ultrafine nio–la2o3–al2o3 aerogel: a promising catalyst for ch4/co2 reforming

Applied Catalysis A: General 213 (2001) 65–71

Ultrafine NiO–La2O3–Al2O3 aerogel: a promisingcatalyst for CH4/CO2 reforming

Zheng Xu a,∗, Yumin Li b, Jiyan Zhang b, Liu Chang b, Rongqi Zhou a, Zhanting Duan a

a Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR Chinab College of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

Received 3 June 2000; received in revised form 7 September 2000; accepted 13 November 2000

Abstract

A newly designed ultrafine NiO–La2O3–Al2O3 aerogel catalyst has been successfully prepared by the combination ofsol–gel method and supercritical drying (SCD) technique for CH4/CO2 reforming. Compared to the conventional impregnatedcatalyst, it exhibits unusual physical and chemical properties, as manifested in very large specific surface area, well-definedpore size distribution and good textural stability. Very high activity and at the same time very low carbon deposition were alsoobserved. It more easily forms homogeneously distributed NiAl2O4 spinel in aerogel catalyst at low heat treatment temperatureand has much higher capacity to adsorb CO2, which may be mainly responsible for its excellent catalytic performance andinsensitive to carbon deposition. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: NiO–La2O3–Al2O3 catalyst; Aerogel materials; Preparation; Methane; Carbon dioxide reforming; Carbon deposition

1. Introduction

At the end of the 20th century, the energy crisisand environmental protection have been two of themost important issues that are related to the continuedexistence and development of human beings. Naturalgas, whose main component is methane, as the thirdand cleanest fossil fuel resource in the world, is apromising alternative energy for petroleum at the be-ginning of the next century. But up to now, the mostsuccessful application of natural gas in industry hasbeen its conversion to liquid fuels or valuable oxy-genated chemicals via synthesis gas. However, theconventional process for methane steam reforming tosyngas (Eq. (1)) is not desirable for the productionof long-chain hydrocarbons due to the high H2/CO

∗ Correspondence author. Fax: +86-10-62770304.E-mail address: [email protected] (Z. Xu).

ratio of product, so methane reforming with carbondioxide to syngas (Eq. (2)) with a low H2/CO ratiohas recently attracted much more attention [1–5].

CH4 + H2O ⇔ CO + 3H2

�H298 = +206 kJ/mol (1)

CH4 + CO2 ⇔ 2CO + 2H2

�H298 = +247 kJ/mol (2)

Moreover, the effective utilization of CO2 has alsobeen attracting attention recently, because it is notonly a main greenhouse gas, but also one of the mostabundant carbon-containing resources [6]. There-fore, some natural gas fields containing considerableamounts of CO2 would be convenient for conversionto syngas on the spot and subsequent transport toremote areas. In addition, CH4/CO2 reforming is alsouseful to reserve and transport energies in the form of

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0 9 2 6 -8 6 0X(00 )00881 -4

66 Z. Xu et al. / Applied Catalysis A: General 213 (2001) 65–71

chemical energy, due to its reversibility and thermaleffect [7]. Thus, the reaction of CH4 with CO2 hasattracted significant attention during this decade inboth academic and commercial areas.

However, CO2 reforming of CH4 is much moreprone to coking than steam reforming because of thehigher C/H ratio in the reactant gases [8]. Moreover,carbon deposition seems to be unavoidable even athigher temperatures as the feedstock is stoichiometri-cally fed. Thus, the catalysts used for steam reform-ing are not suitable for CO2 reforming. So we needto improve or develop new catalysts. Apart from thehigh activity required at high temperatures, the stabil-ity and resistance to coking are also very important forindustrial applications. Some nickel-based catalystshave been reported to be good due to their low costand good catalytic performance [9–12], but the mate-rials used as support usually have low surface areas(from 1 to 30 m2/g), which lead to low utilization ofthe metal.

