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Applied Surface Science 258 (2011) 448– 455

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Applied Surface Science

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Sol–gel based alumina powders with catalytic applications

Maria Cris ana,∗ , Maria Zaharescua , Valluri Durga Kumarib , Machiraju Subrahmanyamb , Dorel Cris ana ,Nicolae Dragana, Malina Raileanua, Mihaela Jitianuc, Adriana Rusua, Gullapelli Sadanandamb,Jakkidi Krishna Reddyb

a Romanian Academy, Ilie Murgulescu Institute of Physical Chemistry, 202 Splaiul Independentei, 060021, Bucharest, Romaniab Catalysis and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, Indiac Department of Chemistry, William Paterson University, 300 Pompton Road, Wayne, NJ 07470, USA

a r t i c l e i n f o

Article history:Received 24 March 2011Received in revised form 12 August 2011Accepted 25 August 2011Available online 3 September 2011

Keywords:NiO/Al2O3 nanopowdersSol–gel processStructural studyCatalytic activityGlycerol reforming

a b s t r a c t

The sol–gel process provides a new approach to the preparation of oxide materials and offers manyadvantages for making catalysts. Since homogeneous mixing can be achieved at the molecular scale, thechemical reactivity of the oxide surface can be greatly enhanced; thus powders with high surface areaand optimized pore size distribution can be obtained at low temperatures. In the present work NiO/Al2O3

sol–gel catalysts were obtained by simultaneous gelation of aluminium isopropoxide and nickel nitrate.A comparative study with pure sol–gel alumina was also realized. By physical–structural studies thechanges induced by the introduction of the Ni precursor, before and after aluminium alkoxide hydrolysiswere highlighted. The introduction of Ni at the beginning of the reaction favors �-Al2O3 crystallization.When Ni is added at the end of reaction, it delays the alumina crystallization and induces the disorderof the lattice. The obtained Ni doped sol–gel derived alumina has been used as catalyst in the finishedform for glycerol reforming to generate H2 for fuel cell applications. Some evaluation results of Ni-dopedalumina combined with TiO2 in photocatalytic glycerol reforming reaction have been included.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Active aluminas with high surface area are well known for theiruse as adsorbents, catalysts, desiccants, coatings, soft abrasivesand advanced polycrystalline ceramics, mainly due to their lowcost, good thermal stability, surface acidity and their interactionwith transition metals. Reactive aluminas do not occur naturallyand generally are prepared by the thermal transformation of alu-minium hydroxides and alumina gels. Solution chemistry offersmany possible routes for “chemical manipulation” and allows var-ious combinations in the synthesis of solids of diverse structures,compositions and morphologies [1].

The catalysts can be prepared from a homogeneous solutionwhich includes not only the metal oxide support precursor, but alsothe metal precursor used as dopant. So, the chemical reactivity ofthe oxide surface can be greatly enhanced and the catalysts with anuniform distribution of the active phase in the support have beenprepared, offering a beneficial alternative to the use of traditionalaluminas obtained by dehydration or pyrolysis of the aluminiumhydrates. The obtained results gave the possibility to optimize thesol–gel reactions starting from commercially available aluminium

∗ Corresponding author. Tel.: +40 213 167 912; fax: +40 213 121 147.E-mail addresses: mcrisan@icf.ro, msidcrisan@yahoo.com (M. Cris an).

alkoxides in order to obtain powders with pre-determined struc-ture and properties.

Ishiguro et al. [2] mentioned that Adkins and Watkins [3] maybe the first who have employed alumina from aluminium iso-propoxide (AIP) as a catalyst and reported superior activity for thedehydration of hexanol to hexane, compared to commercial alu-mina powders. However, a detailed description of the preparationof alumina from AIP was not given.

