Nb2O5 catalysts promoted with Pd and Ru during hydrogen production from ethanol

international journal of hydrogen energy 34 (2009) 3333–3341

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Reactions over Cu/Nb2O5 catalysts promoted with Pd and Ru during hydrogen production from ethanol Christian Gonc¸alves Alonsoa, Andre´ia Cristina Furtadoa, Mauricio Pereira Canta˜ob, One´lia Aparecida Andreo dos Santosa, Na´dia Regina Camargo Fernandes-Machadoa,* a

Departamento de Engenharia Quı´mica, Fundac¸a˜o Universidade Estadual de Maringa´, Av. Colombo, 5790, Bl. D-90, CEP 87020-900, Maringa´, PR, Brazil b LACTEC – Instituto de Tecnologia para o Desenvolvimento, BR 116, Km 98, s/n, CEP 81531-980, C.P. 19067, Curitiba, PR, Brazil

article info

abstract

Article history:

Hydrogen production from reactions between ethanol and steam at 300  C was evaluated

Received 11 December 2008

under low conversion conditions for the Cu/Nb2O5 system promoted with Pd and Ru.

Received in revised form

Parallel reactions occurred on the surface of all samples as it was verified from the

7 February 2009

production of H2, CO2, CH4, CO, C2H4, C2H6, C2H4O and (C2H5)2O. Hydrogen production

Accepted 7 February 2009

occurs mainly from ethanol dehydrogenation and secondly, from steam reforming and

Available online 17 March 2009

ethanol decomposition. Dehydration reactions were also identified and analyzed among others. Addition of Pd and Ru to the catalyst improves product selectivity and it was

Keywords:

verified that Pd-Ru-Cu/Nb2O5 tri-metallic catalyst is the most promising for H2 production

Hydrogen

due to its selectivity and lower deactivation, among all samples tested.

Ethanol

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Steam reforming

reserved.

Parallel reactions Nb2O5 catalyst Noble metals

1.

Introduction

Concerns regarding climate change, especially due to greenhouse gases emission from fossil fuels, and increasing energy demand have influenced the development of cleaner and more efficient technologies for energy generation. Among other technologies hydrogen and fuel cells are remarkable due to its great commercial potential and high energy efficiency. However, hydrogen will be considered a viable fuel only if renewable sources are used for its production [1]. Nowadays, hydrogen is mainly produced via the reforming of natural gas, hydrocarbons and coal [2]. In theory, hydrogen production from biomass or biomassderived liquids such as ethanol can be considered almost

a carbon free process since the carbon dioxide is recycled during crop growing [1]. Ethanol, besides meeting environmental requirements, is considered an important source of hydrogen for the use in fuel cells [3–5], especially in Brazil where ethanol is largely used as automotive fuel [6]. Hydrogen can be obtained from ethanol through catalytic steam reforming reaction [3,4,7,8], which allows the use of ethanol distribution infrastructure as a strategy of hydrogen distribution [1]. Thermodynamic studies have showed that temperatures higher than 230  C are necessary to carry out the reaction of ethanol steam reforming [9] and it was observed that molar ratios H2O/C2H5OH higher than the stoichiometric one prevent coke formation and catalyst deactivation [10,11].

* Corresponding author. Tel.: þ55 44 32614747; fax: þ55 44 32614792. E-mail address: [email protected] (N.R. Camargo Fernandes-Machado). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.02.021

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international journal of hydrogen energy 34 (2009) 3333–3341

