Promoting effect of H2O over macroporous Ce-Zr catalysts in soot oxidation

Molecular Catalysis 474 (2019) 110416

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Promoting effect of H2O over macroporous Ce-Zr catalysts in soot oxidation a,1

a,1

a

a

T

b

Chung Sun Park , Min Woo Lee , Jae Hwan Lee , Eun Jin Jeong , Seong Ho Lee , ⁎ Jin Woo Choungc, Chang Hwan Kimc, Hyung Chul Hamd, Kwan-Young Leea, a

Department of Chemical and Biological Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea Super Ultra Low Energy and Emission Vehicle (SULEEV), Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea Advanced Catalysts and Emission-Control Research Lab, Research & Development Division, Hyundai Motor Group, Hyundaiyeonguso-Ro, Namyang-Eup, Hwaseong-Si, Gyeonggi-Do, 18280, Republic of Korea d Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Gasoline particulate filters Soot oxidation Macroporous Ce1-xZrxO2 Water promoting effect DFT study

Gasoline particulate filters (GPFs) contain small amounts of O2 and excess H2O; however, the effect of H2O on soot oxidation has not been thoroughly explored to date. Thus, it is necessary to understand the effect of H2O on soot oxidation to improve the catalytic performance in GPFs. This study investigates the role of H2O in soot oxidation on macroporous Ce-Zr mixed oxide catalysts (M-CeZr). The results revealed an improvement in the catalytic activity of soot oxidation in the presence of H2O over that afforded under oxygen-only conditions. Since the mechanism of soot oxidation under dry conditions involves the conversion of gaseous oxygen to active oxygen (Ox−) species on the oxygen vacancies of the catalyst, Ox− and oxygen vacancy are critical factors that affect the catalytic performance in the absence of H2O. Notably, when H2O was introduced into the reaction, it was predominantly used as an oxidant rather than gaseous oxygen. Further, the dissociation of H2O into active oxygen over the catalyst surface was not related to the number of oxygen vacancies. Therefore, even when the catalyst comprised few oxygen vacancies, its activity improved under wet conditions. In addition, although the catalysts were damaged by high temperatures, the catalytic performance was maintained in the presence of H2O, unless the morphology of the catalysts collapsed.

1. Introduction Catalysts for the exhaust gases have been developed in the automotive industry [1–8]. Moreover, various technologies also have developed to improve fuel efficiency. Especially, the gasoline direct injection (GDI) engine has been introduced to gasoline vehicles to improve the fuel efficiency and reduce CO2 emissions. However, the use of GDI engines has increased the particulate number (PN) emissions, thereby violating the Euro 6 PN legislations (6.0 × 1011 particulate/ km). To meet the limits set by these regulations, a catalyzed gasoline particulate filter (CGPF), which is similar to a diesel particulate filter (DPF), has been developed [9]. However, unlike the DPF, only a negligible amount of NOx is formed in a GPF. This is due to the upstream three-way catalysts (TWC) that converts NOx to N2 with hydrocarbons and carbon monoxide [10]. Moreover, since the air-fuel ratio of gasoline vehicles is lower than that of diesel, the oxygen concentration is significantly lower than that of diesel. Therefore, many studies that evaluate the performance of catalysts at low oxygen concentrations to

simulate GPF conditions have been reported in the literature [10–15]. Ceria-based catalysts are widely used in automobiles due to their high oxygen storage capacity (OSC) and redox properties that allow easy oxidation and reduction between the Ce4+ and Ce3+ ions [11,12,14,16–19]. Ceria is also well known for generating the active oxygen species (Ox−), which are deeply related to the surface oxygen vacancies (Ce3+-VO + x/2 O2 → Ce4+- Ox−) [16,17]. These Ox− species well combust soot particles over the supplied gaseous O2; thus, ceria is known as a promising material for CGPFs. Moreover, a Ce-Zr mixed oxide synthesized by the addition of zirconium into ceria was reported to exhibit better performance than pure ceria; this was attributed to the partial substitution of Zr, which creates surface deformation and increases the surface oxygen vacancies [1,4,8,20,21]. Additionally, the Ce-Zr mixed oxide presented improved thermal resistance over that of ceria [22]. These results indicated that Ce-Zr mixed oxides are suitable catalysts for application to CGPFs under low oxygen concentrations. In an actual exhaust system, the excessive H2O generated from fuel

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Corresponding author. E-mail address: [email protected] (K.-Y. Lee). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.mcat.2019.110416 Received 17 February 2019; Received in revised form 23 April 2019; Accepted 18 May 2019 Available online 31 May 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.

