Catalyst Deactivation Simulation Through Carbon Deposition in Carbon Dioxide Reforming Over Ni/Cao-Al 2O3 Catalyst

Available online at BCREC Website: http://bcrec.undip.ac.id

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 129 - 136

Research Article

Catalyst Deactivation Simulation Through Carbon Deposition in Carbon Dioxide Reforming over Ni/CaO-Al 2O3 Catalyst

I. Istadi 1) *) , Didi D. Anggoro 1) , Nor Aishah Saidina Amin 2) , and Dorothy Hoo Wei Ling 2)

1) Laboratory of Chemical and Process Engineering, Department of Chemical Engineering, Diponegoro University, Jl. Prof. Soedarto, Kampus Undip Tembalang, Semarang, Indonesia 50275 2) Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai Campus, Malaysia

Received: 10th May 2011; Revised: 16th August 2011; Accepted: 27th August 2011

Abstract

Major problem in CO 2 reforming of methane (CORM) process is coke formation which is a carbonaceous residue that can physically cover active sites of a catalyst surface and leads to catalyst deactivation. A key to develop a more cokeresistant catalyst lies in a better understanding of the methane reforming mecha nism at a molecular level. Therefore, this paper is aimed to simulate a microkinetic approach in order to calculate coking rate in CORM reaction. Rates of encapsulating and filamentous carbon formation are also included. The simulation results show that the studied catalyst has a high activity, and the rate of carbon formation is relatively low. This microkinetic modeling approach can be used as a tool to better understand the catalyst deactivation phenomena in reaction via carbon deposition. Copyright © 2011 BCREC UNDIP. All rights reserved.

Keywords : CO 2 reforming of methane; CORM; Ni/CaOAl 2O3; coke formation; microkinetic modeling; simulation

How to Cite : I. Istadi, D.D. Anggoro, N.A.S. Amin, and D.H.W. Ling. (2011). Catalyst Deactivation Simula tion Through Carbon Deposition in Carbon Dioxide Reforming over Ni/CaOAl 2O3 Catalyst. Bulletin of Chemical Reaction Engineering & Catalysis , 6 (2): 129136

1. Introduction involves the indirect utilization of methane and carbon dioxide to produce valuable synthesis gas. CO 2 reforming of methane (CORM) to produce Utilization of this process will bring both economic synthesis gas has drawn much interest in recent and environmental benefits. years because it consumes methane and carbon The main reaction for the production of dioxide which are both major contributors to the synthesis gas via CORM reaction is given in green house gases. Moreover, the availability of Equation 1. The CORM reaction is usually methane (which is the major component in natural accompanied by the simultaneous occurrence of the gas) has also encouraged development of this Reverse WaterGas Shift reaction (RWGS) as process [1]. The CORM is a catalytic process which written in Equation 2. However, the critical problem in this process is catalyst deactivation caused by carbon deposition (coking). Basically, * Corresponding Author. there are two types of carbon formation in this Telp: (+62247460058); Fax.: (+622476480675); reaction, i.e. encapsulating carbon and filamentous Email: [email protected] (I. Istadi)

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carbon (Figure 1). Therefore, in order to create a more stable catalyst for CORM, great attention has been focused on development of cokeresistant catalysts. Besides that, cost factor is also important to be considered if the catalyst will be applied industrially.

CH 4 + CO 2 ↔ 2CO + 2H 2 (∆H0298K = +247 kJ/mol) (1) CO 2 + H 2 ↔ CO + H 2O ( ∆H0 298K = +41 kJ/mol) (2)

