Performance of Pt/CeO2 catalyst for propane oxidative steam reforming

Lidia Pino *, Antonio Vita, Francesco Cipitı, Massimo Lagana, Vincenzo Recupero

Institute CNR-ITAE, Via Salita S. Lucia sopra Contesse n. 5, 98126 S. Lucia, Messina, Italy

Received 12 December 2005; received in revised form 7 March 2006; accepted 9 March 2006

Available online 18 April 2006

Abstract

Catalytic autothermal reforming of hydrocarbons to hydrogen rich reformate represents an important step in fuel processor development for fuel

cell systems. As a part of a larger research program aimed at developing and testing of such system based on autothermal reforming of light

hydrocarbons, like liquefied petroleum gas (LPG), to be integrated with polymer electrolyte fuel cells (PEM), we investigated the C3H8 oxidative

steam reforming (OSR) over a Pt/CeO2 catalyst prepared by combustion synthesis technique. The catalytic activity, as a function of O2/C3H8, H2O/

C3H8 molar ratios in the feed, reaction temperature and gas hourly space velocity, was evaluated. The experimental tests were used to define the

operating parameters that can assure total fuel conversion and high H2 selectivity, basic elements for the realization of a fuel processor.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Propane oxidative steam reforming; Hydrogen production; Ceria-supported platinum catalyst

www.elsevier.com/locate/apcata

Applied Catalysis A: General 306 (2006) 68–77

1. Introduction

Today, H2 emerges as a clean energy carrier that offers a

valid alternative to fossil fuels, major sources of greenhouse gas

emissions. Fuel cell systems seem to be an answer to this

environmental problem and represent a viable alternative for

clean energy generation.

The wide applications of fuel cell technology, ranging from

portable/micro power, transportation and/or stationary power,

makes pressing the development of fuel processors which are

able to provide a clean, tailored synthesis gas (H2 + CO) to the

fuel cell stack [1,2]. Steam reforming (SR), partial oxidation

(POX), and autothermal reforming (ATR) of fuels, hydro-

carbons or alcohols, are the most suitable processes for H2

production. Catalytic steam reforming, involving the reaction

of steam with fuel is endothermic and requires a large energy

input, that can be considered to be the major drawback of the

process. Catalytic partial oxidation is a more rapid process with

a much higher reaction rate than steam reforming, needs

external cooling and the H2 yield/Cfuel (hydrocarbons) is lower

[3,4].

Autothermal reforming, by combining the exothermic

(partial oxidation) and endothermic (steam reforming) reac-

* Corresponding author.

E-mail address: lidia.pino@itae.cnr.it (L. Pino).

0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2006.03.031

tions, to maintain the required temperature without external

heating, can be considered a better option for converting

hydrocarbons to H2. Temperature, pressure, O2/fuel and H2O/

fuel ratios are independent parameters that determine the

performance of the ATR process. Generally, these parameters

can be selected in order to optimize the performance, in terms

of H2 yield, to avoid coke formation on catalyst or to minimize

the CH4 production [5,6].

ATR catalyst must be active for both steam reforming and

partial oxidation of the selected fuel: various transition metals

(Ni, Co, Fe) or noble metals (Pt, Rh, Pd) supported on oxide

supports, are standard catalyst formulations for autothermal

reforming of hydrocarbons [7,8]. Bimetallic Pt–Ni/d-Al2O3,

with superior performance characteristics compared to mono-

metallic catalysts, has been reported by Caglayan et al. [9].

Many attempts have been recently carried out in these catalyst

formulations to prevent carbon deposition, catalyst deactivation

from sulphur poisoning and in the development of materials

with high thermal and mechanical stability. In this respect, new

catalysts including metal supported on oxide-ion-conducting

substrate such as ceria, zirconia or lanthanum gallate doped

with non-reducible element such as gadolinium, samarium have

been developed at the Argonne National Laboratory [10]. High

reforming activity with lanthana-based perovskites (ABO3),

doped on A- and B-sites ðAxA01�xByB01�yO3�dÞ to improve

catalytic activity and structural stability in reducing and

oxidizing environments, has been reported [11,12].

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–77 69

Recently, ceria as a support was applied to Pt catalyst for

POX of methane: good activity and stability during 100 h of

reaction under operative condition of a demonstrative hydrogen

generation unit were reported [13]. Based on the previous

results, in the present work, 1.13 wt.% Pt/CeO2 has been

applied to the reaction of C3H8 with O2 in the presence of

steam, referred as oxidative steam reforming (OSR). The

influence of O2/C3H8 and H2O/C3H8 molar ratios in the feed,

reaction temperatures and gas hourly space velocity (GHSV) on

catalytic activity has been evaluated.

2. Experimental

Supported 1.13 wt.% Pt/CeO2 catalyst was prepared by

oxalyldihydrazide–nitrate self-combustion synthesis. The

combustion mixture contained ceric ammonium nitrate

[(NH4)2Ce(NO3)6], chloroplatinic acid (H2PtCl6) and oxalyl-

dihydrazide (C2H6N4O2) as fuel (molar ratios 0.99:0.01:2.33)

was dissolved in a minimum volume of water in a borosilicate

dish of 130 cm3 capacity. The dish with this redox mixture was

then introduced into a muffle furnace preheated at 350 8C. The

solution boiled with frothing and foaming with concomitant

dehydration. At the point of its complete dehydration, the fuel

ignites the redox mixture with a flame temperature of ca.

