CO2 methanation over Ni-Ceria-Zirconia catalysts: effect of

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CO2 methanation over Ni-Ceria-Zirconia catalystseffect of preparation and operating conditionsTo cite this article F Ocampo et al 2011 IOP Conf Ser Mater Sci Eng 19 012007

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CO2 methanation over Ni-Ceria-Zirconia catalysts effect of preparation and operating conditions

F Ocampo B Louis A Kiennemann A C Roger University of Strasbourg LMSPC ECPM UMR CNRS 7515 25 Rue Becquerel 67087 Strasbourg FRANCE E-mail annececilerogerunistrafr Abstract Ni-Ceria-Zirconia materials with various CeriaZirconia ratios were prepared by sol-gel and impregnation methods and were subsequently characterized by means of XRD BET TPR and TPO Their catalytic activity and stability were evaluated in the CO2 methanation reaction The main parameters which seem to monitor catalytic performance are the capacity of Ni2+ cations to incorporate into the mixed oxide structure and the obtained ratio between incorporated Ni2+ and surface Ni0 species Catalysts prepared by sol-gel thus exhibited higher methane yield and an improved resistance to deactivation when compared to those obtained by impregnation The process viability was then investigated in potential industrial conditions Operating under moderate pressure was beneficial to catalytic activity whereas working under sub-stoichiometric H2 resulted in a loss of both activity and stability Even though metal sintering and carbon deposits are believed to cause deactivation nickel partial reoxidation and site blocking have to be considered

1 Introduction

Carbon dioxide fixation has received much attention recently since CO2 anthropogenic emissions which have drastically increased in the last decades are believed to directly impact on global warming [1] Among the viable solutions to mitigate CO2 emissions many studies focused on two strategies either its capture and storage [2-3] or its chemical recycling [4-8] However modern society strongly relies on fossil fuels which continuously contribute to further green house effect enhancement It thus appears necessary to find new paths enabling fuels production without releasing additional CO2 in the atmosphere The use of nuclear power and renewable energies such as wind solar or hydraulic power allows the production of ldquocarbon-freerdquo electricity However the process applied in nuclear plants is continuous and thus unable to adapt the production of electricity to the variations of the market demand The same issue is observed when starting from renewable energies for the opposite reason as electricity is produced in a discontinuous process Indeed the major inconvenient of electricity as an energy vector is the impossibility to store it This work deals with its indirect conversion into a fuel easy to store and to transport Electricity obtained via the aforementioned processes could be used to produce hydrogen by water hydrolysis CO2 can then be converted by catalytic hydrogenation into fuels such as methane [5-6 9] methanol or dimethylether [7-8] Indeed increasing amounts of low-cost and concentrated CO2 will be available from carbon sequestration and storage units in a near future Commonly considered as a waste CO2 could thus be used as a cheap raw material This process thus promotes both CO2 and ldquocarbon-freerdquo

Symposium A E-MRS 2010 Fall Meeting IOP PublishingIOP Conf Series Materials Science and Engineering 19 (2011) 012007 doi1010881757-899X191012007

Published under licence by IOP Publishing Ltd 1

electricity Among the possible fuels methane is our target as CO2 methanation remains the most advantageous reaction with respect to thermodynamics when compared to other hydrocarbons or alcohols production [10] The Sabatier reaction has been mainly investigated using Ni-based catalysts on various oxide supports [5-6 11-14] In our previous contribution [9] Ni based catalyst on Ce072Zr028O2 mixed oxides (80 of CeO2 in weight) ie Ni(80-20) were investigated for the first time in CO2 methanation reaction They turned out to be very efficient in terms of activity and stability Their astonishing performances were attributed to their high oxygen storage capacity [15-16] and to their ability to highly disperse nickel [17] Ni2+ cations insertion into the Ce-Zr (CZ) structure improved the redox properties [18] of the material and restricted the metal sintering leading thus to a further improvement in their catalytic performance 5Ni(80-20) (5 Ni in weight) presented the highest specific activity however it also exhibited the smallest CO2 degree of conversion and the lowest stability on stream during 150 h while 10Ni(80-20) was the most efficient In this study we focused on 5 wt Ni loaded catalyst with the aim of enhancing both its catalytic activity and stability It is known that CeZr composition is generally a key factor that strongly influences the catalytic activity of CZ like systems in all type of reactions such as NO reduction in three-way catalysts [19] methane partial oxidation [20] or ethanol and methane reforming [2122] In this work Ni-CZ with several CeO2ZrO2 (CZ) mass ratios was prepared to determine the influence of structural modifications on catalytic activity and selectivity Besides 5Ni(60-40) catalyst was synthesized by two different methods sol gel and wet impregnation in order to compare the catalytic activity of the material resulting from these peculiar preparations All catalysts were tested in a fixed-bed down-flow reactor under the optimal reaction conditions settled down in our previous study [9] Long-term experiments were performed during 67 or 150 h at 350 degC to investigate catalyst deactivation With the aim of taking into account industrial operating constraints the effect of pressure was evaluated And to assess the process viability under changing conditions (discontinued H2 flow) catalytic activity under several CO2H2 ratios was also studied

