CeO2 catalysts for methanol decomposition

Applied Catalysis A: General 268 (2004) 107–113

Characterization and catalytic performances of La doped Pd/CeO2 catalysts for methanol decomposition Kunpeng Sun, Weiwei Lu, Min Wang, Xianlun Xu∗ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Received in revised form 1 March 2004; accepted 15 March 2004 Available online 8 May 2004

Abstract The catalytic behavior of Pd (2 wt.%) catalysts supported on CeO2 and promoted with La2 O3 was investigated for methanol decomposition. The addition of lanthana significantly improved the catalytic activity for methanol decomposition. Over LaPd/Ce-1.00 catalyst, a 100% conversion of methanol can be achieved at around 275 ◦ C, which is about 40 ◦ C lower than the temperature needed for Pd/CeO2 . TPR results showed that the presence of lanthana shifted the reduction temperature of CeO2 to lower values, while it hindered the reduction of PdO due to an accelerated diffusion of oxygen ions in the ceria–lanthana interface. Pdδ+ -like species were formed on the catalyst surface due to the SMSI effect. Moreover, an increase of Pdδ+ /Pd0 ratio accompanied by the shift of the conversion profile to the lower temperature with increase of lanthana loading indicated that the low-temperature decomposition of methanol was very sensitive to the valence of palladium. The reasons for the formation of such Pdδ+ -like species caused by lanthana addition are discussed in detail based on XRD, BET and XPS results. A simplified hypothesis about side reactions, which might occur during methanol decomposition to form by-products in this work, is also proposed. © 2004 Elsevier B.V. All rights reserved. Keywords: Methanol decomposition; Synthesis gas; Palladium; Ceria–lanthana solid solution

1. Introduction Combustion of hydrocarbon fuels is the largest source of the world’s air pollution; it is responsible for the atmospheric presence of CO, CO2 , SO2 , NOx , VOCs and indirectly O3 [1]. Methanol is potentially a cleaner alternative energy resource. Recently the decomposition of methanol to H2 and CO has received growing attention as a method of increasing fuel efficiency for methanol powered vehicles and as a convenient medium for long distance heat transportation. Moreover, the reaction is a highly endothermic reaction and is also applicable to recovery of the waste heat of around 200 ◦ C from industries. For the development of heat recovery systems, new catalysts for methanol decomposition, which are active at relatively low temperature (200–250 ◦ C), are indispensable. While extensive work has been done for decades on methanol decomposition, previous studies on methanol ∗ Corresponding author. Tel.: +86-931-827-7664; fax: +86-931-827-7088. E-mail address: [email protected] (X. Xu).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.03.020

decomposition catalysts have been mostly carried out at 300 ◦ C or above. As catalytic metals, Cu, Pt, Ni, and Pd have been extensively investigated; among these, Pd seemed to be the most effective for methanol decomposition. Recently, Usami et al. [2] tested a number of metal oxides as a support for Pd and found that Pd/ZrO2 , Pd/Pr2 O3 and Pd/CeO2 catalysts prepared by a deposition–precipitation method were active for the selective decomposition of methanol to H2 and CO at low temperature. The authors clarified that the interaction between Pd and ZrO2 affected the catalytic activity of Pd/ZrO2 , in which smaller metal particles and a stronger contact with suitable supports should be advantageous for the catalytic performances. The TOF analysis suggests that CeO2 and Pr2 O3 have more potential as the support of palladium than ZrO2 dose. Lanthana is a particularly attractive support because it permits the combined achievement of high selectivity and specific activity for methanol synthesis. Lanthana-modified Pd has been reported to be very active in the synthesis of methanol from CO and H2 [3–5]. As the reverse reaction of methanol synthesis from CO and H2 , methanol decomposition reaction was also tested

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over a series of Pd/SiO2 catalysts promoted with lanthana [6]. The present studies were undertaken to investigate the influences of doped Lanthana on the surface properties and catalytic performances of Pd/CeO2 for methanol decomposition reactions. X-ray diffraction (XRD), temperature-programmed reduction (TPR), and X-ray photoelectron spectra (XPS) were used as the principal methods for catalyst characterization.