During last several years, great efforts have been focused on development of catalysts for this process incorporating a kinetic inhibition of carbon formation under conditions where deposition is thermodynamically favorable. Nickelbased catalysts and noble metalsupported catalysts such as Rh, Ru, Pd, Pt and Ir were Figure 1 . Schematic of encapsulating and fila found to have promising catalytic performance in mentous carbon on nickel catalyst [1] terms of conversion and selectivity for the CORM process. According to results by Wang et al. [3], experimental system involved a series deactivation noble metalbased catalysts have shown to have scheme for the reaction under supercritical high activity and less sensitive to coking conditions. A comparison of simulation with compared to the Nibased catalysts for the CORM experiment has enabled a validation of the process. In spite of this, high cost and limited structural model and particular reaction scheme availability of noble metals prevent the used in the simulations. commercial use for this reaction. Therefore, it is In actual fact, the key to develop a more coke more practical to develop improved Nibased resistant catalyst lies in a better understanding of catalysts which exhibit stable operation for a long the methane reforming mechanism at a molecular period of time. level [7]. Microkinetic analysis is an examination In order to increase the stability of supported of catalytic reactions in terms of elementary Ni catalysts, some elements were added acting as chemical reactions that occur on the catalytic support modifiers or promoters. According to surface and their relation with each other and with some experimental researches, it has been proven the surface during a catalytic cycle. Microkinetics that the support type and presence of the has, for the most part, focused on analysis for modifiers greatly affect the coking tendency. understanding the reaction mechanism. It has According to FerreiraAparicio et al. [4] who been shown that microkinetic modeling based on prepared and tested two series of transition knowledge about elementary steps and their metals (Co, Ni, Ru, Rh, Ir, Pt)based catalysts energetics, is a very powerful tool for a detailed using silica and alumina in the CORM reaction, it understanding of catalytic processes [8]. This paper was shown that aluminabased catalysts show is aimed to simulate a microkinetic approach in higher specific activity than their respective order to calculate coking rate in CORM reaction. counterparts dispersed on silica. According to Rates of encapsulating and filamentous carbon Quincoces et al. [5], CaOmodified Ni catalysts formation are also included in this study. present high stability and good activity with respect to Ni/Al 2O3. When CaO is added to the 2. Modeling and Simulation Methods support, the formation of filamentous carbon decreases and favors the formation of more First step in this research was to study the reactive carbonaceous species. Recent simulation microkinetic model of methane reforming on Ni on catalyst deactivation due to coke formation catalysts developed by Aparicio [9]. The micro was done by Chigada and coworkers [6]. They kinetic model consists of adsorption and desorption suggested that longrange heterogeneities in the steps of all reactants and products and the surface pore structure of the catalyst are important for elementary reaction steps. The combined reaction determining deactivation behavior, and the for the main CORM reaction and the side RWGS

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 131 reaction is: According to Froment [9], the filamentous carbon formation involves the following process: 2 CO 2 + CH 4 ↔ 3 CO + H 2 + H 2O (3) (a) Dissolution of adsorbedcarbon through Ni: The reactions steps for both main and RWGS **C ↔ C Ni,f (44) reactions are combined and they are presented in (b) Diffusion of carbon through Ni: Table 1 [9]. For each step in the model, the CNi,f ↔ C Ni,r (45) reaction rates for forward and backward reactions (c) Precipitation / Dissolution of carbon: are written. It is assumed that the reaction steps CNi,r ↔ C f (46) are elementary, and the rates are written in terms of partial pressure and surface species. This rate is The adsorbed surface carbon, **C, at first calculated as turnover frequency. Table 2 shows dissolves in the Ni particle, therefore it diffuses the reaction rates for all the steps (Equations 4 and precipitates at rear of the nickel crystallite. 24). The next step in this research was to write the The continuous precipitation of this adsorbed mathematical models in terms of rate of reaction carbon will form filamentous carbon. For this for all the ten surface species and five gaseous process to take place, the carbon concentration in species. The differential equations against time is the layer just below the Ni surface, C Ni,f must written for each of the surface species namely exceed the carbon solubility in Ni, C sat . The higher *CH 3, *CH 2, *CH, *C, *H 2O, *OH, *CHO, *CO, *H the difference between C Ni,f and C sat , the bigger and *, and also the partial pressures for five the driving force for this process of filamentous gaseous species CH 4, CO 2, H 2O, H 2 and CO. The carbon formation. fifteen differential equations of surface species are A simple Langmuir equation was used to then solved simultaneously to obtain the fraction of estimate the concentration of carbon in the sites occupied by each surface species. The site segregation layer is given in Equation 47 [7]. coverage of adsorbed carbon would then be used to calculate the rate of coking. The fifteen differential G  equations are presented in Table 3 (Equations 25 θc xb seg = exp  (47) 39). θ1 c x1 b  RT  In this model, it is assumed that the atomic carbon on the surface is a common intermediate in This equation is used to calculate the weight both the main reaction and the carbon formation fraction of carbon in the segregation layer, x b. The including filamentous and encapsulating carbon. values of θc are obtained from the simulation According to Chen et al. [7], the reaction between results. The value of θc needs to be divided by 2, as the adsorbed carbon on the surface is assumed to one carbon atom occupies two active sites on the be reversible and the sole pathway for catalyst. encapsulating carbon formation, as shown in Equation 48 is used to calculate the Gibbs Equation 40 [6]. energy for the segregation layer in this process [7]:

n **C → n C p (40) Gseg = 10800 – 3.4T (cal/mol) (48)

Therefore, the rate for encapsulating carbon The rate of carbon diffusion through Ni, which is formation is given in Equation 41 [7]. also the rate of filamentous carbon formation, is given in Equation 49 [8]. rcp = k p θcn (41) D   r = C a C −C  (49) The weight of encapsulating carbon is given in d Ni Ni,f sat Ni Equation 42: where C Ni,f is calculated based on x b in Equation Cp = r cp . dt . 12 (42) 46 by multiplying xb with a Ni density of 8900 kg/m 3. The site coverage of encapsulating carbon is All values for constant parameters are obtained calculated from Equation 43 [8]: from Chen et al. [7]. The values are presented in C M Table 4. All the necessary equations were solved = p Ni θ cp using MATLAB Df Ni M c (43)

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Table 1. Reaction steps for the combined reaction [9] Reaction Steps k E CH 4 + 2* → *CH 3 + *H 2.02 x 10 6 53. 9 *CH 3 + *H → CH 4 + 2* 2.50 x 10 10 95.9 *CH 3 + * → *CH 2 + *H 1 x 10 13 115.4 *CH 2 + *H → *CH 3 + * 2 x 10 12 75.4 *CH 2 + * → *CH + *H 1 x 10 13 102.9 *CH + *H → *CH 2 + * 2 x 10 12 75.4 *CH + 2* → **C + *H 1 x 10 13 0 **C + *H → *CH + 2* 1.54 x 10 11 64.4 2 [ CO 2 + 2*H + 2* → **CHO + OH* + * ] 9.97x10 6 (1/T) 0.968 50.0 2 [ **CHO + OH* + * → CO 2 + 2*H + 2* ] 2.9 x 10 15 0 3 [ **CHO + * → **CO + *H ] 5.14 x 1019 0 3 [ **CO + *H → **CHO + * ] 1 x 10 7 23.0 3 [ **CO → CO + 2* ] 5 x 1012 115.0 3 [ CO + 2* → **CO ] 1 x 10 8 0 2*H → H 2 + 2* 1 x 10 13 95.0 H2 + 2* → 2*H 3 x 10 8 0 **C + *OH → **CHO + * 3 x 10 14 65.5 **CHO + * → **C + *OH 1.13x10 21 (1/T) 3.03 90.3 *H + *OH → *H 2O + * 3.08 x 10 11 32.2 *H 2O + * → *H + *OH 4.15 x 10 7 0 *H 2O → H 2O + * 1 x 10 13 64.4 H2O + * → *H 2O 1.78 x 10 6 0

