Highly Dispersed Pt Species with Excellent Stability and Catalytic Performance by Reducing a Perovskite-Type Oxide Precursor for CO Oxidation

A new scheme for the preparation of highly dispersed precious metal catalysts is proposed in this work. Samples of LaCo1−xPtxO3/SiO2 (x = 0.03, 0.05, 0.07, 0.09, and 0.10) were prepared through a simple method of citrate acid complexation combined with impregnation. In a nanocrystallite of LaCo1−xPtxO3, ions of lanthanum, cobalt, and platinum are evenly mixed at the atomic level and confined within the nanocrystallite. In the reduction process, platinum ions were reduced and migrated onto the surface of the nanocrystallite, and the platinum should be highly dispersed owing to the even mixing of the platinum ions in the precursor. When x = 0.05 or lower, the highest dispersion of Pt could be achieved. The highly dispersed Pt is stable, because of the strong interaction between Pt atoms and the support. The catalysts were characterized by BET surface area, temperature-programmed reduction, X-ray diffraction, transmission electron microscopy, CO temperature-programmed desorption, and turnover frequency. Compared with general precious metal Pt catalysts, the LaCo0.95Pt0.05O3/SiO2 catalyst exhibited better activity for CO oxidation, and it maintained stability at a high temperature of 400 °C for 250 h with complete CO conversion.


Introduction
Supported precious metal catalysts are widely used in the industry for their very good catalytic performance. The size of metal nanoparticles (NPs) is a key element in terms of the catalytic activity. For general metal catalysts, only part of the metal atoms or ions serves as the active sites to play the catalytic role during the reaction. To improve the utilization of noble metals, higher distributions of the metal active sites are expected [1,2]. Recently, theoretical and experimental results have proved that highly dispersed catalysts have better catalytic activity and selectivity than common metal particles [3]. Particularly for precious metal catalysts, the highly dispersed catalysts can improve the atom catalytic efficiency substantially, thus reducing the cost. At the same time, the highly dispersed metallic atoms make it possible to study the mechanism of heterogeneous catalysis at the molecular level [4,5].
As mentioned above, highly dispersed catalysts have many advantages, but their preparation is a challenge.
The key problem is the high surface free energy of atoms, which promotes aggregation to form small clusters [6]. The reported preparation methods for highly dispersed catalysts include the following: Ramos-Fernandez et al. [7] used the metal organic framework to prepare PTA-MOF catalysts for CO oxidation and the hydrogenation of toluene. Watanabe et al. [8] prepared highly dispersed Pt + Ru alloy clusters through the co-deposition of platinum and ruthenium oxides at the atomic level for the electro-oxidation of methanol. In 2011, Qiao et al. [9] prepared a catalyst of single atom Pt/FeO x by a co-precipitation method for CO oxidation. A photochemical method was used to synthesize Pd/TiO 2 monatomic catalysts for styrene hydrogenation [10]. Hightemperature heat transfer atom trapping on polyhedral ceria and nanorods was adopted to synthesize thermally stable single-atom platinum-on-ceria catalysts for CO oxidation [11]. The methods above can only be applied to a narrow range, as they are expensive and of low-yield. Therefore, simple and feasible methods for the preparation are needed.
Perovskite-type oxides (PTOs) are mixed oxides with a cubic lattice structure of general formula ABO 3 , where rare earth elements are usually at the A-site positions and the smaller transition metals are located at the B-sites. The large range of tolerance factors (0.75 ≤ t ≤ 1.0) makes it possible to tolerate significant partial substitution. At the same time, oxygen vacancies caused by doping of perovskites with different species are helpful to the activity of many oxidation reactions. PTOs also have other advantages, such as lower cost and higher thermal stability than noble metals. Furthermore, they exhibit excellent partial substitution tolerance for precious metals (Pd, Rh, and Pt) and are capable of recycling the noble metal atoms in and out of the perovskite lattice in oxidizing and reducing conditions, which can prevent the aggregation of the metal atoms [12][13][14]. The specific surface area of a PTO is low, generally below 10 m 2 /g [15]. Loading a PTO on a support with high specific surface area is one strategy to overcome this problem.
In this article, a new scheme for preparing highly dispersed catalyst was proposed, as shown in Scheme 1. Taking advantage of the strong interaction between a PTO and precious metals (Pt, here), by supporting a PTO with high specific surface area on silica, it is possible to overcome their disadvantage of low specific surface area. A PTO of LaCo 0.95 Pt 0.05 O 3 was highly dispersed on SiO 2 , where the ions of Pt should be uniformly dispersed in the PTO lattice. In the reduction process, ions of Pt should be reduced and migrate onto the surface of the PTO to achieve high dispersions. The as-prepared catalyst showed high activity and very good stability to CO oxidation.

