The experiment and modeling of supported Wacker-type catalyst for CO oxidation at high relative humidity

Catalysis Today 242 (2015) 315–321

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The experiment and modeling of supported Wacker-type catalyst for CO oxidation at high relative humidity Li Wang ∗∗ , Wei Wang, Yanhui Zhang, Yanglong Guo, Guanzhong Lu, Yun Guo ∗ Key Lab for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science & Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 17 February 2014 Received in revised form 4 July 2014 Accepted 10 July 2014 Available online 20 August 2014 Keywords: CO oxidation Deactivation Modeling Water

a b s t r a c t The difficulty of maintaining steady reaction is generated when water presents on supported Wacker-type catalyst in CO oxidation. Water can be adsorbed and condensed on catalyst surface, which facilitated Cu species transporting from the surface into the inner part of support. This weakened the contact between Pd and Cu species and caused their existence in low valence. The temperature difference between the co-reduction peak of Pd and Cu species and the individual Cu species in H2 -TPR profile was found to have a linear relationship with the extent of deactivation. A mathematic model was developed and the effects of flow rate, CO concentration, temperature and relative humidity were also analyzed. The mathematic model well described the performance of CO oxidation at high relative humidity. Combined with the characteristic and modeling results, it was known that irreversible deactivation played a key role at lower temperature/higher relative humidity; while reversible deactivation was responsible for the instability at higher temperature/lower relative humidity. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The catalytic oxidation of CO has attracted considerable attention due to the increasing applications in the automotive emission control, trace CO removal in the enclosed atmospheres, and carbon dioxide lasers exhaust abatement [1]. Meanwhile CO catalytic oxidation was often used for academic studies due to its simple and structure sensitive reaction [2]. Moisture is inevitable avoided in practical application, and the moisture concentration in the feed gas is usually near-saturated. Problems are generated when water involved. It is well known that Co3 O4 displays higher CO oxidation activity even at −77 ◦ C, but 10 ppm water will cause the severely deactivation [3]. This phenomenon also occurs to the Hopcalite catalyst (CuO–MnO) which is a commercial CO oxidation catalyst [4]. Gold nanoparticles supported on metal oxide exhibit an extremely high activity for CO oxidation at low temperature, but the deterioration or activitydecline usually happens during its storage, which is attributed to the residual water in the fresh sample [5]. Supported Wacker-type catalyst has been reported to be active in CO oxidation at low temperature, and the reaction mechanism

∗ Corresponding author. Tel.: +86 21 64253703. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Guo). http://dx.doi.org/10.1016/j.cattod.2014.07.011 0920-5861/© 2014 Elsevier B.V. All rights reserved.

is regarded as a co-catalysis process which can be expressed as follow: CO + PdCl2 + H2 O → CO2 + Pd(0) + 2HCl Pd(0) + 2CuCl2 → PdCl2 + 2CuCl 2CuCl + 2HCl +

1 O2 → 2CuCl2 + H2 O 2

For the PdCl2 -CuCl2 catalyst, Pd2+ is the main active component for CO oxidation, and the re-oxidation of reduced cuprous ion (Cu+ ) is the rate-determining step for the overall CO oxidation process. A larger amount of CuCl2 is essential to re-oxidize metal palladium to high valence palladium to maintain the catalytic cycle [6]. Because the reaction rate increases linearly with the increasing amounts of CuCl2 relative to PdCl2 , an amount of CuCl2 larger than stoichiometric is required for high activity [7]. It is well known that the rate of CO oxidation is much higher in the presence than at the absence of water [8]. Water could act as a catalyst; it may participate in CO2 formation steps and may be regenerated via the interaction between oxygen and the reduced forms of the catalyst as suggested in earlier studies [9]. When further rising the water concentration in the feed gas, CO oxidation slows down dramatically. The deactivation at high water concentration is due to the formation of Pd metal [10]. The role of water, either accelerating or inhibiting CO oxidation was determined by the operation conditions.

