Highly dispersed Pt species anchored onto NH₂-Ce-MOFs and their derived mesoporous catalysts for CO oxidation

Abstract

Simultaneously maximizing the dispersion of noble metals and demonstrating optimal activity are of significant importance for designing stable metal catalysts. In this study, highly dispersed ultrafine platinum (Pt) particles with a size of <1.5 nm anchored onto a mesoporous CeO₂ structure have been synthesized by coordinating Pt ions with amino groups in NH₂-Ce-MOFs, followed by high-temperature calcination. It was found that the presence of –NH₂ groups in Ce-MOFs played a crucial role in anchoring Pt species with high dispersion on the MOF framework. Interestingly, the anchored Pt species were beneficial for the formation of Ce–Pt sites during the conversion from Ce-BDC to CeO₂. As a result, the as-prepared catalysts held dense surface peroxo species, responsible for boosting CO oxidation at low temperatures.

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Introduction

Platinum/ceria (Pt/CeO₂) nanocomposites have been extensively studied in various catalytic reactions because of the high oxygen storage capacity of CeO₂ and excellent intrinsic catalytic activity of Pt species, as well as their synergistic effect in catalytic processes.^1–10 To date, numerous researchers have developed various methods to prepare Pt/CeO₂ catalysts, such as the impregnation method,^11,12 deposition-precipitation method,¹³ and co-precipitation method.¹⁴ Although many efforts have been devoted to regulating the structures/compositions to improve the Pt/CeO₂ catalytic activities, the development of an ideal catalyst with unique features, including the mesoporous structure and well-dispersed Pt species, is still challenging and important for optimizing their catalytic performance.

Recently, how to maximize the utilization of noble metals in catalysts by means of ultrafine dispersion (even atomic dispersion) has attracted increasing attention.^15,16 Unfortunately, with the decrease of the noble metal particle size, the drastically increased surface free energy will make the highly-dispersed small metal particles or clusters very energetic and active, thus leading to their aggregation/sintering which may reduce their catalytic activity to some extent. To further address this issue, various strategies have been proposed to stabilize ultrafine noble metal particles. For example, Pd nanoparticles were stabilized through a rattle-type Pd/ZnO@hollow ZIF-8 structure, where the outer ZIF-8 shell protected the catalytically active Pd nanoparticles from poisoning and leaching, giving rise to excellent stability.¹⁷ Cheng et al. prepared Pd@CeO₂ core–shell nanotube catalysts using multi-walled carbon nanotubes as a sacrificial template, in which the noble metal nanoparticles were preserved at high dispersion due to physical confinement and interaction confinement, even after being treated at a high temperature.¹⁸ Our group has obtained ultra-small Pt particles embedded into CeO₂ nanotubes by an interfacial reaction between Ce(OH)CO₃ nanorods and NaOH. During the interfacial reaction, the negatively charged Pt species electrostatically attracted to gradually form Ce(OH)₃ were responsible for the homogeneous mixture of Pt and CeO₂.¹⁹ Considering that the mesoporous structure is beneficial for the diffusion of reactants, it is thus more promising to explore alternative and effective strategies to prepare ultrafine Pt particles anchored onto the mesoporous structured CeO₂.

Metal–organic frameworks (MOFs), with rich porosities, diverse topological structures and multiple possibilities being functionalized, have attracted much attention for catalyst preparation.^20–24 Upon calcination, the MOFs were demonstrated to be a good precursor candidate for the synthesis of metal oxides, in which the MOF-derived metal oxides inherit high porosity and large surface area. Importantly, with the assistance of the uncoordinated atoms in MOFs, noble metal ions could be anchored onto MOFs for generating a high dispersion of noble metal nanoparticles after reduction or calcination. For instance, the uncoordinated N atoms present in the zirconium-porphyrinic MOFs were found to coordinate with noble metal (Ir, Pt, Ru, Au, and Pd) ions; the single noble metal atom was dispersed evenly on the MOF support after suitable reduction.²⁵ The Li group obtained a N-doped carbon supported single Ru atom catalyst by the pyrolysis of Ru³⁺/Zr-MOFs. They found that the free amino (–NH₂) group at the MOF skeleton coordinating with Ru³⁺ played a key role in the formation of a single Ru atom.²⁶ Although Pt/CeO₂ composites and the noble metal particles supported on MOF-derived metal oxides have been reported, catalysts with ultrafine Pt particles supported on CeO₂ derived from Ce-MOFs have not been reported yet. Therefore, it is desirable to extend this strategy to design ultrafine Pt particles that could anchor onto mesoporous CeO₂, particularly assisted by –NH₂ functionalized Ce-MOFs.

