Thermal stability of Pd-containing LaAlO3 perovskite as a modern TWC

Journal of Catalysis 330 (2015) 71–83

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Thermal stability of Pd-containing LaAlO3 perovskite as a modern TWC Dal Young Yoon a, Young Jin Kim a, Ji Hyun Lim a, Byong K. Cho a, Suk Bong Hong a, In-Sik Nam a,⇑, Jin Woo Choung b a School of Environmental Science and Engineering/Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-Dong, Pohang 790-784, Republic of Korea b Advanced Catalysts and Emission-control Research Lab, Powertrain Performance Development, R&D Division, Hyundai Motor Company, 772-1 Jangduk-Dong, Hwaseong 445-706, Republic of Korea

a r t i c l e

i n f o

Article history: Received 6 April 2015 Revised 22 June 2015 Accepted 6 July 2015

Keywords: Pd-based TWC LaAlO3 Metal–support interaction Thermal stability Structure-sensitivity

a b s t r a c t TWC activity and thermal stability of LaAlO3-based Pd catalysts were investigated under simulated gasoline engine exhaust conditions. The LaAlO3-based catalysts revealed a stronger thermal stability than the conventional Al2O3-based ones. Pd-substituted for Al in the LaAlO3 perovskite structure exhibits a stronger thermal stability than that impregnated on the LaAlO3 surface. The increased electron density around Pd induced by the electron donation from the more electronegative La may enhance the thermal stability of PdO by suppressing the decomposition of PdO. The strong interaction between Pd and La appears also contributing to the improvement of the TOFs for the TWC reactions over the Pd-substituted LaAlO3 (i.e., LaAlPdO3) catalyst, as evidenced by CO-chemisorption, TGA, XPS, XANES, H2-TPR, DRIFT and TPD studies. CO and C3H6 oxidation reactions over the LaAlO3-based Pd catalysts are strongly structure-sensitive, while those over the Al2O3-based Pd catalysts are weakly structure-sensitive. The NO reduction and H2 oxidation reactions are strongly structure-sensitive over all the Pd-containing catalysts examined. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The three-way catalyst (TWC) has long been recognized as the most efficient catalytic system for removing air pollutants such as CO, hydrocarbons (HCs) and NOx emitted from gasoline engines operating at near stoichiometric conditions [1–3]. In order to meet the ever-tightening emission standards for gasoline-driven vehicles over the world, the amount of noble metals such as Rh, Pt and Pd required in TWC consistently increases [1]. Pt and Pd have been employed as active metal components for the oxidation of CO and HCs, while Rh has been known to be effective for the reduction of NOx [1,4]. In recent years, Pd has been used as an appropriate alternative for Pt in the modern commercial TWC system, mainly due to its cost advantage when combined with the use of unleaded and low-sulfur gasoline fuel [2]. Since TWCs are typically exposed to extremely high exhaust gas temperature, occasionally reaching to 1050 °C and beyond, they are prone to be deactivated due to the sintering of the noble metals as well as the thermal ⇑ Corresponding author at: School of Environ. Sci. and Eng./Dept. of Chem. Eng. Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Republic of Korea. Fax: +82 54 279 8299. E-mail address: [email protected] (I.-S. Nam). http://dx.doi.org/10.1016/j.jcat.2015.07.013 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

degradation of other catalyst components including catalyst support, most commonly Al2O3 [1,4–8]. Over the past years, perovskite-type oxides (a general chemical formula of ABO3 where the A-site is normally a rare earth or alkali earth metal coordinated with 12 oxygen ions and the B-site is commonly a transition metal coordinated with 6 oxygen ions in an octahedral manner) have been widely investigated for application in heterogeneous catalysis due to their unique adsorption, acid– base and redox properties, as a potential TWC material for improving its thermal stability in particular [9–14]. Nishihata et al. [10] and Tanaka [11] reported that the NO–CO reaction activity of a LaFe0.57Co0.38Pd0.05O3 catalyst was higher than that of the Pd/Al2O3 catalyst, when used as a TWC after thermal aging at 900 °C for 100 h, which was attributed to the strong interaction of Pd with the perovskite structure involving a self-regeneration process under the redox cyclic aging mode – Pd emerging from the perovskites as a metallic Pd under a reducing environment while fully dissolving into the perovskite structure under an oxidizing condition. Tou et al. reported that a Pd/LaMnO3/Al2O3 catalyst revealed a NO–CO reaction activity superior to a Pd/Al2O3 catalyst after thermal aging at 900 °C due to the Pd–LaMnO3 interaction resulting in the suppression of the Pd sintering [12].

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When the LaAlO3 perovskite was employed as the support for Pd, the reduction of PdO to Pd0 on the LaAlO3 surface during the thermal aging at 900 °C was moderated, leading to the improvement of its thermal stability, compared to the Pd/Al2O3 catalyst [13]. However, the Pd-substituted LaAlO3 perovskite for the TWC reaction has not been reported to the best of our knowledge, although the incorporation of Pd into the perovskite structure appears to have strong potential as a way to improve the stability of Pd upon the thermal aging of the perovskite catalyst [10,11]. In the present study, a Pd-substituted LaAlO3 perovskite (LaAlPdO3) has been prepared as a promising TWC to improve the catalyst thermal stability. The catalytic performance has been systematically investigated in comparison with other Pd-containing catalysts under a slightly lean condition before and after thermal aging. The effect of thermal aging on the variation of the physicochemical properties of the Pd-containing catalysts has been examined by a variety of catalyst characterization techniques including CO-chemisorption, N2 sorption, STEM-EDS, H2-TPR, TGA, XANES, XPS, DRIFT and TPD to understand the superior thermal stability of LaAlPdO3.

2. Experimental 2.1. Catalyst preparation The LaAlO3 perovskite was prepared by the citric acid method using La(NO3)36H2O (Yakuri Pure), Al(NO3)39H2O (Junsei) and citric acid in the monohydrate form (Kanto). Details of the procedure are described elsewhere [15]. Pd (0.8 wt.%) was impregnated onto c-Al2O3 (Alfa Aesar) and LaAlO3 by the incipient wetness method with an aqueous solution of Pd(NO3)2 (Aldrich) to prepare Pd/Al2O3 and Pd/LaAlO3, respectively. Pd was also incorporated into the lattice structure of catalyst supports including Al2O3 and LaAlO3 in order to examine the possibility of improving its thermal stability [10,11]. The Pd-substituted Al2O3 catalyst (Al2xPdxO3, x: 0.008 equivalent to 0.8 wt.% of Pd loading) was synthesized by the sol–gel method [16]. An aqueous solution of Pd(NO3)2 (Aldrich) was added into aluminum hydroxide sol formed by the hydrolysis of Al-isopropoxide (Aldrich) in the distilled water containing nitric acid (0.01 M) at 85 °C, followed by vigorous stirring to form a gel.

The Pd-substituted LaAlO3 perovskite (LaAl1yPdyO3, y: 0.016, 0.032 and 0.049 equivalent to 0.8, 1.6 and 2.4 wt.% of Pd loading, respectively) was prepared by adding an aqueous solution of Pd(NO3)2 (Aldrich) along with La- and Al-precursors into distilled water to partially substitute the B-site of the LaAlO3 perovskite. The solution containing the metal precursors and citric acid was processed in the same way as used for the preparation of the LaAlO3 perovskite [15]. The prepared catalysts were then dried overnight at 110 °C, followed by calcination at 700 °C for 5 h in a muffle furnace. The catalysts were systematically aged by using an in-house laboratory aging system in order to investigate their thermal stability [17,18]. The fresh catalyst was charged into the quartz tubular reactor (1 in. o.d.), and then the desired aging temperature and time were controlled by using a three-zone electronic furnace. The periodic cyclic aging was conducted as per the following procedure: background gas containing 10% CO2, 10% H2O and N2 balance for 45 s ? 3% O2 + background gas for 5 s ? background gas for 125 s ? 3% CO + background gas for 5 s (total flow rate: 6 L/min). The catalyst was stabilized first at 600 °C for 16 h, and then aged at 1000 °C for 6 h or at 1050 °C for 6 h, which is equivalent to 4 k-, 65 k- and 140 k-field-aged mileage of the catalyst, respectively, based upon the 2nd order deactivation kinetics (Fig. S1 and Table S1 of Supplementary Material) [17]. The Pd-containing catalysts prepared were designated as Pd(x)  y catalyst, where x and y represent the Pd loading (wt.%) and actual aging temperature, respectively. Details of the catalysts employed in the present study are listed in Table 1. 2.2. Reactor system The TWC activity over the Pd-containing catalysts has been examined in a differential fixed-bed reactor system to evaluate the turnover frequency (TOF) for each reactant including CO, C3H6, H2 and NO. A feed gas mixture containing 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm NO, 1% O2, 10% CO2, 10% H2O and Ar balance (stoichiometric factor, S = 1.17) was introduced into the reactor system at 1 L/min to simulate the gasoline engine exhaust gas conditions. The stoichiometric factor (S) was calculated by Eq. (1) [17,19].

