Nanoporous carbons obtained by carbonization of copolymers impregnated by salts

Using the same mixture of salts, the same way of impregnation, and identical carbonization conditions but different polymeric precursors, it was possible to evaluate the importance of polymer structure for porosity formation in the final products. Also influence of the salts (which was assumed to be the same in all cases) on their porous structure was possible to observed.

Figure 2 presents the changes in porous structure after carbonization of the polymers impregnated by the salts (NiSO₄, AgNO₃, and Gd(NO₃)₃). In all cases, shrinkage of polymer pores characteristic for carbonization was observed. This effect was the strongest for GLY-DVB copolymer, that did not possess elements stabilizing its structure during carbonization. The PSD curve for this carbon shows two well separated regions. The first one, connected with the presence of micropores, is narrow (from 15 to 20 Å), while the second is wide and shows the presence of mesopores.

Shrinkage of NAF-DVB copolymer was smaller. This phenomenon could be explained by the fact that the naphthalene rings stabilized polymer structure and prevented its contraction. Probably this is one of the reasons, why the obtained carbon is thoroughly mesoporous.

BM-DVB was the polymer with very stable porous structure (aromatic rings and nitrogen atoms) consequently after carbonization many of pores were retained. Pore size distribution curve for the carbon C-BM-NiAgGd indicated multimodal system of pores. In case of this material, the influence of the salts mixture on porous structure formation is the most strongly noticed.

Quantitative data of percentage contributions of micro- and mesopores and other parameters characterizing porous structure of the precursor copolymers and all the carbonaceous materials presented in this work are collected in Table 1.

According to the works of Maneva et al. (1990), Kwon et al. (2005), Melnikov et al. (2012) thermal decomposition of the NiSO₄, AgNO₃, and Gd(NO₃)₃ takes place in the range of 230 to 800 ^∘C and leads to releasing reactive gaseous products such as steam, SO₃, NO₂, N₂O₅, and O₂. These gases possess oxidative properties and they are able to interact with the polymeric precursors, which decompose in similar range of temperatures (300–550 ^∘C). The proceeding processes affect the formation of porous structure and functional groups on the surface.

Since the process of impregnation is carried out in aqueous solution, Ag₂SO₄ can precipitate in porous structure of the copolymers. This process causes blocking of porous structure by precipitating settlings and partially prevents of the pores collapse. However, activation with silver sulphate has no practical significance, as thermal decomposition of Ag₂SO₄ takes place at about 1050 ^∘C, that is much higher than maximum temperature of the carbonization in these studies. For that reason SO₂ and O₂ as gases released during decomposition of Ag₂SO₄ can be neglected. Detailed studies of this reaction was given by Savarino et al. (2001).

FTIR spectra for the analyzed carbons, presented in Fig. 3, are similar to each other especially in the wavenumber range of 1700 to 900 cm⁻¹. The bands in this range are the results of the salts presence. The very broad signals indicate variety of chemical species on the surface of the materials. In this region, vibration of single and double bonds carbon-oxygen and carbon-nitrogen are characteristic. Interesting is the fact that there are some characteristic bands (1384, 1252, 1092 and 800 cm⁻¹), which intensities increasing in the series C-BM-NiAgGd → C-GLY-NiAgGd → C-NAF-NiAgGd. These bands are caused by stretching vibrations of bonds O=S=O (1140–1000 cm⁻¹), S=O (1225–980 cm⁻¹), S–O (870–650 cm⁻¹), C–S (800–600 cm⁻¹) present in sulphonic and sulphinic compounds as well as anions thiosulphate (VI), \(\mathrm{S}_{2}\mathrm{O}_{3}^{2-}\) (1660–1620 and 1000–990 cm⁻¹) and sulphate (1130–1080 and 680–580 cm⁻¹) (Socrates 2001; Zieliński 2000, and On-Line Spectroscopic Tool¹). The results of elemental analysis confirm this observation (Table 2). The contents of oxygen and sulphur increases in ratio 1:2:3. Total amounts of Ni, Ag, and Gd are very high and make from 20 to 40 % of mass of the carbon sample, proving that the metals in the form of oxides and salts were partially trapped in the porous structure during heat treatment. The highest contents of gadolinium are observed for carbons, which polymeric precursors possessed broad PSD (Fig. 1). This means the metal was trapped in the network during carbonization. In case of C-NAF-NiAgGd, the contents of nickel and silver are high, while the contents of gadolinium is the lowest. The copolymer NAF-DVB had the narrow pore size distribution, consequently small amount of wide pores was available for gadolinium nitrate to crystallize, so less gadolinium was built in the carbon structure. The above explains, why relatively low values of specific surface areas and total pore volume for materials of this series (and particularly for C-NAF-NiAgGd) were obtained.

In this step of the studies influence of different salts (with the same metal cations) on porous structure formation of the synthesized carbons was evaluated. PSD plots for the prepared carbon materials are presented in Fig. 4. For comparison purposes, PSD curve for carbon, prepared without salts impregnation (C BM) was also included. This material had relatively small values of porous structure parameters (Table 1).

In this case impact of the salts presence on porosity of the final products is very clear. The largest influence on porous structure formation of the carbonaceous materials was observed when chlorides and nitrates were used for impregnation. In both cases specific surface areas reach value of about 300 m²g⁻¹ and PSD curves (Fig. 4) show the presence of ultramicropores (<10 Å), supermicropores (10–20 Å) and relatively narrow mesopores (20–100 Å). However, porous structures of these two materials are quite different.

