110 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133synthesis gas with low hydrogen-to-carbon monoxide ratio, which can bepreferentially used for production of liquid hydrocarbons in the Fischer-Tropschsynthesis network . This reaction has also very important environmentalimplications because both methane and carbon dioxide are greenhouse gaseswhich may be converted into valuable feedstock. In addition, this process haspotential thermochemical heat-pipe applications for the recovery, storage, andtransmission of solar and other renewable energy sources by use of the largeheat of reaction and the reversibility of this reaction system [2,3]. One of themajor problems encountered in the application of this process is rapid deactiva-tion of the catalyst, mainly by carbon deposition [4,5]. During the past decades, the process of carbon dioxide reforming of methanehas received attention, and efforts have focused on development of catalystswhich show high activity towards synthesis gas formation, and are also resistantto coking, thus displaying stable long-term operation. Numerous supportedmetal catalysts have been tested for this process. Among them, nickel-basedcatalysts [6-1 I] and supported noble metal catalysts (Rh, Ru, Ir, Pd and Pt)[12-22] give promising catalytic performance in terms of methane conversionand selectivity to synthesis gas. Conversions of CH 4 and CO 2 to synthesis gasapproaching those defined by thermodynamic equilibrium can be obtained overmost of the aforementioned catalysts, as long as reaction temperature andcontact time are sufficiently high [8,10,12,13]. The catalysts based on noblemetals are reported to be less sensitive to coking than are nickel-based catalysts[8,10,12,13,21-23]. However, considering the aspects of high cost and limitedavailability of noble metals, it is more desirable, from the industrial point ofview, to develop nickel-based catalysts which are resistant to carbon depositionand exhibit stable operation for extended periods of time. Arakawa et al.[24-27] used a Ni/A1203 catalyst to obtain synthesis gas from a mixture ofmethane, carbon dioxide and water. They found that the catalyst deactivatesrapidly by carbon formation on the surface, but addition of vanadium (5-10wt.-%) can decrease, to a certain extent, coke formation. Rapid catalyst deactiva-tion due to carbon deposition on supported Ni catalysts during the C H 4 / / C O 2reaction was observed by many investigators [6,7,16,23,28]. It is generallyclaimed that catalyst deactivation is due to coke formation within the pores ofthe catalyst, which leads to breakup of the catalyst particles. Carbon dioxidereforming of methane over Ni supported on different carriers was studied indetail by Gadalla and co-workers [8,10]. They found that no carbon depositionwas obtained when reaction temperatures higher than 940°C and C O J C H 4ratios larger than 2 were applied. Due to the high temperature, however, thesupport structure was found to be changing and the activity to be decreasingwith time on stream because of reduction of surface area. Swaan et al. studied deactivation of supported Ni catalysts during reforming of methane withcarbon dioxide. They found that N i / Z r O 2, N i / L a 2 0 3 , N i / S i O 2 and N i -K / S i O 2 exhibit moderate deactivation with zero order kinetics. The deactiva-
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 111tion was shown to be due to carbon deposition on Ni from CO disproportiona-tion. Recently, it was observed in this laboratory  that a novel Ni/La203catalyst, properly prepared and activated, is capable of exhibiting good activityand excellent stability. In the present work, the detailed kinetic performance ofthe Ni/La203 catalysts, obtained under various testing conditions is reported. Anumber of characterization techniques, i.e. H 2 and CO chemisorption, X-raydiffraction (XRD) and temperature-programmed desorption (TPD), were em-ployed to study the interactions between Ni and La203 which give this catalystactive and stable performance for carbon dioxide reforming of methane tosynthesis gas.2. Experimental2.1. Catalyst preparation Ni/La203, Ni//T-A1203 and Ni//CaO catalysts were prepared by the wet-im-pregnation method, using nitrate salt as the metal precursor. A weighed amountof nickel nitrate (Alfa Products) was placed in an 100 ml beaker and a smallamount of distilled water was added. After 30 min, the appropriate weight ofsupport (La203, T-A1203 or CaO) was added under continuous stirring. Theslurry was heated to ca. 80°C and maintained at that temperature until the waterevaporated. The residue was then dried at 110°C for 24 h and was subsequentlyheated to 500°C under N 2 flow for 2 h for complete decomposition of thenitrate. After this treatment, the catalyst was reduced at 500°C in H 2 flow for atleast 5 h. A Ni//La203 catalyst obtained by physically mixing appropriateamounts of NiO and La203 was also prepared. The solid mixture was reduced inH 2 flow at 750°C. It is designated as Ni//La203 (physical mixture). While thepresent study focused on the Ni//La203, the other catalysts mentioned wereused for comparison purposes.2.2. Kinetic measurements Kinetic studies under differential conditions, and studies under integralreaction conditions were conducted in a conventional flow apparatus consistingof a flow measuring and control system, a mixing chamber, a quartz-fixed-bedreactor (ca. 4 mm, i.d.), and an on-line gas chromatograph. The feed streamtypically consisted of C H a / C O 2 / H e = 20//20/60 vol.-%. For the kinetic stud-ies under differential conditions, one portion of catalyst (5-10 mg) was dilutedwith 2 - 4 portions of a-A1203. The solid mixture was powdered ( d - - 40 /xm)before being placed at the middle of the reactor tube. Conversions were usuallycontrolled to be significantly lower than those defined by thermodynamic