Table 2. Reaction rates for all the steps

Step Reaction Rates Equation Num- ber 1 CH 4 + 2* → *CH 3 + *H R1 = k 1PCH4 θ*2 (4) 2 *CH 3 + *H → CH 4 + 2* R2 = k 2θCH3 θH (5) 3 *CH 3 + * → *CH 2 + *H R3 = k 3θCH3 θ * (6) 4 *CH 2 + *H → *CH 3 + * R4 = k 4θCH2 θH (7) 5 *CH 2 + * → *CH + *H R5 = k 5θCH2 θ* (8) 6 *CH + *H *CH 2 + * R6 = k 6θCH θH (9) 7 *CH + 2* → **C + *H R7 = k 7θCH θ* 2 (10) 8 **C + *H → *CH + 2* R8 = k 8θCθH (11) 9 2 [ CO 2 + 2*H + 2*….→ **CHO + OH* + *] R9 = k 9PCO2 θH2 θ*2 (12) 10 2 [ **CHO + OH* + * → CO 2 + 2*H + 2* ] R10 =k 10 θCHO θOH θ* (13) 11 3 [ **CHO + * → **CO + *H ] R11 = k 11 θCHO θ* (14) 12 3 [ **CO + * H → **CHO + * ] R12 = k 12 θCO θH 13 3 [ **CO→ CO + 2* ] R13 = k 13 θCO (15) 14 3 [ CO + 2* → **CO ] R14 = k 14 PCO θ* 2 (16) 15 2*H → H 2 + 2* R15 = k 15 θH2 (17) 16 H2 + 2* → 2*H R16 = k 16 PH2 θ* 2 (18) 17 **C + *OH → **CHO + * R17 = k 17 θCθOH (19) 18 **CHO + * → **C + *OH R18 = k 18 θCHO θ* (20) 19 *H + *OH → *H 2O + * R19 = k 19 θHθOH (21) 20 *H 2O + * → *H + *OH R20 = k 20 θH2O θ* (22) 21 *H 2O → H 2O + * R21 = k 21 θH20 (23) 22 H2O + * → *H 2O R22 = k 22 PH2O θ* (24)

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Table 3. Rate equations for surface species and gaseous species

Species Term in rates Equation Num- ber dθ /dt R1R2+R 3R4+R 5R6+R 7R84(R 9)+4(R 10 )+3(R 11 ) (25) H 3(R 12 )(R 15 )+2(R 16 )R19 +R 20 R1R2R3+R 4 (26) dθCH /dt 3 R3R4R5+R 6 (27) dθCH /dt 2 dθ /dt R5R6R7+R 8 (28) CH dθ /dt R7R8R17 +R 18 (29) C R19 R20 R21 +R 22 (30) dθH O /dt 2 dθ /dt 2(R 9)2(R 10 )R17 +R 18 R19 +R 20 (31) OH dθ /dt 2(R 9)2(R 10 )3(R 11 )+3(R 12 )+R 17 R18 (32) CHO dθ /dt 3(R 11 )3(R 12 )3(R 13 )+3(R 14 ) (33) CO dθ /dt 2(R 2)2(R 1)R3+R 4R5+R 62(R 7)+2(R 8) (34) * 4(R 9)+4(R 10 )3(R 11 )+3(R 12 )+6(R 13 )6(R 14 )+2(R 15 )

2(R 16 )+R 17 R18 +R 19 R20 +R 21 R22 R2R1 (35) dPCH /dt 4 2(R 10 )2(R 9) (36) dPCO /dt 2 R21 R22 (37) dPH O /dt 2 R15 R16 (38) dPH /dt 2 dP /dt 3(R 13 )3(R 14 ) (39) CO

Table 4. Constant parameters used in the simulation [7]

