Chemical and Structural Dynamics of Nanostructures in Bimetallic Pt

Subscriber access provided by Stockholm University Library

Article

Chemical and Structural Dynamics of Nanostructures in Bimetallic PtPd Catalysts, Their Inhomogeneity, and Their Roles in Methane Oxidation Haoyu Nie, Jane Howe, Petar Lachkov, and Ya-Huei (Cathy) Chin ACS Catal., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Chemical and Structural Dynamics of Nanostructures in Bimetallic Pt-Pd Catalysts, Their Inhomogeneity, and Their Roles in Methane Oxidation Haoyu Nie,1 Jane Y. Howe,2,3 Petar T. Lachkov,1 and Ya-Huei (Cathy) Chin*,1 1Department

of Chemical Engineering and Applied Chemistry, University of Toronto, M5S 3E5, Canada 2Hitachi

3Department

High-Technologies America Inc., Clarksburg, MD 20871, USA

of Materials Science and Engineering, University of Toronto, Toronto, M5S 3E4, Canada

* To whom correspondence should be addressed Email: [email protected] Tel: (416) 978-8868 Fax: (416) 978-8605

ABSTRACT: This study unravels the diverse sizes and chemical compositions of various nanostructures, from single atoms, monometallic clusters, to bimetallic particles in realistic, supported bimetallic Pt-Pd catalysts. Aberration-corrected scanning transmission electron microscopy, CO infrared spectroscopy, and oxygen uptaketitration studies probe the structural dynamics of these nanocreatures in response to changing gas phase compositions and oxygen chemical potentials, whereas rate assessments in the kinetically controlled regime under differential fuel-lean conditions at 698-773 K elucidate their catalytic roles in C-H bond activation during methane oxidation catalysis. Reductive treatments on Pt-Pd bimetallic catalysts (0.92-3.67 wt% Pt, 1 wt% Pd) lead to redistributions of the metals as Pt single atoms, small Pt clusters (~2 nm), and large Pt-Pd alloy clusters (> 5 nm), and their relative abundances depend largely on the overall Pt-to-Pd atomic ratio. Treatments in incremental O2 pressures at temperatures relevant to CH4-O2 catalysis redisperse the small Pt clusters, thus increasing the density of Pt single atoms, while the remaining clusters retain their metallic bulk. The large Pt-Pd alloy clusters, however, undergo incipient structural reconstruction, forming a thin PdO shell covering a Pt-rich core, driven by the large, negative free energy of PdO formation and the lower surface free energy of PdO than Pt. During CH4-O2 catalysis, Pt single atoms and small Pt clusters are largely inactive. In contrast, the core-shell clusters are highly reactive. On these cluster surfaces, the O2- anions are highly nucleophilic whereas the Pd2+ cations are highly electrophilic, 1 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 35

as they are contacted to the underneath Pt-rich core. They form Pd2+-O2- site pairs that catalyze the kinetically relevant C-H bond cleavage of methane at < 40 kJ mol-1 via the formation of a highly stabilized four-center transition state (H3Cδ−--Pd2+--Hδ+--O2-)⧧, much more effectively than monometallic O* covered Pt or PdO clusters. An increase in the Pt-to-Pd atomic ratio results in excess Pt that presents as inactive Pt single atoms or Pt clusters, thus lowering the overall, ensemble average rate constants. The Pt-to-Pd atomic ratio of ~0.5 is optimal for creating effective Pd2+-O2- site pairs on bimetallic core-shell clusters and minimizing the density of inactive Pt single atoms and clusters for CH4-O2 reactions. KEYWORDS: methane oxidation, C-H activation, Pt-Pd catalyst, core-shell, aberration-corrected STEM; infrared characterization; exhaust emission control