112 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133equilibrium by adjusting the total flow rate (200-400 ml/min). Due to abnor-mal H 2 chemisorption of the N i / L a 2 0 3 catalyst, the reaction rate is expressedin units of mmol/(gca t s), instead of tumover frequency. Rate limitations byexternal or internal mass transfer, under differential conditions were proven tobe negligible by applying suitable criteria. For the studies under integral reactionconditions, one portion of the catalyst ( 1 0 - 5 0 mg) was diluted with up to 10portions of ot-A1203 so as to reduce the temperature gradient along the catalystbed. The solid mixture was pelletized and then crushed and sieved to sizes of ca.1.0 mm. Conversions were controlled to be somewhat lower than those definedby thermodynamic equilibrium. A weak influence of mass and heat transferresistances may exist under these reaction conditions. The temperature of thecatalyst bed was measured by a chromel-alumel thermocouple, and it was keptconstant within + 2°C. Analysis of the feed stream and reaction mixture wasperformed using the TC detector of a gas chromatograph. A carbosieve S-II100/120 column was used to separate H 2, N 2, CO, CH 4, CO 2 and H20. Priorto reaction, the catalyst was reduced again, in situ, at 750°C in H 2 flow for 1 h.2.3. Catalyst characterization2.3.1. H 2 and CO chemisorption H 2 and CO chemisorption on Ni catalysts was studied at room temperature.H 2 chemisorption was determined in a constant-volume high vacuum apparatus(Micromeritics, Accusorb 2100E). The adsorption isotherms were measured atequilibrium pressures between 10 and 300 mm Hg. Prior to adsorption measure-ments, the samples were pre-reduced in H 2 flow at 750°C for 2 h. Uptake of H 2at monolayer coverage of the Ni particles, Vm, was obtained by extrapolation ofthe linear portion of the adsorption isotherms to zero pressure. CO chemisorption was conducted in an apparatus which is connected to aquadrupole mass spectrometer (Fisons, SXP Elite 300 H). The sample, whichwas placed in a quartz-tube, was first reduced in H 2 flow at 750°C for 2 h. Afterpurging with He for 10 min, the sample was cooled to room temperature in Heflow. The adsorbent (i.e. 1.1% CO in He) was then passed through the sample ata stable flow rate of 30 ml/min. The transient response of CO was recorded bythe mass spectrometer. Due to adsorption of CO on the clean Ni surface, acertain degree of delay in the CO response occurred, as compared to thebackground response (when CO was passed through the reactor containing nocatalyst). The differences between the responses were used to determine theuptake of CO on the Ni catalyst.2.3.2. XRD study A Philips PW 1840 X-ray diffractometer was used to identify the main phasesof N i / L a 2 0 3 catalysts, before and after reaction. Anode Cu K t~ (40 kV, 30mA, A = 1.54 A) was used as the X-ray source. The catalyst which had been
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 113exposed to reaction conditions for a certain period of time was quickly quenchedto room temperature and then transferred onto the XRD sample holder formeasurements. The mean nickel particle size was estimated by employingScherrers equation , following standard procedures.2.3.3. TPD Temperature-programmed desorption (TPD) experiments were carried out inan apparatus which consists of a flow switching system, a heated reactor, and ananalysis system. The reactor was a quartz tube of 0.6 cm diameter and 15 cmlength. A section at the centre of the tube was expanded to 1.2 cm diameter, inwhich the catalyst sample, approximately 300 mg, was placed. The outlet of thereactor was connected to a quadrupole mass spectrometer via a heated siliconcapillary tube of 2 m length. The pressure in the main chamber of the massspectrometer was approximately 10 - 7 mbar. The sample was fhst reduced in H 2 flow at 750°C for more than 2 h. Afterpurging with He for 10 min, the sample was cooled under He flow. When thedesired adsorption temperature was reached, the He flow was switched to n 2 orCO flow. After 10 min, the sample was cooled to room temperature under H E o rCO flow, and then the flow was switched to He and the lines were cleaned for2 - 5 min. Temperature programming was then initiated and the TPD profileswere recorded. Calibration of the mass spectrometer was performed with amixture of known composition.3. Results3.1. Catalytic performance3.1.1. Kinetic behaviour of Ni / La203 Fig. 1 shows the alteration of reaction rate, obtained under differentialreaction conditions at 750°C over Ni/y-A1203, N i / C a O and N i / L a 2 0 3 cata-lysts, as a function of time on stream. In each case, l0 mg of catalyst sample,diluted with 20 mg of a-A1203, were charged to the fixed-bed quartz reactor.The feed stream consisted of C H 4 / C O 2 / H e = 2 0 / 2 0 / 6 0 vol.-%, while a totalflow rate of 300 m l / m i n was used. As shown in Fig. 1, the intrinsic rates ofmethane reforming with CO 2 over the N i / y - A I 2 0 3 and N i / C a O catalystsdecrease continuously with time on stream. Although the initial rate overNi/y-A1203 is higher than the respective one over N i / C a O , the deactivationrate of the Ni/~/-AI203 catalyst is also faster than that of the N i / C a O catalyst.In contrast, the rate over the N i / L a 2 0 3 catalyst increases with time on streamduring the initial 2 - 5 h of reaction, and then it tends to be essentially invariablewith time on stream during 100 h of reaction, showing very good stability. Thisleads to the suggestion that new catalytic sites, which are more active and stable
114 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 3.0 Ni Catalyst, 750°C t~0 2.0 O --Al 0 "~Q 1.0 ~ ~ , ~ CaO 0.0 0 8 16 100 Tirne/hFig. 1. Alteration of reaction rate of carbon dioxide reforming of methane to synthesis gas as a function oftime on stream over N i / L a 2 0 3 , Ni/T-A1203 and N i / C a O catalysts. Reaction conditions: P c r t , = 0.2 bar,Ptot = 1.0 bar, C H 4 / C O 2 = 1, T = 750°C, W / F = 2.10 -3 g s / m l , metal loading = 17 wt.-%.towards the c n 4 / / C O 2 reaction are formed on the N i / L a 2 0 3 catalyst surface,following exposure to the reaction mixture. Table 1 reports the reaction ratesobtained over the Ni/~/-AI203, N i / C a O and N i / L a 2 0 3 catalysts at 550, 650and 750°C. Both reaction rates, measured initially and after 5 h of reaction, arepresented. For the N i / L a 2 0 3 catalyst, the rate measured at 650°C and 750°Cafter 5 h of reaction corresponds to the rate at the stable level (see Fig. 1). Therate obtained over the N i / L a 2 0 3 at 550°C shows a very slow increase with timeon stream, which lasts for at least 10 h. The rate at the pseudo-stable level at550°C amounts to ca. 0.18 m m o l / ( g s) which is significantly lower than the oneobtained at 550°C, following first reaction at 750°C for 5 h and decrease oftemperature to 550°C (see Table 1). Apparently, the stable structure of theTable 1Influence of catalyst support on reaction rate at various temperatures over supported Ni catalystCatalyst State Reaction rate ( m m o l / g s) a17 wt.-% N i / 550°C 650°C 750°CLa 203 Initial 0.13 0.52 0.95 After 5 h 0.18 1.58 2.18 (0.56) b (1.60) bT-AI203 Initial 0.56 1.41 2.20 After 5 h 0.23 0.79 1.48CaO Initial 0.10 0.49 1.23 After 5 h - 0.21 0.72a Reaction conditions: Pcrt, = 0.2 bar, Ptot = 1.0 bar, C H 4 / C O 2 = 1, W / F = 2- 10 - 3 g s / m l .b The data were obtained following initial reaction at 750°C for 5 h and decrease of temperature from 750°Cto 650 and 550°C.
