Optimized Design Method for Pt/SiO2-Al2O3 with High NH3-SCO Activity and Thermal Stability.

Pt/SiO2-Al2O3 catalysts were prepared by the traditional impregnation method (IM) and the strong electrostatic adsorption (SEA) process. Differences in particle size, surface chemical state, Pt adsorption site, ammonia oxidation activity, and thermal stability of Pt species were studied systematically. For the fresh catalyst of Pt/SiO2-Al2O3-IM (Pt/SiO2-Al2O3-IM-fresh), Pt species were dispersed unselectively on SiO2-Al2O3, and the large average size (6.6 nm) of Pt species could be observed in a bimodal distribution (ranges of 5.5-6.5 and 8.5-9.5 nm). After the hydrothermal treatment, the Pt size of the aged catalyst (Pt/SiO2-Al2O3-IM-aged) increased significantly, especially Pt particles on SiO2 showed obvious agglomeration and some even increased to 40 nm. Conversely, for the catalyst prepared through the SEA process, Pt species of Pt/SiO2-Al2O3-SEA-fresh were selectively absorbed on Al2O3, the Pt particle size was in the range of 1.5-6.0 nm, and the average particle size was only 2.7 nm. After hydrothermal aging, Pt species did not show obvious agglomeration (the average particle size was 3.2 nm). Above all, Pt/SiO2-Al2O3-SEA presented better catalytic activity and thermal stability than Pt/SiO2-Al2O3-IM, i.e., the temperatures of 50% NH3 conversion for the fresh and aged Pt/SiO2-Al2O3-SEA catalysts were 216 and 223 °C, respectively, much lower than those for Pt/SiO2-Al2O3-IM-fresh (228 °C) and Pt/SiO2-Al2O3-IM-aged (250 °C).

Introduction

To meet low emission limits, NO from vehicle exhaust is reduced usually by NH3.[1−3] Excess NH3 is used to increase NO conversion, which results in NH3 leakage.[4,5] As an alkaline gas, NH3 can react with acidic gases to form secondary aerosols NH4NO3, (NH4)2SO4, and NH4HSO4, which are the main component of PM2.5.[6] Nowadays, the ammonia selective catalytic oxidation technology can prevent unreacted NH3 effectively from vehicle exhaust.[7,8]

Noble metal catalysts have been widely used because of their excellent ammonia oxidation activity, especially Pt.[9−11] However, the problem is the high cost and low thermal stability of Pt-based catalysts. In the ammonia oxidation reaction, improving Pt dispersion and thermal stability is the key for high catalytic performance. In our previous work, ethylenediamine was added to the precursor solution for preparing a Pt-based catalyst, which was beneficial for improving ammonia oxidation activity due to the strong metal–support interaction and high Pt dispersion.[12]

The metal–support interaction could be modified by the adsorption mechanism. The metal oxide material in the precursor solution could react with H+ or OH–, which made the material surface protonate or deprotonate. When the pH of the precursor solution was below the point of zero charge (PZC), the support material could be protonated and adsorbed anions. When the pH of the precursor solution was above the PZC, the support material could be deprotonated and adsorbed cations.[13,14] Hao et al.[15] found that [(NH3)4Pt]2+ could be adsorbed easily over carbons with low PZC in the high pH precursor solution, while [PtCl6]2– could be adsorbed easily over carbons with high PZC in the low pH precursor solution. Zhu et al.[16] proved that the dispersion of Pt could be improved vastly on C, SiO2, and Al2O3 by acidifying or basifying the precursor solution and also presented the relationship between the initial pH of the precursor solution and absorption quantity over the supports with different PZCs.

According to the commercial application, Pt/Al2O3 was widely used in ammonia treatment because of its excellent NH3-SCO catalytic activity.[17,18] However, the thermal stability of the catalyst was still not ideal. In this study, using SiO2-Al2O3 as the support material and H2PtCl6 as the metal precursor, different metal–support interactions between Pt-Al2O3 (Al2O3, PZC = 8) and Pt-SiO2 (SiO2, PZC = 4) were obtained via controlling the initial pH of the precursor solution. For the Pt/SiO2-Al2O3 catalyst prepared by the strong electrostatic adsorption (SEA) method, [PtCl6]2– was absorbed selectively and strongly on Al2O3, which made SiO2 act as a barrier and was beneficial for inhibiting Pt species agglomeration. As a result, Pt/SiO2-Al2O3 prepared by the SEA method presented extremely high NH3-SCO activity and thermal stability, which will provide a good idea for improving the thermal stability of Pt-based catalysts.

