Pollutant emission from gasoline engine powered vehicles is a major source of environmental pollution which is becoming more severe [1, 2]. Hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOx) are the major components in exhaust emission from vehicles [3, 4, 5]. Since the third European emission standard was adopted in 2000, a close-coupled catalyst (CCC) has been widely used to eliminate the HCs emission (60%-80% of the total emitted) during the cold-start period [5, 6]. In order to quickly achieve light-off temperature (T50), the CCC is often installed close to the engine. However, the extremely high temperature (ca. 1100 °C) near the engine can readily lead to the sintering of catalysts. Therefore, a CCC should possess not only excellent catalytic activity but also excellent thermal stability.
The classical components of these systems include Rh, Pt, and Pd as active metal and high surface alumina as the support [7, 8, 9]. The use of Pd as a single active metal component in a CCC has received considerable attention on the basis of its economic aspects (lower cost and more supply of Pd than Rh and Pt) as well as the availability of cleaner fuels and its remarkable activity for oxidation [10]. Pd/Al2O3 is widely regarded as the most active CCC for the catalytic combustion of propane, with excellent activity in the oxidation of HCs. However, its poor stability and the insufficient Pd supply have limited its practical application [11]. During the past several years, considerable attention has been paid to the improvement of catalytic activity of the CCC. To promote the catalytic activity by improving the stability of the support using a stabilizer is a viable means. Besides, another interesting approach is to promote Pd with lower cost metals [12]. Promising results have been obtained with Mn, Cr, or Cu, which have been attributed to the formation of the corresponding alloys (and consequent perturbation of Pd electronic properties) or, such as Mn, to a combination of the effects of alloy formation and Pd-MnOx interaction [12, 13, 14, 15]. Ni is also a potential promoter of Pd activity for these reactions despite legislative restrictions on its use in some countries. The effect of contact between Pd and Ni on promoting catalytic activity is positive, especially on enhancing NOx conversion under stoichiometric or rich atmosphere, e.g., the remarkable activity of Rh for the conversion of NOx that is due to that the nickel aluminate crystallites provide sites for NO adsorption and subsequent transfer to the active Pd sites [16, 17]. Recent reports on the same Pd-Ni catalyst have revealed that Ni induces a promotion of CO oxidation as a consequence of an indirect effect favoring the establishment of contact between Pd and the nanostructured configuration of the Ce-Zr mixed oxide component [16, 18, 19]. In a previous work, it was found that the improvement of the activity of NOx conversion promoted the total catalytic activity of the CCCs [20]. Therefore, the effect of Ni on the activity of HC conversion is worth studying for its lower cost and better activity in promoting NOx conversion. For this purpose, the effects of introducing Ni into a Pd/Al2O3 CCC for the elimination of C3H8 was studied in the present work.
La2O3-stabilized Al2O3 was prepared by the peptizing method. This was calcined at 1000 °C for 5 h in a muffle furnace. The Al2O3 material obtained was used as the support. The CCCs Pd/Al2O3 and bimetallic PdNi/Al2O3 were prepared by the impregnation method employing Pd(NO3)2 (Pd(NO3)2 content of 14.21%, Herarus Holding) and Ni(NO3)2·6H2O (Ni(NO3)2·6H2O content of 98.0%, Kelong Chemical Company) as the aqueous precursor solution. Deionized water was added to these materials to form a homogeneous slurry. The resulting slurry was spread over a honeycomb cordierite (2.5 cm3, Corning, China). Excess slurry was blown away using compressed air. The sample was dried at 120 °C and calcined at 1000 °C for 5 h in a muffle furnace to give the final monolithic catalyst. The Pd and Ni contents were 2.5 and 0.18 g/L, respectively.
The textural properties of La2O3-stabilized Al2O3 were assessed using a Quantachrome SI instrument. To remove surface impurities, the sample was first degassed under vacuum at 300 °C for 3 h prior to the measurement. The surface area and pore volume were 138 m2/g and 0.47 mL/g, respectively.