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 115Table 2Reaction of carbon dioxide reforming of methane to synthesis gas over various catalystsCatalyst(l 0 rag) Solid diluent State Reaction rate a Comment (20 mg) (mmol/gca t s)17 wt.-% N i / L a 2 0 3 a-A1203 Initial 0.95 After 5 h 2.18 stable17 wt.-% N i / L a 2 0 3 La203 Initial 1.21 After 5 h 2.52 stable17 wt.-% N i / L a 2 0 3 b ot_Al203 Initial 0.14 After 5 h 0.04 deactivatingLa203 - Initial 0.02 After 5 h negligible deactivatinga Reaction conditions, T = 750°C, PcH, = 0.2 bar, Ptot = 1.0 bar, CH4//CO 2 = 1, W / F = 2.10 -3 gear s / m l .b Prepared by physically mixing of NiO and La203.Ni/La203 catalyst is favourably produced when the reaction temperatureapplied is higher than 650°C. It is shown that the initial reaction rate overNi/T-A1203 is ca. 2 times higher than the respective ones over Ni/La203 andNi/CaO. However, the reaction rate over Ni/La203 at the stable level is higherthan the ones over the deactivated Ni//3~-AI203 and N i / C a O catalysts. Table 2 shows kinetic results obtained at 750°C over Ni/La203 (physicalmixture), and pure La203 catalysts. Both reaction rates, obtained at zero time onstream and after 5 h reaction, are presented. It is shown that pure La203 exhibitsnegligibly low reactivity towards conversion of C H 4 / / C O 2 t o synthesis gas. TheNi/La203 (physical mixture) catalyst behaves as other nickel-based catalysts,such as Ni/T-A1203 and Ni/CaO, showing continuous deactivation with timeon stream, which is completely different from the behaviour of the Ni/La203catalyst (see Fig. 1). Apparently, only when the two components (Ni andLa203) are in appropriate contact (e.g. prepared by the wet impregnationmethod) can the Ni/La203 exhibit the unique active and stable performance.Finally, it should also be pointed out that the nature of the solid dilution(a-A1203 or La203) seems to play a minor role in affecting the kineticbehaviour of the Ni/La203 catalyst.3.1.2. Influence of structural and operating parameters on kinetic behaviour126.96.36.199. Ni metal loading. Fig. 2 shows the influence of metal loading (3-17wt.-%) on the reaction rate and the stability of Ni/La203 catalyst at 750°C. Thereaction rate is expressed in units of mmol/(gmetaI s). It is observed thatdecreasing the nickel loading on the Ni/La203 catalyst results in increase of thereaction rate, presumably due to enhanced dispersion of Ni on the La203support. Regardless of different metal loadings, a similar pattern, i.e. the rateincreasing with time on stream during the initial several hours of reaction is
116 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 50. Ni/La2Oa, 1023K ~ 40- 30- 2o- 10 wt?/, ~e~.e.-~ o 0 0 0 O0 0 g aC 0 lO. 17 w t ? / , 0 1 I I I 0 5 i0 15 20 25 Time /hFig. 2. Influence of Ni metal loading on reaction rate and stability of the N i / L a 2 0 3 catalyst. Reactionconditions: Pcn, = 0.2 bar, Ptot = 1.0 bar, C H 4 / C O 2 = 1, T = 750°C, W / F = 2-10 -3 g s / m l .observed. After reaching a maximum level, the reaction rate decreases graduallyover the 3 wt.-% Ni/La203 catalyst, but tends to be essentially invariable withtime on stream when the nickel loading in increased to above 10 wt.-%. Itappears that stable performance is favourably obtained over the catalyst withlarge metal particle size.188.8.131.52. Influence of contact time. The influence of contact time on conversionsof methane and carbon dioxide over a 17 wt.-% Ni/La203 catalyst wasinvestigated at 750°C. The feed consisted of C H 4 / C O 2 / H e = 2 0 / 2 0 / 6 0 vol.-%.The alteration of contact time was realized by adjusting both, the amount ofcatalyst (5-30 mg) and the feed flow rate (30-300 ml/min). As shown in Fig.3, both methane and carbon dioxide conversion increases rapidly as contact timeincreases from 0.002 to 0.07 g s / m l . Conversions approaching those expected atthermodynamic equilibrium (i.e. the dotted lines) are already achieved at contacttimes as low as ca. 0.06 g s / m l , which correspond to a superficial contact timeof ca. 0.02 s. The conversions of methane and carbon dioxide obtained at acontact time of 0.06 g s / m l was also studied at various temperatures and theresults are shown in Fig. 4. It is observed that the conversions obtained atvarious temperatures, employing the specified contact time, are approximatelyequal to those expected at thermodynamic equilibrium (i.e. the dotted lines). Thehigh intrinsic activity of Ni/La203 may be related to its absence of strongalkali- a n d / o r alkaline-promoter (La203 has only moderate basicity) on the Nicatalyst. It is well known  that strong basic promoters help to inhibitaccumulation of surface coke but also result in significant reduction of activityof reforming-type reaction.
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 117 1 O0 Equilibrium Level .~ :_-~=_--_ "" 75 O ~a 50 (D  o 25 r,D 0 i i i i 0.00 0.02 0.04 0.06 0.06 Contact Time ( g s / m l )Fig. 3. Influence of contact time on conversion obtained over the Ni/La203 catalyst. The dotted linescorrespond to values expected at thermodynamic equilibrium. Reaction conditions: PcH, = 0.2 bar, Ptot = 1.0bar, C H 4 / C O 2 = l, T = 750°C, metal loading = 17 wt.-%.184.108.40.206. Influence of reaction temperature. The rate of reaction and the stabilityof a 17 wt.-% N i / L a 2 0 3 catalyst was investigated at 550, 650 and 750°C underdifferential reaction conditions and the variation of the rate of reaction withtime-on-stream is shown in Fig. 5. The N i / L a 2 0 3 catalyst was first exposed tothe CH4//CO 2 mixture at 750°C until the reaction rate reached the stable level(see Fig. 1). After this treatment, the reaction rates at 550, 650 and 750°C weremonitored as a function of time on stream for approximately 20 h. It is shown I00 dH(/C02-- 1 t~," Pc~=0.2 bar J $ 75 ] st O /J 50 / / o , tJ ~j / ~  CO~ o 25 • CH4 J t s I I I I I 500 600 700 800 90q Temperature (°C)Fig. 4. Influence of reaction temperature on conversion obtained over the Ni/La203 catalyst using a constantcontact time of 0.06 g s / m l . The dotted lines correspond to conversion expected at thermodynamicequilibrium. Reaction conditions: Pea 4 = 0.2 bar, Ptot = 1.0 bar, CH 4/CO2 = 1, metal loading = 17 wt.-%.
