CO oxidation on metal oxide supported single Pt atoms: The role of the

Article pubs.acs.org/IECR

CO Oxidation on Metal Oxide Supported Single Pt atoms: The Role of the Support Yang Lou and Jingyue Liu* Department of Physics, Arizona State University, Tempe, Arizona 85287, United States S Supporting Information *

ABSTRACT: Supported metal single atoms have demonstrated excellent catalytic performance for many chemical transformations. The effects of support on the catalytic performance of supported single metal atoms, however, have not been clearly elucidated. We carried out a systematic investigation of the effects of supports on CO oxidation by single Pt (Pt1) atoms dispersed on different metal oxides: highly reducible Fe2O3, reducible ZnO, and irreducible γ-Al2O3. It was found that Pt1 atoms on three metal oxides are active for CO oxidation, and the chemical properties of supports determine the catalytic performance of Pt1 single-atom catalysts (SACs). Both the presence of −OH groups on support surfaces and the addition of H2O significantly modify CO oxidation on three SACs and reduce the effects of supports on their catalytic performances. We conclude that the interaction between single metal atoms and support as well as surface properties of supports control the catalytic behavior of SACs.

1. INTRODUCTION Catalysis by supported single atoms has attracted increasing attention.1−13 Supported single metal atoms have demonstrated excellent performances for a variety of important catalytic reactions such as water−gas shift (WGS) reaction,3,9−12 CO oxidation,1,14 methanol reforming,15 hydrogenation,5−8 electrocatalysis,16−18 and photocatalysis.19,20 Single-atom catalysts (SACs), defined as those catalysts that consist of only isolated single atoms dispersed on a support, not only maximize the metal efficiency but also provide unique systems for fundamental study of catalytic processes due to their welldefined active centers. Metal single atoms, if not strongly anchored onto a support surface, are often mobile and tend to aggregate to form clusters or nanoparticles either during the catalyst synthesis processes or under catalytic reaction conditions.4,7,21,22 Metal−support interactions can be utilized to anchor single atoms onto supports, for example, metal atoms embedded into cation vacant sites of metal oxides,1,23 adsorbed onto unsaturated cationic sites (such as pentacoordinate Al 3+ on Al 2 O 3 surfaces),24 and/or bonded onto specific defects or heteroatoms.6 Anchoring between support surfaces and metal atoms not only stabilizes metal atoms under reaction conditions but also may facilitate the dispersion of metal atoms over supports during the synthesis processes.25−27 Many approaches have been utilized to synthesize SACs. For example, atomic layer deposition (ALD) method has been used to prepare SACs.7,16,28 Leaching of metal nanoparticles from supported metal catalysts by cyanide (NaCN or KCN) has been used to prepare SACs.11,29 Metal atoms can be anchored onto the support surfaces when appropriate organic groups © XXXX American Chemical Society

such as ethylene glycolate radicals are used to enhance the interactions between specific metal atoms and selected organic functional groups.5 In this study, we utilize a conventional strong adsorption approach30−33 to directly deposit single Pt atoms (Pt1-containing species) onto preformed metal oxide nanocrystallites or nanowires (NWs) and investigate the catalytic behavior of the synthesized SACs with different postsynthesis treatments and under selected reaction conditions. It has been reported that Au and Pt ionic species dispersed on different types of metal oxides possess similar intrinsic activity (turnover frequency (TOF)) and apparent activation energy for WGS reaction irrespective of the types of supports used.9−12 The authors proposed that the supports act only as physical carriers to support the metal species but are not catalytically or chemically involved in the WGS reaction.12 On the other hand, for CO oxidation on Pt1/FeOx SAC, the nature of FeOx nanocrystallites plays a crucial role in modifying the electronic properties of the individual Pt1 atoms and provides active oxygen species to react with the CO molecules which adsorb on the anchored Pt1 atoms.1 Furthermore, for steam reforming of methanol (SRM), it has been proposed that the interaction between Pt1 and Au1 atoms with ZnO {10−10} nanofacets facilitates electron transfer from the Pt1 and Au1 atoms to the ZnO {10−10} surfaces, resulting in positively charged Pt1 and Au1 atoms.15 Such charge transfer is proposed Received: Revised: Accepted: Published: A