In this work, we have prepared –NH₂ functionalized Ce-BDC (BDC = 1,4-benzenedicarboxylate) in order to anchor Pt species onto NH₂-Ce-BDC by the coordination of Pt with –NH₂. After pyrolysis and hydrogen reduction, the catalyst with ultrafine Pt nanoparticles anchored onto porous CeO₂ was obtained successfully. The high dispersion of ultrafine Pt nanoparticles was verified by HAADF-STEM and XPS characterization. Because of the higher dispersion of Pt species and the existence of Ce–Pt sites and the resultant surface peroxo species, the as-prepared ultrafine Pt/CeO₂ catalyst exhibits superior catalytic activity (e.g. T₅₀ = 60 °C) in comparison with the one prepared from NH₂-Ce-BDC by the impregnation method. The relationship between the structure and the catalytic performance of Pt/CeO₂ nanostructures and the mechanistic fundamentals will be investigated and discussed in detail.

Experimental section

Materials

All chemicals used in this work were of analytical grade, and were used without further purification treatment. Sodium hydroxide (NaOH), hexachloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O) and cerium chloride heptahydrate (CeCl₃·7H₂O) were purchased from Sinopharm Chemical Reagent Co. Ltd. 2-Aminotropic acid (NH₂-BDC) and terephthalic acid (H₂BDC) were purchased from Macklin.

Synthesis of NH₂-Ce-BDC

The synthesis of NH₂-Ce-BDC was carried out according to the method reported in the literature, with minor modification.²⁷ In a typical procedure, 550 mg of NH₂-BDC was dispersed in 110 mL of deionized water, followed by adjusting the pH value to 5–6 using 0.1 M NaOH aqueous solution. Then 484 mg of CeCl₃·7H₂O was added to the solution, which was kept for 40 min after shaking for several seconds. Finally, the precipitate of NH₂-Ce-BDC was obtained after centrifugation and washing in sequence, and drying at 70 °C.

Introducing Pt into NH₂-Ce-BDC (or Ce-BDC)

100 mg of NH₂-Ce-BDC (or Ce-BDC) was dispersed in 20 mL of deionized water and ultrasonicated for 3–5 min to obtain a homogeneous solution, and then 0.18 mL of H₂PtCl₆·6H₂O (1 g·100 mL⁻¹) was added into the solution. The solid samples were separated by centrifugation after reaction at 80 °C for 12 h. Finally, Pt ions anchored onto NH₂-Ce-BDC (or Ce-BDC) were prepared, followed by washing three times with water, and drying at 80 °C overnight before any further characterization.

Synthesis of N-Pt/CeO₂

The as-prepared Pt-decorated NH₂-Ce-BDC was placed in a muffle furnace and calcined at 500 °C for 2 h (heating rate: 5 °C min⁻¹) under air. Then the sample was reduced under a H₂ atmosphere at 300 °C for 2 h (heating rate: 5 °C min⁻¹). For convenience, this sample was denoted as N-Pt/CeO₂.

