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Applying a face-centered central composite design to optimizethe preferential CO oxidation over a PtAu/CeO2eZnO catalyst

Sangobtip Pongstabodee a,c,*, Sutarawadee Monyanon a, Apanee Luengnaruemitchai b,c

aDepartment of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Patumwan,

Bangkok 10330, ThailandbThe Petroleum and Petrochemical College, Chulalongkorn University, 254 Phayathai Road, Patumwan, Bangkok 10330, ThailandcCenter for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University, 254 Phayathai Road, Patumwan,

Bangkok 10330, Thailand

a r t i c l e i n f o

Article history:

Received 11 October 2011

Received in revised form

24 November 2011

Accepted 4 December 2011

Available online 3 January 2012

Keywords:

H2O and CO2 content

Preferential oxidation of CO

Statistical design of experiment

Face-centered central composite

design

* Corresponding author. Department of ChPatumwan, Bangkok 10330, Thailand. Tel.: þ

E-mail addresses: sangobtip@gmail.com,0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.12.023

a b s t r a c t

The catalytic performance for the preferential oxidation of CO over a 1% (w/w) PtAu/

CeO2eZnO catalyst prepared by co-precipitation was investigated using a full 2k factorial

design with three central points and a 95% confidence interval, in order to screen for the

importance of the operating temperature (�C) and the H2O and CO2 contents (%) in the

simulated reformate gas on the CO conversion and selectivity. The catalyst was charac-

terized by TEM, BET, XRD and FTIR. The temperature and CO2 content had a significant

influence on the conversion, whilst the selectivity depended on the temperature only. A

face-centered central composite design was then used to evaluate the optimal conditions

by simultaneously considering the maximal conversion, selectivity and constraints of the

composition of realistic reformate gas. The difference in the estimated response and the

experimental one was within �2% and �3% for routing simulated and realistic reformate

gases, respectively.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction and the carbon monoxide (CO) elimination unit. A H2-rich

Hydrogen is an environmentally friendly fuel of potentially

widespread use, especially for proton exchange membrane

fuel cells (PEMFCs), which are one of the candidate energy

sources for portable power appliances and powering vehicles.

Nevertheless, the requirement of pure hydrogen for PEMFCs

and the issue of the safety of a large capacity hydrogen storage

system are the main limitations of hydrogen applications.

Thus, recently considerable attention has focused upon

producing hydrogen on-board directly. Such technology

currently uses two major units, the hydrogen production unit

emical Technology, Facu662 218 7676; fax: þ662 2sangobtip.p@chula.ac.th2011, Hydrogen Energy P

stream can be obtained following catalytic conversion of

methane, methanol, hydrocarbons or liquid fuels via steam

reforming, partial oxidation or autothermal reforming. Of

these substrates, methanol has received themost attention as

a candidate substrate for producing pure hydrogen on-board

due to the fact that it does not require desulfurization or

pre-reforming processes [1]. Additionally, only minimal coke

formation is obtained during the steam reforming of meth-

anol (SRM) compared to that produced from the other

substrates. However, the H2-rich stream is always contami-

natedwith CO, although the CO concentration depends on the

lty of Science, Chulalongkorn University, 254 Phayathai Road,55 5831.(S. Pongstabodee).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 14750

type of feedstock, operating procedure and the performance

of the catalysts used in the hydrogen production unit. The CO

elimination unit is required in the hydrogen-fuel processor

system since even trace levels of CO show strong chemi-

sorption on the anodic platinum (Pt) electrode of the PEMFC

and lower its performance dramatically. The wateregas shift

(WGS) reaction is then used to reduce the CO level from

w3e10% (v/v) to w1% (v/v), while the preferential oxidation

(PROX) of CO is further employed to clean-up the CO within

acceptable levels (<10 ppm).

Among the current methods in use for the CO elimination

unit, the PROX of CO seems to be one of the most effective

methods with an economic approach and minimal loss of H2.

The main reaction is as follows:

COþ 1=2O2/CO2 DH ¼ �283 kJ=mol (1)

and the undesired side reactions are hydrogen oxidation (Eq.

(2)), WGS reaction (Eq. (3)) and methanation (Eq. (4)):

H2 þ 1=2O2/H2O DH ¼ �241:8 kJ=mol (2)

COþH2O/CO2þH2 DH ¼ �41:2 kJ=mol (3)

COþ 3H2/CH4þH2O DH ¼ �206:1 kJ=mol (4)

The catalysts for the PROX of CO should exhibit a high CO

conversion including a high selectivity with respect to mini-

mization of the undesired side reactions. Furthermore, the

catalysts should be active in the presence of water and CO2

since they are some of the main components in the H2-rich

stream from hydrogen production unit. Therefore, the devel-

opment of catalysts with a suitable performance (rate, sensi-

tivity and selectivity) for the PROX of CO in the realistic

reformate gas is still needed. Catalysts are classified into the

two major categories of noble [2e12] and non-noble catalysts

[2,13e22]. The optimal catalytic activities of supported Pt

catalysts are shifted to a higher temperature when CO2 [5,6] or

water [5,7] or both [5,6] are present in the feedstream,

although some reports have claimed that the supported Pt

catalysts were not deactivated in the presence of CO2 and

water [2,8]. Moreover, adding excess O2 in the H2-rich stream

encourages not only a higher catalytic performance of the

supported Pt catalysts but also a higher level of hydrogen

consumption. Some research reports have highlighted

a significant decrease in the catalytic activities of supported

gold (Au) catalysts in the presence of CO2 alone [4,9] or in the

presence of CO2 and water [2,3,9], whereas the presence of

only a small amount of water enhanced the catalytic activity

of the Au catalysts [3,4]. In our previous work [10], we studied

catalytic performance of a series of PteAu catalysts prepared

by co-precipitation (CP) and single step sol-gel methods (SSG)

for selective CO oxidation. We found that the catalytic

performance over PtAu/Ce1Zn1O2 prepared by co-

precipitation was higher than that of PtAu/CeO2 and PtAu/

ZnO due to a higher metallic surface area and a smaller

particle size. The catalytic activity of supported Au catalysts

depends on the dispersion and size of the Au particles, the

type of support material and the preparation method. Sup-

ported copper [2,13e19,21,22] and manganese [20] catalysts

are effective for the PROX of CO but their activities are

decreased significantly in the presence of water and CO2.