catalysts were reduced in a H2 flow (10% in Ar) at 400 ◦ C for 1.0 h. The methanol feed (LHSV = 1.8 h−1 ) was introduced into the reactor by using a syringe pump. Prior to entering the reactor, the feed was fully vaporized through a quartz vaporizer operating at 180 ◦ C. A glass condenser at about 0 ◦ C was used to separate liquid products from gaseous products. The products such as methanol, methyl formate (MF), and dimethyl ether (DME) were analyzed by an on-line gas chromatograph equipped with a GDX-401 column. H2 , CO, CO2 and CH4 were analyzed using a TDX-01 column.

2. Experimental 3. Results and discussion 2.1. Catalyst preparation 3.1. Characterization of catalysts The supported Pd catalysts with different La/Pd molar ratios, e.g. 0, 0.33, 0.66 and 1.00, were prepared by a deposition–precipitation method [7]. In short, an aqueous solution of 0.25 M Na2 CO3 was added drop-wise to an aqueous mixture of PdCl2 (or PdCl2 and La(NO3 )3 ) and CeO2 powder until the pH value reached 10 under vigorous stirring. The resulting suspension was aged for 3 h under stirring, and was then washed several times with distilled water until no chlorine anion was detected in the rinse water with an AgNO3 reagent. The resulting precipitate was filtered, dried at 120 ◦ C, and finally calcined in air at 400 ◦ C for 3 h. The palladium loading was 2 wt.% in all cases. The resulting catalysts will each be designated as LaPd/Ce, followed by a hyphen and the La/Pd molar ratio. 2.2. Catalyst characterization X-ray diffraction (XRD) data of catalysts were collected on a Rigaku Dmax-B diffractometer (Cu K␣ radiation, 50 kV, 60 mA). The crystallite sizes of CeO2 in these supported catalysts were evaluated from the full width at half maximum of the CeO2 (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peaks by using the Scherrer equation with a correction for the instrumental broadening. XPS were acquired on a PHI 550 photoelectron spectrometer equipped with an Mg K␣ (hν = 1253.6 eV). Binding energies were calculated with respect to C 1s at 285.00 eV. Binding energies were measured with a precision of 0.2 eV. The BET surface areas of catalysts were calculated from the nitrogen adsorption isotherm at 77 K measured in a micromeritics ASAP2010 apparatus. Temperature programmed reduction was conducted using a 60 mg of catalyst (in a fixed-bed quartz reactor) under 10% H2 in Ar (10 ml/min) with a ramping rate of 10 ◦ C/min from 20 to 600 ◦ C. 2.3. Catalytic activity test The methanol decomposition reaction was carried out in a conventional fixed-bed flow reactor. The catalyst (1 ml) was packed in a quartz tube, which was connected with a thermocouple in the catalyst bed. Prior to the catalytic reaction, all

3.1.1. XRD The XRD patterns (not shown) show only the peaks assigned to the cubic CeO2 phase. No reflections of PdO and La2 O3 were detected; no obvious change in the crystallite sizes of CeO2 (see Table 1) was detected. The accurate evaluation of the position of the main reflections of CeO2 corresponding to the (2 0 0), (2 2 0), (3 1 1), (4 0 0), (3 3 1) and (4 2 0) crystallographic planes in La-modified Pd/CeO2 catalysts showed a 2θ shift toward lower angles in the range of 0.04–0.10◦ with respect to the positions of the same peaks in the Pd/CeO2 catalyst (see Table 2). Similar results have been reported by other groups [8,9]. This is probably due to the formation of ceria–lanthana solid solution caused by dissolution of the La3+ ions into the CeO2 lattice, since, the radius of La3+ ion (1.19 Å) is larger than that of the Ce4+ ion (1.09 Å). 3.1.2. TPR TPR profiles of the different Pd catalysts are shown in Fig. 1. Although the TPR profiles exhibited similar spectra, variations (peak intensity and peak position) existed between samples with different La/Pd ratios. Reduction of Pd started at about 60 ◦ C and ended below 160 ◦ C. The two peaks (␣ and ␤) can be attributed to the two-step reduction of Pd2+ , which is in good agreement with Refs. [10,11]. The two stages of reductions of PdO can be represented as follows: 2PdO + H2 → Pd2 O + H2 O