Synthesis of IM-Pt/CeO₂

The IM-Pt/CeO₂ catalyst was prepared by the impregnation method. The loading amount of Pt was determined by the N-Pt/CeO₂ sample. Specifically, the as-prepared NH₂-Ce-BDC sample was calcined at 500 °C for 2 h (heating rate: 5 °C min⁻¹) to convert into CeO₂. Then, 200 mg of the as-prepared CeO₂ was mixed with 5 mL of water, and 0.79 mL of H₂PtCl₆·6H₂O (1 g·100 mL⁻¹) was added, followed by vigorous stirring to obtain a homogeneous mixture. After drying at 80 °C for a few hours, the sample was then calcined at 300 °C for 2 h (heating rate: 5 °C min⁻¹) under air, followed by reduction with H₂ (reduction conditions: 300 °C, 2 h, 5 °C min⁻¹). For convenience, this sample was denoted as IM-Pt/CeO₂.

Sample characterization

The as-prepared samples were characterized by X-ray powder diffraction (XRD) using a Japan Rigaku D/Max-RB rotating anode X-ray diffractometer (Cu Kα radiation, λ = 1.54178 Å). The structure and morphology of the samples were studied using a scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan), a transmission electron microscope (JEOL, JEM 2100F) equipped with an annular dark-field detector and an energy dispersive X-ray spectrometer (EDS). The surface chemical valence states of the samples were characterized using a X-ray photoelectron spectrometer (XPS) with Al Kα radiation. The specific surface area was measured by the Brunauer–Emmett–Teller (BET) method, using a Micromeritics Tristar II 3020 instrument (test temperature: −196 °C). Before the BET test, all the samples were degassed at 150 °C for over 8 h (N₂ atmosphere). The pore size distribution was determined by the BJH method using the N₂ adsorption data. Fourier transform infrared (FT-IR) spectroscopy was used to analyze the samples, and the analysis was carried out using pressed KBr disks in the range of 4000–500 cm⁻¹. Raman spectroscopy measurements were performed using a LabRAM HR800 spectrometer equipped with an excitation laser of 532 nm. The loading of Pt in the samples was determined by ICP-MS using a Thermo Scientific XSeries-2 system. The in situ DRIFTS characterization was performed on a Bruker Tensor II FT-IR spectrometer. A CaF₂ window was selected as the diffuse reflection cell of the in situ diffuse infrared spectroscopy system. The in situ DRIFTS characterization was carried out at 110 °C; the gas flow rate was 50 mL min⁻¹. The CO adsorption was carried out under a 1 vol% CO/N₂ atmosphere. Before the test, the samples were activated at 300 °C for 30 min under a 10 vol% O₂/N₂ atmosphere.

Catalyst tests

Catalytic activity was studied using a continuous flow fixed-bed micro-reactor at atmospheric pressure. In a typical procedure, the system was first purged with high purity N₂ gas and then a gas mixture of CO/O₂/N₂ (1 : 10 : 89) was introduced into the reactor, which contained 50 mg of samples, at a flow rate of 50 mL min⁻¹ (weight hourly space velocity (WHSV): 60 000 mL g⁻¹ h⁻¹; temperature ramping rate: 2 °C min⁻¹). Gas samples were analyzed using an online infrared gas analyzer (Gasboard-3100, China Wuhan Cubic Co.).

Results and discussion

Firstly, XRD was used to characterize the phase and structure of the as-prepared samples. As shown in Fig. 1a, the diffraction peaks of the as-prepared NH₂-Ce-BDC are consistent with those reported in the literature,²⁷ suggesting the successful preparation of NH₂-Ce-BDC. After the introduction of Pt ions, there is no obvious effect on the structure of NH₂-Ce-BDC. Upon calcination at 500 °C, as expected, the NH₂-Ce-BDC was converted entirely into CeO₂, as shown in Fig. 1b, and its diffraction peaks are all consistent with the face-centered cubic phase (JCPDS no. 34-0394). However, it was hard to detect the diffraction peaks of Pt in both the N-Pt/CeO₂ and IM-Pt/CeO₂ samples, probably due to the poor crystallinity, small crystal size and/or low loading amount of Pt species.

Fig. 1 XRD patterns of (a) NH₂-Ce-BDC and Pt-decorated NH₂-Ce-BDC, and (b) N-Pt/CeO₂ and IM-Pt/CeO₂ samples.