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Nomenclature C Cm cal Cout exp

Cout Cp dp Dm Dp Dz kf KH2 O ms M Ne Ns P r Rep RCO Sc Shp t T u U  V Ws z

concentration in the bulk phase, g m−3 concentration at catalyst external surface, g m−3 calculated outlet concentration, g m−3 experimental outlet concentration, g m−3 internal concentration in particle, g m−3 particle diameter, m film mass transfer coefficient, m2 s−1 pore diffusion coefficient relative to particle, m2 s−1 axial diffusion coefficient, m2 s−1 external mass transfer coefficient, m s−1 adsorption constant of H2 O, Pa−1 function of water deposition, mol g−1 molecular weight, g mol−1 number of experiment runs number of samples pressure, Pa radius of particle, m Reynolds number of particle rate of reaction, mol h−1 g−1 Schmidt number Sherwood number of particle time, h temperature, ◦ C velocity, m s−1 flow rate special diffusion parameters saturation water capacity, g g−1 cat bed length, m

Greek letters deactivation coefficient ˛ ˇ water deposition coefficient εb bulk porosity initial bulk porosity of particle εp0 εp bulk porosity of particle ϕ(ms ) deactivation function ϕ objective function for optimization Eq. (17) f particle density, kg m−3 f dynamic viscosity, Pa s The analysis on Xiangyun tunnel in Shanghai (China) showed that the relative humidity (RH) and the air temperature of Xiangyun tunnel varied from 30% to 100% and 0 to 30 ◦ C, respectively, according to seasons. In most cases, the relative humidity was higher than 50% which put the whole reaction system into unstable status. In order to get further understanding on the supported Wacker-type catalyst performance in CO oxidation at high relative humidity, the physical and chemical properties of the fresh and used catalyst were analyzed, and the deactivation model for the process was also developed in this paper. The breakthrough curves were used to obtain kinetic parameters accounting for axial dispersion, external and internal mass-transfer resistances as well as H2 O deposition on inner-face of the catalyst, meanwhile the influences of operation parameters on the CO oxidation performance were also investigated. 2. Experiment 2.1. Catalysts preparation and characterization The supported Wacker-type catalyst was prepared using a conventional incipient impregnation method. The metal precursors were PdCl2 -HCl solution and CuCl2 ·2H2 O. The Pd and Cu loading

were 2 and 6 wt%, respectively. The catalytic activities were carried out in a quartz tube reactor at atmospheric pressure. 1.0% CO in nitrogen was further diluted in air which could obtain different CO concentration gas mixture. The CO gas mixture passed through a water vapor saturator then into catalyst (10–12 mesh, 1 g). Changing the temperature of water vapor saturator can adjust the water concentration in the feed gas. The gas mixture was analyzed by online gas chromatograph equipped with a column packed with carbon molecular sieve, a methanator and FID detector. During these experiments, the concentration of outlet CO is changing from several to thousands ppm with the time, so CO breakthrough time is defined as the time when the outlet CO concentration lower than 1 ppm. A water absorption experiment was conducted at ambient temperature in a quartz U-tube reactor; 0.2 g catalyst was used. 50 ml/min N2 was directed through a water vapor saturator, and then flowed into reactor. The weight of the reactor was measured after 24 h to obtain the water absorption amount. Nitrogen isotherms were measured using an ASAP 2010 apparatus by N2 adsorption at −196 ◦ C. The isotherms were used to calculate specific surface area (SN2 ). X-ray photoelectron spectroscopy (XPS) analysis was performed on Thermo ESCALAB 250 with a monochromatic Al K␣ source (h = 1486.6 eV), operating at 150 W. The binding energy (BE) of adventitious C 1s (284.8 eV) was used as a reference. H2 -TPR experiments were conducted on the quartz tube reactor at atmospheric pressure using 1% hydrogen in argon (40 ml/min) as a reducing gas from 100 to 900 ◦ C at a rate of 10 ◦ C/min. 2.2. Mathematical model In the modeling, the following assumptions were made: the process is operated under isothermal conditions and the packed porosity is constant along the longitudinal axis of the reactor bed; an axial dispersion plug-flow model is adopted to account for nonideal flow in reactor bed; mass transfer between the bulk phase and the solid particle is described by the external-film mass transfer coefficient (kf ), intra-particle mass transfer is characterized by the pore diffusion coefficient (Dp ). 2.2.1. Reactor model The overall mass balance around the fixed bed can be expressed as