Hydrogen selectivity depends on the complex ethanol steam reforming reaction routes [12] and supplying enough steam maximizes H2 production and minimizes ethanol dehydration and decomposition reactions [13]. The development of ethanol steam reforming process is still in the beginning, and there is a great number of papers describing the performance of different catalysts tested under several conditions. In a general way, the research has focused the search for catalysts presenting good activity and good selectivity to H2 and CO2. In this context, catalysts based on Cu [14–18], Ni [8,15,16,18–22], Co [1,5,8,12,20,22,23], Rh [19,20,24,25], Pt [2,12,19,24], Pd [4,12,19,20,24,26], Ru [2,12,24] and Ir [22] supported on different materials have been tested with relative success in quartz micro-reactors and under reagent mixture H2O/C2H5OH heavily diluted in inert gas. Previous studies have demonstrated the importance of the support for ethanol steam reforming reaction [27,28], making it evident that development of a catalyst for industrial use can not only focus the active metal phase but also the composition and nature of the support material. Cu catalyst was tested over different supports made of n-type semiconductor oxides (Nb2O5, TiO2 and ZnO) for hydrogen production from ethanol and steam reaction and it was verified that Cu/Nb2O5 with 5% wt of Cu is the most active [29]. The Cu/Nb2O5 catalyst requires low cost and great availability materials, and therefore has attracted the interest of other research groups [30]. However, Rizzo-Domingues et al. [14] have showed that the system acidity provokes ethanol dehydration and that catalyst promoters are necessary in order to prevent this reaction. In this work, the performance of the 5%Cu/Nb2O5 catalyst is evaluated during ethanol and steam reaction at low conversion of ethanol and at 300  C and atmospheric pressure. Also, the influence of Pd and Ru on catalyst behavior is analyzed, looking for the best alternative for hydrogen production from ethanol steam reforming.

were granulated according to the rules described by Trimm [31], for minimization of diffusional effects of mass and heat in packed bed reactors, resulting in average particle size of 0.6 mm. The following characterization techniques were used: – X-Ray Fluorescence, in a RIX 3100 Rigaku equipment. – X-Ray Diffraction (XRD), in a XRD 6000 Shimadzu equip˚ ), scanning ment, radiation source of CuKa (l ¼ 1.5418 A speed of 2 /min. – Thermogravimetry and Differential Thermal Analysis (TG/DTA), in a simultaneous thermal analysis STA 409C NETZSCH equipment, temperature between 20 and 1000  C, heating speed of 5  C/min, under O2 gas atmosphere and N2 carrier gas at flow rate of 100 cm3/min each. – Textural Analysis (BET), from adsorption/desorption isotherm of N2 at 196.15  C in a Quantachrome Nova1200 equipment, samples previously submitted to N2 flow and in situ heating at 90  C (HY-340) and 200  C (calcined catalysts). – Temperature Programmed Reduction (TPR), in samples with 0.2 g calcined at 500  C/5 h and submitted to temperature range of 10  C/min in the presence of the mixture H2/Ar (1.75/98.25) at flow rate of 30 cm3/min from room temperature to 1000  C. – Temperature Programmed Desorption of NH3 (TPD-NH3), in a Quantachrome Chembet-3000 equipment. Firstly, the samples were treated at 150  C under N2 flow rate of 20 cm3/min by 1 h. Then they were reduced by H2-N2 (5% H2 vol.) mixture at 500  C/30 min. The adsorption of NH3 was carried out at 100  C using NH3-N2 (5% NH3 vol.). Physisorbed NH3 was purged during 2 h under N2 flow rate of 20 cm3/min. The heating rate was then adjusted to 10  C/min, from 100  C to 500  C, to perform the desorption of chemisorbed NH3.

2.2.

2.

Experimental

2.1.

Catalysts preparation and characterization

Samples of the niobium pentoxide (referred to as N) were obtained from niobic acid HY-340, supplied by the Companhia Brasileira de Metalurgia e Minerac¸a˜o, CBMM, after washing with deionized water followed by drying at 90  C/72 h and calcination in furnace at 500  C/5 h. The 5%Cu/Nb2O5 (CN), 1%Pd-5%Cu/Nb2O5 (PCN), 1%Ru5%Cu/Nb2O5 (RCN) and 0.5%Pd-0.5%Ru-5%Cu/Nb2O5 (PRCN) catalysts were prepared using the wet impregnation method from alcoholic solutions of CuCl2$2H2O (VETEC), PdCl2 (VETEC) and RuCl3 (Acros Organics). The corresponding salt solutions were added to wetted Nb2O5 (Nb2O5/deionized H2O ratio equals to 1 g/1.5 cm3), maintaining the samples under agitation at room temperature for 18 h. Catalytic precursors were heated at 55  C at reduced pressure in a rotating evaporator and dried in oven at 90  C/ 24 h. Then, catalytic precursors were hydraulically pressed at 147 kgf/cm2 and calcined at 500  C/5 h. The catalysts obtained