The size, morphology and surface structure of NH₂-Ce-BDC, N-Pt/CeO₂ and IM-Pt/CeO₂ were characterized by SEM measurement. As shown in Fig. 2a and d, most of the NH₂-Ce-BDC particles exhibit a regular cubic shape with a relatively smooth surface. After incorporation of Pt ions and calcination at 500 °C for 2 h, both the obtained N-Pt/CeO₂ and IM-Pt/CeO₂ samples still maintained the cubic morphology of NH₂-Ce-BDC (Fig. 2b, c, e and f). This result was also confirmed by TEM characterization (Fig. S1†). Impressively, a significant porous structure can be observed on the surface of these two samples. It is generally recognized that the porous structure was caused by the removal of organic ligands and the retention of the initial pore structure in NH₂-Ce-BDC during the calcination process.

Fig. 2 SEM images of the samples: (a and d) NH₂-Ce-BDC, (b and e) N-Pt/CeO₂, and (c and f) IM-Pt/CeO₂.

FT-IR characterization was performed to investigate whether there exist an interaction between –NH₂ and Pt. According to the literature, the peaks located at ∼3436 and ∼3360 cm⁻¹ in the FT-IR spectra could be assigned to the stretching vibration of the –NH₂ group, while the peaks at ∼1260 and ∼1624 cm⁻¹ were assigned to the C–N stretching vibration and N–H scissoring vibration, respectively.^28–31 The peaks centred at ∼1541 and ∼1380 cm⁻¹ could be ascribed to carbonyl asymmetric and symmetric stretching vibrations, respectively.^29,32,33 As shown in Fig. 3a and b, the appearance of the FT-IR characteristic peaks of the C–N stretching vibration (1256 cm⁻¹) and the N–H scissoring vibration (1624 cm⁻¹) in NH₂-Ce-BDC is a good proof of the existence of amino groups, while these vibration peaks do not exist in Ce-BDC.³⁴ After introducing Pt ions, the peak intensities of the C–N stretching vibration and the N–H scissoring vibration in NH₂-Ce-BDC decreased slightly. This peak intensity change usually implies an interaction between Pt and –NH₂.³⁴ Although the peak intensity change is not obvious due to the low Pt loading, the new peak at 1653 cm⁻¹ split from the N–H scissoring vibration is another evidence of the interaction between Pt and –NH₂ (Fig. S2a†). In comparison, under the same reaction conditions, the FT-IR spectrum of Ce-BDC, without the –NH₂ group, does not change before and after introducing Pt ions. Moreover, as shown in the DRIFT spectrum (Fig. S2b†), after introducing Pt ions, the intensities of both the N–H asymmetric and symmetric stretching vibration peaks changed obviously, along with a red shift from 3481 and 3366 cm⁻¹ assigned to NH₂-Ce-BDC to 3478 and 3365 cm⁻¹ assigned to Pt-NH₂-Ce-BDC. These results confirmed that there is a coordination between –NH₂ and Pt ions for Pt-NH₂-Ce-BDC.³⁴ After calcination, NH₂-Ce-BDC was transformed into ceria, accompanied by a significant change in the FT-IR spectrum. The introduction of Pt had no effect on the FT-IR spectrum of the ceria derived from NH₂-Ce-BDC (Fig. S3†). The color of the N-Pt/CeO₂ derived from Pt-NH₂-Ce-BDC became darker than the one derived from Pt-Ce-BDC (insets in a & b). ICP-MS analysis showed that the loading amount of Pt in the Pt/CeO₂ derived from the Pt-NH₂-Ce-BDC and Pt-Ce-BDC samples is 1.33 and 0.05 wt%, respectively. Clearly, the –NH₂ groups in NH₂-Ce-BDC acted as anchoring sites for Pt ions, and thus are highly beneficial for the introduction of Pt into the mesoporous CeO₂ nanostructure.

Fig. 3 FT-IR characterization of (a) NH₂-Ce-BDC and Pt-NH₂-Ce-BDC, and (b) Ce-BDC and Pt-Ce-BDC. The insets in (a) and (b) are the digital photographs of the Pt/CeO₂ derived from Pt-NH₂-Ce-BDC and Pt-Ce-BDC, respectively.