2 Experimental

21 Preparation of the catalysts

211 Pseudo sol-gel synthesis The fluorite oxides Ni-CZ were synthesized by a pseudo sol-gel method based on the thermal decomposition of propionate precursors Ni loading was 5 wt (coded 5Ni) Several CZ mass ratios were prepared 4 15 and 025 referenced (80-20) (60-40) and (20-80) and referring to Ce072Zr028O2 Ce05Zr05O2 and Ce014Zr086O2 in terms of molar composition respectively The starting materials were cerium (III) acetate sesquihydrate zirconium (IV) acetylacetonate and nickel (II) acetate tetrahydrate These starting salts were individually dissolved in boiling propionic acid in a concentration of 012 mol L-1 in cation at 100 degC for 1 h leading exclusively to propionate precursors The boiling solutions were mixed during 2 h under reflux and then the solvent was evaporated until a resin was obtained Finally the resin was calcined under air at 500 degC for 6 h with a heating ramp of 2 degC min-1

212 Wet impregnation synthesis 5Ni(60-40) was also synthesized by a wet impregnation method and was referenced 5Ni(60-40) The mixed oxide support was prepared as described in the pseudo sol-gel method The nickel precursor (Ni (II) acetate tetrahydrate) was dissolved in ethanol The latter solution and the support were mixed in a rotary evaporator ethanol was evaporated and nickel impregnated on the support The metal-supported oxide was then dried at 120 degC for 1 h and calcined under air at 500 degC for 6 h with a heating ramp of 2 degC min-1

22 Characterization of the catalysts


2

The obtained metal contents in CZ mixed oxides were determined by Inductively Coupled Plasma (ICP) for elemental analysis (Service Central drsquoAnalyse de Vernaison CNRS France) The specific surface area (SSA) of the different catalysts was determined by N2 adsorption-desorption measurements at 77 K by employing the Brunauer-Emmet-Teller (BET) method (Micrometrics sorptometer Tri Star 3000) Prior to N2 adsorption the sample was outgassed at 200 degC overnight to desorb moisture adsorbed on the surface and inside the porous network X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advance diffractometer with a VANTEC detector side and Ni filtered Cu Kα radiation (15418 Aring) over a 2θ range of 10-90deg and a position sensitive detector using a step size of 005deg and a step time of 1s The crystallite size of the samples was evaluated from X-ray broadening by using the well-known Debye-Scherrer equation Temperature-programmed reduction (TPR) was conducted on a Micromeritics AutoChem II to study the reducibility of the catalysts TPR measurements were carried out on 100 mg of catalyst loaded in a quartz U-tube and heated from room temperature to 900 degC at a heating rate of 15 degC min-1 under 10 H2Ar with a total gas flow of 50 mL min-1 TPR profiles were presented as a function of temperature herein but H2 consumptions were calculated using the profiles expressed as a function of time for which the baseline returns to its initial value Carbon deposits formed during CO2 methanation long test runs were evaluated by means of temperature-programmed oxidation (TPO) on a Pfeiffer vacuum mass spectrometer Prior to TPO a TPD was performed up to 900 degC under pure He in order to clean up the catalyst surface with a heating ramp of 15 degC min-1 using 50 mg Temperature was then cooled down to room temperature The TPO analysis was carried out up to 900 degC at a heating ramp of 15 degC min-1 under a 1 O2He flow