In our early work, Ni/�-Al2O3 catalyst exhibitedhigher initial activity and larger surface area thanNi/MgO, Ni/TiO2 and Ni/�-Al2O3 catalysts at hightemperatures [13], but it is more susceptible to coking.One of the solution methods is the addition of basicmetal oxide as promoters, such as MgO and La2O3.The NiAl2O4 spinel formed by strong interactionof metal with support might also play an importantrole in inhibiting carbon deposition. Nevertheless,the formation of NiAl2O4 spinel was difficult andwas always accompanied by some damage of surfacearea and other properties when using conventionalpreparation methods, such as impregnation followedwith thermal drying. In this paper, a newly designedultrafine Ni-based aerogel catalyst including La2O3as a modifier has been put forward for solving thisproblem. Such a treatment is also an improvement tothose Ni catalysts with ultrafine MgO or Al2O3 assupport reported earlier [14,15].

2. Experimental

2.1. Catalyst preparation

Ultrafine ternary aerogel catalyst was prepared bya combination of sol–gel method and supercriticaldrying (SCD) technique just like the preparation of

simple metal oxide aerogel [16,17]. Other metal ox-ides were added during the sol–gel process to makea multicomponent cogel, and then SCD occurred to-gether. In this way, the catalyst is different from thosewhose active metals were added to the aerogel supportby impregnating or other conventional methods.

The NiO–La2O3–Al2O3 aerogel catalyst was pre-pared by the following steps: starting with aluminumnitrate solution and ammonia, both titrated slowly inboiled water at ambient temperature, the lanthanumand nickel nitrate solution could be added in sequenceby precipitating and depositing on the alumina gel.Vigorous stirring was used until the pH value of thesolution reached 9.0; after aging for 1 h, a homoge-neous greenish cogel had been formed in the vessel.This cogel should be washed and exchanged by alco-hol to form alcogel, and then placed into an autoclaveto experience SCD (533 K, 8.0 MPa), which is anessential step to prevent the structure of gel fromcollapse through getting rid of the capillary pres-sure [18]. Eventually, the light aerogel powders wereobtained when the autoclave was opened.

In this way, the binary NiO–Al2O3 aerogel and sin-gle Al2O3 aerogel powders were also prepared. TheAl2O3 aerogel powder was used as support to makea catalyst by impregnating nickel nitrate solution,which was labeled as aereogel-1. Other binary andternary aerogel catalysts were, respectively, labeledas aerogel-2 and aerogel-3. The conventional ternarycatalyst as comparative non-aerogel was prepared byimpregnating commercial �-Al2O3 (obtained fromTianjin Chemical Engineering Institute) with lan-thanum nitrate and nickel nitrate solution, respec-tively, and then dried at 393 K. All of them werecalcined in air at 823 K for 3 h, then shaped andsmashed into particles with 40–60 mesh.

2.2. Characterization

The BET surface areas and pore size distribution ofcatalysts were measured by a Quantachrom CHEM-BET 3000 system. XRD patterns were taken by usinga Rigaku D/MAX-2038 X-ray diffractometer with CuK� radiation at a scanning rate of 4◦/min. A Shi-madzu DT-20B thermal analysis instrument was usedfor differential thermal analysis (DTA). The appar-ent density of samples was measured in a graduatedcylinder.

Z. Xu et al. / Applied Catalysis A: General 213 (2001) 65–71 67

2.3. Reactivity and coking evaluation

The reactivity of the catalysts were evaluated at at-mospheric pressure using a stainless steel reactor (ca.9 mm i.d.) over the temperature range 873–1173 K.The catalyst bed was approximately 17 mm long andwas typically composed of 500 mg catalyst and in-ert diluents. The pre-reduction by pure H2 was firstrequired at 873 K for 0.5 h, and then the system waspurged by N2. Finally, a feed mixture consistingof CO2/CH4/N2 = 1.0/1.0/0.8 (mol) was used.Gaseous products were analyzed by an on-line gaschromatograph equipped with a thermal conductivitydetector (TCD) and a carbon molecular sieve column,with H2 as a carrier gas. The composition of productswas calculated on a material balance calibrated by N2.

The amount of coking on the catalyst was measuredby the difference of the weight of the catalysts beforeand after reaction.