1000 8C, yielding a voluminous finely dispersed solid product

within about 5 min. The catalyst, pelletized, calcined in air at

800 8C, crushed and sieved to 200–600 mm was tested without

pre-reduction. Pre-treatment is not required since, the strong Pt/

CeO2 interaction that, in the conventionally prepared catalysts

(i.e. impregnated catalyst) was induced by H2 treatment, in the

current catalyst is reached during synthesis [13,14].

Powder X-ray diffraction patterns were done with a Philips

X-Pert 3710 diffractometer, using Cu Ka radiation at 40 kV

and 20 mA. Continuous scans were collected with scan rates of

0.6 and 0.068/min in the 2u ranges 15–758 and 35–708,respectively. Peaks’ positions, widths and lattice parameter

were resolved by fitting profile using the Marquardt–

Levenberg algorithm. The particle size of ceria and platinum

was determined by Scherrer’s equation, assuming a Gaussian

shape of the peaks.

A Philips-CM12 transmission electron microscope (TEM),

operated at 200 kV, was used for catalyst examination. Catalyst

powder in isopropyl alcohol was supported on a holey 400 mesh

copper grid.

TPR was carried out by using 5% hydrogen in an argon

mixture (30 cm3/min, STP), as a reducing gas, in a TPR reactor.

The investigated temperature range was�80 to 1000 8C, with a

heating rate of 20 8C/min; hydrogen consumption was

monitored by a thermoconductivity detector (TCD). The

TCD response was quantitatively calibrated by monitoring

the reduction of known amount of CuO, according to the

procedure elsewhere described [15].

Catalytic activity tests on the oxidative steam reforming of

C3H8 were conducted in a fixed-bed quartz reactor with inner

diameter of 6 mm at atmospheric pressure. The catalyst sample

(ranging between 2.0 and 0.2 g, in dependence to the applied

gas hourly space velocity) was placed between two quartz wool

plugs in the centre of a quartz tube, inserted into a furnace

heated to the reaction temperature and controlled through a

temperature controller. The reaction temperature was measured

and controlled by two chromel–alumel thermocouples. One of

the thermocouples was inserted into a thermowell and centred

within the catalyst bed, while the other one was kept just at the

outlet of the catalytic bed. Temperature profile measurements

along the catalytic bed were carried out by moving the

thermocouple from the inlet to the outlet through the catalyst

bed, while the thermocouple centred outside the quartz reactor

was connected to the temperature controller.

The activity tests were carried out over a temperature

ranging between 650 and 750 8C with gas hourly space

velocity (GHSV, defined as volume per h of the gaseous feed at

0 8C and 1 bar of pressure per volume of the catalytic bed)

increasing from 5000 to 100 000 h�1, corresponding to a

WHSV ranging between 0.3 and 5.7 gC3H8/(gcatalyst h). In the

tests at lower GHSV (5000 h�1) the total flow rate of the

reaction mixture was kept at 75.75 cm3/min; whereas, it was

increased to 151.50 cm3/min in the experiments at higher

GHSV (10 000–100 000 h�1). High purity gases C3H8

(99.999%) with O2 (99.999%) and N2 (99.999%) in a ratio

to simulate air composition, were used in the experiments; the

gas flow rates were kept constant by mass flow controllers. The

liquid feed (H2O) rate, controlled with an isocratic pump

(Agilent 1100 Series), was vaporized and mixed with the

incoming fuel (C3H8, O2, N2) in a vaporizer, properly designed

in order to obtain a water concentration as constant as possible

in the stream and for a wide liquid flow rate range. The transfer

lines of the reaction system were heated to prevent

condensation.

The feed and reaction products were analysed using an

Agilent 6890 Plus gas chromatograph equipped with thermal

conductivity and flame ionization detectors (FID). The CH4,

revealed to the TCD detector, was used as reference compound

in FID analyses. The catalytic tests were initiated by heating the

reactor, from ambient temperature to 650 8C, under N2 flow.

Subsequently, N2 flow was stopped and the reaction mixture

(C3H8, O2, steam, N2) was allowed to flow through the reactor;

a rapid temperature increase that stabilizes within some

minutes to the set up value (650 8C) was observed. The effect of

the reaction temperature, at fixed GHSV, was studied by

increasing progressively the temperature (heating rate = 10 8C/

min) from 650 to 700 8C and subsequently to 750 8C.

On-line chromatographic analysis of the reaction products

was carried out every 20 min during each test; the typical

duration of each catalytic test was 6 h.

The reactants’ conversion and products’ selectivity were

calculated according to the following equations:

Scarbon-containing product ¼Fcarbon-containing product

ðFC3H8 in � FC3H8 outÞn

where Fi, in or out was the molar flow rate of species i measured

at inlet or at outlet of the reactor and n was the stoichiometric

factor between the carbon-containing products and the C3H8.

The nitrogen peak was used for calibration of mass balance.

