High surface area tin oxide

Applied Catalysis A: General 317 (2007) 139–148 www.elsevier.com/locate/apcata

High surface area tin oxide Alfred Hagemeyer *, Zach Hogan, Marco Schlichter, Birgit Smaka, Guido Streukens, Howard Turner, Anthony Volpe Jr., Henry Weinberg, Karin Yaccato Symyx Technologies Inc., 3100 Central Expressway, Santa Clara, CA 95051, USA Received 13 July 2006; received in revised form 18 September 2006; accepted 22 September 2006

Abstract High surface area tin oxides are desirable catalyst carriers for emissions control including CO oxidation, VOC removal/methane combustion and NOx abatement. We have optimized literature procedures as well as developed proprietary recipes for the synthesis of porous SnO2 carriers. Precipitation of SnCl4 from homogeneous solution by urea and by hydrazine, the dissolution of Sn metal powder in HNO3, sol–gel routes from Sn alkoxides and inorganic Sn precursors, acid-induced gelation of K2SnO3, the Pechini method ex Sn(OAc)4 using various organic acids as dispersants and dry and wet thermal decomposition of Sn carboxylates have been investigated and compared. BET surface areas >100 m2/g have been achieved by a variety of methods after calcination in the temperature range 300–500 8C. High surface area tin oxides with excellent sintering resistance (250, 228, 190 and 175 m2/g after calcination at 300, 400, 500 and 600 8C, respectively) have been synthesized by the hydrazine method. Pt/SnO2 catalysts have been prepared by impregnation with Pt tetraamine hydroxide solution, and screened for propylene combustion and CO oxidation activity in a parallel 8  1 process optimization reactor. Light-off takes place at higher temperatures for CO oxidation than for propylene combustion due to Pt poisoning by CO at lower temperatures. High surface areas >200 m2/g could also be achieved by a modified Pechini method using aqueous glyoxylic acid as dispersant for Sn(IV) acetate. Ce–Sn–Co mixed oxides have been synthesized by the glyoxylic acid method, impregnated with Pt–Ru and found to be more active than Pt/Al2O3 for CO oxidation when using CO-only feed, but inferior to the Pt standard when feeding a CO–propylene mixture. # 2006 Published by Elsevier B.V. Keywords: High surface area; Tin oxide; Carriers; Supports; Sol–gel; Pechini; Propylene combustion; VOC removal; CO oxidation; Combinatorial chemistry; Combinatorial catalysis; Heterogeneous catalysis; High-throughput synthesis; High-throughput screening

1. Introduction Tin oxide is an important material for various technological applications such as gas sensors and conductive coatings but has received little attention as carrier for supported catalysts. SnO2-based catalysts have been reported to be active for oxidative dehydrogenations [1,2], catalytic CO oxidation [3], and selective catalytic reduction of NOx by hydrocarbons [4]. Although attempts to support SnO2 on common high surface area oxide supports have been carried out in order to improve dispersion, few reports address the catalytic properties of unsupported high surface area SnO2. Pd on SnO2 has been shown to be active for the NOx reduction into N2 in CO–NO–O2 * Corresponding author. Tel.: +1 408 764 2059; fax: +1 408 748 0175. E-mail address: [email protected] (A. Hagemeyer). URL: www.symyx.com 0926-860X/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apcata.2006.09.040

mixtures [5] as well as for the low temperature oxidation of methane [6,7]. The preparation and characterization of structural, textural and semiconducting properties of high surface area stannic oxides has been reported [8]. The complete oxidation of methane over Pt catalysts supported on high surface area SnO2 has been discussed [9]. We have investigated a variety of methods for the preparation of high surface area tin oxide and its application as catalyst and carrier for CO oxidation and propylene combustion. 2. Experimental 2.1. Synthesis of SnO2 and catalysts 2.1.1. Precipitation by urea An aqueous solution of 0.1 M SnCl4 and 10-fold excess of urea was refluxed at 90 8C for 15 h and the precipitate isolated

