Activating surface lattice oxygen in single-atom Pt/CeO2 catalysis brings industrial applications a step closer

Achieving better fuel efficiency with low greenhouse gas emissions has been the main focus in the development of advanced combustion engines. In order to achieve this goal, the catalyst materials must be active at temperatures < 150 °C. Single-atom heterogeneous catalysts have been shown to offer excellent low-temperature reactivity. Nevertheless, they lack durability at high-temperatures. Researchers at Pacific Northwest National Laboratory, USA, University of New Mexico, USA, Beijing University of Chemical Technology, China and Washington State University, USA have demonstrated that activation of atomically dispersed Pt²⁺ on CeO₂ by steam treatment (hydrothermal aging) at 750 °C decreased the temperature required to achieve 100% conversion (T₁₀₀) of CO from 320 °C to 148 °C with no evidence of deactivation in the catalytic ability besides providing a better thermal stability.

Ce(NO₃)₃·6H₂O was used as the precursor to prepare the CeO₂ polyhedra by thermal treatment at 350 °C for 2 h at in air atmosphere. Platinum was loaded on CeO₂ by incipient wetness impregnation method followed by drying at 80 °C for 12 h. Thermal and hydrothermal aging treatments were performed to prepare the catalysts (Fig.1(a)). The thermally aged (800 °C for 12 h in flowing air) and hydrothermally aged (10% H₂O in Argon at 750 °C for 9 h) CeO₂ catalysts are designated as Pt/CeO₂ and Pt/CeO₂_S, respectively. In the Pt/CeO₂ catalyst (Fig. 1(b)), Pt is atomically dispersed while in the Pt/CeO₂_S catalyst, no sintering of Pt has occurred, and Pt remained atomically dispersed (Fig. 1(c)), even after the steam treatment at 750 °C. The presence of Pt NPs is not evident either in Pt/CeO₂ or Pt/CeO₂_S even in high-resolution STEM images, suggesting atomic dispersion of Pt, which is further substantiated by XRD, EXAFS and XPS.

Fig. 1 (a) Protocol for preparing Pt/CeO₂ and Pt/CeO₂_S catalysts; (b, c)  aberration corrected–STEM images of (b) thermally aged Pt/CeO₂; and (c) hydrothermally aged Pt/CeO₂_S (Circles: Single atoms of Pt)

The mechanism of CO oxidation is evaluated in terms of adsorption of CO over the Pt sites of both Pt/CeO₂ and Pt/CeO₂_S catalysts at 180 °C using diffuse reflectance infrared Fourier-transform spectroscopy. The IR bands at 2096 and 2098 cm^–1 are assigned to the linearly adsorbed CO on isolated ionic Pt²⁺ over the catalysts (Figs. 2(a) and 2(b)). The intensity of the IR band at 2098 cm^–1 did not change appreciably after the flow of CO is stopped over the Pt/CeO₂ catalyst (Fig. 2(a)), suggesting a strong adsorption of CO on the ionic Pt site. In contrast, the substantial decrease in the intensity of the IR band at 2096 cm^–1 suggests that the CO adsorbed on single-ion Pt²⁺ is readily oxidized to CO₂ (Fig. 2(b)). Hence, it is clear that the CO adsorbed on Pt²⁺ in Pt/CeO₂_S catalyst is more reactive than those adsorbed on Pt²⁺ in the Pt/CeO₂ catalyst. Since the Pt on both catalysts exhibits the same atomic dispersion and valence (Pt²⁺), the difference in low-temperature reactivity between these catalysts could be attributed to neighboring lattice oxygen, which is part of the active site and it is reflected in the reactivity of the ionic Pt sites. When compared to the Pt/CeO₂ catalyst, steam treatment roughly doubled the amount of active lattice oxygen in Pt/CeO­₂_S catalyst (Fig. 2(c)). H₂ temperature-programmed reduction (H₂-TPR) analysis (Fig. 2(d)) indicates the presence of two major reduction peaks for the Pt/CeO₂ catalyst: (i) reduction of the surface lattice oxygen in the vicinity of Pt (Pt–O–Ce bond), centered at 184 °C; and (ii) reduction of surface lattice oxygen on CeO₂ distant from Pt, centered at 348 °C. For Pt/CeO₂_S catalyst, in addition to these peaks, an extra peak at 162 °C is also observed, which is due to a new type of active surface lattice oxygen generated during steam treatment.

Fig. 2 Identification of Pt single atoms (Pt²⁺) and active surface lattice oxygen. CO adsorption DRIFTS for (a) Pt/CeO₂ and (b) Pt/CeO₂_S. (c) Time-resolved CO oxidation with surface active lattice oxygen of Pt/CeO₂ catalysts at 300 °C; and (d) H₂-TPR profiles of Pt/CeO₂ catalysts.

Density functional theory calculations and reaction kinetic analyses indicate that the oxygen vacancies from the CeO₂ bulk is redistributed to the CeO₂(111) surface (Fig. 3(a)) as a result of exposure to water at 380 °C. Under steam-treatment conditions, H₂O molecules fill out the oxygen vacancy (VO) over the atomically dispersed Pt/CeO₂ surface, generating two neighboring active O_lattice[H] in the vicinity of Pt (Fig. 3(a)), which are thermodynamically stable up to 767 °C. The proposed reaction mechanism for CO oxidation on isolated Pt on CeO₂(111) surface and the calculated energy profile are shown in Fig. 3(b).  Coordination of only one catalytically active O_lattice[H] site with a Pt atom (Pt²⁺) occurs during the initial stage (Fig. 3(b), intermediate I). The surface O_lattice[H] reacts with CO adsorbed on Pt and creates an VO (Fig. 3(b), intermediate III), which is subsequently filled by adsorption of an oxygen molecule. Deprotonation of the carboxyl intermediate assisted by the newly adsorbed oxygen molecule [(Fig. 3(b), transition state 2 (TS2)] enables generation of CO₂. Reaction between the OO[H] species (Fig. 3(b), intermediate V) and another adsorbed CO results in the formation of an additional CO₂ molecule (Fig. 3(b), TS3). The atomically dispersed Pt/CeO₂_S catalyst surface is recovered after desorption of CO₂, and the catalytic cycle over the steam-treated catalyst surface (2O_lattice[H]) is closed. The improved low-temperature activity of Pt/CeO₂_S catalyst is due to the activation of surface lattice oxygen that is bonded to H, resulting in the formation of hydroxyls on the CeO₂ support in the vicinity of atomically dispersed Pt.

Fig. 3 (a) Steam-treatment on the atomically dispersed Pt/CeO₂ catalyst – generation of active sites by steam treatment is responsible for low-temperature CO oxidation activity (highlighted by dashed green circles); (b) Proposed reaction mechanism for CO oxidation on isolated Pt on a CeO₂(111) surface

The importance of activation of the catalyst support by steam treatment to achieve high reactivity and durability has been demonstrated. The enhanced reactivity of the Pt/CeO₂_S catalyst is not due to the formation of Pt NPs, rather it has been attributed to the activation of surface oxygen on the ceria support. The excellent low-temperature reactivity and better high-temperature durability bring single-atom catalysis of Pt/CeO₂_S closer to reality for many industrial applications.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Lei Nie et al., Activation of surface lattice oxygen in single-atom Pt/CeO₂ for low-temperature CO oxidation, Science 358, 1419–1423 (2017)