Catalytic tests

Catalysts performance was evaluated in an experimental unit consisting of pre-heating system, stainless steel reactor 18 cm long and 2.1 cm ID, condenser and phase separator/collector system. The tests were carried out at 300  C and atmospheric pressure. Gas weight hourly space velocity (30,000 cm3/h gcat), flow rate (1.52 g/min), H2O/C2H5OH molar ratio in feeding (10/1) and catalyst mass (7 g) were kept constant. This mass occupied a volume of 5 cm3 of the reactor bed, corresponding to approximately 1.45 cm of linear length. Reagent mixture feeding to the system was carried out in the liquid phase by means of a Gilson peristaltic pump. Prior to the tests, catalysts were activated in situ with heating steps at 100  C (0.5 h), 200  C (1 h) and 500  C (4 h), under N2-H2 (40 vol.% H2) mixture flow rate of 85 cm3/min approximately. The gaseous reaction products were analyzed in a Trace GC Thermo Finnigan gas chromatograph in line coupled to the gas collector exit and equipped with a thermal conductivity detector (TCD), Porapak N and 13X molecular sieve packed columns. Samples of the liquid product were collected and analyzed in a Varian 3300 gas chromatograph equipped with Carbowax packed column and TCD, and also in a Trace GC

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international journal of hydrogen energy 34 (2009) 3333–3341

Thermo Finnigan equipped with PONA capillary column and flame ionization detector (FID).

Results and discussion

3.1.

Composition and structure

X-rays fluorescence results presented in Table 1 show that the measured composition is similar to the nominal composition used in the preparation method. X-ray diffractograms are presented in Fig. 1. The noncrystalline starting material (HY-340) presents diffraction peaks characteristic to the crystalline niobium pentoxide hexagonal phase (Nb2O5, JCPDS card # 28-0317) after treatment described in Section 2.1. Catalysts samples did not present diffraction lines corresponding to the impregnated oxides, which might be related to the small particles or to amorphous feature of the oxides.

3.2.

Thermal properties

Catalyst

Nominal content (% w/w) Cu

CN PCN RCN PRCN

5.0 5.0 5.0 5.0

Pd

Ru

1.0 0.5

1.0 0.5

Real content (% w/w) Cu 5.4 5.6 5.6 5.6

Pd

Ru

PCN CN Nb2O5

10

15

0.5

30

35

40

45

50

55

60

65

70

75

80

Fig. 1 – X-ray diffractograms for starting material, support and catalysts.

synthesis process, while in the intermediate range it can be attributed to H2O fraction highly adsorbed on the catalytic precursors surface and pores. Mass loss in the upper range corresponds to the release of chloride ions, hydroxylic water and possibly to carbon traces from ethanol used as solvent of metallic precursors.

3.3.

Textural analysis

HY-340 presented type IV isotherm, characteristic of mesoporous solids with condensation due to narrow, crack-shaped pores while Nb2O5 showed type III isotherm, characteristic of non-porous or macroporous solids. The difference between isotherms of the starting material and the catalyst support is due to the phase change during HY-340 calcination, in agreement with TG/DTA and XRD analyses. The catalysts isotherms are similar to that of Nb2O5. Specific surface area results are shown in Table 2. Considerable decrease of surface area was observed after catalysts preparation, which might be consequence of crystallites rearrangement during metal impregnation using ethanol as solvent.

0.4

100 HY-340

98 96

0.3 0.2

94 92

0.1

90

0.0

88

-0.1

86 84

-0.2

82

-0.3 0

1.1 0.4

25

2θ (°)

80 1.1

20

-0.4 100 200 300 400 500 600 700 800 900 1000

Temperature (°C) Fig. 2 – Thermal analysis results for HY-340.

DTA (µV/mg)

Table 1 – Chemical composition of catalysts.

