Single-atom catalyst for high-performance methanol oxidation

Single-atom catalysts have been widely investigated for several electrocatalytic reactions except electrochemical alcohol oxidation. Herein, we synthesize atomically dispersed platinum on ruthenium oxide (Pt1/RuO2) using a simple impregnation-adsorption method. We find that Pt1/RuO2 has good electrocatalytic activity towards methanol oxidation in an alkaline media with a mass activity that is 15.3-times higher than that of commercial Pt/C (6766 vs. 441 mA mg‒1Pt). In contrast, single atom Pt on carbon black is inert. Further, the mass activity of Pt1/RuO2 is superior to that of most Pt-based catalysts previously developed. Moreover, Pt1/RuO2 has a high tolerance towards CO poisoning, resulting in excellent catalytic stability. Ab initio simulations and experiments reveal that the presence of Pt‒O3f (3-fold coordinatively bonded O)‒Rucus (coordinatively unsaturated Ru) bonds with the undercoordinated bridging O in Pt1/RuO2 favors the electrochemical dehydrogenation of methanol with lower energy barriers and onset potential than those encountered for Pt‒C and Pt‒Ru.

P latinum is the most effective element for anodic methanol oxidation reaction (MOR) in direct methanol fuel cells 1,2 . The electrocatalytic activity of Pt is highly dependent on its geometrical structure and the surrounding environment 1 . To improve MOR activity and reduce Pt loading, conventional strategies have focused on 2 (i) tailoring the structure and/or morphology of Pt (e.g., by making hollow/framed 3 or coreshelled 4 Pt); and (ii) hybridizing Pt with other elements (e.g., Co 3 , Ni 5 , Sn 6 , Bi 7 , etc.). However, the Pt in these catalysts is usually assembled as a nanoparticle of diameter greater than 1 nm, leading to unsatisfactory mass activity. Furthermore, in MOR, Pt nanoparticles are susceptible to poisoning by adsorbed intermediates (CO ads ) 5 , resulting in activity loss. Hence, developing new types of Pt-based MOR electrocatalysts with high activity and anti-poisoning capability is of both practical and fundamental significance.
Single-atom catalysts (SACs) are now emerging as a new class of catalysts with extraordinary activity towards many electrocatalytic reactions, including oxygen and hydrogen evolution, oxygen, CO 2 , and N 2 reduction, and hydrogen and formic acid oxidation [8][9][10][11][12] . Pt SACs have utmost utilization of Pt atoms and good capability for CO oxidation 13 . However, the electrochemical dehydrogenation of methanol to CO in MOR requires at least three contiguous Pt atoms 14 . Further, it has been reported that SACs consisting of Pt single atoms supported on carbon nanotubes are inactive towards MOR 15 . Yet, we should note that these studies focused only on the Pt active centers rather than the entire catalysts, thereby neglecting the environment surrounding Pt. In this regard, enhancing the activity of single atomic Pt towards MOR is a scientifically significant and challenging topic.
For SACs, the atomic coordination of single atoms also plays an important role in determining the catalytic activity 16 . It has been shown that the electronic structure and coordination of the central single atoms can be adjusted by tuning the bonds between the single atoms and the substrate [17][18][19][20] . Herein, we designed two types of Pt SACs. Thanks to a simple adsorption-impregnation preparation method 21 , single Pt atoms were immobilized on RuO 2 and carbon black (VXC-72) to obtain Pt 1 /RuO 2 and Pt 1 / VXC-72, respectively. The Pt 1 /RuO 2 SACs showed superb mass activity (6766 mA mg -1 Pt ) and stability towards the MOR, far superior to those of most Pt-based catalysts developed to date. The MOR mechanism including the dehydrogenation of methanol and CO electrooxidation is further studied by density functional theory (DFT), confirming the experimental observation that the prepared SACs are active for the alcohol oxidation reaction. This finding suggests an approach of SACs for direct alcohol fuel cells.