Materials
The catalysts were prepared by a citric acid complexation, combined with an incipient wetness impregnation method. The chemicals involved in the preparation of the catalysts were as follows: La(NO 3  Similarly, Pt/SiO 2 and LaCoPtO x /SiO 2 were prepared by the same method with a mass fraction of 0.5% for Pt, except that no citric acid or ethylene glycol was added during the preparation process. LaCoO 3 /SiO 2 was also prepared by the same method with a mass fraction of 13% for the perovskite.

Catalyst Characterization
Specific surface areas and hole structures were calculated by the BET method and the BJH model. The data came from a Micromeritics apparatus, model ASAP-2020, and were collected at −196 °C. Before the measurements, 200 mg of samples were pretreated for 2 h at 300 °C to remove absorbed air and moisture.
H 2 -temperature-programmed reduction (H 2 -TPR) experiments were performed on a Thermo-Finnigan TP-5079 which was equipped with a thermal conductivity detector. Samples of 150 mg were heated from 30 to 900 °C at a heating rate of 10 °C/min under a reduction gas mixture of 5 vol% H 2 /Ar with a flow rate of 25 cm 3 /min. It should be noted that 19.5 mg of LaCoO 3 and 130.5 mg of quartz sand were mixed for the TPR tests.
Transmission electron microscopy (TEM) measurements were performed on a Technai G 2 F20 transmission electron microscope. The Pt-containing samples were pretreated at 220 °C in hydrogen for 2 h, followed by grinding and dispersing in ethanol. The samples were loaded onto Cu grid for testing.
During CO temperature-programmed desorption (CO-TPD) measurements, the LaCo 1−x Pt x O 3 /SiO 2 catalysts were first reduced in H 2 at 220 °C for 2 h. The chemisorption of CO onto the catalysts was performed in a reactor at atmospheric pressure and 25 °C. The catalyst was outgassed and purged with helium of ultrahigh purity grade (99.9999%, 30 cm 3 /min) at 400 °C for 0.5 h. The sample was purged at 50 °C for 3 h and allowed to adsorb CO under CO atmosphere for 2 h. Then, TPD measurements were completed at a constant heating rate (5 °C/min) from 50 to 500 °C under helium flow (30 cm 3 /min) by an SRS RGA300 quadrupole mass spectrometer. The Pt dispersion (D) was calculated based on the following equation [16]: where M CO is the amount of CO adsorption in μmol; SF is the stoichiometric factor, i.e., the Pt/CO ratio in chemisorption, which is 1 in this study [17]; FW m is the formula weight of the metal Pt; WF m is the weight fraction of the metal Pt in the catalyst; and m cat is the mass of the catalyst.
Based on the Pt dispersion in the catalyst, the turnover frequency (TOF) value at 145 °C was calculated according to the following equation [18]: where X CO is the CO conversion at 145 °C; F CO is the flow rate of CO in mol/s; m cat is the mass of catalyst; X Pt is the Pt loading in the catalyst; D Pt is the dispersion of Pt which was calculated based on the CO-TPD results; and M Pt is the molar weight of Pt (195.1 g/mol).

