Applied Catalysis A: General 210 (2001) 45–53

Bound-state Ni species — a superior form in Ni-basedcatalyst for CH4/CO2 reforming

Zheng Xua,∗, Yumin Li b, Jiyan Zhangb, Liu Changb, Rongqi Zhoua, Zhanting Duanaa Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China

b College of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

Received 5 January 2000; received in revised form 6 June 2000; accepted 21 August 2000

Abstract

The effects of nickel loading, calcination temperature, support, and basic additives on Ni-based catalyst structure andreactivity for CH4 reforming with CO2 were investigated. The results show that the structure of the nickel active phasestrongly depends on the interactions of the metal and the support, which are related to the support properties, the additivesand the preparation conditions. “Free” Ni species can be formed when the interaction is weak and their mobility makes themeasily deactivated by coking and sintering. The effect of strong metal-support interaction (SMSI effect) is different for varioussupports. The formation of solid solution of Ni–Mg–O2 and the blocking of TiOx by the partially reduced TiO2 can bothdecrease the availability of Ni active sites in Ni/MgO and Ni/TiO2. The spinel NiAl2O4 formed in Ni/g-Al2O3 might beresponsible for its high activity and resistance to coking and sintering because it can produce a highly dispersed active phaseand a large active surface area as bound-state Ni species when the catalyst is prepared at high calcined temperatures or withlow nickel loading. The addition of La2O3 or MgO as alumina modifiers can also be beneficial for the performance of theNi/g-Al2O3 catalyst. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Supported Ni-based catalysts; CH4/CO2 reforming; Carbon deposition; Metal-support interaction; Modification of support;Calcination temperature; Nickel loading

1. Introduction

In recent years, increasing attention has been paidto the reforming of methane with carbon dioxide toproduce synthesis gas with low H2/CO ratios for bothresearch and industry [1–3]. This process employs twoof the cheapest and most abundant carbon-containingmaterials, CH4 and CO2, and yields products thatare more preferable feeds for some liquid fuel syn-thesis processes, such as Fischer–Tropsch synthesisand carbonyl synthesis. The process is also of specialinterest due to its potentially friendly effect on the

∗ Corresponding author. Fax:+86-10-6277-0304.

environment by reducing greenhouse gas emissions[4,5]. Moreover, the reversibility and large thermal ef-fect of this reaction can provide easy transport energyfrom remote regions by chemical energy transmissionsystems (CETS) [6]. However, the strong thermody-namic potential for carbon deposition and the veryhigh operating temperature limit its application.

Although it has been reported that supported noblemetal catalysts (Rh, Ru, Ir, Pt and Pd) have promisingcatalytic performances and low sensitivities to car-bon deposition [2,7–10], the unavailability of noblemetals limits their utilization in large-scale processes.Therefore, supported nickel catalysts are a promisingalternative because of their high activity and relatively

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-860X(00)00798-5

46 Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53

low cost. Nevertheless, carbon accumulation on thesurface of nickel-based catalysts and Ni particle sin-tering are difficult to avoid in industrial applications[11,12]. Recent reports stated that MgO and somereducible metal oxide supports such as TiO2, ZrO2,or La2O3 can interact favorably with nickel to signifi-cantly inhibit carbon deposition on the catalyst surface[13–16], so it is thought that the metal-support inter-action may play an important role. However, thesesupports are usually low surface area materials (only1–30 m2/g), so Schmitz et al. [17] proposed that anindustrial nickel-based catalyst should be formed ona mixed oxide support containing a reducible compo-nent to obtain more efficient utilization of the metal.Some basic metal oxides were also useful for inhibit-ing coking [18–21], but there are still some practicalproblems to be settled.

This paper analyzes metal-support interactions toevaluate several important factors. Various supportmaterials including MgO, TiO2, g-Al2O3, a-Al2O3,and SiO2 will be examined. Some basic metal oxidessuch as La2O3, MgO, K2O, and Li2O will be used aspromoters. The effects of nickel loading and calcina-tion temperature on Ni/g-Al2O3 catalyst will be alsostudied by different characterization techniques, suchas TPR, TGA, TEM, XRD, and XPS.