Experimental Section

Preparation of Catalysts

The support material SiO2-Al2O3 (SiO2 with 30 wt %) was provided by Solvay. First, the pH of the H2PtCl6 solution was regulated to 2.7 by NH3·H2O, and then SiO2-Al2O3 was added to the solution and stirred for 12 h. After that, the sample was filtered, washed, dried at 100 °C for 12 h, and calcined at 550 °C for 2 h under H2. The obtained catalyst powder was mixed with appropriate water to form a uniform slurry, which was coated on a cordierite honeycomb (2.2 mL) with a loading of about 160 g/L. After being calcined at 550 °C for 1 h under H2, a fresh monolithic catalyst was prepared and was labeled as Pt/SiO2-Al2O3-SEA-fresh. The catalyst was aged at 800 °C with 5% H2O for 10 h to obtain the Pt/SiO2-Al2O3-SEA-aged catalyst. The Pt content of Pt/SiO2-Al2O3-SEA-fresh was 0.4 wt %, which was determined by inductively coupled plasma (ICP) characterization.

For comparison, Pt/SiO2-Al2O3-IM-fresh (Pt, 0.4 wt %) was prepared by the impregnation method (IM) with SiO2-Al2O3 as a support material. First, H2PtCl6 as a precursor was added in an appropriate amount of water to form an aqueous solution. Then, the support material SiO2-Al2O3 was added into the aqueous solution and stirred with a glass rod to ensure that water entered the material channel, dried at 100 °C for 12 h, and calcined at 550 °C for 2 h under H2. The coating steps were consistent with those of Pt/SiO2-Al2O3-SEA-fresh. Pt/SiO2-Al2O3-IM-fresh was aged at 800 °C with 5% H2O for 10 h to obtain Pt/SiO2-Al2O3-IM-aged.

Catalytic Performance Measurements

The catalyst activity was evaluated in a self-assembled reactor with a gas hourly space velocity of 10 0000 h–1. The reaction gas was a mixture of NH3 (200 ppm), O2 (10%), CO2 (8%), H2O (5%), and N2 and was used as the balance. The transient test was carried out in the range of 500–200 °C. The gas composition before and after the reaction was quantitatively analyzed with an Fourier transform infrared (FTIR) (Antaris IGS-6700) analyzer.

Characterization of Catalysts

CO adsorption was accomplished on an FTIR spectrometer (Thermo Nicolet 6700). First, the sample powder was pretreated at 5% H2/N2 at 400 °C for 30 min. Then, background peaks were collected until cooled to 30 °C under N2 (99.999%). The CO adsorption was carried out at 30 mL/min in CO/N2 (1 vol %) until saturation followed by N2 purging, and the CO adsorption results were obtained after deducting the background.

An in situ reaction of NH3 and O2 was measured using an FTIR spectrometer (Thermo Nicolet 6700). The samples were pretreated at 400 °C for 30 min under reaction gases of 200 ppm NH3, 5% O2, and N2 and then cooled to 80 °C. Background spectra were collected under N2 (99.999%). Then, the gas was switched to the reaction gas, and the in situ NH3 and O2 reaction spectra were recorded in the range of 80–500 °C.

The morphology of the samples was analyzed by transmission electron microscopy (TEM) (Tecnai G2 F20 S-TWIN, FEI Company) measurement. Before testing, the samples in aqueous ethanol were dispersed in a centrifugal machine for 0.5 h, and the acceleration voltage was 200 kV.

The surface areas of the samples were investigated by a MicroActive for ASAP 2460. The pretreatment temperature was 300 °C under vacuum conditions for 1 h. During the test, 0.1 g of the sample was weighed and measured at −195.8 °C. The surface area and pore size distribution were obtained by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

The samples (0.1 g) were pretreated at 400 °C under N2 for 1 h; after that, the temperature was lowered to about 60 °C. The gas was switched to adsorb NH3 for 1 h, and then, N2 was used to purge excess NH3. When the baseline was stable, the temperature was increased to 700 °C at a rate of 10 °C/min.

The crystal structures of the samples were measured by X-ray diffraction (XRD, DX-2700, Dandong, China) using Al K(1486.6 eV) as radiation.