April 10, 2017 May 19, 2017 May 23, 2017 May 23, 2017 DOI: 10.1021/acs.iecr.7b01477 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

powder was dried at 60 °C for 12 h in air. To exclude the possible influence of chloride ions, the catalysts were washed by deionized water, before being dried, several times until no Cl− ions could be detected by saturated AgNO3 solution. The Pt/ Fe2O3 catalyst was calcined at 300 °C for 2 h in N2, and the Pt/ γ-Al2O3 and Pt/ZnO catalysts were calcined at 300 °C for 2 h in air. We initially used N2 for calcination of the Pt/Fe2O3 with the assumption that the use of N2 may enhance the dispersion of Pt on the surfaces of Fe2O3. However, there were no detectable differences on the catalytic activity and the Pt dispersion of Pt1/Fe2O3 catalysts calcined with air or N2, as shown in Figure S2. The actual loadings of Pt, which were measured by ICP-MS (inductively coupled plasma mass spectrometry), on the Fe2O3, ZnO, and γ-Al2O3 are 0.029, 0.034, and 0.044 wt %, respectively. We used low levels of metal loading to guarantee that only singularly dispersed Pt1 atoms were present in the catalysts during the synthesis and the catalytic reaction processes. The synthesized SACs are conveniently denoted, in this paper, as Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3, respectively. 2.2. Evaluation of the Catalytic Performance. The activity of the synthesized SACs was tested for CO oxidation in a fixed-bed reactor at atmospheric pressure. Typically, 50 mg of catalyst was used for each test. The feed gas, containing 1 vol % CO, 4 vol % O2, and He balance, passed through the catalytic bed at a flow rate of 12.5 mL/min (corresponding to the weight hourly space velocity (WHSV) of 15, 000 mL/g·h). The concentrations of CO and CO2 in the outlet stream were measured by an online gas chromatograph (Agilent 7890A equipped with thermal conductivity detector). For wet CO oxidation, the feed gas flowed directly through a water vapor saturator at 20 °C and then into the reactor. The partial pressure of the saturated water vapor at 20 °C is about 2.2 kPa, resulting in a concentration of moisture (H2O vapor) in the feed gas of about 2.2 vol %. The “dry” catalyst was obtained by treating the previously calcined catalyst (calcined at 300 °C for 2 h) at 250 °C for 40 min in 5 vol % O2/He (12.5 mL/min) just prior to catalytic testing without further exposure to atmosphere. This pretreatment was used to eliminate the −OH groups that might have accumulated on the surfaces of the synthesized SAC catalysts. In this paper, when we use the term “dry CO oxidation”, it refers to CO oxidation reaction with CO, O2, and He as feed gas mixture only. When a specified amount of H2O molecules are added into the reaction gas mixture, we label the reaction as “wet CO oxidation”. 2.3. Catalyst Characterization. The loading levels of Pt were measured by ThermoFinnegan iCAP Q quadrupole ICPMS with CCT (collision cell technology). Samples were run in kinetic energy discrimination mode with in-line aspiration of a multielement internal standard. The surface −OH groups were evaluated by Fourier transform infrared spectroscopy (FT-IR, Bruker IFS66 V/S) in the range of 4000−400 cm−1. The X-ray diffraction pattern was recorded on a PANalytical’s materials research diffractometer using Cu Kα radiation (1.54056 Å, 45 kV, and 40 mA), scanning from 15 to 80° with step size of 0.02°. Subangstrom resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images were obtained on a JEM-ARM200F TEM/ STEM with a guaranteed resolution of 0.08 nm. Before microscopy examination, the catalyst powders were ultrasonically dispersed in ethanol, and then a drop of the solution was