The first study of hydrolysis of aluminium alkoxides as a func-tion of water temperature, and structural transformation of theresultant hydroxides has been realized by Yoldas, in 1973 [4].There are several important forms of aluminium hydroxides cor-responding to the stoichiometries: AlO(OH) and Al(OH)3. AlO(OH)generally occurs as boehmite, or in nature as the mineral dias-pore. The true hydroxide Al(OH)3 is much more abundant andoccurs commonly as bayerite and gibbsite. Pseudo-boehmite isfrequently the chosen precursor for many catalysts. Generallyspeaking, aluminas obtained by calcination of well-crystallizedaluminium hydroxides (gibbsite, bayerite and boehmite) showpseudomorphosis to starting hydroxides and have narrow poresdeveloped inside the particles. On the other hand, aluminasfrom poorly crystallized aluminium hydroxides (pseudoboehmiteand alumina gel) have wide pores created between the ulti-mate particles. To avoid diffusion problems, wide-pore aluminasare usually favored in industrial processes, and major parts

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M. Cris an et al. / Applied Surface Science 258 (2011) 448– 455 449

of the alumina support are produced from pseudoboehmite[5].

Since Yoldas [6–8] developed the sol–gel technique, the prepa-ration conditions of monolithic alumina gel have been extensivelystudied by regulating reaction temperature, concentration, pH, andmany factors in the process starting from hydrolysis of aluminiumalkoxide [9].

Yoldas’s papers constituted a basic material of the furtherresearches in the field that has been greatly developed after 1990[10–27].

Most catalysts and sensors are composites in which certainactive materials are supported on carriers. In such materials, perfor-mance is as much affected by the structure of a supported materialas by its particle size, so its structure must be controlled [28].In the past few decades, a considerable effort has been directedtoward the preparation of nanosized nickel and its oxide parti-cles with a high degree of dispersion on alumina due to the factthat usually a fine, highly dispersed nickel phase can provide moreactive sites which are accessible to reactant molecules and availablefor catalysis [29]. Nickel oxide catalysts are used in industry dueto their availability and reasonable cost compared to noble met-als. Steam reforming of hydrocarbons, methanation, hydrogenationand hydrocracking reactions, CO oxidation and N2O decompositionare main applications fields. The nature of the support affects theproperties of the active phase, the activity of the catalyst dependingboth on the dispersion of the metal and metal support interac-tions. Sol–gel process ensures these requirements. A great numberof publications concern with coprecipitated nickel alumina cata-lysts with high nickel loading, but only a few papers with sol–gelnickel alumina [30–43].

The aim of the paper was the development of sol–gel basedalumina powders for catalytic applications. This study investi-gated the effect of Ni-modified �(�)-Al2O3 prepared by sol–gelmethod on the structural and functional properties of the finalcatalyst. A comparative study with sol–gel pure alumina was real-ized. By physical–structural studies, the changes induced by theintroduction of the Ni precursor, before and after aluminium alkox-ide hydrolysis were highlighted. The obtained Ni doped sol–gelderived aluminas have been used as catalysts in the finished formfor glycerol reforming to generate H2 for fuel cell applications. Someevaluation results of Ni-doped alumina combined with TiO2 in pho-tocatalytic glycerol reforming reaction for hydrogen generationhave been included. According to these results, the best powdershave been recommended for catalytic applications.

2. Experimental

For the preparation of the samples, aluminium triisopropylate,Al(O-iC3H7)3 has been used as aluminium precursor, nickel nitrate,Ni(NO3)2·6H2O, as nickel precursor, and H2O for hydrolysis. Thetemperature of reaction was 80 ◦C. The samples were dried at 110 ◦Cfor 24 h and thermally treated at 450 ◦C for 1 h with a heating rateof 1 ◦C/min.

Both un-doped (pure Al2O3, sample A) and Ni-doped Al2O3 pow-ders containing 5 wt.% (sample AN5), 10 wt.% (sample AN10) and20 wt.% (sample AN20) Ni related to Al2O3 were prepared by simul-taneous gelation of both precursors in the sol–gel process. Theintroduction of Ni dopant in the alumina sol has been made in twoprocedures: before (a) and after (b) aluminium alkoxide hydroly-sis. The spinel NiAl2O4 (sample S) has been also prepared by thesol–gel procedure. By thermal treatment of nickel nitrate at 700 ◦C,nickel oxide was prepared (sample N). In order to point out the per-formance of the sol–gel powders as catalysts for glycerol reformingprocess to generate hydrogen, Ni-doped alumina sample contain-ing 30 wt.% Ni has been prepared following impregnation methodon sol–gel derived Al2O3 (sample AN30-imp).