Results and Discussion

Textural Properties of Catalysts

The textural properties of the catalysts are shown in Figure . The results of powder X-ray diffraction (XRD) are presented in Figure a. Compared with the support material SiO2-Al2O3, the peaks of Pt/SiO2-Al2O3-SEA-fresh and Pt/SiO2-Al2O3-IM-fresh could not be changed after introducing Pt by different methods. Figure b presents that all catalysts have the same N2 adsorption–desorption result, which is attributed to the hysteresis loop of type IV. The pore diameter distribution (Figure c) showed that the main pore diameter was located at 1–40 nm. The detailed textural parameters of the catalysts are listed in Table . The surface area of Pt/SiO2-Al2O3-SEA-fresh was 156.5 m2/g larger than those of SiO2-Al2O3 (139.5 m2/g) and Pt/SiO2-Al2O3-IM-fresh (138.7 m2/g). Accordingly, a higher pore volume of 0.62 cm3/g and a smaller pore size of 9.9 nm were obtained by Pt/SiO2-Al2O3-SEA-fresh. This was beneficial for the higher catalytic performance.

Figure 1

XRD patterns of catalysts (a), N2 adsorption–desorption results (b), and pore diameter distribution (c).

Table 1

Textural Parameters of the Catalysts

sample	surface area (m2/g)	pore volume (cm3/g)	average pore diameter (nm)
SiO2-Al2O3	139.5	0.55	10.0
Pt/SiO2-Al2O3-SEA-fresh	156.5	0.62	9.9
Pt/SiO2-Al2O3-IM-fresh	138.7	0.57	10.4

Temperature-Programmed Desorption of Ammonia

The results of temperature-programmed desorption of ammonia are shown in Figure . For SiO2-Al2O3, the peaks below 300 °C were attributed to the physically adsorbed NH3; the peaks above 300 °C were due to the strongly adsorbed NH3. After introducing Pt by the SEA method, the peak below 300 °C was increased, which might be attributed to the high dispersion of Pt species on the surface of SiO2-Al2O3. Compared with Pt/SiO2-Al2O3-IM-fresh, Pt/SiO2-Al2O3-SEA-fresh showed a larger amount of acid sites than that of Pt/SiO2-Al2O3-IM-fresh, which was beneficial for adsorbing the reactant NH3 to improve the catalytic activity.

Figure 2

Results of NH3-TPD.

In Situ DRIFTS Spectra of the NH3 and O2 Reaction

Figure shows the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of the NH3 and O2 reaction. Only NH3 adsorption peaks were observed on Pt/SiO2-Al2O3-IM-fresh and Pt/SiO2-Al2O3-SEA-fresh at 80 °C, demonstrating that there was no reaction between NH3 and O2. For Pt/SiO2-Al2O3-IM-fresh (Figure a), the NH3 adsorption bands in the range of 3000–3500 cm–1 belonged to L acid sites,[23−25] and the bands at 1701 cm–1 were attributed to B acid sites.[26−29] Similarly, for Pt/SiO2-Al2O3-SEA-fresh (Figure b), the NH3 adsorption bands for L acid and B acid sites were in the range of 3000–3500 cm–1 and at 1692 cm–1, respectively. It was worth noting that the bands at 1490 cm–1 found for Pt/SiO2-Al2O3-SEA-fresh were attributed to the B acid site of SiO2.[27,30] This was because the active component Pt was selectively adsorbed on Al2O3, exposing SiO2. The increase of B acid sites could promote the adsorption amounts of NH3, which was favorable for the activity of NH3-SCO. As the reaction progressed, the band at 1553 cm–1 was observed on both catalysts, indicating the formation of NO.[31]

Figure 3

In situ reaction of NH3 with O2 over Pt/SiO2-Al2O3-IM-fresh (a), Pt/SiO2-Al2O3-SEA-fresh (b), Pt/SiO2-Al2O3-IM-aged (c), and Pt/SiO2-Al2O3-SEA-aged (d).