Parameter Value Unit

aNi 0.33 m3/g cat

Csat 1.45 mol C/m 3 Ni

dNi 7.467 x 10 8 m D 0.0046 surface Ni atoms/ total Ni atoms

Dc 5.32 x 10 10 cm 2/s

fNi 0.11 g Ni/g cat

kp 605 mol C/mol of site,s

MC 12.01 g C/mol C MNi 58.69 g Ni/mol Ni R 1.987 cal/mol K T 923.15 K

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3. Results and Discussion carbon formation increases initially at the reaction. This is because the reaction takes place According to Wang and Lu [11] who evaluated very fast in the beginning, and the site coverage the catalyst performance of various Ni catalysts of adsorbed carbon is high. Therefore, this will with different supports, it was discovered that increase the rate of encapsulating carbon both types of Ni/Al 2O3 catalysts, namely Ni/ γ formation. However, after a certain period of Al 2O3 and Ni/αAl 2O3, give very high CO 2 and CH 4 time, the rate starts to decrease until the end of conversions. Figure 2 indicates that the fraction the simulation time. According to Mieville [13] of vacant sites decrease drastically at beginning who studied the kinetics of coking for a reforming of the reaction. This result shows that the process, there is an inverse relationship between catalyst has a high activity. The fraction of vacant the coking rate and the amount of coke formed. sites becomes stagnant after a short time in the This is similar to the Voorhies equation originally reaction. Theoretically, the fraction of vacant sites derived for catalytic cracking. should be decreasing to zero, meaning that all active sites are participating in the reaction. However, this result cannot be achieved in practical. There are certain numbers of sites -10 which are not functioning in the reaction. Some of x 10 these sites are hidden in the catalyst pores, and 6 due to diffusional limitations, they are not reachable by the reacting species. Ding and Yan 5 [12] who conducted their research on MgO and CeO 2 promoters on Ni/Al 2O3 catalysts suggested that this phenomenon happened because of the 4 blockage and coverage of active Ni sites by species originating from the promoters. Therefore, it is 3 assumed that the CaO promoter used in this model could also lead to the same consequences. In this model, the maximum amount of 2 encapsulating carbon is treated as a monolayer of carbon on the Ni surface, and this monolayer is 1 sufficient to block the active sites and consequently deactivate the catalyst. Any Rate of encapsulating Cformation(mol encapsulating cat.h) of g C/ Rate 0 multiple layers of carbon are treated as 0 0.5 1 1.5 2 filamentous carbon in this model. From Figure 3, Time (h) it is observed that the rate of encapsulating Figure 3. Rate of encapsulating carbon formation

-5 1 x 10 5

0.8 4

0.6 3

0.4 2 Fractionvacantof sites 0.2 1 Site coverageSite encapsulatingof C 0 0 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Time (h) Time (h)

Figure 2. Fraction of vacant sites of the catalyst Figure 4 . Site coverage of encapsulating carbon

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-9 x 10 Site coverage and weight of encapsulating 4.5 carbon simulations are depicted in Figure 4 and Figure 5, respectively. Figure 4 shows that the site 4 coverage of encapsulating carbon increases quite 3.5 fast at the beginning of the reaction, and becomes stagnant at the end of simulation time. This can be 3 explained by saying that at the beginning, there 2.5 are a lot of vacant sites on the catalyst, meaning that there is a potentially higher possibility of the 2 formation of encapsulating carbon. As the reaction 1.5 proceeds, most of the active sites are already being 1 occupied, therefore there are fewer sites to be covered, and subsequently the site of encapsulating 0.5 carbon becomes stagnant. In an overall view, the Weightencapsulatingof g cat ) / C (g C 0 site coverage of encapsulating carbon is relatively 0 0.5 1 1.5 2 low, only at about 105 from the total active sites. Time(h) Figure 6 depicts the rate of filamentous carbon formation in which the negative values indicate Figure 5 . Weight of encapsulating carbon that filamentous carbon is not formed in our

-3 model. Many reviews have been done regarding the x 10 formation on filamentous carbon. It has been -0.9 proposed that catalyst deactivation by carbon deposition depends on the amount, type and -0.95 location of carbon formed. Quincoces et al. [5] suggested that catalyst promoted with 3% of Ca -1 presents lesser amount of whisker (filamentous) carbon with respect to Ni/Al 2O3 catalyst. The CaO -1.05 addition inhibits the whisker carbon formation and is therefore more stable. This stability is related to -1.1 the formation of more reactive carbonaceous residues that act as reaction intermediate during -1.15 methane reforming. According to Kim et al. (2000) [14], the minimum metal particle diameter needed -1.2 to form filamentous carbon is 6 nm. The Ni particle size used in the simulation is 112 nm. Ratefilamentous of cat.h) g / CCformation (mol -1.25 Theoretically, filamentous carbon should be formed 0 0.5 1 1.5 2 in this model based on this criterion. However, this Time(h) phenomenon did not happen, and can be attributed Figure 6. Rate of filamentous carbon formation to the inappropriate equations used in the model.

4. Conclusions inappropriate equations used to calculate the rate of filamentous carbon formation. However, this From the simulation results, the rate of research serves as a good starting point for the encapsulating carbon formation and the site subsequent studies in microkinetic modeling. coverage of encapsulating carbon is relatively low.