2.4. H2-TPR and CO2-TPD

Temperature-programmed reduction (TPR) experi-ments were carried out in a quartz tube of 4 mmdiameter with 100 mg sample by raising the tempera-ture from 298 to 1273 K at a rate of 10 K/min under10 vol.% H2 and N2 flow. The outlet of reactor wasconnected to a thermal conductivity detector (TCD)and was described as a signal of consumption ofhydrogen.

The temperature-programmed desorption (TPD)experiments were also carried out using the sameapparatus as TPR experiment. After reduction of thecatalyst at 1073 K, the system was cooled down, andthen carbon dioxide was adsorbed at 573 K for 1 h.Argon gas was used as carrier gas by raising thetemperature to 1173 K at a rate of 10 K/min.

Table 1Physical properties of the aerogel and non-aerogel catalysts

Catalyst Composition SBET (m2/g) Dpa (nm) Vpt

b (cm3/g) ρbc (g/cm3)

Aerogel-1 7.1 wt.% NiO–Al2O3 209.9 10.0 4.6 0.658Aerogel-2 7.1 wt.% NiO–Al2O3 254.6 11.3 4.1 0.053Aerogel-3 7.1 wt.% NiO–La2O3–Al2O3 286.3 10.1 3.7 0.051Non-aerogel 7.1 wt.% NiO–La2O3–Al2O3 145.3 9.7 3.4 0.591

a The average pore diameter.b Cumulative pore volumes.c Bulk density.

Fig. 1. DTA curves of ultrafine aerogel samples.

3. Results and discussion

3.1. Catalyst characterization

Table 1 shows that aerogel catalysts have larger sur-face areas, higher average pore diameters and greatercumulative pore volumes than those of non-aerogelcatalyst, which would be more favorable for inhibit-ing carbon deposition. The far less apparent densityof aerogel-2 and aerogel-3 is characteristic of aerogel.The abnormal case in aerogel-1 should be attributedto the effects of impregnating and the followingthermal treatment used in the preparation process,although the aerogel material was used as support.This is why the surface area and the average porediameter of the aerogel-1 are not as good as those forthe aerogel-2. The largest surface area in aerogel-3was observed, which means the new preparationmethod is also very suitable for the preparation ofmulticomponent-supported catalyst.

The results of DTA provided another support forthis conclusion. As shown in Fig. 1, three peaks were


observed while the aerogel samples were heating inair. The first is an endothermic peak, starting at ambi-ent temperature; this should be attributed to the lossof water. The second is an exothermic peak (about600 K) caused by the decomposition of organic com-pounds, which might be left during the preparationprocess. The third is a small endothermic peak, whichoccurred at about 723 K, should be correspondingto the phase transition temperature of boehmite to�-Al2O3. Obviously, the thermal effects of phasetransition and decomposition of organic compounddecreased as the number of components increase.This suggested that the crystallization of single metaloxide aerogel powder might be more difficult thanthat of binary or ternary one. The addition of the sec-ond or third component could improve the quality ofthe aerogel prepared by sol–gel and SCD techniques.

Fig. 2 reveals that the major crystalline phases ofaerogel-2 and aerogel-3 catalysts after calcination at823 K are NiO and NiAl2O4 spinel, but only �-Al2O3was detected in the XRD pattern of non-aerogel cat-alyst. This should be attributed to the differences ofpreparation methods. Since the active metal speciescould be dispersed only on the surface of the supportwhen the impregnation method was used, thus, nonickel species can be detected by XRD in non-aerogelcatalyst. The XRD pattern of aerogel-1 is similar tothe non-aerogel, but its diffraction peaks are much

Fig. 2. XRD spectra of ultrafine aerogel and non-aerogel catalysts:(a) NiO–La2O3–Al2O3 non-aerogel; (b) NiO–Al2O3 aerogel-1; (c)NiO–Al2O3 aerogel-2; (d) NiO–La2O3–Al2O3 aerogel-3.

broader due to the utilization of aerogel support. Theformation of NiAl2O4 spinel in aerogel catalysts in-dicates that the interaction of metal and support is sostrong that they could be occurred not only on thesurface but also in the bulk. So, a very homogeneousdispersity of nickel species was obtained in this way.Only using aerogel materials as support is not as goodas the method that the supported catalyst was directlyprepared by the combination of sol–gel and SCDtechniques, which can lead to the result of atomic ormolecular level interaction. It is also proper for thepreparation of multicomponent catalyst. Therefore,under moderate thermal treatment conditions, the for-mation of NiAl2O4 spinel can be clearly observed byXRD in NiO–La2O3–Al2O3 aerogel catalyst.