23 Catalytic activity The catalysts were reduced in situ before reaction in a 80 H2N2 stream for 6 h with a total gas flow of 45 mL min-1 at 400 degC with a heating ramp of 2 degC min-1 Carbon dioxide methanation was conducted at atmospheric pressure in a fixed-bed down-flow reactor at 350 degC A thermocouple was inserted in the furnace to measure the pretreatment and reaction temperatures in situ The reactor was heated in a tubular furnace monitored by a temperature controller The flow of reactants was regulated by calibrated mass flow controllers (Brooks) H2 and CO2 were mixed at several H2CO2 ratios ranging from 41 to 21 and N2 was added as an internal standard The total flow rate was set to 55 mL min-1 150 mg of catalyst was loaded into the reactor The gas hourly space velocity was kept fixed at 43000 h-1 The feed and products were analyzed on-line by a micro gas chromatograph (Hewlett Packard Quad Series Micro GC) equipped with TCD alumina poraplot and molecular sieve 5 Aring columns CO2 and H2 conversion CH4 selectivity and CH4 yield were defined as follows

1002

1()6242

22

+++minus=

HCCOCHCO

COX CO

100)(

)(1()

22

222

minus=

in

inH HN

NHX

1002

()462

44 CHHCCO

CHSCH ++

=

100() 424 CHCOCH SXY =

where X is the conversion S the selectivity Y the yield P and (P)in the reactant product molar quantities at the exit and at the entrance of the reactor respectively


3

3 Results and discussion

31 Characterization of fresh catalysts The main characteristics of parent catalysts are presented in table 1 Ni contents were always slightly lower than expected (5 wt) However these values only varied between 47 and 49 wt In contrast CZ ratio values (calculated from CeO2ZrO2 weight ratio) were always higher than expected The discrepancy between experimental and theoretical values increased when raising ceria content in the catalyst formulation 425 against 400 for 5Ni(80-20) 159 against 150 for 5Ni(60-40) and finally 026 with respect to 025 for 5Ni(20-80) The excess of ceria could be due to the hygroscopicity of starting cerium salt This excess being more pronounced at high CeZr ratio led to a variation in the mean molecular mass of the CZ oxide thus explaining the lower experimental values of Ni content when compared to the theoretical ones

Table 1 Elemental analysis obtained by ICP SSA values obtained by BET and crystallite sizes (D) and CZ

lattice parameter (a) obtained by XRD calculations Catalyst ICP Analysis SSA

(m2g-1) D CeZrO2

(nm) D NiO (nm)

CZ ap-cubic (Aring)

D NiO (nm) a

Ni (wt) CZ ratio

5Ni(80-20) 471 425 70 63 263 534 372 5Ni(60-40) 480 159 62 60 208 527 390 5Ni(20-80) 491 026 78 64 144 515 351

5Ni(60-40) 67 65 222 529 507 a after reaction

The specific surface areas varied from 62 to 78 m2g-1 These results are in agreement with values given in the literature for Ni-CZ oxides prepared by pseudo-sol gel methods [21-23] The XRD patterns of the pristine oxides are given in figure 1 The studied mixed oxides were well crystallized after a calcination at 500 degC Two crystalline phases were detected corresponding to CZ mixed oxide and to NiO respectively It appears that NiO amount depended both on CZ ratio and on the preparation procedure Indeed NiO phase content increased together with CZ ratio It is noteworthy comparing 5Ni(60-40) to 5Ni(60-40) as the amount of NiO obtained by impregnation technique was clearly higher than by sol-gel synthesis for a given CZ ratio

Figure 1 XRD of (a) 5Ni(80-20) (b) 5Ni(60-40) (c) 5Ni(20-80) and (d) 5Ni(60-40)