4000 350 0 300 0 250 0 200 0 150 0 100 0 50 0

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

AN-5b

AN-5a

AN-10a

AN-10b

Abs

orba

nce

Wavenumbers [cm-1]

Fig. 1. FT-IR spectra of AN5 and AN10 (a) and (b) samples.

The AN samples have been prepared as follows: 25 g of alu-minium isopropoxide dissolved in 225 mL distilled water weremixed with the required amount of nickel nitrate hexahydrate (atthe start moment for samples “a” and after 1–2 h for samples “b”)and kept stirring for 1–2 h at 80 ◦C.

The sample AN30-imp has been obtained by adding the sol–gelderived Al2O3 to a 30 wt.% aqueous solution of nickel nitrate hex-ahydrate and impregnating using a hot plate.

All the resulted gels were oven dried at 110 ◦C overnight andthen thermally treated at 450 ◦C for 1 h.

The structural and textural characterization of the obtainedsol–gel based alumina nanopowders has been accomplished usingthe following methods:

- Fourier transform infrared spectroscopy with a FT-IR Nicolet 6700spectrophotometer in the 500–4000 cm−1 range;

- X-ray diffraction (XRD) analysis with a Shimadzu XRD 600 diffrac-tometer, using Ni-filtered CuK� radiation (� = 1.5418 A) with ascan step of 0.02◦ and a counting time of 1 s/step, for 2� diffrac-tion angles ranging between 20 and 80◦

. The calculated values ofmicrostructural factors have been obtained from computerizedanalysis of XRD spectra with a proper XRAY5.0 program;

- BET specific surface area (SSA) measurements with a Micromerit-ics analyzer using N2 adsorption at 77 K. Before analysis thesamples were heated at 300 ◦C, under vacuum for 4 h, for out-gasing.

- UV–vis diffuse reflectance spectroscopy on a GBC UV–vis Cintra10e spectrometer, in the wavelength range 200–800 nm.

- SEM analysis of the catalysts on Hitachi S-520 SEM unit. Elemen-tal analysis was carried out using Link, ISIS-300, Oxford, EDAXdetector.

Photocatalytic activity reactions were carried out in gas closedevacuated and deaerated system under UV light irradiation (400 WHg vapor lamp). The reaction was performed in a 150 mL quartzreactor by taking 5 mL glycerol and 45 mL distilled water containing100 mg of catalyst. The evolved gaseous products were analyzed bya gas chromatograph (Shimadzu GC-2014) with molecular sieve 5 Acolumn and thermal conductivity detector using N2 as a carrier gas.

3. Results and discussion

3.1. FT-IR

The IR spectra (Figs. 1 and 2) show for all samples, themetal–oxygen stretching frequencies, in the range 500–900 cm−1

associated with the vibrations of Al–O, Ni–O or Ni–O–Al bonds.

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4000 3500 3000 2500 2000 1500 1000 500

0,0

0,1

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0,3

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0,6

0,7

0,8

AN-30 impA

AN-20a

S

AN-20b

N

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orba

nce

Wavenumbers [cm-1 ]

Fig. 2. FT-IR spectra of A, S, N, AN20 (a) and (b) and AN30-imp samples.

The Al–O stretching bands can be assigned to different coordina-tion states of Al atoms: AlO6 (∼770 cm−1) and AlO4 (∼570 and380 cm−1) [21] (sample A). The shoulder near 870 cm−1 can beassigned to the Al–O vibrational mode localized in the surface layer,and most likely involving deformation of the surface OH groups. TheNi–O stretching frequences are found in the range 400–500 cm−1

