Nanocrystalline Zirconia as Support for Nickel Catalyst in Methane

Energy & Fuels 2006, 20, 923-929

923

Nanocrystalline Zirconia as Support for Nickel Catalyst in Methane Reforming with CO2 Mehran Rezaei* and Seyed Mahdi Alavi Chemical Engineering Department, Iran UniVersity of Science and Technology, Tehran, Iran

Saeid Sahebdelfar Petrochemical Research & Technology Company (NPC-RT), Tehran, Iran

Zi-Feng Yan* State Key Laboratory for HeaVy Oil Processing, Key Laboratory of Catalysis, CNPC, China UniVersity of Petroleum, Dongying 257061, China ReceiVed NoVember 21, 2005

The surfactant-assisted route by using hexadecyltrimethylammonium bromide (CTAB) as surfactant and the combustion method by using sucrose as a chelating agent and template material were used to synthesis the nanocrystalline zirconia as a support for the preparation of nickel catalysts. The supports and catalysts were characterized by X-ray diffraction, nitrogen adsorption, mercury porosimetry, transmission electron microscopy (TEM), thermogravimetric (TG) and differential thermal analysis (DTA), and hydrogen sulfide chemisorption analyses. The catalytic activity measurements showed that the support synthesis method has a strong effect on the catalytic activity and stability of nickel catalysts. The nickel catalyst that was supported on the zirconia prepared by the surfactant-assisted route showed higher activity and stability during the reaction, which was related to the high thermal stability of the support during the reaction.

Introduction During the past decades, the process of carbon dioxide reforming with methane has received attention.1-3 Efforts have focused on development of catalysts that show high activity toward synthesis gas formation and are also resistant to carbon formation, thus displaying stable long-term operation. Nickel might be the optimum active component of the potential catalyst designed.4 Carbon deposition is a fatal problem to nickel-based catalyst in the dry reforming process.5 In addition, it has been accepted that the catalytic activity depends on the nature of the support, active phase precursor, synthesis method, and pretreatment.5,6 Nickel-based catalysts have shown an excellent behavior in this reaction, with an activity comparable to noble metal catalysts.4,7 Ni has been supported on different carriers such as MgO, Al2O3, promoted Al2O3, TiO2, CeO2, etc. However, it tends to deactivate by coke formation,4,5 which is closely related to the catalyst structure and composition.8 The use of supports with low concentration of Lewis acid sites and/or the presence * Address correspondence to either author. E-mail: [email protected] (Z.-F.Y.); [email protected] (M.R.). (1) Chang, J.S.; Park, S. E.; Yoo, J. W.; Park, J. N. J. Catal. 2000, 195, 1-11. (2) Portugal, U. L.; Santos, A. C. S. F.; Damyanova, S.; Marques, C. M. P.; Bueno, J. M. C. J. Mol. Catal. A 2002, 184, 311-322. (3) Cheng, Z. X.; Zhao, X. G.; Li, J. L.; Zhu, Q. M. Appl. Catal. A 2001, 205, 31-36. (4) Rostrup-Nielsen, J. R.; Bak Hansen, J. H. J. Catal. 1993, 144, 38. (5) Wang, S.; Lu, G. Q. Appl. Catal. 1998, 169, 271-280. (6) Wang, S.; Lu, G. Q. Appl. Catal. B 1998, 16, 269-277. (7) Hegarty, M. E. S.; O’Connor, A. M.; Ross, J. R. H. Catal. Today 1998, 42, 225.