4

In an earlier study [24] we have demonstrated that depending on CZ ratio Ni2+ could partially be inserted into the CZ mixed oxide structure thus modifying the lattice parameter along with local coordination While raising ceria content a decreasing proportion of Ni2+ can substitute for Zr4+ in the mixed oxide lattice This is in line with the variations in NiO amounts detected by XRD within the 5Ni-CZ series For the catalyst prepared by impregnation the amount of NiO remained obviously higher than in the corresponding sol-gel catalyst as no Ni2+ was incorporated into the CZ lattice in the latter case The particle size of CZ and Ni oxides as well as CZ lattice parameter were calculated from XRD results The particle size of the CZ mixed oxide was around 60-65 nm The size of the NiO particles varied between 14 and 26 nm Within the sol-gel series the size of NiO particles increased with cerium content as a result from the decrease in substitution of Ni2+ in the mixed oxide and hence to a deeper sintering of rejected NiO The calculated particle size of NiO was slightly higher for 5Ni(60-40) than for 5Ni(60-40) 222 nm with respect to 208 nm The partial insertion of Ni2+ in the CZ oxide was clearly evidenced by the comparison of the pseudo-cubic lattice parameters of 5Ni(60-40) and 5Ni(60-40) Indeed for the same CZ ratio the catalyst prepared by the sol-gel method exhibited a lower lattice parameter than the catalyst prepared by impregnation 527 Aring compared to 529 Aring The lattice parameter of 5Ni(60-40) was consistent with the expected value for a CZ oxide of such composition [3] In contrast the lower lattice parameter of 5Ni (60-40) accounted for a deformation of the lattice by substitution of smaller cations [9 25-27] The specific surface areas of the four studied catalysts were all in the same range ie 60-70 m2g-1

32 Reducibility Figure 2 presents the hydrogen consumption profiles of the four catalysts investigated in the study In our previous contributions devoted to Ni-CZ sol-gel materials [9 24] we have shown that Ni loading greatly improved surface Ce4+ reducibility thus shifting the H2 consumption assigned to this phenomenon from 570 degC to around 320 degC Moreover the partial insertion of Ni2+ in the CZ oxide led to hardly reducible Ni2+ species From figure 2 it appears that a decrease in cerium content in the catalysts within the sol-gel series largely raised the H2 consumption at high temperature while decreasing H2 consumption due to NiO reduction around 400 degC These results accounted for a better incorporation of Ni2+ into the CZ mixed oxide lattice at lower cerium content in line with the observations deduced from XRD experiments The reduction profile of 5Ni(60-40) was typical of an impregnated catalyst with a peak located around 400 degC ascribed to NiO and a characteristic reduction zone between 350 and 600 degC corresponding to the CZ support

Figure 2 TPR of (a) 5Ni(80-20) (b) 5Ni(60-40) (c) 5Ni(20-80) and (d) 5Ni(60-40)


5

33 Catalytic activity Prior to catalytic evaluation in the CO2 methanation reaction the materials were submitted to a reduction pretreatment at 400 degC The stable Ni2+ species integrated in the mixed oxide are not expected to be reduced by this treatment Metallic Nideg is expected to arise from the reduction of the NiO phase

331 Catalytic activity under stoichiometric mixture at atmospheric pressure First the catalysts were tested under stoichiometric CO2H2 mixture (14 ratio) at atmospheric pressure and isothermal conditions (350 degC) The results are presented in figure 3a which displays CH4 yield as a function of time on stream for the different catalysts The initial CH4 yield was the highest for 5Ni(60-40) reaching approximately 80 The two other sol-gel catalysts 5Ni(80-20) and 5Ni(20-80) exhibited similar initial CH4 production close to 72 The impregnated 5Ni(60-40) catalyst led to the lowest initial activity with a methane yield of 58 The thermodynamic yield is 90 under our operating conditions In addition to the initial catalytic activity the behavior as a function of time on stream was very different for the impregnated catalyst with respect to the sol-gel systems Whereas deactivation remained slow and continuous for the latter catalysts the deactivation encountered for 5Ni(60-40) was much more pronounced Deactivation versus time is given in figure 3b It clearly appears that sol-gel preparation led to highly stable catalyst (5Ni(60-40)) when compared to 5Ni(60-40) synthesized by Ni impregnation over a sol-gel prepared CZ support both systems having the same composition After 100 hours on stream the deactivation was only 15 for 5Ni(60-40) whereas it reached 64 for the corresponding impregnated catalyst 5Ni(60-40) Within the sol gel series the deactivation was inversely proportional to activity In other words the more active the catalyst was the more stable it was In a previous work the positive effect of Ni2+ insertion in the CZ lattice on catalytic activity was demonstrated Here we evidence that more than activity itself the presence of Ni2+ in the lattice has a beneficial effect on the stability of the catalyst during the test The detailed catalytic results are summarized in table 2 Along with a decrease in conversion with time on stream deactivation was also characterized by a modification of the selectivity towards reaction products Higher the CO2 conversion was higher methane selectivity was achieved 5Ni(60-40) exhibited an initial CO2 conversion of 797 with a selectivity to CH4 of 993 The other products formed were CO and C2H6 with selectivities of 06 and 01 respectively When the conversion was lower the selectivity to CH4 decreased along with an increase in CO formation The selectivity to ethane remained below 1 in all cases

