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Vol. 26, No 1, 2

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An International Refereed Scientic Journal

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at the Universidad del Zulia

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ISSN: 1315-2076

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

CIENCIA 26 (1,2), 28 - 41, 2018

Maracaibo, Venezuela

Effect of Sn addition in alumina- and silica-supported palladium

catalysts for n-butane dehydrogenation

(Dehydrogenation of n-butane over supported palladium catalysts)

Jonathan E. Mendez *, Douglas R. Rodríguez, Jorge L. Sánchez, Geomar J. Arteaga.

Instituto de Supercies y Catálisis, Facultad de Ingeniería, Universidad del Zulia, P. O. Box 15251,

Maracaibo 4003A, Venezuela.

Recibido: 15-01-18 Aceptado: 06-02-18

Abstract

Dehydrogenation of n-butane was studied over alumina- and silica-supported Pd and Pd-Sn catalysts with

0.5 wt. % of Pd. The reaction was carried out at 500 °C in both continuous ow and pulses systems. The catalysts

were prepared by incipient wetness impregnation. For the bimetallic catalysts, the sequential impregnation

method was used adding rst Sn and then Pd, with Sn/Pd atomic ratios of 0.1, 0.25 and 0.5. The samples

were characterized by XRF, UV-DRS, N2 adsorption, TPR, CO chemisorption and FTIR-CO techniques. On the

supported-alumina catalysts, Pd oxychloride were formed, which is highly resistant to the reduction, and it is

responsible for the high Pd dispersion. On the supported-silica catalysts, the addition of Sn modies both the

reducibility and adsorption capacity of Pd. The effects of Sn were mainly of geometric nature when supported

on alumina, and both geometric and electronic nature when supported on silica. On the supported-alumina

catalysts, the Sn addition increases the selectivity toward butenes, due to the high stabilization of Pd species;

while on the supported-silica catalysts, greater Pd-Sn interactions are evidenced, where the catalyst properties

are strongly inuenced at the lowest content of Sn added. The alumina–supported bimetallic catalysts showed

the best performance for n-butane dehydrogenation among the catalysts investigated herein.

Keywords: Palladium-tin catalysts; Sn/Pd ratio; FTIR-CO; n-butane dehydrogenation.

Efecto de la adición de estaño en catalizadores de paladio soportados sobre

alúmina y sílice para la deshidrogenación de n-butano

(Deshidrogenación de n-butano en catalizadores de paladio soportado)

Resumen

Se estudió la deshidrogenación del n-butano en catalizadores de Pd y Pd-Sn soportados sobre alúmina

y sílice, con una carga de Pd de 0,5 % p/p. La reacción se llevó a cabo a 500 ° C tanto en ujo continuo como

por pulsos. Los catalizadores fueron preparados mediante impregnación a humedad incipiente. Para los

catalizadores bimetálicos, se utilizó la impregnación secuencial, añadiendo primero Sn y luego Pd, para obtener

relaciones atómicas Sn/Pd nominales de 0,1, 0,25 y 0,5. Las muestras se caracterizaron mediante las técnicas

de XRF, UV-DRS, adsorción de N2, TPR, quimisorción de CO y FTIR-CO. En los catalizadores soportados sobre

alúmina, se formó un complejo de oxicloruro de Pd, que es altamente resistente a la reducción y es responsable

de la alta dispersión de Pd. En los catalizadores soportados sobre sílice, la adición de Sn modicó tanto la

reducibilidad como la capacidad de adsorción del Pd. Los efectos del Sn sobre el Pd fueron principalmente

del tipo geométrico en los catalizadores soportados sobre alúmina, y de naturaleza tanto geométrica como

electrónica en aquellos soportados sobre sílice. En los catalizadores soportados sobre alúmina, la adición de

Sn aumentó la selectividad, debido a la alta estabilidad de las especies de Pd; mientras que en los catalizadores

soportados sobre sílice, se evidenciaron mayores interacciones Pd-Sn, donde las propiedades de los catalizadores

se vieron fuertemente inuenciadas con el menor contenido de Sn adicionado. Los catalizadores bimetálicos

soportados sobre alúmina mostraron el mejor desempeño para la deshidrogenación de n-butano.

* Corresponding Author: Tel-fax.: +58 (261) 4128791/97. E-mail: jomendez@ng.luz.edu.ve.

Other E-mail addresses: dorodriguez@ng.luz.edu.ve, jlsanchez@ng.luz.edu.ve, garteaga@ng.luz.edu.ve

29Mendez et al.,/ Ciencia Vol. 26, Número Especial (2018) 28-41

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

Introduction

The importance of catalytic dehydrogenation of

light parafns to olens has been growing due to

the demand for these unsaturated hydrocarbons [1].

They are the starting material for some of the most

important chemical products, such as polymers,

synthetic rubbers and oxygenated compounds for

reformulated fuels. Unfortunately, light olens

are not available as a natural resource; they are

obtained in stream cracking furnaces and produced

in FCC (uid catalytic cracking) plants in reneries.

Catalytic dehydrogenation of light parafns (like

the widely available normal-butane) provides a

second way to obtain light olens, which in turn

can be transformed into more valuable chemicals,

including the methyl tertiary-butyl ether (MTBE),

ethyl tertiary-butyl ether (ETBE), and tertiary amyl

methyl ether (TAME) [1-3].

Nevertheless, the conversion of light parafns

to the corresponding olens is limited by

thermodynamics restrictions due to the strong

endothermic character of this reaction. High

temperatures (530–730 ºC) are required to reach

conversions of nearly 50% for parafns C2–C6. The

dehydrogenation temperatures are higher for light

parafns than for heavy ones [1].

Side reactions like isomerization, thermal

cracking and coking, which are thermodynamically

and kinetically favoured over dehydrogenation

reactions, can occur under operating conditions

(>530 ºC). Therefore, it is highly desirable a catalyst

with a suitable conversion and high selectivity

towards the desired olen, working at lower reaction

temperatures. Besides, the unavoidable formation

of coke on the catalyst surface that results in a

progressive reduction of catalytic activity, could be

minimized at these more benevolent conditions [2].