Therefore, the presence of either water, CO2 or both has

a negative influence on the catalytic PROX of CO. However, it is

important to note that these studies have been investigated

on the assumption of no-interaction between the factors,

called a univariate (one-variable-at-a-time) experimental

approach.

A statistical design of experiment (DOE) was employed in

this work, since it is a powerful tool for process investigation

and optimization [17,23e26], by simultaneously considering

many factors at different levels and their potential interac-

tions. Thus, the catalytic activity for the PROX of CO over a 1%

(w/w) PtAu/CeO2eZnO catalyst was evaluated in terms of the

CO conversion and selectivity with respect to the operating

temperature (�C) and the presence of water (%) and CO2 (%) in

the simulating methanol reformate gas. The importance of

each factor and their interactions were evaluated by a full 23

factorial design. After screening the importance of each of the

three factorsand their interactionson the%COconversionand

selectivity, a face-centered central composite design (FCCCD)

falling under response surface methodology (RSM) was then

applied to optimize the responses. It is worth remarking that,

after determining the optimization, the validation of the

developed models was tested for both simulating reformate

gas and realistic methanol reformate gas. In addition, the

stability of the catalyst under realisticmethanol reformate gas

was evaluated over a continuous 10 h time period.

2. Experimental

2.1. Catalyst preparation

The 1% (w/w) PtAu/CeO2eZnO catalyst was prepared by co-

precipitation. The desired amount of Ce(NO3)3$6H2O (Merck),

Zn(NO3)2$4H2O (Merck), H2PtCl6$6H2O (Fluka) and

HAuCl4$3H2O (Fluka) were mixed simultaneously to make an

aqueous solution and stirred continuously. The mixed solu-

tionwas held at pH 8 by the dropwise addition of 0.5MNa2CO3

aqueous solution as required. Themolar ratios of Pt to Au and

of Ce to Zn were kept constant at 1:1. After aging for 1 h at

80 �C, the precipitated material in suspension was harvested

by filtration,washedwithwarmdeionizedwater several times

to remove the excess ions, dried at 110 �C for 12 h and then

calcined at 500 �C for 5 h.

2.2. Catalyst characterization

The particle morphology of the catalysts was observed by

transmission electronmicroscopy (TEM) using a JEM 2010 TEM

microscope operating at 200 kV in bright and dark fieldmodes.

The BrunauereEmmetteTeller (BET) method for evaluating

the surface area of the catalyst was examined by N2 adsorp-

tion/desorption at �196 �C (Micromeritics ASAP 2020). The

crystalline structure was determined by X-ray diffractometry

(XRD) using a Rigaku X-ray diffractometer system equipped

with a RINT 2000 wide-angle goniometer and using CuKa

radiation (l ¼ 1.54 A) and a power of 40 kV � 30 mA. The

particle diameter was calculated by the DebyeeScherrer

equation at the main X-ray line broadening in each phase.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 1 4751

Fourier transform infrared spectrometry (FTIR) was used to

investigate the functional groups. Solid sample was milled

with potassium bromide (KBr) to form a very fine powder. The

powderwas then compressed into a thin pellet to be analyzed.

The spectra were collected on a PerkineElmer (Spectrum one)

spectrometer equipped with a mercury-cadmium-telluride

(MCT) detector to record wavenumber range of functional

group.

2.3. Catalytic activity measurement

The catalytic activity for the PROX of CO over a 1% (w/w) PtAu/

CeO2eZnO catalyst was investigated at atmospheric pressure.

A 100 mg catalyst sample was placed between two layers of

quartz wool in the middle of a 6 mm ID fixed-bed U-tube

reactor. The simulating reformate gas, which consisted of (all

(v/v)) 1% CO, 1% O2, 0%e10% H2O, 0%e20% CO2 and 40% H2

with the remainder made up of He, was routed to the reactor

at a flow rate of 50 cm3 min�1 by a mass flow controller. The

operating temperature was controlled in the range of

50 �Ce190 �C and monitored by a thermocouple placed in the

center of the catalyst bed. The influent and effluent gases

were routed to an ice-condenser to trap the water and then

analyzed by on-line gas chromatograph (Agilent Technolo-

gies, model 6890N) equipped with a carbosphere column and

a thermal conductivity detector (TCD). Heliumwas used as the

carrier gas. The catalytic activities are expressed in terms of

the % CO conversion and selectivity, which were calculated

based on the CO consumption, as shown below:

CO conversion ð%Þ ¼ ½CO�in � ½CO�out½CO�in

� 100 (5)

CO selectivity ð%Þ ¼ 0:5� �½CO�in � ½CO�out�

½O2�in � ½O2�out� 100 (6)

where [CO]in and [CO]out are the concentrations of CO (% (v/v))

in the feedstream and the effluent, respectively, [O2]in and

[O2]out are the concentrations of O2 (% (v/v)) in the feedstream

and the effluent, respectively.

2.4. Statistical design of experiments

2.4.1. A full 2k factorial designA factorial design was carried out to evaluate the effect of the

operating temperature (�C), H2O content (% (v/v)) and CO2

content (% (v/v)) in the reformate gas, and their interactions

on the catalytic activities for the PROX of CO in terms of the %

Table 1 e Physical properties of the prepared catalysts.

