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ISSN(Print) : 2320 –1967 ISSN(Online) : 2320 –1975

Original Article ChemXpress 5(3), 078-088, (2014)

The study of compensation effect in reverse Boudouard reaction on in presence of activating additives

Pavel Anatolyevich Nikolaychuk*, Aleksandr Vasilyevich Kolesnikov

Department of Analytical and Physical Chemistry, Chelyabinsk State University, 454001, Bratyev Kashirinykh Street, 129, Chelyabinsk, (RUSSIAN FEDERATION) E-mail: [email protected]; [email protected] Received : 05th April, 2014 ; Revised : 03rd June, 2014 ; Accepted : 08th June, 2014

Abstract : In this study the results of calculation of vation energy for calcium, strontium and compounds rate constants for graphite gasification reaction (reverse were plotted. The isokinetic temperatures for various Boudouard reaction) taking into account heat and mass groups of catalysts were calculated. °C are  transfer processes at temperatures of 900-1200 Global Scientific Inc.

presented. The calculation of rate constants of C + CO2 reaction on graphite in presence of calcium, , Keywords : Graphite gasification; Reverse strontium, iron (II) and iron (III) oxides, strontium car- Boudouard reaction; Compensation effect; Isokinetic bonate and metallic iron was performed. The graphs of temperature. linear dependencies of pre-exponential factor on acti-

INTRODUCTION ied very widely. Particularly, the studies concerning the influence of temperature on kinetics of the reverse The interaction of with carbon plays Boudouard reaction reveal the presence of compensa- a major role in many pyrometallurgical high tempera- tion effect. ture processes, where coke, coal and other carbon ma- Compensation effect is the dependency between

terials are used as solid reductants. Many studies are pre-exponential factor (A) and activation energy (EA) devoted to investigation of C + CO reaction in order 2 in Arrhenius equation ( ), which reveals to in crease yield of , which is one of often, when studying heterogeneous catalytic pro- the most important raw materials for chemical produc- cesses[5-7]. In this case values of A and E increase or [1] [2-4] A tion . It was noted in papers , that basic solutions of decrease simultaneously, compensating each other, and the problem of carbon gasification reaction speeding the influence of temperature on rate constant becomes can be obtained from fundamentals of reaction kinet- less substantial. If the relationship between EA and A ics, stream dynamics, diffusion, heat and mass transfer. for multiple reactions strictly obeys linear nature (log A Therefore, the various aspects of this reaction were stud- á â  = + EA), rate constants for all reactions become ChemXpress 5(3), 2014 Original Article79 equal at one certain temperature, which is called It is stated[3], that full amount of active centers per “isokinetic temperature” [7-9] è T . gram of carbon [C[t]] is the sum of amounts of free and The presence of compensation effect was noticed occupied active centers: at catalytic gasification of carbons, activated by various [C[t]] = [C[f]] + [C[o]] (5). [10] additives such as sodium lignosulphate , ferric ni- Assuming that rates of formation and vanishing of [10] [11,12] trate , sodium and potassium carbonates , orga- occupied active centers are equal and using steady- [13] nometallic compounds with V, Cr, Mn, Fe and Co , state approximation, the following kinetic equation can [14] [15] sodium vanadate and others . The influence of tem- be derived: perature on rate constants of C + CO2 reaction and compensation effect in presence of calcium and mag- nesium oxides, strontium oxide and carbonate, iron and (6). iron oxides is the subject of the present work. Expressing [C[f]] from equation (5) and substituting The reverse Boudouard reaction C + CO2 was it into equation (6) yields: [2-4,16-19]     studied very widely , including studies with dif- k [CO ] [C ] = k [CO ] [C ] + k ferent coal types and at various carbon dioxide partial  1  2 [t]  1 2 [o] -1 [CO] [C[o]] + k2 [C[o]] (7). pressures. The mechanism of this reaction is postulated Solving for [C ] from equation (7) gives: to be the following[3,4,16,17,19]: [o] (8).  (1), Dividing both numerator and denominator by k1

