Autothermal reforming of propane for hydrogen production overPd/CeO2/Al2O3 catalysts

Wagner L.S. Faria, Lıdia C. Dieguez, Martin Schmal *

NUCAT/PEQ/COPPE, Federal University of Rio de Janeiro, C.P. 68502, 21945-970 Rio de Janeiro, Brazil

Applied Catalysis B: Environmental 85 (2008) 77–85

A R T I C L E I N F O

Article history:

Received 14 April 2008

Received in revised form 24 June 2008

Accepted 26 June 2008

Available online 19 July 2008

Keywords:

Propane autothermal reforming

Hydrogen production

Ceria supported palladium catalyst

TPSR

A B S T R A C T

Steam reforming, partial oxidation and autothermal reforming of hydrocarbons are important routes to

hydrogen generation for use in fuel cells. The propane autothermal reforming was studied in supported

CeO2/Al2O3 based Pd catalysts, prepared with different Pd precursors. The reaction was carried out

under different feedstock conditions and the catalytic activity was evaluated by temperature

programmed surface reaction (TPSR). The effects of palladium precursors on the activity and selectivity

were studied. Propane autothermal reforming showed three temperature ranges: at relative low

temperatures total oxidation occurs; steam reforming occurs at intermediate temperatures when

oxygen was completely consumed; and, finally, at higher temperatures the CO2 reforming prevails. The

Pd acetylacetonate precursor catalyst was the most active, starting the reforming 100 K less than the Pd

chlorine precursor catalyst. The presence of water in the reaction mixture was responsible for an

increasing propane conversion and H2/CO ratio when compared without water. Cerium oxide (CeO2)

was responsible for the decreasing CO concentration, promoting the water gas shift reaction

(CO + H2O! CO2 + H2). The catalysts were stable with time on stream during 50 h and a small

deactivation occurs only at the beginning.

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1. Introduction

Restrictions of gas emissions in the atmosphere, such ashydrocarbons (HCs), carbon monoxide (CO) and nitrogen oxides(NOx), originated from internal combustion engines, are increas-ing. The industries are being constantly pressed for developingclean technologies, such as fuel cells [1,2]. It offers an efficientconversion of chemical into electrical energy without the emissionof pollutants, generated in engine of internal combustion, beingone of the most promising sources of energy generation in thefuture [3].

Hydrogen can be produced from different processes, such assteam reforming or partial oxidation of hydrocarbons and alcohols,biomass, electrolysis of the water or as by-product of the oilrefining. Among them, steam reforming is probably the mostcommon and cheapest way for hydrogen production [4]. Theautothermal reforming or oxidative steam reforming has the mainadvantage that the initial oxidation reaction is extremelyexothermic, generating heat for the subsequent endothermicreforming reactions [5]. Among various hydrocarbons, propane is

* Corresponding author. Tel.: +55 21 2562 8352; fax: +55 21 2562 8300.

E-mail address: schmal@peq.coppe.ufrj.br (M. Schmal).

0926-3373/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2008.06.031

drawing attention in hydrogen production studies primarilybecause it is a constituent of LPG [6,7]. LPG is a commercial gasthat is easily transported and stored on site [8]. Moreover, due toits composition (short aliphatic C3–C4 chains and absence ofsulphur or other electronegative atoms), LPG is reported to presentsome significant advantages compared to heavier feedstocks,especially in terms of catalyst resistance to deactivation led bylimited carbon deposition during reforming [9]. Propane isproduced in relative high amounts from natural gas and oil cruderefining. For pressures of approximately 9 bar, it is in liquid stateand can be easily stored and distributed.

Three-way catalysts are used in catalytic converters and in anarrow air/fuel ratio, allowing simultaneously the oxidation of HCsand CO, beyond the reduction of NOx [10]. Noble metals, such asPd, Pt and Rh supported on g-Al2O3 have been used. Palladium isvery active for hydrocarbons and CO oxidation. The mainadvantage of Pd is the relatively low price in comparison to othernoble metals [11].

Cerium oxide (CeO2) has been widely used as promoter due toits high oxygen storage capacity (OSC) [12], acting as thermalstabilizer of the support [13,14], allowing better dispersion of themetal [15], and preventing coke formation [16,17].

