribution of oxygen was found. Other variants of thermo-chemical treatment cause a slight increase in the C^daf contribution and slight changes in the content of hydrogen and heteroatoms. These changes are most probably a consequence of gasification of least stable fragments of carbonaceous structure as well as progressing aromatisation of carbon matrix during heat treatment.

According to the data presented in Table 2 all modifications lead also to significant changes in acid-base properties of the peat-based activated carbon. As a result of thermo-chemical treatment the total content of surface functional groups increase, in particular for AN sample, which contain almost three times more groups of acidic character as well as twice more basic surface groups than unmodified carbon (A). In turn, the AT sample, received by means of annealing of starting activated carbon in nitrogen atmosphere is characterised by the highest content (0.93 mmol/g) and simultaneously the greatest prevalence of basic functional groups (almost five times) over acidic ones, what is also confirmed by very high pH value (10.57). This is most probably a consequence of high temperature of annealing (900 °C), which favours formation of greater amount of functional groups of basic nature as well as leads to a decomposition of the majority of acidic groups present on carbon surface. In case of samples subjected to thermal treatment in milder conditions (AH, AU and especially AN) the dominance of basic functional groups is definitely smaller.

As indicated by the data collected in Table 3 and in Figs. 1 and 2, all the samples under investigation show very similar textural parameters, independent on variant of modification. They have medium-developed surface area ranging from 582 to 657 m²/g and porous structure with dominant mesopores. The most developed surface area of 657 m²/g was found for sample AN oxidised with HNO₃ and subjected to thermal treatment in microwave furnace at 600 °C, while the greatest total pore volume of 0.78 cm³/g as well as the most mesoporous character of texture was found for unmodified activated carbon (A). As follows from further analysis of the textural data, modification of starting activated carbon led to generation of some new micropores in its porous structure. This process is most effective in case of AN and AT samples, for which the micropore contribution in total pore volume is by 7 and 4% higher than for unmodified carbon. It is also confirmed by narrower average pore diameter in comparison to AH, AU and particularly to starting activated carbon.

According to the IUPAC classification, the isotherms obtained for all prepared carbons (Fig. 1) belong to type IV, which is characteristic for micro/mesoporous materials. In the range of relative pressure between 0 and ~ 0.45 the adsorption branch of isotherm increase slowly, what indicates the mono- and multi-molecular layer sorption in micro and mesopores. Further increase of relative pressure causes a capillary condensation in mesopores, what is evidenced by wide hysteresis loop, similar to H3 type. In turn, the lack of the horizontal plateau as well as a rapid increase in adsorption capacity at p/p₀ close to 1, suggest that the samples studied have also some amount of macropores. As follows from the course of pore size distribution curves presented in Fig. 2, the main contribution in the porous structure of carbons studied have mesopores with diameters in the range between 3 and 12 nm.

All tested samples were subjected also to thermal and spectroscopic analysis. By using coupled methods, we can get in one analysis, multiple data. Based on thermal studies we get information about thermal stability, how the mass loss is changing during the temperature increase and based on the curve slop we can estimate the tendency of the samples degradation. Activated carbon obtained via physical activation of peat with steam at 800 °C—sample A—is thermally stable up to 545 °C (recorded mass loss 2.5 wt%). Sample AU, which was subjected to impregnation with urea followed by heat treatment at 800 °C has almost identical thermal stability (Figs. 3, 4). Annealing in nitrogen atmosphere at 900 °C or subjecting sample A to oxidation with 25% solution of nitric acid followed by heat treatment at 600 °C enhance the thermal stability, while oxidation with 30% solution of hydrogen peroxide followed by heat treatment at 600 °C diminishes it. But, it should be noted that sample AN, after reaching temperature of 580 °C (recorded mass loss of 2.4 wt%) begins to lose mass very quickly. Nevertheless, the sample annealed in nitrogen atmosphere (AT) is the most stable sample. This is most likely related to the incorporation of nitrogen atoms into the surface carbon layers. Carbons with built-in nitrogen atoms have greater thermal stability (Jankowska et al. 1991). When discussing thermal stability, one should take into account the changes that occur at the beginning of the temperature rise. Figures 5 and 6 shows the changes of intensity of ions m:17 and m:18 which are responsible to the presence of hydroxyl groups and water. Their presence are also visible on Fig. 4, where observed maximum on DTG curves around 40 °C is due to the desorption of the physically absorbed water. By subjecting the material to oxidation process, its surface will acquire a strong hydrophilic character, which will cause strong water sorption. As it shown in Table 1 sample AN is characterized by a high oxygen content, which is also confirmed by MS spectroscopy (Figs. 5, 6) by relatively high content of water and hydroxyl groups. It should be also noted that in the case of sample AU (which was subjected to impregnation with urea), the presence of ions: 17 and 18 can also suggest the presence of ammonia and ammonium.