of basic sites, such as ZrO2, MgO, and La2O3, resulted in enhanced activities, lower carbon deposition rates, and therefore more stable catalysts.9,10 Among these supports, ZrO2 has a high thermal stability as a catalyst support.11-13 ZrO2 has three polymorphs: monoclinic (m-phase, below 1170 °C), tetragonal (t-phase, between 1170 and 2370 °C), and cubic (c-phase, above 2370 °C).14 The high-temperature cubic and tetragonal phases can be stabilized to room temperature by incorporating other dopants in the crystal lattice; these dopants include CaO, MgO, Y2O3, CeO2, etc.15,16 Among them, the tetragonal phase (t-ZrO2) has both acid and basic properties,17 and in addition, it has been found that the t-ZrO2 is the most active phase for some reactions.18 Many methods have been explored in order to get (8) Bradford, M. C. J.; Vannice, M. A. Catal. ReV. Sci. Eng. 1999, 41 (1), 1-42. (9) Lercher, J.A.; Bitter, J. H.; Hally, W.; Niessen, W.; Seshan, K. Stud. Surf. Sci. Catal. 1996, 101, 463. (10) Tomishige, K.; Chen, Y. G.; Fujimoto, K. J. Catal. 1999, 181, 91. (11) Clifford, Y. T.; Hsiao, B.Y.; Chiu, H. Y. Colloids Surf. A 2004, 237, 105-111. (12) Ma, T.; Huang, Y.; Yang, J.; He, J.; Zhao, L. Mater. Des. 2004, 25, 515-519. (13) Lee, M. H.; Clifford, Y. T.; Lu, C. H. J. Eur. Ceram. Soc. 1999, 19, 2593-2603. (14) Luo, T.Y.; Liang, T. X.; Li, C. S. Mater. Sci. Eng. A 2004, 366, 206-209. (15) Ray, J. C.; Saha, C. R.; Pramanik, P. J. Eur. Ceram. Soc. 2002, 22, 851-862. (16) Peshev, P.; Stambolova, I.; Vassilev, S.; Stefanov, P.; Blaskov, V.; Starbova, K.; Starbov, N. Mater. Sci. Eng. B 2003, 97, 106-110. (17) Yamaguchi, T. Catal. Today 1994, 20, 199. (18) Centini, G.; Cerrrato, G.; Angelo, S. D.; Finardi, U.; Giamello, E.; Morterra, C.; Perathoner, S. Catal. Today 1996, 27, 265.

10.1021/ef050384k CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006

924 Energy & Fuels, Vol. 20, No. 3, 2006

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Figure 1. Preparation of zirconium oxide: (a) surfactant-assisted route, (b) sucrose method.

superfine ZrO2 powders with high surface area, such as glycothermal process,19 alcohothermal-SCFD (supercritical fluid drying) process,20 CO2 supercritical drying,21 sol-gel method,22 solid-state reaction method,23 precipitation method, and sucrose method. The sucrose method is a new chemical route for the preparation of nanopowders. Rabindra and co-works prepared nanocrystalline R-Al2O3 with this method.24 Decomposition of sucrose generates excess heat and a huge amount of gases that help to produce the porosity in the final zirconia. In this method, 1 mol of sucrose produces 23 mol of gases as shown in eq 1:

C12H22O11 + 12O2 f 12CO2 + 11H2O

(1)

Precipitation by increasing the pH of a zirconium salt solution resulting in a gelatinous, amorphous hydroxide is wellknown. In this paper, comparison between the structural properties of the prepared samples by sucrose and surfactantassisted route methods was conducted, and their applications as a support for nickel catalysts in dry reforming reaction were investigated. Experimental Section Materials. The starting materials were ZrO(NO3)2‚xH2O (x is ca. 6; Aldrich), hexadecyltrimethylammonium bromide (Fluka), ammonium hydroxide, 25 wt % (Fluka). Sucrose was used as a chelating agent and a template material in the sucrose method. Zirconia Preparation. In the surfactant-assisted route, aqueous ammonia (25 wt %) was added dropwise at room temperature to an aqueous solution containing zirconia precursor and hexadecyltrimethylammonium bromide under rapid stirring, The surfactant to zirconia molar ratio and pH were chosen as 0.5 and 11, respectively. After precipitation, the slurry was stirred for another 30 min and then heated at 80 °C under continuous stirring for 24 h. After refluxing the mixture, it was cooled to room temperature, (19) Inoue, M.; Sato, K.; Nakamura, T.; Inui, T. Catal. Lett. 2000, 65, 79. (20) Hu, J. C.; Cao, Y.; Deng, J. F. Chem. Lett. 2001, 398. (21) Ward, D. A.; Ko, E. I. Chem. Mater. 1993, 5, 956. (22) Aguilar, D. H.; Torres-Gonzalez, L. C.; Torres-Martinez, L. M.; Lopez, T.; Quintana, P. J. Solid State Chem. 2000, 158, 349. (23) Liu, X.; Lu, G.; Yan, Z. F. J. Natural Gas Chem. 2003, 12, 161166. (24) Das, R. N.; Bandyopadhyay, A.; Bose, S. J. Am. Ceram. Soc. 2001, 84 (10), 2421-23.