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while the pore volumes and diameters were estimated from the BJH method. The O2 temperature programmed desorption (O2-TPD) analyses were carried out on a BELCAT-B instrument (BEL Japan Inc.) to examine the amount of absorbed O2. Thus, 50 mg sample was purged with He (50 mL/min) for 60 min at 200 °C to remove the moisture. The sample was then pretreated at room temperature with 1% O2/He (50 mL/min) for 60 min to adsorb the O2. After pretreatment, the sample was heated to 900 °C under He (50 mL/min) at a heating rate of 5 °C/min. The O2-TPD data was recorded with a thermal conductivity detector (TCD). The H2 temperature programmed reduction (H2-TPR) analyses were performed on an AutoChem II 2920 instrument (Micromeritics, USA). Thus, 50 mg sample was heated to 900 °C under a flow of 10% H2/Ar (50 mL/min) at a heating rate of 5 °C/min. During the test, the H2-TPR data were recorded with a TCD. X-ray photoelectron spectra (XPS) were recorded on a PHI 5000 Versa Probe (ULVAC PHI, Japan) equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. The binding energy of the adventitious carbon (C 1s) at 284.8 eV was used as the internal standard.

oxidation influences many oxidation reactions. In CO and hydrocarbon (HC) oxidation, H2O generates surface hydroxyl groups (OH) on the catalysts, which enhance the oxidation and lower the reaction temperature [23,24]. In the presence of NO2, H2O is also related to the production of HNO3, which aids soot combustion at lower temperatures under DPF conditions [7,25]. However, the role of H2O in soot oxidation under GPF conditions has not been adequately studied, even though H2O promotes the soot oxidation activity. Thus, it is necessary to study the H2O promoting effect in soot oxidation under GPF conditions. This study attempts to elucidate the effect of H2O on soot oxidation. As soot oxidation is a solid-solid-gas reaction, the catalytic activity is largely affected by the contact between the soot particles and catalysts. In terms of a catalyst-soot contact area, the macroporous structure enlarges the contact by its high external surface area, thereby exhibiting superior soot oxidation activity [26–28]. Therefore, macroporous Ce-Zr mixed oxide catalysts were synthesized using a range of Zr contents (0, 30, 50, 70, 100 mol%) and their catalytic activities, with and without the presence of H2O, were subsequently compared. A series of analyses revealed a mechanism for soot oxidation in the presence of H2O, as well as the effect of H2O when the catalyst is damaged by high temperatures. The aim of this work is to provide a direction for future research on the catalysts used in GPFs comprising less oxygen and excess water.

2.3. Catalytic activity tests The catalytic activities of all the catalysts used for soot oxidation were evaluated by the temperature-programmed oxidation (TPO) process in a fixed-bed tubular quartz system. Printex-U (Degussa, Germany; particle size =25 nm, surface area = 100 m2/g) was used in the study as model soot. About 0.022 mg catalyst soot mixture (catalyst-to-soot weight ratio of 10:1) was mixed in an agate mortar for 5 min to realize the tight contact mode. Two experiments were performed on each catalyst-soot mixture. To evaluate the catalyst performance under dry conditions, the mixture was fed with a 100 ml/min flow of 1% O2/He (WHSV = 300,000 ml/g∙h) and heated to 700 °C at a heating rate of 5 °C/min. 1–10% H2O/1% O2/He (100 mL/min) was fed to the mixture to evaluate the water promoting effect. The CO2 and CO concentrations in the outlet gas were recorded using a Nicolet iS50 FTIR spectrometer (ThermoFisher Scientific, USA).

2. Experimental 2.1. Catalyst synthesis Seven catalysts: M-Ce, M-Ce7Zr3, M-Ce5Zr5, M-Ce3Zr7, M-Zr, MCeT, and M-Ce7ZrT were prepared according to our previous method [29]. To synthesize the macroporous structure, polymethyl methacrylate (PMMA) spheres were synthetized as the catalyst templates [30,31]. The methyl methacrylate (MMA, 160 mL) monomer and potassium persulfate (KPS, 1.62 g) initiator were added to 640 ml DI water (18.2 MΩ cm). The mixture was then stirred constantly at 70 °C for 2 h with Ar bubbling. After cooling, the spheres were obtained by centrifugation (15,000 rpm, 40 min) and then dried overnight at 70 °C. A series of macroporous catalysts were prepared by the colloidal crystal templating method. Ce(NO3)3∙6H2O (Sigma-Aldrich, USA, 99%) and ZrOCl2∙8H2O (Sigma-Aldrich, USA, 99%) were dissolved in 15 ml of an ethylene glycol-methanol solution (60 vol.% of ethylene glycol) at different molar ratios (10:0, 7:3, 5:5, 3:7 and 0:10) to produce a 2 M mixed solution. Then, dried PMMA was soaked in the solution and stirred for 10 min and the excess solution was removed by filtration. The filtered PMMA template with inorganic precursor were dried overnight at 70 °C and finally removed by calcination at 550 °C for 4 h; M-CeT and M-Ce7ZrT were calcined at 800 °C for 4 h.