118 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 3.0 CH4/eOz at 750"12 ,], I 75oc 2.0 I 0 , 650°C I I v I 0 1.0 I r~ I e~ , 660"C I 0.0 o 5 ,o 20 Tirne/hFig. 5. Alteration of reaction rate as a function of time on stream over the Ni/La203 catalyst. Reactionconditions: T = 5 5 0 , 650 and 750°C, PcH = 0 . 2 bar, Ptot = 1.0 bar, C H 4 / C O 2 = 1, metal loading= 17wt.-%.that the resultant Ni/La203 catalyst does not exhibit any deactivation during 20h of reaction at these temperatures. These results demonstrate the excellentstability of the Ni/La203 catalyst since it is known that even supported noblemetal catalysts, e.g. Rh catalyst, which has been reported to be one of the moststable catalysts for carbon dioxide reforming of methane [12,13,20,21], doessuffer carbon deposition and deactivation at reaction temperatures below 700°C(carbon-free performance over Rh catalysts can be obtained at >/700°C). Theabsence of deactivation of the Ni/La203 catalyst in a wide temperature range(at least 550-750°C) indicates that a new type of nickel-containing species isformed on the surface, following exposure of the catalyst to the reaction mixturefor 2-5 h. Fig. 6 shows an Arrhenius plot of the reaction carried out over the Ni/La203catalyst, within the temperature range of 500-750°C. The upper curve (filledsquares) represents data obtained after the catalyst had reached the stable level,while the lower curve represents data obtained at time-on-stream approachingzero. The apparent activation energy over the Ni/La203 catalyst at the initialstate amounts to ca. 80.0 kJ/mol, which falls within the range of valuesobtained over other types of supported metal catalysts measured in this labora-tory [20,21] and by other groups [15,16]. The apparent activation energyobtained over the Ni/La203 catalyst at the stable level (usually after 2 - 5 h ofreaction) amounts to ca. 62.7 kJ/mol, which is somewhat lower than the one atthe initial state of the catalyst. This implies that the new surface state, formedafter exposure of the Ni/La203 catalyst to the reaction mixture for 2-5 h,provides a reaction pathway of lower apparent activation energy requirements.
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 119 10 02 62.7 k J / t o o l 0 18nO.tial J / m o l 0 k ~0.1 Q W/F=2xlO-~ s g / m l  0.01 0.8 1.0 1.2 1.4 T e m p e r a t u r e (IO00/K)Fig. 6. Sensitivity of reaction rate on temperature over a 17 wt.-% N i / L a 2 0 3 catalyst. The filled squares ( l l )represents the experimental value obtained after establishment of the stable level. The open squares ()represent the value obtained at time on stream approaching zero. Reaction conditions: T = 500-750°C,Pea4 = 0.2 bar, Ptot = 1.0 bar, CH4//CO2 = 1, W / F = 2- 10 -3 g s / m l .220.127.116.11. Influence of gas (pre-)treatment. In order to explore the nature of thenew type of nickel compound at the stable level, the influence of various gaspretreatments, including heating under flow of 02, air, H 2, CO 2, and C H 4 at1023 K for 1-2 h, on the performance of the Ni/La203 catalyst was investi-gated. Table 3 reports the results obtained at the initial state of the catalyst andafter reaching the stable level, following various pretreatments. The pretreatmentof the N i / L a 2 0 3 with CO 2, 02, and air at high temperatures (Experiments No.2, 3, 8, 9) would favour the formation of La202CO 3, NiO and LaNiO 3,respectively. From experiments No. 2, 3, 8 and 9, it is derived that none of thecompounds La202CO 3, NiO and LaNiO 3 is likely to be solely responsible forthe enhancement of the reaction rate. The results obtained in experiments No. 5,6 and 7 indicate that the increase of reaction rate during the initial several hoursof reaction is not due to in situ reduction of incompletely reduced nickel sincenickel is expected to be fully reduced after exposure to pure H 2 flow at 750°Cfor 12 h (experiment No 7). Generally speaking, regardless of what kind ofpretreatment is applied, the initial reaction rate is always lower than the reactionrate at the stable level, suggesting that none of the above pretreatments results inan initial surface state which is analogous to the stable state, which is onlyobtained under reaction conditions. Although the pretreatment affects the rate ofthe initial state to a certain extent, it does not influence significantly the value ofthe reaction rate at the stable level. These results imply that there exists a strongtendency of the Ni/La203 catalyst to form the stable surface structure onlyunder the working reaction conditions. It appears that the stable surface structureconsists of a mixture of several components involving nickel and lanthanum
120 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133oxide, as well as species from the gas phase, which are in equilibrium underworking reaction conditions. Fig. 7 shows the influence of several treatments on the reaction rate over theN i / L a 2 0 3 catalyst, following the establishment of the stable surface state. It isfound that the stable surface is insensitive to exposure of the catalyst to air atroom temperature. It is interesting to observe that when the catalyst is exposedto H 2 (or to 02) at 750°C, following establishment of the stable surface state,evolution of CH 4 (or of CO 2) is registered. Consequently, the stable surfacestructure is altered or destroyed, as indicated by the lower reaction rates whichare obtained upon re-exposing the catalyst to the reaction mixture at the sametemperature. However, the stable surface structure is found to be essentiallyretrievable after several hours of reaction (Fig. 7). These results may imply thatcarbon itself may constitute an imperative component contained in the stablesurface structure. The results of Experiment No. 4 (catalyst was pretreated withCH 4 at 750°C) given in Table 3 show the initial rate is smaller but rather closeto that at the stable level, suggesting that the presence of a certain amount ofcarbon on Ni crystallites favours the enhancement of the reaction rate. Thehigher initial rate might be due to accumulated carbon on the surface whichreact with CO 2 to produce synthesis gas.3.1.3. Integral reactor performance The results presented in the preceding sections were all obtained using adilute reaction mixture, i.e. C H a / C O 2 / H e = 2 0 / 2 0 / 6 0 vol.-%, and the con-versions were usually controlled to be far below those expected by thermody-namic equilibrium. In this section, results of the long-term stability test of theN i / L a 2 0 3 catalyst under integral reaction conversions, with and without Hedilution, are presented. Conversion somewhat lower than the equilibrium onewas achieved. This allows to study the catalytic performance at high conver-sions, while catalyst deactivation, if there is any, can also be easily detected. Fig. 8 shows the alteration of conversion of methane and carbon dioxide, andselectivity to carbon monoxide and hydrogen as a function of time on stream,obtained at 750°C over the N i / L a 2 0 3 catalyst using a feed mixture ofCHJCOJHe = 2 0 / 2 0 / 6 0 vol.-%. Both conversion and selectivity increaseduring the initial several hours of reaction. After this, the conversion andselectivity tends to be essentially invariable with time on stream during 100 h ofreaction. It was found that the weight of the used catalyst was significantlylarger than that of the fresh one, suggesting that a certain amount of carbonformed on the catalyst. The fact that the quantity of carbon present on the usedcatalyst surface under high conversions is much larger than that under lowconversions suggests that carbon may originate from the CO product through theBoudouard reaction (2CO ~ C + CO2). This observation is in harmony withthe earlier study of Swaan et al. . In the present case, the carbon species onthe surface must be either at steady state or in a certain form which does not