3.2. H2-TPR

Fig. 3 shows there are two clear peaks in the TPRcurves of NiO–Al2O3 and NiO–La2O3–Al2O3 cata-lysts: a small low temperature reduction peak (about680 K) attributed to the reduction of NiO and a largehigh temperature reduction peak (about 1043 K),which should be the reduction of NiAl2O4 spinelspecies, according to the result of XRD and the re-port [19]. Obviously, the main active phase in all theabove catalysts might have similar structures to that ofNiAl2O4 spinel, due to the strong interaction of metaland alumina support. However, the degree of the inter-action is different because of the different preparation

Fig. 3. H2-TPR curves of ultrafine aerogel and non-aerogel cata-lysts: (a) NiO–La2O3–Al2O3 non-aerogel; (b) NiO–La2O3–Al2O3

aerogel-3; (c) NiO–Al2O3 aerogel-2; (d) NiO–Al2O3 aerogel-1;(e) NiO powders.


methods and various components. As shown in XRDpatterns, using new preparation can strengthen the in-teraction between components. Therefore, the shapeof TPR curve and starting reduction temperature inaerogel-3 are not the same as the non-aerogel one.The former has higher starting temperature and has anapparently symmetric high temperature peak with notail, which indicates more homogeneous distributionof nickel species in the former, and consistent withthe result of XRD. The main reduction peak areas ofternary catalysts are both larger than that of binaryones, this should be attributed to the contribution ofLa2O3. It has been reported that the lanthanum oxidecan be interacted with nickel oxide and alumina [20].Consequently, many properties of catalyst includingreduction aspect can be further improved.

3.3. CO2-TPD

The results of CO2-TPD experiment also displayedsome advantages of the new preparation method.Fig. 4 shows that the total desorption peak areas ofthe aerogel catalysts are much larger than those ofones prepared by impregnating, even using aluminaaerogel as support. Because the aerogel materialsas ultrafine particles has a lot of advantages, suchas higher surface area, nano-sized particles, greaternumber of atoms or molecules exposed on the sur-face, and so on. Therefore, it exhibited very highadsorptive capacity to carbon dioxide in the aerogelcatalysts, which means the active surface area for theactivation of carbon dioxide is much larger than thatin the common catalysts prepared by conventionalmethods.

A high temperature desorption peak at about1043 K was observed in both kinds of ternary catalysts(Fig. 4b). It should be attributed to the decomposi-tion of some kind of La2O2CO3 species, formed bythe interaction of La2O3 and CO2. Therefore, La2O3can become another active center on the surface ofcatalyst for the activation of CO2. As a “CO2 pool”,it is favorable to eliminate carbonaceous species intime in the interfacial regions of metal Ni and La2O3by the dissociation of carbonates to CO2

− and ad-sorptive oxygen (Oads) [12]. Thus, the adsorptioncapacity of CO2 has been improved dramatically bothby the addition of La2O3 and by this new preparationmethod.

Fig. 4. CO2-TPD curves of ultrafine aerogel and non-aerogelcatalysts: (a) NiO–Al2O3 binary catalysts; (b) NiO–La2O3–Al2O3

ternary catalysts.