The use of weak acid supports in a catalytic system

is also essential to avoid side reactions, like cracking

and isomerization.

Many researchers have been devoted to study

the dehydrogenation of light parafns over Pt-based

catalysts, particularly on bimetallic Pt-Sn supported

catalyst [4-8], due to the benecial effects of tin

as a promoter: tin decreases the size of platinum

ensembles; reducing hydrogenolysis and coking

that require large ensembles of platinum, and/or

acts as electronic modier of Pt, thereby enhancing

the catalyst selectivity and stability. However, few

studies are related to the performance of these

reactions on supported Pd catalysts [1, 9-11].

Particularly, the Pd-Sn system has been studied for

nitrates removal [12, 13], methane oxidation [14, 15]

and selective hydrogenation reactions [16-18].

A catalytic system based on Pd can be an

alternative to the more expensive metals for

dehydrogenation reactions, such as Pt, Ru and Ir.

Studies carried out earlier [9] on the performance of

Pd-Sn/SiO2 catalysts for propane dehydrogenation,

revealed the effects of tin addition in low proportions

(Sn/Pd atomic ratio = 0.14-0.84), and an optimal in

both adsorption capacity of Pd and catalytic activity

was found. Rodriguez et al. [1] studied the n-butane

dehydrogenation over Pd-Ga/Al2O3 catalysts with

different atomic Ga/Pd ratios, and they found that

the effects of Ga were the same as those caused by

Sn for dehydrogenation in Pd-Sn/Al2O3 catalysts.

The aim of this work is to study the effect of Sn/

Pd ratio on the catalytic properties of supported Pd

catalysts for n-butane dehydrogenation reaction

in both continuous ow and pulses at a relatively

low temperature (500 ºC). Alumina and silica were

chosen as support due to its low acid strength, in

order to do not promote cracking reactions that

affect selectivity towards olens. Catalysts were

characterized by X-ray uorescence spectroscopy

(XRF), diffuse reectance UV-vis spectroscopy (UV-

DRS), N2 adsorption, temperature programmed

reduction (TPR), CO chemisorption and FTIR

spectroscopy of adsorbed CO (FTIR-CO).

Experimental

Catalysts preparation

Monometallic Pd and Sn samples were prepared

by incipient wetness impregnation of γ-Al2O3 (Alfa

Aesar, SBET = 210 m2/g, Vpore = 0.76 cm3/g) and

SiO2 (Cabosil M-5, Scintran, SBET = 224 m2/g),

using a rotavapor at 80 ºC, with an aqueous solution

of PdCl2 (Fisher Scientic, 99%) or SnCl4⋅5H2O

(Fisher Scientic, > 99%) in HCl, in order to obtain

Pd and Sn nominal contents of 0.5 and 0.56 wt.

%, respectively. Bimetallic Pd–Sn/Al2O3 and Pd–

Sn/SiO2 catalysts were prepared by sequential

impregnation, adding rst Sn and then Pd. Supports

were previously crushed and sieved (60/80 mesh),

and then calcined at 700 °C for 4 h. The nominal

contents of metals in bimetallic catalysts were 0.5

wt. % Pd and 0.056−0.28 wt. % Sn, (Sn/Pd atomic

ratios between 0.1 and 0.5). After impregnating, the

samples were dried at 120 °C overnight, and then

calcined at 500 °C for 2 h. Pd, Sn and Cl contents were

measured by energy dispersive X-ray uorescence

spectroscopy (XRF) using a Shimadzu EDX-700HS

spectrometer, provided with an Rh gun operated

at 50 keV. Al2O3 and SiO2 supports will be named

Al and Si, respectively. Table 1 summarizes the

notation and chemical composition of the catalysts.

Palabras Claves: Catalizadores de paladio-estaño; relación Sn/Pd; FTIR-CO; Deshidrogenación de

n-butano.

30 Effect of Sn addition in alumina- and silica-supported palladium catalysts...

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

Characterization techniques

BET surface areas of supports and catalysts

were determined from the N2 adsorption isotherms

at -196 ºC, using an automatic Micromeritics ASAP

2010 apparatus after the samples were outgassed at

150 ºC for 2 h. The specic surface area of selected

catalysts are also listed in Table 1.

The catalysts were studied by UV–vis diffuse

reectance spectroscopy (UV-DRS) at atmospheric

conditions, in a Perkin-Elmer spectrometer model

Lambda 2 equipped with a diffuse reectance

accessory, consisting of an integrating sphere of 50

mm coated with BaSO4 to collect the reected light.

The wavelength range varied from 190 to 700 nm.

TPR proles were measured from -50 to 700 °C

in a Vycor U-tube reactor connected to a stainless

steel apparatus provided with a thermal conductivity

detector (TCD), using a mixture of 5% H2/Ar as

reducing gas. Prior to measurements, the sample

(100 mg) was pretreated in air ow at 500 °C for 1 h,

then cooled down in owing Ar to room temperature

and, then, to -50 °C, using an isopropanol-liquid

nitrogen bath, before admitting the ow of reducing

gas. The reactor was freely heated from -50 to room

temperature (RT) by removing the cool bath, and

immediately heated up to 700 °C at 10 °C/min,

using a temperature-controlled furnace.

Dynamic CO chemisorption measurements were

carried out in the same stainless steel apparatus

used in TPR experiments, which was equipped with

a gas injection valve (Valco). Successive CO pulses

(58 µL STP) were sent to the reactor until reaching

the saturation, using He (30 mL/min) as carrier gas.

The catalysts (100 mg) were previously reduced at

400 °C in owing H2 for 1 h, and then the adsorbed

hydrogen was removed by ushing He at the same

temperature for 1 h.