[CO2] gives:

(2), (9). where C[f] is a free active center on carbon surface,  k1 able to reaction, and C is an active center, occupied Defining the value of I and substituting it and [o] 2 k by oxygen atom, k , k–1 and k are rate constants of 2 1 2 the value of K from equation (3) into equation (9) yields: corresponding elementary stages. p Reaction (1) proceeds relatively fast and is charac- terized by following equilibrium constant: (10). Substituting equation (10) into equation (4) gives (3). the expression for overall carbon gasification rate: Here equilibrium concentrations of gases, [CO] and –3 [CO2], are expressed in mol cm and [C[o]] and [C[f]] (11), are the numbers of moles of occupied and free active The values of K and I do not involve any param- centers per one gram of carbon. p 2 Reaction (2), corresponding to carbon transfer from eter that is a function of physical properties of carbon, solid phase to gaseous one, which proceeds on occu- they also are expected to be independent of pressure. pied active centers, is much slower. The dependencies of these parameters on temperature [20] In this case, rate of gasification of carbon sample, sur- are taken from paper . rounded by gas of unified composition, can be ex- The aim of this study is:1) to investigate C + CO2 pressed in following way[3,4]: reaction on graphite in a flow reactor in presence of calcium, magnesium, strontium and iron compounds using (4), kinetic equation (11) and considering heat and mass  dn transfer processes; 2) to calculate rate constants and where N is the rate of carbon transfer from solid c dt evaluate possible compensation effect between activa- –1 – mass of carbon, phase to the gaseous one, mol s , P tion energy and Arrhenius prefactor in presence of study- g. ing additives. . 8O0 riginal Article ChemXpress 5(3), 2014

EXPERIMENTAL Analysis of experimental data The experimental results were presented in form of Performing of experiments dependencies of mass loss of graphite (F) on time (t):

The commercially available graphite powder which (12), comes in the graphite pots was used in all experiments. Ð Ð where ê is current graphite mass, î is initial graphite The graphite was of spectral analysis quality and had  – mass and P = P P is graphite mass loss. the particle size d 0.1 mm and impurities content of no o k  -5 Kinetic curves were conditionally divided into three more than 2 10 weight percent. Activating additives – from start of experiment to time of sections: section I were of analytical quality with particle size d 0.1 mm – from third minute to the moment 3 minutes, section II and purity of 99%. Carbon dioxide from balloons, con- – 70 % graphite mass loss and section III – from of 65 taining no more than 0.1% CO and 0.01% H2 was used. – 70 to 100 % graphite mass loss. The example of The initial samples with the various concentrations 65 of the catalysts were prepared by the mechanical mix- the kinetic curves is shown at Figure 1. The rate con- ing of corresponding amounts of the graphite and the stants were calculated according to section II. In most additive in plastic cups during 10 minutes. A weight taken cases the experimental points in this section were de- from the sample mixture using analytical balance ADV- scribed by straight lines. The slopes of these lines ±0.0001 g was placed ÄF 200 with measurement error of ( Äô) were determined using less squares technique. into alundum bath that was preliminarily calcined at After this the average rates of graphite mass loss at the °C during 1 hour. The bath had been injected into 1200 second section were determined: porcelain tube and placed into thermobalance ATV-14 with carborundum heaters. Carbon dioxide flow was (13). passed through the tube with determined rate. The stud- ÄP ies were performed at temperature range of 900- After dividing the values of Äô by carbon molar °C –1 –1 1200 . Carbon dioxide consumption was 50 cm3 min mass (12.0107 g mol ) the rates of loss of graphite and gas flowing rate in reaction tube was 1.06 and 1.32 amount of moles (graphite consumption rates) were –1 °C respec- cm s at temperatures of 900 and 1200 calculated. tively. The alundum baths used in experiments were According to[23], the average reaction rate inflow weighted before and after the experiment and no mass dn system ( ) can be calculated independently by equa- loss was detected. dt Thermogravimetric analysis was applied to study tion: reaction kinetics. According to paper[21], TGA can be (14), freely used without needing to pay attention to chemi- where N is the average rate of interaction of carbon sorption dynamics if carbon active surface area does c –1 Õ Õ – the amounts of 2 dioxide with graphite, à and à not exceed 312 m g . However, active surface area ,i ,o ÑÎ 3 – [22] at reactor inlet and outlet respectively, cm , Fà of graphite without special processing is much lower , 2 –1 – reaction which allows using of TGA method freely. the rate of incoming substance, mol s , V 3 Thermogravimetric curves obtained from different space volume, cm . The values of N calculated in accordance with equa- weights taken from one sample mixture show good re- c producibility and compatibility with each other. tion (14) were found to be compatible with the values Since the heating area in thermobalance is small, of graphite consumption rate within their ranges of un- certainty. Therefore, the graphite consumption rates there was no possibility to place a gas analyzer close to Än reaction zone in order to avoid decreasing temperature ( Äô) were used for further rate constants calcula- of the gas stream. Therefore, the concentrations of CO tions. and CO2 in downstream were not directly measured. Substituting these values into equation (11), taking They were calculated according to gas flowing rate in into account that carbon mass (P) in equation (11) is reaction tube and graphite mass loss rate. identical to initial graphite mass (Po) and solving the ChemXpress 5(3), 2014 Original Article81