Results
Structure characterization of Pt 1 /RuO 2 . RuO 2 and VXC-72 supports were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) (Supplementary Fig. 1). Pt 1 /RuO 2 and Pt 1 /VXC-72 catalysts with an identical Pt loading of 0.38 wt% were prepared by simple impregnation/adsorption followed by filtration and washing (for more details, see 'Methods' and Supplementary  Fig. 2). Figure 1 shows the catalysts' morphology and structure. As revealed by the bright spots marked as yellow circles in the high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image, individual Pt atoms were randomly dispersed on RuO 2 (Fig. 1a). The magnified HAADF-STEM image of Pt 1 /RuO 2 suggests that the Pt atoms exactly substituted Ru (Fig. 1b). The presence of Pt single atoms was further verified by the intensity profile along with the dashed rectangle in the image (the inset in Fig. 1b), where the Pt atom brightness is more intense than that of Ru due to the higher Z number. The corresponding elemental mapping shows that Pt, Ru, and O were homogeneously distributed throughout the entire Pt 1 /RuO 2 sample ( Fig. 1d-g). For Pt 1 /VXC-72, Pt atoms were also atomically dispersed, as shown in the HAADF-STEM image (Fig. 1c). No Pt clusters/nanoparticles on VXC-72 were observed by low-magnification HAADF-STEM and TEM imaging (Supplementary Fig. 3). Due to the single-atom structure and low Pt loading, Pt peaks were, however, not detectable by XRD (Supplementary Fig. 4).
To study the coordination and electronic configuration of Pt single atoms in the Pt 1 /RuO 2 and Pt 1 /VXC-72, X-ray absorption fine structure (XAFS) spectroscopy and XPS were carried out (Fig. 2). The normalized X-ray absorption near-edge structure (XANES) spectra of Pt 1 /RuO 2 shows stronger white-line intensity at 11,568 eV compared to the Pt foil, suggesting that Pt is positively charged in Pt 1 /RuO 2 due to the electron transfer from Pt to the surrounding O atoms in the RuO 2 support (Fig. 2a) 22,23 . The Pt 1 /VXC-72 catalyst is characterized by a similar Pt-to-VXC-72 electron transfer. However, the white-line peaks suggest that the Pt in Pt 1 /VXC-72 has a lower charge state than the one observed for Pt 1 /RuO 2 . The average Pt oxidation numbers in Pt 1 / RuO 2 and Pt 1 /VXC-72 are estimated to be +2.90 and +1.22, respectively, by integrating the XANES spectra ( Supplementary  Fig. 5) 10,24 . The different charge of Pt in RuO 2 and VXC-72 is likely a result of the different coordination. As revealed by the k 3weighted extended XAFS (EXAFS) at the Pt L 3 -edge, no peak at 2.64 Å from Pt-Pt is observed for either Pt 1 /RuO 2 or Pt 1 /VXC-72, indicating full atomic dispersion for both catalysts (Fig. 2b), consistent with the HAADF-STEM images. Specifically, the R-space spectrum of Pt in Pt 1 /RuO 2 has a dominant peak at 1.62 Å from Pt-O coordination, in agreement with the PtO 2 case (Fig. 2b) 22,23 . In contrast, the dominant peak of Pt 1 /VXC-72 shifted to 1.83 Å (Fig. 2b). To determine the coordination of Pt in both catalysts, the main R-space peaks were fitted against corresponding models (see 'Computational details') by Fourier transform. For Pt 1 /RuO 2 , the coordination number of Pt-O is estimated to be 4 with an average bond length of 1.99 Å (Supplementary Fig. 6 and Supplementary Table 1). For Pt 1 / VXC-72, the Pt-C bond coordination number is 3, and the average bond length is 1.93 Å (Supplementary Fig. 6 and Supplementary Table 1). Also, the wavelet-transform plots of Pt 1 /RuO 2 and Pt 1 /VXC-72 show a maximum at~7.5 Å −1 corresponding to the Pt-O/C bonds (Fig. 2c). No intensity maximum at~12.5 Å −1 corresponding to the presence of Pt-Pt bonds was observed for either catalyst, consistent with the EXAFS (Fig. 2c).
