Article pubs.acs.org/JPCC
Probing BaO Doping Effect on the Structure and Catalytic Performance of Pd/CexZr1−xO2 (x = 0.2−0.8) Catalysts for Automobile Emission Control Linyan Yang, Xue Yang, and Renxian Zhou* Institute of Catalysis, Zhejiang University, Hangzhou 310028, China S Supporting Information *
ABSTRACT: Pd/CeO2-ZrO2-BaO (Pd/CZB) catalysts with 5.0 wt % BaO and different CeO2/ZrO2 molar ratio were synthesized. The modification with BaO significantly promotes the catalytic activity of HC and NOx conversions when the molar ratio of CeO2 to ZrO2 is 2/1, because the substitution of Cex+/Zr4+ ions by Ba2+ promotes the formation of oxygen vacancy as revealed by XRD refinement analysis results and improves the mobility of active oxygen as a result of electronic and structural modifications. Among the Ba-doped catalysts, the crystalline form of the CZB supports gradually transform from cubic and pseudocubic to stable tetragonal phase as the increase of ZrO2 content. Meanwhile, the Raman results show that addition of moderate ZrO2 promotes the formation of structural defects, resulting in higher OSC and NOx storage efficiency. On the other hand, the introduction of moderate ZrO2 facilitates the dissociative adsorption of NO and promotes the deep oxidation of HC. Therefore, Pd/CZB catalysts with CeO2/ ZrO2 molar ratio of 3/1−1/1, especially Pd/CZB21, show a much better catalytic activity of HC and NOx eliminations and a wider dynamic operation window. Zr-rich catalysts present worse catalytic activity of CO oxidation compared with Ce-rich catalysts, which is mainly arising from the decreased amount of active sites. After thermal aging treatment, Zr-rich catalysts (CeO2/ZrO2 < 1) undergo less severe deterioration of the catalytic activity compared with Ce-rich catalysts (CeO2/ZrO2 ≥ 1) as a result of better thermostability, and Pd/CZB11-a presents the best catalytic performance and widest dynamic operation window.
1. INTRODUCTION Three-way catalysts (TWCs) have been the most satisfactory and efficient solution to control and suppress automobile emissions by converting CO, HC, and NOx pollutants into nontoxic matters.1−3 Ceria-zirconia-based solid solutions are widely used as oxygen storage materials in three-way catalysts.4 Taking account of the operating condition of catalysts (continuous exhaust supply and the bed temperature of TWCs can rise to above 1000 °C), their thermal, chemical, and structural stabilities will act as the key factors in determining the three-way catalytic performance.5,6 Furthermore, the strictness of exhaust regulations inevitably demand better efficacy of three-way catalysts, especially for the removing of HC and NOx during cold-start process.7,8 Hence, the generation of the ceria-zirconia-based mixed oxides with high textural/structural stability and outstanding oxygen storage capacity is the main determinant of the development of TWCs that have excellent thermostability and low-temperature activity as well as wide operation window.9,10 As reported in previous literatures,1,10,11 the addition of structural promoters into CeO2-ZrO2 solid solution could increase the concentration of oxygen vacancy as a result of charge equilibrium, thus, accelerating the oxygen mobility and © XXXX American Chemical Society
improving the thermostability. More attention has been paid to alkali and alkaline earth additions lately, considering their higher basicity and better electron-donating ability compared with rare earth and transition earth metals.8,12,13 In our previous work,14−16 we found that the introduction of alkaline earth metals (such as Ca, Sr, and Ba) into Ce0.67Zr0.33O2 solid solution significantly promoted the catalytic activity of NOx and HC conversions and enhanced the thermostability of the corresponding catalysts. Moreover, the operation window of NOx conversion could also be widened as a result of promoted NOx storage capacity, especially for the Ba modified catalysts. Considering that the host/guest cations should preferably induce stress and structural defects because of the difference in the ionic radius and valence state between cations, as well as the change of crystal structure;9,17,18 therefore, the change of host/ guest cations molar ratio could significantly affect the catalytic performance for Pd/CeO2-ZrO2-BaO catalysts. One of the difficulties in handling this system is the large variety of crystalline structures that may exist depending on the Received: October 21, 2015 Revised: January 21, 2016
A
DOI: 10.1021/acs.jpcc.5b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Light-off (T50%) and full-conversion (T90%) temperature of HC, CO, NO, and NO2 over the fresh (A) and aged (B) Pd/CZB catalysts with different CeO2/ZrO2 molar ratio; the conversions of reactants over the fresh (C) and aged (D) Pd/CZ21 and Pd/CZB21 catalysts.