– 950îÑ; – 1000îÑ; Figure 1 : Thermogravimetric curves for the gasification of graphite doped with 0.1 mol % SrCO at: 1 2 – 1050îÑ; – 1100îÑ. 3 3 4 The kinetic parameters were calculated from section II of the curves.  resulting equation for k [C ] gives the expression for is endothermic. At high reaction rates and big amount 2 [t] Ñ ÑÎ the apparent rate constants of reaction + 2: of initial weights it is necessary to consider temperature difference between solid material and gas stream. At (15). – 1200îÑ ÑÎ ÑÎ the temperatures of 900 heat transfer pro- Variables [ ] and [ ] in equation (15) are av- 2 ceeds mainly by emission mechanism. The approximate erage actual gases concentrations on graphite surface. expression for temperature difference calculation is the In flow reactors, even at high flowing speed, in case following[24]: when reaction on surface proceeds with significantly high rate, a difference exist between reacting gas con- (17), centrations in gas phase and on the surface. Taking into ÑÎ account mass transfer from stream to graphite sur-  2 where is Stefan-Boltzmann constant, face[24] was performed according to equation: SB is maximal amount of heat, produced during reaction  / (16), per time unit, is square of outer contour of solid Ò – tempercal ture, K. The dimensions and defini-  –å body, where C = C (1 ), C is gas concentration R(o) R R(o) Ñ – av- tion ranges of variables, needed for calculations accord- (either CO or CO ) on solid reagent surface, 2 Ô – the avRerage ing to formulae (16, 17), are listed in TABLE 1. erage gas concentration in stream, cl flow, referred to the outer contour of the solid reagent, – diffusion coefficient; Õ – powder filling length, – RESULTS AND DISCUSSION D cl – the average gas flowinv g gas kinematical viscosity, U F Determination of initial graphite weight rate in tube. It is well known, that reverse Boudouard reaction The influence of graphite initial weight on gasifica- . 8O2 riginal Article ChemXpress 5(3), 2014