The activity for C3H8 and the single reactions of C3H8 were evaluated in a multiple fixed bed continuous flow microreactor by passing a gas mixture similar to gasoline engine exhaust through the reactor. The gas flows were regulated with mass flow controllers and sent to a blender. The simulated exhaust gas contained a mixture of O2, CO (0.6%), C3H8 (0.06%), NO (0.06%), CO2 (12%), H2O (10%), and N2 (balance). The gas space velocity was 40000 h−1. The individual reactions of C3H8 were the C3H8 + O2, C3H8 + NO, and C3H8 + H2O reactions. The oxidation of C3H8 was studied in an exhaust gas consisting of a mixture of O2 (adjustable), C3H8 (0.06%), and N2 (balance). The reduction of NOx with C3H8 was studied with an exhaust gas including a mixture of NO (0.06%), C3H8 (0.06%), and N2 (balance). The steam reforming reaction of C3H8 was studied in a mixture of H2O (10%), C3H8 (0.06%), and N2 (balance). The C3H8 concentration was analyzed online by a five-component analyzer (FGA-4100, Foshan Analytical Instrument Co., Ltd., China) before and after the simulated gas was passed through the microreactor.
H2 temperature-programmed reduction (H2-TPR) measurement was performed in a quartz tubular microreactor. Each sample (100 mg, 20-40 mesh) was pretreated in a flow of N2 (20 mL/min) from room temperature to 400 °C, maintained at this temperature for 60 min, and then cooled to room temperature under N2 (20 mL/min). The reduction was carried out under a flow of a 5% H2-95% N2 (V/V) gas mixture (20 mL/min) between room temperature and 600 °C at a heating rate of 8 °C/min. The H2 consumption was assessed by a thermal conductivity detector (TCD).
Pd dispersion was determined by CO chemisorption measurement. A catalyst sample of 200 mg was placed in a U-shaped quartz tube, reduced under a flow of H2 (20 mL/min) at 500 °C for 60 min and then purged with pure He (30 mL/min). After cooling to room temperature, pulses of CO gas were passed through the tube and the amount of adsorption was assessed by TCD.
X-ray photoelectron spectroscopy (XPS) data were obtained using a spectrometer (XSAM-800, Kratos Co) with Mg Kα radiation under ultrahigh vacuum (UHV). XPS measurements were conducted with a resolution of 0.9 eV and a step size of 0.05 eV. The X-ray source was powered at 13 kV and 20 mA. The electron binding energy was referenced to the C 1s line of adventitious carbon at 284.8 eV to correct for the effects of charging on the XPS spectra. Surface morphology analysis was carried out using high-resolution transmission electron microscopy (HRTEM) on a JEM-2010 instrument operated at 200 kV. The sample for the HRTEM study was ultrasonically dispersed in ethanol and deposited onto a perforated carbon film supported on a copper grid.
The catalytic activity of the Pd/Al2O3 and PdNi/Al2O3 catalysts before and after aging treatment was tested with the simulated exhaust gas. The results are shown in Fig. 1. Fig. 1(a) presents the C3H8 conversion over all catalysts at different temperatures. Fig. 1(b) shows the light-off temperature (T50) and full conversion temperature (T90) over the different catalysts.
According to Fig. 1(a), the C3H8 conversion increased with the rise of temperature and C3H8 rapidly reached complete conversion. The difference in activity between the catalysts with and without Ni addition was obvious. From Fig. 1(b), the T50 of the fresh Pd/Al2O3 and PdNi/Al2O3 catalysts were 280 and 274 °C, respectively. After the aging treatment, the catalytic activity declined dramatically, and T50 increased to 356 and 325 °C, respectively. The ΔT50 (T50-Aged-T50-Fresh) of the Pd/Al2O3 and PdNi/Al2O3 catalysts were 76 and 51 °C, respectively. A lower T50 represents a higher low-temperature activity of the catalyst. This indicated that the introduction of Ni not only promoted the light-off property but also inhibited the deterioration of light-off activity, promoting the stability of catalytic activity. A superior light-off property is vital to a CCC.
The T90 of the fresh Pd/Al2O3 and PdNi/Al2O3 catalysts were 315 and 305 °C, respectively. Those of the aged catalysts were 379 and 349 °C, respectively. It is interesting to note that the T90 of Pd-only and Ni-added catalysts were increased by 64 and 44 °C, respectively, after the aging treatment. To summarize, the introduction of Ni promoted the catalytic activity dramatically and also increased the stability of the catalytic property during the aging treatment, especially the light-off property.