The influence of sub-stoichiometric condition in the feed O2/C3H8 on the reaction is relatively scarce in the literature. Therefore,

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–8578

in the present work we investigate the influence of the sub-stoichiometric condition in the feed O2/C3H8 and the influence ofthe Pd precursors in the presence of a surface layer of CeO2 on theactivity of the autothermal reforming of propane on Pd/Al2O3

catalysts, as well as the addition of water on the H2 selectivity.

2. Experimental

The CeO2/Al2O3 system was prepared by grafting a ceriumacetylacetonate precursor (Ce(acac)3) (Aldrich Corp.) over alumina(AL-3916P-Engelhard Corp., 208 m2/g and 0.47 cm3/g) surfacehydroxyl groups, as described elsewhere [17]. After grafting, thesample was filtered and washed with toluene (Vetec, 99% P.A.) anddistilled water, followed by drying in an oven at 393 K for 18 h andcalcination under O2 flow at 673 K for 4 h using a heating rate of1 K min�1. The global process of preparation of the CeO2/Al2O3

support (grafting, filtration, drying and calcination) was carried outin six stages due to the low solubility of Ce(acac)3 in toluene.

Three catalysts with 1% (wt) of palladium supported on CeO2/Al2O3 (18% wt CeO2, 193 m2/g and 0.40 cm3/g) were prepared,using different precursor salts of the metal, Pd(acac)2, Pd(NO3)2

and PdCl2, now designated as PdCeacac, PdCeN and PdCeCl,respectively. The PdCeacac catalyst was prepared using wetimpregnation method. Toluene was used to dissolve Pd(acac)2

and the system was then filtered, whereas the PdCeN and PdCeClcatalysts were prepared by wetness impregnation, using solutionsof HCl and HNO3 for dissolution of PdCl2 and Pd(NO3)2,respectively. Finally, the samples were dried at 393 K for 18 hand calcined in an aerated muffle at 773 K for 4 h. Additionally, Pdcatalysts with different precursors were prepared on g-Al2O3,using the same methodology and were denoted as: Pdacac, PdNand PdCl. The crystallite sizes obtained were in the range of 3–8 nm(CeO2) and 3–5 nm (Pd), similar to reported data in the literature[18,19]. The crystallites of Pd/CeO2/Al2O3 catalysts were deter-mined after calcination and reduction at 773 K, using Scherrer’sequation and in particular the Rietveld method on XRD data (notshown).

Table 1 presents the metallic content and chemisorptionresults of the catalysts with different precursors, as reportedpreviously [17]. It shows that depending on the precursor thedispersion is different, however, for the precursor of acetylace-tonate (acac) both Pd/Al2O3 and promoted with Ce (Pd/CeO2/Al2O3) presented equal dispersions. The chloride precursor forPd/Al2O3 displayed similar and high dispersion like the acacprecursor, however, in the presence of CeO2 was reduced by half,indicating formation of larger particles. On the contrary, withnitrate precursor the presence of CeO2 increased the dispersionwhen compared to the Pd without the promoter, evidencingbetter dispersion in the presence of CeO2.

Temperature programmed surface reaction (TPSR) measure-ments were carried out in a multipurpose unit equipped with aquadrupole mass spectrometer (Balzers Prisma-QMS 200). Thesignal intensity of masses 2, 28, 29, 32 and 44, corresponding to H2,

Table 1Properties and chemisorption on catalysts with different precursors

Catalyst Content (%) Volumetric chemisorption

H2 (mmol/mg Pd)

PdCl 0.99 2.38

Pdacac 0.94 2.39

PdN 1.04 0.74

PdCeCl 0.91 1.19

PdCeacac 0.93 2.36

PdCeN 0.99 1.72

CO, C3H8, O2 and CO2, respectively, were monitored continuously.Quantifications were carried through calibration of each compo-nent and expressed in mol%. Contributions of C3H8 and CO2 in thesignal of CO (m/e = 28) and influence of C3H8 in the CO2 signal (m/e = 44) were duly deducted in the calculation of the molar fractionsof the components in the mixture.