2.4. Density functional theory (DFT) calculation The Vienna ab-initio simulation package (VASP) was used for all the calculations based on the spin polarized DFT in this study [32]. The generalized gradient approximation (GGA) of the Perdew − Burke − Ernzerhof (PBE) functional was introduced for the exchange-correlation [33], while the projector augmented wave (PAW) method was used to describe the interaction between the core and valence electrons [34,35]. An energy cutoff of 400 eV for the plane wave basis set was chosen for the expansion of the electronic eigenfunctions. The CeO2(111)-p(3 × 3) surface was constructed with a three O-Ce-O triplelayers (nine atomic layers) and five layers of vacuum with (2 × 3×1) Monkhorst-Pack mesh of k points for Brillouin zone integration. Additionally, the bottom layer was fixed to simulate bulk CeO2, while the other two layers were relaxed. The DFT + U (Ueff =5 eV) method was adopted for the accurate description of the Ce 4f orbital. Zr-doped CeO2 was constructed by substituting a surface Ce atom with a Zr atom. Geometric optimization was carried out using the conjugate gradient method until the remaining forces on the relaxed atoms reached values ≤ 5 × 10−2 eV/Å. The reaction enthalpy (ΔH) and barriers were obtained using the climbing-image nudged elastic band (c-NEB) method with six intermediate images for each reaction step. The zero-point energy (ZPE) of each reaction step was calculated as ZPE = ∑(1/2)hνi, where h and νi are Plank’s constant and the vibrational frequency, respectively. The adsorption energy (Eads) was calculated from the total energy differences:

2.2. Catalyst characterization The catalyst structures were determined by powder X-ray diffraction (XRD; Rigaku SmartLab, Japan) using Cu Kα (λ = 1.5406 Å) radiation. The diffraction data were recorded at the 2θ range 10° to 90° with a scanning rate of 2°/min. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was conducted to measure the mass fractions of Ce and Zr in the catalysts using a JY Ultima2C (Jobin Yvon) at the Korea Basic Science Institute (Seoul Branch). The catalyst morphology was observed by high-resolution scanning electron microscopy (HR-SEM; Hitachi SU-70) at the Korea Basic Science Institute (Seoul Branch). Prior to SEM analysis, the samples were attached on a carbon belt and coated with a Pt beam. Nitrogen adsorption-desorption analysis was performed on a BELSORP-max instrument (BEL Japan Inc.) at −196 °C. The specific surface area was calculated from the BET equation and t-plot method, 2

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the mesopores could not participate in the soot oxidation process. Thus, the actual catalyst-soot contact points were mainly determined by external surface area due to the macropores. The measured external surface area of each catalyst was in the range 22.3–36.5 m2/g in the order: M-Ce5Zr5 > M-Ce3Zr7 > M-Ce7Zr3 > MCe > M-Zr. This is the reverse order of crystal size (Table 1.), which means that the smaller the crystal size, the larger the external surface area. Comparing M-Ce with M-Ce7Zr3, M-Ce showed smaller external surface area than M-Ce7Zr3 even though BET surface area of M-Ce (33.2 m2/g) was larger than that of M-Ce7Zr3 (29.7 m2/g). It is proposed that M-Ce with a larger crystal size (11.3 nm) formed a thicker mesoporous wall, thus M-Ce contained a greater proportion of mesopores than M-Ce7Zr3. After calcination at 800 °C, M-CeT exhibited a decreased external surface area over that of M-Ce due to sintering at high temperatures. However, compared to M-Ce7Zr3, M-Ce7Zr3T maintained its textural properties because of its high thermal stability.

Fig. 1. X-ray diffraction (XRD) results of (a) M-Ce, (b) M-CeT, (c) M-Ce7Zr3, (d) M-Ce7Zr3T, (e) M-Ce5Zr5, (f) M-Ce3Zr7, and (g) M-Zr.

3.2. Catalytic performances Eads = Eadsorbate+surf, – (Esurf + Eadsorbate),

(1)

Fig. 3 and Table 2 display the catalytic performances of the catalysts. The activity of the catalysts was evaluated by the temperature at 20 and 50% soot conversion (T20 and T50, respectively), temperature of maximum soot oxidation rate (Tm). Activation energies (Ea) of the reaction were calculated by Redhead method [38]. The method requires the Tm and the ramping rate during the reaction to determine the Ea if the order and the pre-exponential of the process are known. In this study, we took soot combustion as a first-order reaction according to previous literatures [38–40]. The subsequent text (part 3.2. and 3.3.) focuses on the catalyst calcined at 550 °C; details on M-CeT and MCe7Zr3T will be discussed later. Fig. 3(a) and Table 2 reveal that the activity of the catalyst calcined at 550 °C was in the order M-Ce7Zr3 = M-Ce5Zr5 > M-Ce = MCe3Zr7 > M-Zr under dry conditions (1% O2/He), which is inconsistent with the order of the external surface area. This implies that the catalytic activities are affected by their chemical properties rather than their morphologies in that the external surface areas of the prepared catalysts are in a similar range. On the other hand, the performance of all the catalysts improved in the presence of H2O while the ignition temperature without catalyst was almost unchanged (ΔTm = −4 °C). Moreover, the order of the catalytic activity changed when 10% H2O was introduced. Among these catalysts, the activity of M-Ce displayed the greatest increased (ΔTm = −39 °C), even showing similar activity to M-Ce7Zr3 and M-Ce5Zr5. Additionally, catalytic activity tests with different H2O concentrations (1–30 %) were performed using M-Ce as presented in Fig. S2 and results are summarized in Table S1. As a result, catalyst showed almost the same Tm (418–421 °C) and Ea (129–130 kJ/mol) regardless of H2O concentration. This implies that the presence of H2O is much important than its concentration. This improvement by H2O has been reported to be due to the elimination of adsorbed hydrogen on soot by the oxygen species and the increase in the contact area caused by the gasification of carbon with H2O [41]. However, it is insufficient to account for the different enhancement effect between the catalysts when H2O is introduced. Therefore, we tried to figure out the role of H2O in relation to catalytic chemical properties and surface catalysis during soot combustion.