The RuO 2 support contains Ru 4s with the binding energy at the range of~70-85 eV ( Supplementary Figs. 1b and 7), which is very close to the XPS signature of Pt 4f. Therefore, the Pt 1 /RuO 2 XPS peaks in the~68-82 eV range were deconvolved into Ru 4s and Pt 4f, i.e., Pt 4f 7/2 and Pt 4f 5/2 (Fig. 2d). The binding energies of the Pt 4f were measured to be 73.6 and 76.9 eV for Pt 1 /RuO 2 , and 72.9 and 76.2 eV for Pt 1 /VXC-72. The higher binding energy of Pt 1 /RuO 2 compared to Pt 1 /VXC-72 implies that Pt in Pt 1 / RuO 2 has a higher oxidation state, in agreement with the XANES results. Moreover, the O 1s spectrum of bare RuO 2 was deconvolved into three characteristic peaks at 529.3, 530.2, and 532.3 eV (Fig. 2e), which can be attributed to M-O, M-OH, and adsorbed oxygen, respectively 25 . After the incorporation of Pt single atoms into RuO 2 , the O 1s of Pt 1 /RuO 2 shows a new peak at 531.3 eV. This peak can be attributed to the appearance of oxygen vacancies on the surface resulting from Pt single atoms ( Supplementary Fig. 8) 22,26 . These data support the existence of Pt-O in Pt 1 /RuO 2 , in agreement with the XANES characterization. In addition, the O 1s of Pt 1 /VXC-72 is similar to that of VXC-72 ( Supplementary Fig. 9), indicating that there is no Pt-O interaction in Pt 1 /VXC-72. In other words, Pt-C coordination is dominant in Pt 1 /VXC-72.
The preceding results indicate that Pt is atomically dispersed on both RuO 2 and VXC-72 but surrounded by distinct coordination environments. Pt-O coordination characterizes in Pt 1 /RuO 2 . In contrast, Pt-C coordination is dominant in Pt 1 / VXC-72.
MOR electrocatalytic activity. Although Pt-based nanoparticles have been the most efficient catalysts for MOR, individual Pt atoms on carbon support are inactive towards that reaction 27 . By replacing the carbon support with RuO 2 , the coordination of Pt single atoms changes. Herein, we evaluated in the N 2 -saturated 0.1 mol L -1 KOH and 1 mol L -1 methanol solution the MOR performance of the Pt 1 /RuO 2 SACs. Concomitantly, these materials were compared to Pt 1 /VXC-72 and commercial Pt/C, see Fig. 3. As revealed by cyclic voltammogram (CV) of Pt 1 /VXC-72 SACs, no electrocatalytic peak was detected, indicating that Pt 1 / VXC-72 is inactive towards MOR (Fig. 3a). We also prepared a Pt 1 /VXC-72 catalyst with a higher Pt loading of 1.48 wt%, i.e., Pt/ VXC-72-1.48, which had a negligible MOR activity  ( Supplementary Fig. 10). These results support the literature findings that single atoms or even clusters of Pt on carbon are MOR inactive. Unexpectedly, the CV curves of Pt 1 /RuO 2 SACs displayed oxidation current peaks in both the forward and backward CV scans (Fig. 3b), corresponding