The samples were pretreated passing a flow of 5% O2/He at673 K for 60 min to eliminate carbonated species, probablyformed on ceria [20], and followed by flowing He at 773 K for90 min. Then, the samples were reduced in pure H2 at 773 K for60 min and, finally, cooled down to room temperature in Heatmosphere. The oxidation stage was suppressed for catalystssupported only on g-Al2O3. The same total flow rate (50 ml min�1)was used in all steps.

The feed consisted of 0.5% C3H8 and 0.5R% O2 diluted in He,where R is defined as the O2/C3H8 molar ratio. Experimentscontaining water in the inlet composition were denoted by S,where S represents the H2O/C3H8 molar ratio. The total flow used inall the experiments was 150 ml min�1, with heating rate of10 K min�1 and 25 mg of catalyst. Reactants and balance gas Hewas supplied through mass flow controllers.

For the stability test of the catalysts, runs were performedduring 48 h with time on stream in a fixed bed reactor using thesame experimental conditions as the TPSR test. The exit gases wereanalyzed by a gas chromatograph (9001 Chrompack CP-ColumnHaye Sep D 100/120#) and the calculations based with relation ofthe injection of the non-reacted gas loading (bypass).

3. Results and discussion

3.1. Effect of O2/C3H8 molar ratio

The products distribution and reactant (mol%) as function oftemperature for the PdCeacac catalyst and for R = 5 (O2/C3H8 molarratio) are displayed in Fig. 1. In this case, R = 5 correspond to thestoichiometric amount for total oxidation of propane, according tothe following reaction:

C3H8þ5O2 ! 3CO2þ4H2O; DH298¼ �2046 kJ=mol (1)

Under this condition (R = 5), all propane was practicallyconsumed. The conversion of propane was initiated at 523 Kand reached about 85% conversion at 923 K. The amount ofhydrogen produced was insignificant. The partial oxidationreaction (2) was not observed in this case, because CO and H2

were not detected.

C3H8þð3=2ÞO2 ! 3CO þ 4H2; DH298¼ �229 kJ=mol (2)

Indeed, DRIFTS (‘‘Diffuse Reflectance Infrared Fourier TransformSpectroscopy’’) results (not shown) confirmed this statement, afteradsorbing propane, desorption with He flux and flowing O2,evidencing the formation of CO2 only.

CO (mmol/mg Pd) Ratio CO/H2 Dispersion DH2(%)

3.46 1.45 51

4.57 1.91 51

1.01 1.37 16

1.73 1.45 25

4.61 1.95 50

1.83 1.06 37

Fig. 1. Composition profile of PdCeacac catalyst in R = 5 condition. Fig. 3. Composition profile of PdCeacac catalyst in dry reforming of propane.

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–85 79

The product distribution under reductive conditions, R = 2.5, forthe PdCeacac catalyst is shown in Fig. 2. Propane consumptionstarted around 523 K and increased abruptly at 700 K. Concomi-tantly, oxygen concentration in the reactant mixture decreased untilits complete consumption. Only at this moment, H2 and CO wereformed with simultaneous formation of CO2, in accordance withobservations found by Barbier et al. [21], on a Pt–Rh/CeO2/Al2O3

catalyst, denoting the presence of two distinct regions in thecomposition profile for the autothermal reforming, where H2 isformed by steam reforming with the residual propane and waterproduced in the primary oxidation. However, at high temperatures(>880 K), still CO and H2 increased, reacting with CO2, denotingpropane dry reforming. Therefore, Fig. 2 can be divided in threedistinct regions occurring, respectively: propane oxidation(T < 690 K), oxidation and steam reforming (690 K < T < 880 K)and finally, simultaneous oxidation, steam reforming and dryreforming (T > 880 K).

C3H8þ3H2O ! 3CO þ 7H2; DH298¼ 497 kJ=mol (3)

C3H8þ6H2O ! 3CO2þ10H2; DH298¼ 395 kJ=mol (4)

C3H8þ3CO2 ! 6CO þ 4H2; DH298¼ 620 kJ=mol (5)

Fig. 2. Composition profile of PdCeacac catalyst in R = 2.5 condition.

Indeed, Fig. 3 shows the results of dry reforming of propane at aratio CO2/C3H8 equal 4 (Eq. (5)) producing H2 and CO at a ratio of0.62 at 923 K, evidencing the occurrence of dry reforming duringthe reaction.