Figure 3a Methane yield as a function of time over 5Ni-CZ catalytic systems


6

Figure 3b Catalytic deactivation versus time on stream

Table 2 Effect of CZ mass ratio and impact of the catalyst preparation method on catalytic activity and

selectivity Catalyst Time on

stream (h)

X () S () Y ()

CO2 H2 CH4 CO C2H6 CH4

5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209

332 Characterization after test Carbon deposits were quantified by TPO for 5Ni(60-40) and 5Ni(60-40) catalysts The profiles are given in figure 4 The oxidation occurred around 600 degC in both cases For the sol-gel catalyst the calculated amount was 0016 mmol of C per g of catalyst very close to the one obtained for the impregnated catalyst (0018 mmolcgcat

-1) However when calculated per mol of C converted carbon deposition was much more important for 5Ni(60-40) Indeed selectivity to C deposit was 37 10-5 for 5Ni(60-40) and reached 13 10-4 for 5Ni(60-40) (selectivity defined by 100molCdepositmolCconverted

-1)


7

Figure 4 TPO profiles of 5Ni(60-40) and 5Ni(60-40)

The particle size of NiO was estimated from XRD measurements after test The values are given in table 1 In all cases a sintering effect was evidenced after test especially marked for the catalyst prepared by impregnation For 5Ni(60-40) NiO particle size increased from an initial 222 nm up to 50 nm after reaction Sintering was less important for 5Ni(60-40) with NiO particles reaching 39 nm against 208 nm before test As a conclusion deactivation of 5Ni(60-40) could be due to a sintering effect along with the formation of carbon deposits Both phenomena could be directly related as carbon formation may be favored on larger particles [28] The preparation method had an impact on the modification of the oxide lattice on metallic Ni dispersion and on the stabilization of Nideg particles during the reaction As a consequence the catalyst prepared by impregnation exhibited much lower activity in the CO2 methanation reaction than the catalyst of same composition prepared by sol-gel method as well as a lower stability towards deactivation

333 Catalytic activity under stoichiometric mixture effect of pressure The most promising catalyst of the sol-gel series was studied in the CO2 methanation reaction under pressure The test was first performed under atmospheric pressure with a gradual raise in temperature Then at 350 degC the pressure was increased to 3 5 and 7 atm CH4 yields obtained are presented in figure 5

Figure 5 Methane yield versus temperature and pressure over 5Ni(60-40)


8

One can observe that CH4 yield increased with temperature under atmospheric pressure up to 70 at 350 degC This yield was lower than the initial one obtained under isothermal conditions in section 331 (791) since deactivation was more pronounced during the first hours of reaction Higher pressures led to an improvement in methane yield at 350 degC Under 3 and 5 atm CH4 yield reached 77 and 82 respectively A further raise in pressure to 7 atm did not allow any further improvement in CH4 yield It means that operating under moderate pressure in the CO2 methanation reaction would be beneficial to the process global efficiency favoring the production of methane