where E adsorbate+surf, Esurf and Eadsorbate are the energies of the adsorbate on the catalytic surface, clean surface without adsorbate, and gas phase adsorbate molecule, respectively. 2.5. Isotopic experiment The experiment was conducted using water isotopes (H218O) to elucidate the effect of water on the catalysts. Four catalysts (M-Ce, MCe7Zr3, M-CeT, and M-Ce7Zr3T) were used and except for the use of isotope water, the experimental process was similar to the TPO procedure. The C16O (m/z = 28), C18O (m/z = 30), C16O2 (m/z = 44), C16O18O (m/z = 46), and C18O2 (m/z = 48) were monitored via mass spectrometry (Lab Questor-RGA, Bongil). 3. Result & discussion 3.1. Catalysts solid properties Fig. 1 displays the XRD patterns of the M-Ce(x)Zr(y) samples with various Ce/Zr ratios. M-Ce presents a typical cubic fluoride CeO2 crystal phase with peaks located at 28.5, 33.1, 47.6 56.5, 59.2, and 69.4 corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), and (4 0 0) planes, respectively [36]. All Ce-Zr mixed oxide peaks gradually shifted to higher 2θ values with the increase of the Zr molar ratio. This was due to the substitution of Ce4+ (ionic radius =0.97 Å) with Zr4+ (ionic radius =0.84 Å) ions [37]. The M-CeT and M-Ce7Zr3T peaks, which were calcined at 800 °C, a were sharper than those observed for M-Ce and M-Ce7Zr3. This was attributed to the increase in the crystal size of the catalysts due to sintering at high temperatures. Representative HR-SEM images of the PMMA and catalysts are displayed in Fig. 2. The average particle size of the PMMA is ˜ 330 nm. The catalysts calcined at 550 °C present a macroporous structure with thin walls in the size rage 150–270 nm. The macroporous structure of M-CeT partially collapsed after calcination. On the other hand, MCe7Zr3T maintained the macroporous structure, thereby exhibiting better thermal stability than M-CeT. The results from the nitrogen adsorption-desorption analysis are presented in Fig. S1. The graphs of all the catalysts displayed a hysteresis loop and a steep rise in the relative pressure range 0.9–0.99, thereby indicating the presence of macropores [27]. The overall textural properties of the catalysts are summarized in Table 1. The average mesopore size of the catalysts, calculated via the BJH method, was in the range 8–15 nm. Mesopores were present in the wall of the macroporous structure. However, since the size of these mesopores was smaller than that of soot particles (˜25 nm), the internal surface area of

3.3. Catalyst chemical properties under dry conditions Previous studies have reported that the Ox− species on ceria-based catalysts are the critical factor in soot oxidation under dry conditions, and that the amount of Ox− species can be evaluated by O2-TPD and H2TPR methods. The O2-TPD profiles (Fig. 4) can be divided into three areas: i) the first area, at temperatures < 180 °C, represents the desorption of the 3

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Fig. 2. Scanning electron microscopy (SEM) images of (a) polymethyl methacrylate (PMMA), (b) M-Ce, (c) M-CeT, (d) M-Ce7Zr3, (e) M-Ce7Zr3T, (f) M-Ce5Zr5, (g) M-Ce3Zr7, and (h) M-Zr. Table 1 Ce molar fractions, surface areas, external surface areas, crystal sizes and pore diameters of the catalysts. Catalyst

M-Ce M-CeT M-Ce7Zr3 M-Ce7Zr3-T M-Ce5Zr5 M-Ce3Zr7 M-Zr

Ce molar fractiona

1 1 0.72 0.72 0.54 0.30 0

BET surface area (m2/g)

External surface area (m2/g)

Crystal size (nm)b

33.2 11.7 29.7 27.4 38.9 33.9 26.3

22.9 10.2 28.6 24.2 36.5 30.6 22.3

11.3 33.0 5.17 7.18 4.78 6.29 11.6

Mesopore diameter (nm)c

8.05 13.8 8.58 9.65 13.7 14.4 15.1

Table 2 Combustion temperatures and activation energies (Ea) of soot over the prepared catalysts under different conditions.