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–7770

3. Results and discussion

3.1. Catalyst characterization

XRD pattern of 1.13 wt.% Pt/CeO2 sample is shown in

Fig. 1. The crystalline phase of CeO2 associated with a

scarcely visible Pt metal phase and Pt2Ce alloy (lattice

spacing = 0.2322 A), were identified. The related particle

size, derived by application of the Scherrer equation, results

186 A for CeO2 and 107 A for the Pt, no quantification can

be possible for the Pt2Ce phase. The presence of Pt2Ce alloy

and metallic Pt was highlighted in high-resolution diffrac-

tion pattern; this was carried out with a scan rate of 0.068/min in the 2u range 37–438 and 61–698, as shown in the inset

of Fig. 1. The reported least-square fitting sub-routine,

based on the Marquardt–Levenberg algorithm included in

the X-Pert software, has been employed to resolve each

component.

The CeO2 reflections were slightly shifted to a higher degree

(D2u1 1 1 = 0.058) confirming, as previously reported [13], the

occurrence of a solid solution with substitution of Pt2+/4+ in

the ceria lattice. The unit cell parameter a decreases (a =

5.4026 A), in comparison to a = 5.4113 A calculated for the

pure CeO2 (JCPDS 4-594), resulting in good accordance

with effective ionic radius considerations (rCe4þ ¼ 0:97 A,

rPt2þ ¼ 0:80 A, rPt4þ ¼ 0:625 A). In this respect, the evaluation

of the change in the lattice parameter of CeO2 solid solution as a

function of the guest platinum content, according to as reported

by de Leitenburg et al. [16], can be realised by application of

the empirical relation derived from Kim [17]:

aCe ¼ 0:5413þX

k

ð0:0220Drk þ 0:00015DzkÞmk

where a (nm) is the lattice parameter of the fluorite struc-

tured solid solution at room temperature, Drk is the differ-

ence in ionic radius (rk � rh) between the kth dopant (rk) and

the host cation (rh), Dzk is the valency difference and mk is

the mol% of the kth dopant. Considering PtO as dopant and

Fig. 1. X-ray diffraction patterns of fresh and spent catalyst, compared with the

peak position of the reference oxide CeO2 (JCPDS 4-594). Spent catalyst—

reaction condition: 650–750 8C, O2/C3H8 = 2.0, H2O/C3H8 = 3.6, GHSV

= 5000–100 000 h�1 (inset: least-square fitting of the diffraction peaks in the

XRD patterns of the fresh catalyst compared with the peaks position of the Pt

(JCPDS 4-802) and Pt2Ce phases (JCPDS 17–10)).

the valency difference between Pt2+ and Ce4+ the former

equation becomes

aCe ¼ 0:5413þ ½0:0220ðrPt2þ � rCe4þÞ � 0:0003�mk

introducing the r value and the derived molar content of PtO, a

theoretical value of 5.404 A for aCe can be derived. The

calculated lattice parameter results in sufficient accordance

with the experimental value, giving an indirect evidence of

the solid solution formation.

The presence of additional phases, ascribed to platinum

metal and Pt2Ce, detected in the current analysis with a lower

scan speed (0.6 and 0.068/min) respect to previously reported

data [13], can be due to the reaction conditions reached during

the combustion synthesis. The combustion reaction of the redox

mixture ((NH4)2Ce(NO3)6; H2PtCl6) with C2H6N4O2 (oxalyl-

dihydrazide) as fuel, was very vigorous with a flame

temperature of about 1000 8C: on heating, the nitrate

decomposes producing oxides and nitrogen [18]. Oxygen

and carbon elements, provided by the C2H6N4O2 (or

accessional oxygen from the air), perform different functions:

oxygen gives essential oxidant for combustion; whereas,

carbon not only plays a fuel’s role but also can makes a

deoxidation effect together with the deoxidizing action of

hydrogen. The reducing effect and the decomposition of Pt

oxides, induced by the temperature, can be responsible for the

Pt metal occurrence in the catalyst, as shown in XRD patterns of

Fig. 1 [19,20]. Furthermore, the breaking of the strong bond

between Pt and ceria, caused by the calcinations of the

pelletized catalyst at 800 8C, can lead to the observed metal

segregation, as observed by Hosokawa et al. [21]. Besides, it is

probable that the local reducing conditions associated with high

temperatures, reached during the combustion synthesis, make

suitable conditions for the growth of the alloy at the metal–ceria

interface. The epitaxial growth of the intermetallic compounds

on ceria has been evidenced by Bernal et al. [22] at high

reduction temperature ranges (800–950 8C). Penner et al. [23]

pointed out that the alloying process can start at 4508, but the

probability of a specific alloy formation and the onset

temperature of formation, depends on the experimental

conditions (contact area between metal and oxide, crystallinity

of the oxide, possibility of topotactic growth, etc.). It is

probable that the atomic mixing of the reactants during the

synthesis can facilitate the mutual incorporation of Ce into the

Pt lattice and vice versa.

Fig. 2 shows the TEM image of the Pt/CeO2 catalyst. Metal

particles fairly well dispersed with an average size of 42 A,

despite the poor contrast between the support and the platinum

particles, can be evidenced; while, particles whose the contrast

cannot be clearly interpreted as due to either platinum or ceria

and probably due to partially oxidized state of platinum, can be

envisaged. However, when the support crystals are favourably

aligned small platinum particles are readily seem, as in Fig. 2,

this can validate the difference in the particle size evaluation

from X-ray diffraction and TEM analyses.