(1)

Table 1 Summarized characterization data for the catalysts Catalyst

BET surface area (m2 /g catalyst)

Pd surface area (m2 /g catalyst)

Pd dispersity on surface (%)a

Dce b (nm)

LaPd/Ce-0 LaPd/Ce-0.33 LaPd/Ce-0.66 LaPd/Ce-1.00

58.81 60.15 60.92 61.48

1.51 1.55 1.68 1.82

2.46 2.54 2.79 3.09

25.4 25.9 26.1 25.9

a

Defined by SPd /SBET . Crystallite size of CeO2 evaluated from the FWHM using the Scherrer equation. b

K. Sun et al. / Applied Catalysis A: General 268 (2004) 107–113

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Table 2 XRD results of main reflections of CeO2 in the catalysts Catalyst

Position of main reflections of CeO2 /2θ (◦ ) (1 1 1)

(2 0 0)

(2 2 0)

(3 1 1)

(2 2 2)

(4 0 0)

(3 3 1)

(4 2 0)

LaPd/Ce-0 LaPd/Ce-0.3 LaPd/Ce-0.66 LaPd/Ce-1.00

28.52 28.52 28.52 28.52

33.06 33.06 33.06 33.02

47.48 47.46 47.42 47.40

56.36 56.36 56.34 56.32

59.08 59.08 59.08 59.08

69.48 69.46 69.44 69.38

76.74 76.66 76.66 76.64

79.06 79.04 79.02 79.02

Pd2 O + H2 → 2Pd0 + H2 O

(2)

H2 consumption/Pd (%)

Besides ␣ and ␤ peaks, a third peak (␥) appeared at about 370 ◦ C. Notably, the total hydrogen consumption of the Pd/CeO2 catalyst, which is estimated by integrating the areas of the ␣ and ␤ peaks, slightly exceeded that needed for stoichiometric reduction of PdO (indicated in the inset of Fig. 1), suggesting that the ␥ peak should be attributed to the additional reduction of surface oxygen of CeO2 . This phenomenon is consistent with other reports, which concluded that the presence of Pd could decrease the temperature of surface ceria reduction by about 200 K due to hydrogen activation by noble metals [12]. In the case of La modified catalysts, the ␥ peak was shifted to lower temperature with the increasing of La loading; e.g. the peak obtained on LaPd/Ce-1.00 catalyst occurred at a temperature 30 ◦ C lower than that of the un-promoted catalyst. This suggests that the addition of La to Pd/CeO2 increases the reducibility of CeO2 . As concluded from XRD, the La3+ ions are dissolved into the CeO2 lattice, resulting in the formation of oxygen vacancies in the Ce lattice because of the charge neutralization. Therefore, it is likely that this produces an accelerated diffusion of oxygen ions in the ceria–lanthana solid solution from the bulk to the support surface and from the support to Pd particles, which promoted the reduction of Ce4+ [13,14]. Furthermore, the increase in oxygen transfer