S ¼ ð2CO2 þ CNO Þ=ðCCO þ CH2 þ 9CC3 H6 Þ

ð1Þ

Table 1 Physicochemical properties of Pd-containing catalysts employed in the present study.

a b c d e

Catalyst

BET surface area (m2/g)

Metallic surface area of Pda (m2/g)

Pd dispersiona (%)

Particle size of Pda (nm)

Particle size of Pd0b (nm)

Pd contente (wt.%)

Pd(0.8)/Al2O3-600 Pd(0.8)/Al2O3-1000 Pd(0.8)/Al2O3-1050 Al2Pd(0.8)O3-600 Al2Pd(0.8)O3-1000 Al2Pd(0.8)O3-1050 Pd(0.8)/LaAlO3-600 Pd(0.8)/LaAlO3-1000 Pd(0.8)/LaAlO3-1050 LaAlPd(0.8)O3-600 LaAlPd(0.8)O3-1000 LaAlPd(0.8)O3-1050 LaAlPd(1.6)O3-600 LaAlPd(1.6)O3-1050 LaAlPd(2.4)O3-600 LaAlPd(2.4)O3-1050

113 26.5 12.3 149 18.2 11.3 11.0 7.4 4.8 10.8 8.7 5.1 11.1 6.6 11.6 8.0

0.436 0.050 0.028 0.240 0.031 0.021 0.505 0.094 0.024 0.313 0.077 0.033 0.604 0.073 0.780 0.112

12.5 1.4 0.8 6.9 0.9 0.6 14.5 2.7 0.7 9.0 2.2 0.9 8.9 1.0 7.6 1.0

9 80 141 17 126 180 8 41 163 13 51 116 13 109 15 106

n.d.c n.d.d n.d.d n.d.c n.d.d 68 n.d.c 43 74 n.d.c 37 58 n.d.c 53 n.d.c 52

0.82 0.80 0.79 0.77 0.83 0.78 0.78 0.81 0.82 0.81 0.82 0.81 1.65 1.60 2.39 2.42

Obtained from CO-chemisorption results. Obtained from XRD results. Not determined by XRD results due to the highly dispersed Pd resulting in the absence of specific peak. Not determined by XRD results due to the peak overlap between Pd0 and h-Al2O3. Determined by ICP analysis.

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where Ci is the volumetric concentration of species i. A physical mixture of the catalyst powder and cordierite (catalyst powder: cordierite = 3:7) in the 0.60–0.84 mm sized pellet (20/30 mesh) has been employed for the activity test. The catalyst pellets (0.1 g) were mixed with quartz beads of the same size (1.2 mL), and charged into a 3/8 in. o.d. stainless steel U-type tubular reactor immersed in a molten-salt bath to maintain the catalyst bed in the isothermal condition during the course of reaction, while the gas hourly space velocity (GHSV) was maintained at 500,000 h1 [4,15]. The temperature difference between the inlet and outlet of the catalyst bed was within 1–2 °C over the entire reaction temperature range covered. Under the present experimental condition, the influence of the external mass transfer and internal diffusion resistances on the TWC activity of the Pd-containing catalysts was negligible as discussed in Figs. S2a and S2b of Supplementary Material. The inlet and outlet gas concentrations were determined by an online gas chromatography equipped with a TCD and an FID (Agilent, Model 6890N) and an FT-IR (Thermo Electron Co. Nicolet 5700). The conversion of reactant species i (Xi) was calculated according to Eq. (2):

X i ð%Þ ¼ 100  ðC i;inlet  C i;outlet Þ=C i;inlet

ð2Þ

where C i;inlet and C i;outlet refer to the volumetric concentration of i species in the feed gas stream and the outlet stream of the reactor, respectively. The conversion of each reactant species including CO, C3H6, H2 and NO over the differential reactor was maintained below 15% in the temperature range from 450 to 650 K to keep the reaction in the kinetic regime. The TOF for each reaction over the Pd-based catalyst was calculated according to Eq. (3):

TOF for iðs1 Þ ¼

C f ;i  X i  m ML D MW

ð3Þ

where Cf,i is the feed concentration of species i (mol/L), v is the volumetric flow rate (L/s), ML is the Pd loading on the catalyst (g), MW is the molecular weight of Pd (106.42 g/mol) and D is the Pd dispersion (%) on the catalyst surface obtained from the CO-chemisorption results. The activation energy (Ea) was calculated from the slope of the Arrhenius-type plot of the TOF of each reactant. Prior to the activity test, the catalyst was pretreated in a flow of 0.9% CO, 0.3% H2, 0.6% O2, 10% CO2, 10% H2O and Ar balance (S = 1.0) at 450 °C for 2 h.

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Scanning transmission electron microscopy (STEM) equipped with energy-dispersive X-ray absorption spectroscopy (EDS) operated at 200 kV was employed to optically evaluate the particle size of metals on the catalyst surface (HR-TEM-I: JEM-2100F, JEOL). The specimen for the TEM study was prepared by dispersing the samples in ethanol and ultrasonically mixed for 1 h. The suspension was dropped onto a holey carbon grid supported on a 200 mesh copper grid and dried overnight in an oven (80 °C). X-ray photoelectron spectroscopy (XPS) spectra were collected by using a VG-Scientific ESCALAB 220iXL spectrometer equipped with a Mg Ka X-ray source (hm = 1253.6 eV). The binding energies of each sample were corrected by the C 1s peak of carbon at 284.9 eV [15]. X-ray adsorption near edge spectra (XANES) for Pd K-edge (24,350 eV) were collected at the 7D1 beam line of the Pohang Light Source (PLS). This beam line is equipped with a Si (1 1 1) monochromator and ionization chambers for measuring the transmitted beam intensities. The photon energy was calibrated with a Pd foil, determining the absorption edge energy (E0) as 24,350 eV by the maximum peak from the first derivative of f(E). Temperature-programmed reduction by H2 (H2-TPR) was conducted to examine the redox property of the catalyst and interaction between Pd and catalyst support. 0.2 g of the catalyst pre-oxidized in a flow of 5% O2/Ar at 500 °C for 1 h was cooled down to the 50 °C using liquid N2 and then heated to 500 °C at 2 °C/min under a 5% H2/Ar flow. Details on this experimental procedure are described elsewhere [23]. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were collected by using an FT-IR (Thermo Scientific Nicolet 6700) equipped with an MCT detector and a DRIFT cell (Pike). The spectra were collected with a resolution of 4 cm1 in the range of 400–4000 cm1. Prior to each experiment, the sample was pretreated at 500 °C for 1 h in a He flow, and then exposed to 1% of CO/He for 30 min at 35 °C. Temperature programmed desorption (TPD) was conducted in a U-type quartz tubular reactor containing 0.2 g of the catalyst powder. Before the TPD analysis, each sample was pretreated at 500 °C for 1 h in a flow of 5% H2/He balance (100 mL/min). During the CO-, C3H6-, NO- or CO2-TPD experiments, the adsorption of the probe molecule onto the catalyst sample was carried out at 150 °C with a flow of 1% CO (CO-TPD), 500 ppm C3H6 (C3H6-TPD), 500 ppm NO (NO-TPD) or 10% CO2 (CO2-TPD), respectively, and 10% H2O in He balance for 1 h followed by purging in a He flow at the same temperature for 4 h. The catalyst was then heated from 150 to 750 °C at 10 °C/min under the He flow at 100 mL/min. The outlet gas composition was monitored by an online FT-IR equipped with a 2 m gas-cell (Thermo Electron Co. Nicolet 6700) to identify the molecules desorbed from the catalyst surface.