 ∂C ∂C ∂ C 3kf 1 − εb + ( Cp  − C) +u = Dz r=RP RP εb ∂t ∂z ∂z 2 2

(1)

The initial and Danckwerts boundary conditions at the both ends of the bed are given by the following equations: at t = 0,

C=0

(2)

(3)at t > 0, z = 0, z = L,

Dz ∂C = u(C − Cin ) ∂z

∂C =0 ∂z

(4)

The Wakao–Funazkri correlation [11] was adopted to estimate the film mass transfer coefficient (Dm ) and axial dispersion coefficient (Dz ): Shp = 2 + 1.1Sc 1/3 Re0.6 p

εb Dz = 20 + 0.5ScRep Dm

(5)

where Shp and Rep are the Sherwood and Reynolds numbers relative to particle, and can be calculated as follow Shp =

kf dp Dm

Rep =

dp Uf f

(6)

L. Wang et al. / Catalysis Today 242 (2015) 315–321

Schmidt number (Sc) can be calculated according to: Sc =

f f Dm

(7)

The molecular diffusivity in multicomponent gas (Dm ) was calculated by the Fuller–Schettler–Gridding method [12]: DAB =

VA )

1/3



+(

VB )

(8)

1/3 2

]

2.2.2. Intra-particle mass transfer The mass transfer within a catalyst microsphere can be expressed with the following reaction-diffusion equation:

∂(εp Cp ) 1 ∂ = 2 r ∂r ∂t



r 2 Dp (r, t)

∂Cp ∂r



− RCO

(9)

The water deposition in the particle was governed by

∂ms = RCO ∂t

(10)

The random pore model [13] was used to estimate the pore diffusion coefficient (Dp ) which is calculated by the following equation: Dp (r, t) = Dm ε2p

(11)

where εp is given by εp (r, t) = εp0 − ˇm(r, t)

(12)

together with the initial conditions at

t = 0,

Cp = 0,

m=0

(13)

The mass transfer boundary conditions on the surface of the catalyst are expressed as at

t > 0,

r = Rp ,

Dp

 ∂Cp ) = kf (C − Cp  r=RP ∂r

(14)

The symmetry conditions at the center of the particles are as follows:

∂Cp =0 ∂r

r = 0,

(15)

2.2.3. Kinetic model The reaction kinetic at low temperatures (300–400 K) was explained by a model which assumed the adsorbed CO and O2 react along the perimeters of “island” resulting from the adsorption of these molecules on the surface of the catalyst [14]. 21% O2 in the feedstock was 50 times larger than that of CO (max 4000 ppm), therefore O2 concentration should be considered as constant. Water plays a dual role in CO oxidation, the presence of water promoted CO oxidation for water can act as a catalyst [15]; while raising the relative humidity to 100%, CO oxidation slows down obviously [16]. Considering the effect of water on CO oxidation, the following equation was taken [17]. RCO =

kCO CCO (ms ) 1 + KH2 O CH2 O

(16)

where (ms ) is the deactivation function, expressed as follows: (m) =

1 (1 + ˛m)2

The values of εp0 , ˛, ˇ, kCO and KH2 O were determined by minimizing the difference between calculated and experimental outlet concentrations of CO, using the objective function of ϕ=

Ne Ns  

(17)

The method of finite volume was applied to discretize the space variables z and r of the partial differential Eqs. (1) and (8), and the resulting ordinary differential equations were numerically integrated by Gear’s method.