Fourier transform infrared spectra of adsorbed

CO (FTIR-CO) were recorded at RT on a Shimadzu

IR Prestige 21 spectrometer, at a resolution of 2 cm-1

using around 100 scans. Self-supported samples

(20-24 mg/cm2) were put into an infrared cell

equipped with a KCl window, designed for treating

the samples in a controlled atmosphere. The samples

were submitted to the following pretreatment: (i)

drying in Ar ow at 120 °C for 1 h; (ii) reduction under

owing hydrogen at a programmed temperature

rate (10°C/min) from 120 to 400°C, plus 60 min at

400 °C; (iii) outgassing in Ar at 400°C for 30 min,

and nally cooled down in owing Ar from 400

°C to room temperature. Successive CO pulses in

a He ow were sent to the sample, registering the

IR spectrum elapsed 5 min after each injection, to

follow the formation of the absorption bands. To

obtain the IR spectrum corresponding to saturation,

a CO ow was used for 5 min, and then outgassing

in Ar for 15 min. The spectra presented here were

obtained after subtracting the gas phase and solid

contributions.

Catalytic activity test

Two different n-butane dehydrogenation tests

were carried out, one of them in a continuous

ow reactor and the other one in a system where

Table 1. Chemical composition, surface area and CO/Pd ratio

Catalysts Pd (wt. %) Sn (wt. %) Cl (wt.%) Sn/Pd atomic ratio S

BET

/g) CO/Pd

0.40

Al 0.40 - 0.70 0 - 0.56

0.55

0.045

Al 0.55 0.045 0.54 0.073 210 0.53

0.52

0.091

Al 0.52 0.091 0.58 0.16 - 0.58

0.54

0.19

Al 0.54 0.19 0.66 0.32 215 0.51

0.43

Al - 0.43 0.70 ∞ - -

0.34

Si 0.34 - n.d. 0 - 0.20

0.31

0.061

Si 0.31 0.061 n.d. 0.18 218 0.08

0.36

0.12

Si 0.36 0.12 n.d. 0.30 - 0.09

0.31

0.22

Si 0.31 0.22 n.d 0.64 218 0.06

0.51

Si - 0.51 n.d ∞ - -

n.d.: not detected

a: Based on the Pd loading values obtained by XRF.

31Mendez et al.,/ Ciencia Vol. 26, Número Especial (2018) 28-41

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

pulses of n-butane were sent to the catalyst. The

continuous ow experiment was performed at 500

°C and atmospheric pressure, in a conventional

reaction line built in stainless steel. The switching

of gases was done without mixing or access of air. A

Vycor U-tube (6 mm OD) reactor was charged with

20 mg of catalyst. Samples were pretreated in Ar ow

at 120 °C for 1 h and then reduced in H2 ow at 500

°C for 2 h. The reaction mixture contained n-C4H10

and H2, with a H2:n-C4H10 ratio of 10. A total ow

of 66 mL/min and a GHSV of 200 L·g-1·h-1 were

used. Taking into account that H2 is a by-product

of the dehydrogenation reaction, its presence as a

reagent would not be thermodynamically favorable.

However, it is very important to include H2 in the

feedstock in order to decrease the carbon formation

and hence to moderate the activity decrease along

the reaction time [19].

The pulse experiments were performed by

injecting pulses of the same hydrogen and n-butane

mixture, at the same conditions as the continuous

ow experiments. The catalytic bed was kept under

owing He (30 mL/min) between the injections

of successive pulses. The exit gases were analyzed

in a Perkin-Elmer GC, equipped with an Al2O3/

KCl 50 m x 0.25 mm capillary column and a ame

ionization detector (FID).

Specic activity (A), n-butane conversion and

selectivity were calculated on a carbon basis, using

the FID areas (Ai) and the corresponding response

factors (rfi), according to the following equations:

where Fn-c4 is the n-butane molar ow =

0.000245 mol/min; nPd= Pd moles in the sample

(based on XRF results)

Results and discussion

1. Catalysts characterization

Table 1 shows the values of Pd and Sn loadings

obtained by XRF. It is observed that catalysts

supported on alumina have a Pd loading near to the

nominal composition of 0.5 wt %, meanwhile the ones

supported on silica have lower values of Pd contents

(0.31-0.36 wt. %). This could be caused by the low

afnity of palladium precursor with silica, hindering

the anchoring of Pd anions (as PdCln(H2O)4-n2-n)

on this support [20]. According to Toebes et al. [21],

the nature of the interactions of the precursor metal

salt with a given support depends on the pH of the

solution, the zero point charge (ZPC) of the support,

and the nature of ions to be adsorbed. The ZPC

varies from 7.0 to 9.0 for γ-alumina, and 1.0 to 2.0

for silica [21]. Therefore, during the impregnation

with the Pd solution (PdCl2 in HCl 0.1 N, pH ≈ 1), the

surface of the alumina support becomes positively

charged, and tends to adsorb compensating anions

like PdCl42-, but the opposite occurs in the case of

silica, because of the pH value is in the range of its

ZPC. For the case of Sn content, the values were

slightly lower than the nominal (0.056 – 0.28 wt.

%) for all of the catalysts.

Chlorine contents were also determined by XRF.

Only the catalysts supported on alumina showed

signicant contents of Cl- ions, in the range of 0.54

- 0.70 wt. %, that increased in bimetallic catalysts, i.

e., the Cl content increased with the Sn loading, from

0.54 to 0.66%. This fact was expectable, as the Cl-

ion sources came from both the Pd and Sn solutions

in the preparation of the bimetallic catalysts.

The signicant Cl content found in the Al2O3-

supported catalysts after the calcination treatment

at 500 ºC for 2 hours, suggests that this anion was

initially well anchored to the surface of the alumina,

probably as a supercial complex of Pd and chlorine

(PdxOyClz) which has a high thermal stability [22].

Rodriguez et al. [1] concluded that this palladium

supercial complex was also formed during the

calcination of Pd-Ga/Al2O3 catalysts, considering

these species as mobile complexes that spread over

the alumina surface, which can improve the Pd

dispersion upon reduction.

The BET surface areas (SBET) of some catalysts

are also listed in Table 1. Regardless of the amount

of Pd and Sn added, the values of BET surface area of

the catalysts remain as the corresponding supports

(210 m2/g for alumina and 224 m2/g for silica).

Similar results were found for pore volumes (values

not shown). As expected, the textural properties of

the catalysts are almost the same as those of their

respective supports, due to small amounts of Pd and

Sn on the surface of the alumina and silica.

