Surface acidity and basicity of γ-Al2O3 doped with K+ and La3+ and calcined at elevated temperatures

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  • Thermochimica Acta 397 (2003) 8186

    Surface acidity and basicity of -Al2O3 doped with K+ and La3+and calcined at elevated temperatures

    Hu Zou, Xin Ge, Jianyi ShenLaboratory of Mesoscopic Materials Science, Department of Chemistry, Nanjing University, Nanjing 210093, China

    Received 15 January 2002; received in revised form 15 May 2002; accepted 16 May 2002

    Abstract

    High temperature reactions in industry require catalysts with high stability. Basic metal oxides, K2O and La2O3, were addedto-Al2O3 in order to obtain supports with low acidity and high surface areas at high temperatures. Microcalorimetry and FT-IRwere employed to determine the surface acidity and basicity using ammonia and carbon dioxide as the probe molecules. It wasfound that the addition of basic metal oxides inhibited the transformation of-Al2O3 to the forms such as-Al2O3 and-Al2O3when calcined at 1000 C. Instead, X-ray diffraction (XRD) results indicated the formation of aluminates for the supportedsamples. The 6% K2O/-Al2O3 sample retained high surface area of 188 m2 g1 and strong basicity (170 kJ mol1 for CO2adsorption) when calcined at 600 C. The sample retained the surface area of about 100 m2 g1 when calcined at 1000 C. In thiscase, the sample possessed low acidity and basicity and may be used as a neutral support with high thermal stability. The additionof La2O3 onto -Al2O3 might cause even more loss of surface area when calcined at high temperatures. The formation of aperovskite phase LaAlO3 on the surface of the La2O3/-Al2O3 samples calcined at 1000 C led to the low acidity and basicity. 2002 Elsevier Science B.V. All rights reserved.

    Keywords: Surface acidity; -Al2O3; FT-IR

    1. Introduction

    -Al2O3 is widely used as catalyst support becauseof its high surface area and acidity. However, surfaceacidity is sometimes undesirable and -Al2O3 maylose its surface area when heated at high temperatures.Aria and Machida [1] and Schaper et al. [2] pointedout that -Al2O3 would loss its surface area partly dueto sintering when calcined at 1000 C. When calcinedabove 1000 C, sintering and phase transformation to-Al2O3 are the two major factors for the decreaseof surface area. Researchers tried to dope -Al2O3

    Corresponding author. Tel.: +86-25-3592-909;fax: +86-25-3317-761.E-mail address: jyshen@nju.edu.cn (J. Shen).

    with foreign elements while maintaining the surfacearea [315]. For example, Matsuda et al. [13] reportedthat an effect of adding La2O3 is obviously to re-tard the transformation of -Al2O3 to -Al2O3 andthe associated sintering. Oudet et al. [15] suggestedthat transition of alumina can be thermally stabilizedby surface interactions with a perovskite-type oxide,LnAlO3 (Ln = La, Pr, Nd), and this thermally stablecompound on the surface of alumina has a neutraliz-ing effect on the corundum nucleation areas, inhibitingthe formation of the stable form of alumina. Machidaet al. [7,14] also found that alkaline earth metal oxidessuch as BaO, SrO and CaO have the same stabiliza-tion effects. Later other oxides of elements such as P,Si [8,11], Pr and Nd [15] were used. It seemed thatLa2O3, BaO and SiO2 had the positive effect on the

    0040-6031/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0 0 4 0 -6031 (02 )00329 -5

  • 82 H. Zou et al. / Thermochimica Acta 397 (2003) 8186

    stabilization of surface area of alumina at high tem-peratures.

    The present study deal with the thermal stabilityof -Al2O3 support modified respectively by K+ andLa3+. In particular, the surface acidity and basicity ofthe modified samples were studied in terms of nature,number and strength and were correlated with loadingand calcination temperatures.

    2. Experimental

    The starting -alumina had the surface area of203 m2 g1. The various amounts of basic metals K+and La3+, calculated according to the desired loadingswere introduced onto the -alumina by the incipientwetness impregnation method using the correspond-ing aqueous nitrate solutions. After impregnation, thesamples were dried at 373 K overnight followed bycalcination for 6 h at 600, 800 and 1000 C, respec-tively. Table 1 summarizes the samples used in thisstudy.

    The surface areas of the samples were measured bynitrogen adsorption at196 C using the BET methodon an ASAP-2000 type instrument (Micrometrics Co.,USA). Helium was used as the carrier gas.

    The phases present of the catalysts were deter-mined by X-ray diffraction (XRD) using the RigakuD/Max-RA X-ray diffractometer equipped with a Cutarget and graphite monochromator.