Higher temperature showed increasing H2 formation, whichcould be attributed to the water gas shift (WGS) reaction(CO + H2O! CO2 + H2), however, although CO2 decreases the H2

formation increases, suggesting that equilibrium was not reachedunder such conditions.

3.2. Equilibrium calculations

Equilibrium calculations were done for products as function oftemperature, pressure and composition, based on the Lagrangemultipliers principle [22], and ideal gas hypothesis. In thisprocedure, chemical reactions are not involved directly in any ofthe equations, but the choice of a set of species is entirelyequivalent to the choice of a set of independent reactions betweenthe species. Therefore, it is necessary to know what possibleproducts may be formed.

Fig. 4 shows the profiles of thermodynamic equilibriumcomposition for 1 bar and temperatures varying between 323and 923 K. The simulated inlet gas was the same as the experimentfor R = 2.5, with a feed composition of 0.5% C3H8, 1.25% O2 and98.25% He.

The profiles present interesting characteristics, mainly forhydrogen. The production of hydrogen initiates at low tempera-ture, reaching a maximum around 600 K and, finally, decreaseswith increasing temperature. However, the dry reforming reactiondoes not occur, as observed in the experiments (Fig. 2). Thedecrease of hydrogen at higher temperatures is attributed to thereverse water gas shift (RWGS), highly undesirable, because itconsumes H2 with concomitant formation of CO. Exothermicreactions present high equilibrium constant values at lowtemperatures. For this reason, the oxygen consumption iscomplete at 300 K. Therefore, experiments were performed farfrom equilibrium conditions.

Thermodynamic simulations were done varying the O2/C3H8

ratio (Fig. 5). The productions of H2 and CO were calculated for eachratio, at the point of maximum hydrogen production. For lowvalues of R (0.5 < R < 1.5), small water concentration is observedduring oxidation, which does not allow total conversion of propanein the subsequent reforming reaction and therefore, H2 and CO arelow. For higher R values, when oxidation of propane is complete(R = 5), the amounts of CO and H2 are also very small, since

Fig. 4. Thermodynamic composition simulation on propane autothermal reforming

(R = 2.5).

Fig. 6. Dependence of O2/C3H8 ratio on propane conversion for the PdCeacac

catalyst.

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–8580

practically all propane is converted into CO2 and H2O, as alreadyobserved experimentally (Fig. 1). On the other hand, high amountsof hydrogen can be formed for R between 1.5 < R < 2.5. In thisregion, the highest H2/CO ratio was obtained for R = 2.5 andtherefore, this ratio was used in the experiments aiming toevaluate the influence of different Pd precursors. The H2/CO ratiofollows the same trend of R.

3.3. Autothermal reforming

Fig. 6 presents the propane conversion as function oftemperature, using different O2/C3H8 ratios (1.5 < R < 5) for thePdCeacac catalyst. All profiles under stoichiometric conditions(1.5 < R < 3.5) present a sigmoidal form, starting with thereforming reaction, while Table 2 presents the ignition tempera-ture for steam reforming (Tlight-off). The values of the maximumconversion for each ratio R are represented by Xmax (Table 2), takenat the maximum temperature used in the experiments (923 K).Except for R = 3.5 and R = 5, the propane dry reforming did notoccur. Table 2 shows that the light-off temperature increases whenR increases. It suggests that oxygen inhibits strongly the reforming,

Fig. 5. Thermodynamic simulation of O2/C3H8 effect on hydrogen production.

shifting the light-off temperature to higher temperatures. Thehighest final conversion was obtained for R = 3.5.

Yazawa et al. [23] determined an optimal condition for R,while Zhou et al. [24] observed that the oxidation state ofpalladium is strongly affected with this ratio. They also observedthat for higher values the metallic fraction (Pd0) decreased untilcomplete transformation in the oxidation state (PdO), andconsequently, the activity decreased. XPS results showed thatunder such conditions the metallic fraction at the surface wasonly 12%.