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and washed by centrifugation, dried at 120 8C for 16 h and calcined at 200 and 300 8C, respectively, for 4 h in air. 2.1.2. Precipitation by hydrazine By modifying a literature recipe [8], 150 ml of a 0.6 M solution of SnCl45H2O were added drop wise to 100 g of a commercial 34% aqueous hydrazine solution at ambient temperature. A white precipitate formed immediately. After complete addition the mixture was refluxed for 10 days. The precipitate was washed and centrifuged until no more chloride could be detected. The product was dried for 16 h in air at 120 8C. The dry product was divided into four portions which were calcined at 300, 400, 500 and 600 8C for 2 h in air. Pt/SnO2 catalysts were prepared by incipient wetness impregnation of SnO2 with Pt tetraamine hydroxide solution. 2.1.3. Oxidation of Sn metal Following a literature procedure [8], an aqueous solution of HNO3 (400 ml, 34 vol%) was poured on high purity metallic Sn powder (Sn/HNO3 = 4 molar ratio) immediately releasing reddish-brown NO2 vapors and forming a white precipitate. The mixture was refluxed for 10 h, centrifuged and washed until reaching pH 6.0, dried at 120 8C for 16 h and calcined at 300 8C for 2 h in air. 2.1.4. Sol–gel of Sn alkoxide precursor Four millilitres of isopropanol was added to 7 ml of a 10% (w/v) solution of tin(IV) isopropoxide in isopropanol/toluene (supplied by Alfa #36563). The mixture was stirred for 2–3 min and then 0.1 ml of conc. HCl was added. Gelation occurred after about 2 h. The gel was calcined at 400 8C for 2 h in air. 2.1.5. Spontaneous gelation of stannic acid by ion exchange of Na2SnO3 2.5 g Na2SnO33H2O was dissolved in 10 ml water. The solution was ion exchanged by adding 5 ml DOWEX 50WX8200, shaking until the pH dropped from 14 to 9. The clear colorless supernatant was pipetted off and the remaining water was evaporated at room temperature over 2 days. The solution solidified to a glassy gel after the 2 days. The gel was calcined using the following heat up protocol: 60 8C/2 h ramp/120 8C/ 6 h hold/2 h ramp/400 8C/4 h hold. Post calcination, the residue was washed 4 with 20 ml water each time, followed by recalcination according using a 45 8C/2 h ramp/120 8C/2 h hold/ 1 h ramp/250 8C/4 h hold. 2.1.6. Spontaneous gelation of stannic acid by neutralization of K2SnO3 (a) K2SnO33H2O was dissolved in 10 ml water. Gelation was induced by the addition of dry ice over 1–2 h. The white gel was aged and calcined as summarized in Table 1. Post calcination, the white powder was washed 4 with 20 ml water each time and dried at 250 8C for 4 h in air. (b) Three grams (10 mmol) K2SnO33H2O was dissolved in 10 ml water. Gelation was induced by the addition of 50 drops conc. HNO3 to lower the pH from 14 to neutral. A white voluminous gel formed instantaneously and was aged

Table 1 SnO2 gels by CO2-induced gelation of K2SnO3 K2SnO3 in 10 ml [g]

Aging

Calcination

BET [m2/g]

3 3 3 3 3 4

3 2 1 1 1 1

400 8C/4 h 400 8C/4 h 350 8C/4 h 300 8C/4 h 375 8C/4 h 375 8C/4 h

75 76 91 98 (low yield) 76 85

days weeks day day day day

at room temperature overnight. The gel was calcined according to the protocol 45 8C/2 h ramp/120 8C/6 h hold/ 2 h ramp/400 8C/4 h hold. Post calcination, the yellow compact mass was washed 4 with 20 ml water each time to wash out K, then dried according to the protocol 60 8C/ 1 h ramp/120 8C/2 h hold/1 h ramp/250 8C/4 h hold. 2.1.7. Thermal decomposition of Sn precursors (a) Solid Sn(IV) acetate and Sn(II) acetate (supplied by Gelest) were calcined in the temperature range 350–400 8C to yield yellow and brown powders, respectively. Sn(II) acetate decomposes >238 8C according to Sn(OAc)2 ! SnO + Me2CO + CO2 [10]. (b) Eight hundred and eighty five milligrams Sn(IV) acetate (supplied by Gelest) was dissolved in 10 ml 1 M cold malonic acid to an almost clear, slightly cloudy solution, then stirred at 60 8C overnight to form a milky white suspension/precipitate that slowly sedimented over 1 day. The supernatant was pipetted off and the solid residue was washed with 10-fold excess water, dried at 120 8C for 6 h, and calcined at 300 8C for 4 h in air. Similar recipes were executed for 1 M cold citric and 4 M cold malic acid. Isolated yields were low (less than 50%) thus indicating incomplete decomposition of the Sn-carboxylate complexes. 2.1.8. Hydrolysis of organic Sn acetate solutions (a) One gram Sn(IV) acetate (supplied by Aldrich) was dissolved in 10 ml acac (2,4-pentanedione) at room temperature. To the clear organic solution was added 10 ml 2.43 M aqueous ketoglutaric acid (acetone-1,3-dicarboxylic acid). The two solutions are not miscible and phase separation into two phases occurred (upper organic clear brown phase and lower aqueous cloudy phase with the white SnO2 gel). Solvents were evaporated over 1 week at room temperature. The gel was calcined according to the protocol: 45 8C/2 h ramp/ 120 8C/2 h hold/1 h ramp/200 8C/2 h hold/1 h ramp/350 8C/ 5 h hold. The mass balance calculated from the isolated yield (assuming SnO2 as the final product) showed a surplus of about 10%, indicating the presence of water and/or coke and/ or undecomposed acac/acetate. Higher calcination temperatures or less organic solvent resulted in a closed mass balances. Similar recipes for a variety of acids are summarized in Table 2. (b) One gram Sn(II) acetate Sn(OAc)2 (supplied by Gelest SND2700) was dissolved in 10 ml acac by shaking at room

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Table 2 SnO2 gels by hydrolysis of (1 g Sn(IV) acetate in acac) solutions Acac [ml]

H2O [ml]

Acid

Calcination

BET [m2/g]