TPR profile of the catalyst is shown in Fig. 3. Pure CeO2, as

widely reported in literatures [24,25] shows two principal

reduction peaks centred at 500 and 850 8C; the first peak was

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–77 71

Fig. 2. TEM image of as prepared 1.13 wt.% Pt/CeO2 catalyst.

generally assigned to a reduction of the most easily reducible

surface ‘‘capping’’ oxygen of ceria, whereas reduction states

above 850 8C are attributed to a bulk reduction. The presence of

platinum can induce modifications in the reduction zone at lower

temperatures, whereas it does not affect the high temperature

peak [26,27]. In the present case, the catalytic surface reduction

process, induced by Pt, results in the shift to a lower temperature

of the first peak, recorded at 378 8C. Further peaks at 147 and

20 8C are evidenced. It has been shown [28] that the platinum

oxides reduction temperature depends primarily on the degree of

crystallinity of the oxide (the lower the temperature, the more

crystalline the oxide), the particle size and from the interaction

with the support. XRD analysis showed that Pt is present mostly

as solid solution and in a metallic state; this last one cannot

contribute significantly to the reduction process. From this

consideration, the peaks at lower temperatures should be

associated with well-dispersed Pt oxides and bulk-like crystalline

PtO on the surface undetected by X-ray diffraction.

Rajaram et al. [29] proposed that the formation of a solid

solution induces the generation of oxygen vacancies, which

adsorbs oxygen easily. These very reactive oxygen species can

be reduced easily by H2 at low temperatures. From these

considerations, it can derive that XRD-undetectable platinum-

Fig. 3. Normalized TPR profile of as prepared Pt/CeO2 catalyst.

oxide particles and strong metal–support interaction or solid

solution formation of platinum with ceria lattice (as predictable

from XRD analysis) could be responsible for the observed

reduction profile.

3.2. Catalytic activity

3.2.1. Effect of O2/C3H8 and H2O/C3H8 feed ratios

The C3H8 autothermal reforming (ATR) was considered as a

combination process composed of partial oxidation (1) and

steam reforming (2) reactions, followed by establishment of the

water gas shift (3) and methanation equilibria (4):

C3H8 þ 1:5O2! 3COþ 4H2; DH�298 ¼ �229 kJ=mol (1)

C3H8 þ 3H2O! 3COþ 7H2; DH�298 ¼ þ497 kJ=mol (2)

COþ H2O!CO2 þ H2; DH�298 ¼ �41:09 kJ=mol (3)

COþ 3H2!CH4 þ H2O; DH�298 ¼ �205:21 kJ=mol (4)

Exothermic total oxidation (5) followed by steam reforming

and CO2 reforming (6) reactions:

C3H8 þ 5O2! 3CO2 þ 4H2O; DH�298 ¼ �2046 kJ=mol

(5)

C3H8 þ 3CO2! 6COþ 4H2; DH�298 ¼ þ620 kJ=mol (6)

normally play a role that depends on reactant composition,

temperature, residence time and the catalytic system involved.

Additional side reactions, including cracking of propane (7)

and carbon monoxide to carbon deposition (8):

C3H8!C2H4 þ CH4; DH�298 ¼ þ89 kJ=mol (7)

2CO!CðsÞ þ CO2; DH�298 ¼ �172 kJ=mol (8)

must also be considered; the latter is particularly unwonted and

generally occurs when the O2/C3H8 ratio in the reactant mixture

becomes too low. Addition of steam to reactant mixtures

containing propane and air should in principle lead to higher

H2 selectivity as a consequence of the effect of water on the

equilibria for reactions (2), (3) and (6), giving that the heat

generated from the oxidation can be transferred to the reform-

ing zone.

From a theoretical point of view the ATR process, can be

schematized as

CnHm þ eðO2 þ 3:76N2Þ þ ð2n� 2eÞH2O! nCO2

þ ð2n� 2eþ m=2ÞH2 þ 3:76eN2

where the oxygen (or air) to fuel molar ratio controls the overall

heat balance of the reaction, the water required to convert the C

present in the fuel into CO2 and the maximum H2 yield

achievable in the reaction products [30]. At e = 0 the reaction

operates in steam reforming regime, strongly endothermic, the

maximum amount of H2 produced from propane, can reach

about 77%; increasing the value of e, the H2 concentration

proportionally decreases reaching a theoretical value of about

22% at e = 3, where the amount of oxygen in the feed is

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–7772

Fig. 4. Product composition of the C3H8 autothermal reforming predicted by

thermodynamic equilibrium in function of the temperature (inset: influence of

the increased O2 amount at 650 8C on the product composition).

sufficient to convert all the carbon contained in the fuel to CO2

without further addition of water. For e > 3 water appears as a

reaction product; in this condition, the reaction’s heat can be

determined by the physical state of the produced water (liquid

or gas). In the range 0 < e < 3 by increasing the amount of

oxygen, the reaction becomes less endothermic; thermoneutral

conditions can be predicted at e = 1.116 (DHreaction,298 K = 0)

with a theoretical maximum H2 concentration of 52%.

