Significantly enhanced CO oxidation activity induced by a change in the CO adsorption site on Pd nanoparticles covered with metal–organic frameworks

Yoshimasa Aoyama a, Hirokazu Kobayashi *ab, Tomokazu Yamamoto c, Takaaki Toriyama d, Syo Matsumura cde, Masaaki Haneda fg and Hiroshi Kitagawa *aeh
aDivision of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto, 606-8502, Japan. E-mail:
bPRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
cDepartment of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
dThe Ultramicroscopy Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
eINAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
fAdvanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu 507-0071, Japan
gFrontier Research Institute for Materials Science, Nagoya Institute of Technology, Gokiso-cho, Showaku, Nagoya 465-8555, Japan
hInstitute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

Received 21st January 2020 , Accepted 9th March 2020

First published on 9th March 2020

We report the significantly enhanced CO oxidation activity of Pd nanoparticles covered with [Zr6O4(OH)4(BDC)6] (UiO-66, BDC = 1,4-benzenedicarboxylate). The catalytic activity was much higher than those of Pd and Ru nanoparticles on ZrO2. The origin of the enhancement was suggested to be a change in the CO adsorption properties on Pd nanoparticles.

CO oxidation on metal is very important not only in fundamental science but also in a wide range of industrial fields, including automobiles and fuel cell systems.1 The Pt-group metals, such as Pd, Rh, Ru, and Pt, are representative CO oxidation catalysts.2–4 In particular, Pt and Pd have been investigated extensively and the CO adsorption properties and the CO oxidation mechanism are well understood.5 The CO adsorption properties, such as adsorption strength and site activity on a metal surface, are basically due to CO σ-donation to metal and metal π-back donation to CO.6 As the adsorption properties strongly depend on the electronic interaction between CO and the metal surface, attempts to develop useful CO oxidation catalysts have typically involved metal alloying7 with other elements and/or metal oxide support modification8 for metal–support interaction.

Metal–organic frameworks (MOFs), which consist of metal ions connected by organic bridging ligands to form a porous structure, have attracted much research interest due to their gas storage9,10 and separation abilities.11,12 Because of their high porosity and designability, MOFs have also received increasing interest as a functional support material for metal nanocatalysts. In particular, the hybrid structure in which metal nanoparticles (NPs) are covered with MOFs (NP@MOFs) has an advantage in fully utilizing the properties of both components.13–16 For example, Arshad Aijaz et al. immobilized Pd and Pt NPs into MIL-101 for CO oxidation catalysis.17 Gui-lin Zhuang et al. synthesized Pt NPs loaded in UiO-67 for CO oxidation reaction.18 So far, most studies on hybrid catalysts have utilized MOFs mainly as a platform for immobilizing small metal NPs to prevent aggregation of the NPs during the CO oxidation reaction.19–21 In addition, there have been a limited number of reports comparing the catalytic activity of NP@MOF with that of NPs on a metal oxide support such as Al2O3 or ZrO2.

Herein, we report on the significantly enhanced CO oxidation activity of Pd NPs covered with MOFs. The activity was much higher than that of Pd or Ru NPs on ZrO2, which are known as highly active CO oxidation catalysts. In situ Fourier transform infrared (FT-IR) spectroscopy measurements suggested that the enhancement in catalytic activity originated from a change in the CO adsorption site on the surface of Pd NPs induced by the MOF coating. These results indicated for the first time that a MOF coating could control the adsorption properties of reactants on metal NPs.