2.3. Catalyst characterization To quantify the content of Pd in the prepared catalyst, an inductively coupled plasma-optical emission spectroscopy (ICP-Flame-EOP, Spectro Co) was employed. BET surface areas of the Pd-containing catalysts were measured by N2 isotherms obtained at 196 °C using an ASAP2010 (Micromeritics Instrument Co.). The pulse CO-chemisorption was performed to measure the dispersion and MSA of Pd on the catalyst surface (AutoChem II 2920, Micromeritics Instrument Co). After the reduction of 0.2 g sample powder in a flow of 5% H2/Ar at 450 °C for 1 h, the temperature was cooled down to 35 °C, then CO was introduced to the reactor by pulse injection until the amount of CO uptake was saturated. The total amount of the CO uptake was used to calculate the dispersion of metals assuming 1 molecule of CO adsorbed per surface metal atom [17,20,21]. X-ray diffraction (XRD) patterns of the catalysts were obtained by the XPERT PRO MPD X-ray diffractometer (PANalytical) using Cu Ka radiation and the spectra were collected in the region of 2h = 20–70° at 40 kV and 30 mA of the X-ray gun [22].

3. Results and discussion 3.1. TWC activity over Pd-containing catalysts Fig. 1 shows the temperature dependence of the TOFs for the CO, C3H6 and H2 oxidation, and the NO reduction reactions over the Pd-containing catalysts stabilized at 600 °C for 16 h. The TOFs over the LaAlO3-based Pd(0.8)-600 catalysts are higher than those over the Al2O3-based Pd(0.8)-600 catalysts, for all four reactants. The TOFs for CO and C3H6 oxidation over Pd(0.8)/LaAlO3-600 are the highest among other catalysts as shown in Table S2 of Supplementary Material, while LaAlPd(0.8)O3-600 reveals the highest TOF for H2 oxidation and NO reduction reactions. A similar trend has been observed in the light-off temperature (LOT) results over the Pd(0.8)-containing catalysts stabilized at 600 °C for 16 h obtained from the integral reactor experiments (Fig. S3 of Supplementary Material). Note that the LOTs over Pd(0.8)/Al2O3-600 are lower than those over Al2Pd(0.8)O3-600,

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Fig. 1. TOFs for CO, C3H6, H2 and NO over Pd-containing catalysts stabilized at 600 °C for 16 h. Feed: 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm NO, 1% O2, 10% CO2, 10% H2O and Ar balance. GHSV: 500,000 h1.

although the TOF over the Al2O3-based Pd(0.8)-600 catalysts is nearly identical, irrespective of impregnation or substitution of Pd. The activation energies (Ea) for CO, C3H6 and H2 oxidation and NO reduction reactions over the LaAlO3-based Pd(0.8)-600 catalysts were 120, 202, 60 and 67 kJ/mol, respectively, which were apparently lower than those over the Al2O3-based Pd(0.8)-600 catalysts (138, 216, 76 and 86 kJ/mol), probably due to the promoting role of La in the perovskite structure for TWC reactions [13,24]. Based upon these results, it can be concluded that the LaAlO3 perovskite may be more suitable support for the Pd-containing catalysts compared to Al2O3, and the impregnation of Pd on LaAlO3 is preferable to the substitution thereof into the LaAlO3 structure in order to achieve high initial CO and C3H6 oxidation activities. Interestingly, the activation energy remains essentially constant regardless of Pd loadings from 0.8 to 2.4 wt.% in LaAlPdO3, suggesting that the Pd surface is the active reaction sites of the catalysts. After thermal aging at 1000 °C for 6 h in the periodic cyclic aging, the TOFs for CO and C3H6 oxidation over LaAlPd(0.8)O3-1000 become similar to those over Pd(0.8)/LaAlO3-1000, while LaAlPd(0.8)O3 still exhibits the highest TOF for the H2 oxidation and NO reduction reactions, as shown in Fig. 2 and Table S2 of Supplementary Material. The inferior TWC performance is observed again over the Al2O3-based Pd(0.8)-1000 catalysts with the Ea’s for CO, C3H6, H2 and NO unchanged.

To further investigate the sintering behavior of the Pd-containing catalysts, the catalysts have been aged at 1050 °C for 6 h. After aging, LaAlPd(0.8)O3-1050 exhibits the highest TOF with the lowest LOTs for CO, C3H6, H2 and NO conversions among the Pd(0.8)-containing catalysts aged at 1050 °C for 6 h (Fig. 3 and, Table S2 and Fig. S3 of Supplementary Material). The TOF for each reactant over Al2Pd(0.8)O3-1050 is similar to that over Pd(0.8)/Al2O3-1050 demonstrating that the incorporation of Pd into the Al2O3 structure is not very effective for improving its intrinsic activity, regardless of the aging temperature (Figs. 1–3 and Table S2 of Supplementary Material). Note that the TOFs and Ea’s for CO, C3H6, H2 and NO over LaAlPdO3-1050 remained unchanged, although the Pd loading increased from 0.8 to 2.4 wt.% as also observed over LaAlPdO3-600, indicating no variation of the catalytic activity of Pd and the related reaction mechanism for the TWC reaction. On the other hand, the TOFs for the CO, C3H6, H2 and NO removal reactions over the Pd(0.8)-containing catalysts increase upon thermal aging, and this trend becomes more pronounced with the increase of the aging temperature, especially for the Pd-containing LaAlO3 catalysts, as shown in Table S2 of Supplementary Material. In particular, the TOFs for CO at 250 °C over Pd(0.8)/LaAlO3 and LaAlPd(0.8)O3 increase from 3.79 and 1.74 s1 to 7.53 and 11.7 s1, respectively, after aging at 1050 °C

Fig. 2. TOFs for CO, C3H6, H2 and NO over Pd-containing catalysts aged at 1000 °C for 6 h. Feed: 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm NO, 1% O2, 10% CO2, 10% H2O and Ar balance. GHSV: 500,000 h1.

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Fig. 3. TOFs for CO, C3H6, H2 and NO over Pd-containing catalysts aged at 1050 °C for 6 h. Feed: 1% CO, 500 ppm C3H6, 0.3% H2, 500 ppm NO, 1% O2, 10% CO2, 10% H2O and Ar balance. GHSV: 500,000 h1.

for 6 h while only slight changes of the TOF from 1.16 and 1.15 s1 to 1.60 and 1.64 s1 are observed over the Al2O3-based catalysts, Pd(0.8)/Al2O3 and Al2Pd(0.8)O3, respectively. A similar trend is also observed in TOF for C3H6 as the aging temperature increases, while a significant increase of TOF for H2 and NO is observed over all the Pd-containing catalysts regardless of the catalyst support and preparation method (Table S2 of Supplementary Material). The alteration of TOFs with respect to the aging temperature may be closely related to the structure sensitivity of the reaction, strongly affected by the metal particle size, although their Ea’s remain nearly constant [25–27]. When the Pd MSA obtained by the H2-O2 titration method was used to calculate the TOFs, a similar increasing trend of TOFs with the increasing aging temperature was observed for the TWC reactions, regardless of the catalyst type and aging temperature (Table S3 of Supplementary Material). The deactivation trend of each catalyst was also examined by comparing the difference in T50 (the temperature where the 50% of conversion is achieved) or T20 (the temperature where the 20% of conversion is achieved) with respect to the aging temperature as shown in Fig. S4 of Supplementary Material. Here DT50 and DT20 are defined by the change of T50 and T20, respectively, from the base value of the catalyst stabilized at 600 °C for 16 h. The DT50s for the CO, C3H6 and H2 oxidation reactions, and DT20 for the NO reduction over LaAlPd(0.8)O3 are smaller than those over Pd(0.8)/LaAlO3, indicating the superior thermal stability of LaAlPd(0.8)O3. A similar trend was also observed over the Al2O3-based Pd(0.8) catalyst. Smaller DT50s for CO C3H6 and H2, and DT20 for NO have been observed over Al2Pd(0.8)O3 compared to those over Pd(0.8)/Al2O3. These observations indicate that the incorporation of Pd into the structure of LaAlO3 or Al2O3 is beneficial to the enhancement of the thermal stability of the Pd-containing catalysts. Among the Pd-containing catalysts employed in this study, LaAlPdO3 showed the strongest thermal stability with the apparent increase of TOF, although the oxidation activity of LaAlPdO3-600 was lower than that of Pd/LaAlO3-600, but still higher than the conventional Al2O3-based Pd catalysts. 3.2. Cause of sintering for Pd-containing catalysts 3.2.1. Physicochemical properties To understand the primary cause of the catalyst deactivation, the physicochemical properties of the Pd-containing catalysts were systematically investigated. No loss of the Pd content in each catalyst has been observed upon aging as determined by ICP analysis