Since the ATR process, for a fuel processor system, requires

high efficiency, carbon free operation and full C3H8 conversion

(>99.9%) associated with high H2 selectivity, we have

preliminary investigated the activity of the Pt/CeO2 catalyst

by evaluating the influence of the O2/C3H8 and H2O/C3H8

ratios on the product distribution in the temperature range 650–

750 8C under lightly exothermic conditions (OSR).

The thermodynamic equilibrium composition, calculated by

application of a simplified reaction model, including reactions

((1)–(4)), is reported in Fig. 4; the direct CO2 reforming was

excluded for the much smaller rate compared with steam

reforming [31]. Increasing the temperature from 600 to 800 8C,

the H2 and CO content progressively increases; at a selected

temperature (650 8C), introducing higher amounts of oxygen to

the steam/propane mixture, the H2 and CO content decreases so

how the CH4, whereas an increase in the total oxidation

products (H2O and CO2) was predicted, as shown in the inset of

Fig. 4.

The influence of H2O addition in the feed on the catalytic

activity at 650 8C, with a GHSVof 5000 h�1 by changing the O2/

C3H8 molar ratio from 1.5 to 2.0, has been experimentally

evaluated. The lower O2/C3H8 value was derived from

preliminary partial oxidation experiments and in accordance

with literature values [32,33] that suggest, an optimum C/O ratio

of 0.8 to produce synthesis gas: by decreasing the C/O ratio, the

hydrogen selectivity and propane conversion increase, while the

associated total olefins’ production decreases. The steam

addition (H2O/C3H8 = 2.0–3.6) in the current OSR experiments

determines a decline in the C/O ratio (including O2 and H2O

oxygen), ranging between 0.6 and 0.45, respectively. Increasing

the O2/C3H8 ratio to 2.0, a fine tuning of the C/O ratio was

reached, in order to minimize the by-products’ formation. The

results are summarized in Table 1. At lower O2/C3H8 molar ratio

(1.5) the propane conversion is strongly affected from the H2O

addition in the feed; C3H8 conversion is 80%, with H2O/

Table 1

ATR activity of Pt/CeO2 catalyst at 650 8C with GHSV = 5000 h�1

Feed composition

(molar ratios)

C3H8

conversion (%)

H2

concentration (%)a

H2/CO H2/C

O2/C3H8 H2O/C3H8

1.5 2.0 80 26 (30.4) 3.4 1.5

1.5 3.0 90 27 (31.2) 3.5 1.6

1.5 3.6 92 28 (31.3) 4.4 1.7

2.0 2.0 100 24 (24.8) 3.3 1.4

2.0 3.0 100 25 (25.0) 3.7 1.5

2.0 3.6 100 25 (25.0) 4.1 1.4

a Numbers in parenthesis are the thermodynamic predicted values.

C3H8 = 2, and reaches a level of 90–92% increasing the steam/

fuel ratio; the O2 conversion approaches 100% under all

investigated conditions. The H2 concentration in the reaction

products, compared with the thermodynamic predicted value,

follows the same trend observed with the C3H8 conversion,

becomes 28% with a propane conversion of 92%. The derived

H2/COx ratio, ranging between 1.5 and 1.7, results close to the

partial oxidation value (H2/CO = 1.3), suggesting that C3H8

partial oxidation occurs totally and a small portion of steam

reforming and water gas shift reactions took place. Strong

literature evidences [34,35] pointed out that partial oxidation is a

combination of catalytic combustion where all the oxygen is

consumed, followed by steam reforming of unreacted propane.

Following this mechanism, with O2/C3H8 = 1.5, the combustion

reaction involves a fraction (30%) of the fed C3H8; secondly

steam reforming of unreacted C3H8 with the produced steam and

water gas shift reactions generate H2, CO and CO2.

The remaining C3H8, not involved in the combustion

reaction, in presence of the introduced steam can be involved at

the same time in a secondary steam reforming reaction

(C3H8 + 6H2O! 3CO2 + 10H2) which occurs in excess of

steam. The CO2 produced determines the high H2/CO ratio

observed, as evidenced in Table 1; increasing the H2O/C3H8

ratio in the feed, the contribution of secondary steam and water

gas shift reactions, increases, as revealed by the increase in H2/

CO (from 3.4 to 4.4) and by the decrease in CO selectivity, as

reported by Roh et al. [36,37] for the methane reforming.

Ox Selectivity

CO (%) CO2 (%) CH4 (%) C2H6 (%) C2H4 (%) C3H6 (%)

42 51.75 5.5 0.10 0.14 0.51

43 51.64 5.0 0.09 0.09 0.18

38 57.11 4.6 0.06 0.07 0.16

42 54.40 3.6 – – –

39 58.40 2.6 – – –

36 61.80 2.2 – – –

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–77 73

The occurrence of a strong exothermic reaction at the inlet of

the catalytic bed, followed by reforming reactions, occurring in

the last zone of the bed, is supported by the temperature profiles

along the reactor central axis measured during the tests, as

given in Fig. 5. At the lowest H2O/C3H8 ratio the bed

temperature near the inlet was 54 8C higher than the

temperature measured in the central position; while that

recorded at the outlet of the bed, results lower than the central

temperature. Increasing the H2O/C3H8 ratio, at 3.0 and 3.6,

respectively, the temperature profile becomes more flat, the

temperature near the inlet bed becomes 38 and 20 8C,

respectively, higher than the centre, while proportionally a

temperature decrease in the outlet position was recorded. This

evidence confirms that the propane combustion and reforming

simultaneously proceed with overlapping of the two zones, as

reported by Tomishige et al. [38] with platinum catalyst; while

the temperature measured in the last part of the bed, appears

sufficiently low to suggest the occurrence of water gas shift

reaction to a great extent with the reforming [39].