150 135 120 105

0.0 0.2 0.4 0.6 0.8 1.0

La/Pd molar ratio in catalysts

d

on ceria–lanthana interfaces can help maintain the PdO in a more cationic state, and thus hinder the reduction of PdO. This is evidenced in Fig. 1, where the peaks attributed to reduction of PdO shift to higher temperature and become broader. As a result, the total H2 consumption estimated by integrating the areas of the ␣ and ␤ peaks significantly exceeded the amount needed for PdO reduction. 3.1.3. XPS Table 3 presents the values of the binding energies for Pd 3d5/2 and Ce 3d5/2 , as well as the chemical concentrations of elements on calcined catalysts determined from XPS. Meanwhile, the surface areas of Pd of each catalyst were estimated on the basis of the surface concentration of elements and the BET surface area (see Table 1), assuming that the atomic cross sections are 0.060 nm2 for Pd, 0.050 nm2 for O2− , 0.037 nm2 for Ce4+ and 0.040 nm2 for La3+ , which are calculated from the atomic and ionic radii; that is, the surface area of Pd (SPd ) is calculated by the equation, SPd = SBET αPd cPd /αPd cPd + αM cM + cO . Here, SBET is BET surface area; αPd and αM are atomic ratios of Pd/O and M/O; cPd , cM and cO are the cross sections [2]. Clearly, the atomic ratios of La/Pd on the surface for the catalysts containing lanthana were much higher than the nominal values, which seem to suggest that doped lanthana tends to distribute on the surface of CeO2 . Moreover, the La 3d5/2 BE values were about 834.89 eV; they are substantially higher than the values observed for La2 O3 (i.e. 833.2–833.8 eV), but they are close to the values found for the dispersed “La” phase (i.e. 835.0–836.1 eV) [15–17]. That is to say, the La2 O3 in our catalysts is well dispersed on catalyst surface. Recent XPS studies [18,19] of PdO show that binding energies of Pd 3d5/2 in PdO are at 336.8 eV, the BE values for Pd 3d5/2 of our catalysts, especially for La-modified

c

30 60

90

b

Table 3 Data from XPS analysis of calcined catalysts

a

Catalyst

120 150 180 350 380 410 440 470 500

Temperature( C) o

Fig. 1. H2 -TPR patterns of catalysts: (a) LaPd/Ce-0; (b) LaPd/Ce-0.33; (c) LaPd/Ce-0.66; (d) LaPd/Ce-1.00. Inset: H2 consumption/PdO as a function of lanthana content.

LaPd/Ce-0 LaPd/Ce-0.33 LaPd/Ce-0.66 LaPd/Ce-1.00

BE (eV)

Surface concentration (at.%)

Pd 3d5/2

Ce 3d5/2

Pd

Ce

La

337.60 337.64 337.71 337.77

882.44 882.44 882.42 882.41

1.45 1.54 1.73 1.91

52.23 47.29 45.85 43.53

– 4.35 6.79 8.20

110

K. Sun et al. / Applied Catalysis A: General 268 (2004) 107–113 Pd3d3/2

a

Pd3d5/2

Pd3d3/2

a

Pd3d5/2

b

Pd3d5/2

b

Intensity(CPS)

Intensity(CPS)

Pd3d5/2

330

332

334

336

338

340

342

344

330

346

Binding Energy(eV)

(a)

332

336

338

340

342

344

346

344

346

Binding Energy(ev)

Pd3d5/2

Pd3d3/2

a

Pd3d5/2

334

(b) a

Pd3d3/2 Pd3d5/2

b

Intensity(CPS)

Intensity(CPS)

Pd3d5/2

b

330

332

334

336

338

340

342

344

346

Binding Energy(ev)

(c)

330

332

334

336

338

340

342

Binding Energy(eV)

(d)

Fig. 2. X-ray photoelectron spectra of Pd 3d region for reduced catalysts: (a) LaPd/Ce-0; (b) LaPd/Ce-0.33; (c) LaPd/Ce-0.66; (d) LaPd/Ce-1.00. Broken curves are the results of peak deconvolution using Gauss functions.