Catalyst Status BET surface areaa (m2 g�1)

PtAu/CeO2eZnO Fresh 58.6

PtAu/CeO2eZnO Spentc 54.7

a Determined by BET surface analyzer.

b Determined by XRD from the line broadening of CeO2 (1 1 1) and ZnO (

c Composition of the realistic reformate gas (all (v/v)) was 36.8% H2, 1.1%

CO conversion and selectivity. The other factors that likely

affect the catalytic activities, including the catalyst weight to

total gas flow rate (W/F), catalyst type and the reactor volume

were held constant throughout all the experiments. The

experimental matrix for a full 23 factorial design with three

central points was then designed and employed. The experi-

ments were done in a completely random order in order to

minimize errors due to systematic trends in the factors. The

DesigneExpert 5.0 software package (Stat Ease Inc. Minneap-

olis, USA) was employed to treat the experimental data and to

perform the statistical analysis at a 95% confidence interval,

such as the normal probability of the residues, the Pareto

chart of absolute standardized effect, analysis of variance

(ANOVA) and the % contribution.

2.4.2. Response surface methodology (RSM)After screening the three factors and their interactions for any

significant effect upon the CO conversion and selectivity with

the factorial design (Section 2.4.1), the factors found to be

important (significant influence) were then selected for RSM

analysis. To this end, FCCCDwith the above important factors

was applied sequentially to optimize the conditions for the

PROX of CO by simultaneously considering maximal CO

conversion, selectivity, the composition of realistic reformate

gas and temperature of the feedstream. A standard ANOVA at

a 95% confidence interval was then carried out to analyze the

response surface models.

2.4.3. Validation of the modelThe independent screened factors which were found to have

the major influence on the CO conversion and selectivity

(Section 2.4.1) were randomly selected within the given levels

to investigate the accuracy of the developed model as ob-

tained from the RSM (Section 2.4.2). The other less important

factors were held constant at their respective optimal level. A

set of six experiments were then designed and employed

under a feed condition of simulating and realistic reformate

gases. The residual distribution plot, a statistical analysis tool

for determining the validity of themodel, was then employed.

3. Results and discussion

3.1. Catalyst characterization

The physical properties of the fresh and spent 1% (w/w) PtAu/

CeO2eZnO catalysts are summarized in Table 1. No major or

Pore volumea (cm3 g�1) Crystallite sizeb (nm)

CeO2 ZnO

0.16 7.7 27.1

0.15 7.6 23.7

1 0 1).

CO, 1.1% O2, 8.2% H2O, 11.6% CO2 and 41.2% He.

Fig. 1 e Representative TEM images and derived PteAu particle size distribution for the (a) fresh and (b) spent 1% (w/w) PtAu/

CeO2eZnO catalyst. The composition of the realistic reformate gas was (all (v/v)) 36.8% H2, 1.1% CO, 1.1% O2, 8.2% H2O, 11.6%

CO2, and 41.2% He.

(d)

(e)

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 14752

significant difference between the fresh and spent catalysts in

their BET surface area, pore volume or the CeO2 and ZnO

crystallite size were observed. The average particle size of

catalyst and its size distribution were determined from the

2Theta [degree]

20 30 40 50 60 70 80 90

Inte

nsit

y [c

ps]

Cerianite Zincite

(a)

(b)

Fig. 2 e Representative XRD patterns for the (a) fresh and (b)

spent 1% (w/w) PtAu/CeO2eZnO catalyst. The composition

of the realistic reformate gas was (all (v/v)) 36.8% H2, 1.1%

CO, 1.1% O2, 8.2% H2O, 11.6% CO2 and 41.2% He.

Wavenumber (cm-1)

1000 1500 2000 2500 3000 3500 4000

% T

ran

sm

issio

n

(a)

(c)

(b)

Fig. 3 e Representative FTIR spectra for the (a) fresh 1% (w/

w) PtAu/CeO2eZnO catalyst and (bee) the spent catalyst

with varying (bed) simulated or (e) realistic reformate gas

compositions. The simulated reformate gas compositions

were all (v/v) 40% H2, 1% CO and 1% O2, and then

supplemented with (b) nothing, (c) 10% H2O or (d) 20% CO2.

The realistic reformate gas composition was 36.8% H2, 1.1%

CO and 1.1% O2, 8.2% H2O, 11.6% CO2. In all cases the

remaining proportion of the gas was He.

CO

con

vers

ion

(%)

and

sele

ctiv

ity

(%)

0

10

20

30

40

50

60

70

80

90

100

50

70

90

110

130

150

170

190 50

70

90

110

130

150

170

190 50

70

90

110

130

150

170

190 50

70

90

110

130

150

170

190

Temperature (°C)

(a) (b) (c) (d)

Fig. 4 e The catalytic performance (solid line for conversion; dashed line for selectivity) of the 1% (w/w) PtAu/CeO2eZnO

catalyst over the operating temperature range of 50e190 �C when feeding the feedstream with (all (v/v)) 40% H2, 1% CO and

1% O2 supplemented with (a) nothing, (b) 10% H2O, (c) 20% CO2 and (d) 10% H2O and 20% CO2. In all cases the residual gas

composition was He.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 1 4753

TEM images (Fig. 1) when routing realistic reformate gas as the

feedstream. Black spots, whichwere dispersed throughout the

area, are the bi-metallic phase of Pt and Au, which implies

that the catalysts are well dispersed on the support, and

individual Pt-metallic phases could not be separated from the

Au-metallic phases. The mixed oxide support is visualized as

the grey area. However, the average size of the fresh metallic

particles was smaller, compared to that of the spent catalyst.