to methanol and intermediate products oxidation, respectively 7 . In contrast, the CV scan of the RuO 2 was featureless (Fig. 3c). Further, oxygen vacancies on the RuO 2 surface could not contribute to MOR ( Supplementary Fig. 11). These results suggest that its MOR activity was due to having Pt single atoms on RuO 2 . The Pt mass activity of Pt 1 /RuO 2 was 6766 mA mg -1 Pt at 0.80 V vs. RHE (reversible hydrogen electrode), which is about 15.3 times higher than that of the commercial 20 wt% Pt/C (441 mA mg -1 Pt at 0.92 V vs. RHE) (Fig. 3b, d) and significantly larger than the data for the reported catalysts to date (Supplementary Table 2). The peak current ratio between the forward (I f ) and backward (I b ) scan can be used to demonstrate the CO ads tolerance 28 . The I f /I b of Pt 1 /RuO 2 (3.67) is more than twice that of Pt/C (1.81), suggesting an enhanced anti-poisoning ability for Pt 1 /RuO 2 . Longterm durability is another important criterion to assess the quality of a catalyst 5 . The stability of the Pt 1 /RuO 2 SACs and 20 wt% commercial Pt/C was evaluated by chronoamperometry at -0.1 V (vs. Ag/AgCl). After 10 h, the Pt 1 /RuO 2 mass activity remained at 6463 mA mg -1 Pt with a slight degradation of 4.5% (Fig. 3b). In contrast, commercial Pt/C exhibited an~22% decrease in mass activity (344 mA mg -1 Pt ) (Fig. 3d). After the stability test, the Pt atoms in Pt 1 /RuO 2 were isolated on the RuO 2 support while the Pt nanoparticles in commercial 20 wt% Pt/C were aggregated (Supplementary Figs. 12 and 13).
To reveal the origin of MOR activity, several control samples were used, see Fig. 4. In the first set of samples denoted as Pt 1 / RuO 2 −500 and Pt 1 /RuO 2 −700, Pt 1 /RuO 2 was annealed in air at 500 and 700°C, respectively. After heat treatment, Pt single atoms on RuO 2 were aggregated into nanoparticles with a 1-2 nm diameter for Pt 1 /RuO 2 −500, or even 30 nm diameter for Pt 1 / RuO 2 −700 ( Supplementary Fig. 14 and Fig. 4a, b), respectively. When characterized by CV, Pt/RuO 2 −500 showed a mass activity of 2140 mA mg -1 Pt , a value lower than the 6766 mA mg -1 Pt of Pt 1 / RuO 2 SACs (Fig. 4e). For Pt/RuO 2 −700, the oxidation peak between 0.6 and 1.0 V disappeared, showing no MOR activity. The lower MOR performance of these catalysts can be attributed to the Pt aggregation. As is well known, catalytic activity is associated with the number of active sites 29 and single atoms have many more of those compared to clusters or nanoparticles 30 . As expected, due to the presence of Pt clusters or nanoparticles, Pt/ RuO 2 -0.75 and Pt/RuO 2 -1.48, two materials with higher Pt loading than Pt 1 /RuO 2 SACs, showed decreased mass activity ( Supplementary Fig. 15).