RCN

HY-340

Mass Loss (%)

TG/DTA curves for HY-340 samples without treatment is shown in Fig. 2. The observed 20% mass loss between 25  C and 300  C is attributed to the dehydration of HY-340, agreeing with the material chemical analysis informed by the supplier. The DTA curve shows two peaks, an exothermic one with maximum at temperature of 97  C related to the mass loss observed in the corresponding TG curve, and an endothermic one with maximum at 556  C corresponding to the structural change from the amorphous phase to the hexagonal crystalline Nb2O5 phase, in agreement with the diffractograms of the HY-340 and Nb2O5 samples in Fig. 1. The TG/DTA curves of Nb2O5 are shown in Fig. 3. They did not present any thermal transition, which indicates that the phase transition was complete during HY-340 calcination and that the material crystallinity is not altered between 25 and 1000  C. Fig. 4 shows thermogravimetry curves for the precursors of the CN, PCN, RCN and PRCN catalysts, dried at 90  C/24 h and before calcination. These materials show similar behavior regarding shape and intensity and have total mass losses of 7, 9, 8 and 11%, respectively. Mass loss occurs in three different regions: between 25 and 120  C (3–6% of loss), between 120 and 310  C (loss below 1%) and between 310 and 640  C (4–6% of loss). Mass loss in the lower temperature range is due to release of H2O and also of residual ethanol from catalyst

Relative Intensity (a. u.)

3.

PRCN

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international journal of hydrogen energy 34 (2009) 3333–3341

0.4

100 Nb2O5

98 96

Catalyst

0.2

92

0.1

90

0.0

88

-0.1

86 84

-0.2

82

-0.3

80 0

DTA (µV/mg)

94

Mass Loss (%)

Table 2 – Textural analysis by N2 adsorption/desorption isotherms at L196 8C.

0.3

-0.4 100 200 300 400 500 600 700 800 900 1000

Temperature (°C) Fig. 3 – Thermal analysis results for Nb2O5.

3.4.

Specific surf. area (m2/g) External surf. area (m2/g)a Micro-pore surf. area (m2/g)a Pore total volume (cm3/g) Micro-pore volume (cm3/g)a Pore average ˚) diameter (A

N

CN

PCN

RCN

PRCN

187

31

11

13

10

11.9

174

31

11

13

10

11.9

13

–

–

–

–

–

0.18

0.09

0.03

0.03

0.03

0.02

0.004

–

–

–

–

–

38

114

108

95

128

53.4

a Calculated by t-method from Boer standard isotherms.

Temperature programmed reduction

TPR curves for support and catalysts are shown in Fig. 5, whereas H2 consumption is presented in Table 3. In all samples, the reduction of Nb2O5 from Nbþ5 to Nbþ4 occurs with maximum between 877 and 931  C. Reduction of Cu2þ to Cu0 in CN catalyst occurs at 304  C and the small shoulder at 420  C corresponds to reduction of copper phase strongly adhered to the support. PCN catalyst showed reduction peak from Pd2þ to Pd0 with maximum at 35  C. The addition of Pd, even in small amount, shifts the Cu reduction peak to lower temperatures and the Nb reduction peak to higher temperatures. Reduction from Cu2þ to Cu0 occurs at 164  C with a small shoulder at 95  C, which indicates strong interaction between copper and palladium, facilitating the reduction of copper species. Temperature decrease of Cu reduction was also verified in previous works with Pd-Cu catalysts supported in Al2O3 [32] and in mordenite [33] with metallic Pd promoting the reduction of Cu oxide species. RCN catalyst showed two reduction peaks with maxima at 292  C and 877  C. Mitsui et al. [34] working with ZrO2 and gAl2O3 supported Ru catalysts attributed to RuO reduction the H2 consumption peaks at 195  C and 230  C. Therefore, 100

comparing the reduction curves of the CN and RCN catalysts and the results from Table 3, there is a clear indication that a simultaneous reduction of Cu and Ru occurred, indicating strong interaction between these metals which makes the CuO reduction easier. The PRCN catalyst presented reduction peak of Pd with maximum at 30  C. The wider peak with maximum at 193  C corresponds to the simultaneous Ru and Cu reduction. Both catalysts containing Pd (PCN and PRCN) have showed a positive peak close to the room temperature, corresponding to PdO reduction from Pd2þ to Pd0, followed by a negative peak at about 55  C, either attributed to desorption of weakly adsorbed H2 on the Pd surface or to the decomposition of Pd hydride, previously formed at room temperature [35–38]. The shift of CuO reduction peak to lower temperatures for catalysts promoted with Pd and/or Ru might have resulted from the hydrogen spillover phenomenon, which occurs in noble metals and makes copper species reduction easier. A similar result was observed by Profeti et al. [12] for Co based catalysts promoted with noble metals, Pd and Ru among them.