 tion reaction rate and apparent rate constant (k2 [C[t]]) All further experiments were performed using initial was studied, according to equation (15) taking into ac- weights equal to 0.02 g and CO flowing rate of 50 –1 2 count heat and mass transfer according to equations cm3 min . Temperature difference between gas phase (16,17). The experiments were performed at tempera- and graphite layer was not been taken into account due îÑ ture of 1000 with different weights of graphite, doped to its insignificance at small weights (as mentioned afore). ÑàÎ ÑÎ by 0.5 mol % with flowing rate of 150 cm3 The difference between gas concentrations in steam and –1 2 on graphite surface was taken into account. The values min . The results are presented in TABLE 2.   of were calculated according to equation (16) and The differences between values of (k [C ]) at 2 [t] then actual gas concentrations were calculated. various graphite weights are small, therefore used cal- culation formulae are reliable. The average apparent Studying kinetics of C + CO2 reaction with graph-   –1 –1 rate constant value is (9.61 1.58) 10-5mol s g . Tem- ite doped by various additives perature difference between gas stream and graphite Kinetic data for gasification of pure graphite and layer was insignificant for weights of 0.01 and 0.05 g. graphite with additives of 0.5, 1.0 and 2.0 mol % of At the same time difference between partial pressures calcium oxide, 2.0 mol %of magnesium oxide, 0.2 mol ÑÎ ÑÎ % CaO + 0.8 mol % MgO, 0.1, 0.25 and 0.5 mol % of 2 and in gas stream and graphite layer was substantial. For example, at the rate of interaction of of strontium carbonate 0.5 mol % of strontium oxide  –1 carbon dioxide with graphite equal to 3.16 10-6mol s 0.5 and 1.0 mol % of metallic iron, 1.0 mol % of iron  (II) and iron (III) oxides are presented in TABLE 3 at it was obtained according to formula (16) that =0.39. îÑ ÑÎ temperature range of 900-1200 with temperature al- This means that average value of partial pressure îÑ 2 teration step of 50 . Express in gap parent rate con- in gas phase is 1.64 times higher than that in graphite stant in form of Arrhenius equation, values of activation layer. energy EA and prefactor A are calculated. TABLE 1 : The dimensions and definition ranges of physical The dependencies between E and log A are plot- A – one for calcium and chemical variables, used in calculations according to for- ted for three different catalyst groups mulae (16, 17) and magnesium oxides, one for strontium carbonate and Variable Dimension Definition range oxide and one for iron and iron oxides. These relation-   Dimension less 0 1 ships are approximated using less squares technique. – Ñ Ñ 3  R, R(o) mol m 1 1000 The graphic representations of these relationships are – – Ô 1 2 -4 -1 cl mol s m 10 10 presented at Figures 2, 4 and 6. The values of EA and – 2 1   -4 D m s (1.2 2.0) 10 log A for pure graphite are marked in these figures only Õ for comparison; they were not used for less squares cl m 0.033 –  2 1   -4 technique optimization. v m s (0.95 1.25) 10 – 1   -2 In order to verify isokinetic relationship hypoth- UF m s (0.5 1.0) 10 Ò  esis Arrhenius relations for each group of catalysts are K 0 5 – – –  2 1 4  -8 plotted. Isokinetic temperatures are calculated accord- SB J m s K 5.67 10 [25,26] – ing to method, proposed in . The statistical crite- 1  J s 0.8 16 ria which allow accept or reject isokinetic hypothesis, / 2   -4 [27] cl m (5 20) 10 proposed by the authors of are used.  TABLE 2 : The dependence of graphite consumption rate (N ), apparent rate constant (k [C ]), calculated graphite layer Ò îÑ) c 2 [t] temperature ( / and average carbon dioxide partial pressure on graphite initial weight – ÑÎ  5 – – 2 partial pressure, bar î 6 1 (k2 [C[t–]])/1– 0 Ò Ñ Graphite weight, g Nc /10 mol s 1 1 / in gas stream in graphite layer mol s g 0.01 0.676 0.99 0.964 10.56 1000 0.05 3.18 0.96 0.92 10.21 999 0.2 5.97 0.89 0.63 8.33 997 0.5 9.375 0.83 0.43 9.375 995 ChemXpress 5(3), 2014 Original Article83