samples, are significantly higher than that of PdO. This result implies that Pd2+ ions in the present catalyst system are much more cationic than PdO. Usually, this phenomenon can be understood as the strong metal–support interaction (SMSI) effect. That is, the Pd–O bonding does not belong to Pd–O–Pd but rather to Pd–O–Ce in the Pd–CeO2 interface [20,21]. Cationic Pd2+ species could be formed by the SMSI effect. Higher Pd dispersion and surface area (see Table 1) are achieved in the lanthana modified Pd/CeO2 catalysts, which will induce a stronger metal–support interaction between more “separate” PdO particles and CeO2 . Fig. 2 presents the XPS spectra for Pd 3d of reduced catalysts. The large value of the FWHM of Pd 3d5/2 peak and its asymmetry on the high binding energy side suggested that the Pd 3d5/2 peaks in this work might consist of more

than one contribution. Thus, the Pd 3d5/2 lines were deconvoluted to two peaks (Pd 3da5/2 and Pd 3db5/2 ) with the least-squares fitting routine using the Gauss functions. The BE values Pd 3da5/2 , Pd 3db5/2 and Ce 3d5/2 peaks, as well as the chemical concentrations of elements on reduced catalysts determined from XPS, are tabulated in Table 4. The Pd 3da5/2 peaks centered at 335.47–335.55 eV can be assigned to palladium metal since XPS studies show that Pd 3d5/2 in Pd metal are at around 335.4 eV [18,19]. The Pd 3db5/2 peaks at around 337.70 eV are attributed to Pdδ+ -like (0 < δ > 2) species. Comparing the Pd 3db5/2 BE values of Pdδ+ with those of Pd2+ for calcined catalysts, one can conclude that δ may be close to one, since the valence of Pd generally relates to the binding energy [22]. Notably, a steady increase

Table 4 Data from XPS analysis of reduced catalysts Catalyst

LaPd/Ce-0 LaPd/Ce-0.33 LaPd/Ce-0.66 LaPd/Ce-1.00 a

BE (eV)

Ce4+ (%)

Surface concentration (at.%)

Pd 3da5/2

Pd 3db5/2

335.47 335.47 335.53 335.55

336.63 336.68 336.70 336.73

(0.32)a (0.44)a (0.52)a (0.67)a

Ce 3d5/2

Pd

Ce

La

882.46 882.49 882.49 882.51

1.30 1.40 1.58 1.75

55.65 51.01 49.12 46.70

– 6.06 7.15 11.65

Values in parentheses are the area ratios corresponding to Pd 3da5/2 peak and Pd 3db5/2 peak.

93.7 91.1 88.2 85.7

K. Sun et al. / Applied Catalysis A: General 268 (2004) 107–113

1  µ × 100 Ce = 14  where µ is percentage 4+

(3) µ

of peak area with respect to the total Ce 3d area. Fig. 3 shows the XPS spectra of the Ce 3d for the reduced samples. The percentage of Ce4+ defined above is present in Table 4. A remarkable decrease of the µ peak intensity was observed when the La3+ loading increased. The percentage of Ce4+ showed that the relative concentration of such Ce3+ species increases with the increase of lanthana content. This fact indicates that the presence of lanthana facilitates the reduction of the cerium surface species. A similar result has been obtained from TPR measurements.

Intensity(CPS)

a

b c d

''' 920

3d3/2 910

900

3d5/2 890

880

Binding Energy (eV) Fig. 3. X-ray photoelectron spectra of Ce 3d region for reduced catalysts: (a) LaPd/Ce-0; (b) LaPd/Ce-0.33; (c) LaPd/Ce-0.66; (d) LaPd/Ce-1.00.

sults in a shift of the conversion profile to the lower temperature. For the LaPd/Ce-0 catalyst, a complete conversion of methanol can be achieved at about 320 ◦ C. A temperature as low as 275 ◦ C was needed for 100% conversion over LaPd/CeO2 -1.00 catalyst. The fact suggests that the addition of La to Pd/CeO2 and the La/Pd ratio strongly affect the methanol decomposition activity. It is helpful to suggest some possible elementary steps of methanol decomposition over palladium catalysts in order to gain some insights into the reasons behind the promoted effect of lanthana and the product distribution [26–28]: CH3 OH(g) ↔ CH3 O(a) + H(a)

(4)

CH3 O(a) + H(a) → CH2 O(a) + H2 (g)

(5)

CH2 O(a) ↔ CO(a) + 2H(a)

