Functional CeOx nanoglues for ecient and robust atomically dispersed catalysts

Single-atom catalysts (SACs) exhibit unique catalytic property and maximum atom eciency of rare, expensive metals. A critical barrier to applications of SACs is sintering of active metal atoms under operating conditions. Anchoring metal atoms onto oxide supports via strong metal-support bonds may alleviate sintering. Such an approach, however, usually comes at a cost: stabilization results from passivation of metal sites by excessive oxygen ligation—too many open coordination sites taken up by the support, too few left for catalytic action. Furthermore, when such stabilized metal atoms are activated by reduction at elevated temperatures they become unlinked and so move and sinter, leading to loss of catalytic function. We report a new strategy, conning atomically dispersed metal atoms onto functional oxide nanoclusters (denoted as nanoglues) that are isolated and immobilized on a robust, high-surface-area support—so that metal atoms do not sinter under conditions of catalyst activation and/or operation. High-number-density, ultra-small and defective CeOx nanoclusters were grafted onto high-surface-area SiO2 as nanoglues to host atomically dispersed Pt. The Pt atoms remained on the CeOx nanoglue islands under both O2 and H2 environment at high temperatures. Activation of CeOx supported Pt atoms increased the turnover frequency for CO oxidation by 150 times. The exceptional stability under reductive conditions is attributed to the much stronger anity of Pt atoms for CeOx than for SiO2—the Pt atoms can move but they are conned to their respective nanoglue islands, preventing formation of larger Pt particles. The strategy of using functional nanoglues to conne atomically dispersed metal atoms and simultaneously enhance catalytic performance of localized metal atoms is general and takes SACs one major step closer to practical applications as robust catalysts for a wide range of catalytic Schematic diagrams illustrate the fabrication processes of CeOx/SiO2 supported Pt1 SACs. The [Ce(OH)2]+ and [Ce(OH)]2+ precursor species are produced in situ from Ce3+ reacting with OH- species in a mild alkaline solution. The positively charged Ce-containing species electrostatically adsorb onto negatively charged surfaces of the high-surface-area SiO2 support. After high-temperature calcination, atomically dispersed Ce species self-assemble into crystalline CeOx nanoclusters. These ultra-small CeOx nanoclusters act as functional nanoglues to localize metal atoms and to provide surface active oxygen species. By judiciously adjusting the aqueous solution to 6 ≥ pH ≥ 4 where CeOx nanoclusters are positively charged and SiO2 surfaces are negatively charged, the negatively charged Pt-containing species adsorb only onto the CeOx nanoclusters. Subsequent rigorous washing and high-temperature calcination processes eliminate solution residues and facilitate connement of Pt atoms to only the crystalline CeOx nanoclusters.