The as-prepared samples were further characterized by HAADF-STEM to determine the structure of the tested sample and Pt distribution. As shown in Fig. 4a and g, both the N-Pt/CeO₂ and IM-Pt/CeO₂ samples show a cubic-like morphology with a porous structure, well consistent with the observation in the SEM images. Interestingly, the sensitivity to atomic number Z-contrast of the magnified HAADF-STEM image revealed that there are a number of bright spots in the N-Pt/CeO₂ sample. These highly dispersed bright spots with a size of <1.5 nm could be ascribed to Pt particles (Fig. 4b). In comparison, the size of Pt particles in the IM-Pt/CeO₂ sample is between 2.0 nm and 3.5 nm (Fig. 4h). Although the Pt particles are not highly uniform in these two samples, the size of Pt in IM-Pt/CeO₂ is obviously larger than that of Pt in N-Pt/CeO₂. The interlayer spacings shown in the HRTEM image were found to be ∼0.31 nm and ∼0.27 nm, which could be indexed to the (111) and (200) crystal planes of CeO₂ (Fig. 4c and i), respectively. No lattice spacings corresponding to Pt particles were found, indicating their small particle size and/or low crystallinity. Through the elemental mapping of these two samples (Fig. 4d–f and j–l), it can be found that the Pt, Ce and O elements in these two samples were uniformly distributed. The ultra-small size and high dispersion of Pt particles in the N-Pt/CeO₂ sample are expected to be favorable for catalytic reactions.

Fig. 4 HAADF-STEM (a and b), HRTEM (c), and elemental-mapping (d–f) images of N-Pt/CeO₂, and the images (g–l) corresponding to IM-Pt/CeO₂.

To study the catalytic performance of the as-prepared catalysts, the CO oxidation reaction was chosen as a model reaction to evaluate the performance of the as-prepared N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts. The ICP-MS results showed that the loading of Pt in the IM-Pt/CeO₂ and N-Pt/CeO₂ catalysts was around 1.39 wt% and 1.33 wt%, respectively. As shown in Fig. 5a, the catalytic activity of N-Pt/CeO₂ is superior to that of IM-Pt/CeO₂. Specifically, the CO conversion reaches 100% at ∼110 °C for the N-Pt/CeO₂ catalyst, while the CO conversion is only ∼2% at the same temperature for the IM-Pt/CeO₂ catalyst. Even when the reaction temperature was increased to 250 °C, the CO conversion for the IM-Pt/CeO₂ catalyst is less than 50%. In addition, the N-Pt/CeO₂ catalyst shows excellent stability, evidenced by the fact that the catalytic activity still remains 100% conversion at 110 °C after 5 cycles (Fig. 5b). It is clear that the catalytic activity toward CO oxidation is highly related to O₂ concentration. To further confirm the advantageous structure of N-Pt/CeO₂, we tested the CO oxidation experiment at low O₂ concentrations (3% and 5% O₂). As shown in Fig. S4a,† even if the O₂ concentration was reduced to 3%, our catalyst still exhibits good catalytic performance with T₅₀ < 80 °C. At the same time, the activation energy (E_a) and reaction rate of the two catalysts for the CO oxidation reaction were calculated according to the Arrhenius equation. According to the calculation results (Fig. S4b†), the N-Pt/CeO₂ catalyst has a lower activation energy and a higher reaction rate (N-Pt/CeO₂: E_a = 56.12 kJ mol⁻¹, R(100 °C) = 0.0936 mol_CO mol_Pt⁻¹ s⁻¹; IM-Pt/CeO₂: E_a = 67.09 kJ mol⁻¹, R(100 °C) = 0.0022 mol_CO mol_Pt⁻¹ s⁻¹). Compared with the reported catalysts, the N-Pt/CeO₂ catalyst synthesized in this work still has advantages in the catalytic performance of the CO oxidation reaction (Table S1†).