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 121cause any significant catalyst deactivation, at least within 100 h, as is demon-strated in Fig. 8. Results of a similar long-term stability test, conducted employing undilutedfeed (CH 4/CO2 = 50/50 vol.-%) under otherwise similar conditions, are shownin Fig. 9. Even in this case, after several hours of reaction, both conversion andselectivity tend to be rather stable. Only a small decline of activity withtime-on-stream was observed during the 100 h stability test. It was found that inthis set of experiments even larger amounts of carbon are deposited on thecatalyst. Apparently, the amount of carbon deposited on the Ni/La203 catalystis related to the partial pressure of CO, although the present global observationsshow that these carbon species on the Ni/La203 catalyst do not result insignificant catalyst deactivation. It is found that the slow deactivation which isobserved in Fig. 9 could be largely eliminated by addition of small quantities(1-5%) of oxygen in the feed.3.2. Catalyst characterization3.2.1. XRD study The major crystalline phases of the Ni/T-A1203 and Ni/La203 catalystswere examined by XRD and are described in Table 4. The results show thatT-A1203 and NiA1204 crystalline phases exist in the reduced Ni/y-A1203catalyst (fresh). The NiA1204 phase, which is not easily reducible, shouldoriginate from the reaction between NiO and A1203 . No metallic Ni crystallinephase is observed in Ni/AI203 (Table 4). Only metallic Ni and La203crystalline phases are found in the reduced Ni/La203 catalyst (fresh). Since themost prominent peak of Ni is well resolved from those of La203 (Fig. 10a), itallows to estimate properly the Ni particle size using the XLBA method (X-rayline broadening analysis). By employing Scherrers equation [31 ], it is estimatedTable 3Influence of pretreatment of 17 wt.-% Ni/La203 catalyst on reaction rates at the initial and stable levelsExperiment Pretreatments Favorable Rate for CO formation ( m m o l / g s) bNo. compound a Initial Stable1. No treatment - 0.13 1.912. CO2,750°C,2 h La202CO 3 0.07 1.443. 02,750°C, 2 h NiO 0.34 1.764. CH 4, 750°C, lh C on Ni 1.23 1.425. H2,750°C, 2 h Metallic Ni 0.94 2.106. H2,750°C, 5 h Metallic Ni 1.10 1.907. H2,750°C, 12 h Metallic Ni 0.67 2.008. Air, 850°C, 10 h LaNiO 3 0.19 1.719. Air, 850°C, 10 h; then H2, 1023 K, 2 h - 0.40 1.60a This compound is expected to be formed in preference following the stated pretreatment.b Reaction conditions: PcH, = 0.2 bar, Ptot = 1.0 bar, C H 4 / C O 2 = 1, W / F = 2- 10 -3 g s / m l . T = 750°C.
122 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 3.0 Treatment 2.4 $ i i o" u l , ~ z x zOal::~ 1.8 I i b I:1 1.2 After H~ a t 750"0 for 2 h 0.6 • After e x p o s e d to air at 30"C a After Oz a t 750"C for 2h 0.0 o 2a Time/hourFig. 7. Effect of various treatments on reaction rate over the N i / L a 2 0 3 catalyst, following establishment ofthe stable surface state. Reaction conditions: P C H 4 = 0 . 2 bar, Ptot = 1.0 bar, C H 4 / C O 2 = 1, T = 7 5 0 " C ,W / F = 2 . 1 0 -3 g s / m l .that the average Ni particle size present on La203 support is of the order of 330A. The major crystalline phase of the working Ni/La203 catalyst was alsostudied by XRD. The catalyst which had been exposed to the reaction mixture(i.e. C H 4 / C O 2 / H e = 2 0 / 2 0 / 6 0 vol.-%) at 750°C for a certain period of timewas quickly quenched to room temperature and transferred to the XRD appara-tus. Fig. 10 shows the alteration of the XRD spectra obtained over the I00 q• gl, • o 80 I "f• ~•n • 60 m CO ~ e l e e t i v i t y n H= ~ e l e e t i v i t y A C0= 0 o n v e r s i o n • 0H4 C o n v e r s i o n 40 , , , , 0 25 50 75 i00 Time/hourFig. 8. Alteration of conversion of CH 4 and CO2, and selectivity to C O and H 2, obtained over a 17 wt.-%N i / L a 2 0 3 catalyst, as a function of time on stream. Reaction conditions: PCH4 = 0.2 bar, Ptot = 1.0 bar,C H 4 / C O 2 = 1, T = 750°C.
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 123 I00 75 50 O 120 Selectivity • Hz Selectivity 25 1:1 COz Oonverllion • (~H, Conversion I I I I I 0 25 50 75 100 Time/hFig. 9. Alteration of conversion of CH 4 and CO 2, and selectivity to CO and H2, obtained over a 17 wt.-%Ni/La203 catalyst, as a function of time on stream. Reaction conditions: Pen4 = 0.5 bar, Ptot = 1.0 bar,C H 4 / C O 2 = 1, T = 750°C.N i / L a 2 0 3 catalysts as a function of time on stream. It is shown that the catalystexperiences a profound change in its bulk phase, following exposure to theCH4/CO 2 mixture at 750°C. While the Ni and La203 phases which existed inthe fresh N i / L a 2 0 3 catalyst disappear, La202CO 3 (type II) and La202CO 3(type Ia) phases are formed, following more than half hour of reaction time. Theproportion of the La202CO 3 (type II) and La202CO 3 (type Ia) only changesslightly with time on stream, up to 100 h of reaction. The formation ofLa202CO 3 phase should be the result of the reaction between La203 and theCO 2 gaseous reactant. However, the occurrence of this reaction should beaccompanied by a process which brings about the disappearance of the Nicrystalline phase.Table 4Various parameters of Ni/7-AI203 and Ni/La203 catalystsCatalyst Crystalline phase a Uptake/cm3g~al Ni particle size H2 CO /~17 wt.-% Ni/~-AI203 b ~-AI203 NiAI204 0.99 1.97 -17 wt.-% Ni/La203 Ni, La203 0.33 0.22 330 ¢ 1.100 d 3.240 ea Crystalline phase was determined by XRD measurements.b Since no Ni crystalline phase was detected by XRD in the Ni/3,-AI~O 3 catalyst, the Ni particle size is notestimated due to uncertainty in the shape of the Ni particles.c The Ni particle size was derived from XRD results.d The Ni particle size was derived from the uptake of H 2 chemisorption assuming that H/Nisurfae e = 1.e The Ni particle size was derived from the uptake of CO chemisorption assuming that CO/Nisunace = 1.