10 5 2 10 10 5 5 5 5 5 5 5

10 5 2 10 10 5 4 5 4 – 5 5

– – – 10 drops each of HNO3, malonic, acetic, ketoglutaric 2.43 M ketoglutaric 2.43 M ketoglutaric 1 ml formic 1 ml cyclopentane carboxylic acid 1 ml glacial acetic 5 ml 1 M HNO3 1 ml propionic 1 ml butyric

350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h

109 107 76 143 160 195 92 129 110 97 121 107

Table 3 SnO2 gels by hydrolysis of organic solutions of inorganic (non-alkoxide) Sn precursors Precursor 1g 1g 1g 1g 1g 1g 1g 1g 1g 1g 1g 1g

Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV) Sn(IV)

acetate acetate acetate acetate acetate acetate acetate acetate acetate acetate acetate acetate

Solvent

Gelation agent

Calcination

BET [m2/g]

5 ml acac 30 ml toluene 40 ml isopropanol 20 ml MIBK + 3 ml acac 5 ml ethylene glycol 5 ml ethylene glycol 5 ml ethylene glycol 10 ml glacial acetic 10 ml acetic/water 1:1 20 ml acetic/water 1:1 5 ml formic acid 1 ml formic acid (!)

2 ml 2.43 M ketoglutaric 10 ml 1.43 M ketoglutaric 10 ml 1.43 M ketoglutaric 7 ml 1.43 M ketoglutaric 10 ml 2.43 M ketoglutaric 10 ml H2O 10 ml 1 M HNO3 10 ml 2.77 M ketoglutaric 10 ml 2.77 M ketoglutaric 8 ml 3.12 M ketoglutaric 5 ml 2 M ketoglutaric 10 ml 2 M ketoglutaric

350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 350 8C/4 h 350 8C/4 h

135 163 163 144 197 93 107 201 200 179 116 149

temperature, then 10 ml water was added. A white emulsion formed. The yellowish-whitish suspension stayed liquid/ low viscosity even after 1 week of aging, however, the sample solidified after 2 weeks of aging to a yellow gel which was calcined according to the protocol: 60 8C/2 h ramp/120 8C/2 h hold/1 h ramp/200 8C/2 h hold/1 h ramp/ 350 8C/5 h hold (BET: 118 m2/g). (c) The Sn precursor was dissolved in the organic solvent at room temperature, the gelation agent was added, the system was mixed by shaking, the gel was aged/solvent evaporated at RT over 1–5 days, and the gel was calcined as detailed in Table 3. 2.1.9. Modified Pechini and related methods [11] (a) The Sn precursor was dissolved in the aqueous dispersant/ gelation agent at room temperature, the system was aged/ solvent evaporated at RT over 1–5 days, and the gel was calcined as detailed in Table 4. (b) Solid Sn(IV) acetate was dissolved in the aqueous acid at RT as detailed in Table 5, the resulting clear solution was calcined without aging according to the heat up protocol: 60 8C/2 h ramp/120 8C/2 h hold/1 h ramp/200 8C/2 h hold/ 1 h ramp/calcination temperature/calcination time. (c) Solid Sn(IV) acetate was dissolved in aqueous acid to a clear solution and then dried/calcined without aging using the heat up ramps and calcination conditions as indicated in Table 6.

(d) Solid Sn(IV) acetate was dissolved in aqueous glyoxylic acid to a clear solution and then dried/calcined without aging using the heat up ramp 55 8C/4 h ramp/120 8C/4 h hold/1 h ramp/calcination temperature/4 h hold with calcination temperature as indicated in Table 7. (e) A mixed metal solution was formed by mixing aqueous 50% glyoxylic acid, Ce nitrate solution, Co acetate solution, solid Sn(IV) acetate as indicated in Table 8. The mixed metal solution was calcined without aging according to the protocol 60 8C/2 h ramp/120 8C/2 h hold/1 h ramp/200 8C/ 2 h hold/1 h ramp/325 8C/4 h hold [11]. For secondary screening in tubular fixed-bed reactors (see Section 2.3 below), approximately 500 mg of each catalyst were prepared by classical incipient wetness impregnation. The particle fraction 180–425 mm resulting from crushing and sieving was used. Carrier powders were pelletized with a cold Table 4 SnO2 by modified Pechini method Precursor

Gelation agent

Calcination

BET [m2/g]

0.5 g Sn(IV) acetate

10 ml 2 M ketoglutaric

350 8C/4 h

175

1 ml Sn(II) acac

15 ml 3.12 M ketoglutaric

350 8C/5 h 450 8C/2 h

61 50

1 ml Sn(II) acac 1 g SnCl4

10 ml 2.32 M ketoglutaric 20 ml 2.7 M ketoglutaric

400 8C/5 h 350 8C/4 h

52 161

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Table 5 SnO2 by modified Pechini method from Sn(IV) acetate precursor Precursor

Gelation agent

Calcination

BET [m2/g]

885 mg Sn(OAc)4 2.465 g Sn(OAc)4 885 mg Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4 1.2 g Sn(OAc)4 883 mg Sn(OAc)4 966 mg Sn(OAc)4 2.067 g Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4 1 g Sn(OAc)4