to facilitate the adsorption of reaction intermediates during SRM and thus modify the reaction pathways.15 A recent report suggested that when Pt atoms were incorporated into a zeolite (H-ZSM-5) by atomic layer deposition (ALD), the catalytic activity of such a system for low temperature (below 150 °C) CO oxidation was reported to be negligible.28 The adsorption strength of CO molecules onto single metal atoms, however, needs to be carefully evaluated because charge transfer between single metal atoms and support surfaces may strongly depend on the nature of the metal and the electronic structure of the support surfaces. To understand how the surface properties of different types of metal oxide supports affect the catalytic performance of supported metal SACs, we conducted a systematic investigation on a series of SACs that consist of Pt1 atoms dispersed onto (1) Fe2O3 (highly reducible), (2) ZnO (reducible), and (3) γ-Al2O3 (irreducible) supports. We hypothesized that the supported metal atoms interact differently with metal oxide surfaces of different redox properties. Our experimental results confirm that for CO oxidation reaction, the interaction between single metal atoms and support surfaces as well as surface functional groups of the supports control the catalytic behavior of SACs. To investigate the effect of support surfaces as well as the surface functional groups, for example, −OH groups, on the catalytic behavior of the synthesized SACs, we conducted the following catalytic evaluations: (1) wet CO oxidation (a small amount of H2O was added into the feed gas) on dry SACs (calcined prior to catalytic testing), (2) dry CO oxidation (without H2O in the feed gas) on dry SACs, (3) dry CO oxidation on as-synthesized SACs, and (4) dry CO oxidation on −OH functionalized SACs. Our experimental results reveal that for CO oxidation reaction, the presence of −OH groups on the support surfaces and the addition of H2O into the reactant mixture significantly modify the catalytic behavior of the supported Pt1 SACs: when −OH groups and H2O are present, the chemical nature of the support surface becomes less important. When the support surfaces are dry and when the reactant mixture does not contain H2O, the CO oxidation on Pt1 SACs, however, strongly depends on the reducibility of the support. Although we use Pt as an example, these results are general and may apply to other types of supported metal SACs and thus provide insights into the fundamental understanding of the catalytic nature of SACs.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Fe2O3 nanocrystallites were synthesized by a precipitation method.34,35 The iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, Sigma-Aldrich) was used as a precursor salt, and sodium carbonate (Na2CO3, Sigma-Aldrich) was used as a precipitant. The resultant solid powder precipitates were dried at 60 °C for 12 h and calcined at 350 °C for 4 h in air. The X-ray diffraction result confirms that the prepared Fe2O3 is mainly in the form of α-Fe2O3, as shown in Figure S1. The ZnO nanowires were synthesized by a vapor transport process.36 The γ-Al2O3 was purchased from Inframat Advanced Materials. Pt1 atoms were dispersed onto the surfaces of Fe2O3, ZnO, and γ-Al2O3 by a strong adsorption method.30−33 The corresponding amount of Pt (H2PtCl4· 6H2O, Sigma-Aldrich) was first deposited onto the surfaces of Fe2O3 nanocrystallites, γ-Al2O3 nanopowders, and ZnO nanowires. The pH values of the Pt-containing solutions were finely controlled for each support material. After being aged at room temperature for 2 h, washed, and filtered, the solid B

DOI: 10.1021/acs.iecr.7b01477 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Low-magnification (a, c, and e) and atomic resolution (b, d, and f) HAADF/STEM images of Pt1/Fe2O3 (a and b), Pt1/γ-Al2O3 (c and d), and Pt1/ZnO (e and f) unambiguously show that the synthesized SACs consist of only isolated individual Pt atoms without the presence of Pt clusters or nanoparticles dispersed onto the corresponding metal oxide supports.

for all the Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs. Because three different supports possess different activity for CO oxidation, we need to subtract the contribution of the support surfaces when we compare the catalytic behavior of the Pt1 atoms supported on different supports. Because the Pt loading is extremely low, we can approximate the reaction rates due to the presence of Pt1 atoms by subtracting the reaction rates of the supports from the reaction rates of the corresponding SACs. This treatment is justifiable as long as the total number of the supported Pt1 atoms is much less than the total number of the surface atoms of the support material. The TOF is defined, in this case, as the amount of CO molecules converted over one Pt atom in one second, calculated based on eq 2. When the TOF was evaluated, the CO conversion rate was adjusted to below 15% by varying the space velocity to eliminate the thermal and diffusion effects:

put onto a lacey carbon coated TEM Cu grid. Low magnification images were extensively screened to make sure that Pt nanoparticles were not present in the as-synthesized or used SACs. High magnification images were used to verify the presence of individual Pt atoms and the absence of Pt clusters in SACs. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of CO adsorption on Pt1/Fe2O3 and Pt1/γ-Al2O3 SACs with a Pt nominal loading of 1.0 wt % were obtained on a Nicolet Nexus 6700 spectrometer with an MCT detector. The DRIFT sample cell was equipped with a ZnSe window. The DRIFT spectra were collected with a resolution of 4 cm−1 and 64 scans in Kubelka−Munk units. The samples were pretreated at 250 °C for 40 min by N2. The in situ DRIFT experiments were conducted with the sample chamber filled with 5 vol % CO/He, and the sample temperature was 25 °C. 2.4. Kinetic Measurements. The specific reaction rate (calculated based on eq 1) and apparent activation energy were measured under a reaction condition of 1 vol % CO, 4 vol % O2, and He balance in the temperature range of 140 to 180 °C

r= C

CCOXCOVPatm (mol ·s−1·g −cat1) mcat RT

(1)


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Figure 2. Specific rate of dry CO oxidation over dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs (a). Feed gas composition: 1 vol % CO, 4 vol % O2, and He balance with a WHSV of 15 000 mL/g·h. Apparent activation energy (Ea) of dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs for dry CO oxidation (b).

TOF =

(rcatalyst − rsupport)NA NPt

(s−1)

spectra (Figure S5) clearly show that the −OH groups were present on the surfaces of the as-synthesized SACs.40,41 These SACs were calcined at 300 °C for 2 h and then stored under ambient conditions prior to the FT-IR experiments. Figure S5 also clearly shows that the surface −OH groups are almost nondetectable immediately after calcination at 250 °C for 40 min. Therefore, we can assume that the −OH groups were not present on the dry SACs. Figure 2a shows the specific activities for dry CO oxidation on the dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs. These SACs clearly exhibit different specific activities. Because the specific activity reflects the normalized (normalized to per gram of Pt) conversion rate, Figure 2a clearly demonstrates that the catalytic behavior of the Pt1 atoms strongly depends on the type of the support material. Figures S6a−c display the plots of the catalytic activity under different reaction conditions. The activity of the second cycle test repeats the results of the first cycle test, suggesting that the as-synthesized SACs were stable during CO oxidation reaction, which is in line with our STEM images of used SACs (Figures S4a−c). Figure 2b displays the Ea values of the dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs for dry CO oxidation reaction. The apparent activation energies of dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs are 67.2 ± 2.9, 78.1 ± 0.2, and 91.2 ± 0.5 kJ/mol, respectively. For dry CO oxidation, Pt1 atoms supported over different metal oxides exhibit drastically different specific activity and Ea. These experimental results suggest that the active centers for CO oxidation, which are assumed to consist of the Pt1 atoms and their nearby neighbors of the support, behave differently on different supports. It is highly plausible that due to the influence of the support, Pt1 atoms possess different binding properties to the reactant, intermediate, and product molecules.42−44 The TOF values for dry CO oxidation on the three dry SACs are shown in Table 1. In the reaction temperature range of

(2)

where the rcatalyst and rsupport are reaction rates of catalysts and supports, respectively, mcat is mass of catalyst in the reactor bed (g), CCO is concentration of CO in the feed gas, V is total flow rate (m3/s), XCO is the conversion rate of CO, R is the molar gas constant (8.314 Pa·m3·mol−1·K−1), T is temperature (K), Patm is atmospheric pressure (101.3 KPa), NA is the Avogadro constant (6.02 × 1023 mol−1), and NPt is the number of Pt atoms per gram of catalyst.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized Pt1 SACs. With low levels of Pt loading, characterization of the synthesized SACs becomes difficult by spectroscopy or diffraction techniques. Aberration-corrected STEM techniques, especially the HAADF imaging mode (HAADF-STEM), however, can be routinely used to visualize the highly dispersed Pt1 atoms and determine if metal nanoclusters and/or nanoparticles are present in the fresh or used SACs.1,37 As clearly shown in the HAADF-STEM images in Figure 1, isolated individual Pt atoms were uniformly dispersed on the surfaces of the Fe2O3, ZnO, and γ-Al2O3 supports. Low magnification HAADF-STEM images (Figures 1a, c, and e) confirmed the absence of any Pt particles or clusters in the synthesized SACs. By examining numerous low/high magnification HAADF-STEM images (Figures S3a−c) obtained from different regions of the catalyst samples, we unambiguously concluded that the synthesized Pt1/Fe2O3, Pt1/ZnO, and Pt1/γAl2O3 SACs contained only isolated Pt1 single atoms dispersed onto the metal oxide supports. For the used Pt1/Fe2O3, Pt1/ ZnO, and Pt1/γ-Al2O3 SACs, single Pt atoms were still retained after reaction, and no Pt clusters or particles could be observed on the surfaces of Fe2O3, ZnO, or γ-Al2O3 supports, as shown in Figures S4a−c, which confirms that the measured catalytic activity and kinetic data originated from the single Pt1 atoms rather than the Pt clusters or particles. 3.2. Dry CO Oxidation on Dry Pt1 SACs. The surfaces of metal oxides are generally covered by −OH groups,38,39 and the amount of the −OH groups as well as their adsorption strength depend on the surface chemistry and structure of the specific metal oxide supports. The representative peaks of the FT-IR