124 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 4000 • La20s A Ni 3000 • LazO~COa(type II) a La~OtCOs (type la) = 2000 o r..) 1000 25 30 35 40 45 50 55 60 2eFig. 10. XRD spectra obtained over a 17 wt.-% N i / L a 2 0 3 catalyst exposed to CH 4 / C O 2 for variable periodsof time at 750°C. (a) Fresh sample; (b) after 0.5 h; (c) 2 h and (d) 100 h.3.2.2. H e and CO chemisorption The uptake of H 2 at room temperature is used to determine the dispersion ofnickel on the support, assuming that each surface metal atom chemisorbs onehydrogen atom, i.e. H/Nis,rfac e = 1. Blank experiments show that the amount ofH 2 chemisorbed on bare supports is negligibly small. It is found that the H 2uptake of Ni/y-Al203 and Ni/La203 are rather low, only amounting to ca.0.99 and 0.33 cm3/g, respectively (Table 4). These correspond to Ni dispersionof ca. 3.0 and 1.0%, respectively. Since no metallic Ni particles are observed byXRD in the Ni/3,-A1203 catalyst, the apparent low nickel dispersion on the highsurface area ~/-A1203 carrier should be largely due to the formation of NiA1204,which is not capable of chemisorbing hydrogen at room temperature. Therelatively higher H 2 uptake on the Ni/A1203, as compared to the Ni/La203,may be due to high dispersion of the remaining metallic Ni particles (most ofnickel is in the form of NiA1204) which could not be detected by XRD. Theunusually low nickel dispersion on La203 appears, at least partially, to be due toformation of large nickel particles on the relatively low surface area ( < 5 m2/g)carrier, as revealed by the XRD study (Table 4). However, as described above,the Ni particle size based on the XRD results is of the order of 330 ,~ which isstill much smaller than the one (ca. 1.000-1.100 ,~) derived from H 2 chemisorp-tion (1.0% dispersion). CO chemisorption at room temperature was studied by measuring the COresponses upon passing 1.1% CO through the catalyst. The area between theresponse curves corresponding to the empty and loaded reactors is equal to theuptake of CO on the catalyst, which could contain adsorbed CO and possiblyother surface carbon species originating from CO. During CO chemisorption,the transient response of CO 2 was also monitored. Blank experiments show thatthe amount of CO chemisorption on the bare supports (,/-A1203 and La203) is
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 125undetectably small. It was observed that a small amount of CO 2 was alsoevolved from the Ni/T-A1203 catalyst upon passing through 1.1% CO, suggest-ing that CO disproportionation occurs during CO chemisorption at room temper-ature. However, no CO 2 evolution was registered from the Ni/La203 catalyst.It can be calculated that the CO uptake on the N i / L a 2 0 3 and Ni/y-AI203catalysts amounts to ca. 0.22 and 1.97 ml/gca t respectively (see Table 4).Assuming that each surface Ni atom chemisorbs one CO molecule, i.e.C O / N i s , rface = 1, the number of surface Ni atoms on the Ni/T-A1203 derivedfrom CO uptake amounts to ca. 5.5. 1019 atoms/gca t, which is close to thevalue derived from the H 2 uptake (5.6-10 t9 atom/gca t or 0.99 cm3/gcat)(Table 4). Considering the uncertainties in the stoichiometric ratio of CO/Nisurfaceand the occurrence of CO disproportionation, the results obtained by COchemisorption on the Ni/T-AI203 catalyst are in surprisingly good agreementwith those of H 2 chemisorption. This could be due to the high Ni loading (17wt.-%) which may favour CO chemisorption in the form of linearly boundadsorbed species (CO/Nisurra~e = 1). Previous studies [33,34] showed that areliable estimation of Ni particle size on A1203 could be obtained for catalystscontaining more than 3 wt.-% metal. For the case of the Ni/La203 catalyst, theCO uptake only amounts to ca. 10% of the respective one on the Ni/AI203catalyst. The Ni particle size of the Ni/La203 catalyst, derived from the COuptake, is about 3-10 times larger than that derived from XRD and H 2chemisorption (Table 4). Apparently, CO chemisorption on the N i / L a 2 0 3catalyst is significantly suppressed.3.2.3. Temperature-programmed desorption experiments TPD profiles of H 2 from the Ni/La203 and Ni/T-A1203 catalysts wereobtained following H 2 adsorption at 25 and 400°C. The TPD profiles of H 2from N i / L a 2 0 3 and Ni/~/-AI203 are shown in Figs. 11 and 12, respectively.Two desorption peaks at ca. 120 and 280°C are observed from the N i / L a 2 0 3catalyst which has adsorbed H 2 at 25°C. As adsorption temperature is raisedfrom 25 to 400°C, the quantity of desorbed H 2 increases significantly (Fig. 11),which might imply that H 2 adsorption on the Ni/La203 catalyst is partly anactivated process. The major desorption peak from the Ni/La203 is shiftedfrom ca. 120 to 165°C, and a new peak at ca. 200-220°C appears, as theadsorption temperature is raised from 25 to 400°C. It seems that hydrogenoriginating from adsorption at higher temperature, tends to desorb at highertemperatures. The H2-TPD profile from the Ni/T-A1203 catalyst are very different fromthose from the Ni/La203 catalyst (Fig. 12). The quantity of hydrogen desorbedfrom the Ni/y-A1203 is found to be about 2.5-3 times that of the N i / L a 2 0 3catalyst. At least five discernible peaks at ca. 120, 220, 320, 440 and 520°C canbe distinguished on the Ni/y-AI203 catalyst after H 2 chemisorption at 25°C.While the first three peaks at 120, 220 and 320°C may correspond to the
126 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 Ni/La203 6OOppra b 25 125 225 325 425 525 T e m p e r a t u r e (*C)Fig. 11. TPD profilesof H2 obtainedovera 17 wt.-%Ni/La203 afteradsorption(a) at 25°Cand (b) at 400°C./3 = 28°C/min.respective three peaks on the N i / L a 2 0 3 (Fig. 11), the two peaks, at 440 and520°C, are absent from the N i / L a 2 0 3 catalyst. These two peaks correspond tostrongly bound H species, probably the hydride species or the hydrogen speciesin the subsurface layers of the metal catalyst . The population of hydrogenspecies under these two peaks accounts for about 15-20% of all hydrogenspecies adsorbed. The proportion of the first three peaks at 120, 220 and 320°Con the Ni/~/-Al203 is also found to be different from that of the respectivepeaks on the N i / L a 2 0 3 . While the major hydrogen species desorb at ca. 120°Cfrom the N i / L a 2 0 3 , they remain on the Ni/3,-A1203 surface at temperatures I 800ppm Ni/A120a i , , , , i . . . . i . . . . i 25 275 525 775 T e m p e r a t u r e (°C)Fig. 12. TPD profiles of H2 obtainedover a 17 wt.-% Ni/3,-AI203 after adsorption(a) at 25°C and (b) at400°C. /3 = 23°C/min.