Macropore diameter (nm)d

Catalyst

176 119 243 257 267 264 259

Pure soot M-Ce M-Ce7Zr3 M-Ce5Zr5 M-Ce3Zr7 M-Zr M-CeT M-Ce7Zr3T

a

Calculated by Ce/(Ce + Zr) based on ICP-AES data. Average crystal size were measured by X-ray crystal diffraction (XRD). c Average mesopore diameters were estimated using the Barrett-JoynerHalenda (BJH) method. d Average macropore diameters were measured from the scanning electron microscopy (SEM) images.

1% O2 (dry conditions)

1% O2, 10% H2O (wet conditions)

T50 (oC)

Tm (oC)

Ea (kJ/ mol)a

T50 (oC)

Tm (oC)

Ea (kJ/ mol)a

588 446 426 428 454 569 493 447

609 457 435 437 464 585 503 456

167 137 132 133 138 162 146 137

576 409 410 411 426 559 462 411

605 418 421 421 435 572 472 418

166 129 130 130 133 160 140 129

ΔTm(oC)

−4 −39 −14 −16 −29 −13 −31 −38

b

a

The pre-exponential factor was assumed as 109 min−1.

displays two broad peaks at 471 and 629 °C, which can be explained by three sequential steps: i) the generation of eOH groups on the ceria surface by the dissociation of the supplied H2 molecules ii) H2O desorption via the combination of the supplied H and eOH groups with surface reduction of Ce4+ to Ce3+ and the formation of anionic vacancies (first peak); and iii) diffusion of the bulk oxygen to the surface vacancies (second peak) [14,19,28,42]. Therefore, the first peak area is considered as a criterion to evaluate the amount of surface oxygen (Ox−), which is related to the performance of soot oxidation [12,18,19,27]. Fig. 5 reveals that over all the catalysts, M-Ce7Zr3 and M-Ce5Zr5 display the largest peak corresponding to their high soot oxidation activities under dry conditions. Therefore, both O2-TPD and

physiosorbed oxygen species (O2). ii) The second area, in the temperature range 180–500 °C, represents the desorption of the chemisorbed Ox− species. iii) Finally, the third area, at high temperatures > 500 °C, is assigned to the desorption of the bulk lattice oxygen (O2-) [26]. The second area (Ox−) is therefore related to the activity of soot oxidation. As a result, M-Ce7Zr3 and M-Ce5Zr5 presented the largest amounts of Ox− species, corresponding to their high soot oxidation performance under dry conditions. The H2-TPR profiles of the catalysts are presented in Fig. 5. M-Ce

Fig. 3. CO2 and CO concentrations during the temperature-programmed soot oxidation reactions. Reaction conditions: (a) 1% O2/He (100 mL/min), (b) 10% H2O/ 1% O2/He (100 mL/min), catalyst/soot = 10/1, tight contact, heating rate =5 °C/min. Solid line: CO2 concentration, Dashed line: CO concentration. 4

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oxygen is quantified by the [OV]/([OV]+[OL]+[OC]) [43] and Ce3+/ Ce4+ ratio since the Ce3+ concentration is related to the creation of oxygen vacancies [11,44,45]. M-Ce7Zr3 displays higher [OV] and Ce3+ ratio than M-Ce after dry condition pretreatment (Table 3). This is consistent with previous studies in which the amount of active oxygen species increased when Zr was added to ceria, thereby improving the performance of the ceriabased catalyst under dry conditions [19,20,46]. On the other hand, the [OV] and Ce3+ ratio in M-Ce and M-Ce7Zr3 increased and became similar to each other after wet condition pretreatment, which corresponds to the reaction result under wet condition in Fig. 3. This increase in the Ce3+ ratio is due to the generation of oxygen vacancies. It is suggested that generation of oxygen vacancies under wet condition caused the enhanced catalytic activity. Moreover, the increase of the [OV] ratio is attribute to the formation of eOH groups and active oxygen species on the ceria surface, since it has been reported that surface eOH groups are formed at the oxygen vacancy sites of ceria in presence of H2O [47,48]. There have been several reports that the H2O promotes the catalytic activity in various oxidation reactions. Weibin Li et al. showed that the methane oxidation activity of Co-Mn mixed oxide catalysts was improved in the presence of H2O, but no further studies on the reaction mechanism have proceeded [49]. Roberto Caporali et al. explicated the promoting effect of H2O in CO oxidation by elucidating the reaction mechanism of CO oxidation in the presence of H2O. According to their study, CO reacted with H2O derived species (H2O or eOH group) rather than O2 to form CO2 via COOH [23]. Chunlei Wang et al. also suggested the similar promoting effect of H2O in CO oxidation using single-atom Pt/CeO2 catalyst based on experiments and DFT calculations [24]. However, a mechanistic study on soot oxidation in the presence of H2O have not proceeded. Thus, we tried to investigate the mechanistic study on soot oxidation under wet condition in order to figure out the role of H2O, which enhances the catalytic activity.