The apparent equilibrium constant for the WGS reaction

(Kexp), based on the concentrations of the appropriate gases in

the effluent stream, decreases from 1.8 to 1.7 with the increase

of steam in the reactant mixture. The equilibrium constant for

this reaction (KWGS,913), calculated from thermodynamic

parameters (KWGS = exp(�DG8/RT)), at the exit bed tempera-

ture results KWGS = 2.07. This confirms that the approach to

equilibrium for the WGS reaction was from the CO + H2O side.

Besides, the observed temperature profiles are in substantial

agreement with Holmen and co-workers [40] for the oxidative

steam reforming of propane with Rh catalyst supported on

Al2O3 foam. The higher temperature peaks, observed by the

Authors at a residence time of 0.105 s, should be dependent on

the catalyst activity and morphology.

The selectivity of lower hydrocarbons (CH4, C2H6, C2H4 and

C3H6) proportionally decreases with the increasing in the H2O/

C3H8 ratio. Increasing the H2O/C3H8 ratio the reaction becomes

less exothermic and the thermal cracking, that can generate in

prevalence CH4 and C2H4, was minimized. Additional, C3H6 and

CH4 formation occurs by the possible side reaction during steam

reforming: desorption of the dehydrogenated C3 carbon species

Fig. 5. Dependence of H2O/C3H8 molar ratio in the feed on the temperature

profile of the catalytic bed under C3H8 oxidative steam reforming carried out at:

T = 650 8C, O2/C3H8 = 1.5 and GHSV = 5000 h�1.

can generate propene ðC3H8 þ 12O2!C3H6 þ H2OÞ, whereas a

more complex process (hydrogenation of a C1 carbon species or

the subsequent hydrogenation of CO, CO + 3H2! CH4 + H2O)

can be responsible for the CH4 formation [41,42].

The high C3H6 selectivity (related to C2–C3-products)

decreases with the steam addition in the reactant mixture,

suggesting that the O2 activation lines a predominant role in

the product distribution: this evidence can be tentatively

rationalized as follows. Actually, two different mechanisms

for the catalytic partial oxidation of hydrocarbons have been

proposed in literature: catalytic combustion followed by H2O/

CO2 reforming and water gas shift reactions [43–45] or a

pyrolysis mechanism in which CO and H2 are produced

directly and are followed by their consecutive oxidation [46–

49]. Besides, it is widely accepted that the steam reforming

reaction proceeds via the dissociative adsorption of C3H8 on

the catalyst surface whereas the support provides the sites for

water activation into hydroxyl group [50,51]. In both

processes, hydrocarbon and/or O2 dissociate rapidly on

metallic Pt sites, while the presence of ceria can support

the water dissociation and transfer the produced oxygen to the

supported metal, reducing the coke formation [52]. During the

oxidative steam reforming, C3H8 dissociative adsorption

occurs on metallic Pt surface, while both steam and O2 can

competitively adsorb on Pt and ceria. The presence of Pt ions

in the CeO2 matrix, as derived from the catalyst characteriza-

tions, includes the concomitant presence of Ce3+ associated

with anionic vacancies that become additional sites for the

adsorption of gaseous species: C3H8 and O2 adsorption, more

favoured that the H2O adsorption, occurs. Since the surface

defects of ceria are sites for oxygen adsorption, the dynamic

equilibrium between gaseous oxygen, adsorbed oxygen and

bulk lattice-oxygen can be involved in the propene formation.

This equilibrium can give high C3H6 content, when the steam

addition in the feed is insufficient to convert all C3H8, as

occurs at H2O/C3H8 = 2.

Creaser et al. [53,54] in the kinetic modelling of C3H8

oxidative dehydrogenation with oxide catalyst, report that the

oxygen lattice selective produces propene whereas total

oxidation involves an oxygen adspecies (O�), arising from

gaseous oxygen activation with the lattice oxygen O2�

(12O2 + O2� ! 2O�). The proposed Mars–van Krevelen

mechanism [55], with superficial O2� anion, leads to the

formation of anionic vacancies and subsequent reoxidation of

the surface by gaseous O2. In presence of an oxygen deficient

composition at the catalyst surface, which has occurred in the

current sample, the gaseous oxygen can fill the surface oxygen

vacancies with the creation of an electron hole, modelled as a

substitutional O� species at the ceria surface:

Vo�� þ 12O2ðgÞ !Oo

x þ 2h�

where Vo�� is an anionic vacancy, Oox is an oxygen anion in a

normal site and h� is an electron hole [56], resulting insufficient

to the further oxidation of adsorbed propene. Increasing the

steam content, the decreasing in the C3H6 selectivity can be

ascribed by the increase in the propane conversion.