10 ml 98% pyruvic acid 10 ml 4 M tartaric acid 10 ml 88% lactic acid 5 ml 1 M malonic + 5 ml 98% pyruvic 5 ml 1 M malonic + 5 ml 98% pyruvic 10 ml 50% glyoxylic acid 10 ml 50% glyoxylic acid 5 ml 50% glyoxylic + 5 ml 2.43 M ketoglutaric 5 ml 50% glyoxylic 10 ml 50% glyoxylic + 2 ml ethyleneglycol 2.5 ml 25% glyoxylic acid 5 ml 25% glyoxylic acid 10 ml 25% glyoxylic acid 5 ml 25% glyoxylic acid 5 ml 25% glyoxylic acid 2.5 ml 25% glyoxylic acid

450 8C/4 h 450 8C/4 h 450 8C/4 h 450 8C/2 h 375 8C/2 h 350 8C/4 h 325 8C/5 h 325 8C/5 h 325 8C/3h 350 8C/6 h 350 8C/5 h 350 8C/5 h 350 8C/5 h 325 8C/4 h 300 8C/4 h 300 8C/4 h

45 51 48 51 97 89 123 179 147 98 96 123 120 149 194 168

Yellowish Yellow Yellow Yellowish Yellowish Whitish Yellowish Yellowish Yellowish Yellow Yellow Yellow Yellow Yellow Orange Orange

Table 6 SnO2 by modified Pechini method (acid screening) Precursor

Dispersant

State

Calcination

BET [m2/g]

Yield [mg]

Theory [mg]

Temperature ramp: 45 8C/150 min ramp/120 8C/6 h hold/160 min ramp/200 8C/2 h hold/63 min ramp/325 8C/4 h hold 1 g glycolic in 10 ml H2O Colorless solution 325 8C/4 h 136 450 1 g Sn(OAc)4 1 g Sn(OAc)4 0.5 g glycolic in 5 ml H2O Colorless solution 325 8C/4 h 118 456

425 425

Yellow Yellowish

Temperature ramp: 45 8C/4 h ramp/120 8C/4 h hold/2 h ramp/325 8C/4 h hold 1 g oxalaa in 10 ml H2O Yellow solution 325 8C/4 h 1 g Sn(OAc)4 Temperature ramp: 45 8C/150 min ramp/120 8C/6 h hold/160 min ramp/200 8C/2 h hold/63 min 0.5 g oxalaa in 10 ml H2O Yellow solution 325 8C/4 h 1 g Sn(OAc)4 0.75 g oxalaa in 10 ml H2O Yellow solution 325 8C/4 h 1 g Sn(OAc)4 1 g Sn(OAc)4 10 ml 1 M citric Colorless solution 325 8C/4 h 1 g Sn(OAc)4 10 ml 1 M tartaric Colorless solution 325 8C/4 h Temperature ramp: 45 8C/4 h ramp/120 8C/4 h hold/2 h ramp/310 8C/4 h hold 0.5 g oxalaa in 10 ml H2O Yellow solution 310 8C/4 h 1 g Sn(OAc)4 1 g Sn(OAc)4 10 ml 0.5 M tartaric Colorless solution 310 8C/4 h

190 ramp/325 8C/4 h 134 145 160 145 207 213

457

425

Brownish

hold 438 452 468 450

425 425 425 425

White Yellow Yellow White

444 482

425 425

Yellow Brown

Citric acid HOOC–CH2–C(OH)(COOH)–CH2–COOH; glycolic acid (hydroxyacetic acid) HO–CH2–COOH; glyoxylic acid OHC–COOH; ketoglutaric acid (acetone-1,3-dicarboxylic acid) HOOC–CH2–CO–CH2–COOH; oxalacetic acid HOOC–CH2–CO–COOH; tartaric acid HOOC–CHOH–CHOH–COOH.

Table 7 SnO2 by modified Pechini method (glyoxylic acid method) Sn(OAc)4 [mg]

Stock solution glyoxylic acid aq. [wt%]

Dispense volume stock solution [ml]

Dispense volume water [ml]

Calcination temperature [8C]

Ratio acid/metal [mol/mol]

Metal concentration [mol/l]

BET-SA [m2/g]

800 800 800 750 750 750 700 700 700 600 600 600 600 600 600

50 50 50 50 50 50 50 50 50 50 50 50 50 50 50

1.66 2.08 2.49 1.17 2.33 3.50 1.63 2.00 2.36 2.18 1.56 0.93 2.18 1.56 0.93

2.37 1.95 1.54 2.61 1.44 0.27 1.89 1.53 1.16 0.84 2.89 2.09 2.27 1.46 2.09

290 290 290 320 320 320 285 285 285 300 300 300 280 280 280

4.00 5.00 6.00 3.00 6.00 9.00 4.50 5.50 6.50 7.00 5.00 3.00 7.00 5.00 3.00

0.56l 0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.56 0.38 0.56 0.38 0.56 0.56

161.0 180.8 201.4 120.4 146.4 115.1 171.7 207.8 231.1 173.0 159.3 133.3 192.4 173.6 163.8

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Table 8 Ternary Ce–Co–Sn by modified Pechini method (glyoxylic acid method) Glyoxylic acid (50% aq.) [ml]

Ce(NO3)3 1 M[ml]

Co acetate 1 M [ml]

Sn(IV) acetate [mg]

Comp. (mol.)