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 127higher than 200°C. Four peaks at ca. 200, 320, 400 and 530°C are discerniblefrom the Ni/-y-AIzO 3 catalyst which has adsorbed H 2 at 400°C. The peak at ca.120°C which is observable at low adsorption temperature is now not discemible.This may be due to the enhancement of the peak at ca. 200°C, due to theactivated adsorption, which possibly shields the peak at 120°C. A previousH2-TPD study  showed that H 2 adsorption on Ni/A1203 is an activatedprocess and the uptake of H 2 is increased with increasing adsorption tempera-ture. These observations are in agreement with the present results. The general characteristics revealed by H 2-TPD experiments are: (1) a largeramount of hydrogen is desorbed from the Ni catalysts which have been exposedto hydrogen at higher temperature. It seems that H 2 adsorption on the Nicatalysts is partly an activated process; (2) the H - N i bond on the Ni/T-A1203appears to be stronger than that on the Ni/La203, suggesting that there mightexist a certain kind of interaction between Ni and La203 which leads toweakening of H - N i bond; and (3) the quantity of hydrogen desorbed from theNi/7-AI203 catalyst is about 2.5-3.0 times that of the N i / L a 2 0 3 catalyst. Thisis in harmony with the results obtained by isothermal H 2 chemisorption at 25°C(Table 4). CO-TPD profiles from the Ni/La203 and N i / 7 - A l z O 3 catalysts were ob-tained following CO adsorption at 25°C for 10 min. The response of CO 2 wasalso recorded in order to monitor the occurrence of CO disproportionationduring the process of increasing the temperature. No CO or CO 2 was observedto desorb from the Ni/La203 catalyst, presumably because the amount of COa n d / o r CO 2 desorbed was too small to be detected a n d / o r because the CO 2produced is strongly adsorbed on the La203 support, e.g. in the form ofLa202CO 3 which does not decompose in the temperature range applied. Incontrast, a group of intense CO and CO 2 peaks were observed to desorb fromthe Ni/3,-AI203 catalyst. Referring to the results of CO chemisorption whichshow that the CO uptake on the Ni/La203 catalyst is negligibly small and thatthe uptake of CO chemisorption on the Ni/y-A1203 is about 10 times largerthan on the N i / L a 2 0 3 catalyst (Table 4), the above observation is reasonable.Fig. 13 shows the TPD profiles of CO and CO 2 (as a product from COdisproportionation) from the Ni/7-AI203 catalyst. Three pairs of CO and CO 2peaks are observed at 90-110, 240 and 360°C. It is interesting to note that COand CO 2 desorb at approximately the same temperatures. This could be ex-plained by the fact that the mobile CO species, after overcoming the activationenergy barrier, can partly desorb into the gas phase and partly attack theneighbouring oxygen adatoms to form CO 2. It is also possible that the COwhich has already desorbed from the surface readsorbs and then reacts withsurface oxygen adatom to produce CO 2 : CO(g) + O ( a ) ~ CO2(a ) ~ CO2(g )
128 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 I 600ppm Ni/A120z v o o r.) 25 225 425 625 T e m p e r a t u r e (°C)Fig. 13. TPD profiles of CO and CO2, following CO adsorption at 25°C over a 17 wt.-% Ni/~/-La203catalyst. 13 = 28°C/min.The surface oxygen species originate from the dissociation of CO whichprobably takes place at lower temperatures, as described in the section on COchemisorption. The amount of CO 2 desorbed is found to be 2.1 times that ofCO. This means that a significant extent of CO disproportionation occurs on theNi/~/-AI203 catalyst. CO chemisorption/desorption on Ni/A1203 catalystswas studied in detail by Zagli et al. . More than half of the CO was found todisproportionate to CO 2 and carbon. In view of global observations, the presentCO-TPD results on the Ni/3,-AI203 catalyst are in agreement with previousstudies [37,38]. From the results of CO chemisorption (Table 4) and CO-TPD(Fig. 13), it can be derived that the Ni/A1203 catalyst favours CO chemisorp-tion and disproportionation to CO 2 and C, much more than the Ni/La203catalyst.4. Discussion One of the major problems encountered in the process of reforming ofmethane with carbon dioxide to synthesis gas over Ni-based catalysts is rapidcarbon deposition, which leads to blocking of active sites and decrease ofactivity. However, in contrast to other nickel-based catalysts (e.g. Ni/T-A1203and Ni/CaO) which exhibit continuous deactivation with time on stream,essentially no deactivation is observed over the Ni catalyst supported on La203.Moreover, the reaction rate over the Ni/La203 catalyst increases with increas-ing time on stream during the initial several hours of reaction. This leads to thesuggestion that the La203 support plays a key role, affecting the kineticbehaviour of the Ni/La203 catalyst. It may be deduced that the reaction overthe Ni/La203 catalyst occurs mainly at the Ni-La203 interface.