2. Experimental

2.1. Catalyst preparation

Supported Ni catalysts were prepared using the wetimpregnation method with nitrate salts as the metalprecursors. The support materials wereg-Al2O3,a-Al2O3, TiO2, SiO2 and MgO, all obtained fromthe Tianjin Chemical Engineering Institute. Afterimpregnation, the precursor was dried at 393 K, andthen calcined in air at 823 K for 4 h. The Ni loadingwas normally 11.8 wt.%. Some Ni/g-Al2O3 catalystswere calcined individually at 1073 and 1173 K. OtherNi/g-Al2O3 catalysts were prepared with Ni loadingsfrom 2.4 to 23.7 wt.%.

The Ni/g-Al2O3 catalysts were prepared with basicmetal oxides such as La2O3, MgO, K2O and Li2O aspromoters. All the mixtures were dried at 393 K andcalcined at 823 K. The La2O3 and MgO precursorswere also introduced by impregnation before impreg-

nating nickel nitrate on the support to prepare thecomposite support. Other conditions were the sameas above.

2.2. Reactivity evaluation

The reactivity of the catalysts were evaluated atatmospheric pressure using a quartz tube reactor(ca.12 mm i.d.) over the temperature range 873–1173 K.The catalyst bed was approximately 10 mm long andwas typically loaded with 500 mg catalyst. The cat-alyst was first pre-reduced by pure H2 at 873 K for0.5 h, and then purged by N2. A feed mixture consist-ing of CO2/CH4/N2 = 2.0/1.0/0.8 (mol) was used.The gaseous products were analyzed by an on-line gaschromatograph, which was equipped with a thermalconductivity detector (TCD) and a carbon molecularsieve column. The product composition was calcu-lated by material balance and calibrated by N2.

2.3. Coking measurement by thermogravimetricanalysis

Thermogravimetric analysis (TGA) was used tomeasure the rate and amount of coking on the catalystsurface using a quartz micro reactor. Typically, 20 mgof catalyst was put into a little basket under the feedstream of CO2/CH4/N2 = 77/60/103 (volume).After it had been reduced under a flowing gas mix-ture of 50 vol.% H2 and 50 vol.% N2 at 1073 K for0.5 h, it was purged and cooled by flowing N2. Thenthe testing was carried out at 973 K for 40 min. Thecoking rate refers to the constant rate in the uniformspeed stage of the TG curves. The amount is the netincrease of weight during the reaction for 40 min.

2.4. Temperature-programmed reduction experiment

Temperature-programmed reduction (TPR) wasconducted in a quartz tube of 4 mm diameter with a50 mg sample by raising the temperature from 298 to1173 K at a rate of 10 K/min under argon with 4 vol.%H2.

2.5. Characterization

A Quantachrom CHEMBET 3000 system was usedto determine the surface area. A Rigaku D/MAX-2308

Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53 47

X-ray diffractometer with Cu Ka radiation at a scan-ning rate of 4◦/min was used for XRD. The sur-face state of elements in catalyst was analyzed by aPHI1600 ESCA SYSTEM X-ray photoelectron spec-trometer (XPS) with Mg Ka radiation (1253 eV), anda JEM100-CX-II electron microscope was used forthe TEM images.

3. Results

3.1. Catalytic performance

The activities of various supported Ni catalysts andNi/g-Al2O3 with different basic metal oxides as pro-moters were tested at 873 and 1173 K. The results(listed in Table 1 ) indicate that, except for Ni/TiO2,

Table 1Conversion of methane and carbon dioxide at 873 and 1173 K oversupported Ni-based catalysts (0.1 MPa, CO2/CH4 = 2, GHSV=4400 h−1)

Catalyst (wt.%) Temperature(K)

Conversion of (%)

CH4 CO2

11.8%Ni/g-Al2O3 873 58.6 42.61173 100 71.1

11.8%Ni/a-Al2O3 873 57.6 42.51173 100 71

11.8%Ni/MgO 873 58.6 42.21173 100 71

11.8%Ni/SiO2 873 57.6 40.71173 100 71.4

11.8%Ni/TiO2 873 5.6 4.91173 19.2 19.5

11.4%Ni–2.9%K2O/Al2O3 873 56.6 41.31173 100 70.1

11.4%Ni–2.8%Li2O/Al2O3 873 59 43.41173 100 70.5

11.4%Ni–2.9%MgO/Al2O3 873 59 42.91173 100 70.5

11.5%Ni–2.3%La O3/Al2O3 873 60.5 44.31173 100 71.7

11.4%Ni/Al2O3–5.8%La2O3 873 61.9 46.81173 100 73.7

11.8%Ni/Al2O3–3.0%MgO 873 61.3 43.11173 100 72.0

the supported catalysts all displayed very high activity.Close to 100% of the CH4 was converted within theTCD detection limitations. The Ni/g-Al2O3 catalystexhibited excellent performance over the temperaturerange of 873–1173 K with a substantial increase in thereactant conversion when lanthanum oxide or magne-sium oxide was added. Furthermore, the effect becamestronger if they were added as composite supportsthrough modifying the alumina, which is more suitablefor MgO, because it has almost no effect on the activity