exp

cal (Cout,j,i − Cout,j,i )



exp 2

i=1 j=1

0.00143T 1.75



1/2 PMAB [(

317

(18)

(Cin,i )

where Ne is the number of experimental runs, and Ns is the number of samples per run. The BFGS algorithm was adopted to search for the optimum fitting value of the parameters. 3. Results and discussions 3.1. The textual property and characterization of catalysts N2 adsorption–desorption and XPS analysis results of catalysts are shown in Table 1. For the fresh catalyst, the surface area was 159 m2 /g, but a sign of decrease was found in all the tested samples. When higher relative humidity was displayed in feed gas, more water would easily be deposited on hydrophilic support, especially on Al2 O3 , which led to H2 O capillary condensation in small pores. The adsorption water experiment results showed that adsorption water capacity could be determined by the operation parameters. 23% adsorption water capacity was obtained at 0 ◦ C, while 15% at 25 ◦ C. Adsorption water capacity at 0 ◦ C was nearly 1.5 times more than that at 25 ◦ C. H2 O capillary condensation caused micro-pore blocked and surface area decreased. The deposited water not only covered the active sites but also resisted the reactants transferring to active sites [18]. The deactivation by H2 O capillary condensation could be reactivated by heat treatment, so it was called reversible deactivation. XPS was used to characterize the surface composition and chemical state of active metal. The spectra of Cu 2p or Pd 3d are numerically fitted. The binding energy (B.E.) of 932.2 and 934.6 eV were assigned to Cu+ and Cu2+ , while 337.8, 336.5 and 335.0 eV was assigned to Pd2+ , Pd+ and Pd0 . The chemical states surface and their atom concentration are summarized in Table 1. For the fresh catalyst, Pd existed in the form of Pd2+ and Cu in the form of Cu+ and Cu2+ , respectively. The surface atom concentration of Pd2+ was 0.38, and the surface atom concentration of Cu2+ and Cu+ was 0.41 and 0.26 respectively. The data in Table 1 also indicated that Pd and Cu species existed in high-valence on fresh catalyst, for much amount of low-valence Pd species (Pd0 , Pd+ ) and Cu species (Cu+ ) appeared on spent catalyst. The concentration of active component (Pd2+ , Cu2+ ) on spent ◦ ◦ catalyst operated   at 25 C was much higher than that at 0 C. The ratio of Cu/ Pd decreased  with the increasing of deposited  water on surface. The ratio of Cu/ Pd on fresh catalyst surface was 1.76, while it decreased to 1.14 after the catalyst treated at 0 ◦ C relative humidity 100%. ICP analysis (not shown) suggested that there   were no Cu and Pd loss after reaction. The decline of Cu/ Pd ratio meant that some Cu species dissolved in the condensed water on the surface and migrated into the pores which would expose the previously covered Pd species. The migration of Cu species broke the close-knit structure of the Pd-Cu species and decreased their interactions which led to low-valence Pd species (Pd0 , Pd+ ) and Cu species (Cu+ ) appeared on spent catalyst. H2 -TPR experiments were also conducted to investigate the redox behavior of samples and the results are shown in Fig. 1. The fresh sample showed an intense TPR signals at 152 ◦ C and a shoulder peak at 250 ◦ C. The intense peak (˛) was ascribed to the co-reduction of Pd and Cu species, while the shoulder peak (ˇ) was the reduction of bulk copper species that was not interacted with palladium phase [19]. The TPR profile of spent catalyst A was similar with that of fresh catalyst, but the two peaks were at 145 and 273 ◦ C,

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L. Wang et al. / Catalysis Today 242 (2015) 315–321

Table 1 Textural properties and XPS analysis of samples. Surface area (m2 /g)

Samples

Fresh Spent Ac Spent Bd a b c d

Surface atom concentrationa

159 143 131

R

atio of atom concentrationb



Pd2+

Pd+

Pd0

Cu2+

Cu+

Cu+ /

0.38 0.29 0.21

– 0.07 0.11

– 0.10 0.17

0.41 0.32 0.22

0.26 0.40 0.51

0.39 0.56 0.70

Cu





Cu/

Pd

1.76 1.57 1.14

Fitting of XPS  results spectra. + 2+ 0 Cu = Cu+ + Cu2+ , Pd = Pd + Pd + Pd . The catalyst was deactivated at 25 ◦ C,RH: 100%. The catalyst was deactivated at 0 ◦ C,RH: 100%.