UV-DRS spectra of Pd-Sn catalysts supported

on alumina and silica are presented in Figures 1

and 2, respectively. It can be observed that samples

containing palladium (Fig. 1(a) – (d)) display similar

spectra, exhibiting bands at 214, 245, 275 and 425

nm.

32 Effect of Sn addition in alumina- and silica-supported palladium catalysts...

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

The band observed at 214 nm can be associated

to the charge transfer (Pd⋅ Cl) of PdCl2 [22],

whereas the strong band at 245 nm is assigned to

charge transfer Pd ⋅ O, and the band at 275 nm has

been attributed to PdxOyClz surface complex [22].

The band at 425 nm is related to d-d transition in

PdO species interacting strongly with the alumina

support. It is noteworthy that the unsupported

PdO phase, exhibits a signal around 490 nm, which

was not observed in the spectra shown in Fig. 1.

Furthermore, the UV spectrum of the Sn0.43Al

sample (Fig. 1e) shows a broad band in the range

225-300 nm. This broad signal is also assigned to

the charge transfer Sn ⋅ O [22]. In summary, these

results lead to the existence of a Pd and Cl phase on

the surface of the support, which can be strongly

stabilized.

Figure. 1. UV–DRS spectra for Al2O3-supported

catalysts: (a) Pd0.40Al; (b) Pd0.55Sn0.045Al;

Sn0.43Al; and (f) Al.

On the other hand, samples supported on

silica show UV-DRS spectra different from those

observed for the ones supported on alumina. UV-

DRS spectrum of Pd0.34Si (Fig. 2a) presented

a wide band in the d-d transition region around

490 nm, related to the electronic structure of PdO

crystals in supported Pd catalysts [23]. The other

signals in the charge transfer region at 250-300 nm

are contributions of the silica. In the spectra of the

bimetallic Pd0.31Sn0.061Si and Pd0.31Sn0.22Si

catalysts (Fig. 2(b) – (c)), similar bands as those in

the monometallic Pd0.34Si catalyst can be observed,

which indicates the presence of PdO phase without

any interaction with the silica support. Finally,

the signals at 200-350 nm exhibited for Sn0.51Si

sample (Fig. 2d), are ascribed to the Sn⋅O charge

transfer and silica support.

Figure. 2. UV–DRS spectra for SiO2-supported

catalysts: (a) Pd0.34Si; (b) Pd0.55Sn0.061Si; (c)

Pd0.31Sn0.22Si; (d) Sn0.51Si; and (e) Si.

The TPR proles of the catalysts supported on

alumina are shown in Figure 3. The monometallic

and bimetallic Pd-Sn catalysts show very similar

reduction proles (Figs. 3a-3d) with low intensity

signals at 19, 50-80, and 300 ° C, and a wide and

extensive signal in the temperature range of 80-260

ºC. The peaks at 19 and 50-80 ºC can be assigned

to the reduction of crystalline PdO with different

sizes, where the largest particles are reduced at

lower temperatures [1, 11, 24]. The signal with the

great broadness (80-260 ºC) has been associated

to the reduction of different species of PdxOyClz

[1, 11], while the peak at 300 ºC is attributed to the

reduction of PdO particles interacting strongly with

the support [25].

Figure3. TPR proles for Al2O3-supported

catalysts: (a) Pd0.40Al; (b) Pd0.55Sn0.045Al; (c)

Pd0.52Sn0.091Al; (d) Pd0.54Sn0.19Al and (e)

Sn0.43Al.

33Mendez et al.,/ Ciencia Vol. 26, Número Especial (2018) 28-41

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

For Sn0.43Al sample, a broad reduction signal

between 300 and 650 °C is observed. This signal

could be attributed to reduction of Sn4+ to Sn2+

and/or Sn0 [4, 26, 27], so that it indicates some

resistance to reduction.

TPR proles of the catalysts supported on silica

are presented in Figure 4. For bimetallic catalysts

(Figs. 4b-4d), the reduction peaks observed at

low temperatures (< 100 °C), correspond to the

reduction of large PdO particles [1, 11, 24], in

contrast to that observed in the catalysts supported

on alumina. For Pd0.34Si (Fig. 4a), the reduction

peaks observed at 21 and 38 °C, could be associated

to the reduction of PdO particles with different sizes.

In addition, the negative signal at 60 °C is ascribed to

the decomposition of β-hydride Pd phase (β-PdHx),

whose presence indicates the existence of large

palladium particles [24].

Figure 4. TPR proles for SiO2-supported

catalysts: (a) Pd0.34Si; (b) Pd0.31Sn0.061Si;

Sn0.51Si (insert)

Some changes occurred in the reducibility of

these catalysts when increasing the Sn/Pd ratio.

For the Pd0.31Sn0.061Si catalyst, the TPR prole

shows signals at 33 and 44 °C, followed by the

decomposition of β-hydride Pd phase (PdHx) at 59

°C. For the Pd0.36Sn0.12Si and Pd0.31Sn0.22Si

catalysts (Fig. 4(c) – (d)), similar reduction signals

to those obtained in the monometallic sample are

observed, and can also be attributed to the reduction

of PdO particles and decomposition of palladium

hydride. However, in these bimetallic samples

with the highest Sn/Pd ratio, the negative signal

decreases in intensity, probably due to a geometric

effect of Sn on palladium ensembles, hindering the

β-PdHx phase, either by the formation of bimetallic

Pd-Sn particles [28] or by the dilution of Pd particles

by Sn atoms [29].

Other signals centered at 240-250 ºC observed

for Pd0.36Sn0.12Si and Pd0.31Sn0.22Si catalysts,

can be ascribed to the reduction of Sn4+ catalytically

assisted by Pd0, thus indicating a close interaction

between Pd and Sn in the bimetallic catalysts. As

can be seen in the insert in Fig. 4, the reduction of

Sn4+ occurred at higher temperatures (around 464

ºC) in the Sn0.51Si sample [27].

The values of CO/Pd ratio obtained from

chemisorption measurements are also listed in Table

1. For catalysts supported on alumina, the values of

CO/Pd ratio remained in the range of 0.51-0.58,

despite of the presence of tin. These results display

a high Pd dispersion, probably as a result of the

formation of PdxOyClz, promoted by the presence

of Cl [29]. Pd species were strongly stabilized, which

is in agreement with UV-DRS (Fig. 1) and TPR (Fig.