Different authors [24–26] claim that the active phases duringthe oxidation of propane consist of small metallic (Pd0) sites andPdO particles. Probably under reductive conditions (R = 3.5) at790 K, Pd0 particles still exist, keeping the catalyst active. Here,approximately 30% of propane was converted under suchconditions due to steam reforming. Additional experiments usingDRIFTS in situ reaction (not shown) revealed that along thereaction CO was adsorbed on the metallic sites and PdO particlesunder autothermal condition (R = 2.5). On the other hand, XRD insitu (not shown) results under similar conditions displayed onlymetallic palladium, suggesting that at the surface both sites areactive during the reaction although in the bulk phase metallicpalladium predominate.

Autothermal reforming under reductive conditions, varying O2/C3H8 ratios are presented in Table 3. Between 1.5 < R < 3.5 higheramounts of hydrogen are produced. Although for R = 3.5 theconversion of propane is high (Fig. 6) the hydrogen concentration islow. For R = 1.5 the hydrogen formation is high at highertemperatures (T > 823 K), however, the H2/CO ratio is lowercompared to R = 2 and R = 2.5. In a narrow temperature range(723 < T < 823 K) and for R = 2.5, the hydrogen formation was thehighest among all ratios and still the H2/CO ratio was relativelyhigh (2–2.5). Higher temperatures favor higher amounts of

Table 2Temperature ‘‘light-off’’ and maximum conversion of propane as a function of

feedstock condition

Feedstock condition Tlight-off (K) Xmax (%)

R = 1.5 645 88

R = 2 665 84

R = 2.5 700 95

R = 3.5 790 98

Table 3Hydrogen production according to the temperature at various feedstock conditions

Temperature

(K)

H2 production (%) H2/CO ratio

R = 1.5 R = 2 R = 2.5 R = 3.5 R = 1.5 R = 2 R = 2.5 R = 3.5

723 0.56 0.67 0.67 0 2.28 2.49 2.10 0

773 0.95 0.95 1.09 0 1.89 2.33 2.39 0

823 1.33 1.19 1.33 0.81 1.63 2.03 2.12 2.40

873 1.82 1.44 1.53 0.92 1.48 1.93 1.93 2.37

923 1.95 1.86 1.83 0.87 1.49 1.87 1.84 2.12

Fig. 7. Effect of Pd precursor on composition profile (R = 2.5).

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–85 81

hydrogen for R = 1.5, but in the temperature range where reformernormally works (T = 773 K), the H2/CO ratio was higher for R = 2.5.For R = 2 the H2/CO ratio is high but the hydrogen concentration islow when compared to R = 2.5. Finally, for total oxidation (R = 5)hydrogen was not produced.

Fig. 7 displays the product distribution for R = 2.5 of thePdCeacac and PdCeCl catalysts. The profiles of PdCeacac andPdCeN catalyst are very similar. For this reason, the PdCeN profilewas not included in Fig. 7. There is a noteworthy difference relatedto the start-up reforming temperature. It starts around 800 K forthe PdCeCl catalyst, 100 K higher than for the PdCeacac catalyst.The presence of residual oxygen seems to inhibit the reforming,and only after total consumption of oxygen, H2 and CO commence.In addition, CO and hydrogen form concomitantly, but around850 K the CO concentration is higher than CO2. For R = 2.5 thereforming reaction (3) prevails in relation to the reaction (4) above850 K. Concerning the hydrogen concentration, the PdCeacaccatalyst reached 1.9%, whereas the PdCeCl 1.2%. It should beobserved that the reactants are diluted in He (98.25%) andtherefore, these values are of this order of magnitude. Contrary tothe PdCeacac catalyst, the dry reforming was not seen in thePdCeCl catalyst.

Repeated runs evidenced reproducibility of the results. Runswere repeated with different masses and temperatures withascending and descending cycles. Results of product distributionand activity were reproducible with different precursors.

The presence of residual chlorine ions on Pd/Al2O3 catalystsaffects strongly the catalytic activity, according to the literature[27–32]. On the other hand, the negative effect of residual chlorideis reversible and the activity may increase after elimination ofchlorine ions, according to Pieck et al. [33]. Schmal et al. [26]observed that the addition of water increased significantly theconversion of propane after approximately 10 h of reaction. Theincreasing activity with time on stream in this case was attributedto the elimination of chlorine due to water generated in theoxidation step, as follows:

Cl�Pd � þH�Pd � ! HCl þ Pd�

where * denotes an active site.The lower performance of the PdCeCl catalyst (Fig. 7) is due to

blocking Pd sites. Water released during the oxidation is consumedduring steam reforming, hindering chlorine removal, affecting thecatalytic performance.