In parallel, an additional sample, denoted as Pt 1 /RuO 2 -H, was prepared by reducing the Pt 1 /RuO 2 SACs in H 2 at 80°C. In that sample, the RuO 2 support was reduced to metallic Ru with a little RuO 2 left as confirmed by XRD ( Supplementary Fig. 16). The different structure led to a changed coordination environment for Pt single atoms. The HAADF-STEM of Pt 1 /RuO 2 -H shows that the Pt atoms are isolated on the support (Fig. 4c). Compared to that of Pt 1 /RuO 2 , the EXAFS spectra of Pt 1 /RuO 2 -H show a prominent peak at 2.49 Å due to Pt-Ru bond (coordination number n = 9), and the 1.62 Å peak of Pt-O is negligible (Fig. 4d,  Supplementary Fig. 17 (Fig. 4e). Instead, Pt 1 /RuO 2 -H shows enhanced capacitive current and polarization current starting from~0.9 V in the forward scan, resulting from higher relative Ru content in Pt 1 /RuO 2 -H than Pt 1 /RuO 2 . In consideration of the coordination of Pt-O in Pt 1 /RuO 2 and Pt-C in Pt 1 /VXC-72, it can be concluded that the coordination of Pt single atoms governs MOR activity. On the basis of the preceding results, the origin of MOR activity in Pt 1 /RuO 2 SACs can be attributed to: (i) the existence of large numbers of atomic Pt sites; and (ii) the environment surrounding Pt single atoms (thanks to RuO 2 , Pt single atoms turned into MOR-active catalysts). The CO resistibility of the catalysts was evaluated by CO stripping tests 6,9 , see Fig. 5. For commercial Pt/C, a main peak appeared in the first forward scan with onset and peak potentials at 0.448 and 0.711 V (Fig. 5a), respectively, where the latter value has been previously attributed to CO electrooxidation taking place on Pt nanoparticles 5 . Downsizing the Pt nanoparticles to the atomic scale in Pt 1 /VXC-72 reduces the onset potential of CO electrooxidation to~0.276 V (Fig. 5b), a value much lower than that of commercial Pt/C (0.448 V). This reduction indicates that Pt single atoms trigger CO electrooxidation, consistent with the literature, where Pt single atoms have been shown to weaken CO adsorption, thereby facilitating the oxidation of CO at a lower potential 9 . Compared to Pt 1 /VXC-72 and commercial Pt/C, Pt 1 / RuO 2 SACs triggered CO electrooxidation at a lower onset potential of 0.152 V with a minute peak around 0.673 V (Fig. 5c), suggesting an enhanced anti-poisoning capability for Pt 1 /RuO 2 . Such a change indicates that introducing RuO 2 as a support favors CO electrooxidation. As is well known, oxophilic RuO 2  boosts water dissociation, which, in turn, facilitates the formation of adsorbed OH and the oxidative removal of CO ads on Pt sites, which, in turn, facilitates their regeneration 5,6 . As shown in Fig. 5d, RuO 2 alone electrooxidized CO at an low potential of 0.076 V. Hence, both single atomic Pt and RuO 2 favor the electrooxidative removal of CO, leading to the superior stability of Pt 1 /RuO 2 .
Ethanol oxidation reaction (EOR) tests in alkaline medium show an~5.7-fold higher mass activity of Pt 1 /RuO 2 compared to benchmark Pt/C (2824 vs. 498 mA mg -1 Pt ) ( Supplementary  Fig. 18). These results suggest that Pt 1 /RuO 2 also has remarkable potential for alcohol oxidation reactions.
MOR mechanism. To understand the MOR mechanism, three different models were constructed to simulate the catalytic reaction. The first system had a Pt atom located at the Ru cus (1-fold coordinatively unsaturated Ru) site of the RuO 2 (110) surface (Pt-RuO 2 (110)) corresponding to Pt 1 /RuO 2 . The second model had a Pt atom bonded to three C atoms on graphene (PtC3) corresponding to Pt 1 /VXC-72. The final model had a Pt substituting a surface Ru in Ru(0001) (Pt-Ru(0001)) corresponding to Pt 1 / RuO 2 -H, see 'Methods' and Supplementary Figs. 19-21. The calculated MOR free energy diagrams of these three samples are shown in Fig. 6.
For the MOR on Pt-RuO 2 (110), the CH 3 OH molecule preferentially adsorbs on the site above Ru cus with a free energy of -0.54 eV (Fig. 6a and Supplementary Fig. 22). Thanks to the highly active O br , the scission of O-H bond of the adsorbed CH 3 OH molecule (CH 3 OH* → CH 3 O* + H*) proceeds spontaneously (Fig. 6a). For this elementary step, the cleaved H atom bonds to its neighboring O br with a reaction free energy of -0.26 eV ( Supplementary Fig. 22b and Supplementary Table 3). More importantly, the removal of H* (H* + OH -→ H 2 O + e -) to leave the sole adsorbate of CH 3 O* is slightly endothermic with a free energy requirement of 0.17 eV at pH = 13 (Supplementary Table 3). Considering that the O-H bond scission is energetically favorable, we may propose that the MOR starts with the break of the O-H bond in the methanol molecule and follows the CH 3 OH* → CH 3 O* → CH 2 O* → CHO* → CO* pathway 32,33 .