Fig. 5 (a) CO conversion as a function of temperature for the N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts, and (b) catalytic cycles of N-Pt/CeO₂ (CO/O₂/N₂ (1 : 10 : 89), total flow rate: 50 mL min⁻¹).

It is well known that the specific surface area and pore size distribution of the catalyst have an important effect on its catalytic performance. The BET surface area was calculated and compared in this work. Based on the N₂ adsorption–desorption isotherms (Fig. 6a), the BET surface area of pure CeO₂ derived from NH₂-Ce-BDC was calculated to be about 59.8 m² g⁻¹. In contrast, upon the incorporation with Pt, the BET surface areas of N-Pt/CeO₂ and IM-Pt/CeO₂ are 63.7 and 57.5 m² g⁻¹, respectively. In addition, there is no significant difference in the pore size and size distribution (Fig. 6b). This is reasonable since both CeO₂ supports are derived from the same NH₂-Ce-BDC precursor, and the low loading of Pt (∼1.3 wt%) has little effect on blocking the pores in CeO₂. Thus, the specific surface area and pore structure are not the main factors determining the difference in the catalytic performance of these two Pt/CeO₂ catalysts.

Fig. 6 (a) N₂ adsorption–desorption isotherms, and (b) pore size distribution curves for pure CeO₂ derived from the NH₂-Ce-BDC, N-Pt/CeO₂ and IM-Pt/CeO₂ samples.

XPS characterization was performed to study the surface composition and elemental chemical states of the catalysts. The data were calibrated with standard peaks of C 1s (284.6 eV). As shown in Fig. 7a, the peaks centred at v (882.2 eV), v′ (888.7 eV), v′′ (898.1 eV), u (900.7 eV), u′ (907.4 eV), and u′′ (916.6 eV) are characteristic of Ce(IV). No significant characteristic peak of Ce(III) was observed, indicating that Ce is mainly present in the form of Ce(IV) in these two catalysts. From the Pt 4f spectrum (Fig. 7b), the peaks of the N-Pt/CeO₂ catalyst can be deconvoluted into four contribution peaks. The peaks centred at ∼70.8 eV and ∼72.2 eV are characteristic peaks of Pt⁰ and Pt²⁺, respectively.^35–37 Clearly, a small amount of Pt⁰ and Pt²⁺ species is present in the N-Pt/CeO₂ catalyst, while only the Pt²⁺ species is present in the IM-Pt/CeO₂ catalyst, which is well consistent with the H₂-TPR result (Fig. S5†). Impressively, compared with the IM-Pt/CeO₂ catalyst, the N-Pt/CeO₂ catalyst has a weaker Pt 4f XPS signal, which further indicates the high dispersion of Pt species on the catalyst surface.³⁸ In the case of the O 1s spectrum (Fig. 7c), the peaks centred at ∼529.0 eV and ∼531.0 eV could be assigned to the lattice oxygen and surface oxygen species, respectively. Calculated from the peak area, the percentage of the surface oxygen species for the N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts is 25.9% and 19.5%, respectively.

Fig. 7 XPS spectra of the N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts: (a) Ce 3d, (b) Pt 4f and (c) O 1s, and (d) Raman spectra of the N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts. The inset is a partially enlarged picture of the Raman spectra.

Raman spectroscopy is another effective technique to study the structure of catalysts. As shown in Fig. 7d, the peak centred at ∼464 cm⁻¹ in both the samples could be attributed to the characteristic peak of the triply degenerate F_2g symmetric vibration (Ce–O–Ce stretching). Interestingly, the peaks centred at ∼262, ∼683 and ∼831 cm⁻¹ are present in N-Pt/CeO₂, while these peaks are negligible in the case of IM-Pt/CeO₂ (inset in Fig. 7d). These peaks are associated with the formation of Frenkel-type oxygen defect sites and surface peroxo (O₂²⁻) species, which is assigned to the O–O stretching vibration of dioxygen species bound to two-electron defects on the CeO₂ surface.³⁹ This demonstrates that the densities of coordinatively unsaturated Ce sites and surface peroxo species over N-Pt/CeO₂ are much higher than those in IM-Pt/CeO₂.⁴⁰