Several authors [5,34] did not discard the hypothesis ofmethane formation during the hydrogenolysis (6) and methana-tion (7). They observed cracking of propane for high tempera-tures (T > 900 K), which may occur, generating methane, ethaneand ethylene. In our experiments, these compounds were notobserved.

C3H8þ2H2 ! 3CH4 (6)

CO þ 3H2 ! CH4þH2O (7)

3.4. Effect of water

The addition of water (propane–oxygen feed) was investigatedin the PdCeacac catalyst, varying water concentration (S = H2O/C3H8 molar ratio), where S = 3 corresponds to reaction (3) and S = 6to the reaction (4). Results are shown in Fig. 8. Autothermalreforming for R = 2.5 in the absence of water and steam reforming(S = 6) were included as reference. Notice that S = 6 representsteam reforming of propane without oxygen.

Water did not change the autothermal reforming cycle (R = 2.5,R = 2.5, S = 3 and R = 2.5, S = 6), because CO and H2 formedindependent of water in the inlet gas. However, the reforminginitiated at lower temperature (about 50–70 K) with water in theinlet gas. Propane conversion was high with addition of water.Hydrogenolysis and methanation did not occur under suchconditions.

However, small amount of water (R = 2.5, S = 3) increased thehydrogen production. At 823 K, the hydrogen concentrationincreased 50%. Also, the CO2 concentration increased with lessCO formation. Therefore, the effect of water on the selectivity wascalculated according to the following equation:

SCO ¼100 %COxð Þ

3ð%C3H8 in�%C3H8 outÞ

Table 4 presents the selectivity of CO and CO2 for three differenttemperatures (723, 773 and 823 K). Results show that waterinfluenced the remarkable selectivity of CO and CO2. Withincreasing water content, CO2 increased and CO decreased, whichis interesting, keeping the CO concentration low in the outletgas. On the other hand, the temperature causes the opposite,augmenting the CO concentration, which is compensated byincreasing the water content. Indeed, CO2 is formed throughreaction (4) around 700 K, and at higher temperature (�823 K)through reaction (3), generating CO. Through reforming alone(S = 6), CO increases, enhancing the hydrogen production (Fig. 8).

Fig. 8. Effect of water content on composition profile for PdCeacac catalyst.

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–8582

Fig. 9 shows the H2/CO ratio in the presence of water. For R = 2.5this ratio reaches 2.5 for wide temperature range, whereas withaddition of water (S = 6) it increases two-fold.

Resini et al. [35] presented results of the propane steamreforming on Pd–Cu/Al2O3 catalysts, using S = 6. Hydrogen formedonly above 800 K. For low propane conversions (�10%), theyobserved high selectivity to propylene and CO2, attributed to thepropane dehydrogenation (8) and water gas shift reactions (9),respectively. At higher temperatures (�1000 K) propane conver-sion increased, resulting preferentially the formation of ethyleneand methane according reaction (10). Aartun et al. [5] observedsimilar results for propane autothermal reforming with Rh catalystsupported on a Fe–Cr–Al alloys. The hydrogen was formed at 773 Kand increased up to 900 K. Ethylene and methane were also formedat higher temperatures (>950 K) by cracking (10).

C3H8 ! C3H6þH2 (8)

CO þ H2O $ CO2þH2 (9)

C3H8 ! C2H4þCH4 (10)

Table 4Influence of the water on selectivity of products containing carbon for the PdCeacac

catalyst

Condition Temperature (K)

723 773 823

SCO (%) SCO (%) SCO (%) SCO (%) SCO (%) SCO (%)

R = 2.5 25.2 74.8 34.4 65.6 45.2 54.8

R = 2.5, S = 3 24.8 75.2 31.0 69.0 37.3 62.7

R = 2.5, S = 6 22.6 77.4 25.8 74.2 30.0 70.0

S = 6 34.5 65.6 48.2 51.8 57.3 42.7

Whittington et al. [36] observed total conversion of propane forthe Pd/CeO2/Al2O3 catalyst at 773 K (R = 13). Kolb et al. [37] observedtotal propane conversion at 1023 K with a Rh–Pt/CeO2/Al2O3

bimetallic catalyst and a ratio S = 6. The catalyst was stable over6 h with time on stream. Barbier et al. [10] using similar catalyst,observed 95% conversion of propane at 723 K for autothermalreforming (R = 2, S = 30). According to these authors, Pt promotesoxidation while Rh the reforming reaction. In conclusion, ourcatalyst seems to be more active and selective compared withreported results in the literature.