3.5.

Temperature programmed desorption of NH3

Table 4 shows relative acidity data obtained from desorption curves of NH3, which are available for starting material, support and the catalysts.

99

193

98

929

96 95 94

CN

93

RCN PCN

92 91 90 89

PRCN

H2 Consumption (a. u.)

30

97

Mass Loss (%)

HY-340

48

877

RCN

164 35

931

95

PCN

304 877

46

917

88 0

PRCN

292

CN Nb2O5

100 200 300 400 500 600 700 800 900 1000 1100

Temperature (°C) Fig. 4 – Thermogravimetry profile for dry catalytic precursors.

0

100

200

300

400

500

600

700

800

900 1000

Temperature (°C) Fig. 5 – TPR profile for support and catalysts.

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international journal of hydrogen energy 34 (2009) 3333–3341

Table 3 – H2 Consumption from TPR profiles. H2 Theoretic consumption (mmol/gcat)

H2 Real consumption (mmol/gcat)a

Reduction percentage (%)

0.85 0.98 1.04 0.99

0.69 0.92 0.95 0.96

81.18 93.88 91.35 96.96

CN PCN RCN PRCN

a Calculated from TPR profiles.

The Nb2O5 preparation process reduced the niobium acid acidity to a bit more than half, compared to starting material, while Cu incorporation decreased even more the acidity, possibly due to the occupation of support acid sites and to its basic nature. The CN, PCN and RCN catalysts have different quantities of acid sites but they are comparable in acid strength since the maximum desorption occurs at the same temperature range. PRCN catalyst presented lower acidity compared to other promoted catalysts, but similar to the CN catalyst. However, it showed increase in acid strength due to the appearance of a second ammonia desorption peak at 334  C. Besides the occupation of exposed acid sites by active phase metals, the decrease of catalysts relative acidity might also be caused by decreasing the number of sites related to the reduction in specific surface area, according to Table 2.

3.6.

Catalytic tests

Catalysts performance evaluation for ethanol reforming was carried out by measuring ethanol conversion (Xethanol), selectivity to products (Sp) and catalyst deactivation (Dcat), according to equations 1–3 below, where F represents the molar flow rate of the reagents and products.   out Fin ethanol  Fethanol  100% Xethanol ¼ Fin ethanol Fout Pp

Sp ¼

isC2 H5 OH isH2 O

Dcat ¼

Fout i

(1)

 100%

(2)

t¼8h Xt¼1h ethanol  Xethanol  100% Xt¼1h ethanol

Table 4 – Temperature Programmed Desorption results (TPD-NH3).

HY-340 Nb2O5 CN PCN RCN PRCN

(4)

2C2H5OH / (C2H5)2O þ H2O

(5)

The increase of selectivity to ethene with catalyst deactivation is attributed to the acidity of Nb2O5 support, according to Breen et al. [19], who have related ethene formation via ethanol dehydration reaction to the acid character of alumina support. In addition, it was observed a slight increase of selectivity to ethane (C2H6) probably via partial hydrogenation of ethene formed, according to equation 6: C2H4 þ H2 / C2H6

Desorbed NH3 (mmol/gcat)

Temperature ( C)

0.306 0.178 0.051 0.088 0.072 0.049

259 262 255 250 231 221 and 334

(6)

The selectivities to acetaldehyde (C2H4O), carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO) indicate that hydrogen production occurs mainly via ethanol dehydrogenation (equation 7). According to Zhang et al [40], the ethanol dehydrogenation to acetaldehyde is the primary reaction, and acetaldehyde is then decomposed to CO and CH4 (equation 8) that is further reformed to H2 and CO (equation 9), and the

25

(3)

Ethanol conversion results for the support and CN, PCN, RCN and PRCN catalysts are presented in Fig. 6. The reaction