BET-SA [m2/g]

5 5 5 5 5 10 10

1 2.5 2 1.25 1 5 5

1.5 1.5 2 2.5 3 5 5

88.5 36 36 44 36 1116 2960

Ce0.2Sn0.5Co0.3 Ce0.5Sn0.2Co0.3 Ce0.4Sn0.2Co0.4 Ce0.25Sn0.25Co0.5 Ce0.2Sn0.2Co0.6 Sn0.24Ce0.38Co0.38 Sn0.45Ce0.27Co0.27

125 72 95 137 30 106 103a

a

Calcination: 325 8C/3 h followed by 350 8C/2 h.

isostatic press (Carver model 3912) before crushing and sieving. After the first impregnation, the catalysts were equilibrated at ambient temperature for 2 h and then either dried and impregnated a second time or calcined at 300 8C if no second or third impregnation steps were necessary (in the case of all-compatible metal precursor solutions). In particular, supported Ru and Ru-Pt catalysts have been prepared by incipient wetness impregnation of these Ce–Sn–Co mixed oxide carriers with Pt tetraamine hydroxide and Ru nitrosyl nitrate solutions and then calcined using the following heat up protocol: ramp up 25–120 8C with 0.5 K/min, 120 8C hold for 2 h, ramp up 120–200 8C with 1.33 K/min, 200 8C hold for 2 h, ramp up 200–300 8C with 2 K/min, 300 8C hold for 2 h. Since Ru and Pt solutions are incompatible, sequential impregnation was applied (Ru first, then drying, Pt last). 2.2. Characterization BET surface area, N2 adsorption isotherms and pore size distributions were measured either on a Coulter SA3100 or Micromeritics Tristar 3000 surface area and porosity analyzer. 2.3. Catalytic tests Catalysts and carriers were screened in a Symyx highthroughput process optimization reactor for propylene combustion and CO oxidation activity [12–15]. The reactor consisted of eight parallel fixed-beds with a flow-distributed feed, in which a stream selection valve selected one of the reactor effluents for rapid serial GC analysis. The feed consisted of 1% CO and/or 0.15% propylene in CDA (clean dry air) at a space velocity of 25,000 h1. The reactor temperature was increased from 40 to 200 8C in 10–20 8C increments in order to produce light-off curves for CO oxidation and propylene combustion both as pure components in CDA as well as a mixed COpropylene-CDA feed. GC analysis used a TCD for quantitative detection of CO, CO2, H2O, propylene and O2. 3. Results The highest surface areas achieved for the different synthesis methods are summarized in Table 9. BET surface areas >100 m2/g are possible using a variety of methods after calcination in the temperature range 300–500 8C. The highest

surface area tin oxides have been obtained by the hydrazine method and by the glyoxylic acid method. SnO2 (hydrazine) shows excellent sintering resistance (Fig. 1). The isotherm is classified as type II (Fig. 2). The pore size distribution for SnO2 (hydrazine) in Fig. 3c is relatively broad and characterized by a large fraction of mesopores and macropores whereas the corresponding distributions for mixed Sn–Ce–Co (glyoxylic) and SnO2 (K2SnO3 gelation) are narrower and taper off at about 20 and 5 nm, respectively (Fig. 3a and b). The SnO2 (hydrazine) carriers (calcined at 300 8C, 234 m2/ g, 0.75 ml/g pore volume) were impregnated with active metals (mixed solution of Co(NO3)2 and Ru(NO)(NO3)3 first, followed by drying, and then Pt(NH3)2(NO2)2 second, followed by calcination at 300 8C) and screened for CO oxidation and VOC combustion activity in an eight-channel parallel fixed-bed reactor with FTIR or GC analytics. Fig. 4 shows the resulting light-off curves for both propylene and CO for several potential catalysts using a mixed propylene-CO-CDA feed. For Pt/SnO2, high CO oxidation activity is observed for the high surface area SnO2 support > 200 m2/g used in Fig. 4. Lightoff for CO oxidation occurred at higher temperatures than for propylene combustion due to Pt poisoning by CO at lower temperatures. Moreover, Pt/SnO2 is seen to be inferior to the commercial 0.5% Pt/Al2O3 benchmark (supplied by Alfa) [24]. Referring to Table 9, also noteworthy are the synthesis methods based on aqueous organic acids as dispersants (hydrolysis of Sn carboxylate complexes and modified Pechini method) resulting in surface areas of about 200 m2/g. Best results have been obtained for glyoxylic acid, oxalacetic acid, tartaric acid and ketoglutaric acid. These novel Pechini acids have a low decomposition temperature in common, in the temperature range of 300–350 8C, thus allowing relatively low calcination temperatures and concomitant mitigation of surface sintering. These acids mostly decompose into volatiles (e.g. ketoglutaric acid ! acetone + CO2) and are less prone to coking. The isolated powder is yellowish with the mass balance almost closed (assuming Sn acetate ! SnO2) thus indicating the absence of larger amounts of coke deposits. On the other hand, the high boiling traditional Pechini acids citric and malic yield black materials when calcined <350 8C and would require temperatures of the order of 400 8C to burn off coke completely. Fig. 5 summarizes the glyoxylic acid preparations as a function of synthesis variables. The highest surface areas