Fig. 9. Effect of water content on H2/CO ratio for PdCeacac catalyst.

Fig. 11. Influence of CeO2 on H2/CO ratio in R = 2.5 condition.

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–85 83

The literature indicates [26,38–40] that water inhibits theoxidation reaction (first stage of the autothermal reforming). Thedissociative adsorption of water occurs on the palladium sitesleading to the formation of surface hydroxides. These, in turn, aremuch less active diminishing the reaction rate. On the other side,water is responsible for the elimination of residual chloride in thecatalyst, blocking the active sites [26,33]. According to Wang et al.[41], water is also important for the elimination of coke formationduring steam reforming. Our results show that water increases CO2

and H2 formation during steam reforming and reduces CO.

3.5. Effect of the ceria

Fig. 10 shows the product distribution for R = 2.5 for differentpalladium precursors (acac and nitrate) supported on CeO2/Al2O3

and g-Al2O3. Significant differences were observed in the hydrogenproduction. For both precursors, the presence of ceria modifies thepropane autothermal reforming. The hydrogen production on thecatalyst with ceria is higher than on the catalyst without ceria. At773 K, for example, the H2 concentration for PdCeacac, Pdacac,PdCeN and PdN is equal to 1.2, 0.8, 1.35 and 0.5%, respectively.Comparing the catalysts with and without ceria, oxygen does affectthe start up reforming reaction in both cases.

Whittington et al. [36] observed that cerium shifted theconversion of propane (T50%) to higher temperature for Pd andPt supported on Al2O3 catalyst. However, under reformingconditions the T50% was reduced drastically for all catalysts, ingood agreement with our results. Although the activity with ceriawas lower than without ceria, during the reforming it was stillhigh, compensating the low initial activity, affecting the entireprocess (oxidation + reforming) positively. Tagliaferri et al. [42]also observed the negative effect of ceria in the propane oxidation,

Fig. 10. Influence of CeO2 on composition profile for differ

while Guimaraes et al. [43] observed different behavior betweenoxidation and reforming for the Pd/CeO2/Al2O3 catalyst. Indeed,CeO2 supplies the oxygen deficiency due to the initial consumptionthrough the oxidation reaction. This factor contributes for thelower availability of metallic sites for adsorption of reactants andconsequently inhibits the reaction.

H2/CO ratios for different catalysts are displayed in Fig. 11. Thecatalysts with ceria presented high H2/CO values (around 2.5 forPdCeacac and PdCeN). The product selectivity (for carboncompounds) is presented in Table 5.

ent Pd precursors in autothermal reforming (R = 2.5).

Table 5Influence of CeO2 on selectivity of products containing carbon

Catalyst Temperature (K)

723 773 823

SCO (%) SCO (%) SCO (%) SCO(%) SCO (%) SCO (%)

PdCeacac 25.2 74.8 34.4 65.6 45.2 54.8

Pdacac 36.0 64.0 45.2 54.8 55.0 45.0

PdCeN 29.5 70.5 38.1 61.9 47.8 52.2

PdN 22.0 78.0 38.4 61.6 50.4 49.6

Fig. 13. Propane conversion with time on stream in R = 2.5 condition.

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–8584

Results show that those catalysts with ceria presented thehighest CO2 selectivity, with exception of the PdN catalyst at 723 K.The order in the CO2 selectivity is: PdCeacac > PdCeN > PdN > P-Pdacac. It should be noted the CO2 selectivity on the PdCeacaccatalyst is higher than on PdCeN, but the homologous without ceriachanged (PdN > Pdacac).