Material

C2H5OH / C2H4 þ H2O

Nb2O5 CN PCN RCN PRCN

20

Xethanol (%)

Catalyst

parameters were chosen in order to keep ethanol conversion below 10%, allowing performance evaluation under conditions with minimum mass transfer effects. However, the PRCN catalyst has exceeded the others with ethanol conversion above 10% during a great part of the test. Fig. 7 shows catalytic deactivation for catalysts and support between t ¼ 1 h and t ¼ 8 h. The addition of copper, palladium and ruthenium to Nb2O5 support strongly influences the catalytic activity and consequently the catalyst life. Activity loss, observed for all catalysts after 8 h of reaction, is probably due to coke formation on the catalyst surface, as it was verified for Al2O3, SiO2 and MgO supported cobalt catalysts by Batista et al. [39]. Selectivity results are presented in Figs. 8–12. The pure support selectivity (Nb2O5, Fig. 8) indicates support strong tendency to production of ethene (C2H4) and diethyl ether (C2H5OC2H5), both reaction products of ethanol dehydration, as indicated in equations 4 and 5:

15 10 5 0

0

1

2

3

4

5

6

7

8

Time (h) Fig. 6 – Ethanol conversion for support and catalysts at 300 8C.

9

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international journal of hydrogen energy 34 (2009) 3333–3341

80

CH4

CO

CO2

C2H4

C2H6

C2H4O

(C2H5)2O

60

60

40

50

20

40

SP (%)

Catalyst Deactivation (%)

70

H2

30 20

1.2 0.9 0.6

10

0.3

0 N

CN

PCN

RCN

0.0

PRCN

0

Catalyst

1

2

3

4

5

6

7

8

9

Time (h) Fig. 7 – Catalysts deactivation after 8 h.

later is converted to CO2 by the water gas shift reaction (equation 10). However in this case, as the selectivities to C1 products are very small, hydrogen could also be produced via direct ethanol steam reforming (equation 11), ethanol decomposition (equation 12) and via coke formation through ethene decomposition (equation 13). The last reaction could possibly explain the fast deactivation of catalysts [41].

Fig. 9 – Product selectivity for CN catalyst (scale: same as Fig. 8).

C2H4 / 2C þ 2H2

(13)

C2H5OH / C2H4O þ H2

(7)

C2H4O / CO þ CH4

(8)

CH4 þ H2O / CO þ 3H2

(9)

CO þ H2O / CO2 þ H2

(10)

C2H5OH þ 3H2O / 2CO2 þ 6H2

(11)

The selectivity to acetaldehyde for CN catalyst, shown in Fig. 9, confirms that hydrogen production occurs via ethanol dehydrogenation (equation 7). During the test, a gradual increase of selectivity to ethene was observed, concomitantly with the decrease of selectivity to acetaldehyde, which might be related to a relative acidity increase of the catalytic surface. The lower selectivity to dehydration products (ethene and diethyl ether) for CN catalyst favors the occurrence of ethanol dehydrogenation reaction and can be explained by lower acidity of CN catalyst compared to the pure support, as seen in Table 4. Increase of selectivity to ethene, decrease of selectivity to acetaldehyde and stability of selectivity to hydrogen agree with increasing hydrogen production via coke production reactions, possibly from ethanol, according to equation 14:

C2H5OH / CH4 þ CO þ H2

(12)

C2H5OH / 3H2 þ CO þ C

H2

CH4

CO

CO2

C2H4

C2H6

C2H4O

(C2H5)2O

60

(14)

H2

CH4

CO

CO2

C2H4

C2H6

C2H4O

(C2H5)2O

60

40

50

20

SP (%)

SP (%)

40 1.2 0.9

2.5 2.0 1.5

0.6

1.0

0.3

0.5

0.0

0.0 0

1

2

3

4

5

6

7

Time (h) Fig. 8 – Product selectivity for support Nb2O5.

8

9

0

1

2

3

4

5

6

7

Time (h) Fig. 10 – Product selectivity for PCN catalyst.

8

9

international journal of hydrogen energy 34 (2009) 3333–3341

H2

CH4

CO

CO2

C2H4

C2H6

C2H4O

(C2H5)2O

3339

gasification reactions could explain higher selectivity to CO in comparison with the selectivity to CH4.