The XRD patterns of the fresh and spent catalysts are dis-

played in Fig. 2, where all the peaks correspond to the mixed

oxide support. There were no characteristic peaks of Pt

(2q ¼ 39.8� and 46.2�) or Au (2q ¼ 38.2�, 44.4� and 77.6�), which

suggests that the Pt and Au particles were highly dispersed on

surface of themixed oxide support and/or that the actual Pt or

Au loading level (<1%) in the catalysts is too low to be detec-

ted. Alternatively, the bi-metallic phase (Pt and Au) particles

could be too small to be detected by XRD. Regardless, these

Table 2 e Experimental matrix (full 23 factorial designwith threPROX of CO over a 1% (w/w) PtAu/CeO2eZnO catalyst.

Factors Variables Unit

A Temperature �CB H2O content %

C CO2 content %

Standard order Run order A B

1 9 �1 �1

2 10 1 �1

3 6 �1 1

4 3 1 1

5 11 �1 �1

6 2 1 �1

7 1 �1 1

8 4 1 1

9 5 0 0

10 7 0 0

11 8 0 0

results are consistentwith the TEM images. The peaks at 28.6�,33.2�, 47.5�, 56.3�, 59.1�, 69.4�, 76.7�, 79.1� and 88.4� correspondto the cerianite phase (CeO2), whereas the diffraction peaks at

31.8�, 34.6�, 36.2�, 62.9� and 68.0� are from the zincite phase

(ZnO). The intensity peak of the fresh catalyst was higher than

that of the spent ones, supporting that the fresh catalysts had

a higher crystalline contentwhen compared to the spent ones.

The crystallite size of the metal oxide phases (CeO2, and ZnO),

as calculated by the DebyeeScherrer equation from the X-ray

line broadening of the (1 1 1) diffraction peak for CeO2 and the

(1 0 1) diffraction peak for ZnO, are the same for the spent and

fresh catalysts except that the peak intensity is larger in the

fresh catalysts (Table 1; Fig. 2).

The functional group(s) on the surface of the fresh and

spent catalysts was evaluated by FTIR analysis, with an

example of the obtained spectra shown in Fig. 3. The fresh

catalyst showed an absorbance peak at around

e central points) for, and the results of, the evaluation of the

Low (�1) Medium (0) High (1)

50 120 190

0 5 10

0 10 20

C CO conversion (%) CO selectivity (%)

�1 25.7 70.0

�1 47.5 20.9

�1 22.6 70.5

�1 53.6 25.0

1 7.91 66.6

1 42.0 19.9

1 6.92 66.0

1 45.2 21.3

0 87.2 37.1

0 83.1 33.0

0 83.9 40.0

Fig. 5 e Normal probability plot of the effects for a full 23

factorial design with three central points when using the %

CO (a) conversion and (b) selectivity as the response.

0 10 20 30 40 50 60

A

B

C

AB

AC

BC

ABC

Curvature

5.65

8.72

0.41

0.93

0.80

1.37

3.16

1.35

46.52

Standardized Effect

0 10 20 30 40 50 60

A

B

C

AB

AC

BC

ABC

Curvature

7.50

55.67

1.28

0.18

4.91

3.38

11.82

1.30

31.30

Standardized Effect

a

b

Fig. 6 e The Pareto diagram for a full 23 factorial design

with three central points when using the % CO (a)

conversion and (b) selectivity as the response. The

absolute standardized value of the effect of each factor and

its interaction appear at the right of each bar.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 14754

3100e3700 cm�1 with the center at w3500 cm�1, which

represents the OeH stretching mode [27,28]. The intensity of

this OeH peak in the fresh catalyst is higher than that of the

spent catalyst when no H2O was added to the feedstream. In

addition, the spectra of the spent catalysts displayed absor-

bance bands in two further regions; 1200e1700 cm�1 and

2852e2960 cm�1. The vibrational stretching frequencies at

1200e1700 cm�1 correspond to carbonate species, formed

from chemisorption of CO2 on the catalyst surface [28,29]. The

bands at 1520e1550 cm�1 and 1360e1385 cm�1 range repre-

sent the asymmetric and symmetric stretching of unidentate

carbonate species, respectively. The bands at 1610e1640 cm�1

are OeCeO asymmetric stretches of the bidentate carbonate

species. The CeH stretching peak at 2852 and 2960 cm�1

represents either formate [30] or methoxy groups [31]. These

results agree with the work reported by Martınez-Arias et al.

[32]. When H2O was added to the feedstream, the intensity of

the OeH peak in the spent catalyst was higher than that of the

fresh one (Fig. 3a and c), whilst the addition of CO2 (Fig. 3d) or

the co-addition of H2O and CO2 (Fig. 3e) to the feedstream

resulted in a broadened band for the OeH peak and a higher

intensity of the carbonateetype and formate peaks compared

to that seen in the fresh catalyst (Fig. 3a).

3.2. Catalyst activities

The catalytic activity of the 1% (w/w) PtAu/CeO2eZnO catalyst,

in terms of the % CO conversion and selectivity, at an oper-

ating temperature range of 50 �Ce190 �C when routing the

Table 3 e ANOVA results of the % CO conversion and selectivity data obtained from the full 23 factorial design with threecentral points for the PROX of CO over a 1% (w/w) PtAu/CeO2eZnO catalyst.