filtered, and washed, first with deionized water and finally with acetone in order to remove the surfactant. The final product was dried at 100 °C for 24 h under flowing air and calcined at 600 °C for 10 h or at 800 °C for 0.5 h. A diagram of the preparation is shown in Figure 1a. Sucrose Decomposition Approach. ZrO2 was synthesized using sucrose to metal-ion mole ratio of 6:1. For ZrO2 synthesis with 6:1 sucrose to metal ion ratio, 0.04057 mol of ZrO(NO3)2‚xH2O was dissolved in 50 mL of deionized water to prepare a metal salt solution. Sucrose solution was prepared separately by adding 0.243 mol of sucrose into 180 mL of deionized water. Two solutions were then mixed and continuously stirred to obtain a homogeneous solution. The solution was then heated at 90 °C for 90 min on a hot plate with continuous stirring to lead the ions to react with sucrose completely, resulting in a brown dark solution. The solution will be heated on the hot plate at 110 °C for dehydration and will continue until the solution changes into a viscous dark brownish gel. After that, this gel will be heated to 200 °C overnight in oven for complete dehydration, which resulted in a black foamed mass. The foamy black mass will be ground into powders and calcined in a furnace at 600 and 800 °C for 10 h and 30 min, respectively. After calcination, carbon will be oxidized from the black precursor powders to form a white powder. A schematic of the processing steps for this method is given in Figure 1b. Catalyst Preparation. Supported nickel catalysts were prepared by impregnating pellets of prepared zirconia by different methods with an aqueous solution of nickel nitrate. For making some pellets of zirconia, the powders of zirconium oxide calcined at 600 °C for 10 h were pressed under 50 kN to obtain some pellets (dpellet ) 15 mm) with high mechanical strength. The obtained pellets were calcined at 800 °C for 30 min and used for impregnating with nickel nitrate solution. After impregnation, the pellets were dried at 80 °C and calcined at 450 °C. Previous to reaction, the different samples were reduced with a pure hydrogen flow (GHSV ) 2000 L/kgcat‚h) at a heating rate of 10 °C min-1 from room temperature to 525 °C and then held at 525 °C for 4 h. Characterization. The surface areas (BET) were determined by nitrogen adsorption using a nitrogen adsorption analyzer (Micromeritics ASAP 2010) at 77.3 K. The pore size distribution was determined by a Quantachrome Hg-porosimeter. The XRD patterns were recorded on an X-ray diffractometer (Philips-PW-1840) using a Cu KR monochromatized radiation source and a Ni filter in the range 2θ ) 5° to 70°. The amounts of tetragonal and monoclinic ZrO2 present in the ZrO2 samples and the crystallite sizes were estimated as reported elsewhere.25 Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out in a Netzsch (25) Su, C.; Li, J.; He, D.; Cheng, Z.; Zhu, Q. Appl. Catal. A 2000, 202, 81-89.

Nanocrystalline Zirconia Supports

Energy & Fuels, Vol. 20, No. 3, 2006 925

Figure 2. TG/DTA plots for (a) ZrO2-P and (b) ZrO2-S precursors.

STA 409 system in a static air atmosphere at a heating rate of 10 °C/min. TEM investigations were performed with Philips CM200 FEG UltraTwin operated at 200 kV. Images were recorded with a Gatan model 794 CCD camera. The nickel surface areas was determined by chemisorption of hydrogen sulfide as described elsewhere26 (conditions: H2S/H2 ) 15 × 10-6 vol/vol, 550 °C. 100 h). The metal surface area was calculated by assuming a monolayer of 44.5 × 10-9 g of S‚cm-2 for nickel and corresponding to 0.5 sulfur atom per nickel atom (S/Ni ) 0.5) on the (100) surface.26 The nickel area is calculated by using S0 ) 440 wt ppm equivalent to 1 m2‚g-1, with S0 being the sulfur capacity of the catalyst (µg of S/g of Ni). A mean nickel particle diameter can thus be estimated from26 dNi ) 3 × 10

X 3 Ni

be calculated from26 D ) 0.034 ×

S0 ) 1.01 × 102/dNi XNi

(3)

The activity of the catalyst was performed in a fixed bed reactor operated at atmospheric pressure. The feed flows were 1.6 L/h of H2, 4 L/h of CH4, and 16 L/h of CO2. A specific weight of catalyst was loaded into the tube reactor made of stainless steel (6 mm i.d.). A thermowell with external diameter of 2 mm was placed in the center of the reactor. The effluent was analyzed with an online GC (HEWLETT 5890) equipped with a thermal conductivity detector (TCD).