60

C2H5OH þ H2O / 2CO þ 4H2

(15)

C þ H2O / CO þ H2

(16)

SP (%)

50 40 2.5 2.0 1.5 1.0 0.5 0.0 0

1

2

3

4

5

6

7

8

9

Time (h) Fig. 11 – Product selectivity for RCN catalyst (scale: same as Fig. 10).

Figs. 10 and 11 present the selectivity to products for PCN and RCN catalysts, respectively. In comparison to CN catalyst, ethene and diethyl ether production is smaller, which means that addition of palladium or ruthenium to the CN catalyst decreases even more the occurrence of ethanol dehydration reactions. Hydrogen production occurs preferably via ethanol dehydrogenation reaction with little contribution of ethanol steam reforming reaction, estimated by CO2 production, and of ethanol decomposition reaction, estimated by CH4 and CO production. Selectivity to products for PRCN catalyst is presented in Fig. 12. The selectivity to hydrogen and acetaldehyde is almost 100 times higher than the selectivity to any other product including ethene, whose selectivity keeps increasing slowly. Hydrogen production occurs mainly via ethanol dehydrogenation (equation 7) with the participation of decomposition reactions (equations 12 and 14) and, possibly, of partial ethanol reforming and gasification reactions, showed in equations 15 and 16, respectively. The partial reforming and

H2

CH4

CO

CO2

C2H4

C2H6

C2H4O

(C2H5)2O

60

SP (%)

50 40 2.5 2.0 1.5 1.0

The combined effect of Pd and Ru gave to CN catalyst the properties observed when they were added separately, producing the sum of individual effects on the catalyst performance. The PRCN catalyst, besides a greater resistance to deactivation, also showed higher selectivity to the formation of C1 products (CO2, CO and CH4) via ethanol decomposition reaction without significant alterations in the activity to ethanol dehydrogenation. All catalysts promoted with Pd and/ or Ru presented higher CO contents than observed for CH4, which can be explained by the coke production via direct ethanol decomposition reaction (equation 14) [42] or by incomplete ethanol reforming (equation 15) [1]. In general, the results obtained for catalysts based on copper are in agreement with those observed by Benito et al. [43]. They have verified that copper favors ethanol dehydration leading to ethene production, which is easily decomposed to carbon and deposited on the active phase, poisoning the catalyst and causing its deactivation.

4.

Conclusions

Nb2O5 oxide presented good initial activity for hydrogen production and fast deactivation associated with the increase in activity to dehydration reactions. The incorporation of copper to Nb2O5 has caused significant decrease of the BET area and of the support acidity. Such modifications led to the formation of new sites and improved catalytic activity and selectivity. The Pd and Ru promoters have made CuO reduction easier and improved the CN catalyst performance. Ru has promoted catalytic activity but has not altered the distribution of products. Pd has modified catalytic selectivity favoring the formation of C1 products (CO2, CO and CH4). PRCN catalyst has presented better distribution regarding the formation of C1 products, showing greater potential for hydrogen production and for further studies on ethanol steam reforming. Catalytic sites with higher acid strength have had no influence on performance due to reduction of dehydration reactions. The promotion of the CN catalyst with both Pd and Ru has significantly improved the system performance regarding hydrogen production, ethanol conversion and the resistance to deactivation. Pd and Ru promoters have also made the niobia supported copper catalyst more selective to dehydrogenation reaction in relation to dehydration reactions.

0.5 0.0 0

1

2

3

4

5

6

7

8

9

Time (h) Fig. 12 – Product selectivity for PRCN catalyst (scale: same as Fig. 10).

Acknowledgements This work was funded by Parana State Energy Company (Companhia Paranaense de Energia, COPEL GeT), and for its support the authors are very grateful. C.G.A. would like to

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international journal of hydrogen energy 34 (2009) 3333–3341

thank LACTEC, CNPQ and catalysis group of the Rio de Janeiro Federal University (NUCAT/PEQ/COPPE/UFRJ) for XRF analysis. M.P.C. would like to thank Eng. Mario Cesar do Nascimento (COPEL GeT) for his helpful contribution to this work.

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