As illustrated in Fig. 6a, the subsequent activations of the C-H bond require energy barriers of 0.  34 . Nevertheless, the activation barrier on Pt-RuO 2 (110) was predicted to be only 0.49 eV, which is lower than the 0.71 eV we computed for pure RuO 2 (110) (Fig. 6a and Supplementary Fig. 23). This barrier reduction can be mainly attributed to the upshift in CH 2 O* energy due to Pt, which lowers the energy difference between the initial and the transition state of TS2. The final dehydrogenation of CHO (CHO* → TS3 → CO*) was predicted to have a barrier of 0. 29 Fig. 6b, c for comparison. The adsorption free energy of CH 3 OH on the PtC3 and Pt-Ru(0001) are -0.03 and 0.15 eV, respectively. Both values are less negative than that computed for Pt-RuO 2 (110) (-0.54 eV), implying a weaker interaction between both PtC3 and Pt-Ru(0001) and the methanol molecule in comparison to Pt-RuO 2 (110). Moreover, the energy barriers needed to cleave the O-H and C-H bonds are significantly higher than those of Pt-RuO 2 (110) (Fig. 6a-c). The much higher barriers compared to those on Pt-RuO 2 (110) indicate that the MOR is energetically costly for both PtC3 and Pt-Ru(0001) substrates. These computational results agree with experiments as both suggest that Pt 1 /VXC-72 and Pt 1 /RuO 2 -H are inert towards MOR. As for Pt-RuO 2 (101), the highest reaction activation barrier (0.79 eV) is much lower than that of both PtC3 Fig. 6 MOR mechanism. a-c Calculated reaction free energy and energy barriers for methanol oxidation to CO at: a Pt-RuO 2 (110), b PtC3, and c Pt-Ru(0001). d Calculated reaction free energy and energy barriers for CO oxidation on Pt-RuO 2 (110). In (a), the relative energies of CH 2 O* and corresponding TS2 computed for RuO 2 (110) are shown for reference.
We also theoretically studied the stability of Pt-RuO 2 (110) with respect to CO poisoning, or equivalently, the oxidation CO into CO 2 . When OH* is present, CO oxidation proceeds through the CO* + OH* → TS1 → cis-COOH* → TS2 → trans-COOH* → TS3 → trans-COOH* → CO 2 * pathway 35 , as shown in Fig. 6d. The rate-limiting step of the overall reaction was predicted to be the cis-COOH* → trans-COOH* step (i.e., the rotation of the H atom in cis-COOH*) with activation barrier of 0.35 eV. This value is even smaller than the barrier computed for the dehydrogenation of formaldehyde on Pt-RuO 2 (110), implying that the presence of Pt facilitates the full oxidation of methanol into CO 2 on RuO 2 (110). We further note that the trans-COOH* → CO 2 * step (i.e., the desorption of CO 2 *) has a positive ΔG of 0.33 eV, indicating that the release of CO 2 is spontaneous. It is worth mentioning that the adsorption free energies of CO on the PtC3 and Pt-Ru(0001) are -0.81 and -0.92 eV (Fig. 6b, c), respectively. Both values are much more negative compared to that computed for Pt-RuO 2 (110) (-0.54 eV), suggesting an increased interaction between CO and the PtC3/Pt-Ru(0001). According to the Brønsted-Evans-Polanyi relationship 36,37 , higher binding energy correlates to higher activation energies and lower thermodynamic driving force for the reaction. Therefore, we predict that the CO electrooxidation on PtC3 and Pt-Ru(0001) is much more energetically unfavorable with sluggish kinetics in agreement with the experiments above.