Table 1. TOF Values of Pt1 Single Atoms on Different Metal Oxide Supports for Dry CO Oxidation

D

temperature (°C)

Pt1/Fe2O3 (× 10−2/s)

Pt1/ZnO (× 10−2/s)

Pt1/γ-Al2O3 (× 10−2/s)

140 160 180

15.0 33.2 89.6

5.3 16.5 39.0

0.3 0.9 2.6


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Figure 3. (a) Specific rate of wet CO oxidation over dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs with a feed gas composition of 1 vol % CO, 4 vol % O2, and He balance and a space velocity of 15 000 mL/g·h (2.2 vol % H2O was added to the feed gas) and (b) apparent activation energy (Ea) for wet CO oxidation on dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs.

140−180 °C, the TOF values of Pt1/Fe2O3 are larger than those of the Pt1/ZnO, which are much larger than those of Pt1/ γ-Al2O3. At 140 °C, the TOF of a Pt1 atom supported on Fe2O3 and ZnO is about 50 and 18 times higher than that of a Pt1 atom supported on γ-Al2O3. Such a huge difference in the TOF value of one Pt1 atom suggests that under dry CO oxidation reaction conditions, the surface properties of the supports control the catalytic behavior of the dry SACs. Density functional theory (DFT) calculations suggest that for the Pt1/Fe2O3 SAC, about 0.45 electrons transfer from the Pt1 to the Fe2O3 support.1 The excessive electrons are distributed to the Fe2O3 support.45,46 Such a strong electronic interaction between the Pt1 atom and the Fe2O3 surface generates partially vacant 5d orbitals on the Pt1 atom, which subsequently reduce the adsorption strength of CO molecules and the reaction barrier for CO oxidation.1 The Pt1 atoms occupying the Fe cation vacancy sites facilitate electron transfer and reduce the activation energy of creating oxygen vacancies in the Fe2O3 support.1 For the Pt1/Fe2O3 SAC, electronic interaction between the Pt1 atom and the Fe2O3 surface modifies both the adsorption behavior of the CO molecules and the activation of the surface oxygen vacancies on the Fe2O3 support, resulting in significantly enhanced activity for dry CO oxidation because both the CO adsorption and O2 activation have been simultaneously promoted.47,48 For pure γ-Al2O3 support, the CO molecules bind extremely weakly onto the coordinated and unsaturated Al3+ sites because the heat of CO adsorption is as low as 9.5 ± 1.0 kJ/mol, close to the sublimation enthalpy of CO (7.6−8.1 kJ/mol).49 Furthermore, pure Al2O3 support is unable to activate O2 molecules.50,51 Thus, γ-Al2O3 is considered as inert for CO oxidation. When Pt1 atoms are deposited on γ-Al2O3 surface, only about 0.05 electrons can be transferred from a Pt1 atom to the γ-Al2O3 support,52 indicating the interaction between single Pt1 atoms and γ-Al2O3 is much weaker than that of Pt1/Fe2O3 SAC, which can be attributed to the reason why the TOF of Pt1 atom on the γ-Al2O3 support is much lower, as shown in Table 1. However, the Pt1 atoms play the most important role in activating both CO and O2 molecules in this case.4 For Pt1/γAl2O3 SAC, once an O2 molecule adsorbs on the Pt1 site, the O2 molecule gains one electron in an antibonding orbital to form O2− species, and then the activated O2 molecule will react

