CO 2 Reforming of CH 4 on Co-Containing Supported Catalysts

Mono- and bimetallic cobalt-containing catalysts supported on alumina have been investigated in the reaction of interaction between carbon dioxide and methane at variation of experiment temperature and pressure. It was shown, that the bimetallic catalysts have a high activity in this reaction in compare with monometallic ones. The main reaction products are carbon oxide, hydrogen, water and oxygenates. The yield of latter reaches 30% at certain conditions (P > 0.5 MPa, T < 853K). The maximum conversion of both methane (100%) and carbon dioxide (94%) is reached at lower pressure (0.1MPa) and 1023˚ . In these conditions the synthesis-gas is a main reaction product. One of the advantages of the bimetallic catalysts is their resistance to coke formation.


Introduction
ÑÎ 2 +ÑÍ 4 = 2ÑÎ + 2Í 2 (1) The interaction between carbon dioxide and methane was investigated by Fischer and Tropsch yet in 1926 [1]. From all metals studied by them cobalt and nickel catalysts were the most preferabSle for this process [1]. But there is a very strong coke formation on these catalysts. The noble metals are characterized by a low level of coke formation and more promising for process. Nevertheless the coke formation occurs on all catalysts. It discourages the use of catalysts under high pressure.
In this paper the interaction between carbon dioxide and methane on mono-and bimetallic cobaltcontaining catalysts modified by different noble metals and supported on alumina has been investigated. The choice of catalysts was based on the literature data about preference of Co in this reaction [1] and noble metals as stable to coke formation [2][3][4][5][6]. Moreover the same catalysts have been investigated in our previous works and showed high activity in Fisher-Tropsch Synthesis [7][8][9][10][11]. It was supposed that synthesis-gas in case of its formation is able to following reactions with formation of different organic compounds under moderate temperature and pressure.

Experimental
Catalysts (content of each metal is 2.5 weight %) were prepared by impregnation method. Co:M ratio in the bimetallic catalysts was 1:1. Temperature of catalyst reduction was 473-673K. The process was carried out in a flow stainless steel reactor under pressure 0.1 -2.0 MPa and varying experiment temperature from 473 to 853K, and in a quartz flow microreactor in the temperature range of 473-1373K at atmospheric pressure. Space velocity was varied from 500 to 2000 hr -1 . Ratio CO 2 :CH 4 was constant 1:1, content of each gas in the initial reacting mixture with Ar was 10%.
The sets were combined with gas chromatographs (GC) equipped with thermal conductivity detector for analysis of H 2 , Ar, CO, CH 4 , O 2 , CO 2 (columns: molecular sieves and activated coal) and flame-ionization detector (FID) for analysis of hydrocarbons (column: modified alumina) during the reaction. Analysis of oxygenates and water after reaction and their collection in special cooled trap (separator) was carried out by GC equipped with FID on columns: Carbowax/ Eurasian ChemTech Journal 2 (2000) 41-44 Carbopak and Poropak N, as well as by IR-spectroscopy. The degree of surface carbonization was determined by TGA. In addition, after reaction the thermo-programmed hydrogenation was carried out for definition of carbon-containing species on catalyst surface.
Catalysts were investigated by X-Ray analysis, TPR and IR-spectroscopy.

a) CO 2 + CH 4 reaction
Some results of CO 2 + CH 4 reaction study on monoand bimetallic Co-containing catalysts are summarized in the Table 1. Co/Al 2 O 3 -catalyst has the lower activity in this process in compare with bimetallic ones (see Table 1). Introduction of the second metal to the composition of catalyst implies the significant increase of their activity. Conversion degree of initial compounds depends on the nature of catalyst and experiment conditions. Basically an unequal consumption of both carbon dioxide and methane is observed on all catalysts. The main reaction products are carbon oxide, hydrogen, water and oxygenates. In some cases oxygen is formed at the beginning of process. It has been shown that the yield of oxygenates depends on the nature of the second modified metal and experiment conditions ( Table 1). Table 1 The comparative characteristics of activity of 5% Co-M (1:1)/Al 2 O 3 catalysts in the reaction of CO 2 +CH 4 (CO 2 : On all catalysts the conversion of both reactants grows with increase of experiment temperature and decrease of pressure. It needs to note that ethane formation and increase of oxygenates' yield over these catalysts are observed with pressure increase. There are no oxygenates at atmospheric pressure. The maximum yield of oxygenates (20-30%) is observed on catalyst modified with M 4 . Oxygenates consist of C 1 -C 2 alcohols (≥ 95%) and traces of acids (T exp = 853 K, P= 0.1-1.5 MPa, space velocity =1000 hr -1 ). On other catalysts the liquid fraction consists basically of water.
The advantage of the investigated catalysts is their resistance to coke formation. The coke formation is not verified by TGA. The weight loss during increase of temperature up to 1173K is less than 4%. It occurs because of water elimination at ~ 373K. The formation of carbon-containing species is not observed during thermo-programmed hydrogenation of catalysts after reaction at temperature increase from 298 to 1373K. Water always produced during reaction on these catalysts may promote the suppression of coke formation. According to [12] oxygen suppresses coke formation to a greater degree than water. Oxygen is produced on the investigated catalysts at the reaction beginning.
b) The mechanism of CO 2 + CH 4