In addition to the activation energy barriers, onset potentials were also calculated ( Supplementary Fig. 25). The potentialdetermining step (i.e., the elementary step with the highest onset potential) was found to be, for all three catalysts, the stripping off methoxyl proton (CH 3 O* → CH 2 O*). Accordingly, the onset potentials were predicted to be 0.26, 0.92, and 0.33 V for Pt-RuO 2 (110), PtC3, and Pt-Ru(0001), respectively. Both the lower onset potential and smaller activation barriers suggest that Pt 1 / RuO 2 SACs are more favorable towards the methanol oxidation than Pt 1 /VXC-72 and Pt 1 /RuO 2 -H, in agreement with experimental results.
To unravel the mechanisms underpinning the high MOR activity, we calculated the projected density of states (PDOS) and charge analysis for both Pt and Ru cus atoms in the Pt-RuO 2 (110). It is apparent that only the spin-up state of the dz 2 orbital of Pt is filled while the dz 2 orbital of Ru cus is empty ( Supplementary Fig. 26). Consistent with the PDOS results, the charge density difference also suggests that more electrons are depleted for the Ru cus than the Pt atom ( Supplementary Fig. 27). Specifically, Bader charge analysis suggests that Ru cus and Pt on Pt-RuO 2 (110) lose 1.61 e and 1.22 e, respectively (Supplementary Table 4). Thanks to the empty antibonding dz 2 orbital, a stronger interaction between the adsorbate (e.g., CH 3 OH) and Ru cus than that of Pt is predicted, in agreement with the calculated adsorption free energies ( Supplementary  Fig. 22a-c). The orbital interaction between adsorbate, e.g., CH 3 OH, and the dz 2 orbital is shown in Supplementary Fig. 28. In the case of Ru cus , for which the dz 2 orbital is empty, the methanol molecule is well stabilized due to the favorable two-center two-electron interaction. Instead, the anti-bonding dz 2 orbital of Pt is filled with one spin-up electron, resulting in the less-favorable two-center threeelectron interaction, thereby destabilizing the adsorbate. Specifically, the filled anti-bonding Pt orbital leads to a weaker adsorption free energy of formaldehyde on Pt-RuO 2 (110) (-0.26 eV) compared to that of RuO 2 (110) (-0.88 eV) (Fig. 6a).The increased energy towards CH 2 O* thus lowers the activation barrier (TS2) for the subsequent dehydrogenation (Fig. 6a). We further evaluated the PDOS of the d orbitals with a Pt atom placed on different substrates. Compared to Pt-Ru(0001), the d orbitals of Pt on Pt-RuO 2 (110) are characterized by greater delocalization (Supplementary Fig. 29) 32 as well as a larger density near the Fermi level. These features facilitate electron transfer and contribute to the enhanced MOR activity of Pt 1 / RuO 2 35 . In contrast, the Pt PDOS of PtC3 is small at the Fermi level ( Supplementary Fig. 29), suggesting that Pt 1 /VXC-72 is an ineffective MOR catalyst 9 . Besides, the additional d electrons in the Pt of PtC3 and Pt-Ru(0001) cause stronger repulsion to the coming ligand, e.g., CH 3 OH, thus weakening the adsorption of methanol and hindering MOR.

Discussion
We successfully prepared Pt single atoms characterized by distinct coordination environment by leveraging a simple impregnation-adsorption method and tailoring the support. The Pt 1 /RuO 2 SACs not only had a superb mass activity of 6766 mA mg -1 Pt but also CO resistibility towards the MOR. Pt 1 / RuO 2 was shown to be a significantly better catalyst compared to the commercial Pt/C benchmark. Theoretical simulations revealed that the Pt-O 3f -Ru cus with O br in Pt 1 /RuO 2 favored dehydrogenation of methanol with a low energy barrier and onset potential compared to that of Pt-C and Pt-Ru bonds. Moreover, CO at Pt 1 /RuO 2 can be removed via electrooxidation at low potentials, leading to enhanced anti-poisoning capability. The discovery of Pt 1 /RuO 2 SACs provides an approach to explore Pt SACs for MOR and related alcohol oxidation reaction.