3) results. On the other hand, the high values of the

CO/Pd ratio corresponded with that obtained by

Daley et al. [30], who reported a high Pd dispersion

(ca. 50%) when the catalysts were subjected to

chlorination treatment, and it was ascribed to the

interactions between hydroxyl groups of the support

and anions of precursor salt.

For the catalysts supported on silica, the values

of CO/Pd ratio were signicantly lower. In this case,

they were highly dependent on the presence of Sn.

For the monometallic Pd0.34Si catalyst, a CO/

Pd ratio of 0.20 was obtained, decreasing with the

content of tin to 0.06-0.09 for the bimetallic Pd-Sn/

SiO2 catalysts.

The decrease in CO chemisorption with the

presence of tin can be attributed to a possible

blocking and/or substitution of Pd atoms by Sn

and/or the formation of alloys at high tin content

[29]. Moreover, the Sn atoms could also cause a

weakening of the strength of CO adsorption on Pd.

Changes in the stoichiometry of CO adsorption on

Pd due to the Sn addition, considering the different

forms of adsorbed CO, has been reported by Sales

et al. [29].

FTIR-CO spectra are presented in Figures 5-8.

The results were expressed as a function of CO

coverage on Pd (θCO). For the monometallic catalyst

Pd0.40Al (Fig. 5b), four bands were observed at

2155, 2125, 1990 and 1945 cm-1. The intensity of

these bands progressively increased with θCO,

and another band arose at 2085 cm-1 when the

saturation was reached (Fig. 5f). Besides, the band

at 1990 cm-1 shifted to lower frequency (1980 cm-1).

34 Effect of Sn addition in alumina- and silica-supported palladium catalysts...

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

Figure 5. FTIR spectra of CO adsorbed for the

Pd0.40Al catalyst at increasing CO coverages: (a)

θ=0, (b) θ=0.20, (c) θ=0.27, (d) θ=0.31, (e) θ=0.36

y (f) θ=1.

CO adsorption on supported Pd catalysts depends

greatly on the Pd oxidation state [31], and the

location of Pd atoms in different Pd crystal planes

(111, 100, 110), with different coordination numbers

(9, 8, and 7, respectively) [29]. Hence, the band at

2155 cm-1 is attributed to the interaction of CO with

residual surface Pd2+ ions (linear carbonyls Pd2+ -

CO), which are stabilized by neighbouring chloride

ions, present in all supported alumina catalysts

after calcination (Table 1), and were not removed by

the reduction treatment (400 ºC for 60 min under

H2). The band at 2125 cm-1 was attributed to the

linear CO adsorption on Pd+ ions [31], which are

also stabilized by neighbouring chloride ions. The

weak band at 2085 cm-1 is ascribed to linear CO

species adsorbed on metallic Pd [32]. The rest of the

bands located at 1980 and 1945 cm-1 are ascribed to

CO bridged species located on different Pd crystal

planes and/or submitted to dipole–dipole coupling

of different intensities. Therefore, the CO:Pd

stoichiometry ratio can be far from 1.

In the bimetallic Pd-Sn/Al2O3 catalysts, the

spectra exhibited the same bands observed for the

monometallic one (2155, 2125, 2085, 1990 and 1945

cm-1), but slightly shifted to lower wavenumbers.

Thereby, only the infrared spectra obtained for

the bimetallic Pd0.55Sn0.045Al catalyst (Sn/Pd =

0.073) are shown (Fig. 6).

Figure 6. FTIR spectra of CO adsorbed for

the Pd0.55Sn0.045Al catalyst at increasing CO

coverages: (a) θ=0, (b) θ=0.19, (c) θ=0.25, (d)

θ=0.30, (e) θ=0.36 y (f) θ=1.

The assignments of the IR bands for the Al2O3-

supported catalysts, indicate that a fraction of Pd

species remained in oxidized state, as Pd+ and/or

Pd2+, even after reduction at 400 °C in H2 for 1 h,

while the rest of Pd is completely reduced to Pd0.

It is worth noticing in the FTIR-CO spectra in Figs.

5 and 6 that the intensity of the band in the region

35Mendez et al.,/ Ciencia Vol. 26, Número Especial (2018) 28-41

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

of linear carbonyls is greater than bridged ones, at

lower θCO (Figs. 5b and 6b), i. e., CO is preferentially

adsorbed on Pd sites stabilized with Cl- ions. As θCO

increased, Pd+ and Pd2+ were saturated, allowing

to increase CO adsorption on Pd0 atoms.

For the silica-supported catalysts, FTIR-CO

spectra of monometallic and bimetallic catalyst

are shown in Figures 7 and 8, respectively. For the

Pd034Si catalyst, bands are observed initially at

2087 and 1905 cm-1 (Fig. 7b). As θCO increased,

a band appeared at 1990 cm-1, and then the bands

at 1905 and 1990 cm-1 become shoulders, which

are overlapped with a new band at 1975 cm-1 when

saturation was reached. The band at 2087 cm-1 is

assigned to the linear adsorption of CO on metallic

Pd [32], while the bands located between 1990 and

1905 cm-1 are ascribed to bridged CO species, also

over metallic Pd [29, 32]. According to the spectrum

in Fig. 7f, there is a higher proportion of bridged CO

(COB) than linear CO (COL) species, resulting in a

value of COL/COB ratio of 0.30 after integration of

the signals, which indicates the largest Pd ensembles

in this sample.

Figure 7. FTIR spectra of CO adsorbed for the

Pd0.34Si catalyst at increasing CO coverages: (a)

θ=0, (b) θ=0.23, (c) θ=0.43, (d) θ=0.67, (e) θ=0.72

y (f) θ=1.

However, for the bimetallic catalysts (Fig. 8),

the COL/COB ratio increased from 0.30 to 0.68.

This is explained by a dilution of Pd by Sn on the

catalyst surface layer, and an ensemble effect should

account for the preferential suppression of bridged

CO species which requires the presence of adjacent

Pd atoms [29, 32]. Nevertheless, the presence of

tin seems to weaken the CO adsorption strength, as

linear entities are less rmly bonded than bridged

ones.