Fig. 4. O2 temperature-programmed desorption (O2-TPD) profiles of (a) M-Ce, (b) M-Ce7Zr3, (c) M-Ce5Zr5, (d) M-Ce3Zr7, and (e) M-Zr.

3.5. Mechanism of soot oxidation in the presence of H2O The isotope experiment was conducted to determine the source of active oxygen species under wet conditions. The labelled H218O was introduced to investigate whether H218O was used as an oxygen source on M-Ce, M-CeT, M-Ce7Zr3, and M-Ce7Zr3T. Fig. 7 illustrates the MS signal of the products (C16O, C18O, C16O2, C16O18O, and C18O2) during combustion under wet conditions. The experimental results revealed that all the products contained 18O (C18O, C16O18O, and C18O2) and no products with only 16O (C16O, C16O2) were observed; this is totally different from the results observed under dry conditions (Fig. S3). An active oxygen species generated from gaseous oxygen adsorbed on the oxygen vacancy of the catalyst combusts soot particles under dry conditions [11,12,14]. However, the isotope experiment results suggest that soot oxidation uses 18Ox− species supplied from H218O in the presence of H2O; this follows a different mechanism from that reported in previous studies. Thus, a new mechanism for soot oxidation under wet conditions was next elucidated. In order to understand the mechanism of active oxygen formation from H2O, DFT calculations were conducted with CeO2 and Zr-doped CeO2 models. The O2 and H2O adsorption energies (Eads) on the clean surface and the surface with oxygen vacancies (VO) were first calculated (Table 4). Positive O2 Eads values (+0.05 eV on CeO2 and +0.01 eV on Zr-doped CeO2) were obtained on both clean surfaces implying the weak physisorption of O2. The Eads values of H2O were lower than those of O2 on the surface with VO in both the CeO2 and Zr-doped CeO2 models (−0.19 eV and −0.29 eV, respectively). These results indicate that under wet conditions, H2O adsorption is preferred over O2 adsorption on the clean surfaces in both models. However, the Eads values of O2 were lower than those of H2O on the surface with VO in both the CeO2 and Zr-doped CeO2 models. Studies have reported that H2O molecules adsorbed on the VO react

Fig. 5. H2 temperature-programmed reduction (H2-TPR) profiles of (a) M-Ce, (b) M-Ce7Zr3, (c) M-Ce5Zr5, (d) M-Ce3Zr7, and (e) M-Zr.

H2-TPR explain the high performance of the M-Ce7Zr3 and M-Ce5Zr5 catalysts under dry conditions. However, O2-TPD and H2-TPR have not been conducted under wet conditions and thus, it is necessary to introduce other analyses to investigate the effect of H2O. 3.4. Effect of H2O on the catalyst XPS analysis was performed to observe the change in the oxidation state of Ce in the catalyst under wet and dry conditions. M-Ce and MCe7Zr3, which present the most controversial activities and chemical properties, were analyzed after two different pretreatments: the first samples were pretreated under dry conditions (1% O2/He) at 550 °C for 4 h and the second samples were aged under wet conditions (10% H2O/ 1% O2/He) at 550 °C for 4 h. The O 1s spectra of M-Ce and M-Ce7Zr3 after different pretreatments are illustrated in Fig. 6 (a) and (c), respectively. The spectra can be deconvoluted into three peaks [43,44]: i) a peak at the binding energy 529.18 eV assigned to the lattice oxygen (OL); ii) a peak at ˜531.55 eV representing the surface oxygen species such as eOH groups and active oxygen in the defects (OV); and iii) a peak at 532.45 eV due to chemisorbed carbonates or water (OC). Fig. 6(b) and (d) illustrate the respective Ce 3d spectra of M-Ce and M-Ce7Zr3 after different pretreatments. The main features are composed of ten peaks: six peaks are attributed to Ce4+3d (u1-6: 882.2, 888.6 898.1, 900.9, 907.0, and 916.6 eV) and four peaks to Ce3+3d (v1-6: 880.3, 884.2, 899.3, and 902.3 eV) [12,14]. Generally, the concentration of the surface-active 5

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Fig. 6. X-ray photoelectron spectroscopy (XPS) images of (a, b) M-Ce, and (c, d) M-Ce7Zr3 with various pretreatments.

