Ni-Carbon Mineral Sorbent - Catalyst of Sulfur Dioxide Sorption

The morphology and structure of Ni-carbon mineral sorbent-catalysts on the basis of local mineral raw material were studied by methods of thermoprogrammed desorption and electron microscopy. The specific surface, filamentary carbon diameter sizes and sorption characteristics of sulfur dioxide adsorption were determined. It was shown that the adsorption capacity of carboncontaining sorbents is influenced by: nature of metals of varying valence, specific surface and density of patterns. Physical and chemical sorption of sulfur dioxide was shown experimentally.


Introduction
At present the environment protection, particularly environmental air pollution prevention, is still one of the most urgent ecological problems. It is well known that the waste gases of many industrial productions and refinery plants contain such sulfur component emissions as H 2 S, SO 2 , SO 3 etc., affecting not only the animals and plants, but also inducing corrosion of metal constructions and oil pipelines.
In the process of catalytic purification of sulfur oxide-containing waste gases the catalysts are poisoned by sulfur oxides [1], that results in catalyst deactivation [2][3]. The regeneration of poisoned catalysts requires extra power consumption that increases the catalyst costs. Moreover, insufficient availability and expensive costs of catalysts based on noble metals (Pt, Pd), as well as substantial costs of technology for manufacture both of carriers and catalysts prevent their large-scale applications, particularly in processes of exhaust gas purification.
The synthesized carbon-mineral sorbents, i.e., carbonized clays may be used to purify waste gas emissions of sulfur oxides, being the cheapest material that requires no substantial expenses relative to clay mining and catalyst manufacture technology costs [4][5][6].
The X-ray phase analysis (RPA) results prove that the natural clays contain oxides of metals showing variable valence, which being activated by carbonization, improve adsorption and catalytic process features.
The data obtained by electron microscopy and other physico-chemical investigation methods [6][7] showed that the tubular-filamentary nanosctructure carbon formed in certain carbonization conditions is promoting extension of specific surfaces and structure porosity.
This paper deals with the results of investigations of SO 2 adsorption on the carbonized clay from Narinkol region of the Republic of Kazakhstan.
The catalysts were prepared by clay impregnation with nickel acetate solution. The samples pre-molded in cylinders of d = 1-2 mm and d = 5-6 mm were dried at ambient temperature (12 hours) and then were calcinated at 423 K (2 hours) and 623 K (3 hours).
The thermo-catalytic synthesis of carbon-mineral catalysts and sorbent was carried out by propanebutane mixture pyrolysis at 80cm 3 /min. flow rate within 673-1073 K temperature range during 10-30 minutes. After the process was completed, the argon is fed into the reactor and cooled down to ambient temperature.
The SO 2 adsorption based on the carbonized sample was studied in the flow reactor by thermoprogrammed desorption (TPD) method within 298-773 K temperature range at 17 K/min. rate and further chromatographic analysis of desorption products. The analysis of O 2 , CO 2 and SO 2 was proceeded using «Gasochrome-3101» and «LCM-8 MD» gas chromatographs.
The interactions between SO 2 and carbonized samples (0.5 g) pre-treated under vacuum at 3×10 -5 Pa and then exposed at 673 K temperature during 2 hours, were proceeded at 423 K in He (40 cm 3 /min) flow 0.08-0.1 vol. % SO 2 .
The electron microscopy investigations were carried out using the 120000× microscope «EM-125 K».

Results and discussion
The mechanism of catalytic formation of carbon deposits on pure oxides was thoroughly studied by R.Ya.Buyanov et al. [7][8][9]. The obtained results show that carbon is formed in fine-dispersion graphite structures having morphological patterns differing from each other in their carbide mechanism. Under certain experimental conditions the morphological structure changes and carbon deposits in the form of tubular filaments.
The obtained experimental results are summarized in Table 1. As is seen from the table, the specific surface of the clay increases from 5.21 m 2 /g to 9.23 m 2 / g with carbonization temperature (T carbon. , K) rise from 673 K to 823 K. The further T carbon. increase results in specific surface decrease.
The microscopy images (Fig. 1) evidence that the carbonization of clays is followed by the formation of filamentary nanostructure carbon and its diameter depends on the carbonization temperature. The diameters increase with temperature (Table 1).
Moreover, one can see from the images (Fig. 2), that the metal atoms are mainly located on the nanotube ends. This phenomenon is the most evident on the carbonized clay with deposed nickel (Fig. 3, 4), where quite numerous nanotubes with nickel deposed on their ends are observed. In case of samples with deposed cobalt and copper, the filamentary fibril carbon was not found on the electron microscopy images, evidencing the selectivity of fibril carbon formation on various metals.  Table 1 Effects of carbonization temperature on the specific surface of clays and filamentary carbon According to the authors [5], the filamentary fibril carbon is a catalytic active center and, moreover, it has a particular property to remove metal from the volume up to the surface.
The electron microscopy images (Fig. 5) were obtained after SO 2 adsorption at 423 K and further SO 2 desorption proceeded at 298-773 K from the carbonized samples having nickel deposits (1 % weight). It is seen from these image that sulfur dioxide (loose form) is adsorbed both on the fibril carbon and on the metals bounded with fibril carbon. After thermo-desorption the surface of sorbent-catalysts samples became uneven with etched sites (Fig. 6).
According to the thermo-desorption data (Fig. 7), after SO 2 adsorption proceeded at 423 K, SO 2 is released in the form of two peaks from the nickeldeposed (1-5 % weight) samples carbonized at different temperatures and from non-modified clay.
The comparative analysis of TPD spectra of SO 2 (Fig. 8, Table 2), after its adsorption at 423 K, from non-carbonized clay and from the clay carbonized at 723 K, showed that SO 2 adsorbs more effectively (3.73 cm 3 /g) on the carbonized clay than on noncarbonized (0.35 cm 3 /g) one. In case of nickelcontaining catalysts the SO 2 adsorbs still more effec-  tively and adsorption depends on nickel content in the clay (% weight). The adsorption of sulfur dioxide increases with the increase of nickel content in the clay. The maximal amount (13.52 cm 3 /g) of adsorbed SO 2 falls at the clay containing 5 % nickel and minimal amount (5.01 cm 3 /g) of adsorbed SO 2 falls at the clay containing 0.1 % nickel. The similar dependence of SO 2 adsorption is observed for carbonization temperature and the higher is the carbonization temperature, the more SO 2 is adsorbed. However, at temperatures over 723 K the decreasing in total fraction of adsorbed SO 2 to 6.93 cm 3 /g (T carbon. = 873 K) and 1.0 cm 3 /g (T carbon. = 1073 K) is observed.
The filamentary carbon that we have found is a catalytic active center showing selective interactions both with deposed metals and with metals present in natural sorbents, besides it is the center of SO 2 adsorption. Moreover, the active carbon has a particular property to remove metals from the volume onto the sorbent surface being a main factor to direct involving of metals in adsorption and catalytic processes.
As appears from TPD data the substantial increase of SO 2  and modified catalysts on the clay basis, is related to the formation of active fibril carbon and its interactions with SO 2 . The catalysts can be arranged by amounts of adsorbed SO 2 at T adsorp. = 423 K in the following series: C-Ni/clay > C-Cu/clay > C-Co/clay > carbonized clay > non-carbonized clay

Conclusions
Thus, the clay activation by overcarbonization leads to increase porosity and specific surfaces of sorbents and contribute to the procedure of physical and chemical sorption of sulfur dioxide and that was confirmed by the values of calculated activation energy of sulfur dioxide sorption process.