Figure 8. FTIR spectra of CO adsorbed for

the samples after saturation (θCO=1) with owing

CO: (a) Pd0.34Si; (b) Pd0.31Sn0.061Si; (c)

Pd0.36Sn0.12Si and (d) Pd0.31Sn0.22Si.

On the other hand, the maxima of the signals in

FTIR-CO spectra tend to change progressively with

the Sn/Pd atomic ratio, i.e., peaks initially located at

2087 and 1975 cm-1 for the monometallic Pd0.34Si,

shifted to 2080 and 1990 cm-1, respectively, for

the bimetallic Pd-Sn/SiO2 catalysts. Therefore,

the presence of tin can be affecting the electronic

properties of palladium in these silica-supported

catalysts [32].

2. Catalytic activity measurements.

2.1 Continuous ow reactor

The measurements were carried out at 500 ºC in

a differential ow reactor (n-butane conversion less

than 10%), and the results were expressed as moles

of n-butane converted per mole of Pd per minute,

i.e., specic activity (min-1), as a function of time

on-stream (TOS). It is known that dehydrogenation

36 Effect of Sn addition in alumina- and silica-supported palladium catalysts...

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

reactions are metal-catalyzed non-demanding

reactions [33]. On the other hand, it is important

to indicate that the Sn0.43Al and Sn0.51Si catalysts

did not exhibit catalytic activity for the n-butane

dehydrogenation under the reaction conditions.

The specic activity of the Al2O3-supported

catalysts as a function of TOS is shown in Fig. 9.

The Pd0.40Al catalyst displayed an initial specic

activity of 44 min-1, suffering a fast deactivation

in the rst 15 min of TOS, and then the activity

decreased progressively along the reaction time.

Deactivation was also observed in the rest of the

catalysts, probably as a result of coke deposition on

the catalyst surface [19]. At these reaction conditions,

the formation of coke would build up on both the

metal and support sites [34, 35]. However, at the

beginning of the reaction, rst coke is deposited

on Pd atoms of high coordination number (cluster

or ensembles) and, then, coke blocks the Pd atoms

of low coordination number like corners or edges.

Therefore, low coordinated Pd palladium atoms are

more resistant to deactivation by coke [35].

Figure 9. Specic activity at 500 ºC as a

function of TOS for the Al2O3-supported catalysts.

The initial activity and deactivation rate

decreased upon Sn addition, in good agreement

with that observed by other researchers in Pt-Sn [4]

and Pd-Sn catalysts [10]. This deactivation occurred

mainly in the rst 15 minutes of TOS, and the catalyst

with the highest Sn content (Pd0.54Sn0.19Al, Sn/

Pd = 0.32) displayed the smallest activity. These

effects of the Sn addition on the catalytic behavior of

Pd supported on alumina can be explained mainly in

terms of geometric effects, as the majority of Sn in

the bimetallic catalysts blocked the Pd atoms of high

coordination number. Besides, the FTIR-CO results

(Figs. 5 and 6) show the lack of any electronic effect

of tin on palladium, probably because the electronic

properties of palladium were already modied by

the presence of chlorine.

Fig. 10 shows the specic activity for n-butane

dehydrogenation on SiO2-supported catalysts.

Specic activities of this catalysts were lower than

those obtained in alumina supported catalysts

(Fig. 9). The Pd0.34Si catalyst exhibited an initial

activity of 31 min-1, then the activity decreased

monotonically with TOS. The addition of a small

amount of Sn (Pd0.31Sn0.061Si) decreased the

initial activity, but after 15 min of TOS, this catalyst

showed the same activity as the monometallic one.

Figure 10. Specic activity at 500 ºC as a

function of TOS for the SiO2-supported catalysts.

The addition of a greater amount of Sn (Sn/Pd=

0.30 and 0.64) decreased the catalytic activity from

initial values of 23 and 16 min-1, respectively, to

nal values around 3 min-1. This decrease can be

attributed to both geometric and electronic effects of

Sn on Pd in concordance with CO chemisorption and

FTIR-CO results. Moreover, TPR results suggested

that Pd and Sn were in intimate contact on this

support, due to the possible formation of bimetallic

Pd-Sn particles after reduction. In the silica-

supported Pd catalysts, predominates the existence

of high coordination Pd atoms, indicated by the low

Pd dispersion (CO measurements). Furthermore, all

of the silica-supported Pd catalysts were deactivated

at the same extent in spite of the presence of Sn (Fig.

10).

Table 2 compiles the selectivity to all butenes

(initial: SD,0 and nal: SD,f), selectivity to

isomerization (initial: SI,0 and nal: SI,f) and the

deactivation parameter ∆R/R0, where ∆R is the

difference between the nal and initial reaction

rates (R0 and Rf, respectively).

37Mendez et al.,/ Ciencia Vol. 26, Número Especial (2018) 28-41

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at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

The addition of Sn decreased the ∆R/R0 values

only in the Al2O3-supported Pd catalysts, and this

decrease seems to be determined by the minimum

quantity of the promoter added. For the Al2O3-

supported catalysts, the deactivation parameter

values are similar to that obtained by Ballarini et

al. [19] and Bocanegra et al. [36] in monometalic

Pt(0.5%) and bimetallic Pt(0.5%)Sn(0.3%) catalysts

supported on γ-Al2O3/α-Al2O3 and MgAl2O4,

respectively. In the case of SiO2-supported catalysts,

the surface modication caused by Sn affected

structure-sensitive reactions, such as hydrogenolysis

rather than coking.

The Sn addition increased the selectivity toward

butenes in both Al2O3- and SiO2-supported

catalysts. For the Pd-Sn/Al2O3 catalysts, the

selectivity increased with the Sn content and the

time on-stream, achieving a selectivity of 94% for

the Pd0.54Sn0.19Al catalysts at 180 min of TOS;

probably Sn and coke blocked the most active

Pd sites, responsible for the reactions leading to

hydrogenolysis. Pd in the PdxClyOx phase appears

to be the most active species, because the electronic

properties of Pd were modied by means of charge

transfer Pd⋅Cl [22]. It should be pointed out that

an electronic effect of Sn on Pd was not observed for

this series of catalyst. In addition to dehydrogenation

and hydrogenolysis products, isobutene was also

observed: an isomerization selectivity between 2-4

% was registered in all Pd-Sn/Al2O3 catalysts [37].