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Table 9 Synthesis of high surface area SnO2 [16,17] Synthetic method

Precursor

ppt/gelation agent

Calcination

BET [m2/g]

Precipitation

SnCl4

Urea

Precipitation [8,9]

SnCl4

Hydrazine

Oxidation [8] Sol–gel

Sn(0) powder Sn i-propoxide Na2SnO3 K2SnO3

Nitric acid Hydrochloric acid Ion exchanger CO2 (dry ice)

K2SnO3

Nitric acid

200 8C 300 8C 300 8C 400 8C 500 8C 600 8C 300 8C 400 8C 400 8C 350 8C 375 8C 400 8C

185 168 250 228 190 175 75 88 54 91 85 110

SnIV(OAc)4 SnII(OAc)2 SnIV(OAc)4 SnIV(OAc)4 SnIV(OAc)4

– – Citric acid Malic acid Malonic acid

350 8C 375 8C 300 8C 350 8C 300 8C

39 52 96 92 133

Ketoglutaric Ketoglutaric Ketoglutaric Ketoglutaric Ketoglutaric

350 8C 350 8C 350 8C 350 8C 350 8C

195 118 197 201 149

285 8C/4 h 290 8C/4 h 310 8C/4 h 310 8C/4 h 350 8C/4 h

231 201 207 213 175

Thermal decomposition Dry Wet

Hydrolysis of Sn acetate solutions in organic solvents SnIV(OAc)4/acac SnII(OAc)2/acac SnIV(OAc)4/eg SnIV(OAc)4/acetic SnIV(OAc)4/formic

acid acid acid acid acid

Modified Pechini SnIV(OAc)4 SnIV(OAc)4 SnIV(OAc)4 SnIV(OAc)4 SnIV(OAc)4

>200 m2/g have been achieved for glyoxylic/Sn ratios between 5 and 7 mol/mol, net weights of the Sn acetate precursor of 700–800 mg, and calcination temperatures of 280–300 8C. For constant calcination temperature and Sn initial weight, the surface area increases with acid/metal ratio. However, for too high glyoxylic acid amounts the surface area starts decreasing, possibly due to the exotherm caused by burning off the organic matrix during calcination.

Fig. 1. BET surface area of SnO2 (prepared using various methods) as function of calcination temperature.

Glyoxylic acid Glyoxylic acid Oxalacetic acid Tartaric acid Ketoglutaric acid

The solubility of Sn(IV) acetate in aqueous glyoxylic acid is very high thus allowing high productivities (space-time-yields) and rendering the ‘glyoxylic acid method’ a convenient and flexible tool for the synthesis of porous tin oxides. Moreover, as Table 8 reveals, mixed metal oxides with high surface areas can also be synthesized, even in the presence of (hard to stabilize) base metals (such as Co in Table 8). Fig. 6a–d shows light-off curves for Ru and Ru-Pt impregnated Ce–Sn–Co mixed oxide carriers (three Ce–Sn–Co carriers with surface areas of 125, 95 and 137 m2/g, respectively). Undoped

Fig. 2. N2 adsorption/desorption isotherm for SnO2 (hydrazine).

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4. Discussion

Fig. 3. Pore size distributions for (a) Sn–Ce–Co mixed oxide prepared by glyoxylic acid method, (b) SnO2 prepared by K2SnO3 gelation and (c) SnO2 prepared by hydrazine method.

Ce0.2Sn0.5Co0.3Ox and the standard 0.5% Pt/Al2O3 are also included for comparison. For propylene combustion using the combined feed, the Ru-Pt/CeSnCo samples are comparable to the commercial Pt catalyst lighting off at 180 8C, whereas the undoped CeSnCo oxide is much less active and starts converting propylene at >200 8C. For CO oxidation in CO-only feed, all RuPt doped samples as well as the undoped CeSnCo carrier are superior to the Pt/Al2O3 standard, whereas in combined feed the mixed oxides appear to be inhibited by propylene and do not lightoff until at least 20 K higher temperature than Pt/Al2O3.

A variety of synthesis techniques have been used to prepare metal oxide materials. These techniques include conventional precipitation, the Pechini, or citric acid combustion process, and a variety of sol–gel techniques. Typical precipitation methods utilize stable, acidic metal salts in solution. The solution is combined with a base that increases the pH of the metal salt solution and destabilizes the metal salts to form metal hydroxides and/or metal carbonates that precipitate out of the solution. This reaction results in counter-anions of the metal salt, such as nitrates or chlorides, and the counter-cations of the base, such as Na, K or NH4 being present. After the precipitation, it is usually desirable to remove the ions from the base and the salt by washing, usually with a solvent such as water. However, this does not typically remove all of the impurities. The precipitate is still typically contaminated with 0.1–0.5% of an ion from the base (e.g. Na). The particle size of the precipitate is usually big enough (micron-sized) to allow filtering and isolation of the powder. If the powder is washed several times to remove most of the ions and reduce the ion content to 50–100 ppm the powder typically no longer sediments, but floats due to the lack of electrolyte, thus making filtration difficult as the filter is typically clogged by the nanosized particles, which are difficult to isolate.