One of the most important characteristics of the catalyst is its reducibility of active PdOx species, which impacts the catalytic property of the catalyst. The H2-TPR measurements of the Pd/Al2O3 and PdNi/Al2O3 catalysts before and after the aging treatment are shown in Fig. 2. There were two peaks in the TPR profiles, which were denoted as α and β, respectively. However, the α peak was only seen in the profile of the aged Pd/Al2O3 catalyst. The β peak is from hydrogen consumption, and the negative α peak corresponds to the decomposition of palladium hydride [21]. A larger area reduction peak of the active component is beneficial for the superior catalytic activity of the catalyst (the peak area corresponds to the adsorbed H2 consumption, and a larger peak area means a larger amount of the active component) [22].
For the fresh catalysts, the reduction temperature of peak β was similar (around 115 °C), but the peak area of the PdNi/Al2O3 catalyst was larger than that of the Pd/Al2O3 catalyst. This demonstrated that the amount of active PdOx species on the supported PdNi/Al2O3 catalyst was larger, which is beneficial for enhanced catalytic performance. After the aging treatment, significant changes in the reduction characteristics occurred for the catalysts. The area and reduction temperature of the β peak of the aged catalysts became smaller and lower than the fresh catalysts, indicating that the amount of active species decreased and the catalyst was sintered during the aging treatment. However, the area of the β peak of the PdNi/Al2O3 catalyst was still larger than that of the aged Pd/Al2O3 catalyst, implying a larger amount of active PdOx species. Another obvious change after the aging treatment was that the negative α peak from the decomposition of palladium hydride appeared in the profile of the aged Pd/Al2O3 catalyst at 93 °C. Metallic Pd0 can dissociate H2 molecules into H atoms, which can form palladium hydride with the large crystallites of metallic Pd0 at room temperature. However, the formation of palladium hydride on the highly dispersed Pd was considerably suppressed [23]. Moreover, metallic Pd0, which showed poorer activity for C3H8 than the active PdOx species, was generated through the reduction or decomposition of PdOx after the aging treatment. To summarize, large crystallites of metallic Pd0 appeared only on the aged Pd/Al2O3 catalyst, indicating that the doping with Ni effectively inhibited the sintering of the active component and the formation of metallic Pd0.
The dispersion of the active component on the supported catalyst is vital to the catalytic performance of the catalyst. CO chemisorption is effective for determining the dispersion of metal Pd over the support. The dispersion of active Pd on the Pd/Al2O3 and PdNi/Al2O3catalysts before and after the aging treatment was determined by CO chemisorption. Simplício et al. [24] reported that a higher dispersion of the active component and smaller particle size produce a higher active surface area, therefore enhanced catalytic performance of catalyst can be obtained. It was assumed that the stoichiometry of Pd to CO is 1:1. The average particle size, d (nm), was approximated by 1.12/D [25], where D (%) is the dispersion of Pd. The Pd active surface area, S (m2/g), was obtained using the formula V0Nδ/22.4wp, where V0 is the consumption of CO (L), N is Avogadro’s constant (6.023 × 1023), δ is the atomic cross section of Pd (0.06 nm2) [26], w (g) is the mass of catalyst, and p (%) is the mass fraction of Pd.
The results listed in Table 1 show that both the fresh and aged PdNi/Al2O3 catalysts exhibited higher Pd dispersion and Pd active surface area compared with the Pd-only catalyst. The active Pd particle size in PdNi/Al2O3 was also smaller than that in the Pd/Al2O3 catalyst. Moreover, the decline of active Pd dispersion in the PdNi/Al2O3 catalyst was less than that in the Pd/Al2O3 catalyst during the aging treatment. These results are in good agreement with the above TPR results. These data indicated that the addition of Ni to the Pd/Al2O3 catalyst increased the stability and dispersion of the active PdOx species.
In order to investigate the changes in the chemical state of Pd element, the fresh and aged catalysts were studied by XPS. Fig. 3 presents the Pd 3d photoelectron peaks of the catalysts. No Ni peak was seen on the surface of the Ni-promoted catalyst due to the low concentration of Ni added.