For the silica-supported catalysts, higher

selectivity values toward butenes were observed. The

monometallic Pd0.34Si exhibited dehydrogenation

selectivity of 77 % at the beginning (1 min) and 98%

at the end (180 min of TOS) of the experiment. These

results indicate that Pd in metallic state (Pd0), as in

the silica support, exhibits high selectivity toward

dehydrogenation. In all Pd-Sn/SiO2 catalysts, the

dehydrogenation selectivity was 100 %. The SiO2-

supported catalyst with the lowest Sn loading (Sn/

Pd = 0.18) showed the best performance of these

catalysts, considering both the selectivity toward

butenes and the dehydrogenation activity. It is

worth mentioning that isobutene was not observed

as a product, probably due to the low acidity

of the support, that could not catalyse skeletal

isomerization reactions.

2.2 Pulse experiments.

In order to elucidate the catalytic behavior

for n-butane dehydrogenation over the different

mono and bimetallic supported catalysts during

the rst steps of reaction, a study of the catalytic

performance was carried out by sending pulses of

the H2/n-butane reactant mixture to the reactor.

The n-butane conversion for the indicated

Al2O3- and SiO2-supported Pd-Sn catalysts as a

function of the number of H2/n-butane pulses,

are shown in Figure 11. For the Al2O3-supported

catalysts, it can be seen that the Pd0.40Al sample

showed a conversion of 49% in the 1st pulse and

43% in the 12th pulse, while the Pd0.55Sn0.045Al

and Pd0.54Sn0.19Al catalysts showed higher

conversions of 64-68% in the 1st pulse and 46-47%

in the 12th pulse. The lower conversion obtained on

the Pd0.40Al catalyst is due to the smaller amount of

Pd atoms exposed on the surface, considering both

the lower Pd loading and the similar Pd dispersion

(Table 1). The conversion for the bimetallic catalysts

supported on alumina are in agreement to CO

chemisorption results (Table 1), where the CO/Pd

ratio was unaffected by the Sn content. Therefore,

conversion values observed in Figure 11 conrm

that Pd active sites were unaffected by Sn. Also, the

conversion decreases with pulse number, due to the

Table 2. Initial and nal selectivity for dehydrogenation (SD,0, SD,f), selectivity for isomerization (SI,0,

SI,f) and deactivation parameter (∆R/R0; ∆R: initial reaction rate [R0]−nal reaction rate [Rf]) in the

continuous ow reactor at 500 ºC.

Catalyst S

D,0

D,f

I,0

I,f

∆R/R

(%)

0.40

Al 50 89 3 4 40

0.55

0.045

Al 44 83 3 4 17

0.52

0.091

Al 63 92 3 3 25

0.54

0.19

Al 68 94 2 2 20

0.34

Si 77 98 - - 51

0.31

0.061

Si 100 100 - - 48

0.36

0.12

Si 100 100 - - 72

0.31

0.22

Si 100 100 - - 71

SD,0, SI,0: measured at 1 min of the reaction beginning. SD,f, SI,f: measured at 180 min of time of stream.

R0: measured at 15 min of the reaction beginning. Rf: measured at 180 min of time of stream.

38 Effect of Sn addition in alumina- and silica-supported palladium catalysts...

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

signicant coke deposition at this stage. This fact

was also observed by Pirngruber et al. [37] in their

studies of n-butane dehydroisomerization.

Figure 11. n-butane conversion at 500 ºC as

a function of the pulse number for the indicated

catalysts.

In the case of the catalysts supported on silica,

the Pd0.34Si sample showed a conversion of 14%

in the 1st pulse, and then gradually decreases

with subsequent pulses, until reaching about 8%

in the 12th pulse. For the Pd0.31Sn0.061Si and

Pd0.31Sn0.22Si catalysts, lower conversion values

were obtained, ranging between 3-6% along the

twelve pulses sent to the reactor. In this case, the

n-butane conversion decreased with the Sn loading

(or Sn/Pd atomic ratio). Similar to the continuous

ow experiments, the catalytic activity of the SiO2-

supported catalysts were lower than the Al2O3-

supported ones, but it should be taken in account that

both Pd loading and dispersion were lower in these

catalysts (Pd = 0.31-0.34% and CO/Pd = 0.06-0.20,

respectively). The decrease in n-butane conversion

with the amount of Sn is in agreement with CO/Pd

ratio. Nevertheless, the lower deactivation degree

in the n-butane dehydrogenation with the pulse

number indicates a lower coke deposition rate in

these catalysts.

The initial and nal selectivity for the n-butane

dehydrogenation in pulse experiments are listed

in Table 3. For the Al2O3-supported catalysts, was

observed a high selectivity for hydrogenolysis in all

of the pulses of reactant mixture sent to the reactor,

while the selectivity toward butenes were unaffected

by Sn content, ranging from 2-4% in the 1st pulse

and 5-8% in the 12th pulse, which means that, as

mentioned above, in the course of the reaction coke

deposition affected the hydrogenolysis reactions.

On the other hand, it is worth noticing that the

deactivation parameter increased in the bimetallic

catalysts, from 12 to 28-31%, but this change was

attributed to a higher amount of Pd active sites

available for the reaction (and, thus, the higher coke

formation rate) rather than the presence of Sn.

Table 3. Initial

and nal

selectivity for dehydrogenation (SD,0, SD,f), selectivity for hydrogenolysis

(SH,0, SH,f) and deactivation parameter (∆X/X0; ∆X: initial conversion [X0]−nal conversion [Xf]) in

the pulse experiments at 500 ºC.

Catalyst S

D,0

D,f

H,0

H,f

∆X/X

(%)

0.40

Al 4 8 96 92 12

0.55

0.045

Al 2 5 98 95 28

0.54

0.19

Al 2 6 98 94 31

0.34

Si 19 29 81 71 43

0.31

0.061

Si 72 90 28 10 31

0.31

0.22

Si 78 89 22 11 19

a: measured at 1st pulse of the reactant mixture.

b: measured at 12th pulse of the reactant mixture.