Source Sum of squares DFa Mean square F-value Probability (P-value) Contribution (%)

(a) Response: % CO conversionb

Model 2317.08 7 331.01 68.87

A 1959.69 1 1959.69 407.73 0.0024 22.99

B 3.37 1 3.37 0.70 0.4907 0.04

C 279.54 1 279.54 58.16 0.0168 3.28

AB 22.88 1 22.88 4.76 0.1608 0.27

AC 48.27 1 48.27 10.04 0.0868 0.57

BC 0.067 1 0.067 0.014 0.9170 0.0007

ABC 3.26 1 3.26 0.68 0.4965 0.04

Curvature 6197.43 1 6197.43 1289.41 0.008 72.70

Residual 9.61 2 4.81 0.11

Cor Total 8524.13 10

(b) Response: % CO selectivityc

Model 4358.08 7 622.68 50.73

A 4327.29 1 4327.29 352.64 0.0028 95.43

B 3.62 1 3.62 0.29 0.6416 0.08

C 20.03 1 20.03 1.63 0.3296 0.44

AB 3.78 1 3.78 0.31 0.6346 0.08

AC 1.30 1 1.30 0.11 0.7760 0.03

BC 1.71 1 1.71 0.14 0.7447 0.04

ABC 0.34 1 0.34 0.028 0.8824 0.01

Curvature 152.03 1 152.03 12.39 0.0421 3.35

Residual 24.54 2 12.27 0.54

Cor Total 4534.65 10

a DF ¼ Degrees of freedom. A, B and C are as defined in Table 2.

b R-Squared ¼ 0.9959 and Adj. R-Squared ¼ 0.9814.

c R-Squared ¼ 0.9944 and Adj. R-Squared ¼ 0.9748.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 1 4755

feedstream with various compositions are presented in Fig. 4.

The catalytic performance decreased with any further

increase in the operating temperature after achieving the

maximum % CO conversion, but the temperature required to

attain the maximum % CO conversion changed with the

different feedstream gas compositions. Without the addition

of H2O and/or CO2 to the feedstream (Fig. 4a) the % CO

conversion increased 3.72-fold from w25%ew93% as the

temperature increased from50 �C to the optimal at 90 �C. Here,

CO reacted with O2 to produce CO2 which could then be

chemisorbed on the catalyst surface as carbonate, as evi-

denced in FTIR spectra (see Fig. 3b). Further increasing the

temperature from 90 �C to 190 �C significantly (1.98-fold)

decreased the % CO conversion to w47% because of the

competition of the undesired reaction of H2 oxidation. The

obvious evidence for that is provided by catalytic activity

results themselves, the OeH intensity increase providing only

indirect support that H2O has been formed (see Fig. 3b). CO

selectivity was decreased when increasing the operating

temperature. Overall, a w93% of the maximum %CO conver-

sion with w41% of its selectivity was achieved at 90 �C.When 10% (v/v) H2O was added to the feedstream a similar

trend of results were seen to that obtained without H2O and

CO2. And discussed above, with an optimal %CO conversion at

90 �C and declining thereafter with increasing temperature,

but not as sharply as that seen without water. That is with the

addition of 10% (v/v) H2O to the feedstream, the % CO

conversion was some 5%e10% higher at each respective

operating temperature of >90 �C than that seen without the

addition of water. The presence of H2O in the feedstream thus

increases the catalytic activity for CO oxidation. Indeed, as

evidenced from the FTIR analysis, the spent catalyst from the

reaction with added H2O to the feedstream (Fig. 3c) showed

a higher intensity peak of the hydroxyl group and a lower

intensity of the carbonate and formate vibrational stretching

peaks compared to that seen without the addition of H2O and

CO2 (Fig. 3b). The slight positive effect of the presence of H2O

on the catalytic activity is likely to be due to the provision of

hydroxyl groups from the water for the CO oxidation reaction

to take place, and so results in a shift in the % CO conversion.

When adding H2O to the feedstream, a lower accumulation of

the carbonate intermediate and formate species on the cata-

lyst surface was observed even though the % CO conversion

was enhanced, which indicates that water may have attacked

and decomposed some of the carbonate intermediate on the

catalyst surface.

The addition of CO2 in the feedstream caused a dramatic

decrease in the maximum % CO conversion and selectivity

(Fig. 4c), with only a w56% conversion and w28% selectivity

compared to that seen without the addition of CO2 and H2O in

feedstream. The operating temperature for themaximal % CO

conversion was shifted from 90 �C to 150 �C. These results are

in agreement with those reported by Schubert et al. [4] and

Panzera et al. [33], where the presence of CO2 also significantly

increased the maximum conversion temperature. This

dramatic decrease in the catalytic performance was likely to

be due to CO2 chemisorption that results in the formation of

carbonate intermediates and formate species on the catalyst

surface, as evidenced in the FTIR spectra (Fig. 3d). The accu-

mulation of these species on the catalyst surface could block

Fig. 7 e Main effect plot with its response for the % CO (a) conversion and (b) selectivity.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 14756

the active sites for the PROX of CO. Thus, the presence of CO2

in the feedstream has a significant negative effect on the

performance of the catalyst.

The co-addition of 10% (v/v) H2O and 20% (v/v) CO2 to the

feedstream was somewhat intermediate between that seen

with the addition of either 10% (v/v) H2O or 20% (v/v) CO2 alone,

but potentially biasedmore toward that of 20% (v/v) CO2. From

Fig. 4d, the % CO conversion profile was much lower than that

obtained without the co-addition of CO2 and H2O, or the

addition of 10% (v/v) H2O, to the feedstream but was only

slightlyhigher than that seenwith theadditionof 20% (v/v) CO2

to the feedstream. It thus appears that the negative affect of

the addition of 20% (v/v) CO2 upon the % CO conversion by the

catalyst is greater then thepositive effect of the additionof 10%

(v/v) H2O, although this requires further counter-titrations for

confirmation. Themaximum%CO conversion and selectivity,

obtained at 130 �C, was aroundw63% andw30%, respectively.

The operating temperature for the maximal % CO conversion

was shifted lower compared to that seen with the addition of

CO2 (150 �C), as has been reported before [3e6,9,19,20,22].

Comparing the FTIR spectra (Fig. 3dee) suggests that a lower

level of formation of the carbonate intermediate and formate

species on the catalyst surface occurred when co-adding H2O

and CO2 to the feedstream than when adding only CO2. It is,

however, surprising that a higher intensity of the hydroxyl

regionwas observed compared towhen adding only CO2 to the

feedstream.The lowernegative effect of the co-additionofH2O

and CO2 to the feedstream on the catalytic activities compared

to that for the addition of CO2 only can probably be explained

by the fact that water can provide hydroxyl groups where the

reaction takes place. Carbonate intermediate on the catalyst

surface may be attacked and decomposed by water [34], and

results ina reducedaccumulationof the formatespecieson the

active sites of the catalyst.