(Table 1). The structural characteristics of the Pd-containing catalysts were examined by the XRD analysis as shown in Fig. 4. The typical XRD pattern of the amorphous c-Al2O3 phase (JCPDS 29-1486) has been observed over the Al2O3-based Pd(0.8)-600 catalysts including Pd(0.8)/Al2O3-600 and Al2Pd(0.8)O3-600 without any Pd related species such as PdO and metallic Pd, mainly due to the relatively small amount of Pd (0.8 wt.%) in a highly dispersed form on the catalyst surface (Fig. 4a). For the Al2O3-based Pd(0.8)-1000 catalysts, c-Al2O3 is transformed to a mixture of h(JCPDS 86-1410) and a-Al2O3 (JCPDS 46-1212), while it is predominantly transformed to a-Al2O3 after the thermal aging at 1050 °C for 6 h. However, the formation of the aggregated Pd on the surface of the aged catalyst could not be verified by the present XRD study due to the peak overlapping of the metallic Pd and h-Al2O3 formed. For the perovskite-based catalysts, the rhombohedral LaAlO3 phase (JCPDS 31-0022) without any segregated phases such as La2O3 and Al2O3 is clearly formed at 2h = 23.5°, 33.5°, 41.3°, 48.0°, 54.1° and 59.8° regardless of their Pd loading (Fig. 4b). As the aging temperature increases, the typical perovskite peaks become apparent with the advent of the new XRD peaks due to the metallic Pd (JCPDS 46-1043) in the perovskite-based catalysts prepared in this study (Outset of Fig. 4b). For the catalysts aged at 1050 °C for 6 h, the metallic Pd with the average particle size of 58 nm (calculated by the Scherrer equation) has been formed on the surface of LaAlPd(0.8)O3-1050, which is smaller than that on Pd(0.8)/LaAlO3-1050, 74 nm. It is indeed indicative of the superior thermal stability of the Pd-substituted perovskite catalyst, LaAlPdO3 (Table 1). The particle size of Pd0 on LaAlPdO3-1050 decreases only slightly with the increase of the Pd loading, although the XRD peak intensity of the metallic Pd phase increases noticeably. To directly measure the actual particle size of Pd on the Pd(0.8)-containing catalysts, the STEM images have been collected (Fig. 5 and Fig. S5 of Supplementary Material). Each of grains observed in the STEM results was analyzed by EDS to be able to distinguish the Pd particle from the catalyst supports including Al2O3 and LaAlO3 (outset of Fig. 5a). As shown in Fig. 5, the particles size (7 nm) of the segregated Pd on LaAlPd(0.8)-600 apparently increases to a size larger than 70 nm, as the aging temperature increased to 1050 °C. Note that the part of the substituted Pd in LaAlO3 was detached from the perovskite structure to form segregated metallic Pd after the stabilization at 600 °C for 16 h, as evidenced by the HRTEM image (Fig. S6 of Supplementary Material). A similar increasing trend of the Pd particle size has also been

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Fig. 4. XRD patterns of Al2O3-based (a) and LaAlO3-based Pd catalysts (b).

observed over the other catalysts with the increase of the aging temperature, regardless of the catalyst formulation and preparation method, although the Pd particle sizes obtained from the TEM images differ somewhat from those by XRD (Fig. 5 and Fig. S5 of Supplementary Material). As listed in Table 1, the BET surface areas of the catalysts decrease as the aging temperature increases. Particularly, a drastic BET area decrease in Pd(0.8)/Al2O3 and Al2Pd(0.8)O3 from 113 and 149 m2/g to 12.3 and 11.3 m2/g, respectively, is observed between the catalysts stabilized at 600 °C and those aged at 1050 °C, mainly due to the phase transformation of c-Al2O3 to h- and a-Al2O3 as determined by XRD analysis (Fig. 4a and Table 1) [28]. However, nearly 45% of the BET surface areas of the LaAlO3-based catalysts still remains even after their thermal aging at 1050 °C for 6 h as listed in Table 1, although the absolute areas of the perovskite-based catalysts are apparently smaller than those of the Al2O3-based catalysts. The increase of the LOTs for CO, C3H6, H2 and NO over the Pd(0.8)-containing catalysts with respect to the increasing aging temperature shown in Fig. S3 of Supplementary Material may be understood by the decrease of the catalyst Pd MSA listed in Table 1. For the catalysts stabilized at 600 °C for 16 h, the Pd(0.8)-impregnated LaAlO3 catalyst exhibits a greater Pd MSA than the Pd(0.8)-substituted LaAlPdO3 catalyst. It may be attributed to the partial incorporation of Pd into the sublattice of LaAlO3 for the LaAlPdO3 catalyst [29]. Similarly, the Pd MSA of Pd(0.8)/Al2O3-600 is greater than that of Al2Pd(0.8)O3-600. Note that the particle size of Pd on Al2PdO3 and LaAlPdO3 obtained from CO-chemisorption results (Table 1) may have been overestimated, since the Pd incorporated into the structure of Al2O3 or LaAlO3 has not been properly accounted for [29]. As the aging temperature increases, the Pd MSA gradually decreases, mainly due to the agglomeration of Pd particles by thermal sintering. For the catalysts aged at 1050 °C for 6 h, nearly 95% of the Pd MSA disappeared on the Pd-impregnated catalysts, Pd (0.8)/LaAlO3 and Pd(0.8)/Al2O3, while nearly 10% of the Pd MSA still remained on the Pd-substituted catalysts, LaAlPd(0.8)O3 and Al2Pd(0.8)O3, indicative of the enhanced thermal stability of Pd by its incorporation into the structure of the supporting materials including LaAlO3 and Al2O3 (Table 1). The moderated decreasing trend of the Pd MSA with the increasing aging temperature may be one of the primary reasons for maintaining the smallest DT50s for CO, C3H6 and H2 oxidation, and DT20 for NO reduction reactions

