Effect Of Different Alkali-assisted Deposition Precipitation Methods On The Durability Of Three-way Catalysts

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Abstract: The aging treatment always brings about severe deactivation of three-way catalysts(TWCs) so improving the durability is one goal of TWCs design. In this work,Pt/Ceo aZrosLao.osPro.osO2(Pt/CZ)TWCs were prepared by two different alkali-assisted deposition precioitation method, urea or ammonia. The effects of different alkali-assisted on the physicochemical properties, catalytic activity and durability were analyzed. It was found that the Pt/CZ catalyst prepared by urea-assisted deposition precipitation method (UDP) had poor aging resistance but the Pt/CZ catalyst prepared by ammonia-assisted (ADP)showed better anti-aging ability. Based on the results of XRD, CO adsorption, XPS and Hz-TPR, comparatively larger particle size of Pt species and stronger interaction for ADP make it more difficult to sinter together during the aging treatment, which is favorable for maintaining catalytic activity. Therefore, the ADP catalyst shows a bright future for industrialization.

Keywords: three-way catalysts; deposition precipitation; alkali-assisted; durability

Three-way catalysts (TWCs) are one of the most important catalysts in purifying exhaust gas from gasoline-powered vehicles 12l, which are able to effectively convert hazardous carbon monoxide (CO), nitrogen oxides(NO.), hydrocarbons (HC) to harmless carbon dioxide(CO2), water (HO) and nitrogen (N2) simultaneously34l. However, aging is the main cause of catalyst deactivation due to the serious influence on the structural and textural properties, metal dispersion, oxygen storage capacity (OSC) and redox ability5-61. cistanche wirkung It is necessary to design TWCs with high stability to meet increasingly durability requirementl7-81 Researchers have done a lot of work to improve the thermal stability of TWCs[9-10], suppressing phase transformation and preventing the sintering of active noble metal. No matter which strategy, the typical formulation of TWCs is unchanged because of their preferable poisoning resistance and higher thermal stability[10-l1]. Generally, changing the preparation method is a simple and easy route to improve the durability of TWCs[12-13],

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The deposition precipitation method is one of the main preparation methods for industrial catalysts, which is done in the presence of an existing support, and a precursor solution usually forms metal hydroxides by injection of the precipitantl4]. Thomas, et al.[15]used deposition precipitation method to prepare Co/TiO2 catalysts using urea hydrolysis or ammonia evaporation. Moreau, et al.[16]prepared Au/TiO2 catalyst by deposition precipitation method with NaOH as precipitant. The deposition precipitation method has been widely used in the preparation of catalysts, but it is rarely used in the preparation of noble metal TWCs. Therefore, it is valuable to study the deposition precipitation method for the preparation of TWCs.

In the present work, we have prepared ceria-zirconia-supported platinum (Pt/CZ) catalysts with different alkali-assisted deposition precipitation method, and a Pt/CZ catalyst prepared by impregnation method as reference catalyst. The effect of different alkali-assisted deposition precipitation methods on the stability of TWCs were investigated.

Experimental 1

1.1 Catalyst Preparation

The metal precursor was H-PtCl6(0.10 g/mL) aqueous solution and the loading content of Pt was lwt%. The support material, a mixed oxide of CeO-ZrO-La-O3-Pr2O5 (the mass ratio of 40/50/5/5,CZ) was supplied by Rhodia Company. The benchmark Pt/CZ was prepared by the traditional impregnation method (IM). The catalyst prepared by the deposition-precipitation method assisted with urea(molar ratio of urea/Pt=50) was labeled as UDP. Another catalyst prepared by the deposition-precipitation method assisted with ammonia at pH=9 was labeled as ADP. All samples were dried in an oven at 90 ℃ for 1 d and calcined at 550 ℃ for 3 h in air. The as-prepared powders were sprinkled onto cordierite to obtain the monolithic catalyst with a loading capacity of (160±5)g/L. citrus bioflavonoids Afterward the monolithic catalysts were calcined at 550 ℃ for 3 h in air to obtain the fresh catalysts. The fresh catalysts were hydrothermally treated in 10vol% H2O/air at 750 ℃ for 13 h to obtain aged catalysts, which were labeled as IM-a, UDP-a and ADP-a, respectively.

Cistanche can anti-aging

1.2 Catalyst characterization

The textural properties of the catalysts were obtained by Quantachrome automated surface area and pore size analyzer (Autosorb SI). The powder X-ray diffraction (XRD) patterns were recorded on Philips Company(PW 1730). The dispersion of Pt species was performed on CO pulse adsorption and Fourier transforms infrared spectrometer (FT-IR, Thermo Nicolet 6700). X-ray photoelectron spectroscopy (XPS) experiments were carried out on an electron spectrometer (XSAM-800). Hydrogen-temperature programmed reduction (Hz-TPR) was performed in a tubular quartz microreactor and recorded by a thermal conductivity detector(TCD).