with a CO molecule to produce a carbonate species which decomposes to release CO2.4,52,53 The catalytic performances of the dry Pt1/Fe2O3 and Pt1/γAl2O3 SACs clearly demonstrate that the redox property of the support surfaces determines the metal−support interaction that subsequently influences their catalytic performances. For the dry Pt1/ZnO SAC, the redox ability of ZnO is much lower than that of the Fe2O3 but much higher than that of the γ-Al2O3. DFT calculations suggest that there is significant electron transfer from Pt1 atoms to ZnO15 and this charge transfer may significantly modify the electronic structure of the neighboring ZnO surfaces.54 Such electronic interactions may also modify the electronic orbitals of the Pt1 atom to form partially vacant 5d orbitals, which reduces the adsorption strength of the CO molecules.1 The Pt1 atoms occupying the Zn cation vacancy sites facilitate the formation of oxygen vacancies on the ZnO surfaces.15 Similar to the enhancement of dry CO oxidation by the Pt1 atoms on the Fe2O3, the presence of Pt1 atoms on the ZnO surfaces significantly promotes the activity for dry CO oxidation (Table 1). Moreover, the in situ DRIFT spectra (Figure S7) show that for the Pt1/γ-Al2O3 SAC, the peak at 2060 cm−1 can be attributed to CO adsorption on the Pt1 atoms.4 For the Pt1/ Fe2O3 SAC, there is a broad peak centered at 2076 cm−1, which can be assigned to CO adsorption on Pt1 atoms.1 The different CO-Pt bands on Pt1/Fe2O3 and Pt1/γ-Al2O3 SACs indicate that the electronic characteristics of the supported Pt1 atoms depend on the chemical nature of the support. The obvious shift from 2060 to 2076 cm−1 on the CO adsorption on Pt1 atoms supported on different metal oxides clearly confirms that the electronic interaction between Pt1 atoms and support is determined by the surface properties of the metal oxide support. Our experimental results clearly confirm that for dry CO oxidation over dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs, the kinetic parameters such as TOF and Ea strongly depend on the nature of the support surface, which governs the interactions between the Pt1 atoms and the support. For dry CO oxidation reaction, the surface properties of the supports in supported metal SACs play a dominant role in determining their catalytic performance. E


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Figure 4. (a) Apparent activation energy (Ea) of uncalcined Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs for dry CO oxidation and (b) FT-IR spectra of −OH groups on uncalcined Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs.

3.3. Wet CO Oxidation on Dry Pt1 SACs. It has been reported that for WGS reactions, the presence of −OH groups and alkali ions facilitates the catalytic activity of supported Au and Pt ionic species and that the effects of the supports on the catalytic performance are negligible.9−12 For catalytic reactions over noble metals, the presence of H2O in the reactant mixture significantly promotes the catalytic performance and modifies the reaction pathways.55−59 To investigate the influence of H2O on the activity of dry Pt1/Fe2O3, Pt1/ZnO, and Pt1/γ-Al2O3 SACs, 2.2 vol % H2O was added into the feed gas. The measured activity and kinetic parameters for wet CO oxidation (Figure 3a and b) confirm that the presence of H2O significantly increases the specific reaction rate and decreases the apparent activation energies for all the synthesized SACs. As shown in Figure S8, the catalytic activity was greatly increased when H2O is added to the feed gas, while the catalytic activity is significantly dropped when H2O is removed from the feed gas. Similar experimental results were reported for CO oxidation over Au/TiO2; it was proposed that the weakly adsorbed water can take part in the catalytic reaction, and the coverage of weakly adsorbed water on the support surfaces largely influences the catalytic performance.41 For wet CO oxidation, both the dry Pt1/Fe2O3 and Pt1/ZnO SACs exhibit similar apparent activation energy (∼50 kJ/mol). For Pt1/γAl2O3, the apparent activation energy is about 10 kJ/mol higher than that of the Pt1/Fe2O3 and Pt1/ZnO SACs. The TOF values of the Pt1 atoms supported on the three different types of supports are still different, suggesting that the nature of the support still exerts some influence on the CO oxidation reaction even in the presence of H2O. By comparing the TOF values of the Pt1 atoms under dry (Table 1) and wet (Table S1) conditions, one can clearly conclude that the presence of H2O in the reaction mixture significantly reduces the influences of the chemical nature of the supports on the TOF values of the supported Pt1 atoms. The promoting effects of H2O on the reaction barrier and activity were proposed to originate from the direct involvement of H2O in the catalytic processes of CO oxidation.41,55,57,59,60 Activated oxygen can react with H2O to form *OH, which further reacts with CO via a smaller barrier route compared to that of the direct reaction of CO with oxygen species to form CO2 to yield reactive *COOH.55,57 Further reaction between *COOH and *OH to form CO2 can be easily accomplished with an extremely low reaction barrier.55,60 As a result, the