over the perovskite-based catalysts as the aging temperature increases (Fig. S4 of Supplementary Material). Note that a similar trend in the variation of the Pd MSA of Pd-containing catalysts was obtained from both the CO-chemisorption and the H2–O2 titration methods regardless of the catalyst type and aging temperature (Table S3 of Supplementary Material) [30]. Indeed, the Pd MSA has been commonly recognized as a useful criterion for representing the number of active reaction sites formed on the surface of TWC and it plays a crucial role for determining the catalytic activity of Pd-based TWCs [17]. However, the variation of the TWC activity of the Pd-containing catalysts could not be fully explained solely by the alteration of their Pd MSA alone, listed in Table 1. The LOTs over LaAlPd(0.8)O3-600 are lower than that of Pd(0.8)/Al2O3-600 (Fig. S3 of Supplementary Material), although the Pd MSA of the former catalyst (0.313 m2/g) is smaller than that of the latter (0.436 m2/g) indicating that the intrinsic TWC activity of Pd over each Pd-based catalyst may depend on the catalyst formulation and preparation method [31]. 3.2.2. State of Pd The chemical state of Pd on the Pd-containing catalysts may be crucial for its TWC activity and thermal stability [32]. To determine the characteristic of Pd on the catalysts employed, Pd 3d XPS spectra were examined (Table 2 and Fig. S7 of Supplementary Material). As listed in Table 2, the binding energy (BE) of the Pd 3d5/2 for both Pd(0.8)/Al2O3-600 and Al2Pd(0.8)O3-600 is 335.8 eV, indicating that the Pd may exist in a state between PdO (336.5 eV) and Pd0 (335.1 eV) [32–34]. However, the BE of the Pd 3d5/2 for Pd(0.8)/LaAlO3-600 shifts to a lower BE compared to that for the Al2O3-based Pd-600 catalysts and a further shift is observed over LaAlPd(0.8)O3-600, indicating the formation of the less oxidized PdO1x species on the surface of the perovskite-based catalyst [32,34]. This BE of LaAlPdO3-600 remains nearly constant with the increase of the catalyst Pd loading from 0.8 to 2.4 wt.%, as shown in Table 2 and Fig. S7 of Supplementary Material. Since the electronegativity of La (1.1) is lower than that of Pd (2.2), the electron donation from La to Pd may readily occur, resulting in the higher electron density around Pd in the perovskite-based catalysts compared to that in the Al2O3-based catalysts [32]. As shown in Table 2, both the BE of Pd 3d5/2 and the surface atomic concentration of Pd decreased with the increasing aging temperature for all the Pd-containing catalysts examined,

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Fig. 5. STEM images of LaAlPd(0.8)O3 with respect to aging temperature; LaAlPd(0.8)O3-600 (a), LaAlPd(0.8)O3-1000 (b) and LaAlPd(0.8)O3-1050 (c). Red circle: Pd particle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

indicative of the alteration of the state of Pd from PdO to Pd0 and the decline of the Pd dispersions, in consistency with the CO-chemisorption and H2–O2 titration results presented in Table 1 and Table S3 of Supplementary Material [8]. The decline of the surface atomic concentration of Pd upon thermal aging was more severe on the Al2O3-based Pd(0.8)-600 catalysts than that on the perovskite-based catalysts such as Pd(0.8)/LaAlO3-600 and LaAlPd(0.8)O3-600 (Table 2). Particularly noteworthy is that LaAlPd(0.8)O3-1050 exhibited the most stable and interesting aging behavior; it exhibits the smallest decrease of both the BE of Pd 3d5/2 and the surface atomic concentration of Pd on its surface from its base values obtained for LaAlPd(0.8)O3-600. This may be attributable to the facile electron

transfer from La to Pd in the LaAlO3-based Pd catalysts, especially in LaAlPdO3 catalysts [32]. Note also that the BEs of the Pd 3d5/2 on LaAlPdO3-1050 are hardly affected by their Pd content. In addition, the decomposition temperatures of PdO are higher over the LaAlO3-based Pd(0.8) catalysts than those over the Al2O3-based counterparts, as confirmed in the thermo-gravimetric (TG) analysis (Fig. S8 of Supplementary Material). This can be understood by the higher electron density around Pd by the electron donation from La as evidenced in the XPS results, resulting in a stronger bonding of oxygen (an electron acceptor) to Pd while suppressing the electron transfer from oxygen to Pd required for the thermal decomposition of PdO (Pd(II)O ? Pd0 + 1/2O2) [19,32].

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Table 2 BE of Pd 3d5/2 and surface atomic concentration of Pd, and distribution of Pd species on Pd-containing catalysts.

a b c

Catalyst

Binding energy of Pd 3d5/2a (eV)

Surface atomic concentration of Pda (%)

PdOb (%)

Pd0b (%)

Pd(0.8)/Al2O3-600 Pd(0.8)/Al2O3-1000 Pd(0.8)/Al2O3-1050 Al2Pd(0.8)O3-600 Al2Pd(0.8)O3-1000 Al2Pd(0.8)O3-1050 Pd(0.8)/LaAlO3-600 Pd(0.8)/LaAlO3-1000 Pd(0.8)/LaAlO3-1050 LaAlPd(0.8)O3-600 LaAlPd(0.8)O3-1000 LaAlPd(0.8)O3-1050 LaAlPd(1.6)O3-600 LaAlPd(1.6)O3-1050 LaAlPd(2.4)O3-600 LaAlPd(2.4)O3-1050

335.8 335.2 335.1 335.8 335.2 335.1 335.6 335.4 335.3 335.5 335.4 335.4 335.5 335.4 335.5 335.4

0.7 0.1 0.1 0.5 0.1 0.1 0.8 0.6 0.3 0.6 0.5 0.4 0.9 0.7 1.1 0.8

71 35 30 72 37 33 64 47 42 61 54 51 n.a.c n.a.c n.a.c n.a.c

29 65 70 28 63 67 36 53 58 38 46 49 n.a.c n.a.c n.a.c n.a.c

Obtained by Pd 3d5/2 XPS analysis. Determined by XANES spectra using linear combination fitting. Not analyzed.

The state of Pd formed on the surface of the Pd-containing catalysts was further examined by their XANES spectra. Since Pd mainly exists in either form of Pd2+ or Pd0, regardless of the catalysts examined, the distribution of the Pd species observed from the Pd K-edge XANES spectra can be quantified by the linear combination fitting (LCF) using the IFEFFIT packages (ver. 1.2.11d), as shown in Fig. 6 [35]. Note that the curve-fitting of XANES spectra was conducted with the reference materials (PdO and Pd foil) in the range of the absorption edge energy, E0 between 20 and +30 eV with a typical agreement of R-factors = 0.0002. The distribution of Pd species determined is listed in Table 2. The results from the XANES LCF indicate that the Pd(0.8)/Al2O3-600 contains a large amount of PdO (71%) relative to that of Pd0 (29%). Similarly, 72% of PdO and 28% Pd0 were observed over Al2Pd(0.8)O3-600. However, the LaAlO3-based Pd(0.8)-600 catalysts have smaller amounts of PdO than Pd(0.8)/Al2O3-600 due to their higher electron density of Pd by the electron donation from La [32]. Indeed, the lower electronegativity of La than Pd results in the formation of the less-oxidized

PdO1x species on the catalysts surface with the low oxidation state of Pd, as also observed in the XPS results [34]. The distribution of the Pd species on the catalyst surface is notably altered by the increase of aging temperature. In particular, the amount of PdO in Pd(0.8)/Al2O3 and Al2Pd(0.8)O3 significantly decreases from 71% and 72% to 30% and 33%, while 42% (Pd/LaAlO3) and 51% (LaAlPdO3) of PdO still remains on the perovskite-based catalysts as the aging temperature increases up to 1050 °C. These observations demonstrate again that La plays an important role in improving the stability of PdO on LaAlPdO3 during thermal aging owing to the effective electron donation from La to Pd, in consistency with the TGA and XPS studies.

3.2.3. Metal–support interaction Presented in Fig. 7 are the H2 consumption peaks in H2-TPR experiments over the Pd(0.8)-containing catalysts with respect to the aging temperature. Pd(0.8)/Al2O3-600 showed a single symmetric H2 consumption peak at 9 °C (a-peak) due to the reduction of PdO cluster highly dispersed on the surface of the Al2O3 support with a negative H2 consumption peak at 57 °C attributed to the production of H2 by the decomposition of palladium b-hydride (b-peak) formed at low temperatures during the H2 treatment [33,36]. As the aging temperature increased, the intensity of the a-peak decreased (Table 3) with a slight shift of the peak temperature from 9 to 5 °C, in consistency with the deceased Pd MSA due to the sintering of Pd particles (Table 1) [33]. A quite similar behavior was also observed over Al2Pd(0.8)O3-600 and the intensity of the a-peak apparently decreased as the aging temperature increased. Compared to the TPR profiles over the Al2O3-based Pd(0.8)-600 catalysts, Pd(0.8)/LaAlO3-600 reveals an additional peak (c-peak) attributable to the reduction of PdO interacting with the LaAlO3 perovskite support which may be closely related to the enhanced activity of Pd(0.8)/LaAlO3-600 for CO and C3H6 oxidation reactions [37,38]. Indeed, this strong interaction of Pd with LaAlO3 may contribute to the stabilization of Pd in an oxidized form bonded to La and Al in the LaAlO3, helping to complete the oxidative catalytic cycle involving the lattice oxygen coordinated to Pd with a lower activation barrier through the Mars van Krevelen mechanism [39]. The reduction temperature of the a-peak on Pd(0.8)/LaAlO3-600 was higher than that on the Al2O3-based Pd(0.8)-600 catalyst, indicating that the interaction between Pd and La suppresses the reducibility of PdO cluster dispersed on

Fig. 6. Pd K-edge XANES spectra of Pd(0.8)-containing catalysts; Pd(0.8)/Al2O3 (a), Al2Pd(0.8)O3 (b), Pd(0.8)/LaAlO3 (c) and LaAlPd(0.8)O3 (d).