Results and Discussion (2)

Thermal Analyses. Figure 2 shows the TG/DTA plots for the ZrO2 precursors. For the sample prepared by surfactant-

where dNi is given in nm, and where XNi is the wt % of nickel in the reduced state. The corresponding dispersion (%) of nickel can

(26) Rostrup-Nielsen, J. R. Catalytic Steam Reforming; Haldor topsøe A/S: Nymollevej 55, DK-2800 Lyngby, Denmark, 1984.

S0


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Figure 3. XRD patterns for (a) ZrO2-P (600 °C), (b) ZrO2-P (800 °C), (c) ZrO2-S (600 °C), and (d) ZrO2-S (800 °C). Table 1. Phase Composition and Crystallite Sizes of the Samplesa crystallite size (nm) sample

calcination conditions

ZrO2-S ZrO2-S ZrO2-P ZrO2-P

600 °C/10 h 800 °C/0.5 h 600 °C/10 h 800 °C/0.5 h

tetragonal wt % t(101) t(112) t(211) m(11h1) m(111) 81 47 100 100

13.2 25.4 7.2 8.5

8.9 11.2 5.7 6.6

13.4 18.4 4.7 5.4

12.4 27.1

13.2 24.4

a S, corresponding to the sucrose method. P, corresponding to the surfactant-assisted route.

assisted route (ZrO2-P), Figure 2a, the DTA curve presents four major peaks in which two are endothermic and the others are exothermic. The first endothermic peak appears at low temperature about 100 °C, which corresponds to the elimination of residual water, and the second one appears at about 300 °C, which is due to the removal of hydroxyl group bonded on the surface of zirconia. Simultaneously, the first exothermic peak at 400 °C is attributed to the oxidation of organic residues, and the second one at 570 °C, usually called “glow exotherm”, is attributed to the crystallization of amorphous zirconia. Simultaneously, the TG curve, curve (a) in Figure 3, is leveled off at about 600 °C, meaning that the organic residues have been removed completely. For the sample prepared by sucrose method (ZrO2-S), Figure 2b, the exothermic decompositions take place between 270 and 520 °C, which resulted in a weight loss of about 90% that is attributed to the removal of hydroxyl groups and the burning and removal of sucrose, which is accompanied by exothermic reaction. Phase Analyses. The crystallite phase compositions for these samples were measured by XRD analysis, and the obtained results are shown in Figure 3 and Table 1. Figure 3 indicated that ZrO2-S, which was calcined at 600 °C, was a mixture of the tetragonal and monoclinic phases. By increasing the calcination temperature up to 800 °C, the tetragonal weight percent decreased and sintering occurred, resulting in an increase in crystallite sizes. For the ZrO2-P at both calcination temperatures, just the tetragonal phase appeared, and the crystallite sizes are much smaller than of that the ZrO2-S. These results indicated that the preparation method has a strong influence on the crystallite phase and also the crystallite size of the zirconium oxide. The thermodynamically most stable ZrO2 phase at room temperature is the m-phase. Probably, the nanosize effect of