Catalytic Performance Test for CO Oxidation
The catalytic activity measurements were performed on a fixed bed micro-reactor at atmospheric pressure. Firstly, a 100 mg sample was reduced at 220 °C for 2 h in H 2 atmosphere with a heating rate of 5 °C/min. The feed gases contained 1% CO, 1% O 2 , and 98 vol% N 2 and reached the flow rate of 40 mL/min in total. The catalyst sample was well distributed from 40 to 60 mesh. A thermocouple was inserted into the catalyst bed and used to measure the reaction temperature. The reaction exhaust was analyzed by an online gas chromatograph, SP-3420, which was equipped with a TCD and a 5 A molecular sieve column. The performance of samples was determined by CO conversion, which was defined as follows [19]: where [CO] in is the concentration of CO in the feed gases and [CO] out is the concentration of CO in the effluent stream.

N 2 Adsorption and Desorption Isotherms
Nitrogen adsorption-desorption isotherms and the BJH pore size distribution of SiO 2 and LaCo 0.95 Pt 0.05 O 3 /SiO 2 are shown in Fig. 1. The isotherms belong to Type IV under IUPAC classifications [20], which closely relates to the properties of a mesoporous structure. As shown in Fig. 1, the most probable size and the specific surface area declined slightly from 9.45 nm and 369.8 m 2 /g for the SiO 2 support to 8.76 nm and 326.6 m 2 /g for LaCo 0.95 Pt 0.05 O 3 /SiO 2 , respectively, which is likely caused by the blockage of some mesopores by the NPs of the PTO. The average pore size (12 nm) is larger than the size of the PTO (below 9 nm), which can be seen in Fig. 4a-c, meaning that the NPs of the PTO can enter the mesopores. The specific surface area of the SiO 2 support is large enough to highly disperse the PTO of LaCo 0.95 Pt 0.05 O 3 when the amount of the PTO loaded is 13 wt%.

Temperature-Programmed Reduction (TPR)
The TPR profiles are shown in Fig. 2. The reduction curves of LaCoO 3 have been well studied, where the peak at the low temperature of approximately 400 °C is attributed to the reduction of Co 3+ to Co 2+ , and the peak at high temperature of approximately 570 °C is attributed to the reduction of Co 2+ to metallic Co 0 [21,22]. Compared with LaCoO 3 , for LaCoO 3 /SiO 2 , the peak at around 570 °C moved toward a higher temperature, which is due to the effect of the carrier. Compared with LaCoO 3 /SiO 2 , the TPR peaks for LaCo 1−x Pt x O 3 /SiO 2 shifted back to lower temperatures, and the magnitude of the shift increased with the increase in Pt content in the catalyst, which agrees with the shift due to the hydrogen spillover from the sites of Pt as pointed out in previous works [23,24]. The most important information is that the outlines of the TPR profiles of LaCo  [3,17,19]. Additionally, the content of Pt in LaCo 1−x Pt x O 3 /SiO 2 is small; therefore, no reduction peaks corresponding to the ions of Pt were detected. The TPR results indicate that after reduction, LaCo 1−x Pt x O 3 /SiO 2 was converted to Pt/LaCoO 3 /SiO 2 , which is further confirmed by the following XRD and TEM results. Figure 3A shows XRD patterns of the unsupported PTOs of LaCo 1−x Pt x O 3 . The characteristic diffraction peaks of the perovskite phase at 2θ = 32.9, 33.3, 47.5, and 59.0° are clearly seen, and no peaks corresponding to other impurity phases could be observed, suggesting that the PTOs were synthesized well. From the enlarged pattern of LaCo 1−x Pt x O 3 in Fig. 3B, it is seen that the diffraction peaks of the perovskite phase shifted to smaller values of 2θ, which means that the crystal lattice spacing of the perovskite phase increased. The shift increased with the increase in the Pt content in the PTOs. This should be indicative of the imbedding of Pt ions into the lattice of PTO. The size of the Pt ion is larger than the size of the Co ion, leading to the increase in the lattice spacing. Since no diffraction peak corresponding to platinum or platinum oxide was detected, entering of Pt ions into the lattice of PTO is supported.