TABLE 3 : The data on gasification rate (N ), apparent rate Ñ Ñ c + 0.5 mol % SrCO3 + 0.5 mol % SrO  î – – – – Ò Ñ 7  5 7  5 constant (k2 [C[t]]), activation energy (EA) and pre-exponen- / Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 tial factor (A) for gasification of graphite with various addi- mol s 1 mol s 1 g 1 mol s 1 mol s 1 g 1 tives 900 0.54 0.37 2.08 1.57 Ñ Graphite without +0.5 mol Ñ ÑàÎ 950 1.65 1.24 5.47 4.61 î additives ( ) % Ò Ñ – – – – / 7  5 7  5 1000 5.12 4.88 5.30 10.4 Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 mol s 1 mol s 1 g 1 mol s 1 mol s 1 g 1 1050 18.1 20.9 11.9 32.9 – – 900 1.8 1.34 1100 32.2 48.8 21.9 57.6 – – – 1 ± 12.8 ± 10.7 950 2.94 2.56 EA/kJmol 337.3 245.9 ± 0.526 ± 0.441 1000 0.5 0.38 6.97 6.09 log A 9.557 6.156 Ñ + 0.5 mol % Fe Ñ + 1.0 mol % Fe

1050 1.18 0.91 14.5 15.0 Ò îÑ – – – – / 7  5 7  5 Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 1100 1.14 1.19 21.1 23.6 mol s 1 mol s 1 g 1 mol s 1 mol s 1 g 1 – – 1150 1.89 1.58 1000 0.72 0.48 1.88 1.27 – – 1200 3.11 2.75 1050 1.7 1.33 2.85 2.36 – 1 ± ± 10.7 EA/kJmol 141.2 16.9 201.1 1100 3.19 2.63 5.04 4.28 ± 0.246 ± 0.441 1150 5.59 4.92 9.69 9.05 logA 0.438 4.045 Ñ ÑàÎ Ñ ÑàÎ 1200 12.1 13.6 23.5 28.5 î +1 mol % +2 mol % – Ò Ñ – – – – 1 ± 14.0 ± 25.3 / 7  5 7  5 EA/kJmol 249,2 234,3 Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 1 1 1 1 1 1 ± 0.533 ± 0.968 mol s mol s g mol s mol s g log A 4.915 4.642 Ñ Ñ 900 0.87 0.61 0.66 0.47 + 1.0 mol % FeO + 1.0 mol % Fe2O3 Ò îÑ – – – – / 7  5 7  1 950 1.95 1.47 1.20 0.88 Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 mol s 1 mol s 1 g 1 mol s 1 mol s 1 g 1 1000 4.61 3.53 5.30 4.49 1000 1.79 1.46 1.83 1.55 1050 8.85 7.39 11.9 10.6 1050 3.17 2.62 4.17 3.63 1100 17.2 17.7 21.9 24.9 1100 5.34 4.76 7.44 10.4 – 1 ± 5.1 ± 22.0 EA/kJmol 223.5 279.2 1150 17.8 19.0 24.8 32.9 ± 0.209 ± 0.907 log A 4.724 7.031 1200 29.8 41.4 33.2 50.1 Ñ ÑàÎ – ± 30.8 ± 18.3 Ñ Î + 0.2 mol % E /kJmol 1 268.7 286.0 +2 mol % Mg A î + 0.8 mol % MgO ± 1.178 ± 0 Ò Ñ – – – – / 7  5 7  5 log A 6.081 6.906 .699 Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 1 1 1 1 1 1 mol s mol s g mol s mol s g (a) Calcium and magnesium oxides 950 0.83 0.61 1.67 0.86 It should be noted that catalytic activity of CaO is 1000 1.09 0.82 2.16 1.64 much better than that of MgO. For example, rate of 1050 3.19 1.57 10.8 10.2 ºC increases in graphite gasification reaction at 1100 1100 4.28 3.43 17.2 17.8 – following sequence: pure graphite (C) >C + 2% 1 ± 25.8 ± 48.7 EA/kJmol 161.6 304.7 MgO>C + 0.2% CaO + 0.8% MgO>C + 0.5% ± 1. ± 1.967 log A 1.623 040 7.887 CaO>C + 2% CaO. Reaction rate of sample of C + Ñ Ñ î + 0.1 mol % SrCO3 + 0.25 mol % SrCO3 1.0% CaO slightly drops out of this sequence. There- Ò Ñ – – – – / 7  5 7  5 Nc /10– (k2 [C[t–]])/1– 0 Nc /10– (k2 [C[t–]])/1– 0 fore, addition even of 0.2 mol % of calcium oxide speeds 1 1 1 1 1 1 mol s mol s g mol s mol s g up the reaction 5 more times than addition of 2.0 mol 950 0.66 0.48 0.98 0.72 % magnesium oxide. However, addition of MgO raises 1000 1.57 1.24 2.4 1.89 activation energy and prefactor values dramatically. 1050 6.46 6.27 10.7 10.0 As can be seen from TABLE 3, both values of EA 1100 27.7 37.1 30.0 43.8 and A grow with increment of of added calcium oxide – 1 ± 47.7 ± 36.7 EA/kJmol 407.3 389.1 amount. This is clearly shown at Figure 2. Therefore, ± 1.92 ± 1.48 log A 11.96 11.38 compensation effect occurs. . 8O4 riginal Article ChemXpress 5(3), 2014