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–7774

Fig. 6. Conversion of C3H8, H2 concentration and carbon-containing products

selectivity as a function of the H2O/C3H8 and reaction temperature during the

oxidative steam reforming of C3H8 (at O2/C3H8 = 1.5, GHSV = 5000 h�1) with

CeO2 in absence of Pt.

The influence of gas-phase reactions in the formation of

ethylene and propylene at high reaction temperatures, as occurs

near the first part of the bed, cannot be excluded. In the

literature concerning the oxidative dehydrogenation of hydro-

carbons, this aspect was widely discussed. Schmidt and co-

workers [32,57] on short contact-time reactor, in the case of

ethane and propane oxidative dehydrogenation (with Pt–

aAl2O3 catalyst) exclude the influence of gas-phase reactions

in the formation of ethylene and propylene and a purely

heterogeneous mechanism and kinetic model were proposed.

Lødeng at al. [58] have shown that the formation of ethylene

from ethane, in the presence of a Pt catalyst at milliseconds

contact time and a temperature range of 700–900 8C, occurred

in the gas phase; with propane it is more difficult to avoid

homogeneous reactions due to its high reactivity.

Forzatti and co-workers [59,60] using a structured reactor

with annular configuration, pointed out that the gas-phase

reactions were active and extremely selective in the formation

of propylene and ethylene from C3H8 at high temperatures

(above 650 8C).

In the present case, additional experiments have been carried

out with CeO2 in absence of Pt, in order to elucidate the

contribution from gas-phase reactions and reactions induced by

the support. To compare these experiments with previous

results (Table 1), the tests were carried out at the maximum

temperature recorded during previous catalytic tests. The

results are shown in Fig. 6.

The C3H8 conversion decreases with the increase in the H2O/

C3H8 ratio, consistent with the proportional decrease in the

reaction temperature; the O2 appears totally converted (not

shown) in all tests. The related H2 concentration in the obtained

products and the CO selectivity, follow the observed trend in the

C3H8 conversion (decrease with the increase in the H2O/C3H8

ratio) remaining substantially lower than that obtained with Pt

catalyst. This behaviour, according to Holmen and co-workers

[40], suggests that heterogeneous reactions, taking place on Pt

particles, are the main contribution to the formation of H2 and

CO. The CO2 selectivity increases with a decrease in the

temperature (or at increased H2O/C3H8) and results much

higher as compared to the Pt catalyst (Table 1), indicating that

carbon dioxide was the main product. The selectivity to

hydrocarbons (C2H4, C3H6, CH4 and C2H6) was noticeably

higher than when Pt was present on catalyst, suggesting that

homogeneous reactions and reactions induced by CeO2 take

place. The high presence of adsorbed oxygen atoms on the ceria

surface, as can be supposed from the total O2 conversion not

strictly related to the C3H8 conversion, can contribute to the

formation of a surface alkyl group by extraction of a hydrogen

atom from propane, as reported by Huff and Schmidt [32].

Then, both homogeneous reactions and reactions induced by

the support can be involved in the by-products’ formations.

The results of the catalytic activity at O2/C3H8 ratio to 2.0

(Table 1) show that the complete C3H8 conversion was reached

with all the investigated H2O/C3H8 ratios; the absence of C2–C3

hydrocarbons associated with a low amount of CH4, which

decreases with increased steam introduction in the feed, was

recorded. Two different effects may occur by adding an excess

of oxygen in the feed: a faster combustion of C3H8 with all of

the O2 and an increase in the reforming reaction rate due to the

exothermicity of combustion reaction, leading to a lower H2

content in the reformate.

The CO selectivity decreases and hence the H2/CO increases,

while the H2 content in the products, close to thermodynamic

value, remains almost constant. Because, optimal conditions for

a fuel processor combine the highest fuel conversion with high

H2 concentration in absence of by-products, the last reported

conditions appear to be a suitable compromise. Further

improvements in the catalyst performance are currently being

studied aimed at a future practical application.

3.2.2. Effect of gas hourly space velocity

The catalytic activity was investigated by changing the

GHSV from 10 000 to 100 000 h�1, in the temperature range

650–750 8C at fixed O2/C3H8 and H2O/C3H8 molar ratios, 2.0

and 3.6, respectively. The flow rate of the reaction gas mixture

was unchanged, whereas the amount of catalyst was decreased

progressively. The results are summarized in Table 2 and Fig. 7.

The C3H8 conversion was slightly effected by the variation in

the GHSV, in the investigated temperature range; while the total

oxygen conversion was reached in all tests. The comparison

between the results of the experiment sets indicates that when

the GHSV increased, the H2 concentration and the CO

selectivity decreased, while the selectivity to CO2 increased.