For the MOF coating on Pd NPs, we selected a Zr-based MOF [Zr6O4(OH)4(BDC)6] (UiO-66, BDC = 1,4-benzenedicarboxylate) because of its high porosity, large specific surface area, and high thermal stability.22 We synthesized the hybrid Pd@UiO-66 by a two-step synthesis. At first, Pd NPs were prepared using a chemical reduction method. An amount of 80 mL of aqueous Na2PdCl4 (1425 mg, 4.8 mmol) solution was added to 200 mL of an aqueous solution containing poly(N-vinyl-2-pyrrolidone) (PVP) (1050 mg, 9.5 mmol), L-ascorbic acid (1500 mg, 9.7 mmol), KBr (125 mg, 1.1 mmol), and KCl (4625 mg, 62.0 mmol). The mixture was heated at 80 °C under magnetic stirring for 3 h. After cooling to room temperature, the product was collected by centrifugation and washed twice with a mixture solution of water, acetone, and diethyl ether. For the preparation of Pd@UiO-66, we used zirconium propoxide as a precursor of UiO-66.23,24 In a typical synthesis of Pd@UiO-66, the obtained Pd NPs (30 mg) were mixed with zirconium propoxide (128.4 μL, 0.29 mmol), 1,4-benzenedicarboxylic acid (50 mg, 0.30 mmol) and acetic acid (6.5 mL) in dimethylformamide (DMF) (41 mL). The mixture was placed in an oven and heated at 65 °C for 15 h. The product was collected by centrifugation and washed with DMF twice, followed by soaking in methanol for 1 day three times. For reference, ZrO2 supported Pd NPs (Pd/ZrO2) were also prepared (see experimental procedures in the ESI). From inductively coupled plasma atomic emission spectroscopy, the content of Pd in Pd@UiO-66 was estimated to be 1.0 wt%. In contrast, the content of Pd in Pd/ZrO2 was estimated to be 1.3 wt% using X-ray fluorescence measurements.

Transmission electron microscopy (TEM) images showed that the mean diameter of the Pd NPs before and after UiO-66 coating was 7.7 ± 1.0 and 8.0 ± 1.0 nm, respectively, and the size of Pd NPs remained unchanged (Fig. 1a, b and Fig. S1, ESI). The mean diameter of Pd NPs in Pd/ZrO2 was 7.8 ± 0.9 nm (Fig. S2, ESI). The powder X-ray diffraction (XRD) pattern of the composite consisted of diffractions from both the Pd NPs and the UiO-66 lattice (Fig. 1c). From the Le Bail fitting of the XRD pattern of the composite, the crystal size of Pd NPs in the composite was determined to be 7.6 nm, which is a similar value to the mean diameter estimated from the TEM image. The lattice constants of Pd and UiO-66 components in the composite were 3.903(1) Å and 20.8077(8) Å, respectively (Fig. S3, ESI). These values were consistent with those of pristine Pd NPs and UiO-66 (3.8898(5) and 20.8238(5) Å, respectively), and are in good agreement with the previously reported values.22,25 IR spectra (Fig. S4, ESI) and thermogravimetric measurements (Fig. S5, ESI) also confirmed the existence of UiO-66 in the composite. The N2 sorption isotherm at 77 K of the composite showed a typical type-I sorption behavior derived from the microporosity of UiO-66 (Fig. S6, ESI).

image file: d0cc00566e-f1.tif
Fig. 1 TEM images of (a) Pd NPs and (b) Pd@UiO-66. Scale bar = 20 nm. (c) XRD patterns of Pd@UiO-66 (red), UiO-66 (cyan), and Pd NPs (black). The radiation was Cu-Kα.

To determine the composite states of Pd@UiO-66, we performed high-angle annular dark-field scanning TEM (HAADF-STEM) (Fig. 2a) and elemental mapping with energy-dispersive X-ray (EDX) spectroscopy. Fig. 2b and c show the EDX maps of Pd-L and Zr-L corresponding to the constituent elements of Pd NPs and UiO-66, respectively. An overlay map of Pd-L and Zr-L is presented in Fig. 2d. These mapping results revealed that the Zr elements of UiO-66 are distributed around the surface of the Pd NPs. These results indicate that the Pd NPs were successfully covered with UiO-66.

image file: d0cc00566e-f2.tif
Fig. 2 (a) HAADF-STEM image and EDX maps of (b) Pd-L and (c) Zr-L obtained from Pd@UiO-66. (d) Reconstructed overlay image of the maps shown in panels (b) and (c) (cyan, Zr; yellow, Pd); scale bar = 20 nm.