To reveal the active sites of the N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts in CO oxidation catalytic reactions, the in situ DRIFTS analysis was performed in a “CO-N₂” mode at 110 °C. According to the literature, the characteristic peaks of CO₂ were centred at ∼2362 and ∼2335 cm⁻¹. As shown in Fig. 8(a and b), the peak positioned at ∼2172 cm⁻¹ is attributed to CO adsorption on Ce⁴⁺ in the CO adsorption process. It is noted that there is one dominant vibrational mode at ∼2062 cm⁻¹ in N-Pt/CeO₂, and one at ∼2117 cm⁻¹ in IM-Pt/CeO₂. The peak located at ∼2062 cm⁻¹ was identified as the carbonyl stretching of CO linearly occupied on the atop geometry of the Pt atom in the Pt–Ce structure,⁴¹ while the peak at ∼2117 cm⁻¹ could be assigned to CO adsorbed on ionic Pt species.⁴² Both dominant peaks increase with the prolongation of time, although their intensities are different. Impressively, the signal of CO₂ corresponding to the peaks at ∼2362 and ∼2335 cm⁻¹ appeared. The increase of the CO₂ signal could be attributed to the direct reaction between CO and the surface active oxygen species in the N-Pt/CeO₂ catalyst via the well-known Mars–van Krevelen mechanism. Of note, the N-Pt/CeO₂ catalyst presents a stronger intensity than IM-Pt/CeO₂ at the maximum CO₂ signal, suggesting a better CO activation ability of N-Pt/CeO₂ than that of IM-Pt/CeO₂. Upon purging with N₂, the adsorption intensity of CO on the Pt species in both the catalysts decreased with time (Fig. 8c and d). However, for the peak at ∼2117 cm⁻¹ in IM-Pt/CeO₂, there is not much change in its intensity after CO was stopped and N₂ was purged, demonstrating that CO is strongly bound to ionic Pt, consistent with previous studies.^41,42

Fig. 8 In situ DRIFT spectra of the N-Pt/CeO₂ and IM-Pt/CeO₂ catalysts. CO adsorption and N₂ purging process of catalysts: (a and c) N-Pt/CeO₂ and (b and d) IM-Pt/CeO₂, respectively.

Based on the above-mentioned Raman and XPS results and in situ DRIFTS analysis, we proposed that the Pt species anchored onto Ce-BDC may be beneficial for the formation of a strong interaction between Ce and Pt during the conversion from Ce-BDC to CeO₂. The Pt–Ce sites in the N-Pt/CeO₂ catalyst induce a relatively high density of the surface peroxo species, which are more oxidative than molecular O₂, and hence boost CO oxidation under the reported conditions. However, for the IM-Pt/CeO₂ catalyst, the strongly bound CO likely blocked the access of molecular O₂ and made IM-Pt/CeO₂ inactive.

Conclusions

In summary, we have successfully prepared CeO₂ supported ultrafine Pt particles with a size of <1.5 nm. The newly developed method involves the introduction of Pt species into NH₂-modified Ce-MOF through the coordination between –NH₂ and Pt, followed by a thermal decomposition process. It was found that the presence of –NH₂ in Ce-MOF played a key role in the introduction and dispersion of Pt species. Combining various characterization techniques, we propose that the N-Pt/CeO₂ catalysts hold high density surface peroxo species, which may be the main reason for increasing CO oxidation at a low temperature. In the case of IM-Pt/CeO₂, the CO strongly adsorbed on ionic Pt blocks the access of substrates, leading to its inferior catalytic performance. This work opens a new path to improve the dispersion of noble metals on CeO₂ by introducing noble metal ions in/onto the Ce-MOFs modified by functional groups for better catalytic performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21878121), the Natural Science Foundation of Shandong Province (Grant No. ZR2018MB004), and Shandong ShenNa Smart Advanced Materials Co. Ltd.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr05626j

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