This can suggest that, in the adopted conditions, a dominant

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–77 75

Table 2

ATR activity of Pt/CeO2 catalyst tested at O2/C3H8 = 2.0 and H2O/C3H8 = 3.6

T (8C) GHSV (h�1) C3H8 conversion (%) H2/CO H2 concentration (%) Kexp KWGSa

650 10000 100 3.4 25.0 2.07 1.97

650 30000 100 3.7 24.8

650 50000 97 4.2 24.3

650 100000 92 4.4 22.5

700 10000 100 3.0 25.0 1.65 1.55

700 30000 100 3.2 25.0

700 50000 98 3.3 24.7

700 100000 97 3.5 24.5

750 10000 100 2.7 24.8 1.35 1.25

750 30000 100 2.8 24.4

750 50000 99 2.8 23.8

750 100000 98 3.0 23.4

a Calculated at the temperature of reaction.

pathway for CO formation over the catalyst via the reforming

reactions of C3H8 with CO2 and H2O occurs. The high affinity

for oxygen of the Pt–CeO2 catalyst, as confirmed from the total

O2 conversion, can be responsible for the observed products

composition [61]. The H2/CO ratio decreases with an increase

Fig. 7. GHSV effect on the C3H8 oxidative steam reforming on the carbon-

containing products selectivity (reaction conditions: O2/C3H8 = 2.0, H2O/

C3H8 = 3.6) at increasing reaction temperature: (a) 650 8C; (b) 700 8C; (c) 750 8C.

in the reaction temperature, suggesting that the approach to

equilibrium water gas shift reaction takes place from the CO2–

H2 rich side (CO2 + H2! CO + H2O) simultaneously with

propane reforming [39]. This can be confirmed both by the slow

decrease in the H2 concentration and by the apparent

equilibrium constants: the evaluated value (Kexp), at

GHSV = 10 000 h�1 for the temperature range 650–750 8C,

results lightly greater to the corresponding equilibrium

constant, as shown in Table 2.

Besides, a small increase in the H2/CO ratio with increasing

the GHSV was observed; the temperature profiles of the

catalytic bed, for the more representative GHSV (depicted in

Fig. 8), showed that the temperature remains quite high,

limiting the water gas shift reaction. Probably, the excess of

steam in the feed, related to the adopted oxidative steam

reforming reaction (C3H8 + 2O2 + 2H2O! 3CO2 + 6H2),

associated with a lightly lower C3H8 conversion, recorded at

50 000–100 000 h�1, determines an increase in the steam

concentration relative to CO concentration, thus promoting the

WGS reaction favoured at high concentration of steam [62].

The increase in the CH4 and C2–C3 selectivity, revealed

exclusively at 650 8C with high GHSV (50 000 and 100 000

h�1), can be related to the decrease in the amount of catalyst.

Fig. 8. Dependence of GHSV on the temperature profile of the catalytic bed

under reaction carried out at 650 8C with O2/C3H8 = 2.0 and H2O/C3H8 = 3.6.

L. Pino et al. / Applied Catalysis A: General 306 (2006) 68–7776

Fig. 9. H2 concentration and carbon-containing products selectivity as a

function of time on stream for the C3H8 oxidative steam reforming carried

out with the Pt/CeO2 catalyst. Reaction conditions: 650 8C, O2/C3H8 = 2.0,

H2O/C3H8 = 3.6, GHSV = 25 000 h�1.

Furthermore, from the temperature profile (Fig. 8) it is visible

that the temperature in front of the catalytic bed is sufficiently

high to support the occurrence of the gas-phase reactions.

According to Holmen and co-workers [40], these reactions, away

from the catalytic bed lead to more by-product formation, than

when there is a closer interplay between homogeneous and

heterogeneous reactions such as for the catalyst. It is likely that

increasing the GHSV, we have a greater contribution of gas-

phase reactions before the entrance to the catalytic bed allowing

the observed formation of by-products.

3.2.3. Time on stream effect

The catalyst shows constant activity over 6 h of reaction

carried out at GHSV of 25 000 h�1 with the above cited O2/

C3H8 and H2O/C3H8 ratios (2 and 3.6, respectively); total

oxygen and propane conversion (not shown) was reached. The

H2 concentration in the products, as reported in Fig. 9, was

about 25% which is close to the thermodynamically predicted

value. The CO, CO2 and CH4 selectivity are consistent with

previous results reported in Table 1.

Fig. 10. TEM image of spent catalyst after reaction at 650–750 8C with O2/

C3H8 = 2.0, H2O/C3H8 = 3.6 and GHSV = 5000–100 000 h�1.

The spent (about 100 h of total reaction) catalyst, tested

under the different experimental conditions object of the

present study, was characterized by means of XRD and TEM

analysis. The related XRD pattern compared with the fresh

sample is depicted in Fig. 1; no substantial aggregation of Pt

particles has occurred, Pt particle size increases from 107 to

110 A after the catalytic tests, while the shift to high degree in

the ceria reflections remains unchanged. Disappearance of the

alloy diffraction features was observed, probably connected

with oxidative reaction [63].

TEM investigation carried out on spent sample confirms the

growth of platinum particles to 54 A, as shown in Fig. 10, and

the absence of carbon deposition.

4. Conclusions

As a part of the development of a LPG reforming system for

hydrogen production, we investigated the oxidative steam

reforming of propane over a ceria-supported platinum catalyst

prepared by combustion synthesis. The catalyst reveals

promising catalytic activity and stability during several hours

of operation in a wide range of gas hourly space velocity;

hydrocarbons by-products are detected as traces at high GHSV.

The catalyst characterizations reveal the presence of

platinum as metal and ionically substituted state in the ceria

support. Absence of carbon deposition was observed.

Acknowledgement

Fundings from MIUR-Italy are gratefully acknowledged.

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