C-BM-Cl possesses mainly mesopores with pore size distribution in the range of 35 to 95 Å. The contribution of micropores in the structure of this material is only 5.5 % with the major part of ultramicropores.

Pore volume of C-BM-NO₃ is almost two times smaller than that of C-BM-Cl, and its structure is micro-mesoporous. Over 43 % of pores form ultramicropores and supermicropores. In case of this carbon, PSD of mesopores is narrower and is in the range of 35 to 60 Å. The observed differences are the result of properties of the salts used for impregnation, particularly their thermal properties such as melting point, initial decomposition temperature, the way of decomposition and the type of released products.

The chlorides possess their melting temperatures in the range of 300–770 ^∘C, which means they still remain in the form of crystals (especially KCl) when polymeric precursors decomposition process begins. This is the reason, why the collapse of polymeric structure is delayed in time and more mesopores are retained. The main gaseous products released from CuCl₂⋅2H₂O and FeCl₃⋅xH₂O while heating are H₂O (from dehydration of crystals), Cl₂ and HCl (Serban et al. 2004; Kanungo and Mishra 1996; potassium chloride MSDS²). Modification caused by these gases is observed in the FTIR spectra as chlorinated functional groups (Fig. 5). Two bands in the ranges of 1300–1060 and 650–480 cm⁻¹ are broad and complex therefore they can be attributed to several types of vibrations. Waging of C–H bonds in CH₂–Cl groups (1300–1150 cm⁻¹), stretching of C–Cl bonds in aromatic compounds (1100–1080 cm⁻¹), and deformations of –COCl groups in acyl chlorides (1300–900 cm⁻¹) are responsible for existence of the former, while stretching of aliphatic C–Cl bonds (850–550 cm⁻¹) and inorganic species e.g. \(\mathrm{ClO}_{3}^{-}\) (630–615, 510–480 cm⁻¹) for the latter. The other signals can be attributed to cyclic anhydrides (stretching of C–O–C bonds—950–910 cm⁻¹), vibrations of –CCl₃ groups (830–700 cm⁻¹) and vibrations C–H bonds in Cl–C–H groups (3080–2900 cm⁻¹) (Socrates 2001; Zieliński 2000; and On-Line Spectroscopic Tool).

Melting and decomposition processes of the nitrates proceed in lower temperatures (150–400 ^∘C). Released oxidative gases such as H₂O, O₂, HNO₃, NO₂, and NO (Freeman 1957; potassium nitrite MSDS;³ Ding et al. 2002; L’vov and Novichikhin 1995; Morozov et al. 2003; Wieczorek-Ciurowa and Kozak 1999) react immediately with polymeric precursor decomposing at the same time. This explains why, C-BM-NO₃ has the most microporous structure among the presented materials. On the surface of this carbon, nitrogen containing functional groups are present. The bands in the range of 1615–1530 and 1290–1220 cm⁻¹ are caused by asymmetric and symmetric stretching of N=O bonds present in nitrates, nitrites nitro, and nitrosocompounds, respectively. The signal at 1090 cm⁻¹ may be assigned to C–N bonds in aromatic species (∼1100 cm⁻¹), but also to secondary alcohols (1130–1000 cm⁻¹). Narrow and strong signal at about 800 cm⁻¹ is the second nitrites characteristics vibration, besides above mentioned the one at 1250–1230 cm⁻¹ (Socrates 2001; Zieliński 2000; and on-line spectroscopic tool).

The carbon prepared by impregnation with sulphates has similar specific surface area to the non-impregnated carbon—C BM, and its total pore volume is even three times lower. No micropores are accessible for N₂ and widths of mesopores are in the range of 42 to 100 Å. FTIR spectra of this material show only two but intensive signals coming from sulphur containing compounds. Both of them are complex indicating that many different functionalities could reveal their presence. In the range of 1630 to 1500 cm⁻¹ signals given by anions \(\mathrm{S}_{2}\mathrm{O}_{3}^{2-}\) (1660–1620 cm⁻¹), vibrations of C=N bonds (1685–1580 cm⁻¹) and C=S ones in thioamides and thiolactons (1570–1460 cm⁻¹) can be observed. The band in the range of 1170 to 1080 cm⁻¹ come from superposition of signals of following species: sulphinic acids (1100–1090 cm⁻¹), sulphinic acid esters (1140–1125 cm⁻¹), sulphones (1170–1110 cm⁻¹), thioesters (1210–1080 cm⁻¹), thioamides and thiolactons (1140–1090 cm⁻¹), and sulphates (1130–1080 cm⁻¹) (Socrates 2001; Zieliński 2000; and On-Line Spectroscopic Tool).

According to papers by Petkova and Pelovski (2008), Mianowski and Bigda (2004), and Masset et al. (2006) copper and iron sulphates decompositions carry out gradually in the range of 250 to 700 ^∘C and finally lead to copper (II) oxide and iron (III) oxide. The gases such as steam, SO₃, SO₂, and O₂ are also evolved slowly and gradually. For this reason their oxidative properties are not so strongly marked, as in case of nitrates or chlorides. Potassium sulphate is thermally stable compound and decomposes at 1067 ^∘C (according to its International Sheet of Chemical Safety⁴). This salt is unchanged in carbonization condition. High amount of potassium in C-BM-SO₄ proves, that the salt was trapped in porous structure of the material and could not be removed by washing. This conclusion explain the lowest value of total pore volume and absence of micropores in this material.