These correspond to four different reactions: i) vacancy formation followed by O2 adsorption (Eq. 3); ii) O2 adsorption followed by O2 dissociation (Eq. 4); iii) spillover of the lattice oxygen (Eq. 5); and iv) H2O adsorption followed by H2O dissociation (Eq. 6). The reaction energetics and activation barriers of all the reaction steps on the CeO2 and the Zr-doped CeO2 surfaces are presented in Fig. 8 and summarized in Table 5. Under dry condition, reactions described in Eqs. 3–5 can occur on the clean surfaces. Vacancy formation followed by O2 adsorption (Eq. 3) and O2 adsorption followed by O2 dissociation (Eq. 4) display the lowest energy barriers. Because of the week O2 adsorption on the surface (Table 4), Eq. 3 is the preferred reaction to produce O* species in both the CeO2 and Zr-doped CeO2 models. This indicates that the active oxygen species are mainly formed by the O2 molecules adsorbed on the VO, as reported in oxidation reaction studies [52]. Thus, oxygen vacancy formation would play an important role in the soot oxidation under dry conditions. Under wet conditions, H2O adsorption and dissociation (Eq. 6) can additionally occur on the clean surfaces. Negligible energy barriers were obtained for H2O dissociation into H* and OH* species (Fig. 8 and Table 5). Additionally, the barriers for OH dissociation (2.34 eV on CeO2 and 1.74 eV on Zr-doped CeO2) are lower than those observed for Eq. 3. This indicates that active oxygen formation from H2O is preferred over that from O2 on the non-defective surface. Therefore, H2O can be used as an active oxygen source on non-defective surfaces under wet conditions, as illustrated in the isotope experiments. Even though the soot oxidation reaction can proceed via H2O dissociation, an O2 supply is also necessary. Soot oxidation with 10% H2O and no O2 presented a much lower reaction rate than that observed under wet conditions (Fig. S6). Presumably, the surface hydrogen (H*) produced by H2O decomposition poisons the catalyst surface since the H2 formation energy from the H* species without oxygen is very high [50]. According to the isotope experiments, the consumption of 16O2 under wet conditions is similar to that observed under dry conditions (Fig. S7). However, the soot oxidation products (CO, CO2) mostly contained 18O from H218O (Fig. 7). This means that 16O2 did not react

Table 3 Quantitative analyses of the ratios related to active oxygen and oxygen vacancies in the X-ray photoelectron spectroscopy (XPS) results of M-Ce and MCe7Zr3 after various pretreatments. Catalyst

M-Ce M-Ce7Zr3 a b

Dry (1% O2)

Wet (1% O2, 10% H2O)

[OV] ratioa

Ce3+ ratiob

[OV] ratioa

Ce3+ ratiob

0.243 0.254

0.561 0.617

0.304 0.301

0.703 0.701

Calculated by [OV]/([OV] + [OL]+[OC]). Calculated by Ce3+/Ce4+.

with the lattice oxygen (OL) and dissociate into surface eOH species as follow [50,51]: H2O(g) + OL + VO → 2OH* (2OL+2H*)

(2)

Thus, the energetics of the surface eOH formation on both the CeO2 and Zr-doped CeO2 models (Fig. 8 and Table 5) were next calculated. The results revealed that OH* formation can be favored over O* formation by O2 adsorption, despite the stronger O2 adsorption over that of H2O. The surface defects would be covered with surface eOH groups under wet conditions, attributed to the increased surface oxygen species, represented by the [OV] ratio in the XPS spectra, after wet condition pretreatment. Thus, since the VO would be covered with surface eOH groups, active oxygen (O*) species would be produced on the nondefective surface. To investigate the source of active oxygen formation, four different mechanisms for the production of active oxygen (O*) on a clean surface without surface defects were considered: OL + 1/2O2(g) → VO + O2(g) → O*

(3)

O2(g) → O2* → 2O*

(4)

OL → VO + O*

(5)

H2O(g) → H2O* → H* + OH* → 2H* + O*

(6) 6

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Fig. 7. C16O, C18O, C16O2, C16O18O, and C18O2 production signals as a function of soot oxidation over (a) M-Ce, (b) M-CeT, (c) M-Ce7Zr3, and (d) M-Ce7Zr3T in presence of isotope water (H218O). Reaction conditions: 10% H218O/1% 16O2/He (100 ml/min), catalyst/soot = 10/1, tight contact, heating rate =5 °C/min. Table 4 Adsorption energies (Eads) of O2 and H2O on each surface. Surface

Table 5 Reaction enthalpy (ΔH) and activation barrier of each reaction on the CeO2 and Zr-doped CeO2 surfaces.

Eads (eV)

Reaction Eads, CeO2 Zr-doped CeO2

Clean Vacancy Clean Vacancy

O2

+0.05 −2.37 +0.01 −1.68

Eads,

Reaction energy (ΔH)

H2O

−0.19 −1.53 −0.29 −1.47

OL → VO+1/2 O2(g) (Vacancy formation) VO+O2(g) → O* VO+H2O(g) → H2O* H2O*+OL → 2OH* (OL+2H*) O2(g) → O2* O2* → 2O* OL → VO+O* H2O(g) → H2O* H2O* → H*+OH* H*+OH* → 2H*+O*

with soot particles but with other species on the catalytic surface. Thus, it is suggested that 16O2 formed H216O with the H* species and regenerated the surface poisoned by H*. In other words, under wet conditions, 16O2 removes the surface H* while H218O combusts soot particles.