nanoclusters uniformly distributed on high-surface-area SiO 2 support via a scalable/practical synthesis process.
Uniform CeO x nanocluster islands were synthesized by strong electrostatic adsorption (SEA) of charged species from aqueous solution 21 , as schematically illustrated in Fig. 1. The point of zero charge (PZC) of the high-surface-area SiO 2 (278 m 2 /g) is ~3.6 (ref. 21), implying that its surface is negatively charged in an aqueous solution with pH > 4.0. Control of the solution OHconcentration and adsorption time yielded soluble cationic [Ce(OH) x ] y+ species that quickly adsorbed onto the SiO 2 surfaces, leading to (after a hightemperature calcination) formation of uniformly dispersed CeO x nanoclusters. Short adsorption time (< 3 min) usually produces uniformly coated Ce species while extended adsorption time leads to formation of large CeO 2 particles (Extended Data Fig. 1a-b). High-angle annular dark-eld scanning transmission electron microscope (HAADF-STEM) images showed uniform coating of the mesoporous SiO 2 support with Ce species as a result of the SEA process (Extended Data Fig. 1c-e). Subsequent calcination at 600° C produced individual crystalline CeO x nanoclusters stably anchored onto SiO 2 surfaces ( Fig.   2a and Extended Data Fig. 1f-g). The CeO x loading was 12 wt%, determined by inductively coupled plasma mass spectrometry (ICP-MS), and the average CeO x nanocluster dimensions were 1.8 nm × 2.1 nm (Extended Data Fig. 1h-i), with ellipsoid shapes. Atomic-resolution HAADF-STEM images (Fig. 2b) show that all the as-synthesized CeO x nanocluster are well crystallized and some show visible surface steps.
Powder X-ray diffraction (XRD) patterns of the as-synthesized CeO x /SiO 2 show broad diffraction peaks that represent a cubic uorite structure (Fig. 2c). In comparison, the two control samples (12 wt% CeO 2 nanoparticles (NPs) on SiO 2 , denoted as CeO 2 NPs/SiO 2 , and pure CeO 2 powders, made by an impregnation or precipitation method, as stated in Fig. 2c Fig. 2h), in agreement with the slight shift of the XRD peaks to lower angles (Fig. 2c).
X-ray photoelectron spectroscopy (XPS) data ( Fig. 2d and Extended Data Fig. 2d) show 28.7 %, 10.9 % and 8.4% Ce 3+ species in CeO x /SiO 2 , CeO 2 and CeO 2 NPs/SiO 2 , respectively 12,23 . The signi cantly increased number of Ce 3+ sites on the as-synthesized CeO x nanoclusters suggests more anchor sites for metal atoms 8, 15,24 . The fact that XPS probes surfaces of nanometers in depth 23 implies that the Ce 3+ /(Ce 3+ + Ce 4+ ) ratio (estimated from the XPS data) can be used to estimate the average composition of the CeO x nanoclusters since the heights of the as-synthesized CeO x nanoclusters are less than 2 nm.
The estimated composition of the as-synthesized CeO x nanoclusters is CeO 1.86 . Analyses of H 2 temperature-programmed reduction (H 2 -TPR) showed that the reduction temperature of "bulk" oxygen from the CeO 1.86 nanoclusters took place at ~492 °C (290 °C lower than that of CeO 2 powder) (Extended Data Fig. 2i), suggesting that full reduction of the as-synthesized CeO 1.86 nanoclusters is much easier than that of CeO 2 NPs, providing a route to facile formation of oxygen species. The as-synthesized ultrasmall, isolated CeO 1.86 nanoclusters are crystalline in nature, possess high-number density of surface defect sites, and provide labile oxygen species-well-suited to strong bonding of isolated Pt atoms. The Pt L III -edge X-ray absorption near-edge structure (XANES) and the Fourier transform radial distribution functions of the k 3 -weighted extended X-ray absorption ne structure (EXAFS) spectra of the 0.4 wt% Pt/CeO 1.86 /SiO 2 catalyst are displayed in Fig. 3a Fig. 7a-c), implying that the Pt-O-Ce bonds were broken and the Pt atoms moved on reducible CeO 2 surfaces. After H 2 reduction treatment, the sizes of Pt particles on CeO 2 powders were smaller than those on SiO 2 (Extended Data Fig. 7a-b and 6c- show any sign of sintering, even after H 2 reduction treatment at 300 °C for 10 h, as evidenced by DRIFTS spectra and HAADF-STEM images ( Fig. 3d and Extended Data Fig. 8a-b). To further probe the stability of Pt atoms localized on the CeO 1.86 nanoglue islands, catalyst samples were exposed to H 2 at 400 °C to 600 °C. Even under such harsh reduction conditions, the Pt atoms remained cationic (Extended Data Fig. 8c-e). For catalysts that were reduced above 500 °C, however, signi cantly blue-shifted CO absorption peaks were observed, probably suggesting a major modi cation of the  Fig. 7d-i) show that although the Pt atoms did not migrate onto the SiO 2 surfaces to form larger particles they did sinter to form small Pt clusters on the CeO 2 NPs-because some of the larger CeO 2 NPs evidently contained more than one Pt atom. The CO DRIFTS spectra (Extended Data Fig. 7g-h) (Fig. 3d) Fig. 8h). The absence of Pt particles with sizes >1 nm in diameter in these highly reduced catalysts is a clear manifestation of localization of the Pt species onto the CeO 1.86 nanoglue islands even when the Pt species become mobile on their own CeO 1.86 nanoglue islands-no cross-movement of Pt species among the CeO x nanoclusters. These results rmly demonstrate that our localization design applies not only to supported metal atoms but also supported subnanometer metal clusters (Extended Data Fig. 8i), signi cantly expand practical applications of our nanoglue localization strategy in contrast to previous stabilization approaches 24,28-30 .
For many catalytic reactions, the active phase is usually activated by H 2 reduction prior to a desired catalytic reaction. For metal-oxide-supported SACs, H 2 treatment at temperatures > 200 °C usually causes sintering of metal atoms 12,13 . Because of such detrimental sintering effects, many SACs were directly used without being activated by such a H 2 reduction treatment, hindering the true measurement of the catalytic performance of the as-prepared SACs. Due to saturation by oxygen ligands, many SACs may not show catalytic activity after a moderate-to high-temperature calcination treatment. Since our 0.4 wt% Pt/CeO 1.86 /SiO 2 SACs resist sintering during H 2 activation processes, we can quantitatively evaluate how the H 2 activation process affects CO oxidation on CeO 1.86 nanoglue supported Pt 1 atoms. Prior to the H 2 reduction treatment, the as-synthesized 0.4 wt% Pt/CeO 1.86 /SiO 2 SAC had relatively low activity for CO oxidation (Fig. 4), in agreement with reports on Pt 1 /CeO 2 SACs 8, [12][13] . The H 2 activated Pt/CeO 1.86 /SiO 2 SAC, however, achieved 50% and 90% CO conversion at 133 °C and 142 °C, respectively. For comparison, the T 50 (temperatures for 50% CO conversion) for the activated Pt/CeO 1.86 /SiO 2 (impregnation-IMP)) and activated Pt/SiO 2 (IMP) catalysts were 171 and 227 °C (Fig. 4b), respectively. In particular, the H 2 activation process had a much bigger effect on impregnated Pt species supported on reducible CeO 1.86 nanoclusters than on Pt species supported on nonreducible SiO 2 , because Pt/CeO 1.86 /SiO 2 (IMP) showed higher activity and Pt dispersion than Pt/SiO 2 (IMP) after H 2 reduction (Extended Data Fig. 9a-d).
The turn-over-frequency (TOF) and speci c reaction rate (normalized by Pt mass) under similar CO oxidation conditions were evaluated for the various catalysts (Extended Data Fig. 9e The CeO x nanoglue islands not only localize Pt atoms to prevent sintering but also provide facile oxygen during CO oxidation reaction. Our strategy to con ne the movement of metal atoms by isolated nanoglue islands extends to metals other than Pt (Extended Data Fig. 10). The use of functional nanoglues on robust high-surface-area supports to localize metal atoms can be broadly applied to creating a wide range of robust single-atom catalysts for a plethora of important catalytic transformations.