XRD Results
The XRD patterns of the supported catalysts of LaCo 0.95 Pt 0.05 O 3 /SiO 2 and SiO 2 are shown in Fig. 3C. Besides the broad diffraction peaks corresponding to SiO 2 , weak diffraction peaks corresponding to the perovskite phase could be detected, indicating that the PTOs were successfully supported on silica. The peak intensity of PTO increased with the increase in PTO loading amount, and when the loading amount of PTOs is 13 wt%, diffraction peaks of perovskite could hardly be detected, suggesting that the PTOs were highly dispersed, which is consistent with the TEM results in Fig. 4. After reduction at 220 °C, the NPs with an interplanar crystal spacing of 2.72 Å, consistent with crystal spacing of plane [110] for LaCoO 3 , can be found in Fig. 4d. The slight decrease in the lattice spacing from 2.74 Å in Fig. 4b to 2.72 Å here would be attributed to platinum ions' escaping from the perovskite lattice. At the same time, no platinum lattices can be detected for LaCo 0.95 Pt 0.05 O 3 /SiO 2 , indicating that platinum is highly dispersed on the support. It agrees well with XRD results and our assumption in Scheme 1.

TEM Images
The representative images of LaCo 0.90 Pt 0.10 O 3 /SiO 2 (Pt loading is 1 wt%) reduced at 220 °C are shown in Fig. 4e, f; the crystallites of LaCoO 3 can be seen. The image in Fig. 4f shows a typical metallic Pt NP. The crystal lattice spacing is 2.27 Å, which is in accordance with the crystalline plane [111] of platinum metal. The result indicates that too much Pt added resulted in the comparatively poor dispersion of Pt atoms.