Z. Zhang, X.E. Verykios / Applied Catalysis A: General 138 (1996) 109-133 129 In the present Ni/La203 catalyst, Ni dispersion is very low. Based on theresults of XRD (Table 4), the average Ni particle size of a 17 wt.-% Ni/La203catalyst is of the order of 330 A. Results o f H 2 and CO chemisorption give amean Ni particle size of ca. 1100 and 3200 A, respectively. Although differenttechniques may result in different metal particle sizes, the significant difference(3-10 times) can not be simply attributed to uncertainties of the techniques. Inany event, the three techniques applied show that the Ni particle size is large onthe La203 support. Only the peripheral sites of such large Ni crystallites can bereadily affected by metal-support interaction of any kind, while the majority ofthe surface nickel sites are essentially unaffected [33,34,36,39]. Apparently, thiscan not explain the observation that the uptakes of H 2 and CO are significantlyreduced (Table 4), with respect to the Ni particle size estimated by XRD. Alsothe reaction occurring on such large metallic Ni crystallites (which are essen-tially unaffected by any metal-support interaction) should lead to continuousdeactivation, as observed over other Ni-based catalysts (Fig. 1). Therefore, analternative explanation should be sought. La203 has been widely used as a support of transition metals for COhydrogenation. Bell and his co-workers [40-42] reported that high activity andselectivity for methanol synthesis (a process which does not need the cleavageof the C - O bond) can be achieved when Pd is supported on La203 carrier. Theyfound [40-42] that a thin covering of the La203 support lies on a portion of thePd surface, thereby changing the chemisorptive behaviour to a great extent (e.g.suppressing CO chemisorption). Similarly, it can be proposed that, for thepresent Ni/La203 catalyst, a portion of the Ni surface is decorated by lan-thanum species (e.g. LaO x) originating from the La203 support. The LaO xspecies which are decorating the Ni crystallites may interact with metallic Ni toform a new type of surface compound or synergetic sites at the interfacial areawhich are active and stable towards the reaction of CH 4/CO2 to synthesis gas.The unusual suppression of CO and H 2 chemisorption of large Ni particles onthe Ni/La203 catalysts can thus be attributed to blocking of Ni sites by theLaO x species. As shown by Swaan et al.  and the present kinetic study, catalystdeactivation is mainly due to carbon deposition from the Boudouard reaction(i.e. CO disproportionation). From the studies of CO chemisorption and CO-TPDon the Ni/La203 and Ni/y-A1203 catalysts (Fig. 13), it is shown that CO isfavourably chemisorbed on the Ni/3,-Al203 and then disproportionated to CO 2and C. CO chemisorption on the Ni/La203 catalyst is significantly retarded.Apparently, these differences between the Ni/T-AI203 and Ni/La203 catalystsare affecting the stability of the catalysts. The suppression of CO chemisorptionand CO disproportionation over the Ni/La203 catalyst appears to be related tothe blocking of Ni sites by the lanthanum species. The nature of the interaction between Ni and La203 or the lanthanum specieswhich are decorating the Ni crystallites is unclear at this moment. The XRD
130 Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133results show that while the Ni and La203 phases, which existed in the freshNi/La203 catalyst, disappear, La202CO 3 phase is formed, after more than halfan hour of reaction time (Fig. 10). This may suggest that the interaction betweenNi and lanthanum species is related with the formation of La202CO 3 species. Arecent isotopic labelling study  shows that the oxygen species from theLa202CO 3 participate, to a significant extent, in formation of CO and CO 2 withinteraction of C H 4 / O 2 mixture, presumably via fast exchange between gaseous02 and the oxygen species from La202CO 3. It may be reasoned that underreaction conditions the La202CO 3, which is formed by reaction between La203and CO 2 also participates in formation of product CO. On the other hand, it isknown that CH 4 only weakly adsorbed on La203 [44,45] while it easily crackson metallic Ni at high temperatures [44-47]. Thus, it may be proposed thatunder the C H 4 / C O 2 reaction conditions, CH 4 mainly cracks on the Ni crystal-lites to form H 2 and surface carbon species (CH x species), while CO 2 prefer-ably adsorbs on the La203 support or the LaO x species which are decorating theNi crystallites in the form of La202CO 3. At high temperatures, the oxygenspecies of the La202CO3 may participate in reactions with the surface carbonspecies (CH x) on the neighbouring Ni sites, to form CO. Due to the existence ofsuch synergetic sites which consist of Ni and La elements, the carbon speciesformed on the Ni sites are favourably removed by the oxygen species originatingfrom La202CO 3, thus offering an active and stable performance. In the absence of La203, carbon deposition on the Ni crystallites results incatalyst deactivation due to blocking of surface catalytic sites. In the absence ofNi, breaking of the C - H bonds of the CH 4 molecule on L a 2 0 3 , in the presenceof CO 2, becomes the slow step. It is recalled [48,49] that, even in the presenceof a much stronger oxidant, 02, the rate of CH 4 activation on L a 2 0 3 is slow,requiring superficial contact times of 1-10 s which are significantly longer thanthose required for C H 4 / C O 2 reaction over the N i / L a 2 0 3 catalyst (cp. Fig. 3).Therefore, the reaction rate over pure La2Oa(in the absence of Ni) is expected tobe very low, as has been experimentally verified (Table 2). Based on the mechanism described above, it is easy to interpret the observa-tion that significant amounts of carbon are deposited on the Ni/La203 catalyst,presumably on the Ni crystallites, while the catalyst does not exhibit anysignificant deactivation. This can be attributed to the fact that the catalyticreaction is occurring at the Ni-La203 interfacial area which is not significantlyaffected by carbon deposition on the surface of Ni crystallites (as long as noexcess carbon is accumulated, blocking totally the surface of the Ni crystallites).The fact that the reaction rate is increased during the initial hours of time onstream could be explained by a slow process of establishment of the equi-librium concentration of the La202CO 3 as well as other surface carbon specieson the Ni crystallites. In summary, the Ni/La203 catalyst provides a new reaction pathway occur-ring at the Ni/La203 interface. It is proposed that while CH 4 cracks on Ni
Z. Zhang, X.E. Verykios /Applied Catalysis A: General 138 (1996) 109-133 131crystallites, C O 2 favourably adsorbs on the La203 support, in the form ofLa2OECO 3. The reaction between oxygen species, originating from the L a 2 0 3support, and carbon species, formed upon cracking of C H 4 on Ni crystallites,gives active and stable catalytic performance for carbon dioxide reforming ofmethane to synthesis gas, in spite of significant carbon deposition on the surfaceof Ni crystallites. It should be mentioned that the mechanism described aboveoffers a reasonable explanation to the present observations. Certainly, otherpossible explanations, e.g. the reaction proceeding via formate a n d / o r hydroxylintermediates on the interfacial area, can not be excluded.5. Conclusions The following conclusions can be drawn from the results of the present studyof carbon dioxide reforming of methane to synthesis gas over the N i / L a 2 0 3catalyst. (1) While a continuous catalyst deactivation is experienced over Ni/3,-A1203and N i / C a O catalysts, the reaction rate over N i / L a 2 0 3 is found to increasewith time on stream during the initial 2 - 5 h of reaction, and then tends to beessentially constant with time on stream, displaying very good stability. (2) A superficial contact time larger than approximately 0.02 s is sufficientfor the N i / L a 2 0 3 catalyst to reach equilibrium conversions (Pcri, = 0.2 bar,C H 4 / / C O 2 = 1). The apparent activation energy over the N i / L a 2 0 3 catalyst atthe stable level and at the initial stage of the reaction (reaction time approachingzero) is found to amount to ca. 62.7 and 80.0 K J / m o l , respectively. (3) The N i / L a 2 0 3 catalyst exhibits stable performance over a wide tempera-ture range (T~> 550°C). Although significant carbon deposition is occurring,especially when concentrated feed is used, and the reaction is operating underintegral conditions, only a small degree of deactivation is recorded during 100 hof time on stream. (4) XRD results show that the fresh N i / L a 2 0 3 catalyst consists of Ni andLa203 phases. After exposure to the reactant mixture at 750°C for more thanhalf hour, the N i / L a 2 0 3 catalyst experiences a profound change in its bulkphase, being transformed into La202CO 3. (5) Results of H 2 and CO-TPD reveal that the H - N i bond is weakened whileCO disproportionation is unfavoured on the N i / L a 2 0 3 catalyst, as compared tothose on the Ni/A1203 catalyst. (6) A comparison of H 2 and CO uptake and Ni dispersion by XRD showsthat H 2 and CO uptakes are significantly suppressed, by 3 - 1 0 times, suggestingthat a portion of the Ni surface is blocked by lanthanum species.
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