Understanding the CO Oxidation on Pt Nanoparticles Supported on MOFs by Operando XPS

Abstract

Metal‐organic frameworks (MOFs) are playing a key role in developing the next generation of heterogeneous catalysts. In this work, near ambient pressure X‐ray photoelectron spectroscopy (NAP‐XPS) is applied to study in operando the CO oxidation on Pt@MOFs (UiO‐67) and Pt@ZrO2 catalysts, revealing the same Pt surface dynamics under the stoichiometric CO/O2 ambient at 3 mbar. Upon the ignition at ca. 200 °C, the signature Pt binding energy (BE) shift towards the lower BE (from 71.8 to 71.2 eV) is observed for all catalysts, confirming metallic Pt nanoparticles (NPs) as the active phase. Additionally, the plug‐flow light‐off experiments show the superior activity of the Pt@MOFs catalyst in CO oxidation than the control Pt@ZrO2 catalyst with ca. 28 % drop in the T50% light‐off temperature, as well as high stability, due to their sintering‐resistance feature. These results provide evidence that the uniqueness of MOFs as the catalyst supports lies in the structural confinement effect.

Highly Dispersed Pt Nanoparticles on N-Doped Ordered Mesoporous Carbon as Effective Catalysts for Selective Hydrogenation of Nitroarenes

“…As depicted in Figure 4a, the Pt 4f XPS spectra of Pt/CMK-3 showed symmetric doublet peaks at the binding energy of 71.7 and 75.2 eV, respectively, which were attributed to Pt 0 . However, this binding energy value was higher than the binding energy of bulk Pt (4f7/2 = 71.2 eV) [28][29][30], indicating the interaction between Pt particles and the CMK-3. When the CMK-3 support was treated with 2-methylimidazole and the N species were introduced into the matrix, the binding energies of Pt 0 4f were shifted to higher values with increasing N content, implying the existence of an interaction effect between Pt and N species due to a charge-transfer.…”

Confined Pt11+ Water Clusters in a MOF Catalyze the Low‐Temperature Water–Gas Shift Reaction with both CO2 Oxygen Atoms Coming from Water

“…Powder Xray diffraction (PXRD) patterns (Figures S8-9) and N 2 adsorption isotherms at 77 K( Figure S10) for 2 and 3 are similar after the PS treatment, with calculated Brunauer-Emmett-Teller (BET) surface areas of 1142-1170 m 2 g À1 , which confirm that the porous structures are not affected either by the replacement of Ni 2+ by Pt 2+ cations or after NaBH 4 reduction. Quantitative X-ray photoelectron spectroscopy (XPS) of 2 and 3 ( Figure S11) shows that half of the Pt 2+ atoms in 3 (73.7 eV) have been reduced to alow-valence oxidation state after treatment with NaBH 4 (72.2 eV), and this value fits better that of unsaturated Pt d+ species confined in MOFs [6] or ligated to thiols, [7] which corresponds to formal Pt 1+ rather than to any typical signal for Pt 0 (< 72.0 eV). [8] Xray absorption near-edge structure (XANES) analysis of 3 ( Figure S12) shows ad iscretely higher intensity for Pt in 3 2+ (2)a nd the Na + and Pt 1 1+ (3)s ites.…”

“…Pt 1+ species are rarely described in the literature and the MOF-supported Pt 1 1+ constitutes an exceptional case of as ingle-atom catalyst (SAC) without strong coordinating ligands or surfaces. MOF 2 crystallizes in the Cmmm space group of the orthorhombic system, with the Pt 2+ cations located in the preferential sites of the hydrophilic octagonal pores [virtual diameter of 2.2 nm] previously occupied by the Ni 2+ cations of 1.R obustness together with flexibility of the net acts in response to Pt 2+ exchange merely with ad istortion of the pores shape,a ccounting for ac rystalline phase transition from atetragonal to orthorhombic system (Figures S1-S5, see also 6 ] 2+ (some NH 3 molecules were not found from DF map in Figures S5-S7). ( Figure 1c,s ee also Table S1 in Supporting Information).…”

“…MOF 2 crystallizes in the Cmmm space group of the orthorhombic system, with the Pt 2+ cations located in the preferential sites of the hydrophilic octagonal pores [virtual diameter of 2.2 nm] previously occupied by the Ni 2+ cations of 1.R obustness together with flexibility of the net acts in response to Pt 2+ exchange merely with ad istortion of the pores shape,a ccounting for ac rystalline phase transition from atetragonal to orthorhombic system (Figures S1-S5, see also 6 ] 2+ (some NH 3 molecules were not found from DF map in Figures S5-S7). Quantitative X-ray photoelectron spectroscopy (XPS) of 2 and 3 ( Figure S11) shows that half of the Pt 2+ atoms in 3 (73.7 eV) have been reduced to alow-valence oxidation state after treatment with NaBH 4 (72.2 eV), and this value fits better that of unsaturated Pt d+ species confined in MOFs [6] or ligated to thiols, [7] which corresponds to formal Pt 1+ rather than to any typical signal for Pt 0 (< 72.0 eV). Powder Xray diffraction (PXRD) patterns ( Figures S8-9) and N 2 adsorption isotherms at 77 K( Figure S10) for 2 and 3 are similar after the PS treatment, with calculated Brunauer-Emmett-Teller (BET) surface areas of 1142-1170 m 2 g À1 , which confirm that the porous structures are not affected either by the replacement of Ni 2+ by Pt 2+ cations or after NaBH 4 reduction.…”

“…Thev ery robust and highly crystalline MOF Ni 2 [5] (Figure 1a) was exchanged with [Pt(NH 3 ( Figure 1c,s ee also Table S1 in Supporting Information). MOF 2 crystallizes in the Cmmm space group of the orthorhombic system, with the Pt 2+ cations located in the preferential sites of the hydrophilic octagonal pores [virtual diameter of 2.2 nm] previously occupied by the Ni 2+ cations of 1.R obustness together with flexibility of the net acts in response to Pt 2+ exchange merely with ad istortion of the pores shape,a ccounting for ac rystalline phase transition from atetragonal to orthorhombic system (Figures S1-S5, see also 6 ] 2+ (some NH 3 molecules were not found from DF map in Figures S5-S7). In contrast, the crystal structure of 3 in Figure 1c reveals the formation of Pt 1 1+ single metal cations confined into the,n ol onger distorted, hydrophilic octagonal channels of the P4/mmm tetragonal anionic Ni 4 2+ Cu 6 2+ openframework net (Figures S3b,S 4a, S5c-d and S8, and Table S2), together with unchanged Pt 2+ ···Pt 2+ dimers in square pores and hydrated charge-counterbalancing alkali Na + cations in the preferential cationic sites.T he poorer accessibility to the square pores for solvated NaBH 4 is most likely at the reason that the Pt 2+ ···Pt 2+ intradimers are not reduced in 3,a sa ssessed by the persistence of the Pt 2+ -NH 3 bands in FT-infrared spectroscopy (IR, Figure S5).…”