Figure c shows the in situ DRIFTS spectrum of the ammonia oxidation reaction for Pt/SiO2-Al2O3-IM-aged. The bands between 3000 and 3500 cm–1 were attributed to NH3 adsorption at the L acid sites, while the band at 1704 cm–1 was attributed to NH3 adsorption at the B acid sites.[25−29] Different from Pt/SiO2-Al2O3-IM-fresh, the peak of 1490 cm–1 was observed on Pt/SiO2-Al2O3-IM-aged, which might be due to the serious agglomeration of Pt species during the aging process, resulting in some exposed SiO2.[27,30]Figure d shows the in situ DRIFTS of Pt/SiO2-Al2O3-SEA-aged for the NH3-SCO reaction. After aging, the peak at 1490 cm–1 decreased compared with that of Pt/SiO2-Al2O3-SEA-fresh. As the temperature increased, the peak of the product NO (1553 cm–1) could be observed on Pt/SiO2-Al2O3-SEA-aged at 200 °C, while NO could be found at 250 °C on Pt/SiO2-Al2O3-IM-aged, indicating that the catalytic activity of Pt/SiO2-Al2O3-SEA-aged was superior to that of Pt/SiO2-Al2O3-IM-aged, and the peak strength of the products enhanced with increasing temperature.[11]

CO-FTIR Results

The CO-FTIR spectra for Pt species of Pt/SiO2-Al2O3-IM-fresh and Pt/SiO2-Al2O3-SEA-fresh catalysts are provided in Figure a. The CO adsorption peaks of Pt/SiO2-Al2O3-IM-fresh and Pt/SiO2-Al2O3-SEA-fresh were at 2060 and 2056 cm–1, respectively, which are attributed to CO linear adsorption on the Pt metal state.[19] In addition, the band of CO on Pt/SiO2-Al2O3-SEA-fresh was at a lower wavenumber, indicating a higher electron density of the Pt species than that of Pt/SiO2-Al2O3-IM-fresh.[20]Figure b shows the CO-FTIR results of Pt/SiO2-Al2O3-IM-aged and Pt/SiO2-Al2O3-SEA-aged after the aging treatment. The band of CO adsorption on Pt/SiO2-Al2O3-SEA-aged was at 2071 cm–1, while the adsorption peak of Pt/SiO2-Al2O3-IM-aged was at 2097 cm–1, indicating that the Pt species of Pt/SiO2-Al2O3-SEA-aged still had a higher electron density, which could promote the NH3-SCO activity.

Figure 4

IR spectra of CO adsorption after pretreated by H2/N2 over Pt/SiO2-Al2O3-IM-fresh and Pt/SiO2-Al2O3-SEA-fresh (a) and Pt/SiO2-Al2O3-IM-aged and Pt/SiO2-Al2O3-SEA-aged (b).

TEM

Figure shows the TEM and Pt particle size distribution results. Figure a shows the morphology of Pt/SiO2-Al2O3-IM-fresh. The support material SiO2-Al2O3 presented two kinds of morphologies. One was filamentous, attributed to Al2O3; the other was flocculent, assigned to SiO2. For Pt/SiO2-Al2O3-IM-fresh, the particle size of Pt species loaded on SiO2 was larger than that on Al2O3. Furthermore, the Pt particle size of Pt/SiO2-Al2O3-IM-fresh was mainly distributed in the range of 2.5–13.5 nm, and the average particle size was 6.6 nm (Figure b). However, it is worth noting that an obvious bimodal distribution could be observed in the ranges of 2.5–7.5 nm and 7.5–13.5 nm. Combined with the results shown in Figure a, Pt species on SiO2 agglomerated more easily, resulting from the different metal–support interactions between Pt-Al2O3 and Pt-SiO2. Therefore, different particle sizes were observed on the SiO2/Al2O3 material. After hydrothermal aging, the Pt particle size of Pt/SiO2-Al2O3-IM-aged increased significantly, especially Pt species on SiO2, even increasing to over 40 nm (Figure c). The Pt species of Pt/SiO2-Al2O3-IM-aged were mainly distributed in the range of 2.5–47.5 nm, and the average particle size was 11.4 nm (Figure d). Similarly, the particle size of Pt species for Pt/SiO2-Al2O3-IM-aged was also bimodal in the ranges of 2.5–27.5 and 27.5–47.5 nm, indicating that the agglomeration degrees of Pt on Al2O3 and Pt on SiO2 were significantly different.

Figure 5

TEM images of Pt/SiO2-Al2O3-IM-fresh (a), Pt/SiO2-Al2O3-IM-aged (c), Pt/SiO2-Al2O3-SEA-fresh (e), and Pt/SiO2-Al2O3-SEA-aged (g). Platinum particle size distribution for Pt/SiO2-Al2O3-IM-fresh (b), Pt/SiO2-Al2O3-IM-aged (d), Pt/SiO2-Al2O3-SEA-fresh (f), and Pt/SiO2-Al2O3-SEA-aged (h).