(6)

2H(a) ↔ H2 (g)

(7)

100

Methanol Conversion/%

in Pd+ /Pd0 ratio (corresponding to the peak area ratio, tabulated in Table 4) with the lanthana content was observed. As proposed in TPR measurements, the introducing of lanthana into CeO2 lattices promoted the oxygen transfer from the bulk to the support surface and from the support to the Pd particles. The increase in oxygen mobility can facilitate the reduction of the CeO2 and maintain the Pd in a more cationic Pdδ+ state [23]. Moreover, the quantitative XPS analysis showed that surface concentration of Pd decreased while surface concentration of La and Ce increased after reduction, due to migration of cerium and lanthana on the surface of Pd [7]. This will result in a closer contact (strong metal support interaction (SMSI)) between support and metal. Shen and Matsumura [21] proposed that a kind of chemical bond, such as Pd–O–Ce, was formed in the Pd–CeO2 interface due to their strong interaction, which was also considered to facilitate the formation of Pdδ+ species. In the case of CeO2 , no valuable information about whether CeO2 has been reduced or not was obtained from the changes in BE values of Ce 3d5/2 in this work. Moreover, the XPS spectra of cerium compounds exhibit complicated features due to overlapping peaks if both Ce4+ and Ce3+ are present. Fortunately, it has been reported that a satellite peak (µ ) of the Ce 3d line, which arises exclusively from Ce4+ due to transition from the 4f0 initial state to the 4f0 final state, is absent from the Ce 3d spectra of pure Ce3+ species [24,25]. Thus, the “degree of reduction” of Ce4+ measurement can be estimated by the µ parameter. According to Shyu et al. [24], the integral area of the µ peak with respect to the total Ce 3d area (Ce 3d3/2 and Ce 3d5/2 ) could be translated into percentage of Ce4+ with the relative error in the range of 10%. The µ peak should constitute around 14% of total Ce 3d area. According to the linear dependence of percentage µ on percentage Ce4+ reported in the literature, the percentage of Ce4+ could be estimated by

111

80

60

40

3.2. Catalytic activity for methanol decomposition 20

Methanol was decomposed mainly to CO and H2 over these catalysts, accompanied by CH4 , CO2 , methyl formate (MF), and dimethyl ether (DME) as detectable by-products. Fig. 4 shows a plot of methanol conversion versus reaction temperature. It can be seen that increasing La loading re-

180 200 220 240 260 280 300 320

Reactiom Temperature/ºC Fig. 4. Methanol conversion as a function of reaction temperature for (䊏) LaPd/Ce-0, (䊐) LaPd/Ce0.33, (䊉) LaPd/Ce-0.66 and (䊊) LaPd/Ce-1.00.

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K. Sun et al. / Applied Catalysis A: General 268 (2004) 107–113

bond of the methoxyl group will be weakened, resulting in the acceleration of the abstraction of the C–H bond. It was also reported that cationic palladium may reduce the activation energy of the rate-determining step, and that the lower activation energy results in the higher activity at low reaction temperature [31]. As discussed above, the Pdδ+ species are detectable from the XPS analysis, and the percentages of such Pdδ+ species significantly increase by introducing lanthana into the catalyst system. For the La2 O3 -modified Pd/CeO2 system, a direct relationship between Pd dispersion (see Table 1) and the catalytic activity was also observed. Although the decomposition of methanol over Pd catalysts has been studied widely in the field of surface science, it is still difficult to select exact reaction paths to explain the formation of by-products. The formation of MF over palladium supported on zinc oxide has been reported by Iwasa et al. as follows [32]:

(CH 3 ) 2 O + H 2 O (11) (10 )

CH 3 OH

(12)

HCOOCH 3

(13)

↑ + CH 3 O(a )

CH 3 O(a ) + H(a ) ↓ + CH 3 O(a )

HCOOCH 3 (a ) + H(a ) ↓ (16) CO 2 + H 2

(14)

CO + H 2 (17 )

CH 4 + H 2 O

(15)

↓ + H 2O CO 2 + H 2

Fig. 5. Schematic diagram of mechanism for the formation of by-products during methanol decomposition.