Fig. 4. Light-off curves for Pt impregnated SnO2 (hydrazine) for CO oxidation and propylene combustion (a) propylene conversion in propylene only feed, (b) propylene conversion in combined CO/propylene feed, (c) CO conversion in CO-only feed and (d) CO conversion in combined CO/propylene feed.

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Fig. 5. SnO2 synthesis by glyoxylic acid method: markers sized by BET surface area, colored by net weight of Sn acetate precursor. Calcination temperature, glyoxylic/Sn ratio and net weight of Sn precursor are the principal parameters for high surface areas to be achieved.

In order to avoid the ion contamination issue, precipitation with urea or hydrazine (which both decompose into volatiles upon boiling the solution) have been found to give comparable results to the use of other bases, such as NaOH or Na2CO3. Hydrazine or urea can be advantageous, since the precipitation agent is almost completely removed leaving little or no countercations. Hydrazine decomposes upon boiling into nitrogen, hydrogen and water, and the anion of the metal precursor (such as a chloride) is also removed from the system as a volatile gas, such as HCl. Urea breaks down to ammonia and CO2 with the ammonia released being the actual base/precipitation agent thus forming NH4Cl or NH4NO3 salts that may partly evaporate and partly reside in the solution. The solutions have to be heated

to about 90 8C or refluxed during precipitation and aging thus adding to the energy cost. Furthermore, in applications where high surface areas are desired, precipitation methods have been found to produce porous materials with BET surface areas significantly less than those achieved by sol–gel methods. The Pechini, or citrate method [18–22], involves combining a metal precursor with water, citric acid and a polyhydroxyalcohol, such as ethylene glycol. The components are combined into a solution which is then heated to remove the water. A viscous oil remains after heating. The oil is then heated to a temperature that polymerizes the citric acid and ethyleneglycol by polycondensation, resulting in a solid resin. The resin is a matrix of the metal atoms bonded through oxygen

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Fig. 6. Light-off curves for Ru and Ru-Pt impregnated Ce–Sn–Co mixed oxides for CO oxidation and propylene combustion (a) propylene conversion in propylene only feed, (b) propylene conversion in combined CO/propylene feed, (c) CO conversion in CO-only feed and (d) CO conversion in combined CO/propylene feed.

to the organic radicals in a cross-linked network. The resin is then calcined at a temperature typically above 500 8C to burn off the polymer matrix, leaving a porous metal oxide. The Pechini method is advantageous in that it utilizes components that are inexpensive and easy to handle. However, the method results in materials having BET surface areas substantially lower than those materials created using precipitation and sol– gel methods. Typical sol–gel methods utilize metal alkoxide precursors in organic solvents with an aqueous inorganic acid, such as nitric acid or hydrochloric acid. The inorganic acid acts as a catalyst allowing the water to hydrolyze the metal alkoxide bonds in a hydrolysis reaction by protonation, forming a metal hydroxide and an alcohol. Subsequent condensation reactions involving the metal hydroxide units reacting with other metal hydroxide units or remaining metal alkoxides result in the metal molecules bridging, and water and alcohol being created. As the number of bridged metal molecules increases, agglomeration occurs, forming irregular agglomerates and eventually growing into a three-dimensional amorphous polymer network, or a gel. The remaining water and alcohol, which is a neutral non-ionic unreactive organic solvent, is evaporated from the system leaving little to no traces of the former metal counter-anion behind. The gel is then calcined, resulting in a porous, solid metal oxide. While the current sol–gel processes produce porous metal oxide materials having surface areas superior to those produced by precipitation and the Pechini method, the

method suffers from the alkoxide precursors being expensive, flammable, viscous and difficult and dangerous to handle. For SnO2, it can be seen from Table 8 that high surface areas >100 m2/g are possible by precipitation, sol–gel techniques as well as Pechini and related methods. The various synthesis methods we have developed complement each other with respect to achievable surface area/porosity and residual contaminations, in particular: Synthesis method

Advantage

Disadvantage

SnCl4 precipitation K2SnO3 gelation Pechini method

Ultra high surface area Inexpensive Flexible; mixed metal oxide synthesis

Chloride contamination Potassium contamination Carbon contamination

When screening alternative Pechini acids (different from citric and malic) we discovered that significant surface area gains can be realized by resorting to organic acids such as glyoxylic or ketoglutaric acid that are easily decomposable at low calcination temperatures in the range 300–350 8C without concomitant formation of refractory coke. Therefore, we propose a modification of the Pechini method using these novel acids as dispersants for the synthesis of porous tin oxide and other metal oxides [11]. Although we did not investigate the mechanism in detail, experimental evidence suggests a different mechanism from the classical Pechini method that is characterized by the metals being entrapped in a matrix of