the ZrO2 crystallites leads to the thermal stabilization of the t-phase.27,28 Garvie suggested that the difference in the surface energy between the tetragonal and the monoclinic phases could cause the tetragonal phase to be thermodynamically stable for very small crystals.29,30 As shown in Figure 4, the zirconium oxides prepared by the surfactant-assisted route and sucrose method have similar morphology, and the nanoparticles are closely sintered together and have a slightly irregular, rounded shape. For ZrO2-S, the particle sizes are bigger, and the structure seems to be denser. Particle sizes in ZrO2-P, which was calcined at 600 °C, are from 4 to 10 nm in diameter, which are in a good agreement with the obtained crystallite size by XRD, while particles in ZrO2-S are bigger (from 12 to 20 nm). Increasing the calcination temperature up to 800 °C resulted in an increase in the particle sizes of the ZrO2-P to 6-15 nm. BET and Mercury Porosimetry Analyses. The specific surface areas of the samples at different calination conditions are reported in Table 2. As it can be seen, ZrO2-P has a higher surface area than that of ZrO2-S. Increasing the calcination temperature up to 800 °C led to a decrease in specific surface areas caused by nanoparticle sintering. The pore size distributions were determined by pressing the calcined powder of ZrO2-S and ZrO2-P at 600 °C to pellets followed by calcination at 800 °C for 30 min. The pore size distributions are plotted in Figure 5, and the pore volume and average pore diameters are given in Table 3. Figure 5 indicated that the ZrO2-P pellets have a mesoporous structure with very small pore sizes, where the ZrO2-S pellets are a mixture of mesoand macropores, and the pore size distribution is much broader than that of ZrO2-P. The porosity of the ZrO2-P pellets is higher than that of the ZrO2-S pellets, resulting in higher total pore volume for this sample. Catalyst Structural Studies. The nickel catalysts supported on the synthesized zirconium oxides by surfactantassisted route and sucrose methods were prepared according to the method described earlier. The nickel contents in both of them were chosen to be about 9%. The XRD patterns and the sulfur capacity results were performed for these catalysts, and the obtained results are shown in Figure 6 and Table 4. Figure 6 indicates that the peaks corresponding to 2θ ) 44.5° (Ni(111)) in the Ni/ZrO2-S are sharper than that of the Ni/ZrO2-P, indicating the lower dispersion of nickel in this catalyst. The sulfur capacity results indicated that the Ni supported on ZrO2-P has a high sulfur capacity, which means that it has the high nickel surface area in comparison to the nickel catalyst supported on ZrO2-S. The nickel crystallite sizes of the Ni/ZrO2-P catalyst were between 10 and 30 nm (Figure 7). As can be seen, the obtained nickel crystallite sizes by sulfur capacity were different from the crystallite sizes that were obtained by XRD. Generally smaller nickel particle sizes are indicated by XRD than by other methods. The main reason is that nickel particles are polycrystalline: XRD gives an average size of the crystalline domains, which are smaller than the total particle size estimated by chemisorption and transmission electron microscopy. Activity Measurements. Before the catalytic test, the reduced catalyst pellets were crushed and sieved to obtain grains with (27) Skandan, G.; Foster, C. M.; Frase, H.; Ali, M. N.; Parker, J. C.; Hahn, H. Nanostruct. Mater. 1992, 1, 313. (28) Garvie, R. C.; Goss, M. F. J. Mater. Sci. 1986, 21, 1253. (29) Garvie R. C. J. Phys. Chem. 1965, 69, 1298. (30) Garvie, R. C. J. Phys. Chem. 1978, 82, 218.



Figure 4. TEM pictures of (a) ZrO2-S (calcined at 600 °C), (b) ZrO2-P (calcined at 600 °C), and (c) ZrO2-P (calcined at 800 °C). Table 2. Specific Surface Areas for the Samples at Different Calcination Conditions sample


BET area (m2‚g-1)

sample


BET area (m2‚g-1)

ZrO2-P ZrO2-P

600 °C/10 h 800 °C/0.5 h

168 115

ZrO2-S ZrO2-S

600 °C/10 h 800 °C/0.5 h

48.6 28.9

a particle size between 0.3 and 0.5 mm. The conversion of methane during the CO2 reforming tests were calculated by

conversion of methane ) yH2 + yCO - FH2IN/DFEX 4yCH4 + yH2 + yCO - FH2IN/DFEX

(4)

In this equation, FH2IN and DFEX are feed flow of hydrogen and total dry exit flow, respectively. Total dry exit flow was calculated via the carbon balance. F and y are gas flow rate and molar fraction of i in the feed gas or the effluent gas, respectively. For determining the stability of the catalysts, a temperatureprogrammed cycle was used. According to this program, the methane conversions were calculated in the heating step from

Figure 5. Pore size distributions for the pellets of (2) ZrO2-P and (b) ZrO2-S.