3.3. Factors screening in a full 23 factorial design

Based on the catalytic activities for the PROX of CO, the

importance of the three independent factors (Section 3.2) on

the catalytic activity was evaluated by using a full 23 factorial

design with three central points and using the % CO conver-

sion and selectivity as responses. The factor levels on the

Table 4 e Experimental variables for a faced-centered central composite design (FCCCD) response surface methodology(RSM) with three central points for the PROX of CO over a 1% (w/w) PtAu/CeO2eZnO catalyst.

Factors Variables Unit Low (�1) Medium (0) High (1)

A Temperature �C 90 120 150

C CO2 content % 0 10 20

Standard order Run order A C CO conversion (%) CO selectivity (%)

1 9 90 0 93.0 41.0

2 2 150 0 67.9 30.0

3 8 90 20 16.3 59.9

4 10 150 20 56.1 28.2

5 5 90 10 54.7 50.5

6 1 150 10 62.0 29.1

7 4 120 0 80.3 35.4

8 3 120 20 38.6 44.6

9 7 120 10 59.4 37.1

10 11 120 10 58.2 40.0

11 6 120 10 57.2 40.0

Note: The H2O content in the feedstream was held at the medium level (5% (v/v)), as given in the factorial design (Table 2).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 1 4757

natural scale were encoded in the dimensionless scale as þ1

for the high level, 0 for the central point and �1 for the low

level, as shown in the upper section of Table 2. The statisti-

cally designed set of experimental matrix and the responses

are presented in the lower section of Table 2.

A normal probability plot of the effect estimates was then

constructed in order to evaluate each independent factor and

its interactions for both the % CO conversion and selectivity

(Fig. 5). Based on the zero value of the abscissa at a 50%

normal probability level, these graphs could be divided in two

regions, one of a positive influence and the other of a nega-

tive influence on the response, each with a normal proba-

bility of less than 50%. The factors and interactions that were

positioned outside of the normal probability line are those

that have a significant influence on the response. With

respect to the % CO conversion (Fig. 5a), the H2O content in

Table 5 e ANOVA results of the % CO conversion and selectivitya 1% (w/w) PtAu/CeO2eZnO catalyst.

Source Sum of squares DFa M

(a) Response: % CO conversionb

Model 3959.85 3

A 81.18 1

C 2825.34 1

AC 1053.33 1

Lack of fit 4.41 5

Residual 2.38 2

Cor Total 3966.64 10

(b) Response: % CO selectivityc

Model 909.22 3

A 686.94 1

C 115.37 1

AC 106.92 1

Lack of fit 1.60 5

Residual 5.47 2

Cor total 916.29 10

a DF ¼ Degrees of freedom. A and C are as defined in Table 2.

b R-Squared ¼ 0.9983, Adj. R-Squared ¼ 0.9976 and Adeq. Precision ¼ 12

c R-Squared ¼ 0.9923, Adj. R-Squared ¼ 0.9890 and Adeq. Precision ¼ 52.

the feedstream and the interactions between all three factors

had no significant influence on the response at the 95%

confidence interval. Only the operating temperature and CO2

content in feedstream had a significant effect on the % CO

conversion, being positive for the operating temperature and

negative for the CO2 content, respectively. For the % CO

selectivity, only the operating temperature had a significant

negative effect on the % CO selectivity, giving a lower CO

selectivity with increasing temperature. This statistical

analysis is consistent with the observed experimental

activity measurements (see Fig. 4).

The Pareto chart (Fig. 6) displays the absolute standardized

effect at a 95% confidence interval for both the % CO conver-

sion and selectivity responses. With respect to the % CO

conversion response, only the operating temperature and CO2

content in the feedstream expressed an absolute value higher

data derived from the FCCCD RSM for the PROX of CO over

ean square F-value Probability (P-value)

1319.95 1359.42

81.18 83.61 <0.0001

2825.34 2909.82 <0.0001

1053.33 1084.82 <0.0001

0.88 0.74 0.6604

1.19

303.07 300.10

686.94 680.19 <0.0001

115.37 114.24 <0.0001

106.92 105.86 <0.0001

0.32 0.12 0.9757

2.74

7.66.

38.

0

5

10

15

20

90 105 120 135 150

A: Temperature

C: C

O2

cont

ent

29.55

42.20

54.84

80.12

67.48

54.56 49.27 43.98 38.69 33.40

Fig. 8 e Contour plot of the RSM model derived % CO ( )

conversion and ( ) selectivity and (shaded portion) the

optimal region. The plot was formed by overlaying the

yield of the w% CO conversion and selectivity responses,

the CO2 content in the realistic reformate gas and the

temperature of the feedstream.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 14758

than 7.50, and so had a significant influence on the % CO

conversion. For the % CO selectivity, only the operating

temperature had an absolute standardized value higher than

5.65, and so had a significant influence on the CO selectivity.

These results were also confirmed by a normal probability plot

of the effect estimates. Moreover, an appearance of curvature

was observed in the % CO conversion and selectivity

responses. The absolute value of curvature was around 55.67

(7.43-fold) and 8.72 (1.54-fold) for the % CO conversion and

selectivity responses, respectively. However, it is surprising

that the addition of H2O in the feedstream had no significant

Table 6 e Validation of FCCCD using various operating temperrealistic reformate gas.