Fig. 1. Conversion of CO2 as a function of reaction tempera-ture over Ni/g-Al2O3 catalyst. (a) Effect of calcination temper-ature (11.8 wt.% Ni/g-Al2O3) (+) 823; (h) 1073; (e) 1173 Kand (b) effect of Ni loading (calcination temperature is 823 K)(+) 1173; (4) 1073; (h) 973; (e) 873 K. Reaction conditions:P t = 0.1 MPa, CO2/CH4 = 2.0, GHSV= 4400 h−1.

48 Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53

Table 2Coking results on supported Ni-based catalysts measured by TGA at 973 K, CO2/CH4 = 1.3; compound determined by XRD; specificsurface areaa

Catalyst Surface area (m2/g) Compoundby XRD

Coking rate (mgC/g cat. min)

Amount of coke(wt.%/40 min)

Ni/g-Al2O3 101.0 g-Al2O3 21.8 42.2Ni/a-Al2O3 2.0 NiO, a-Al2O3 0.2 0.6Ni/MgO 22.1 MgO 4.1 2.0Ni/SiO2 319.5 NiO, SiO2 3.1 3.0Ni/TiO2 50.3 NiO, TiO2 0 0Ni–K2O/Al2O3 109.6 NiO, Al2O3 12.4 38.9Ni–Li2O/Al2O3 101.8 NiO, Al2O3 14.1 31.5Ni–MgO/Al2O3 103.7 NiO–MgO solid solution 14.4 32.7Ni–La2O3/Al2O3 110.9 g-Al2O3 6.7 18.5Ni/Al 2O3–La2O3 86.6 g-Al2O3 5.4 19.9Ni/Al 2O3–MgO 96.1 MgAl2O4 5.6 20.3

a Coking rate comes from the stage where its coking rate is constant according the TG curves.

when added after the Ni precursor. However, regard-less of its addition method, lanthanum oxide alwaysfacilitates the conversion of both methane and carbondioxide and is more effective than any other additive.

The effects of calcination temperature and Ni load-ing on the activity of Ni/g-Al2O3 are shown in Fig. 1.A very large fluctuation of CO2 conversion was ob-served for temperatures from 873 to 1173 K with thecatalyst calcined at 1173 K. The reactivity was verylow below 973 K but higher than the others at 1173 K.A similar phenomenon was observed when the Niloading is only 2.4 wt.%. The results suggest that theeffects of calcination temperature and Ni loading onthe Ni/g-Al2O3 activity are not significant over a widerange, but high calcination temperatures and low Niloadings greatly influence the activity.

3.2. TGA coking measurement

The rate and amount of coking on the catalyst sur-face during the methane reaction with carbon diox-ide for 40 min at 973 K measured by TGA are listedin Table 2. The carbon deposition easily takes placeon the surface of Ni/g-Al2O3, but can be inhibitedby adding a basic metal oxide such as K2O, Li2O,MgO or La2O3. The effects of La2O3 and MgO asadditives are greater than the others as with the activ-ity. In addition, no carbon deposition occurred on theNi/TiO2.

Fig. 2 shows the effect of Ni loading on the cokingrate with the Ni/g-Al2O3 catalyst. Unlike the activity,

the Ni loading greatly influences the coking rate. Niloadings<7.1 wt.% had low carbon deposition, but thecarbon deposition increased dramatically as the load-ing increased. This indicates that carbon depositionwith the Ni/g-Al2O3 catalyst is related to the quantityof active metal and is more sensitive than the reform-ing reaction.

3.3. Temperature-programmed reduction experiments

TPR profiles of H2 from various supported Ni-based catalysts and Ni/g-Al2O3 prepared at different

Fig. 2. Effect of Ni loading on coking rate over Ni/g-Al2O3 cata-lyst during CH4/CO2 reforming for 40 min. Reaction conditions:P t = 0.1 MPa,T = 973 K, CO2/CH4 = 1.3.

Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53 49

Fig. 3. H2-TPR profiles obtained over various supported Ni cata-lysts: (a) Ni/g-Al2O3; (b) Ni/MgO; (c) Ni/TiO2; (d) Ni/SiO2; (e)Ni/a-Al2O3.

calcined temperatures are shown in Figs. 3 and 4.Low temperature reduction peaks (660–700 K) wereobserved as the main peaks for Ni/a-Al2O3, Ni/TiO2and Ni/SiO2, just as with the reduction of unsupportednickel oxide. However, compared to Ni/a-Al2O3 andNi/SiO2, the reduction curve of Ni/TiO2 is narrow,which means that the H2 consumption is low and nomore reduction occurred above 773 K with Ni/TiO2.

Fig. 4. H2-TPR profiles obtained over 11.8 wt.% Ni/g-Al2O3 cat-alyst calcined at different temperatures: (a) 823; (b) 1073; (c)1173 K.

This phenomenon should be attributed to the specialinteraction of Ni with TiO2, It has been reported thatTiO2 can be partially reduced to TiOx at high temper-ature, and that TiOx may block the active Ni specieson the interfacial surface of the metal and support[22], which might be the reason why the activity andcarbon deposition with Ni/TiO2 are both very lowafter reduction at 873 K.

With Ni/g-Al2O3 and Ni/MgO, at least two reduc-tion peaks appeared at higher temperatures (above773 K), but the size and the shape of the peaks werevery different for Ni/g-Al2O3 and for Ni/MgO. TheNi/g-Al2O3 peak area is several times larger than theNi/MgO peak area. So we suggest that the reductionof Ni on Ni/MgO is more difficult than on Ni/g-Al2O3for the same conditions, which would lead to very lowutilization of the metal on Ni/MgO.

With Ni/g-Al2O3, different Ni species formed onthe surface ofg-Al2O3 corresponding to the differ-ent reduction peaks, which are both different fromthose formed with Ni/a-Al2O3, Ni/TiO2, Ni/SiO2 andNi/MgO. When the calcination temperature was in-creased to above 1073 K, the other reduction peaksdisappeared and only one large peak remained withhigh reduction temperatures above 1073 K. This peakarea was almost twice the size of the lower temperaturepeaks. This implies that most other Ni species formedon the support surface changed to this species at thistemperature and could only be reduced above 1073 K.

3.4. Characterization of catalyst

The TEM images of two Ni/g-Al2O3 catalysts withdifferent Ni loadings clearly show the particle sizechanges after the reaction (Fig. 5). All particle sizessignificantly increased during the reaction at hightemperatures, which means that sintering occurredto some degree. However, the size is significantlysmaller for the catalyst with 3.9 wt.% Ni loading thanfor the one with 11.8 wt.% both before and after thereaction. The low Ni loading increases the availabilityof the Ni/g-Al2O3 catalyst for high metal dispersityand increases its resistance to heat and coking.

The specific surface area of supported Ni catalystsvaried greatly from 2 to 320 m2/g with the differentsupport materials (Table 2), but the support materialarea was not directly related to the catalytic reactivityof the catalyst.

50 Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53

Fig. 5. TEM images of Ni/g-Al2O3 catalysts (∗100k) (a) 3.9 wt.% Ni/g-Al2O3; (b) 11.8 wt.% Ni/g-Al2O3; (c) used 3.9 wt.% Ni/g-Al2O3;(d). used 11.8 wt.% Ni/g-Al2O3.

The catalytic compounds detected by XRD are alsolisted in Table 2. Consistent with the H2-TPR profiles,the crystalline phase of nickel oxide were observed onNi/a-Al2O3, Ni/TiO2 and Ni/SiO2, but were not de-tected on Ni/MgO and Ni/g-Al2O3. This implies thatthe interaction of Ni with the supportsa-Al2O3, TiO2and SiO2 is weak and that nickel oxide is only dis-persed to the “free state”, as microcrystal particles onthe surface of the support. Similar results were ob-served for Ni/g-Al2O3 with K2O or Li2O as promot-ers. A strong nickel oxide diffraction peak occurredwith the addition of K2O and Li2O, which may bethe reason that the catalytic performances with themis not as good as without any additives.

No apparent nickel oxide crystalline phase existedwith Ni/MgO and Ni/g-Al2O3 which indicates that thedispersity of Ni is very high compared to the other

supported catalysts, even with additives such as MgOand La2O3. Adding MgO after the nickel precursorhas very poor results due to the formation of a solidsolution between nickel oxide and magnesium oxide,which would greatly reduce the active nickel species[23].