334 Catalytic activity under atmospheric pressure effect of CO2H2 ratio The effect of H2 sub-stoichiometry was studied under atmospheric pressure for the 5Ni(60-40) system The behavior of the catalyst under these peculiar conditions is of interest when considering the conversion of CO2 with H2 provided by water electrolysis based on excess electricity In such a process the production of H2 would be discontinuous only obtained when the electricity demand is low Therefore the catalyst performance under a lack of H2 with respect to the stoichiometry of the CO2 methanation reaction had to be established in order to better define the global process CH4 yields obtained under isothermal conditions and under CO2H2 ratios of 1-3 and 1-2 are given in figure 6a and compared to the yield obtained under stoichiometric conditions (ratio 1-4) Decreasing H2 concentration in the reactive flow led to a loss of activity CH4 yields dropping off to 621 for ratio 1-3 and 333 for ratio 1-2 In all cases CH4 yields decreased slightly with time on stream Figure 6b shows the conversion of the limiting reactive (H2) versus time on stream One can note that the conversion decreased faster when the concentration of hydrogen was lowered That means that the lack of H2 modified the ageing of the catalyst favoring its deactivation The effect of deactivation was the same as detailed in section 311 The detailed catalytic results are presented in table 3 When the conversion of the limiting reactant decreased the selectivity to methane also decreased while the production of CO was enhanced (up to 88 CO selectivity for 393 H2 conversion under a CO2H2 ratio of 1-2) Studies are under progress to further characterize the catalysts after reaction in these peculiar conditions to clearly evidence the cause of the increase in deactivation observed under deficient H2 reacting mixtures Metal oxidation during reaction and site blocking by an excess of CO2 have to be considered

Table 3 Effect of CO2H2 ratio on catalytic activity and selectivity Catalyst Time on

stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9

Figure 6a Effect of CO2H2 ratio expressed in terms of methane yield versus time on stream for 5Ni(60-40)

Figure 6b Effect of CO2H2 ratio expressed in terms of H2 conversion versus time on stream for 5Ni(60-40)

4 Conclusions

In this study 5Ni-CZ catalysts were synthesized with CZ mass ratios ranging from (80-20) to (20-80) Two different preparation methods were used giving rise to 5Ni(60-40) and 5Ni(60-40) obtained by sol-gel and impregnation procedures respectively The impact of CZ composition and synthesis path on the physical properties and on the catalytic activity of the materials was investigated Based on catalytic results one can argue that the main parameters which seem to monitor catalytic activity selectivity to methane and stability towards deactivation are the capacity of Ni2+ cations to incorporate themselves into the CZ structure and the obtained ratio between incorporated Ni2+ and surface Ni0 species 5Ni(60-40) led to the most promising performance among the various CZ compositions evaluated Moreover 5Ni(60-40) was clearly more active and stable than its impregnated version 5Ni(60-40) thus highlighting the crucial role of incorporated Ni2+ Catalytic experiments under 3 to 5 atm suggested that the process global efficiency would be improved by operating the reaction under moderate pressures CO2 methanation with sub-stoichiometric H2 led to a loss of H2 conversion faster deactivation and an increase in CO production The process might thus encounter viability issues when run under changing conditions in the reactant flow (discontinued H2 flow) Work is under progress to further study catalyst behavior and regeneration working under successive variations in CO2H2 ratio Eventually catalytic deactivation is believed to be due to metallic nickel sintering and carbon deposits However metal oxidation during the reaction and site blocking by an excess of unreacted CO2 present at the surface of the catalyst have to be considered


10

Acknowledgments This work was financially supported by the French Government in the frame of a Ministerial Grant The authors are grateful to Yvan Zimmermann for his technical assistance

References [1] Creutz C Fujita E 2001 Carbon Dioxide as a Feedstock Carbon Management (Washington DC) [2] Wu Y Chan CW 2009 Expert Syst Appl 36 9949-60 [3] van Alphen K van Rujiven J Kasa S Hekkert M Turkenburg W 2009 Energy Policy 37 43-55 [4] Song C 2006 Catal Today 115 2-32 [5] Weatherbee GD Bartholomew CH 1981 J Catal 68 67-76 [6] Yamasaki M Habazaki H Asami K Izumiya K Hashimoto K 2006 Catal Commun 7 24-28 [7] Meliaacuten-Cabrera I Loacutepez Granados M Fierro JLG 2002 J Catal 210 273-84 [8] Olah G A Goeppert A Prakash G K S 2006 Beyond Oil and Gas The Methanol Economy