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polymer resin. The new method more closely resembles a sol– gel process because often we observe gel formation when evaporating the solvent (either upon prolonged standing at room temperature or when heating up for drying). Whether the gel is composed of metal oxide bridges and/or a metal carboxylate network requires further investigation. Ketoglutaric acid, for instance, constantly decomposes (into acetone and CO2) upon stirring thus resulting in high losses of acid from the system until pH has risen to a point where gelation occurs. However, the mass balance (for the dried gel or in case of a too low calcination temperature) indicates considerable residual organics in the gel that must be burned off during calcination. In summary, the proposed new method is as easy and flexible to carry out as the classical Pechini method but allows the very high surface areas to be achieved that are characteristic of a sol– gel process and advantageously utilizes common inorganic Sn precursors (rather than alkoxides). 5. Conclusion Tin oxides with BET surface areas >200 m2/g and excellent sintering resistance are achievable by the hydrazine synthesis method. Pt/SnO2 is active for low temperature VOC combustion but poisoned by CO co-feed shifting the light-off curves towards higher temperatures. High surface area SnO2 is also available by a novel modified Pechini method using aqueous solutions of thermally labile organic acids as dispersants for inorganic Sn precursors such as Sn acetate. The concept consists of selecting easy-todecompose organic acids that allow low temperature calcinations thus preventing surface area loss by sintering. Highest surface areas of about 200 m2/g were achieved with glyoxylic, oxalacetic, tartaric and ketoglutaric acid which decompose in the temperature range 300–350 8C into volatiles without excessive coking. Calcination temperature, glyoxylic acid/Sn ratio and net initial weight of Sn precursor are the principal parameters for high surface areas to be achieved and must be carefully optimized in small increments. We have demonstrated that high surface areas are achievable by means of aqueous routes after careful optimization of synthesis parameters, without resorting to organic solvents, sol–gel, supercritical drying or templates/hydrothermal synthesis. Our combustion synthesis is also well suited for the preparation of mixed oxides from mixed metal solutions in aqueous organic acids. We have been investigating the CO oxidation reaction over Ru–Co supported on many carrier materials and carrier grades by high-throughput primary screening [13–15,23,24] and have found the highest activity for the highest surface area materials,

in the order active carbon 1000 m2/g > ceria 300 m2/g > SnO2 200 m2/g > ZrO2 100 m2/g. The differences are more clearly resolved than in the current case of SnO2 dealing with smaller surface area differences among the SnO2 samples screened. Acknowledgement We wish to thank V. Sokolovskii for helpful discussions and advice. References [1] J. Shen, R.D. Cortright, Y. Chen, J.A. Dumesic, Catal. Lett. 26 (1994) 247–257. [2] T. Tagawa, S. Kataoka, T. Hattori, Y. Murakami, Appl. Catal. 4 (1982) 1–4. [3] M.J. Fuller, M.E. Warwick, J. Catal. 29 (1973) 441–450. [4] Y. Teraoka, T. Harada, T. Iwasaki, T. Ikeda, S. Kagawa, Chem. Lett. (1993) 773–776. [5] D. Amalric-Popescu, F. Bozon-Verduraz, Catal. Lett. 64 (2000) 125–128. [6] K. Sekizawa, H. Widjaja, S. Maeda, Y. Ozawa, K. Eguchi, Catal. Today 59 (2000) 69–74. [7] O. Demoulin, M. Navez, F. Gracia, E.E. Wolf, P. Ruiz, Catal. Today 91–92 (2004) 85–89. [8] N. Sergent, P. Gelin, L. Perier-Camby, H. Praliaud, G. Thomas, Sens. Actuators B 84 (2002) 176–188. [9] L. Urfels, P. Gelin, M. Primet, E. Tena, Preprints CAPoC6, Brussels, October 22–24, 2003. O20, pp. 203–212. [10] Gelest Catalog: Metal-Organics for Material & Polymer Technology, 2001. [11] Patent pending. [12] S. Bergh et al., U.S. Patent Appl. No. 2002-0048536 entitled ‘‘Parallel Flow Process Optimization Reactor’’ (published April 25, 2002). [13] A. Hagemeyer, et al. ACS Spring Meeting, San Diego, CA, March 13–17, 2005. [14] S. Cypes, et al. AIChE Spring Meeting, Atlanta, GA, April 10–14, 2005. [15] G. Streukens, et al. Fourth International Conference on Environmental Catalysis, Heidelberg, Germany, June 5–8, 2005. [16] A. Hagemeyer, et al. 19th NAM Meeting, Philadelphia, PA, May 22–27, 2005. [17] B. Smaka, et al. 10th International Symposium on Catalyst Deactivation, Berlin, Germany, February 5–8, 2006. [18] US 3330697 to M.P. Pechini, 1967. [19] H.M. Reichenbach, P.J. McGinn, J. Mater. Res. 16 (4) (2001) 967. [20] H.M. Reichenbach, P.J. McGinn, Appl. Catal. A 244 (2003) 101. [21] US 6372686, US 6352955, US 5977017, US 5939354, US 2002/0042341, WO 97/37760 to Catalytic Solutions Inc. [22] Z. Hui, P. Michele, J. Mater. Chem. 12 (2002) 3787–3791. [23] S. Cypes, A. Hagemeyer, Z. Hogan, A. Lesik, G. Streukens, A.F. Volpe Jr., W.H. Weinberg, K. Yaccato, in: J. Margitfalvi (Ed.), High Throughput Screening of Low Temperature CO Oxidation Catalysts Using IR Thermography, Combinatorial Chemistry & High Throughput Screening CCHT, Special Issue on Catalysis, 2007, in press. [24] G. Streukens, Master Thesis, Technical University of Darmstadt, Germany, 2004.