As shown in Figure e,5f, Pt species of Pt/SiO2-Al2O3-SEA-fresh were uniformly distributed on Al2O3, and fewer Pt species were located on SiO2. Different from Pt/SiO2-Al2O3-IM-fresh, the Pt particle sizes of Pt/SiO2-Al2O3-SEA-fresh were in the range of 1.5–6.0 nm, and the average particle size was 2.7 nm, which was only 40% of the Pt average particle size of Pt/SiO2-Al2O3-IM-fresh. After hydrothermal aging, Pt species did not show obvious agglomeration and still showed uniform dispersion (Figure g), and the particle size of Pt species of Pt/SiO2-Al2O3-SEA-aged only increased to 3.2 nm, much smaller than that of Pt/SiO2-Al2O3-IM-aged (Figure h).

Ammonia Oxidation Performances

Figure a,6b shows the NH3 conversion results. NH3 conversion increased with increasing temperature. The T90 values (the temperature when NH3 conversion was 90%) of Pt/SiO2-Al2O3-IM-fresh and Pt/SiO2-Al2O3-SEA-fresh were 243 and 233 °C, respectively, and their T50 values (the temperature when NH3 conversion was 50%) were 228 and 216 °C, respectively, indicating that the activity of the catalyst prepared by the SEA method was significantly higher than that prepared by the impregnation process. After hydrothermal aging, T50 and T90 for Pt/SiO2-Al2O3-SEA-aged were 228 and 243 °C, respectively, much lower than those for the Pt/SiO2-Al2O3-IM-aged catalyst (250 and 271 °C, respectively). Compared with the catalytic activity before and after aging, the T50 of the catalyst prepared by the impregnation method increased by 22 °C, while the T50 of the catalyst produced by the SEA method just increased by 7 °C, indicating that the catalyst obtained by SEA showed higher activity and stability; even Pt/SiO2-Al2O3-SEA-aged exhibited higher activity than Pt/SiO2-Al2O3-IM-fresh. The results of N2 selectivity are shown in Figure c. There were slight differences between both the fresh catalysts and of the aged catalysts. As the temperature increased, the N2 selectivity decreased. The tendency of the byproducts N2O and NO are shown in Figure d,6e. As the temperature increased, the amount of the byproduct N2O was first increased and then decreased, and the largest amount of the byproduct N2O was obtained at about 300 °C. The amount of the byproduct NO increased with the increase of temperature; N2 selectivity showed an opposite tendency.

Figure 6

NH3 conversion (a), T50 and T90 of NH3 conversion (b), N2 selectivity (c), N2O formation (d), and NO formation (e) over Pt/SiO2-Al2O3-IM-fresh, Pt/SiO2-Al2O3-SEA-fresh, Pt/SiO2-Al2O3-IM-aged, and Pt/SiO2-Al2O3-SEA-aged catalysts.

Pt/SiO2-Al2O3-SEA-fresh with a higher surface area and larger acid content was beneficial to adsorbing a larger amount of NH3. In addition, the higher Pt dispersion of Pt/SiO2-Al2O3-SEA-fresh presented more active sites, which was related to the higher NH3-SCO activity than that of Pt/SiO2-Al2O3-IM-fresh. After hydrothermal aging, the Pt species of Pt/SiO2-Al2O3-IM-aged displayed extreme agglomeration, sharply decreasing its catalytic performance compared with Pt/SiO2-Al2O3-SEA-aged.

Conclusions

Pt/SiO2-Al2O3 catalysts were prepared with different methods. Pt/SiO2-Al2O3-SEA-fresh prepared by the strong electrostatic adsorption (SEA) method had higher dispersion and thermal stability due to PtCl62– and could be adsorbed selectively on the Al2O3 of SiO2-Al2O3, and SiO2 served as a barrier to inhibit Pt agglomeration. The Pt average particle size of Pt/SiO2-Al2O3-SEA-fresh was 2.7 nm and increased to only 3.2 nm after hydrothermal aging, which was significantly lower than those of Pt/SiO2-Al2O3-IM-fresh (6.6 nm) and Pt/SiO2-Al2O3-IM-aged (11.4 nm). Finally, the catalytic performances of NH3-SCO for the two types of catalysts were obviously different. The NH3-SCO catalytic activity of Pt/SiO2-Al2O3-SEA-fresh was superior, and its T50 and T90 were 216 and 233 °C, respectively. After hydrothermal aging, T50 and T90 were still as low as 223 and 239 °C, respectively, even lower than those of Pt/SiO2-Al2O3-IM-fresh.