By looking at the Figs. 3 and 4 it can be certainly stated that the sample AH has the lowest thermal stability. At the very beginning of the temperature rise, a large mass loss of 3.3 wt% till 190 °C is noticeable (DTG curve maximum recorded at 100 °C). At this temperature, the mass loss is no longer so rapid. This mass loss is not only related to the presence to ions: 17 and 18 but also to the presence to ions: 29, 41, 42, 43 (which are only recorded for this sample) (Fig. 7). These ions are most likely responsible for the presence of the following groups: CHO, CH₃CN, NCO(+H). Presented data indicate, that the carbon surface was successfully oxidized. Based on data collected in Fig. 8 it can be concluded, that all tested sample around temperature of 525 °C starts to emit ion: 44, which probably is responsible for the presence of carboxyl group.

The FT-IR spectra of the samples in the region of C–H and –OH groups are presented in Fig. 9. There are no bands indicating the presence of C–H (–CH₂ or –CH₃) groups in the carbon material. The intense band in the 2400–2300 cm^− 1 range indicates the presence of the carbonization product, namely carbon dioxide, which is probably trapped in the carbon structure and not removed during drying. The band of CO₂ is visible also in 700–600 cm^− 1 range (peak at ~ 680 cm^− 1, Fig. 10), but only in AT, AH and AN samples spectra. In the case of other samples, the peak of CO₂ is inverse (2400–2300 cm^− 1) and there is no peak visible at ~ 680 cm^− 1. This is probably due to the fact that there is less CO₂ remained in the pores of these carbon samples that is in carbon black standard (used for background collection).

The fingerprint region is the most suitable for oxygen groups identification (Fig. 10), although all bands in this region have relatively low intensity. Region below 1300 cm^− 1 is difficult to interpret in detail because C–O stretching modes (ethers, esters, alcohols), as well as C–OH bending modes (alcohols, amides) absorb in the same frequency range. The bands below 1050 cm^− 1 may also have inorganic compounds contribution (i.e. silica and other inorganic compounds), which is caused by their presence in the starting material (peat moss). These modes are coupled with other vibrations and do not exhibit well defined group frequencies (Bellamy 1980). Some assignments are nevertheless possible. The bands at 1218, 1145, 1084, 570, 535 and 495 cm^− 1 could be due to C–O and C–O–C in ether structures and/or C=C stretching in aromatic structures (C–C skeleton vibrations) (Gomez-Serrano et al. 1999). The intensities of those peaks are the lowest in the case of A and AU samples. The band at ~ 930 cm^− 1, absent in A and AU samples, may be attributed to N–O stretching in oximes or out-of-plane deformation vibration in –COOH groups (Socrates 2001). Additional band at 1461 cm^− 1, which is present only in AH sample spectra, indicate the presence of C–C=O in-plane deformation vibration of –COOH groups present on the samples surface (Socrates 2001). Unfortunately, this cannot be confirmed by analysing the spectra above 1400 cm^− 1 (not shown) due to the wide, ragged band in the range of 1450–1850 cm^− 1 (hydrogen bonded O–H stretching vibrations; unremoved water). The PAS spectra have also inverse peaks (about 1050–1000 cm^− 1), the presence of which is probably due to the fact that the carbon black standard used for normalizing the PAS spectra has higher intensity of bands in this range than the analysed sample. Nevertheless, the presence of oxygen functional groups in analysed region indicates that the modification with oxidizing agents was effective, although the amount of these groups is very small, most probably as a consequence of further thermal treatment in microwave furnace.