1.3 Activity tests

The catalytic performance were evaluated using a fixed-bed continuous flow reactor by passing through a simulated gasoline engine exhaust gas mixture, which was composed of CO(4600×10-6),CGH6(220×10-),C,Hs (110x10~3),Hz(1533×10~),NO(1250×10~0),CO2(10%), H2O(10%), O2(3600×10-6)and Nz(balance gas). The gas hourly space velocity (GHSV) was controlled at 5×10+h'.

2 Results and discussion

2.1 Textural and structural characterization

The textural parameters of the support and catalysts are listed in Table 1. It can be seen that the specific surface area and pore volume of samples are similar with each other. However, the average pore radii of UDP-a and ADP-a are smaller than that of IM-a. This may be attributed to the that the Pt species are more likely to enter the small pores of support in the preparation of UDP and ADP. The presence of Pt in the small pores is conducive to maintaining the pores in the aging processli7].

The X-ray diffraction patterns and relative intensity of Pt (111) are shown in Fig.1.It is observed from Fig.1(a)that the diffraction peaks of all samples are characteristic of cubic CeOz-ZrOz (CZ) phase 8. There are no peaks of Pt or PtO, in IM and UDP, which may be attributed to the good dispersion of Pt species on support. However, it is clear to find out that the diffraction peaks of Pt in ADP, confirming the existence of larger Pt particles in ADP. After aging, the peaks identified as Pt phase are observed for all samples due to the agglomeration of noble metal. The relative intensity of Pt (111) peak of these catalysts are shown in Fig. 1(b). The order of diffraction peak intensity can be recorded as UDP-a>IM-a> ADP-a. This indicates that Pt particles are the smallest in the ADP-a.

2.2 CO chemisorption results

The dispersion of Pt species was evaluated by CO pulse adsorption method, and the obtained results are depicted in Fig. 2(a). The dispersion of Pt species for fresh samples follow the order of UDP>IM> ADP which coincides with the XRD results. For the aged samples, the dispersion of Pt species sharply declines owing to the aggregation of noble metal particles.

The CO adsorption experiment was also performed on FT-IR equipment and the obtained CO-FTIR spectrum were presented in Fig. 2(b, c). The IR band exhibited in figures are assigned to CO species linearly adsorbed on Pt [19] For the fresh samples, the intensity of band decreased with an order of IM ≈ UDP> ADP， which reflects the dispersion of Pt species to some extentl201. As for aged samples, compared with fresh samples, the band strength of UDP-a decreases more greatly, while that of ADP-a samples decreases more slightly, which indicates that ADP owns good hydrothermal stability. This is consistent with the results of XRD analysis.

2.3 XPS analysis

X-ray photoelectron spectroscopy (XPS) experiment was performed to clarify the surface elemental states of samples. The relative amounts of Ce, O and Pt are listed in Table 2, and the obtained spectra of Ols, Ce3d and Pt4f are illustrated in Fig. 3. The spectra of Ols (Fig. 3(a, d))are composed of two characteristic peaks with the corresponding binding energy (BE) values of 529.2 and 531.2 eV. One at 529.2 eV is denoted as lattice oxygen (Olat), and the other at 531.2 eV is attributed to surface adsorbed oxygen (Oats)21l.The spectra of Ce3d(Fig.3(b,e))are deconvoluted into eight peaks corresponding to four pairs of spin-orbit doublets. The peaks labeled as v, v”, v""and u, u", u""are assigned to Ce4", and an additional doublet (v', u') are arisen from Ce3+ speciesl201. cynomorium benefits For the XPS spectra of Pt4f(c,f),the peak at 70.7 eV is assigned to Pt° and that at 72.4 eV is ascribed to PtO. Moreover, the peaks locating at 74.0 eV is attributed to PtO,I221.

As shown in Fig. 3(a, d), Oads and Olatt exist in all samples. The ratio of Oad/O for fresh samples is in the sequence of UDP>IM>ADP, but the aged samples exhibit a different sequence with an order of UDP-a >ADP-a>IM-a. It is observed slight decrease of Oads for IM and UDP, but slight increase for ADP.As well known, more Oads can enhance catalytic activity under vehicle exhaust atmosphere.

Ce³+ and Ce4+ coexist on the surface. The analysis of the relative ratio of Ce**reveal that conspicuous decline of the ratio of Ce³'/Ce occurs after aging in case of UDP，slight decline for IM-a, while slight increase for ADP-a. The change trend is consistent with that of Oads. Because the existence of Ce³* has an important influence on oxygen vacancy which is conducive to adsorbing more oxygen from the gasl23]. Therefore, the oxygen vacancies in ADP are well maintained during hydrothermal aging treatment, which is favorable for keeping redox property as well as the catalytic performance.