The effects of high calcination temperature on theperformance of Ni/g-Al2O3 can be attributed to theformation of a new compound NiAl2O4 spinel de-tected by XRD. As shown in Fig. 6, the primary crys-talline phase of Ni/g-Al2O3 changed fromg-Al2O3to NiAl2O4 spinel as the calcination temperature wasraised from 823 to 1173 K, which is in agreement withthe changes of reduction peaks from two to one andthe reduction temperature from low to high. Therefore,the difficulty in reducing NiAl2O4 spinel is probablyresponsible for the very low activity observed at low

Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53 51

Fig. 6. XRD patterns obtained over 11.8 wt.% Ni/g-Al2O3 calcinedat different temperatures: (a) 823; (b) 1073; (c) 1173; (d)g-Al2O3.

reaction temperatures after the catalyst was calcinedat 1173 K.

Fig. 7 displays the change of the Ni active phasestructure with Ni loading increasing for Ni/g-Al2O3as measured by XRD. When the loading is<7.1 wt.%,there is no evidence of a Ni crystalline phase. For load-ings >11.8 wt.%, a NiO microcrystal phase appearedwith the signal intensity increasing with the loading.Between them, the signals for the NiO phase are veryweak, but broadened significantly.

The above results indicate that the interaction of Niwith g-Al2O3 is strong and complicated. The resultingstructure of the active phase could be affected by bothcalcination temperature and nickel loading.

Table 3XPS results of Ni/g-Al2O3 catalyst with or without La2O3 modifier before or after CH4 reforming with CO2

Sample Ni2p3/2 (eV) La3d5/2 (eV) O1s (eV) Al2p3/2 (eV)

Fresh Ni/g-Al2O3 856.2 531.3 74.4Used Ni/g-Al2O3 856.4 853.5 531.4 74.4Fresh Ni/Al2O3–La2O3 856.2 836.0 837.3 531.3 74.4Used Ni/Al2O3–La2O3 856.2 853.3 836.4 838.3 531.3 74.4g-Al2O3 531.3 74.4

Fig. 7. XRD patterns obtained from NiO (a);g-Al2O3 (b) andNi/g-Al2O3 with Ni loading 2.4 (c); 3.9 (d); 7.1 (e); 11.8 (f); 16.4(g); and 23.7% (h).

The binding energies of various elements on thesurface of Ni/g-Al2O3 with and without La2O3 werestudied by XPS. As shown in Table 3, compared tog-Al2O3, the BE values before and after the reactionfor O1s and Al2p3/2 did not change for either cata-lyst, but after the reforming reaction, the BE valuesof Ni2p3/2 with the Ni/g-Al2O3–La2O3 catalyst wereless than the values in the Ni/g-Al2O3 catalyst, al-though they are both NiO (853.4 eV). At the sametime, the BE value of La3d5/2 increased by 0.4 eV andits shake-up peak value increased up to 1.0 eV. Thisresult reveals that the main valence state of nickel is

52 Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53

its oxidative form with a valence of two during thereaction, which cannot be completely reduced to zerovalence metal under the reduction conditions, but canbe continuously reduced under the reaction conditions.In addition, La2O3 must be involved in the reformingreaction and is itself oxidized so that Ni remains inthe lower valence state.

4. Discussion

One of the major problems encountered in theprocess of methane reforming with carbon dioxideto yield syngas over Ni-based catalysts is rapid de-activation caused by carbon deposition or sinteringwith high temperature and high pressure reactionconditions. g-Al2O3 is a very suitable support forhigh temperature reactions due to its large surfacearea and thermal stability, but the Ni/g-Al2O3 cata-lyst has been shown to be very susceptible to carbondeposition in many reports and in our experimentalresults. However, there could be several kinds of Niactive phases formed on the surface ofg-Al2O3, suchas microcrystalline nickel oxide and spinel NiAl2O4,which have been studied by various techniques, suchas, TPR and XPS [24,25]. The microcrystalline nickeloxide has been reported as the “free state” of theNi active phase because of its mobility, which leadsto migration, aggregation and growth of particles athigh temperatures so that the dispersion of the activephase rapidly decreases, which is one of the mainreasons why the supported Ni-based catalyst is easilydeactivated.