(Weinheim Wiley-VCH) [9] Ocampo F Louis B Roger A C 2009 Appl Catal A 369 90-96 [10] Inui T Takeguchi T 1991 Catal Today 10 95-106 [11] Chang F W Kuo M S Tsay M T Hsieh M C 2003 Appl Catal A 247 309-20 [12] Du G Lim S Yang Y Wang C Pfefferle L Haller G L 2007 J Catal 249 370-79 [13] Yamasaki M Komori M Akiyama E Habazaki H Kawashima A Asami K Hashimoto K 1999

Mater Sci Eng A 267 220-26 [14] Aksoylu A E Misirli Z Oumlnsan Z I 1998 Appl Catal A 168 385-97 [15] Laosiripojana N Assabumrungrat S 2005 Appl Catal A 290 200-11 [16] Monte R Fornasiero P Kaspar J Rumori P Gubitosa G Graziani M 2000 Appl Catal B 24 157-

67 [17] Trovarelli A 2002 Catalysis by Ceria and Related Materials (Udine Imperial College Press) [18] Strobel R Krumeich F Pratsinis S E Baiker A 2006 J Catal 243 229-38 [19] Haneda M Shinoda K Nagane A Houshito O Takagi H Nakahara Y Hiroe K Fujitani T

Hamada H 2008 J Catal 259 223-31 [20] Silva P P Silva F A Portela L S Mattos L V Noronha F B Hori CE 2005 Catal Today 107-108

734-40 [21] Youn M H Seo J G Cho K M Park S Park D R Jung J C Song I K 2008 Int J Hydr Energy

33 5052-59 [22] Koubaissy B Pietraszek A Roger A C Kiennemann A 2010 Catal Today 85 207-218 [23] Kumar P Sun Y Idem R O 2007 Energy Fuels 21 3113-23 [24] Ocampo F Louis B Kiwi-Minsker L Roger A C 2010 Appl Catal A DOI

101016japcata201010025 [25] Nedyalkova R Niznansky D Roger A C 2009 Catal Commun 10 1875-80 [26] Ambroise E Courson C Kiennemann A Roger A C Pajot O Samson E Blanchard G 2009

Topic Catal 52 2101-07 [27] Stronek L Majimel J Kihn Y Montardi Y Tressaud A Feist M Legein C Buzareacute J-Y Body M

Demourgues A 2007 Chem Mater 19 5110-21 [28] Zhang J Wang H Dalai A K 2008 Appl Catal A 339 121-29


11

CO2 methanation over Ni-Ceria-Zirconia catalysts effect of preparation and operating conditions

F Ocampo B Louis A Kiennemann A C Roger University of Strasbourg LMSPC ECPM UMR CNRS 7515 25 Rue Becquerel 67087 Strasbourg FRANCE E-mail annececilerogerunistrafr Abstract Ni-Ceria-Zirconia materials with various CeriaZirconia ratios were prepared by sol-gel and impregnation methods and were subsequently characterized by means of XRD BET TPR and TPO Their catalytic activity and stability were evaluated in the CO2 methanation reaction The main parameters which seem to monitor catalytic performance are the capacity of Ni2+ cations to incorporate into the mixed oxide structure and the obtained ratio between incorporated Ni2+ and surface Ni0 species Catalysts prepared by sol-gel thus exhibited higher methane yield and an improved resistance to deactivation when compared to those obtained by impregnation The process viability was then investigated in potential industrial conditions Operating under moderate pressure was beneficial to catalytic activity whereas working under sub-stoichiometric H2 resulted in a loss of both activity and stability Even though metal sintering and carbon deposits are believed to cause deactivation nickel partial reoxidation and site blocking have to be considered