The catalyst used in this model also presented a high activity which attributed to the effect of Al 2O3 Nomenclature which serves as a good support for Ni. The simulation results also showed that filamentous aNi Ni surface area in catalyst (m 2/g cat) carbon is not formed in this model. Although the A Stoichiometric matrix addition of CaO promoter has been proven to CNi,f Carbon concentration below the Ni inhibit the filamentous carbon formation, the surface (mol C/m 3 Ni) factor of Ni particle size has shown that there CNi,r Carbon concentration on the support side should be a significant amount of filamentous (mol C/m 3 Ni) carbon formed. This could be due to the

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Cp Weight of encapsulating carbon (g C/g Art. Energy and Fuels . 10: 896904. cat) [4] FerreiraAparicio, P., GuerreroRuiz, A. and Cf Weight of filamentous carbon (g C/g RodriguezRamos, I. (1998). Comparative Study cat) at Low and Medium Reaction Temperatures of Csat Carbon solubility in Ni (mol C/m 3 Ni) Syngas Production by Methane Reforming with dNi Diffusion path length of carbon in Ni Carbon Dioxide over Silica and Alumina (m) Supported Catalysts. Applied Catalysis A: D Dispersion of Ni in catalyst (surface Ni General . 170: 177187. atoms/total Ni atoms) [5] Quincoces, C.E., Dicundo, S., Alvarez, A. M., and Dc Effective diffusivity for carbon in Ni Gonzalez, M.G. (2001). Effect of Addition of CaO (cm 2/s) on Ni/Al 2O3 Catalysts over CO2 Reforming of E Activation energy for reaction (kJ/mol) Methane. Materials Letters . 50: 2127. fNi Weight fraction of Ni in catalyst (g Ni/g [6] Chigada, P.I., Wang, J., AlDuri, B., Wood, J., cat) Rigby, S.P. (2010). Modelling of pore structure k Preexponential factor in Arrhenius evolution during catalyst deactivation and equation (mol/site.s) comparison with experiment. Chemical kp Turnover frequency for encapsulating Engineering Science . 65: 55505558. carbon (mol C/site.s) [7] Chen, D., Lødeng, R., Anundskas, A., Olsvik, O., m Number of elementary reaction steps and Holmen, A. (2001). Deactivation during MC Molecular weight of carbon (g C/ mol C) Carbon Dioxide Reforming of Methane over Ni MNi Molecular weight of Ni (g Ni/mol Ni) Catalyst: Microkinetic Analysis. Chemical Pi Partial pressure of species (bar) Engineering Science . 56: 13711379. rcp Rate of encapsulating carbon formation [8] Chen, D. (2002). Personal Communication . (mol C/g cat.h) r Rate of filamentous carbon formation [9] Aparicio, L. M. (1997). Transient Isotopic Studies and Microkinetic Modeling of Methane Reforming (mol C/g cat.h) over Nickel Catalysts. Journal of Catalysis . 165: R Gas constant (cal/mol K) 262274. Ri Rate of reaction (s 1) t Time of reaction [10] Froment, G.F. (2000). Production of Synthesis T Reaction temperature (K) Gas by Steamand CO2 Reforming of Natural Gas. Journal of Molecular Catalysis A: Chemical . xb Weight fraction of carbon in 163: 147156. segregation layer (g C/g Ni) θi Fraction of sites covered by surface [11] Wang, S. and Lu, G.Q.M. (1998). CO 2 Reforming species of Methane on Ni Catalysts: Effects of the * Active sites on catalyst Support Phase and Preparation Technique. Applied Catalysis B: Environmental . 16: 269277. b Stoichiometric number of reaction Gseg Gibbs energy for segregation layer of Ni [12] Ding, R. G., and Yan, Z. F. (2001). Structure (cal/mol) Characterization of the Co and Ni Catalysts for Carbon Dioxide Reforming of Methane. Catalysis H° 298 Standard enthalpychange of reaction (J mol 1) Today . 68: 135143. [13] Mieville, R. L. (1991). Coking Kinetics of Reforming. Catalyst Deactivation . 68: 151167. 5. References [14] Kim, J. H., Suh, D. J., Park, T. J., and Kim, K. L. (2000). Effect of Metal Particle Size on Coking [1] Lunsford, J. H. (2000). Catalytic Conversion of during CO 2 Reforming of Methane over Ni Methane to More Useful Chemical and Fuels, a alumina Aerogel Catalysts. Applied Catalysis A: Challenge for the 21st Century. Catalysis General . 197: 191200 . Today . 63: 165174.

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