(∼490 and 435 cm−1) [29] (sample N). The characteristic Al–O–Nifrequencies (450–800 cm−1) (sample S) are also observed in thepowder obtained after hydrolysis of the precursors, which indicatesthat the Ni–O–Al framework forms and remains intact during thehydrolysis process. It is probable that the transition metal–oxygenvibrations of the spinel overlap with the Al–O bands [44,45]. Forthe Ni-doped alumina powders (samples AN), IR spectra visual-ized a slight shift of the absorbance maxima position of M–O bandstoward smaller wavenumbers. It is found that after doping, theM–O absorbance peaks have not been markedly disturbed. Thisindicates that no very extensive interaction between the nickel andalumina oxide occurred at this stage (450 ◦C) or the interaction waslimited only to a small number of active sites. All samples showbroad and intense hydroxyl stretching frequencies [�(OH)] whichcan be assigned to the overlapping of bands due to the surfaceadsorbed water (∼3400 cm−1) [21,44]. The bands at ∼1640 cm−1

assigned to the bending vibration of water molecules [ı(H2O)] [21]together with residual C–H bands [ı(CH)] (1460–1500 cm−1) andCO2 adsorbed (2300–2400 cm−1) [46] are also observed.

3.2. XRD

XRD patterns identified �(�)-Al2O3 (ASTM no. 04-0875) and NiO(ASTM no. 47-1049) as crystalline phases for samples AN5a, AN10(a, b), AN20 (a, b). Both mentioned phases belong to the cubic sys-tem. Beside �(�)-Al2O3 and NiO, in the case of sample S the spinelphase NiAl2O4 (ASTM no. 10-0339) having also cubic symmetry, hasalso been identified. The results of computerized profile analysis arepresented in Table 1.

The � values have been calculated based on 〈D〉 and 〈S〉microstructural factors [47]. The few profiles (generally 3 for eachsample) are very large covering an angular domain of up to ∼10◦.The great width of these profiles indicates that the crystalline orderof the �(�)-Al2O3 as predominant phase is of short range which isexpected for the sol–gel prepared nanopowders.

The evolution of main microstructural factors (see histogramsfrom Figs. 3 and 4) indicates the fact that the variation of lattice

Fig. 3. The histograms of lattice strain 〈S〉, crystallite size 〈D〉 and unit cell volume(UCV) variations for �(�)-Al2O3 phase.

strains is well correlated with that of the crystallites size: with thedecrease of lattice strain, the order of the crystallite structure couldbe extended.

The dots lines from the presented histograms are non-linearregressions (second degree equations) obtained by least-squares

Fig. 4. The histograms of lattice strain 〈S〉, crystallite size 〈D〉 and unit cell volume(UCV) variations for NiO phase.

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Table 1Microstructural factors calculated from the computerized profiles analysis of the XRD spectra (lattice constants, mean crystallite size (〈D〉) and internal microstrains (〈S〉).

Sample Phase Cryst. system a [A] UCV [A3] 〈D〉 [A] 〈S〉 × 10−3 � × 10+14 [m−2]

A Al2O3 cubic 7.8789 (97) 489.1 (1.8) 57 (10) 8.5 (2) 32.6AN5a Al2O3 cubic 7.9167 (119) 4.96.2 (2.2) 43 (9) 9.7 (3.7) 49.6

NiO cubic 4.1198 (315) 69.9 (1.6) 4019 (680) 0.24 (3) 0.02AN10a Al2O3 cubic 7.9157 (232) 496 (4.4) 64 (16) 7.6 (3.6) 26.1

NiO cubic 4.1620 (227) 72.1 (1.2) 1106 (372) 0.43 (24) 0.2AN20a Al2O3 cubic 7.9625 (86) 504.8 (1.6) 135 (96) 3.1 (4.1) 5.1

NiO cubic 4.1854 (172) 73.3 (9) >5000 <0.1 <0.01AN5b Al2O3 cubic 7.8903 (281) 491.2 (5.2) 34 (8) 13.4 (5.2) 87AN10b Al2O3 cubic 7.9491 (86) 502.3 (1.6) 58 (10) 5.3 (3.3) 19.9

NiO cubic 4.1638 (160) 72.2 (8) 4462 (1832) 0.07 (8) 0.01AN20b Al2O3 cubic 7.8944 (208) 492 (3.9) 65 (22) 4.1 (6.4) 13.6

NiO cubic 4.1646 (76) 72.2 (4) 4611 (1959) 0.25 (5) 0.02S Al2O3 cubic 7.9261 (44) 498 (1) 99 (12) 3.9 (8) 8.6