For the catalysts supported on silica, all of

them exhibited higher selectivity toward butenes,

especially the bimetallic ones. The monometallic

catalyst showed a selectivity to dehydrogenation

between 19-29 %, meanwhile selectivity values were

from 72% to 90% for the bimetallic catalysts. As it

was expected from previous results (Table 1, Fig. 10),

deactivation parameter decreased with Sn content

in these catalysts, from 43 to 19 %, as an evidence of

Pd-Sn interactions on the surface of the silica.

The differences in the selectivity between the

catalysts supported on alumina and those supported

on silica, point out the support´s role in Pd active

sites. From results obtained in the pulse experiments,

it can be concluded that coke deposition rate was

lower in the catalysts supported on silica, and the

39Mendez et al.,/ Ciencia Vol. 26, Número Especial (2018) 28-41

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at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

selectivity toward butenes increased with the Sn

addition. These results are explained by a geometric

and electronic effects of Sn on Pd, through the

dilution of Pd ensembles and the change in the Pd

nature upon Sn addition.

From results discussed above, it could be proposed

a model of the arrangement of metal particles on

the surface of the alumina and silica, in bimetallic

catalysts. On alumina, Pd atoms are highly stabilized

by chlorine species, which increased d-state

vacancies due to a charge transfer from palladium to

chlorine, as it was mentioned in the UV-DRS results

of these solids (Figure 1), which in turn hinder the

Pd-Sn interactions. The representation of these

particles on the surface of alumina is schematized in

Fig. 12, where two groups of Pd sites predominate:

“sites A” representing the Pd particles interacting

with Cl- by means of PdxOyClz surface complex, and

“sites B” representing the Pd particles interacting

with Sn. Most of the Pd is included in “site A”, which

explains why similar characterization results were

obtained (UV-DRS, TPR, CO chemisorption, FTIR-

CO). Also, these Pd species are the most active in

the n-butane dehydrogenation, and the strength

of n-butane adsorption on Pd is high enough to

favor the rupture of C-C (cracking) rather than C-H

(dehydrogenation) bonds [9]. Instead, on “sites B” Pd

particles are free of Cl, allowing the interaction with

Sn, and represent the minority of Pd. On this type

of sites, the hydrocarbon adsorption is weakened,

hindering C-C bond ruptures, and favoring the

dehydrogenation reactions toward butenes. During

the n-butane dehydrogenation in continuous ow

reactor, “sites A” were blocked by coke deposition

at the beginning (rst 15 min of TOS) and, then, the

reaction proceeded on “sites B”, where differences

in catalytic performance of the Al2O3-supported

catalysts upon Sn addition were observed. This

statement is supported by FTIR-CO results, which

indicated that CO is preferentially absorbed on Pd+/

Pd2+ species.

Figure 12. Schematic representation of the

distribution of metal particles on the surface of the

supports.

On silica, only the “sites B” predominates, as the

characterization results (XRF, UV-DRS, TPR and

FTIR-CO) showed the absence of any chlorinated

species, and the existence of large Pd ensembles,

which showed lower activity and higher selectivity

toward butenes and coking. Therefore, it is proposed

that both geometry and electronic effects of Sn on

Pd are involved in the catalysts supported on silica.

The results obtained herein showed a

commitment in the activity-selectivity relationship

for the n-butane dehydrogenation. In a continuous

ow reactor, Al2O3-supported catalysts displayed

better activity and stability for TOS higher than 15

min, with a selectivity toward butenes up to 94% for

the Pd0.54Sn0.19Al catalyst. In the case of the SiO2-

supported catalysts, all the samples displayed a high

dehydrogenation selectivity (100%), but low activity

and the worse stability on TOS.

Conclusions

The Pd species present in the Pd and Pd-Sn

supported catalysts depends on the nature of the

support. In this sense, when supported on alumina,

palladium exists primarily in two forms: (i) as

Pd oxychloride (PdxClyOx), where the electronic

properties of the metal are modied by the presence

of chlorine, by means of a charge transfer Pd⋅Cl.

This phase is highly resistant to reduction, as some of

the palladium species remained as Pd+/Pd2+ even

after reduction at 400ºC for 1 h. Besides, this Pd-

Cl interaction leads to a high metal dispersion and

high activity in the n-butane dehydrogenation, but

poor selectivity towards butenes; (ii) as Pd metallic

(Pd0), where a Pd-Sn interaction can be involved,

and a higher selectivity toward butenes can be

obtained. Over silica, only Pd0 exists after reduction,

with a very low dispersion and activity. The Al2O3-

supported bimetallic (Pd-Sn) catalysts showed a

better behavior for n-butane dehydrogenation than

the SiO2-supported ones, considering both the

activity and stability. Also, high selective toward

butenes was observed for the Pd0.54Sn0.19Al

catalysts (Sn/Pd = 0.32). These catalysts showed

the best performance for n-butane dehydrogenation

between all catalysts investigated here.

In spite of the poor performance showed by the

SiO2-supported catalysts, a better Pd-Sn interaction

was evidence in these catalysts. The performance

of the Al2O3-supported catalysts tested herein

for n-butane dehydrogenation can be compared

with the Pt-base catalysts, which encourages to

further studies of the supported Pd catalysts for this

reaction.

Acknowledgements

This research was partially supported by the

Consejo de Desarrollo Cientíco y Humanístico of

the Universidad del Zulia (CONDES), Venezuela,

40 Effect of Sn addition in alumina- and silica-supported palladium catalysts...

Scientic Journal from the Experimental Faculty of Sciences,

at the Universidad del Zulia Volume 26 Especial N° 1, 2, Enero - Junio 2018

under the Projects CC-0145-08 and CC-0090-12.

The authors also thanks to Dr. Alberto Campos,

of the Laboratorio de Desarrollo de Métodos de

Análisis, of the Facultad Experimental de Ciencias

de la Universidad del Zulia, by his assistance in the

chemical analysis of the catalysts.

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