To investigate the catalytic activity in CO oxidation, the catalyst that included 1.0 mg of Pd was loaded into a fix-bed flow reactor with quartz wool. A gas mixture of CO/O2/He (CO/O2/He: 0.5/0.5/49 cm3 min−1) was passed over the catalysts at 35 °C, and the catalysts were then heated to 300 °C at a rate of 5 °C min−1. The products were analyzed using a quadrupole mass spectrometer. The catalytic activity test was repeated five times. The temperature dependence of the CO conversion rate for Pd@UiO-66 is shown in Fig. 3a. In the first run, the temperature corresponding to 50% conversion (T50) of CO to carbon dioxide (CO2) was 225 °C, which is higher than that of Pd/ZrO2 (200 °C) (Fig. S7a, ESI). With the repetition of the measurement, the T50 of Pd@UiO-66 significantly decreased, whereas that of Pd/ZrO2 remained unchanged. The plot of T50vs. measurement cycle for Pd@UiO-66 and Pd/ZrO2 is shown in Fig. 3b. The result for Ru NPs supported on ZrO2 (Ru/ZrO2) was also plotted (Fig. S7b, ESI).

image file: d0cc00566e-f3.tif
Fig. 3 (a) CO conversion rate of Pd@UiO-66. The reactions were carried out under a flow of 1% CO/1% O2/He at a total flow rate of 50 cm3 min−1 STP. (b) T50vs. measurement cycle for Pd@UiO-66 (red), Pd/ZrO2 (blue) and Ru/ZrO2 (green).

In the first cycle, the CO oxidation activity of Pd@UiO-66 (T50 = 225 °C) was lower than that of either Pd/ZrO2 (T50 = 200 °C) or Ru/ZrO2 (T50 = 208 °C). Surprisingly, the second cycle measurement showed a drastic enhancement of CO oxidation activity for Pd@UiO-66. Finally, the T50 of Pd@UiO-66 was 150 °C in the fifth cycle, whereas the results for Pd/ZrO2 and Ru/ZrO2 were 200 and 190 °C, respectively. These results indicated that Pd@UiO-66 exhibited excellent catalytic activity with T50 lower by 40 K than that of Ru NPs. The enhanced CO oxidation activity of Pd@UiO-66 is slightly higher than that of the best monometallic catalyst of face-centered cubic (fcc) Ru NPs.26 The drastic enhancement in CO oxidation activity for Pd@UiO-66 was reproduced with another sample (Fig. S8, ESI). The pristine structure of the composite, including the mean diameter of Pd NPs, and the crystallinity and porosity of UiO-66 were maintained after the CO oxidation measurements (Fig. S6 and S9, ESI). Considering that the enhancement of CO oxidation activity was not observed by pretreatment at 300 °C under He flow (Fig. S10, ESI), both high temperature and reaction gas are important to enhance the catalytic activity. It should also be noted that Pd NPs supported on UiO-66 and Pd NPs coated with 1,4-benzenedicarboxylic acid exhibited much lower CO oxidation activity than Pd@UiO-66 (Fig. S11 and 12, ESI). Therefore, UiO-66 coating on Pd NPs contributes to the improvement of catalytic activity of Pd NPs.