Synthesis of Pt 1 /RuO 2 and control catalysts. The Pt 1 /RuO 2 catalysts were prepared by an impregnation-adsorption method. Briefly, 50 mg of RuO 2 were dispersed in 80 mL of distilled water. Next, 0.25 mL of a H 2 PtCl 6 ·6H 2 O solution (2 mg mL -1 ) was added dropwise; stirring at 70°C for 5 h followed this. The product was obtained by filtration, repeated washing with water and ethanol, and drying at 70°C overnight. Pt 1 /VXC-72 was prepared in a similar manner by using VXC-72. Pt 1 /RuO 2 -H catalyst was obtained by reducing Pt 1 /RuO 2 in H 2 at 80°C. The Pt 1 /RuO 2 -500 and Pt 1 /RuO 2 -700 catalysts were obtained under the heat treatment of Pt 1 /RuO 2 in the air at 500 and 700°C, respectively.
Catalyst characterization. The structure and morphology of the catalysts were characterized by XRD (PANalytical X'pert Pro, Cu Kα radiation), XPS (Axis Ultra DLD), and TEM (JEOL-2010F). The electron paramagnetic resonance (EPR) spectra were recorded at room temperature by using JEOL JES-FA200. The HAADF-STEM and EDS mapping were carried out using a double Cs-corrected FEI-Themis microscope operated at 300 kV. The STEM images were obtained with a convergent semi-angle of 25.1 mrad. The HAADF collection angle was 48-200 mrad. The XAFS at the Pt L 3 -edge was obtained on beamline 10C at the Pohang Light Source (PLS) in the Pohang Accelerator Laboratory (PAL), Republic of Korea. The XAFS data were processed and analyzed using ATHENA and ARTEMIS 38 . The Pt content in the catalyst was measured by ultraviolet visible spectroscopy (UV-vis, Lambda 20, Perkin Elmer). An appropriate amount of a H 2 PtCl 6 ·6H 2 O solution (2 mg mL -1 ) was added to deionized water and diluted to 100 mL. After the adsorption of Pt species by RuO 2 in solution, the residual solution was measured by UV-vis, and by the comparison of the absorbance with the working curve, the Pt content in RuO 2 was estimated.
Electrochemical characterization. The electrochemical tests were conducted using a CHI 900D workstation (CH Instruments). A three-electrode setup was used and all tests were conducted at 25°C. A graphite rod was used as a counter electrode; (KCl saturated) Ag/AgCl was selected as the reference electrode. To prepare the working electrode, 1 mg of catalyst and 1 mg of VXC-72 were ultrasonically dispersed in a mixture of 200 μL of water, 50 μL of ethanol, and 20 μL of Nafion (Dupont, 5 wt%). Next, 5 μL of the above ink was then placed onto a glassycarbon electrode (4 mm in diameter) and dried at room temperature. In an N 2purged 0.1 mol L −1 KOH and 1 mol L −1 methanol/ethanol solution, the MOR/ EOR electrocatalytic activity was evaluated by CV (scan rate was set at 50 mV s −1 ). CV scans were performed until a convergent response was recorded. For the CO stripping tests, CO was adsorbed on the catalyst by bubbling CO in 0.1 mol L −1 KOH for 10 min at a potential of −0.95 V vs. Ag/AgCl. CO in the solution was eliminated by bubbling the electrolyte with N 2 for 20 min. Stripping tests were carried out from −0.95 to 0 V at 50 mV s −1 for two consecutive scans.