1. INTRODUCTION Natural gas is an attractive energy carrier and a chemical feedstock for producing syngas and hydrogen.1-6 It contains predominantly methane, small amounts of higher alkanes, and other impurities.1,2 Its use in industrial processes, power plants, and automobile engines, however, leads to the emission of methane, a potent greenhouse gas that traps ~25 times more heat than carbon dioxide in the atmosphere, when averaging over a 100-year period.7,8 Methane abatement is kinetically challenging, because it requires activating the strongest C-H bond among all hydrocarbons (BDE = 439 kJ mol-1).9 Often, the initial C-H bond activation is kinetically formidable and limits the overall catalytic turnovers.3,4 Effective catalysts, typically transition metals or oxides, would stabilize the C-H activation transition state, lowering the activation free energy changes to kinetically feasible values. Since the C-H activation barrier directly relates to the stability of the activated complex,3 the quest of finding an effective catalyst involves understanding what the active site structures are and how these structures stabilize the activated complex. The measured C-H activation barriers range from 75-108 kJ mol-1 on metal (75 kJ mol-1 on Pt,3 81 kJ mol-1 on Ir,9 84 kJ mol-1 on Pd4, 85 kJ mol-1 on Ni,10 and 108 kJ mol-1 on Rh10) and 61-110 kJ mol-1 on metal oxide (61 kJ mol-1 on PdO,4 110 kJ mol-1 for NiO11), much larger than those required for cleaving the weaker C-H bonds in higher hydrocarbons (e.g., 58 kJ mol-1 for C2H6 and 48 kJ mol-1 for C3H8 on PdO12). Despite extensive research efforts, this barrier remains large—the lowest barrier reported previously is 61 kJ mol-1 on PdO and increases to 84 kJ mol-1, as PdO reduces to metallic Pd.4 Previous studies have established that PdO remains as one of the most effective catalysts for methane combustion over a wide range of gas compositions and temperatures, including those encountered in the exhaust gas streams of automobile and stationary engines.1,2,12-14 Recent studies have shown that Pt incorporation into Pd catalysts could improve the long-term durability and tolerance of the catalysts to H2O and SOx impurities.2,15-18 2 ACS Paragon Plus Environment

Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Despite extensive research, the promotional effect of Pt in bimetallic Pt-Pd catalysts has remained an active research topic, and the existing reports on this effect are contradictory to each other—some have reported larger conversion rates on Pt-Pd than unpromoted PdO catalysts,13,15 whereas a few others have reported the exact opposite, that rates actually decrease after Pt incorporation.14-16 The exact reason for the curious increase in reaction rates on selected bimetallic Pt-Pd systems and not the others has not been rigorously established. Yet, a clear understanding on the nature of such active sites and their catalytic roles is required for designing more effective CH4-O2 catalysis. The lack of a satisfying explanation is caused, for the large part, by the structural complexity of these bimetallic catalysts, when dispersing the metals on high surface area substrates and exposing them to oxidative atmospheres. On this realistic, complex catalytic structure, the support is not innocent, but instead it affects the shapes, chemical compositions, and structures of Pt-Pd clusters.1,19 In addition, the bimetallic clusters undergo active, dynamic reconstruction with changing temperatures and feed compositions (from fuel-lean to fuel-rich), often encountered during the practical operation of catalytic converters.20,21 Depending on the contacting atmosphere and temperature, one or both metals in the bimetallic clusters may become oxidized and even segregated from each other.6,18,22-28 Adding to the complexity are the different extents of interactions between the metal atoms (Pt and Pd) and the support, between metal cations (Pt2+ and Pd2+) and the support, between the two metals (Pt-Pd), and also between a metal and a cation (Pt-Pd2+). In addition, cluster diameters dictate the contributions of surface free energies in the overall thermodynamics and, in turn, their thermodynamically stable phase structures.20,22,23,29-32 These complexities have made the site inhomogeneity nearly unpreventable in a realistic catalyst, rendering the detailed understanding of the bulk and surface structures, distributions of these structures, and their catalytic properties difficult. Yet, in a realistic catalyst, multiple types of active sites, each with their unique catalytic role, are present and often work together. The significance of their catalytic roles may change, as they undergo reconstruction with changing oxidant-to-fuel ratios. With the above considerations in mind, we embark on the journey to interrogate the dynamics and inhomogeneity of the Pt-Pd catalysts, unavoidable when dispersing both metals onto Al2O3 support, mimicking the real, working catalysts, in which the various interactions among the metal, oxide, and also support matter. Through systematic studies on an array of bimetallic Pt-Pd catalysts with changing Pt-to-Pd atomic ratios, we probe the surface and bulk dynamics and chemical compositions of metal/alloy clusters, dispersed on Al2O3 support, in both reductive and oxidative atmospheres relevant to CH4-O2 catalysis with spherical aberrationcorrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), infrared spectroscopy, and oxygen uptake-titration studies and then interrogate their catalytic roles in methane oxidation with rate assessments in the kinetically controlled regime. Our study answers the following questions of: (i) how many types of metal/alloy clusters and what are their respective chemical states, structures, and compositions 3 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 35