The spinel NiAl2O4 is usually considered to be the“fixed or bound-state” of the Ni active phase due toits strong interaction with the support [26]. Althoughreduction is difficult, at least above 1073 K [27], itexhibits very high activity at high temperatures and isresistant to coking and heat once reduced, as shownby the Ni/g-Al2O3 catalyst prepared at high calcinedtemperature or very low Ni loading. In this case, thenickel ions tend to be highly dispersed and can be“fixed” by getting into the alumina lattice, in tetrahe-dral and/or octahedral sites in the support. So althoughno Ni species were detected by XRD with Ni loadings<7.1 wt.%, “surfacial NiAl2O4 spinel” may be formedto some degree [28], perhaps only on the surface ofthe support. The TPR profiles of Ni/g-Al2O3 catalyst

calcined at 823 K proved that the NiAl2O4 spinel wason the surface of the support. The reduction tempera-ture for this species is lower than for that formed in thebulk at high calcination temperatures, but higher thanthat for crystalline NiO formed on the Ni/a-Al2O3catalyst.

Since the reduced Ni species is the active phaseresponsible for the activation of CH4, it will lead totwo parallel reactions: CO2 reforming and coking byCH4 dehydrogenation. The dehydrogenation is moresensitive to the active phase structure than the reform-ing and usually requires more Ni atoms as “ensemblesites” [29,30]. Since coking is an irreversible reac-tion, its rate will progressively increase with loadingand will be more significantly influenced by alter-ations of the active phase. It has also been reportedthat the structure of NiAl2O4 spinel can effectivelyinhibit carbon deposition [31], as with the result forNi/g-Al2O3 catalyst with Ni loadings not more than7.1 wt.%. Therefore, a Ni/g-Al2O3 catalyst producedwith low metal consumption not only has excellentperformance but also can resist heating and coking,if the active phase is properly prepared.

The different interactions of the additives with theother components and the effect of the method usedto combine the additives indicate that the interactionis a complex process, which is not only determinedby its basicity. Although the coking rate decreasedwith the addition of K2O or Li2O, the activity wasalso reduced due to the formation of large NiO crys-talline particles. MgO is ineffective as a promoter dueto the formation of a solid solution, but it is usefulas a modifier for alumina. La2O3, a rare earth oxide,is very effective at inhibiting coking and improvingthe activity and stability, which indicates that a redoxreaction occurs in the interfacial region between Niand La, and that electrons are transferred from Lato Ni to maintain Ni at a lower valence, which pro-motes the activation of CH4. La2O3 can also easilyadsorb CO2 to form some carbonates [32], whichwill favor the activation of CO2 and accelerate theelimination of surface carbonaceous species such asCHx (x = 0–3), which come from the dehydrogena-tion of CH4. So the conversion of both CH4 and CO2are improved and resistance to coking is also signif-icantly improved, which may be the reason why theLa2O3 added after the nickel precursor is also a goodpromoter.

Z. Xu et al. / Applied Catalysis A: General 210 (2001) 45–53 53

5. Conclusions

Metal-support strong interaction (SMSI) effectsplay a very important role in the formation of theactive phase of nickel and are mainly responsible forthe performance of nickel based catalysts in the re-action of CO2 with CH4. The “free state” NiO, oftenformed as the major active phase over Ni/a-Al2O3and Ni/SiO2 or Ni/g-Al2O3 with the addition ofK2O or Li2O as promoters because of no significantmetal-support interaction, is considered to be respon-sible for catalyst deactivation. Smaller Ni active sur-face area caused by the formation of a solid solutionor blockage of the support species over Ni/MgO orNi/TiO2 is also undesirable.

g-Al2O3 as the support has a strong tendency tointeract with Ni and other metal oxides such as MgOand La2O3 to form a composite support and can easilyform a new “bound-state” Ni species such as spinelNiAl 2O4, when prepared by wet impregnation at highcalcined temperatures or low Ni loadings. These Nispecies provide large surface areas which increase thereactivity and reduce the coking rate, after reductionon stream at high temperatures, these “bound-state”Ni species remain highly dispersed due to their verylow mobility, so they have long life and high stability.

La2O3 is a valuable additive for Ni-based cata-lysts during CH4/CO2 reforming, especially as a mod-ifier of the alumina support. La2O3 does not changethe structure of the “bound-state” Ni species and canreduce nearby Ni species to form Ni–La2O3 inter-faces with synergetic sites, facilitating electron trans-fer between Ni and La ions to maintain the Ni at alow valence, which would promote activation of thereactant and elimination of coking.

Acknowledgements

The State Key Laboratory of C1 Chemical Enginee-ring of China supported part of this research.

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