NiO cubic 4.1734 (11) 72.69 (6) >5000 <0.1 <0.01NiAl2O4 cubic 8.1625 (191) 543.8 (3.8) ∼4100 ∼0.07 <0.01

AN-30 imp Al2O3 cubic 7.9302 (135) 498.2 (2.5) 75 (28) 0.80 (85) 2.46

a—lattice parameter; UCV—unit cell volume; D—mean crystallite size; S—internal microstrains; �—density of the lattice defects.

method. They allow to more easily standing out the tendency ofevolution of structural modifications of the resulted phases.

By the introduction of the Ni precursor before aluminium alkox-ide hydrolysis (samples a) the alumina crystallites with biggermean size and smaller lattice strain than in the case of samplesb, have been obtained. For all the samples the mean crystallitesize of �(�)-Al2O3 phase varied in the range 34–135 A, while forNiO it was >5000 A. Sample AN5b presents the highest microstrainvalue, 〈S〉 = 13.4 × 10−3, the lowest mean crystallite size 〈D〉 = 34 Aand except sample A it is the only one in which the crystallineNiO was not found. Contrary, for sample AN20a, 〈S〉 has the low-est value (3.1 × 10−3) which is well correlated with the extendedobserved structural order (〈D〉 = 135 A). At the same time, the esti-mation of the density of the lattice defects (�) for sample AN5bassumes the presence of a great number of defects that couldpresent a great reactivity. As a result the catalytic yield of AN5bsample should be increased. The high disorder degree of thissample also results from Fig. 5 which presents XRD patterns,experimental (dotted line) and the identified profile after fittingfor �(�)-Al2O3 phase from AN5a, AN10a, AN5b and AN10b sam-ples.

It is known that the wide and flat profiles indicate a high disorderdegree associated with the short-range distance order and withhigh internal microstrains. High and narrow profiles show a wellordered crystalline structure with high-range distance order and

Fig. 5. XRD patterns experimental and after fitting for �(�)-Al2O3 phase for AN5a,AN10a, AN5b and AN10b samples.

with small microstrains. It clearly results that the worst crystallizedis sample AN5b.

The mean crystalline sizes for NiO phase, where it was identified,are greater than 5000 A; except AN10a sample, where 〈D〉 = 1106 A.They progressively grow with the increase of Ni concentration.

3.3. BET specific surface area measurements

The specific surface area (SSA) and the pores size distribution ofa catalyst or catalyst support are some of the most important factorsthat influence its catalytic activity. Accordingly, a great value of SSAand control of pore size and volume will lead to improvement ofthe catalytic performance. The results of the BET measurements arepresented in Table 2.

The BET surface areas values are in the 200–325 m2/g range.Ni-doped Al2O3 samples have the surface areas bigger then theun-doped, pure alumina. For the Ni-doped samples, surface areasdecrease with the increase of the Ni concentration. The differencein the surface area observed between AN20a and AN20b samplesis considerable and indicates that micropores are collapsed to formmesopores in AN20b. This could be seen in the increase of the aver-age pore radius and in the decrease of both pore volume and BETsurface area, respectively (Table 2).

3.4. SEM-EDAX

The morphology of the samples after thermal treatment at450 ◦C clearly shows that addition of Ni after the Al precursorhydrolysis (in “b” series samples) results in fine dispersion of NiOcrystallites against the alumina background; whereas addition ofNi before the precursor hydrolysis resulted in NiO crystallites withclear alumina background (in “a” series samples). This is also sup-ported by the increase in the mean crystallite size of alumina in “a”series samples (Table 1). In order to illustrate these observations,

Table 2Textural properties of the samples.

Sample BET surfacearea [m2/g]

Pore volume[cm3/g]

Average poreradius [A]

A 207.4 0.3245 24.90AN5a 317.5 0.5217 32.43AN5b 321.2 0.5555 33.89AN10a 293.8 0.4855 29.87AN10b 300.0 0.5001 30.15AN20a 222.8 0.2239 20.03AN20b 191.5 0.2039 21.24

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Fig. 6. SEM images of the A, S, AN5a, AN5b and AN10b samples.