after reductive treatments? (ii) how do the various types of clusters undergo structural and chemical transformation during CH4-O2 reactions at high oxygen chemical potentials? (iii) what are the active site structures on these clusters and their catalytic roles in C-H activation? (iv) how does a change in the Pt-to-Pd atomic ratio affect the relative site abundance? (v) why do bimetallic Pt-Pd catalysts with varying Pt-to-Pd atomic ratios exhibit different reactivities? Our study shows the incipient structural evolution on the series of Pt-Pd catalysts, in response to the different gaseous environments. After reductive treatments, Pt single atoms, small Pt clusters (~2 nm), and large Pt-Pd alloy clusters (> 5 nm) co-exist. Controlled oxidative treatments lead to oxygen dissolution into the large alloy clusters and concomitant Pd migration from the inner core to the outer, near-surface region, forming PdO shells covering the underneath Pt-rich cores. On the surfaces of these core-shell clusters, Pd2+ cations are highly electrophilic and O2- anions are highly nucleophilic. Together, they form Pd2+-O2- site pairs with exceptional reactivity towards the initial, kinetically relevant C-H activation, exhibiting an extremely low barrier of 35-43 kJ mol-1. In contrast, the Pt single atoms and small Pt clusters are much less reactive. As the Pt-to-Pd atomic ratio increases, the fraction of inactive Pt sites increases, because excess Pt is unable to incorporate into the Pt-Pd clusters. Increasing the Pt content in the bimetallic clusters also results in a less oxidized PdO shell, thereby reducing the nucleophilicity of O2- anions. The diversity of active site quantities and structures in Pt-Pd catalysts leads to different rates, because these rates reflect the kinetic properties, averaging over all types of active sites, weighted by their respective abundance. The optimal Pt-to-Pd atomic ratio, established here, is useful for designing bimetallic catalysts without using excess Pt for effective methane abatement.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Monometallic Pt and Pd Clusters and Bimetallic Pt-Pd Clusters Dispersed on Al2O3 Support. Supported Pt (0.92 wt% or 1 wt%), Pd (1 wt%), and Pt-Pd (0.92 wt% Pt-1 wt% Pd, 1.83 wt% Pt-1 wt% Pd, and 3.67 wt% Pt-1 wt% Pd, which correspond to atomic ratios of 0.5Pt:1Pd, 1Pt:1Pd, and 2Pt:1Pd, respectively) on Al2O3 catalysts were prepared using the incipient wetness impregnation method. First, Al2O3 support (10 g, Sasol Germany, PURALOX TH 100/150, 153 m2 g−1 surface area, 0.96 cm3 g−1 pore volume, < 75 μm particle size) was treated in stagnant air by heating to 973 K at 0.083 K s-1 and then holding isothermally for 4 h. For synthesizing the monometallic catalysts, an aqueous solution of doubly deionized water (> 18.2 MΩ cm, 298 K) containing either the Pt(NH3)4(NO3)2 (Sigma-Aldrich, CAS#20634-12-2, 50.6% Pt) or Pd(NO3)2·2H2O (Sigma4 ACS Paragon Plus Environment

Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Aldrich, CAS#10102-05-3, 37.8% Pd) precursor was impregnated dropwise onto the treated Al2O3 support in a ceramic mortar under constant grinding using a pestle. For synthesizing the bimetallic catalysts, similar procedures were adapted, except that the aqueous solution contained both the Pt and Pd precursors. After the impregnation steps, the mortar containing the catalyst samples was sealed with a layer of Parafilm and placed in the ambient environment for 12 h to allow for slow drying. These samples were then held in stagnant air at 393 K and dried for 12 h prior to heating in flowing dry air (Linde, 99.99%, 0.33 cm3 gcat-1 s-1) at 0.033 K s−1 to and holding isothermally at either 673 or 973 K for 4 h. Next, the samples were treated in flowing H2 (Linde, 99.999%) at 1.67 cm3 gcat−1 s−1 by heating at 0.033 K s−1 to and then holding isothermally at treatment temperatures between 773 and 965 K for an hour. Afterwards, the samples were cooled down in He to ambient temperature and then held in flowing 5% CO/He (Linde, certified standard) at 1.67 cm3 gcat-1 s-1 for one hour. The surfaces of the metal/alloy clusters in these samples were saturated with adsorbed CO, thus avoiding the potential bulk oxidation of the clusters,33,34 before exposure to ambient air. The catalyst notations, which include information on their chemical compositions, metal contents, and specific treatment conditions, are summarized in Table 1. 2.2. Catalyst Characterizations. 2.2.1. Isothermal Oxygen Uptakes at 313 and 773 K. Oxygen chemisorption experiments were carried out using a volumetric adsorption-desorption apparatus at 313 K. Catalyst powders (0.5 g) were loaded into a sample chamber (quartz, 8.1 mm ID), heating in flowing H2 (Linde, 99.999%, 1.67 cm3 gcat-1 s-1) at 0.033 K s−1 to the corresponding temperature (given in column “H2” in Table 1), followed by holding isothermally at the temperature for one hour. Afterwards, the reactor was evacuated under dynamic vacuum (10-5 Pa) at the temperature for 12 h before cooling to 313 K for the chemisorption measurements. Two consecutive oxygen adsorption isotherms, each with step-wise increases in O2 pressure, were carried out, during which the chamber pressures were monitored with a pressure transducer (MKS, 120AA Baratron). In the first uptake measurement, doses of 1-2 μmol O2 (Linde, 99.99%) were sequentially introduced into the sample chamber at 5 min intervals; during each O2 dose, the pressure of the chamber was increased by ~0.001-0.5 kPa. Upon reaching ~13 kPa, isothermal oxygen uptakes were completed and the sample was evacuated under dynamic vacuum (10-5 Pa) at 313 K for 0.5 h. The second uptake measurement was then conducted following the same method. The amount of O2 uptake, per gram of catalyst, was determined by extrapolation of the plateau regions of the two isotherms to zero O2 pressure. The amount of chemisorbed oxygen was estimated from the difference between these extrapolated values. The fractional metal dispersion (D = Ms/M, where Ms and M denote surface and total metal atoms, respectively, given in column “Os/M” in Table 2) of the catalyst was determined by assuming an atomic O/Ms adsorption stoichiometry of unity. The average cluster diameter of each catalyst was determined by assuming hemispherical cluster shapes and cluster density equaled that of its bulk phase, i.e., bulk Pt at 21.5 g 5 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 35

cm-3, bulk Pd at 12.0 g cm-3, and bulk Pt-Pd at 16.8 g cm-3 (an average value of Pt and Pd densities) based on the following equation: 𝑑𝑎𝑣𝑔,

𝑂2 =

6𝑣𝑚 𝐷𝑎𝑚

(1)