interaction
By IR-spectroscopic measurements of both combined adsorption of CO 2 and CH 4 and reaction between CO 2 and CH 4 over 5% Co-M 4 /Al 2 O 3 the weak absorption bands have been detected in the region of 2800-2900 cm -1 assigned to adsorbed CH x -species (õ=1-3). Also in the IR-spectrum the bands are present at 2330 ñm -1 , which could be assigned to physically adsorbed CO 2 . The bands at 2000 and 2090 cm -1 could be assigned to bridgely and linearly adsorbed CO respectively and intensive bands at 1410 and 1580 cm -1 are concerned to carbonate-carboxylate species.
The reactions of separate decomposition of carbon dioxide and methane over 5% Co-M 4 /Al 2 O 3 have been studied. It was shown that 100% decomposition of methane occurs at Ò=1273Ê and atmospheric pressure. While the maximum degree of CO 2 decomposition reaches 25% at Ò=673Ê. With further temperature increase until 1273K CO 2 conversion does not increase and varies from 16 to 20%.
At interaction of ÑÎ 2 +ÑÍ 4 the maximum conversion of both CH 4 (100%) and CO 2 (94%) is observed at lower temperature (1023Ê). With further increase of temperature until 1273K the conversion degree of both components does not change.
It is possible to suppose, that the process of interaction between CO 2 and CH 4 on these catalysts represents two parallel reactions of decomposition of each initial compound (equations 2 and 3). The different conversion degree of both reagents may be an indirect evidence of it in some cases, when only synthesis-gas is formed. Each of these reactions is promoted by intermediates produced during the other reactions (equations 4 and 5):  (6) The obtained results are in agreement with mechanism suggested by other authors for CO 2 + CH 4 interaction on supported Pd catalysts [4].
At medium pressures (0.5-1.5 MPa) and not too high temperatures (773-873Ê) produced ÑO ads and Í ads involve into further interaction on the studied catalysts with alcohol and acid formation.
Nevertheless these results do not exclude the formation of «active» carbide, which immediately involves in further interactions.

Conclusions
Earlier we have reported that cobalt is able to form the nano-structures of cluster type at interaction with such noble metals as Ir and Pt on the surface of alumina [7][8][9][10][11]. The properties of bimetallic catalysts drastically differ from monometallic ones. It was observed that bimetallic clusters were very stable and showed the high activity in reactions with participation of C 1 -molecules, in particular in Fischer-Tropsch Synthesis and CO 2 hydrogenation.
The study of the similar catalysts in ÑÎ 2 +ÑÍ 4 reaction demonstrates that the adding of second metal to the composition of Co-containing catalysts increases their activity in this process too. At moderate conditions (Ò exp =853Ê, Ð= 1.0 ÌPà) the conversion of both carbon dioxide and methane is 30-40% depending on the nature of the second metal. The maximum conversion of both methane and carbon dioxide reaches 100 and 94% respectively at 1023Ê and Ð =0.1 MPa.
It was shown that it is possible to produce some organic compounds by one step during the process CO 2 +CH 4 on the same catalyst. The yield of oxygenates reaches 30% at certain conditions (P=1.0 MPa, T ≤ 853K).
The advantage of the synthesized catalysts is their resistance to coke deposition.
The obtained data allow to consider these catalysts as high effective ones for reforming of CH 4 by CO 2 and their further industrial use.