CeO2 (eV)

Zr-doped CeO2 (eV)

+2.88 +0.51 −1.53 −1.53 +0.05 +0.90 +3.10 −0.19 −0.08 +1.16

+1.81 +0.13 −1.48 −1.12 +0.01 +0.66 (1.83) +1.91(2.52) −0.29 −0.43 (0.01) +0.93 (1.74)

(2.89) (3.13) (0.09) (2.34)

treated catalysts, both M-CeT and M-Ce7Zr3T, showed decreased catalytic activity under dry condition. The decrease in dry condition activity in both M-CeT and M-Ce7Zr3T is due to the reduction in the [OV] and Ce3+ ratios, which respectively represent the surface oxygen species and the oxygen vacancies, after heat treatment (Fig.S8). When the catalyst is exposed to high temperatures, the oxygen vacancies decrease

3.6. Effect of H2O on the thermally deactivated catalysts The effect of H2O on the thermally treated catalysts was also investigated. The catalytic activities, XPS results, and morphologies of the thermally treated catalysts are summarized in Table 6. Thermally

Fig. 8. Reaction energetics of the reactions on (a) CeO2 and (b) Zr-doped CeO2 surfaces. 7

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Finally, M-CeT presented a lower activity than M-Ce under wet conditions due to the collapsed morphology.

Table 6 Catalytic activities, X-ray photoelectron spectroscopy (XPS) results, and morphology factors of each catalyst before (M-Ce, M-Ce7Zr3) and after (M-CeT, MCe7Zr3T) heat treatment (HT). Catalyst

Catalytic activity

XPS result

These conclusions reveal that the presence of H2O changes the surface state of the catalyst and participates in soot oxidation as an oxidant. Accordingly, the effect of H2O is a necessary factor in the study of GPF catalysts. Thus, catalysts that are able to activate water well show great potential for application as GPF catalysts.

Morphology Factor

Dry Tm (oC)

Wet Tm (oC)

[OV] ratioa

Ce3+ ratiob

External surfaces area (m2 g−1)

Macropore diameter (nm)

457 503 435 456

418 472 421 418

0.243 0.196 0.254 0.211

0.561 0.456 0.617 0.532

22.9 10.2 28.6 24.2

176 119 243 257

Acknowledgments M-Ce M-CeT M-Ce7Zr3 M-Ce7Zr3T a b

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; NRF2016R1A5A1009592).

Calculated by [OV]/([OV] + [OL]+[OC]). Calculated by Ce3+/Ce4+.

Appendix A. Supplementary data

due to an increase in the particle size [53]. On the other hand, M-Ce7Zr3T displayed a similar wet condition activity to that of M-Ce7Zr3, despite the decreased [OV] and Ce3+ ratios. This implies that under wet conditions, the performance is not strongly affected by the oxygen vacancies because active oxygen can be produced on a non-defective surface via H2O dissociation. Thus, under wet conditions, the morphology of the catalyst, which directly affects the contact points between the soot and catalyst, is considered more important than the oxygen vacancies. As the textural properties of MCe7Zr3T were maintained after heat treatment, M-Ce7Zr3T had similar contact points to those of M-Ce7Zr3 and retained its catalytic activity. M-CeT presented an increased catalytic activity under wet conditions over that observed under dry conditions. However, this catalyst still presented lower wet condition activity than M-Ce. This decrease in catalytic activity was attributed to the loss of contact points due to the collapsed morphology of M-CeT. This indicates that if the morphology of a catalyst is maintained after heat treatment, the catalyst can retain its original performance in the presence of H2O even if the oxygen vacancy sites decrease. Therefore, the thermal stability of the catalyst is an important factor in the wet condition reaction.

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4. Conclusion In this study, characterization and soot oxidation activity of macroporous Ce-Zr catalysts under dry and wet conditions led to the following conclusions: - M-Ce7Zr3 and M-Ce5Zr5 exhibit the highest activity for soot oxidation under dry conditions. Zr-doping to ceria created additional surface oxygen vacancies, confirmed via O2-TPD, H2-TPR, and XPS analyses. - The activity of all the catalysts was greater under wet conditions than under dry conditions. The ratio of OV, associated with the surface-active oxygen, also increased after pretreatment with H2O. The DFT study suggested that the increased active oxygen is generated by the dissociation of H2O and that the dissociation process is not deeply related to the surface oxygen vacancies. Therefore, M-Ce presented a similar performance to that of M-Ce7Zr3, despite the lower oxygen vacancies, when H2O was introduced. - After the catalysts were aged at high temperatures, the oxygen vacancies in M-Ce and M-Ce7Zr3 decreased and the morphology of MCe collapsed. Under dry conditions, the reduction in oxygen vacancies led to a decrease in the catalytic activity of M-CeT and MCe7Zr3T. In the presence of H2O, both M-CeT and M-Ce7Zr3T displayed higher catalytic activities than those observed under dry conditions. Moreover, the performance of M-Ce7Zr3T was the same as that of M-Ce7Zr3. This indicates that the mechanism of soot oxidation in the presence of H2O is not deeply related to the oxygen vacancies but to the contact points between the soot and catalysts. 8

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