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Fig. 7. H2-TPR profiles of Pd(0.8)-containing catalysts.

the surface of the LaAlO3 support [36]. It is worth noting that Pd(0.8)/LaAlO3-600 does not show the negative H2 consumption peak (b-peak), probably due either to the overlap of a- and c-peaks or to the incomplete reduction of PdO to Pd in the temperature range around 60 °C, since the b-hydride can be formed only on the metallic Pd species [36,38]. Interestingly, the b-peak is observed over Pd(0.8)/LaAlO3 as the aging temperature increases

upon thermal aging, and its intensity increases with the increase of the aging temperature, while the TPR intensities of a- and c-peaks decrease. The a- and c-peaks shift toward a lower temperature 38 and 58 °C to 18 and 33 °C, respectively, over Pd(0.8)/LaAlO3-1050, while remaining nearly constant over the Al2O3-based Pd(0.8)-1050 catalysts. Similar to those over Pd(0.8)/LaAlO3, the b-peaks are observed over LaAlPd(0.8)O3 upon aging, and the a- and c-peaks over LaAlPd(0.8)O3 have also shifted to the lower temperature from 35 and 62 °C to 21 and 35 °C, respectively, as the aging temperature increases to 1050 °C, along with the increase of TOF (Table S2 of Supplementary Material). This indicates that the redox property of Pd was improved with the moderation of the Pd–La interaction. In particular, the shift of the c-peak related to the oxidized Pd interacting with LaAlO3 reveals that the lattice oxygen coordinated to Pd becomes more facile in its mobility due to the weakening of the Pd–La interaction after thermal aging, which is beneficial for the oxidation reaction [39,40]. Thus, the weakened interaction between Pd and La may lead to the increase of TOFs for the CO and HC oxidation reactions after thermal aging. In addition, the intensity of the c-peak over LaAlPd(0.8)O3-1050 is stronger than that over Pd(0.8)/LaAlO3-1050 mainly due to the incorporation of Pd into the perovskite structure of LaAlO3 leading to the effective electron transfer from La to Pd, which may be the primary cause for its stronger thermal stability as discussed earlier. The Pd–La interaction was further examined by a DRIFT study of CO adsorption on the Pd(0.8)-containing catalysts, as shown in Fig. 8. The specific IR peak at around the wavenumber of 1930 cm1 may be assigned to the bridged CO adsorbed onto Pd+ species [41]. The peak at around 2091 cm1 is due to the linear CO adsorbed onto the Pd (1 1 1) plane, and the bridged CO adsorption peak on Pd (1 0 0) or Pd (1 1 1) is observed at around 1993 cm1 [42–44]. The DRIFT peak intensities of both the linear and bridged CO adsorbed onto Pd (1 1 1) and Pd (1 0 0)/(1 1 1) are apparent on the surface of both Pd(0.8)/Al2O3-600 and Al2Pd(0.8)O3-600 with a relatively weak IR shoulder peak due to the CO adsorbed onto the Pd+ species. Note that the stronger peak intensity of CO adsorption on Pd0 compared to that on PdO might be due to the reduction of PdO to Pd0 upon the adsorption of CO onto the catalyst surface even at room temperature [41]. The adsorption peaks for CO on the surface of Pd(0.8)/LaAlO3-600 and LaAlPd(0.8)O3-600 shift to the lower IR frequencies which may be attributed to the electron-donation from La to Pd in the LaAlO3-based Pd catalysts [45]. As the aging temperature increases, the peak intensities of both the linear and bridged CO adsorbed onto Pd (1 1 1) and Pd (1 0 0)/(1 1 1) decrease, and the adsorption peak of CO on Pd+ species over the Pd-containing catalysts disappears upon thermal aging, indicating that the sintering of Pd particles and the

Table 3 Amount of H2 consumed and molecules desorbed over Pd-containing catalysts. Catalyst

Amount of H2 consumed during H2-TPR (lmol/g)

Amount of NO desorbed during NO-TPD (lmol/g)

Amount of C3H6, CO and CO2 desorbed during C3H6-TPD (lmol/g)

Amount of CO2 desorbed during CO-TPD (lmol/g)

Amount of CO2 desorbed during CO2-TPD (lmol/g)

Pd(0.8)/Al2O3-600 Pd(0.8)/Al2O3-1000 Pd(0.8)/Al2O3-1050 Al2Pd(0.8)O3-600 Al2Pd(0.8)O3-1000 Al2Pd(0.8)O3-1050 Pd(0.8)/LaAlO3-600 Pd(0.8)/LaAlO3-1000 Pd(0.8)/LaAlO3-1050 LaAlPd(0.8)O3-600 LaAlPd(0.8)O3-1000 LaAlPd(0.8)O3-1050

73.0 47.7 29.1 53.7 36.2 26.5 105.2 79.1 46.7 133.6 86.7 57.6

5.8 3.1 2.0 5.4 2.7 2.1 11.1 4.8 2.3 17.5 7.3 3.9

11.8 7.1 4.0 7.6 4.6 3.2 14.4 9.2 4.9 12.6 7.8 5.4

17.5 3.8 3.4 14.7 3.3 3.1 21.6 5.6 3.3 16.8 6.4 4.1

5.1 2.4 1.4 4.5 2.0 1.6 7.2 3.1 1.9 8.8 5.2 3.0

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~1930 cm-1: bridged CO adsorption on Pd+ species ~1993 cm-1: bridged CO adsorption on Pd (100)/(111) ~2091 cm-1: linear CO adsorption on Pd (111) Fig. 8. DRIFT spectra of Pd(0.8)-containing catalysts. Feed composition: 1% CO and He balance. Temperature: 35 °C.

reduction of PdO to metallic Pd proceed simultaneously. In particular, the intensity of the IR peak assigned to the linear CO adsorbed onto the Pd (1 1 1) on the LaAlO3-based Pd(0.8) catalysts is much stronger than that on the Al2O3-based counterparts, suggestive of a greater amount of Pd in the (1 1 1) plane exposed on the surface of the LaAlO3-based catalysts compared to that on the Al2O3-based counterparts.