Regarding Pt species, it is easy to detect that platinum species mainly exist in oxide state for the existence of Pt-O-Ce interaction as well240.It can be detected Table 2 that the value of Pt**/Pt follow the order of ADP>UDP>IM. ADP has the highest Pt*/Pt ratio which can be explained by the stronger interaction between Pt and support. The stronger interaction is conductive to maintaining Pt species at a more stable state. After aging, the surface concentration of Pt decreases to some extent. The platinum species mainly exist as Pt2* and PtP that PtO? decompose to Pt metal under oxidizing conditions at above 600 ℃B! What should be mentioned is that UDP-a has higher relative percent of Pt", which suggests that the stability of UDP-a is poor. However, ADP-a has relative higher ratio of Pt2*, which also proves that ADP has a better ability to resist hydrothermal treatment.

In conclusion, from the analysis of XPS data, ADP prepared by ammonia-assisted deposition precipitation method possess stronger metal-support interaction to resist aging treatment.

2.4 H2-TPR studies

The Hz-TPR profiles are displayed in Fig. 4.It is observed that IM has two reduction peaks. One peak at 198 ℃ is assigned to the reduction of PtO, and Ce species that are effectively promoted by noble metal for the spillover effectl25]. The other peak at 380 ℃ is assigned to the reduction of a very low percent of surface un-activated oxygen. UDP also has two reduction peaks. The first peak at 174 ℃ is lower than that of IM, implying that UDP possess better reducibility than IM due to the better dispersion of Pt species. Attention should be paid to ADP. In the range of low temperature, ADP has a main reduction peak and two shoulder peaks with the corresponding temperature at 106, 150 and 174 ℃. Combining the analysis above, the lower reduction temperature can be ascribed to the stronger interaction in ADP. In addition, the three reduction peaks can be ascribed to the reduction of the PtO, species and Ce species with different degree of interaction. This can be explained as a consequence of the existence of small and big particles in ADP. desert hyacinth The reason for the smaller area of reduction peak is that the big Pt particles have a weaker promoting effect on the support.

After hydrothermal aging treatment, the reduction peaks for aged catalysts shift to higher temperature due to the sintering and agglomeration of active components. It should be noted that the temperature of the main reduction peak of UDP-a shift from 174 ℃ to 202 ℃, while that of ADP-a only shift from 150 ℃ to 164 ℃. The result suggests that the ADP undergone slighter sintering and the redox properties were well maintained, which proved the better hydrothermal stability in case of ADP.

2.5 Catalytic performance

The conversion curves of CO,NO, C3Hs and C3H6 for all series of samples are depicted in Fig. 5. For fresh catalysts, UDP exhibits better CO conversion activity than the reference sample IM due to the higher dispersion of Pt species and higher content of Oas. ADP exhibits worse catalytic activity for all pollutions due to the worse dispersion of Pt species.

As for the aged samples, the catalytic activities are in the sequence of ADP-a>IM-a>UDP-a. The activity results exhibit a converse sequence with the fresh samples. In order to compare durability carefully, the light-off temperature (Tso, the temperature where pollutions obtained 50% conversion), and the △T (defined as To of aged samples-Tso of fresh samples) are summarized in Table 3. △T is an important parameter to evaluate the stability of TWCs. The To of UDP-a shifts to higher temperature with △T of 87 ℃ for CO, 54 ℃ for NO, 55 ℃ for C3Hg and 59 ℃ for C3H6. flavonoid extraction method pdf This may be ascribed to that the smaller particles are easily agglomerated together, which leads to the sharp decline of dispersion of Pt species and catalytic activity. A similar situation is also found in IM-a, which To has an increase of 41, 44, 43,44 ℃ respectively. Whereas the △T of Tso over ADP is 16 ℃ for CO,17 ℃ for NO,22 ℃ for C,Hand 18 ℃for C3H6. This indicates that ADP has better ability to maintain catalytic activity against hydrothermal treatment. ADP possesses bigger Pt particles and stronger interaction in essence, which are favorable for enhancing the catalytic stability.

3 Conclusions

We have prepared Pt/CZ catalysts by two different alkali-assisted deposition precipitation methods. The effects of two different alkali on the physicochemical properties, catalytic activity and durability of the catalysts were investigated.

Some important results and conclusions are presented as follows: (1) UDP showed poor aging resistance and the catalytic activity decreased seriously after hydrothermal aging treatment. (2) ADP exhibited good anti-aging ability, with little physicochemical properties and catalytic activity changed after aging. (3) The different alkali-assisted deposition precipitation methods have an important influence on the initial particle size of Pt and interaction in the catalyst, resulting in a great difference in durability.

This article is extracted from Journal of Inorganic Materials Jun., 2021