3.4. Catalytic performance

The reactivity of the catalysts for CH4/CO2 reform-ing is shown in Fig. 5. In the temperature range of873–1173 K, the NiO–Al2O3 aerogel-1 catalyst exhib-ited significantly higher conversion of methane thanthat of NiO–La2O3–Al2O3 non-aerogel catalyst. Al-though the low conversion over aerogel-2 or aerogel-3has been observed at lower reaction temperatures, theycan be restored at higher temperatures (above 1073 K)and are superior to the other catalysts even at very highconversion. Fig. 5 also shows when the reduction tem-perature was enhanced from 873 to 1073 K; the con-version of methane over ultrafine NiO–La2O3–Al2O3catalyst has also increased significantly, and can reachthe maximum value among all of the catalysts evenat lower temperatures. This should be attributed to


Fig. 5. Activity of ultrafine aerogel and non-aerogel catalyst forCH4/CO2 reforming (reduction temperature at 873 K; reactionconditions: 1 atm, CO2/CH4 = 1.0, GHSV = 8200 h−1). Note:aerogel-3′: the aerogel-3 catalyst reduced at 1073 K.

the reduction of a great amount of NiAl2O4 spinelspecies and the formation of new active center.

The amounts of coke on the above catalysts afterreaction are listed in Table 2. They decreased in thefollowing order: non-aerogel, aerogel-1, aerogel-2 andaerogel-3. Obviously, the addition of lanthanum oxide,a new preparation method, even using aerogel materi-als as support are all responsible for inhibiting carbondeposition over NiO–Al2O3 catalyst. Therefore, muchless coke on aerogel-3 catalyst was observed in spiteof the difference of reduction temperature.

Fig. 6 shows that the performance of the ternaryaerogel catalyst is excellent and stable when the re-action last for 20 h at 1173 K. The conversions ofCH4 and CO2 would reach to the stable state veryquickly, and the values both approximate to 96.0%.The yield of objective products with 1.0 of CO/H2ratio can reach 96.9%. In addition, there was almostno carbon deposition observed on this catalyst and

Table 2Amount of coke on used catalysts measured by weight difference method

Catalyst Composition Temperature of reduction (K) Amount of coke (wt.%/h)

Non-aerogel NiO–La2O3–Al2O3 873 12.0Aerogel-1 NiO–Al2O3 873 10.6Aerogel-2 NiO–Al2O3 873 4.1Aerogel-3 NiO–La2O3–Al2O3 873 2.8Aerogel-3 NiO–La2O3–Al2O3 1073 2.7

Fig. 6. Activity stability of NiO–La2O3–Al2O3 aerogel catalyst forCH4/CO2 reforming at 1173 K (other reaction conditions: 1 atm,CO2/CH4 = 1.0, GHSV = 8200 h−1): (a) yield of (CO + H2) orH2O as a function of time on stream; (b) CO/H2 ratio of productsas a function of time on stream.

the average content of water in the tail gas is <1.0%,which means a very high selectivity can be obtained,even at higher reaction temperatures or reductiontemperatures. Thus, it can be concluded that theNiO–La2O3–Al2O3 aerogel catalyst, prepared by thecombination method of sol–gel and SCD techniques,


exhibits excellent catalytic performance for CH4/CO2reforming, and is not sensitive to carbon deposition.

4. Conclusion

An ultrafine ternary aerogel NiO–La2O3–Al2O3 ascatalyst has been successfully prepared by very care-fully controlling the sol–gel process in the beginningand subsequent SCD in the following. It is a com-pletely novel technique to get a really homogeneousmultiple component aerogel in one step to obtain thefavorable interactions between components, undermoderate thermal treatment conditions.

The NiO–La2O3–Al2O3 aerogel catalyst exhibitsmany unusual physical properties, such as very largespecific surface area, low apparent density, and well-defined pore size distribution. It is a very promisingcatalyst for CH4/CO2 reforming due to its propertiesof ultrafine particles and homogeneous distributionof bound-state Ni species. Another point worthy tobe mentioned is the introducing of La2O3 into theaerogel catalyst. It is a good promoter to increase theadsorptive capacity of CO2 by interaction with nickeland/or modification with alumina, as well as otherimproved properties. All of them are responsible forthe reforming reactivity and coking resistivity.

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ultrafine nio–la2o3–al2o3 aerogel: a promising catalyst for ch4/co2 reforming

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