CZB13, and CZB14. The corresponding supported Pd-only catalysts with Pd content of 1.0 wt % were prepared by conventional wet impregnation method with an aqueous H2PdCl4 as metal precursor.21 The fresh catalysts calcined at 600 °C for 2 h were designated as Pd/CZB41, Pd/CZB31, Pd/ CZB21, Pd/CZB11, Pd/CZB12, Pd/CZB13, and Pd/CZB14, respectively. Furthermore, a portion of each catalyst was further aged at 1100 °C in air for 4 h in order to investigate the thermal aging resistance, and the corresponding catalysts were denoted as Pd/CZB41-a, Pd/CZB31-a, Pd/CZB21-a, Pd/CZB11-a, Pd/ CZB12-a, Pd/CZB13-a, and Pd/CZB14-a. 2.2. Catalytic Activity Test. Catalytic tests corresponding to reaction mixture at the stoichiometric point (O2 (0.745%)C3H6 (0.067%)-C3H8 (0.033%)-CO (0.75%)-NO (0.1%)-NO2 (0.03%) and balance Ar at a space velocity of 43000 h−1) were carried out with a fixed-bed continuous flow reactor, and the concentration of CO, NO, NO2, and HC (C3H6 and C3H8) were quantified by an online Bruker eq 55 FTIR spectrometer coupled with a multiple reflection transmission cell (Infrared Analysis Inc., path length 10.0 m). The air/fuel ratio (λ) was defined as λ = (2νO2 + νNO + 2νNO2)/(νCO + 9νC3H6 + 10νC3H8) (ν is the concentration of each gas in units of volume percent), and λ = 1 was used in all the activity measurement. The dynamic operation window experiments were carried out at 400 °C by adjusting the concentration of O2 and analyzed by Hiden QIC-20 mass detector working in electron impact (EI) mode at 70 eV. The NOx operation window was measured when the λ value is up to
composition and temperature along with the corresponding impact that each type of crystallographic modification has upon the catalytic activity.19 For example, Zr-rich CeO2-ZrO2 solid solution exhibits apparently higher thermostability compared with Ce-rich samples, but may induce phase change from cubic to tetragonal phase.20 In relation to catalytic properties, oxygen mobility is favored by a cubic geometry, whereas the tetragonal or monoclinic symmetries limit its mobility through the lattice. On the basis of that, to gain a further insight into the doping effect of BaO on structure−activity relation in CeO2-ZrO2 solid solution, we synthesized a series of Pd/CZB catalysts with different CeO2/ZrO2 molar ratio and fixed loading content of BaO (5 wt %). The structural/textural properties, oxygen and NOx storage ability were investigated by Rietveld analysis of Xray diffraction (XRD) patterns, Vis/UV-Raman and NOx adsorption/desorption-MS. Besides, the influence of CeO2/ ZrO2 molar ratio on the formation and reactivity of surface intermediates was investigated by means of in situ DRIFTS.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A series of Ba-modified ceriazirconia mixed oxides with different CeO2/ZrO2 molar ratios were prepared by a conventional coprecipitation method combined with a supercritical drying technique.14 The additive content of BaO was 5.0 wt % and the theoretical molar ratio of CeO2/ZrO2 were 4/1, 3/1, 2/1, 1/1, 1/2, 1/3, and 1/4, respectively. The fresh supports were calcined at 600 °C for 4 h in air and labeled as CZB41, CZB31, CZB21, CZB11, CZB12, B
DOI: 10.1021/acs.jpcc.5b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Time-course curves of CO/N2 (A, D), NOx (B, E), and CO2/N2O (C, F) over the fresh and aged Pd/CZB catalysts with different CeO2/ ZrO2 molar ratio.
NOx adsorption/desorption-MS was conducted in a microreactor equipped with a mass spectrometer apparatus (Hiden QIC-20). The catalysts were pretreated at 450 °C for 0.5 h under 20% O2/Ar flow gas and were subsequently exposed to the reaction gas (0.1% NO-0.03% NO2-0.745% O2, Ar balance) at 100 °C 0.5 h. Then, the samples were heated up to 600 °C (10 °C/min) in highly purified Ar stream. Dispersion of Pd species was calculated on the basis of CO chemisorption using CHEMBET-3000 (Quantachrome Co.). The catalyst (0.2 g) was reduced by purified H2 at 400 °C for 1 h, then purged at 400 °C by He for 0.5 h and cooled down to room temperature. CO pulses were injected from a calibrated online sampling value. CO adsorption was assumed to be completed after three successive peaks showed the same peak areas. A CO/Pd stoichiometry of 1 was used for calculations. DRIFTS spectra were collected using a Nicolet 6700 FTIR equipped with a MCT detector. All spectra were obtained with a resolution of 4 cm−1 and an accumulation of 32 scans. The catalysts were loaded into an environmental diffuse reflectance IR cell designed to work under controlled temperature and flowing gas. The samples were in situ pretreated in high purity Ar stream (10 mL/min) at 450 °C for 0.5 h. Subsequently, the background spectra of each sample at different temperature were collected prior to the introduction of the reacting gases. DRIFTS reaction experiments were conducted under the simulated reaction condition (the simulated gas concentration and space velocity were the same as the catalytic activity test), and spectra were recorded as a function of temperature at the interval of 50−400 °C.
1.15 (rich oxygen condition) from 0.90 (lean oxygen condition). After reaction for 20 min, the λ value was adjusted from 1.15 to 0.90 in order to measure the HC/CO operation window. 2.3. Characterization Techniques. BET surface areas and N2 adsorption−desorption isotherms were determined by N2 physisorption at 77 K using a TriStar II 3020 (Micromeritics Inc.) apparatus, after degassing the samples in vacuum (