100

fresh catalyst spent catalyst A spent catalyst B

TCD single (a.u.)

CO conversion (%)

80

60

fresh catalyst reactivated catalyst A reactivated catalyst B

40

20

0

100

200

300

400

-30

-20

3.2. Testing of regenerated catalysts The results of fresh and regenerated catalysts on CO oxidation performed at 30 ◦ C, relative humidity 100%, CO concentration 1500 ppm are displayed in Fig. 2. The deactivated catalyst was regenerated in air at 40 ◦ C for 12 h before testing. CO oxidation activity of the fresh and regenerated catalyst B were similar as a function of temperature, while obvious activity

10

20

30

Temperature ( C)

0

respectively. Three peaks, 143, 222 and 307 ◦ C were observed in the H2 -TPR profile of spent catalyst B, and the peak at 222 ◦ C ( ) was attributed to the Cu species partly interacting with Pd species. The appearance of third peak (307 ◦ C) suggested that the contact between Pd and Cu species deteriorated. The temperature difference of samples between ˛ and ˇ peak was 98, 130 and 164 ◦ C, respectively. The shifting of reduction temperature in H2 -TPR profiles to higher temperature also suggested the worsening redox properties of catalysts, which was attributed to the deposited water on catalyst surface. The water facilitated Cu species transporting from surface to the inner part of the support which weakened the contact between Pd and Cu species and led to both Pd and Cu species existed in low-valence on catalyst surface [20]. The deactivation by the variation of the composition and chemical state is known as irreversible deactivation, which usually cannot be regenerated. It can be concluded from the above discussion that the presence of water in feed gas is the cause of the reversible and irreversible deactivation. These two kinds of deactivation are relevant to each other, and the dominant deactivation is determined by the operation parameters. In either case, the deactivation puts the whole reaction system into unsteady status.

0 o

Temperature ( C) Fig. 1. H2 -TPR profiles of the samples.

-10

Fig. 2. CO activity of the different samples.

differences between fresh and reactivated catalyst A were found in Fig. 2. 92% CO conversion was obtained on the fresh catalyst at 0 ◦ C; while on the reactivated catalyst B the temperature shifted to 20 ◦ C with the same CO conversion. The temperature difference between the fresh and the reactivated catalysts was due to the irreversible deactivation and the higher the temperature difference the dominant the irreversible deactivation. The obvious temperature difference with the same CO conversion suggested the great variance of chemical state on catalyst surface. Combined with the CO oxidation testing results of regenerated catalysts and H2 -TPR experiments; it was known that the temperature difference between the co-reduction peak of Pd and Cu species and the individual Cu species in TPR profile had a linear relationship with the extent of deactivation. When taking the XPS analysis and TPR results into account, it could be easily understood that irreversible deactivation played a leading function when catalyst operated at lower temperature and higher relative humidity. On the contrary, reversible deactivation came to a dominate form at higher temperature and lower humidity. 3.3. The modeling results 3.3.1. The effect of flow The effect of gas flow rate on CO oxidation was investigated and the results are shown in Fig. 3. The gas flow rate affected the thickness of the boundary layer surrounding the particles and the external mass-transfer coefficient. The external mass-transfer coefficient is a function of the gas flow rate. From the results it was known that the value of external mass-transfer coefficient of the mathematic model was suitable within the experimental range. With the increasing of the gas flow, the thickness of the boundary layer will decrease which benefit the mass-transfer and lead to