Among the catalysts aged at 1050 °C for 6 h, the strongest peak intensity of both the linear and bridged CO adsorbed onto Pd (1 1 1) and Pd (1 0 0)/(1 1 1) is observed on the surface of LaAlPd(0.8)O3-1050, in consistency with the catalyst Pd MSA and atomic surface concentration of Pd obtained by CO-chemisorption and XPS results as listed in Tables 1 and 2, respectively. Also, a lower IR frequency for the bridged CO adsorbed onto Pd (1 0 0)/(1 1 1) is observed over LaAlPd(0.8)O3-1050 (1983 cm1) compared to that over Pd(0.8)/Al2O3-1050 (1993 cm1), Al2Pd(0.8)O3-1050 (1993 cm1) and Pd(0.8)/LaAlO3-1050 (1987 cm1), indicating the stronger interaction of Pd with LaAlO3 by the substitution of Pd into the perovskite structure, in agreement with XPS, XANES and H2-TPR results [34]. The metal-support interaction between Pd and La in the LaAlO3-based Pd catalysts was further investigated in a TPD study using NO as a representative electron acceptor molecule [46,47], as shown in Fig. 9. Presented in Fig. 9a is the NO-TPD over the Pd(0.8)-containing catalysts with respect to aging temperature. The NO desorption temperatures decrease in the following order: Al2Pd(0.8)O3 > Pd(0.8)/Al2O3 > Pd(0.8)/LaAlO3 > LaAlPd(0.8)O3. For each catalyst, both the amount and temperature of NO desorption decrease with the increasing the aging temperature (Table 3). In addition, there is only one desorption peak of NO over the Al2O3-based Pd catalyst, while there is a small shoulder peak at a high temperature (HT) in addition to the large peak at a low temperature (LT) over the LaAlO3-based Pd catalysts, suggesting a convolution of two peaks. The NO-TPD profiles over the Al2O3 and LaAlO3 supports shown in Fig. 9b indicate that both supports are good adsorbents for NO under the experimental conditions employed in this study [48]. The amount of NO desorption decreases with the increasing aging temperature, which may be due partly to the decreased BET surface areas resulting from the sintering of the support particles. Fig. 9c is the difference profile of the NO-TPD data obtained by subtracting the support profile in Fig. 9b from that in Fig. 9a. Clearly in Fig. 9c, the Al2O3-based Pd catalyst reveals a single NO desorption peak, while the LaAlO3-based Pd catalyst reveals two distinct peaks. Two peaks over the LaAlO3-based Pd catalysts represent the effect of the interaction between Pd and La in the LaAlO3-based Pd catalyst, and the LT peak is associated with LaAlO3 interacting with Pd, while the HT peak is due to Pd

Fig. 9. Effect of the catalyst support during the NO-TPD. NO-TPD profiles of Pd(0.8)-containing catalysts (a) and supports (b), and the subtracted NO-TPD profiles of Pd(0.8)-containing catalysts by those of the support (c).

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interacting with LaAlO3 [47]. In addition, the HT peak over the LaAlO3-based Pd catalyst is observed at higher temperature compared to that over Al2O3-based Pd catalysts due to the increased electron density around Pd, resulting in the strong adsorption of NO on Pd [47,48] (Fig. S9 of Supplementary Material). The enlarged outset of Fig. 9c clearly indicates that the effect of the Pd–La interaction on NO adsorption is much greater over LaAlPd(0.8)O3 than that over Pd(0.8)/LaAlO3. In particular, both the amount of NO adsorbed onto the LaAlO3 support and Pd over LaAlPd(0.8)O3 are higher than those over Pd(0.8)/LaAlO3. The effect of the Pd–La interaction on C3H6 adsorption was also investigated in a TPD study using C3H6 as a representative electron-donor molecule [46]. Presented in Fig. 10a are the C3H6-TPD profiles over the Pd(0.8)-containing catalysts, which is the sum of the desorption profiles of C3H6, CO and CO2 [49]. The formation of CO and CO2 is mainly due to the steam reforming (SR) reaction (C3H6 + H2O) during the desorption and re-adsorption processes at the temperatures above 250 °C [50]. Apparently, the desorption temperatures over the LaAlO3-based Pd catalysts are lower than those over the Al2O3-based ones for each aging temperature. This observation indicated that the increased electron density of Pd by the Pd–La interaction suppresses the adsorption of C3H6 (electron donor), resulting in the weakened C3H6 adsorption on Pd [24,51]. The total amount of molecules desorbed during C3H6-TPD decreases with the increasing aging temperature, which may be due partly to the decrease of the Pd MSA upon catalyst aging, as shown in Tables 1 and 3. In addition, the desorption peak temperatures of the LaAlO3-based Pd(0.8) catalysts shifted to lower temperatures as the aging temperature increased, whereas those over the Al2O3-based counterparts were nearly constant. As expected, a similar desorption pattern is also observed in the CO-TPD profiles except the desorption temperatures are shifted to lower temperatures by about 100 °C as shown in Fig. 10b [51]. Note that CO2 was primary desorption species during the CO-TPD in the presence of H2O, due to both the water–gas shift and CO disproportionation reactions [50,52] (Fig. S10 of Supplementary Material). The surface basicity of all the Pd-containing catalysts was examined by a TPD study using CO2 as an acidic probe molecule, as shown in Fig. 10c [45]. As presented in Table 3 and Fig. 10c, the amount of CO2 desorbed from the LaAlO3-based Pd(0.8) catalysts is greater than that from the Al2O3-based Pd(0.8) catalysts,

81

regardless of the aging temperature. This can be ascribed to the higher basicity of LaAlO3 than that of Al2O3 resulting from the incorporation of La into the LaAlO3 structure [45]. In particular, LaAlPd(0.8)O3 exhibits a greater CO2 adsorption capacity than that of Pd(0.8)/LaAlO3, which implies that the overall surface basicity is higher on LaAlPd(0.8)O3 than that on Pd(0.8)/LaAlO3 and is consistent with the results of the NO-TPD studies. Note that the CO2 desorption temperatures are hardly affected by the catalyst formulation or aging temperature. 3.3. Structure sensitivity of TWC reaction According to the CO-chemisorption, XRD and TEM results, the Pd particle size gradually increases with the increase of the aging temperature for all the Pd-containing catalysts examined in this study (Table 1 and Fig. 5 and, Fig. S5 of Supplementary Material), and so does the TOF of all the reactants including CO, C3H6, H2 and NO. This is a clear indication of the structure sensitivity of all the TWC reactions over the Pd-containing catalysts, although the degree of the sensitivity depends on each individual reaction. Thus, the structure-sensitivities of the major four TWC reactions over the four different Pd-containing catalysts were investigated. The very weak structure-sensitivity of CO and C3H6 oxidation reactions as well as the strong structure sensitive nature of the NO reduction reaction over the Al2O3-based Pd catalysts has been reported in the literature [26,27]. However, the strong structure-sensitivity of all the TWC reactions is observed over LaAlO3-based Pd catalysts. For better understanding of the structure-sensitivity of TWC reactions over the LaAlO3-based Pd catalysts, it is useful to recognize that the physicochemical characteristic of Pd in the LaAlO3-based Pd catalysts can be significantly altered with the increase of the Pd particle size, thereby affecting their adsorption properties for the TWC reactants [27] and eventually leading to the alteration of the reactivity of TWC reactions over the catalysts. For example, the H2 consumption peak temperatures in the H2-TPR as well as the desorption temperatures in the CO-, C3H6- and NO-TPD profiles shift to lower temperatures with the increase of the Pd particle size over the LaAlO3-based Pd catalysts. This decreasing trend of the desorption temperature with the increasing Pd particle size on the LaAlO3-based Pd catalysts appears to be the primary reason behind the increasing TOF of TWC reactions

Fig. 10. C3H6 (a), CO- (b) and CO2-TPD (c) profiles of Pd(0.8)-containing catalysts.

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with the increasing Pd particle size, since the TWC reactions are typically suppressed by the much stronger adsorption of CO, C3H6 or NO on the Pd surface compared to the dissociative adsorption of O2 or H2 [40,46,51]. In contrast, they are hardly altered over the Al2O3-based Pd catalysts, except for the result of NO-TPD. In addition, the electron density of Pd in the LaAlO3-based catalysts is higher than that in the Al2O3-based counterparts due to the electron donation from La to Pd, as observed in XPS, TG and XANES results. The interaction between Pd and La results in the stabilization of PdO under the high exhaust gas temperature, and may also contribute to weaken the adsorption strength of reactants including CO, C3H6 and NO on the catalyst surface with the downward shift of the desorption temperatures during NO-, CO- and C3H6-TPDs as the Pd particle size increased, thereby providing more favorable conditions for the TWC reactions at low temperatures [53]. In addition, the weakening of the Pd–La interaction upon thermal aging may lead to the more facile transfer of lattice oxygen coordinated to Pd for the oxidative catalytic cycle, which may be another reason for the increase of TOFs for TWC reactions [39,40]. DRIFT results (Fig. 8) have indicated that more (1 1 1) facets of Pd particles are exposed on the surface of the LaAlO3-based Pd(0.8) catalysts than those on the Al2O3-based counterparts. Also, the linear CO adsorbed on Pd (1 1 1) still remains on the LaAlO3-based Pd (0.8) catalysts after the increase of the aging temperature up to 1050 °C, whereas it nearly disappears on the Al2O3-based Pd(0.8) catalysts, confirming the more abundant presence of Pd (1 1 1) facets on the LaAlO3-based Pd catalysts than those on the Al2O3-based ones. Vesecky et al. reported that the TOF of the NO–CO reaction on Pd (1 1 1) was higher than that on Pd (1 0 0) due to the poisoning by the atomic nitrogen strongly adsorbed on Pd (1 0 0) [26]. These observations suggest that the more facile desorption of N-atoms adsorbed on Pd (1 1 1) over the LaAlO3-based Pd catalysts than that over the Al2O3-based Pd catalysts may result in the higher TOF of the NO reduction over the LaAlO3-based Pd catalysts. Hence, the superior TOF of the NO reduction reaction over LaAlPdO3 may be attributable to the combination of the abundant Pd(1 1 1) phase on its surface and the abundant basic sites on LaAlPdO3. Note that the role of the O2 vacancy in the TWC performance of the LaAlO3-based Pd catalysts was negligible under the present experimental condition, as confirmed by O2-TPD results (Fig. S11 of Supplementary Material).