Catalysts with ceria showed at the beginning of the reformingreactions (temperature around 700–800 K) high CO2 molarfractions (Fig. 10). Different authors [44–46] reported theoccurrence of the WGS reaction (CO + H2O! CO2 + H2) with noblemetals supported on CeO2. Therefore, experiments were carriedout aiming to evaluate the influence of the catalyst with ceriacontent (PdCeacac) for these reactions and compared to a catalystwithout ceria (Pdacac). The feed consisted of a mixture 2% CO and2% H2O diluted in He. The results are presented in Fig. 12.

Fig. 12 shows that the catalyst with ceria is the most active forthe WGS. Consumption of CO for the PdCeacac catalyst initiatearound 500 K and reached maximum conversion around 650 K.Then the CO conversion decreased with increasing temperature.On the other hand, the Pdacac catalyst showed low conversions forall temperatures. It suggests that the WGS reaction cannot bediscarded, in particular for the catalysts with ceria. Due to thisreaction at temperatures next to the reforming beginning(�700 K), the CO2 selectivity is high, however in opposite, at

Fig. 12. Influence of CeO2 in water gas shift reaction.

higher temperatures (�823 K), the selectivity decreases andpresents low CO conversions, in accordance with Fig. 12. Indeed,with increasing temperature the reforming region causes adecrease in CO2 selectivity, which can be attributed to the twodistinct phenomena: occurrence of the WGS reaction at tempera-tures around 700 K and the reforming reaction (4), generating CO2,occurring preferentially at a first moment in the reforming region.Therefore, the selectivity of CO increases continuously withincreasing temperature. For PdCeacac catalyst, the decrease inhydrogen yield after 640 K is a thermodynamic effect that limitsthe reaction, discarding any catalyst deactivation under suchconditions. Furthermore, on Pdacac catalyst, the hydrogenequilibrium was not achieved.

Fig. 13 shows the conversion of propane with time on streamfor the autothermal reforming (R = 2.5) on PdCeacac and PdCeNcatalysts. The catalysts were stable up to 48 h, after smalldeactivation in the first hours. The initial conversions on PdCeacacand PdCeN were 73%, in accordance with the TPSR results wherethe conversions were 79 and 77% for the PdCeacac and PdCeNcatalysts, respectively. After 48 h, when the temperature waschanged to 873 K the conversion reached 95%. TPSR results showedan initial conversion of 95% at 873 K under similar feed conditions.Therefore it can be inferred that the small deactivation occurs onlyduring the first hours of reaction. Additionally, through thermo-gravimetry, coke formation was not detected in catalysts even afteraging for 48 h.

4. Conclusions

High production of hydrogen was obtained by autothermalreforming of propane on Pd/CeO2/g-Al2O3 catalysts, under sub-stoichiometric O2/C3H8 ratio in presence and absence of water.

According to thermodynamic calculations, the optimum O2/C3H8 ratio for hydrogen production is between 1.5 and 2.5. In thisinterval, the results showed for R = 2.5 the highest value for H2/CO.

The PdCeCl catalyst was less active. The weaker performance isattributed to the presence of chlorine ions at the surface,competing for active sites on Pd.

The catalyst prepared with acac precursor was the most active,because the reforming temperature was 100 K lower than for thechloride precursor. Additionally, the H2/CO ratio and the H2

production on this catalyst were the highest.

W.L.S. Faria et al. / Applied Catalysis B: Environmental 85 (2008) 77–85 85

The addition of water to C3H8–O2 feed increased the propaneconversion and lowered the reforming temperature, about 50–70 K.

For all conditions, methane formation was not detected,indicating the absence of hydrogenolysis, methanation and propanecracking reactions. Additionally, the selectivity of products (CO andCO2), showed that CO diminished with water addition.

The Pd/CeO2/Al2O3 system presented higher hydrogen produc-tion, compared to Pd/Al2O3. The presence of CeO2 increased the H2/CO ratio and lowered the CO selectivity. The CeO2 influencespositively to a large extent the high activity in the WGS reaction.The catalysts were stable for 50 h with small deactivation in thefirst hours of reaction.

Acknowledgements

The authors would like to acknowledge CNPq, Finep (Pronex)for the financial support. One of the authors (Wagner Faria) wouldlike to thank FAPERJ for the scholarship.

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