Ò Figures 3 shows the Arrhenius plots and MgO. Isokinetic temperature  is equal to 1084 î   1  Ñ. (ln (k 2 [C[t] ]) f( T )) for reactions with additives of CaO 18

Figure 2 : The dependency of prefactor logarithm (log A) on activation energy (E ) for graphite gasification reaction with – pure graphite (C); – C + 0.2 mol % CaO + 0.8 mol % MgO; – CA + 0.5 mol % CaO; – C + 1 mol % CaO; presence of: 1 2 3 4 – C + 2 mol % CaO; – approximated line, log ± 1)  -3  – (3.7 ± 0.3), R2 5 6 A = (38 10 EA = 0.9977.

– 0.2 mol % CaO + 0.8 mol % MgO; – 0.5 Figure 3 : Arrhenius plots for graphite gasification reaction with additives of: 1 2 – 1.0 mol % CaO; – 2.0 mol % CaO; – isokinetic temperature Ò ± 18) K. mol % CaO; 3 4 5 è = (1357 ChemXpress 5(3), 2014 Original Article85

(b) Strontium carbonate and oxide in this case. The data on rate constants of C + CO reaction in The Arrhenius plots for corresponding reactions are 2 shown at Figure 5. Isokinetic temperature can be cal- presence of strontium compounds are listed in TABLE  îÑ. 3. As can be seen from table, at temperature range of culated very precisely and is equal to 1171 3 îÑ 900-1000 the highest gasification rate is achieved (c) Iron and iron oxides with addition of strontium oxide. The apparent rate con- In cases of adding metal iron to graphite gasifica- stant grows with strontium carbonate content increase î äî tion reaction speeding is observed only up to 900 C, in from 0.1 0.5 mol % at all studied temperatures. At addition, reaction rate grows significantly with increas- low temperatures catalytic activity of strontium carbonate ing of additive amount. Iron oxides increase rate of is less effective, than that of strontium oxide, because Ñ+ÑÎ 2 reaction even more. In relation to iron com- reaction rates with addition of strontium carbonate are pounds linear dependency between log A and EA is also lower, than with oxide. However, when temperature present and has high approximation rate (see Figure 6). îÑ, increases up to 1100 reaction rates become rela- However, isokinetic temperature is determined with large tively equal. This is because strontium carbonate be- uncertainty (see Figure 7). Moreover, statistical verifi- îÑ gins to decompose attemperatures above 1020 form- cation of isokinetic hypothesis[26] fails in this case, and it ing strontium oxide. is not possible to argue about reliable compensation. [28] The dependency between log A and EA for reac- According to it is quite possible that observed cor- tion with presence of strontium compounds is depicted relation between log A and EA is a consequence of ran- at Figure 4. With activation energy decrease, prefactor dom data scarce. drops also; therefore compensation effect is present According to[28] the formation of CO comes easier