Fig. 6 presents the micrographs of the A, S, AN5a, AN5b, AN10bsamples.

The EDAX results are shown in Table 3. It could be seen that Nidopant at lower concentrations participates in compound forma-tion with alumina support (non-stoichiometric Ni aluminate, Niinteraction on surface) independent of sequence of dopant addi-tion, i.e. pre or post hydrolysis. The interaction of Ni with thealumina is shown by the maximum micro strain exhibited by 5aand 5b samples (Table 1).

With increase of Ni amount (%) the compound formation isfavored which is seen by the decreased micro strain of the formedstructure (stoichiometric Ni aluminate, Ni diffused into the bulk).These conclusions are supported by UV–vis DRS measurements.

Table 3EDAX analysis.

Catalysts O (wt.%) Al (wt.%) Ni (wt.%)

S 21.9 11.4 66.7AN5a 56.4 41.8 1.8AN5b 54.6 42.6 2.8AN10a 49.8 42.1 8.1AN10b 49.8 37.7 12.5AN20a 42.8 33.8 23.4AN20b 41.7 29.7 28.6

3.5. UV–vis DRS

Fig. 7 shows the UV–vis DRS results of both “a” and “b” seriesalong with Ni–Al spinel. NiO which is visible light sensitive on inter-action with alumina sol is seen developing a band spreading in

200 250 300 350 400 450 500 550 600 650 700 750 800

AN10a

AN10b

AN20a

AN20b

AN5b

AN5a

s

Abs

orba

nce(

a.u.

)

Wavelength (nm)

Fig. 7. UV–vis DRS of S, AN5a, AN5b, AN10a, AN10b, AN20a, and AN20b samples.

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the range of 250–400 nm. Its intensity increased with the Ni con-tent. However the band formation sequence is proportional to theamount (%) of Ni interacted in both series. For example sampleAN10a with 8.1% Ni absorbed less than sample AN10b with 12.6% Niindicating that catalyst forms better in the later one (AN10b). Thesame observation is valid for AN5a, 5b and AN20a, 20b samples.The small peak at ∼377 nm could be assigned to the octahedrallycoordinated Ni2+ species in the NiO lattice [48].

3.6. Catalytic activity and structure-activity correlation

The obtained Ni doped sol–gel derived aluminas have beenused as catalysts in the finished form for glycerol reforming togenerate hydrogen for fuel cell applications. Biodiesel and glyc-erol are produced from the transesterification of vegetable oils andfats with alcohol in the presence of a catalyst. Hydrogen could beefficiently produced at mild conditions around 500 K by aqueous-phase reforming (APR) and also around 773 K by thermal reformingprocess of glycerol:

C3H8O3 + 3H2O → 7H2 + 3CO2

Some evaluation results of Ni-doped alumina combined withTiO2 in photocatalytic glycerol reforming reaction for hydrogengeneration have been included in order to understand the natureof the compound formed by the interaction of Ni with alumina soland its role in enhancing the photo-catalytic activity of TiO2. Thecatalytic activity results are presented in Figs. 8 and 9. From Fig. 8 itis clearly seen that photo-catalytic activity of sample S is the high-est, which is explained by the presence of both nickel aluminateand NiO (see Table 1). EDAX analysis shows the presence of veryhigh Ni amount (%) on the surface of this compound, confirmedby the maximum UV–vis DRS absorption band in the 250–400 nmdomain.

Taking sample S as standard due to its maximum photo-catalyticactivity in producing hydrogen from glycerol, the activity of thesol–gel samples (“a” and “b” series) are evaluated for the factorsaffecting their activity. It is quite interesting to note that Ni addedprior to hydrolysis favors �-Al2O3 crystallization while Ni addedpost hydrolysis delayed it, inducing the disorder of the lattice andpromotes its dispersion at the surface. Thus the general trend ofactivity observed is higher for samples from“b” series and it is max-imum at 10 wt.% Ni loading, sample AN10b being almost competingwith standard sample S in producing maximum hydrogen produc-tion activity. The surface Ni amount (%) on sample S is five timeshigher than the Ni content (%) observed on sample AN10b (seeTable 3). This clearly shows that Ni interacted with alumina and

Fig. 8. Photocatalytic hydrogen production activity over NiO/Al2O3 catalysts versustime of UV irradiation (h).