Operating condition CO co

Temperature (�C) CO2 content (%) Estimation

(a) Simulated reformate gas

90 10 54.8

120 10 58.5

130 0 76.0

150 0 67.7

150 20 56.7

170 0 59.3

(b) Realistic reformate gas

100 11.6 50.9

110 11.6 53.0

120 11.6 55.1

130 11.6 57.1

140 11.6 59.2

150 11.6 61.3

Note: The simulated reformate gas consisted of (all (v/v)) 40% H2, 1% CO, 1

rest as He, whilst the realistic reformate gas composition from the SRM u

CO2 and 41.2% He.

effect on the % CO conversion and selectivity responses at the

95% confidence interval.

The ANOVA of the catalytic performance is shown in Table

3. For the % CO conversion response, only the operating

temperature and CO2 content in the feedstream were signifi-

cant, their relative importance (% contribution) being w23.0%

and 3.28% for the operating temperature and CO2 content in

the feedstream, respectively. With respect to the % CO selec-

tivity response, only the operating temperature was an

important factor with a 95.43% contribution. In addition, the

relationship between the important factors and the response

was not linear, since the probability of a curvature was

P¼0.008and0.0421 (72.7%and3.35%contribution) for the%CO

conversion and selectivity responses, respectively. The

magnitude of the adjusted-R2 (Adj R-square) termwas close to

the coefficient of determination (R-square), which implies that

non-significant termshave been included in themodels [26,35]

for CO conversion and selectivity. In order to verify the

curvature, the mean changes that occurred in the response

when changing the level of the factor from a lower level

through the central point to a higher level were plotted (Fig. 7).

The average of the response value for all three factors studied

did not correspond to the average of the response value at the

central point,whichsuggested that there shouldbe aquadratic

term in the % CO conversion and selectivity models.

From the results of statistical analysis, it can be concluded

that an operating temperature and CO2 content in feedstream

have a significant effect on the CO conversion whist only

operating temperature has an influence on CO selectivity.

Therefore, the operating temperature and CO2 content in

feedstream were employed for a surface analysis design in

order to achieve an optimal CO conversion and CO selectivity.

3.4. Response surface methodology (RSM)

After screening for the important factor(s) that influence the%

CO conversion and selectivity responses using a full 23

atures and CO2 contents when feeding (a) simulated and (b)

nversion (%) CO selectivity (%)

Experiment Estimation Experiment

54.7 50.3 50.5

59.4 39.6 40.0

76.4 33.4 33.7

67.9 29.7 30.0

56.1 28.1 28.2

58.3 26.0 25.7

49.4 48.0 46.9

53.7 44.1 45.2

56.3 40.3 39.9

56.3 36.6 35.7

60.0 32.6 33.2

62.4 28.8 28.4

% O2, 5% H2O, the indicated amount of CO2 (shown in Table) and the

nit consisted of (all (v/v)) 36.8% H2, 1.1% CO, 1.1% O2, 8.2% H2O, 11.6%

Predicted value (%)

0 10 20 30 40 50 60 70 80 90 100

Res

idua

l (%

)

-5

-4

-3

-2

-1

0

1

2

3

4

5

Predicted value (%)

0 10 20 30 40 50 60 70 80 90 100

Res

idua

l (%

)

-5

-4

-3

-2

-1

0

1

2

3

4

5a

b

Fig. 9 e Residual plots of the response surfacemodel for the

% CO (a) conversion and (b) selectivity when routing ( )

simulated and ( ) realistic reformate gases.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 1 4759

factorial design with central points, a FCCCD with two inde-

pendent screened factors was performed in order to achieve

the optimum condition for CO conversion and selectivity. The

criterion of this design and the level of the screened factors

were chosen based on the previous full 23 factorial design

(Section 3.3), as shown in Table 4. Based on the components of

the realistic reformate gas from the SRM unit, the statistical

analysis and catalytic activities, the H2O content in the feed-

stream was held constant at a medium level. An appropriate

RSM model was generated in coded terms, as shown in Eq. (7)

for the % CO conversion and Eq. (8) for the % selectivity

responses:

CO conversion ð%Þ ¼ þ58:52þ 3:68A� 21:70Cþ 16:23AC (7)

CO selectivity ð%Þ ¼ þ39:60� 10:70Aþ 4:38C� 5:17AC (8)

whereA is the operating temperature (�C), C is the CO2 content

in the feedstream (% (v/v)) and AC is the interaction between

the operating temperature and the CO2 content in the feed-

stream. The conditions that yielded the optimal CO selectivity

(59.9%), at 90 �C and 20% (v/v) CO2, however, yielded the lowest

% CO conversion (16.3%). Conditions that then lead to an

improved % CO conversion typically yield a reduced % CO

selectivity, but not in a linear manner. Thus, the optimization

of conditions for both % CO conversion and selectivity is dis-

cussed below using overlay plots.

ANOVA analysis of the FCCCD results revealed that both

the temperature and the CO2 content and their interaction

were a significant influence on the % CO conversion and

selectivity (Table 5). Note that the lack of fit in the developed

models was not significant, implying that the independent

factors studied were adequate to represent the actual rela-

tionship between these two factors and the responses within

the selected range. The R-Squared value provides a variability

measurement in the estimated response valuewhen using the

factors and their interaction. The Adj R-Squared values of

0.9976 and 0.9890 for the % CO conversion and selectivity

response, respectively, being very close to 1 revealed the

accuracy of the response surface quadratic model [36,37].

Indeed, only 0.24% and 1.1% of the total variation in the % CO

conversion and selectivity responses, respectively, could not

be explained by the models. Accordingly, there was very little

difference between the R-Squared and Adj R-Squared values.

When monitoring the signal to noise ratio by adequate

precision (Adeq Precision), it has been suggested that the

signal is adequate when the ratio was greater than 4. Here the

signal to noise ratio for the % CO conversion and selectivity

were 127.66 and 52.38, respectively, and so displayed an

adequate model perception, and explained the good agree-

ment between the estimated and experimental response

values for both the CO conversion and selectivity.