Figure 4 : The dependency of prefactor logarithm (log A) on activation energy (E ) for graphite gasification reaction with – pure graphite (C); – C + 0.1 mol % SrCO – C + 0.25 mol %A SrCO – C + 0.5 mol % SrCO – C presence of: 1 2 3; 3 3; 4 3; 5 – approximated line, log ± 5)  -4  – (2.7 ± 0.2), R2 + 0.5 mol % SrO; 6 A = (361 10 EA = 0.9995. . 8O6 riginal Article ChemXpress 5(3), 2014

– 0.5 mol % SrO; – 0.5 mol % SrCO Figure 5 : Arrhenius plots for graphite gasification reaction with additives of: 1 2 ; 3 – 0.25 mol % SrCO – 0.1 mol % SrCO – isokinetic temperature Ò ± 3) K. 3 è 3; 4 3; 5 = (1444

Figure 6 : The dependency of prefactor logarithm (log A) on activation energy (E ) for graphite gasification reaction with – pure graphite (C); – C + 0.5 mol % Fe; – C + 1 mol % Fe; – CA + 1 mol % FeO; – C + 1 mol % Fe presence of: 1 2 3 4 5 2O3; – approximated line, log ± 5)  -3  – (6.3 ± 1.4), R2 6 A = (46 10 EA = 0.9724. ChemXpress 5(3), 2014 Original Article87

– 0.5 mol % Fe; – 1.0 mol % Fe; – 1.0 mol Figure 7 : Arrhenius plots for graphite gasification reaction with additives of: 1 2 3 – 1.0 mol % Fe – isokinetic temperature Ò ± 118) K. è % FeO; 4 2O3; 5 = (1254 – C bonds, that should be broken, are weakened. if C 3. The relationship between electron work functions This can take place, if graphite crystal lattice transfers of the oxides and their catalytic activity was noticed. an electron to intermediary metallic ion or if covalent bond forms between carbon matrix and metal atom. REFERENCES According to data in present study the following ’man, T.V.Bukharkina; Ki- consequence of gasification reaction rate increase can [1] N.A.Kurbatova, A.R.El Ñ be provided: (without additives) > Fe netics and , 52(5), 739 (2011). >MgO>FeO>CaO>SrO. It is almost coincident with [2] A.A.Lizzio, H.Jiang, L.R.Radovic; Carbon, 28(1), consequence of oxides ranged with respect to electron 7-19 (1990). work function decreasing. Therefore, catalytic proper- [3] S.Ergun; Journal of Physical Chemistry, 60(4), 480 (1956). ties of noted compounds can be linked with their ability [4] H.J.Grabke; Materials and Corrosion, 50(12), 686 to donate electrons, which take part in active centers – C bonds and increasing (1999). formation, weakening C [5] W.C.Conner Jr.; Journal of Catalysis, 78(1), 238 graphite gasification rate. (1982). [6] G.C.Bond, M.A.Keane, H.Kral, J.A.Lercher; Ca- CONCLUSIONS talysis Reviews: Science and Engineering, 42(3), 323 (2000). 1. Kinetics of reverse Boudouard reaction on graphite [7] L.Liu, Q.X.Guo; Chemical Reviews, 101(3), 673 was studied with presence of additives of calcium, (2001). magnesium, strontium, iron (II) and iron (III) ox- [8] W.Linert, R.F.Jameson; Chemical Society Reviews, ides, strontium carbonate and metallic iron. Appar- 18, 477 (1989). [9] J.J.Rooney; Catalysis Letters, 50(1-2), 15 (1998). ent rate constants are calculated. [10] A.P.Dhupe, A.N.Gokarn, L.K.Doraiswamy; Fuel, 2. The dependencies between activation energy and 70(7), 839 (1991). pre-exponential factor for various catalysts were [11] S.Li, Y.Cheng; Fuel, 74(3), 456 (1995). studied. Compensation effect was revealed. [12] G.Skodras; Central European Journal of Chemistry, Isokinetic temperatures were calculated. 11(7), 1187 (2013). . 8O8 riginal Article ChemXpress 5(3), 2014

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