4. Conclusion A Pd-substituted LaAlO3 perovskite catalyst (LaAlPdO3) has been developed to enhance the thermal stability of the Pd-containing catalysts. The LaAlO3-based Pd catalysts have exhibited higher TWC activity and stronger thermal stability than the Al2O3-based counterparts under simulated gasoline engine exhaust conditions. The incorporation of Pd into the structure of LaAlO3 or Al2O3 appears to be a potential and effective way to improve the stability of Pd upon thermal aging. As the aging temperature increases, Pd-substituted for Al in the LaAlO3 perovskite structure exhibits a higher catalytic activity and stronger thermal stability than that impregnated on the LaAlO3 surface. The high electron density of Pd induced by the effective electron donation from La to Pd due to the strong Pd–La interaction in LaAlPdO3 appears to enhance the thermal stability of PdO during the thermal aging, leading to the suppression of Pd sintering. This interaction between Pd and La in LaAlPdO3 may also contribute to the increase of TOFs for CO and C3H6 oxidation reactions with the downward shift of desorption temperatures during COand C3H6-TPDs as the Pd particle size increases, manifesting their strong structure-sensitivity. NO reduction appeared to be the

strong structure-sensitive reaction regardless of the catalysts examined and the superior NO reduction activity of LaAlPdO3 may be attributable to the combination of the increased basic sites on LaAlPdO3 induced by the Pd–La interaction and the abundant Pd(1 1 1) phase on its surface. In summary, the strong Pd–La interaction in LaAlPdO3 is believed to be the primary cause of the highest TOFs and the lowest LOTs for the conversions of CO, C3H6, and NO over the LaAlPdO3-1050 catalyst among other catalysts aged at 1050 °C for 6 h. Acknowledgments This work was supported by Hyundai Motor Company (‘‘Development of perovskite-based LNT catalyst and thermally stable TWC’’), and NCRI (2012R1A3A-2048833) and BK 21-plus programs through the National Research Foundation of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.07.013. References [1] R.M. Heck, R.J. Farrauto, S.T. Gulati, Catalytic Air Pollution Control: Commercial Technology, second ed., John Wiley & Sons Inc., New York, 2002. [2] H.S. Gandhi, G.W. Graham, R.W. McCabe, J. Catal. 216 (2003) 433. [3] J. Kaspar, P. Fornasiero, N. Hickey, Catal. Today 77 (2003) 419. [4] I. Heo, D.Y. Yoon, B.K. Cho, I.-S. Nam, J.W. Choung, S. Yoo, Appl. Catal. B: Environ. 121–122 (2012) 75. [5] J.C. Jiang, X.Q. Pan, G.W. Graham, R.W. McCabe, J. Schwank, Catal. Lett. 53 (1998) 37. [6] S. Bernal, G. Blanco, J.J. Calvino, J.M. Gatica, J.A. Pérez-Omil, J.M. Pintado, Top. Catal. 28 (2004) 31. [7] C. Mira, J.J. Calvino, J.A. Pérez-Omil, J.M. Rodríguez-Izquierdo, S. Bernal, Catal. Today 180 (2012) 174. [8] I. Heo, J.W. Choung, P.S. Kim, I.-S. Nam, Y.I. Song, C.B. In, G.K. Yeo, Appl. Catal. B: Environ. 92 (2009) 114. [9] H. He, H.X. Dai, C.T. Au, Appl. Catal. B: Environ. 33 (2001) 65. [10] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T. Okamoto, N. Hamada, Nature 418 (2002) 164. [11] H. Tanaka, Catal. Surv. Asia 9 (2005) 63. [12] A. Tou, H. Einaga, Y. Teraoka, Catal. Today 201 (2013) 103. [13] Y. Matsuo, N. Suzuki, K. Kiguchi, A. Furukawa, US Patent No. 7,858,809 B2, 2009. [14] S. Keav, S.K. Matam, D. Ferri, A. Weidenkaff, Catalysts 4 (2014) 226. [15] D.Y. Yoon, E. Lim, Y.J. Kim, J.H. Kim, T. Ryu, S. Lee, B.K. Cho, I.-S. Nam, J.W. Choung, S. Yoo, J. Catal. 319 (2014) 182. [16] Y. Luo, J. Hao, Z. Hou, L. Fu, R. Li, P. Ning, X. Zheng, Catal. Today 93–95 (2004) 797. [17] S.B. Kang, I.-S. Nam, B.K. Cho, C.H. Kim, S.H. Oh, Chem. Eng. J. 259 (2015) 519. [18] C.C. Webb, B.B. Bykowski, SAE 2003-01-0663 (2003). [19] S.B. Kang, S.J. Han, S. B Nam, I.-S. Nam, B.K. Cho, C.H. Kim, S.H. Oh, Top. Catal. 56 (2013) 298. [20] R. Strobel, F. Krumeich, W.J. Stark, S.E. Pratsinis, A. Baiker, J. Catal. 222 (2004) 307. [21] S.B. Kang, S.J. Han, S.B. Nam, I.-S. Nam, B.K. Cho, C.H. Kim, S.H. Chem, Eng. J. 207–208 (2012) 117. [22] D.Y. Yoon, E. Lim, Y.J. Kim, B.K. Cho, I.-S. Nam, J.W. Choung, S. Yoo, ACS Comb. Sci. 16 (2014) 614. [23] S.B. Kang, S. B Nam, B.K. Cho, I.-S. Nam, C.H. Kim, S.H. Oh, Catal. Today 231 (2014) 3. [24] H. Muraki, K. Yokota, Y. Fujitani, Appl. Catal. 48 (1989) 93. [25] S.S. Mulla, N. Chen, L. Cumaranatunge, G.E. Blau, D.Y. Zemlyanov, W.N. Delgass, W.S. Epling, F.H. Ribeiro, J. Catal. 241 (2006) 389. [26] S.M. Vesecky, D.R. Rainer, D.W. Goodman, J. Vac. Sci. Technol. A 14 (1996) 1457. [27] D.R. Rainer, M. Koranne, S.M. Vesecky, D.W. Goodman, J. Phys. Chem. B 101 (1997) 10769. [28] B. Beguin, E. Garbowski, M. Primet, Appl. Catal. 75 (1991) 119. [29] K. Zhou, H. Chen, Q. Tian, Z. Hao, D. Shen, X.I. Xu, J. Mol. Catal. A: Chem. 189 (2002) 225. [30] K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, J. Catal. 179 (1998) 431. [31] V.G. Papadakis, C.A. Pliangos, I.V. Yentekakis, X.E. Verykios, C.G. Vayenas, Catal. Today 29 (1996) 71. [32] T. Kobayashi, T. Yamada, K. Kayano, Appl. Catal. B: Environ. 30 (2001) 287. [33] Q. Wang, G. Li, B. Zhao, M. Shen, R. Zhou, Appl. Catal. B: Environ. 101 (2010) 150.

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