Fig. 9. Photocatalytic hydrogen production activity over TiO2, NiO/Al2O3 combinatesystems versus time of UV irradiation (h).

Table 4Hydrogen production activity over TiO2, NiO/Al2O3 combinate systems under UV light irradiation.

No. of sample Catalyst Amount of H2 (�mol/h/g) Expected amount of H2 (�mol/h/g)

1 TiO2 − 50 mg 3002 TiO2 − 100 mg 6003 5a (50 mg) 1104 5b (50 mg) 1405 5a (50 mg) + TiO2(50 mg) 410 110 + 300 = 4106 5b (50 mg) + TiO2(50 mg)a 770 140 + 300 = 4407 10a (50 mg) 1608 10b (50 mg) 2009 10a (50 mg) + TiO2(50 mg) 330 160 + 300 = 460

10 10b (50 mg) + TiO2(50 mg) 630 200 + 300 = 50011 20a (50 mg) 7512 20b (50 mg) 12013 20a (50 mg) + TiO2(50 mg) 370 75 + 300 = 37514 20b (50 mg) + TiO2(50 mg) 430 120 + 300 = 42015 S(NiO·Al2O3) 210

a Synergistic activity.

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may be considered as the possible site for the formation of the bandstructure that in turn exhibits the photo-catalytic activity.

It is interesting to note that when this catalyst is studied incombination with a photocatalyst like TiO2 (Degussa with surfacearea 55 m2/g, particle size 27 nm), the blended systems (physicalmixtures) show only a combined activity, and no mutual promo-tion (see Table 4). However, such a synergistic activity is observedwhen TiO2 is studied in combination with sample AN5b. This effectis possible due to the electron scavenging property of this sam-ple that could be explained by its observed microstrains (13.4)(see Table 1) which minimizes the electron–hole recombinationon TiO2. All these observations confirm that Ni forms defect Ni–Alcompounds as mixed oxides NiO·Al2O3 and non-stoichiometric orprespinelic NiAl2O4 which is responsible for absorbing the lightradiation, showing photo-catalytic activity like in sample AN10b.Thus the results are in agreement with the structural data.

Ni added to the sol after hydrolysis of alumina precursor andphysically combined with TiO2 ensures the synergistic perfor-mance.

4. Conclusions

• Pure and Ni-doped Al2O3 sol–gel nanopowders were prepared bycontrolled hydrolysis-condensation of aluminium isopropoxide.

• The influence of introduction of the Ni dopant (before and afterhydrolysis) on the nanopowders structure was established. Theintroduction of Ni at the beginning of the reaction favors �(�)-Al2O3 crystallization. When Ni is added at the end of reaction,it delays the alumina crystallization and induces the disorder ofthe lattice. This fact was confirmed by the structural parametervalues (small crystallite sizes and high internal strains).

• The obtained Ni-doped sol–gel derived alumina has been used ascatalyst in the finished form for glycerol reforming to generateH2 for fuel cell applications. Some evaluation results of Ni-dopedalumina combined with TiO2 in photocatalytic glycerol reformingreaction have been included. Ni added to the sol after hydrolysisof alumina precursor and physically combined with TiO2 ensuresthe synergistic performance.

• Ni forms defect Ni–Al compounds as mixed oxides NiO·Al2O3 andnon-stoichiometric or prespinelic NiAl2O4 a stable compound(perhaps NiAl2O4) which is responsible for absorbing the lightradiation, showing photo-catalytic activity. It can act as a pho-tocatalyst and an electron scavenger that could be explained bythe presence of microstrains, which minimizes the electron–holerecombination on TiO2. Thus the results are in agreement withthe structural data. Present work is a first kind of such an attemptunder UV irradiation and our results could be a good start for thedesign of TiO2–NiO/Al2O3 combined materials.

Acknowledgment

This work was supported by the Bilateral Indo-Romanian Coop-eration Program (2007–2009).

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