1.0

1.0

0.8

0.8

0.6

0.6

Cout/Cin

Cout/Cin

L. Wang et al. / Catalysis Today 242 (2015) 315–321

0.4

319

1000 ppm 2050 ppm 3200 ppm

0.4

3

43 ml/m 3 60 ml/m 3 100 ml/m

0.2

0.2

0.0

0.0 0

50

100

0

150

20

40

60

Time (h) Fig. 3. Experimental (dots) and modeled (line) outlet concentration of CO for different flow at RH = 100%, and T = 303 K

the higher conversion. The lower gas flow also increased the contact time between CO and O2 , namely to increase CO oxidation activity. From the experiment results, it was known that lower gas flow would be more feasible to get higher CO removal efficiency.

3.3.2. The effect of relative humidity Fig. 4 shows the effects of moisture concentration on the catalytic stability of CO oxidation catalyst at 30 ◦ C. With the increasing relative humidity in the feed gas from 55% to 100%, the catalytic activity remained above 90% shorten from 96 to 52 h. The results suggested that higher relative humidity played a negative effect on CO oxidation. With the increasing of relative humidity, more water would be deposited on catalyst. The deposited water not only covered the active sites which hindered mass transfer but also affected the distribution of composition and chemical state.

The more amount of the water deposited on catalyst, the faster deactivation the catalyst would be. 3.3.3. The effect of temperature The outlet of CO over the time as a function of temperature at 0, 10, 20 and 30 ◦ C is plotted in Figs. 5–8. It was found that the ratio of outlet and inlet CO concentration varied with the inlet CO concentration and temperature. The slope of the ratio increased with the increase of CO concentration at the same temperature. This was caused by limited catalyst activity instead of the increased deactivation rate, since the same amount of deposited water on catalyst led to the same deactivation rate. It was also known from Figs. 5 to 8 that the slope of the ratio decreased with the increase of temperature at the same CO concentration. With the temperature increasing from 0 to 30 ◦ C shown in Figs. 5 and 8, the breakthrough time at similar CO inlet concentration level (about 2000 ppm) increased from 2 to 30 h, and the time

RH 100% RH 75% RH 55%

Cout/Cin

0.6

0.4

0.2

0.0 0

100

Fig. 5. Experimental (dots) and modeled (line) outlet concentration of CO for different inlet concentration at RH = 100% and T = 273 K.

1.0

0.8

80

Time (h)

50

100

150

200

Time (h) Fig. 4. Experimental (dots) and modeled (line) outlet concentration of CO for different RH at CO 2830 ppm and T = 303 K

320

L. Wang et al. / Catalysis Today 242 (2015) 315–321 Table 2 Optimum model parameters at different RH.

1.0

RH (%)

0.8

100 75 55

εp0

˛ × 104

ˇ × 105

KCO (mol h−1 g−1 )

KH2 O (Pa−1 )

0.42 0.41 0.39

1.02 0.82 0.69

4.16 3.32 2.35

65.7 65.7 65.7

2.0 2.0 2.0

Cout/Cin

0.6 Table 3 Optimum model parameters at different temperature.

3200 ppm 2000 ppm 1050ppm

0.4

0.2

0.0 0

20

40

60

80

100

120

Time (h) Fig. 6. Experimental (dots) and modeled (line) outlet concentration of CO for different inlet concentration at RH = 100% and T = 283 K

Temperature (K)

εp0

˛ × 104

ˇ × 105

KCO (mol h−1 g−1 )

KH2 O (Pa−1 )