overall reaction barrier can be greatly reduced when H2O presents on the SAC surface compared with that on dry surface. This is consistent with our experimental results that the apparent activation energy of CO oxidation over three SACs under wet conditions is much lower than that under dry conditions, suggesting the presence of H2O may have enhanced the activation of oxygen. 3.4. Dry CO Oxidation on Uncalcined Pt1 SACs. To better understand the influence of −OH groups on the nature of the fabricated Pt1/ZnO, Pt1/Fe2O3, and Pt1/γ-Al2O3 SACs, the uncalcined catalysts, which were dried at 60 °C for 12 h, without further calcination were directly tested for dry CO oxidation. Figure 4a shows that the uncalcined Pt1/ZnO, Pt1/ Fe2O3, and Pt1/γ-Al2O3 SACs exhibit similar TOF values (Table S3) and apparent activation energies (around 71 kJ/ mol) for dry CO oxidation. These experimental results demonstrate that for dry CO oxidation at low temperatures, the uncalcined Pt1 SACs are catalytically similar regardless of the chemical nature of the support surfaces. The FT-IR spectra (Figure 4b) obtained from the uncalcined Pt1/ZnO, Pt1/Fe2O3, and Pt1/γ-Al2O3 SACs clearly reveal a strong, broad peak in the range of 3750−2500 cm−1, suggesting the presence of large amounts of −OH groups on the uncalcined Pt1 SACs.40,41 During the process of synthesis via strong adsorption method, the Pt complexes are thought to bind onto the support surfaces through the surface −OH groups.61 The drying at 60 °C for 12 h does not eliminate the −OH groups. We propose that the Pt1 single atoms are most probably surrounded by the −OH groups, and the direct interaction between the Pt1 atoms with the support surfaces is significantly weakened. The interaction between the Pt1 atoms and the −OH groups may play a dominant role in determining the electronic state of the Pt1 atoms and consequently their bonding properties to the CO molecules. As a result, the influence of the support on the catalytic behavior of the supported Pt1 atoms becomes less important. The lower TOF values of the uncalcined Pt1 SACs are still not fully understood. The surface residues from the wet chemistry synthesis may play a role as well. Under this specific condition, the surface −OH functional groups, instead of the surface chemistry of the support materials, play the central role in determining the catalytic activity of the Pt1 atoms. Therefore, the TOF value and the apparent activation energy of the uncalcined SACs should have similar values irrespective of the nature of the support. From this perspective, the supported F


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Figure 5. CO conversion rate of the dry and soaked Pt1/Fe2O3 SACs at 180 (a) and 230 °C (b) to test the stability of the −OH functional groups on the surfaces of Pt1/Fe2O3 SAC with a feed gas composition of 1 vol % CO, 4 vol % O2, and He balance, and a space velocity of 30 000 mL/g·h. (c) FT-IR spectra of −OH groups on a Pt1/Fe2O3 SAC before and after CO oxidation reaction at 230 °C. The soaked Pt1/Fe2O3 SAC was prepared by soaking the calcined Pt1/Fe2O3 SAC in H2O for 12 h and was then dried at 60 °C for 5 h.

soaked Pt1/Fe2O3 SAC and the soaked Pt1/Fe2O3 SAC after the catalytic test at 230 °C. As shown in the FT-IR spectrum (Figure 5c), the strong and broad peak in the range of 3750− 2500 cm−1 suggests the presence of large amounts of −OH groups in the soaked Pt1/Fe2O3 SAC.41 After about 20 min of CO oxidation reaction at 230 °C, the −OH groups in the soaked Pt1/Fe2O3 SAC clearly disappeared. Similarly, the −OH groups on the surfaces of the uncalcined Pt1/Fe2O3 catalyst are also not stable at temperatures higher than 230 °C, as shown in Figure S9. By correlating the presence and disappearance of the −OH groups with the change in conversion rate, we can conclude that (1) the presence of the −OH functional groups on the Pt1/Fe2O3 SAC enhances the CO oxidation rate and (2) the −OH functional groups are not stable at higher reaction temperatures.