A relatively straightforward approach to optimizing the

counter trending CO conversion and selectivity (Table 4) is to

overlay the contour plot for each response, as shown in Fig. 8.

To achieve the optimal condition, the CO2 content in the

realistic reformate gas (9%e12%) and temperature of the

feedstream (w100 �Cew120 �C) were also used as constraints

for optimizing the responses. The optimal condition, which

was estimated by simultaneously considering the CO

conversion and selectivity responses and constraints, is in the

operating temperature range of w100 �Cew115 �C and a CO2

content of 9%e11% (v/v) in the feedstream (shaded portion of

Fig. 8). Under these optimal conditions the maximal CO

conversion and selectivity were in the range of w55%ew61%

and w44%ew48%, respectively, which are in good agreement

with the experimental results.

3.5. Validation of the RSM models

To investigate the accuracy of the above RSM models (Section

3.4), the effect of the operating temperature and CO2 content

in the feedstream were experimentally varied, whilst the H2O

content in the simulated reformate gas was held constant at

5% (v/v). Table 6 shows the % CO conversion and selectivity of

an individual representative experiment along with the esti-

mated responses under the simulated and realistic reformate

Time (min)

0 50 100 150 200 250 300 350 400 450 500 550 600

CO

con

vers

ion

(%)

and

sele

ctiv

ity

(%)

0

10

20

30

40

50

60

70

80

90

100

CO conversionCO selectivity

Fig. 10 e Stability test of the PROX of CO unit over a 1% (w/

w) PtAu/CeO2eZnO catalyst under realistic SRM conditions

(36.8% H2, 1.1% CO, 1.1% O2, 8.2% H2O, 11.6% CO2, and 41.2%

He, all (v/v)) at the optimum condition (obtained from

statistical analysis). (PROX unit condition: operating

temperature [ 120 �C).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 7 4 9e4 7 6 14760

gases. The realistic reformate gas composition from the SRM

unit was comprised of (all (v/v)) 36.8% H2, 1.1% CO, 1.1% O2,

8.2% H2O, 11.6% CO2 and 41.2% He. The realistic reformate gas

was routed directly from the SRM unit to the PROX of CO unit.

The estimated responses were found to be very close to the

experimentally derived ones in all cases. Indeed, when the

difference in the estimated and experimental response

values, in terms of their % residuals, were plotted against the

predicted value, the distribution of the residuals with regard

to the response does not follow a trend for either response

(Fig. 9). For the simulated and realistic reformate gases, all the

residuals were within �2% and �3%, respectively, for both the

CO conversion and selectivity responses, indicating a high

degree of accuracy for the models. Since the RSM analysis

provided an adequate approximation of the true response

function, and so the analysis is approximately equivalent to

analysis of the actual system, then the experimental design in

this work can be applied to optimize the CO conversion and

selectivity over a 1% (w/w) PtAu/CeO2eZnO catalyst.

3.6. Stability test

The stability test of the PROX of CO unit under the realistic

condition was performed at an operating temperature of

120 �C (detailed in Fig. 10). The catalytic activity of the 1% (w/

w) PtAu/CeO2eZnO catalyst was found to be stable at this

operating condition for 10 h, with no significant difference

being observed in the % CO conversion or selectivity between

the fresh and spent catalysts (Figs. 1 and 2; Table 1).

4. Conclusions

The effects of the operating temperature (�C) and the H2O and

CO2 contents (%) in the simulated reformate gas on the

catalytic performance for the preferential oxidation of CO over

a 1% (w/w) PtAu/CeO2eZnO catalyst that was prepared by co-

precipitation was investigated. The % CO conversion profile

from the co-addition of 10% (v/v) H2O and 20% (v/v) CO2 was

much lower than that obtainedwithout the co-addition of CO2

and H2O or the addition of 10% (v/v) H2O to the feedstream but

was only slightly higher than that seen with the addition of

20% (v/v) CO2 to the feedstream. As evidenced from the FTIR

spectra, a lower level of formation of the carbonate interme-

diate and formate species with a higher intensity of the

hydroxyl region on the catalyst surface occurred when co-

adding H2O and CO2 to the feedstream than when adding

only CO2. For screening the importance of these factors on the

catalytic activity, a full 23 factorial design with three central

points was applied. Statistical analysis at a 95% confidence

interval revealed that the operating temperature and CO2

content in the feedstream had a significant influence on the

CO conversion response, whilst only the operating tempera-

ture was significant for the CO selectivity. However, a curva-

ture was observed, which suggests a quadratic term in the

models of the CO conversion and selectivity responses is

required. Therefore, variation of the two important factors

(temperature and CO2 content) and FCCCD with three central

points were performed to evaluate the optimal condition by

the simultaneous consideration of the maximal % CO

conversion and selectivity, the constraint of the CO2 content

in the realistic reformate gas (9e12% (v/v)) and temperature of

the feedstream (w100e120 �C). The optimal condition was

found to be at w100e115 �C and 9e11% CO2 content in the

feedstream, yielding a maximal CO conversion and selectivity

ofw55e61% andw44e48%, respectively. The difference in the

estimated and the experimental response was within �2%

and�3% for routing the simulated and realistic reformate gas,

respectively. No decrease in the catalyst performance was

observed over a 10 h test period. The physical properties of the

fresh catalysts were not different from the spent one. The

experimental design in this work can be applied to optimize

the CO conversion and selectivity over PtAu/CeO2eZnO

catalysts.

Acknowledgments

The authors are grateful to the Center for Petroleum, Petro-

chemicals and Advanced Materials, Chulalongkorn Univer-

sity, the National Research University Project of CHE, the

Ratchadaphiseksomphot Endowment Fund (Project code:

EN276B), the Department of Chemical Technology and The

Petroleum and Petrochemical College, Chulalongkorn

University, Thailand, for financial support.

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