The binding energy of metallic Pd0 and PdO is 335.3 and 336.8 eV, respectively, according to the references [27, 28]. From the results given in Fig. 3, it can be seen that the binding energy of the Pd 3d5/2 electrons of all the catalysts was between 335.3 and 336.8 eV. This result indicated that the Pd species in these catalysts were in a partially oxidized state, Pdδ+ (0 < δ < 2), which is more active than metallic Pd0 [29]. Compared with the Pd-only catalysts, the Pd 3d5/2 binding energy of both the fresh and aged PdNi/Al2O3 catalysts was shifted to a higher value by 0.2 and 0.5 eV, respectively. This was likely due to the electronic effect of Ni, leading to the transfer of electrons from PdOx to NiOx. Various additives can be used to modify the active phase by changing the oxidation state and by modifying the electronic environment of the active component [30]. By considering the observed changes in the total catalytic activity, the higher oxidation state of the active PdOx species may be one reason for the superior activity of the Ni-promoted catalyst. After the aging treatment, the binding energy of the Pd-only catalyst and Ni-promoted catalyst decreased by 0.4 and 0.1 eV, respectively. The decrease in the binding energy of Pd 3d5/2 due to aging can be explained by particle growth [31], indicating that the introduction of Ni to the Pd-only catalyst inhibited the sintering and growth of active PdOx species during the aging treatment. By considering the observed changes in the above catalytic activity, the higher oxidation state and smaller particle size of the active PdOx species may be one reason for the superior activity of the Ni-promoted catalyst.
HRTEM studies play a relevant role in the nanostructure characterization of noble metal dispersed on a support [32]. Fig. 4 presents HRTEM micrographs of the aged catalysts. The corresponding elemental mapping of Pd is inserted in the images. As presented in Table 1, the fresh catalysts showed a higher dispersion of Pd species, especially the Ni-promoted catalyst. Therefore, the highly dispersed Pd species were hard to observe owing to a lack of contrast with the support. Only the aged catalysts were examined by HRTEM. It can be seen from the following micrographs that the promoting effect of Ni on inhibiting the sintering of the catalyst was positive. After the aging treatment, PdOx particles (labeled by the arrows) were observed on the images owing to the lower dispersion (shown in Table 1). It can be seen from Fig. 4 that the Ni-promoted catalyst showed smaller PdOx particles compared with the Pd-only catalyst, which is in accordance with the CO chemisorption results. A smaller particle size and higher dispersion of the active PdOx species were the main reasons that the PdNi/Al2O3 catalyst still has good catalytic activity after the aging treatment.
In order to further study the effect of Ni on the overall catalytic activity of the Pd-only catalyst, specific reactions of C3H8 were assessed at different temperatures. These are shown in Fig. 5. The main single reactions relevant to the elimination of C3H8 are direct oxidation (HC + O2 → CO2 + H2O) and steam reforming (HC + H2O → CO + H2) of C3H8, and the reduction of NOx with C3H8 (NOx + HC → N2 + CO2 + H2O).
Fig. 5(a) presents the C3H8 conversion of the C3H8 + O2 reaction at different temperatures. The oxidation reaction of C3H8 was studied in a C3H8/O2/N2 atmosphere. The changes in the light-off and full conversion temperatures are summarized in Table 2. The conversion of C3H8 by direct oxidation did not increase as rapidly as that of the overall reaction with increasing temperature: the ΔT values (ΔT = T90 - T50) obtained for the fresh Pd/Al2O3 andPdNi/Al2O3 catalysts were 57 and 55 °C, respectively. Shinjoh et al. [33] proposed that either C3H8 or an intermediate species (such as partial oxidation products) strongly adsorbed on the catalyst surface result in self- inhibition by restricting the adsorption of O2. For the fresh catalysts, the catalytic activity of PdNi/Al2O3 catalyst was similar to that of the Pd/Al2O3 catalyst due to sufficient active sites. However, the aged PdNi/Al2O3 catalyst presented better activity than the aged Pd/Al2O3 catalyst, in accordance with the discussion above. The T50 of theaged Pd/Al2O3 and PdNi/Al2O3 catalysts was 404 and 398 °C, respectively. T90 of the aged Pd/Al2O3 and PdNi/Al2O3 catalysts was 450 and 435 °C, respectively. In addition, the conversion of C3H8 over the Ni-promoted catalyst was already 100% at 450 °C, while the conversion over the Pd/Al2O3 catalyst was only 90%. With regard to the oxidation reaction, therefore, the PdNi/Al2O3 catalyst presented a better catalytic activity and low temperature property compared to the Pd/Al2O3 catalyst. This was because the addition of Ni increased the oxidation state of the PdOx species and accelerated the re-oxidation of Pd to PdOx, as discussed in XPS results, both of which are helpful to the adsorption of O2.