CO(a) ↔ CO(g)

(8)

In the elementary steps above, reaction (5), i.e. the abstraction of C–H bond in methoxyl group is considered to be the rate-determining step. In this step, the decomposition of such a methoxyl group is promoted in the presence of cationic palladium species, which have been reported to be more active than zero-valent Pd for both methanol decomposition to syngas and methanol synthesis from syngas [2,29,30]. When Pd takes a partly oxidized state in catalysts, an electron is withdrawn from the methoxyl group to Pd and the C–H

2CH3 O(a) → HCOOCH3 (a) + H2

Reaction (9) is slower than the decomposing of the resulting surface methyl formate species to surface carbon monoxide and hydrogen species. After reviewing

12

3.5

10

3.0

Selectivity(%)

2.5

Selectivity(%)

(9)

2.0 1.5 1.0 0.5

8 6 4 2

0.0

0

-0.5 140 160 180 200 220 240 260 280 300 320 340 0

Reaction Temperature( C)

(a)

160

180

200

220

240

260

280

300

320

340

0

Reaction Temperature( C)

(b)

0.20

0.3

0.2

Selectivity(%)

Selectivity(%)

0.15

0.1

0.0

0.05

0.00

160 180 200 220 240 260 280 300 320 340 (c)

0.10

0

Reaction Temperature( C)

160 180 200 220 240 260 280 300 320 340 (d)

Reaction Temperature(0C)

Fig. 6. Effect of reaction temperature on by-product distribution over (䊏) LaPd/Ce-0, (䊐) LaPd/Ce0.33, (䊉) LaPd/Ce-0.66 and (䊊) LaPd/Ce-1.00: (a) CH4 ; (b) CO2 ; (c) MF; (d) DME.

K. Sun et al. / Applied Catalysis A: General 268 (2004) 107–113

several related references, we proposed a simplified hypothesis about side reactions, which might occur during methanol decomposition to form by-products in this work (see Fig. 5). Fig. 6 presents the selectivity profiles of the observed by-products with reaction temperature. The selectivity of CH4 formed by reaction (15) increased with reaction temperature on all the catalysts. The selectivity of CO2 on LaPd/Ce-0 catalyst is close to be zero; a small quantity of CO2 was formed at low temperature over La modified catalysts, and its selectivity increased gradually with further increase in reaction temperature. MF formation also increased with temperature and then decreased with further increasing temperature. Notably, increasing La loading results in a shift of the selectivity profile of MF to the lower temperature, accompanied by a decrease in selectivity. Therefore, we can infer that CO2 observed at low temperature over lanthana-contained catalysts might be produced by side reactions (16) and (17). A further increase in reaction temperature will suppress reaction (17) since the WGS reaction is exothermic. Thus, the gradual increase in selectivity of CO2 over all the catalysts tested in the higher temperature region was considered to be the result of reaction (16). DME might be produced by methanol dehydration on acidic sites of catalyst surface, which is an exothermic reaction. DME formation presents a similar selectivity profile to that of MF. This is probably due to the addition of lanthana partly neutralizing the surface acidic sites.

4. Conclusions In short, the following can be concluded from the present study: 1. The addition of a small quantity of lanthana to Pd/CeO2 significantly improved the catalytic activity for methanol decomposition. Cationic palladium species, e.g. Pdδ+ , are more active than zero-valent Pd for low-temperature decomposition of methanol to syngas. 2. The introduce of lanthana into CeO2 strongly accelerated the oxygen diffusion on the ceria-lanthana interface. As a result, the diffusion of oxygen from the ceria-lanthana interface to supported metal facilitated the formation of more Pd␦+ species. 3. The addition of lanthana improved the surface area and dispersion of Pd. On the one hand, the increasing amount and dispersion of surface Pd on CeO2 can promote the catalytic activity directly. On the other hand, this will

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