273 283 293 303

0.38 0.39 0.39 0.41

7.67 4.21 1.32 1.02

7.26 7.03 6.27 4.16

10.2 24.5 40.5 65.7

5.8 4.3 3.1 2.0

for CO conversion decreased from 80% to 20% also increased from 30 to 150 h. With the temperature arising, there were more active sites available which led to the catalyst TOF increased accordingly, so higher CO conversion would be easily achieved at higher temperatures compared with lower temperatures. Although the same relative humidity (100%) was employed in feed gas, water content in gas varied with temperatures. When the temperature rose from 0 to 30 ◦ C, saturated vapor pressure of water in gas increased almost 7 times from 0.61 to 4.2 kPa, but water adsorption experiments displayed the amount of deposited water on catalyst conducted at 0 ◦ C was only 15% higher than that at 30 ◦ C. The results suggested that there were much water deposited on catalyst surface at lower temperature and it had a higher influential on the catalyst stability. The mathematical model was built to determine the parameters by minimization of the objective function, and the fit values of the parameters are shown in Tables 2 and 3. It was seen in Table 2 that temperature had contrary tendency on reaction rate constant and water adsorption constant. The rising of temperature made the reaction rate constant increased while adsorption constant decreased. The kinetic parameters obtained by Arrhenius equation was: KH2 O = 1.5 × 10−4 e24,011/RT , kCO = 1 × 109 e−41,530/RT , respectively.

Fig. 7. Experimental (dots) and modeled (line) outlet concentration of CO for different inlet concentration at RH = 100% and T = 293 K.

It was shown in Tables 2 and 3 that the value of deactivation coefficient (˛) and water deposition coefficient (ˇ) decreased in the temperature range of 0 ◦ C to 30 ◦ C; while they presented an increasing trendy when relative humidity arising from 55% to 100% at 30 ◦ C. The value of the water deposition coefficient (ˇ) and the deactivation coefficient (˛) obtained from modeling was relatively lower at higher temperature (30 ◦ C) or lower relative humidity (55%). The smaller value of water deposition coefficient indicated the less amount of water deposited on catalyst, which would cause eversible deactivation due to pore blocking and active sites losing. Higher water deposition coefficient mostly associated with the higher deactivation coefficient, because more water would rapidly deposit on catalyst surface at low temperature (0 ◦ C) and high relative humidity (100%). Large amount of water facilitated Cu species transferring and broke the close-knit structure of the Pd-Cu species which led to the irreversible deactivation. 4. Conclusion

Fig. 8. Experimental (dots) and modeled (line) outlet concentration of CO for different inlet concentration at RH = 100% and T = 303 K

The presence of water on catalyst surface decreased the catalyst surface area and facilitated the transportation of Cu species from the surface to the inner part of support. This transportation impaired the contact between Pd and Cu species which caused their existence in low-valence form. The temperature difference

L. Wang et al. / Catalysis Today 242 (2015) 315–321

between the co-reduction peak of Pd and Cu species and the individual Cu species in TPR profile was found to have a linear relationship with the extent of deactivation. When water existed in feed gas, it would make the system unstable. A mathematical model was developed to describe the behavior of CO oxidation at 100% relative humidity, accounting for the effects of water adsorption on catalyst, external-film and porediffusion mass transfer mechanisms. The model agreed well with the experimental breakthrough curves and satisfactorily explained the performance of CO oxidation at high relative humidity. H2 O adsorption heat and CO oxidation activation energy on catalyst was 24 and 41.5 kJ/mol, respectively. A direct correlation was found between deactivation and operation parameters. Water deposited coefficient and deactivation coefficient was higher at low temperature or high relative humidity, because irreversible deactivation played a key role in deactivation. On the contrary, reversible deactivation was responsible for unsteady state of catalyst at higher temperature and lower relative humidity with lower water deposited coefficient and deactivation coefficient. Acknowledgements This work was financially supported by the National Basic Research Program of China (2013CB933201, 2010CB732300), National Natural Science Foundation of China (21171055, 21207037), the “Shu Guang” Project of the Shanghai Municipal Education Commission and the Fundamental Research Funds for the Central Universities. References [1] W.Y. Hernández, M.A. Centeno, S. Ivanova, P. Eloy, Cu-modified cryptomelane oxide as active catalyst for CO oxidation reactions, Appl. Catal. B 123–124 (2012) 27–35. [2] S. Royer, D. Duprez, Catalytic oxidation of carbon monoxide over transition metal oxides, ChemCatChem 3 (2011) 24–65.

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