Declarations
Data availability. The data that support the ndings of this study are available from the corresponding author upon reasonable request.

Figure 1
Schematic diagrams illustrate the fabrication processes of CeOx/SiO2 supported Pt1 SACs. The [Ce(OH)2]+ and [Ce(OH)]2+ precursor species are produced in situ from Ce3+ reacting with OH-species in a mild alkaline solution. The positively charged Ce-containing species electrostatically adsorb onto negatively charged surfaces of the high-surface-area SiO2 support. After high-temperature calcination, atomically dispersed Ce species self-assemble into crystalline CeOx nanoclusters. These ultra-small CeOx nanoclusters act as functional nanoglues to localize metal atoms and to provide surface active oxygen species. By judiciously adjusting the aqueous solution to 6≥pH≥4 where CeOx nanoclusters are positively charged and SiO2 surfaces are negatively charged, the negatively charged Pt-containing species adsorb only onto the CeOx nanoclusters. Subsequent rigorous washing and high-temperature calcination processes eliminate solution residues and facilitate con nement of Pt atoms to only the crystalline CeOx nanoclusters.

Figure 2
Electron microscopy, X-ray diffraction and XPS characterization of CeOx nanoglue islands dispersed onto high-surface-area SiO2. a, Low magni cation HAADF-STEM image of the as-prepared CeOx nanoclusters conformally coating the high-surface-area SiO2 support. b, Atomic-resolution HAADF-STEM image of crystalline CeOx nanoclusters. c, Powder XRD patterns of pure CeO2 NPs and CeOx/SiO2 (arbitrary units).
The inset shows broadening and red-shift of peak position from CeOx nanoclusters. d, Ce 3d XPS spectra obtained from SiO2 supported CeOx nanoclusters (top panel) and pure CeO2 powders (bottom panel).
The CeOx nanoclusters clearly contain much higher amount of oxygen vacancies (represented by the higher % of Ce3+) than those of the well-crystallized CeO2. Figure 3