Catalytic Performance
To study the catalytic performance of the supported Pt catalysts, CO oxidation was chosen as the probe reaction, which is an extensively used probe and a very important reaction in environmental protection [25]. CO conversions over the catalysts with the same platinum content of 0.5 wt% are shown in Fig. 5a. The complete CO oxidation temperatures are 240, 260, and 320 °C for LaCo 0.95 Pt 0.05 O 3 /SiO 2 , LaCoPtO x /SiO 2 , and Pt/SiO 2 , respectively. Meanwhile, the highest CO conversion is about 90% for LaCoO 3 /SiO 2 at a temperature of 480 °C. It indicates that the catalyst prepared according to Scheme 1 is highly active.
Considering the similar synthesis process and perovskite content between LaCo 0.95 Pt 0.05 O 3 /SiO 2 and LaCoO 3 / SiO 2 , the huge difference in activity comes from the small amount of metallic platinum. Reducing LaCo 1−x Pt x O 3 /SiO 2 resulted in Pt/LaCoO 3 /SiO 2 , as stated in the discussion about TPR results, over which CO is adsorbed on the highly dispersed platinum and would react with the oxygen species adsorbed in the perovskite vacancies [26]. The mechanism on LaCoO 3 /SiO 2 is generally proposed that oxygen species, adsorbed at the oxygen vacancies on LaCoO 3 , react with CO adsorbed on Co 3+ adjacent to the oxygen vacancies to generate CO 2 [26]. Platinum atoms are much more active than Co 3+ , therefore leading to the much better activity of LaCo 1−x Pt x O 3 /SiO 2 . For Pt/SiO 2 , CO adsorbed on Pt atoms reacts with oxygen activated on the Pt surface [27], and the platinum species loaded with the regular impregnation method at a calcination temperature of 700 °C is not highly dispersed, thus showing obviously inferior activity. As for LaCoPtO x /SiO 2 , a PTO was not formed, which means that Pt is not in close contact with the oxide of cobalt, and the dispersion of Pt atoms would not be very high;   Figure 5b shows the variations of CO conversion over the series of catalysts of LaCo 1−x Pt x O 3 /SiO 2 with the reaction temperature. The complete CO conversion temperatures are 280, 240, 230, 215, and 205 °C over the catalysts, when x is 0.03, 0.05, 0.07, 0.09, and 0.10, respectively, which increases with the increase in the amount of platinum present. This is in the expectation, as a higher content of platinum would provide more active sites. The TOFs, CO converted on per atom of platinum per second, decreased slightly with the increase in platinum content as listed in Table 1, suggesting that elevating the dispersion could improve the utilization efficiency of the noble metal platinum.
As shown in Fig. 6, LaCo 0.95 Pt 0.05 O 3 /SiO 2 shows excellent stability at the high temperature of 400 °C during 240 h testing, as complete CO conversion was maintained from the start to the finish. At the same time, the dispersion of Pt atoms slightly decreased from 88.3 to 85.1% after the stability test, and no Pt NPs could be observed from TEM images.
It has been reported that under oxidation conditions, Pt, Pd, or Rh atoms on the surface of a PTO could enter into the lattice of the PTO support, thus acting as the lattice ions of the PTO. Under reduction conditions, the Pt/Pd/Rh ions in a PTO would be reduced to metal atoms and migrate out onto the surface of the PTO. The "entering into" and "migrating out" may cycle under certain atmospheres, meaning that Pt/ Pd/Rh species may be cycling between the surface and lattice of a PTO [13,14,34,35]. This cycling might function for LaCo 0.95 Pt 0.05 O 3 /SiO 2 in the CO oxidation reaction process, considering that the reaction gas mixture contains CO/ O 2 at a ratio of 1:1. Therefore, the highly dispersed platinum atoms showed excellent sintering resistance ability. At least, the Pt species could interact with the support of LaCoO 3 , and the interaction would prevent the Pt atoms from moving, therefore showing excellent sintering resistance ability. To combine the above results, the process shown in Scheme 1 is analyzed as follows. A PTO of LaCo 1−x Pt x O 3 was highly dispersed on SiO 2 with a high surface area, and the platinum ions should be uniformly dispersed in the PTO lattice. In the reduction process, platinum ions should be reduced and migrate out from the lattice onto the surface of LaCoO 3 . In the lattice of LaCo 1−x Pt x O 3 , platinum ions were uniformly dispersed; therefore, in the reduction process, platinum ions should migrate out evenly and highly disperse on the surface of LaCoO 3 . When the content of platinum in LaCo 1−x Pt x O 3 is low, the platinum atoms (migrated from the lattice of LaCo 1−x Pt x O 3 ) would be highly dispersed. When x in LaCo 0.95 Pt 0.05 O 3 /SiO 2 is 0.05 or below, the characterization results suggest that the content of platinum is low enough, as when x = 0.03 and 0.05, the dispersions of Pt are 92.5 and 88.3%, respectively, and no Pt NPs could be found by using high-resolution TEM. It is accepted that Pt atoms can activate CO and the oxygen vacancies can activate oxygen, and the two activated species therefore bond to generate CO 2 , leading to the high activity for CO oxidation.

Conclusions
Highly dispersed Pt catalysts can be prepared by using PTOs as the precursor via a simple method of citrate acid complexation combined with impregnation. Specifically, in LaCo 0.95 Pt 0.05 O 3 /SiO 2 , the PTO of LaCo 0.95 Pt 0.05 O 3 could be highly dispersed on SiO 2 supports, and after reduction, Pt/ LaCoO 3 /SiO 2 was generated, where the platinum was highly dispersed, since the platinum ions were uniformly dispersed in the precursor of LaCo 0.95 Pt 0.05 O 3 . It is interesting to note that the highly dispersed Pt exhibited excellent resistance to sintering, being attributed to the confinement of Pt atoms by LaCoO 3 on SiO 2 . Thus, the prepared catalyst showed high activity and excellent stability for CO oxidation.