450 to 650 °C and also in the cooling step from 650 to 450 °C. The differences in methane conversions at the same temperatures were investigated. Figure 8a indicated that methane conversion for Ni/ZrO2-P is much higher than that of Ni/ZrO2-S


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Table 3. Pore Volume Results Determined by Mercury Porosimetry

sample

total pore vol (mL/kg)

RMEAN mesopore (nm)

RMEAN total (nm)

% porosity

BET area (m2‚g-1)

ZrO2-S ZrO2-P

201.9 212.9

14.1 2.52

19.7 4.73

42.5 51.4

20.5 105

Table 4. Sulfur Capacity Results for the Prepared Samples

sample

sulfur capacity (wt ppm)

dNi (nm)a

dNi(111) (nm)b

dNi(200) (nm)b

Ni area (m2/g)

Ni dispersion (%)

Ni/ZrO2-S Ni/ZrO2-P

900 1240

30 21.7

19.6 13.5

22 9.5

2.04 2.81

3.36 4.65

a

Determined by sulfur capacity results. b Determined by XRD analysis.

Figure 6. XRD patterns of the reduced catalysts: (a) Ni/ZrO2-P and (b) Ni/ZrO2-S.

Figure 8. (a) Methane conversion and (b) activity of different catalysts: (2) Ni/ZrO2-P and (0) Ni/ZrO2-S, GHSV ) 108000 L/Kgcat‚h.

Figure 7. TEM pictures of Ni/ ZrO2-P.

and also that the deactivation is also very slow. For Ni/ZrO2-S, there is a high difference in methane conversion at the heating and cooling steps at the same temperatures. It indicated that the preparation method of the catalyst support has a strong effect on the performances of the catalysts. The turnover frequencies of these catalysts at different temperatures were calculated from eq 5,26 and the obtained results are shown in Figure 8b:

N ) 4.78 × 103 × rw/S0 ) 10.86 × rs

(5)

where rw and rs are intrinsic rates, mol of hydrocarbon‚h-1‚(g of catalyst)-1 and per m2 of nickel surface, respectively. Figure 8b indicated that Ni/ZrO2-P has higher activity than that of Ni/ ZrO2-S and also that the activity remained constant, but Ni/ ZrO2-S lost its activity very quickly.

Figure 9. Methane conversion at 650 °C: (2) Ni/ZrO2-P, (9) Ni/ ZrO2-S, CO2/CH4:4, GHSV ) 108000 L/Kgcat‚h.

The stability of these catalysts was determined at 650 °C during 10 h, and the obtained results are shown in Figure 9. As can be seen, Ni/ZrO2-P showed high stability, but Ni/ZrO2-S lost about 55% of its initial conversion after 10 h. The deactivation of nickel catalysts during the CO2 reforming of methane is mainly due to carbonaceous deposits and metal sintering. Herein, carbon formation is thermodynamically limited under the experimental conditions used, but metal sintering caused by support sintering and phase transformation appears to deactivate the catalysts. It is also approved by the crystallite growth and elevation of the tetragonal-to-monoclinic ratio produced during the reaction. The crystallite sizes of the ZrO2-P



the reverse water gas shift reaction occurs simultaneously with the CO2 reforming of CH4. Conclusion

Figure 10. H2/CO ratio for different catalysts: (2) Ni/ZrO2-P and (9) Ni/ZrO2-S.

catalyst after the reaction showed no change, and ZrO2-P remains in pure tetragonal form, while the crystallite size of ZrO2-S increased and the tetragonal phase completely transformed to the monoclinic phase. The H2/CO ratios at different temperatures are shown in Figure 10. As can be seen, the H2/ CO ratios are less than 1 and are between 0.53 and 0.58 because

Nanocrystallines of zirconium oxides, which were synthesized by surfactant-assisted route by using CTAB as surfactant, have a good potential as a support for nickel catalyst in the dry reforming reaction. This support showed a high thermal stability toward high calcination temperature, without significant change in crystallite size and tetragonal weight percent. The zirconium oxide, which was prepared by sucrose as a chelating agent, has a low thermal stability. The activity measurements for the nickel catalyst supported on these two types of zirconium oxides showed high activity and stability on the nickel catalyst supported on zirconium oxide, which was prepared by the surfactant-assisted route, while the other catalyst showed lower activity and lost about 55% of its activity after 10 h of reaction. These studies showed the main reason for catalyst deactivation is support sintering (mainly) and phase transformation. This is revealed by the crystallite growth and the tetragonal-tomonoclinic transition produced during the reaction. EF050384K

Nanocrystalline Zirconia as Support for Nickel Catalyst in Methane

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