1 Introduction

Carbon dioxide fixation has received much attention recently since CO2 anthropogenic emissions which have drastically increased in the last decades are believed to directly impact on global warming [1] Among the viable solutions to mitigate CO2 emissions many studies focused on two strategies either its capture and storage [2-3] or its chemical recycling [4-8] However modern society strongly relies on fossil fuels which continuously contribute to further green house effect enhancement It thus appears necessary to find new paths enabling fuels production without releasing additional CO2 in the atmosphere The use of nuclear power and renewable energies such as wind solar or hydraulic power allows the production of ldquocarbon-freerdquo electricity However the process applied in nuclear plants is continuous and thus unable to adapt the production of electricity to the variations of the market demand The same issue is observed when starting from renewable energies for the opposite reason as electricity is produced in a discontinuous process Indeed the major inconvenient of electricity as an energy vector is the impossibility to store it This work deals with its indirect conversion into a fuel easy to store and to transport Electricity obtained via the aforementioned processes could be used to produce hydrogen by water hydrolysis CO2 can then be converted by catalytic hydrogenation into fuels such as methane [5-6 9] methanol or dimethylether [7-8] Indeed increasing amounts of low-cost and concentrated CO2 will be available from carbon sequestration and storage units in a near future Commonly considered as a waste CO2 could thus be used as a cheap raw material This process thus promotes both CO2 and ldquocarbon-freerdquo


Published under licence by IOP Publishing Ltd 1


2 Experimental






2



1002

1()6242

22

+++minus=

HCCOCHCO

COX CO

100)(

)(1()

22

222

minus=

in

inH HN

NHX

1002

()462

44 CHHCCO

CHSCH ++

=




3





(m2g-1) D CeZrO2

(nm) D NiO (nm)

CZ ap-cubic (Aring)

D NiO (nm) a

Ni (wt) CZ ratio

5Ni(80-20) 471 425 70 63 263 534 372 5Ni(60-40) 480 159 62 60 208 527 390 5Ni(20-80) 491 026 78 64 144 515 351





4





5





6




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11


2 Experimental






2



1002

1()6242

22

+++minus=

HCCOCHCO

COX CO

100)(

)(1()

22

222

minus=

in

inH HN

NHX

1002

()462

44 CHHCCO

CHSCH ++

=




3





(m2g-1) D CeZrO2

(nm) D NiO (nm)

CZ ap-cubic (Aring)

D NiO (nm) a

Ni (wt) CZ ratio

5Ni(80-20) 471 425 70 63 263 534 372 5Ni(60-40) 480 159 62 60 208 527 390 5Ni(20-80) 491 026 78 64 144 515 351





4





5





6




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11



1002

1()6242

22

+++minus=

HCCOCHCO

COX CO

100)(

)(1()

22

222

minus=

in

inH HN

NHX

1002

()462

44 CHHCCO

CHSCH ++

=




3





(m2g-1) D CeZrO2

(nm) D NiO (nm)

CZ ap-cubic (Aring)

D NiO (nm) a

Ni (wt) CZ ratio

5Ni(80-20) 471 425 70 63 263 534 372 5Ni(60-40) 480 159 62 60 208 527 390 5Ni(20-80) 491 026 78 64 144 515 351





4





5





6




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11





(m2g-1) D CeZrO2

(nm) D NiO (nm)

CZ ap-cubic (Aring)

D NiO (nm) a

Ni (wt) CZ ratio

5Ni(80-20) 471 425 70 63 263 534 372 5Ni(60-40) 480 159 62 60 208 527 390 5Ni(20-80) 491 026 78 64 144 515 351





4





5





6




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11





5





6




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11





6




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11




stream (h)

X () S () Y ()


5Ni(80-20)

initial 50 150

715 540 411

693 521 384

985 972 947

09 22 46

06 06 07

704 525 389

5Ni(60-40)

initial 50 150

797 706 659

761 682 647

993 986 982

06 12 16

01 02 02

791 696 647

5Ni(20-80)

initial 50 150

730 618 524

709 602 501

990 985 978

09 13 20

01 02 02

723 609 512

5Ni(60-40)

initial 50 100

598 307 248

565 283 229

973 884 842

26 109 149

01 07 09

582 271 209



-1)


7






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11






8




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11




stream (h)

X () S () Y ()


R 1-4

initial 50 67

797 706 689

761 682 666

993 986 984

06 12 14

01 02 02

791 696 660

R 1-3

initial 50 66

648 514 485

731 566 532

985 960 954

14 37 43

01 03 03

621 481 450

R 1-2

initial 50 67

360 234 218

687 425 393

952 913 908

47 84 88

01 03 04

333 208 193


9



4 Conclusions



10













11



4 Conclusions



10













11













11

CO2 methanation over Ni-Ceria-Zirconia catalysts: effect of

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Transcript of CO2 methanation over Ni-Ceria-Zirconia catalysts: effect of