To study the steam reforming reaction of C3H8, the conversion at different temperatures was determined under a C3H8/H2O/N2 environment. The results are shown in Fig. 5(b). The activity for the C3H8 + H2O reaction was different from those for the C3H8 + O2 and C3H8 + NO reactions (Fig. 5(c)). C3H8 can reach full conversion more quickly and the deterioration during the aging treatment was much less. For the fresh catalysts, the T50 of the Pd/Al2O3 catalyst was similar to that of PdNi/Al2O3 catalyst, as shown in Table 2. However, the Ni-promoted catalyst exhibited superior high temperature activity compared to the Pd-only catalyst, which was in good agreement with the total catalytic activity. T90 of the Pd/Al2O3 and PdNi/Al2O3 catalysts was 372 and 353 °C, respectively. After the aging treatment, T50 and T90 increased to higher values due to the sintering of catalysts. T50 of the aged Pd/Al2O3 and PdNi/Al2O3 catalysts increased to 375 and 369 °C, respectively. T90 of the aged Pd/Al2O3 and PdNi/Al2O3 catalysts increased to 397 and 394 °C, respectively. The activity of the Ni-promoted catalyst was still better than that of Pd-only catalyst due to more activesites, especially at temperatures higher than 400 °C. The support is the principal site for water activation, and the metal sites are responsible for the activation of the HC in the steam reforming reactions [34]. In addition, H2O weakens the adsorption of C3H8 or its intermediates. With the addition of Ni, not only was the activation of C3H8 adsorbed on Pd sites enhanced, but the conversion of C3H8 due to the SR reaction increased.
Fig. 5(c) shows the catalytic activity of the C3H8 + NO reaction, which was tested in a C3H8/NO/N2 atmosphere at different temperatures. According to Fig. 5(c), the reduction activity of all the catalysts was weaker compared with the direct oxidation (Fig. 5(a)) and steam reforming reaction (Fig. 5(b)). The reactions between adsorbed species on the noble metal surface and desorption of the product are the key steps [35]. The more difficult desorption of NO, C3H8 or an intermediate species, and insufficient active sites are the complicated reasons for the lower catalytic activity of the C3H8 + NO reaction. For the fresh catalysts, T50 of the Pd/Al2O3 and PdNi/Al2O3catalysts was 374 and 367 °C, respectively. However, when the temperature increased to 400 °C, the conversion over Pd/Al2O3 and PdNi/Al2O3 catalyst was only 59.1% and 63.4%, respectively. After the aging treatment, the activity of both the Pd/Al2O3 and PdNi/Al2O3 catalysts decreased dramatically, although the activity of the Ni-promoted catalyst was still superior to that of the Pd-only catalyst. T50 of the PdNi/Al2O3 catalyst was 436 °C and the conversion at 450 °C was 58%, much higher than that of the Pd/Al2O3 catalyst (40%). The positive effect of Ni on improving the activity for the reduction reaction may be attributed to that the nickel aluminate crystals provided sites for NO adsorption [17].
To summarize, the doping with Ni obviously promoted the C3H8 conversion over the aged catalyst, especially for the single reaction of C3H8 + NO. This result was attributed to the superior activity of Ni for NOxconversion, and the higher oxidation state of the PdOx species in the PdNi/Al2O3 catalyst [16].
Addition of Ni to a Pd/Al2O3 catalyst plays a promoting role in the total activity of C3H8 conversion as well as the single reactions of C3H8, especially the C3H8 + NO reaction. TPR, XPS, HRTEM, and CO chemisorption results revealed that the addition of Ni improved the stability of the active PdOx species and increased the oxidation state of the active component.
2015, Vol. 36 Issue (7): 994-1000
PDF (852 KB)
2015, Vol. 36 Issue (7): 994-1000
PDF (852 KB)