To discuss the origin of the significant enhancement in CO oxidation activity of Pd@UiO-66 shown in Fig. 3, we measured the FT-IR spectra of CO adsorbed on Pd/ZrO2 and Pd@UiO-66. The samples were pretreated under the reaction gas at 150, 200, 250, and 300 °C. After the pretreatment, the samples were cooled to 50 °C and FT-IR measurements were carried out under CO gas flow. For Pd/ZrO2 pretreated at 150 and 200 °C, absorption bands assignable to hollow-bonded CO on Pd atoms were observed at 1901 cm−1 for 150 °C and 1886 cm−1 for 200 °C (Fig. 4a).27 With higher pretreatment temperatures, at 250 and 300 °C, new IR bands appeared, which were assignable to bridge-bonded CO (1970 to 1930 cm−1) on the Pd surface.27 However, the hollow-bonded CO bands still remained. For Pd@UiO-66, linear-bonded CO bands (2050 to 2020 cm−1)27 were observed at 150 and 200 °C, in addition to the hollow-bonded CO bands, as with Pd/ZrO2 (Fig. 4b). Interestingly, at 250 and 300 °C, the hollow-bonded CO bands disappeared, and linear and bridge-bonded CO bands became prominent. These pretreatment temperatures correspond to the temperature where the conversion of CO to CO2 reaches 100%. Therefore, the activity enhancement of Pd@UiO-66 observed from the second cycle (Fig. 3a) was considered to have originated from the change in the CO adsorption site on Pd NPs, caused by the high treatment temperature. In fact, we confirmed that the linear and bridge-bonded CO on Pd NPs is reactive for the oxidation reaction by the time-dependent in situ FT-IR measurements of Pd@UiO-66 (Fig. S13, ESI).

image file: d0cc00566e-f4.tif
Fig. 4 In situ IR spectra of (a) Pd/ZrO2 and (b) Pd@UiO-66 under 0.3% CO/He flow. Four spectra were obtained at 50 °C after pretreatment at 150 °C (cyan), 200 °C (blue), 250 °C (orange) and 300 °C (red).

The in situ FT-IR spectra in Fig. 4 suggested that the enhancement of oxidation activity for Pd@UiO-66 correlated with the change in the CO adsorption site on Pd NPs caused by the MOF coating at high treatment temperatures. It is considered that the CO oxidation over a Pd surface follows the Langmuir–Hinshelwood mechanism: a co-adsorbed O atom and a CO molecule react on the metal surface.5 The CO molecules at the hollow sites are strongly stabilized due to the π-back donation from the metal to the CO molecule, and the reactivity of adsorbed CO is expected to be much lower than that of CO adsorbed at top and bridge sites.28,29 Therefore, for Pd@UiO-66, the CO adsorption site changing from a hollow to a bridge or top site on Pd NPs potentially contributed to the enhanced CO oxidation activity, while the catalytic activity of Pd/ZrO2 remained unchanged due to the existence of hollow-bonded CO molecules. In addition, an electronic interaction between Pd NPs and UiO-66 was not observed using X-ray photoelectron spectroscopy before and after the CO oxidation reaction (Fig. S14, ESI). Hence, the enhancement of CO oxidation activity should be attributed to reconstruction at the interface between Pd NPs and UiO-66, i.e. the Zr6 cluster or BDC ligand of UiO-66 may block the hollow sites for CO adsorption on Pd NPs. The detailed mechanism of the change in CO adsorption via the 1st cycle of CO oxidation is under investigation, including theoretical calculations.

In summary, we obtained a significantly enhanced CO oxidation activity of Pd NPs covered with UiO-66. The activity of Pd@UiO-66 was much higher than those of Pd/ZrO2 or Ru/ZrO2. In addition, the activity was also superior to the fcc Ru NPs reported as the best monometallic CO oxidation catalyst. The origin of the enhancement of CO oxidation activity was suggested to be a CO adsorption site change from a hollow to a bridge or top site on Pd NPs. These results give the first example of control of the adsorption properties of reactants on metal NPs by a MOF coating. We hope that these new findings will contribute to the further development of useful MOF-based metal catalysts for reactions related to the CO molecule.

This work was supported by the Japan Science and Technology Agency (JST) PRESTO (No. JPMJPR1514), the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (B) (No. 17750056), Core Research for Evolutional Science and Technology (CREST) and ACCEL from JST, and Grants-in-Aid for JSPS Fellows (JP 17J10102) from JSPS. STEM observations were performed as part of a program conducted by the Advanced Characterization Nanotechnology Platform sponsored by MEXT, Japan.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cc00566e

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