    Table 1Surface areas and phases of -Al2O3 and -Al2O3 supported samples calcined at different temperatures

    Sample Calcinationtemperature (C)

    Surface area(m2 g1)

    Phases by XRD

    -Al2O3 600 203 -Al2O3800 161 -Al2O3

    1000 82 -Al2O3, -Al2O3, -Al2O3

    6 wt.% K2O/-Al2O3 (1280mol Kg1) 600 188 -Al2O3800 161 -Al2O3, K3AlO3

    1000 107 -Al2O3, K3AlO310 wt.% La2O3/-Al2O3 (700mol Lag1) 600 178 -Al2O3

    800 135 -Al2O31000 57 -Al2O3, LaAlO3

    25 wt.% La2O3/-Al2O3 (2000mol Lag1) 600 126 -Al2O3800 95 -Al2O3

    1000 36 -Al2O3, LaAlO3

    Microcalorimetric measurements for the adsorptionof NH3 and CO2 were carried out using a Tian-Calvetheat-flux apparatus, which has been described else-where [16]. The microcalorimeter was connected toa gas-handling and volumetric adsorption system,equipped with a Baratron capacitance manometer(MKS, USA) for precision pressure measurement.The differential heat of adsorption versus adsor-bate coverage was obtained by measuring the heatsevolved when doses of a gas (25mol) were ad-mitted sequentially onto the catalyst until the surfacewas saturated by the adsorbate. Ammonia and carbondioxide with a purity of 99.99% were used. Beforemicrocalorimetric measurements, the samples weretypically dried under vacuum at 350 C for 1 h, cal-cined in 500 Torr O2 at 400 C for 2 h, and evacuatedat 400 C for 2 h. Microcalorimetric adsorption of am-monia and carbon dioxide were performed at 150 C.

    Infrared spectra were collected with an IFS66VVacuum-type FT-IR Spectrophotometer (Bruker Co.Ltd., German). Each spectrum was recorded at 2 cm1resolution with 32 co-added scans. Sample pelletswere formed with a thickness of 2030 mg cm2.The samples were loaded into a quartz cell equippedwith CaF2 windows. The treatment procedure of thesamples for IR was the same as for microcalorimet-ric adsorption studies. Ammonia and carbon dioxidewere dosed onto the sample at 150 C for 0.5 h. Thecell was then isolated, cooled to room temperatureand evacuated. Infrared spectra were then collected.

  • H. Zou et al. / Thermochimica Acta 397 (2003) 8186 83

    Each reported spectrum is the difference betweenthe spectrum of the clean sample and the spectrumcollected after dosing an adsorbate.

    3. Results and discussion

    3.1. -Al2O3 and K+/ -Al2O3

    Table 1 shows the BET surface areas of all thesamples with various loadings and calcination tem-peratures. Fig. 1 shows the diffraction patterns ofthe -Al2O3 and 6% K2O/-Al2O3 calcined at var-ious temperatures. It is seen that the -Al2O3 wastransformed into -Al2O3 and -Al2O3 phases whencalcined at 1000 C. No -Al2O3 or -Al2O3 phasewas detected for the K2O/-Al2O3 sample calcinedat 1000 C. The sample exhibited mainly the -phasewith a new phase K3AlO3. The new phase K3AlO3was formed by the solid reaction between K2Oand -Al2O3 which inhibited the transformation of-Al2O3 into -Al2O3 and -Al2O3 phases. The 6%K2O/-Al2O3 sample remained high surface area(161 and 107 m2 g1, respectively) when calcined at600 and 1000 C, respectively.

    The acidity and basicity of the -Al2O3 and 6%K2O/-Al2O3 calcined at various temperatures werecharacterized by the microcalorimetric adsorptionmethod using ammonia and carbon dioxide, respec-tively, as the probe molecules. Figs. 2 and 3 showthe results. The -Al2O3 calcined at 600 C exhib-ited the initial heat of about 125 kJ mol1 and the

    Fig. 1. X-ray diffraction patterns for -Al2O3 calcined at 600 C(a) and 1000 C (b) and for 6% K2O/-Al2O3 calcined at 600 C(c), 800 C (d) and 1000 C (e).

    Fig. 2. Differential heat vs. adsorbate coverage for adsorption ofNH3 at 150 C on -Al2O3 (), and on 6% K2O/-Al2O3 calcinedat 600 C () and 1000 C ().

    saturation coverage of about 420mol g1 for NH3adsorption. The addition of 6% K2O almost killedall the acid sites with heats higher than 40 kJ mol1when the sample was calcined at temperatures higherthan 600 C. In fact, the acid sites with heat of40 kJ mol1 for ammonia adsorption are weak. Onthe other hand, the -Al2O3 exhibited the initial heatof about 132 kJ mol1 and the saturation coverage ofabout 55mol g1 for CO2 adsorption. The additionof K2O greatly enhanced the basicity, especially forthe sample calcined at 600 C. In particular, the 6%K2O/-Al2O3 sample calcined at 600 C exhibited

    Fig. 3. Differential heat vs. adsorbate coverage for adsorption ofCO2 at 150 C on -Al2O3 (), and on 6% K2O/-Al2O3 calcinedat 600 C () and 1000 C ().

  • 84 H. Zou et al. / Thermochimica Acta 397 (2003) 8186

    Fig. 4. FT-IR spectra collected after NH3 adsorption at 150 Cfollowed by evacuation at room temperature on -Al2O3 (a), andon 6% K2O/-Al2O3 calcined at 600 C (b) and 1000 C (c),respectively.

    the initial heat of 170 kJ mol1 and saturation cover-age of about 275mol g1 for CO2 adsorption. Evenfor the sample calcined at 1000 C, the initial heatand coverage for CO2 adsorption were remained tobe about 92 kJ mol1 and 100mol g1, respectively.

    Fig. 4 shows the infrared (IR) spectra collectedafter exposure of the -Al2O3 and K2O/-Al2O3 sam-ples to ammonia at 150 C. Five bands around 686,1619, 1478, 393 and 1246 cm1 were observed forthe -Al2O3 sample calcined at 600 C. The bandsaround 1686, 1478 and 1393 cm1 are due to the de-formation modes of NH4+ formed by the interactionof NH3 with B