metal SACs can be tuned by functionalizing the support surfaces with desirable functional groups for specific catalytic reactions. When the appropriate functional groups are stable during a selected catalytic reaction, the supported metal SAC can be considered as a single-site heterogeneous catalyst (SSHC), and such strategies should be pursued and may bridge heterogeneous catalysis to homogeneous catalysis.13 3.5. Dry CO Oxidation on −OH Functionalized Pt1/ Fe2O3 SAC. To investigate the stability of surface −OH groups during the CO oxidation reaction, the calcined Pt1/Fe2O3 SAC was fully soaked in deionized H2O for 12 h and then was dried at 60 °C for 5 h (labeled as soaked Pt1/Fe2O3). Figure 5a shows the CO conversion rates for dry CO oxidation (at 180 °C) on both the soaked and dry Pt1/Fe2O3 SACs. The activity of the soaked Pt1/Fe2O3 SAC is about 2 times higher than that of the dry Pt1/Fe2O3 SAC, and both SACs are stable during the CO oxidation reaction. When the CO oxidation reaction is conducted at 230 °C, however, the conversion rate of the soaked Pt1/Fe2O3 SAC drops to that of the dry Pt1/Fe2O3 SAC after about 20 min (Figure 5b). After this initial drop, both the soaked and dried SACs yield similar conversion rates and are stable. Figure 5c displays the FT-IR measurement on the

4. DISCUSSION AND CONCLUSION Our systematic experimental investigations confirm that for CO oxidation reaction, in the absence of −OH/H2O on the surfaces of Pt1 SACs, both the activity and the relevant kinetic parameters of Pt1 atoms are directly influenced by the nature of G


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acquiring the FT-IR data. The authors gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science and the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University.

the support materials. The direct interaction of Pt1 atoms with the coordinatively linked oxygen atoms of the support induces charge transfer between the metal and the support surface. Such direct electronic interaction modifies both the adsorption capability of the Pt1 atoms and the reactivity of the support and thus tunes the catalytic performance of the supported Pt1 SACs. The active centers may consist of (Pt1−Om−), where m refers to the number of oxygen ions of the support material that directly interact with the supported Pt1 atoms. For dry CO oxidation on dry Pt1 SACs, the degree of such electronic interaction and thus the catalytic performance of the Pt1 SACs depend on the reducibility of the support material. The presence of H2O in the reactant mixture significantly reduces the influence of supports on the catalytic performances of the Pt1 SACs. When −OH functional groups are associated with the Pt1 atoms, the chemical nature of the support materials seems to become less relevant, and the support surface may act purely as a carrier for the Pt1 atoms. For CO oxidation over −OH functionalized Pt1 SACs, the interaction between Pt1 atoms and the surface −OH groups may play a more significant role in determining the charge state of the Pt1 atoms and their electronic interaction with the support atoms becomes less important. In this specific case, the (Pt1-(OH)n) entities, instead of the (Pt1−Om−) moieties, become the active centers, and the reducibility of the support surfaces may become irrelevant.9−12 For dry CO oxidation, the −OH functionalized Pt1 SACs seem to be stable at low temperatures. At higher reaction temperatures, however, the −OH functional groups become unstable, and the activity of the Pt1 SACs strongly depends on the chemical nature of the support materials. When H2O is added to the reactant mixture, the promoting effect of the H2O molecules significantly modifies the effect of the chemical nature of the support materials. However, even in this case, the activity of the Pt1 SACs still depends on the chemical nature of the support materials. Our systematic study and experimental design reported in this paper are general, and the approach can be extended to understand the behavior of other types of supported metal SACs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01477. X-ray diffraction patterns, catalytic activity charts, aberration-corrected HAADF/STEM images, FT-IR results, DRIFT spectra, and TOF values and ratios(PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yang Lou: 0000-0002-8310-8150 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported the National Science Foundation under Grant CHE-1465057. We thank Dr. Bin Mu, Mr. Mitchell Armstrong and Mr. Bohan Shan for assistance with H


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CO oxidation on metal oxide supported single Pt atoms: The role of the

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