where 𝑣𝑚 is the average volume occupied by a single metal atom in the bulk phase, 𝑎𝑚 is the average surface area occupied by an exposed metal surface atom, and 𝐷 is the dispersion. The 𝑣𝑚 and 𝑎𝑚 values for Pt are 15.10×10-3 nm3 and 8.07×10-2 nm2 and those for Pd are 14.70×10-3 nm3 and 7.93×10-2 nm2, respectively.35 For Pt-Pd alloy, the averages of these values are used. The oxygen contents in these clusters at 773 K were measured using the same apparatus. Following the same treatment in H2 (Linde, 99.999%) and evacuation under dynamic vacuum (10-5 Pa), the temperature of the sample chamber was held isothermally at 773 K throughout the oxygen uptake measurements. In the first uptake measurement, doses of 1-5 μmol O2 (Linde, 99.99%) were introduced sequentially into the sample chamber at a duration of 1 h dose-1. During this step, the pressure was increased by ~0.001-1.25 kPa per O2 dose to ~25 kPa. After the last O2 dose, the equilibrium was attained, when the pressure change per hour was less than 0.05 kPa, which corresponds to a maximum error of 0.9% from the reported O/M ratios. Upon reaching the equilibrium, the sample was evacuated under dynamic vacuum (10-5 Pa) at 773 K for 0.5 h and the second uptake measurement was then carried out following the same method. The second O2 isotherm was subtracted from that of the first one. This data treatment removes the small contributions of the weakly adsorbed oxygen and thus gives the oxygen contents of the clusters, including those bound strongly on the cluster surfaces and in the cluster bulk, at 773 K. 2.2.2. Scanning Transmission Electron Microscopy. The 1Pt-1Pd-T-Rd and 1Pt-1Pd-T-Ox catalyst powders (treatment details in Table 1) were dispersed onto a holey-carbon film supported on 200-mesh Cu TEM grid (Ted Pella Inc.). Scanning transmission electron microscopy (STEM) was carried out using a Hitachi HD-2700 spherical aberration-corrected scanning transmission electron microscope at 200 kV with a probe size of 80 pm and a probe current of ~100 pA. STEM imaging process and nanoparticle size analysis were proceeded using ImageJ software (Version 1.4.3.67, NIH, USA). The diameter of each nanoparticle (dk, where k represents a specific analyzed nanoparticle) was determined as a function of its area (Ak) measured using ImageJ software assuming a spherical shape of the nanoparticle:36 𝑑𝑘 =

4𝐴𝑘 𝜋

6 ACS Paragon Plus Environment

(2)

Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2.2.3. In Situ Infrared Spectroscopic Study of CO Adsorption and CO Oxidation Probe Reaction on Pt, Pd, and Pt-Pd catalysts. In situ infrared spectroscopy was carried out in the transmission mode using an environmental cell equipped with KBr windows mounted in a Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 70, MCT detector, max. 0.40 cm-1 resolution). For each experiment, the catalyst sample was pressed into a self-supporting wafer (~1.3 cm diameter and ~25 mg cm-2 surface density) using a die set (Across International, 30 mm ID) and hydraulic press (Specac) at 63 MPa for 1 min. The wafer was then loaded onto the infrared cell and one of the following in situ pre-treatment steps was carried out: (1) heating at 0.083 K s-1 in flowing H2 (0.83 cm3 s-1, Linde, 99.999%) to and then holding isothermally at 573 K for 0.5 h, followed by flushing in flowing He (0.83 cm3 s-1, Linde, 99.999%) for 0.5 h or (2) heating at 0.083 K s-1 to 573 K in flowing 20% O2/He mixture (0.83 cm3 s-1, prepared by mixing 99.99% O2 and 99.999% He, Linde) and holding isothermally for 1 h. The pre-treatment conditions for the various catalysts were summarized in Table 1, column “Pre-treatment atmosphere (573 K)”. The samples were then cooled to 296 K and subsequently exposed to a flowing 2% CO/He mixture (prepared by mixing certified standard 5% CO/He and 99.999% He, Linde) at 0.83 cm3 s-1 for 10 min, above which the absorbance remained unchanged with time. The infrared cell was then purged by flowing He (Linde, 99.999%) at 0.83 cm3 s-1 in order to remove the CO(g) from the chamber. Upon the complete removal of all gas-phase CO, detected by the disappearance of the CO rotational side band at 2180 cm1,37

the infrared spectra in the range of 500-4000 cm-1 were measured at 296 K by averaging 16 scans (0.40 cm-1

resolution). All spectra were normalized with the Al-O vibrational band at 1010 cm-1

38

in order to allow

quantitative comparisons of the integrated peak intensities among the samples. Afterwards, for the 0.5Pt-Rd, 1Pd-Rd, 0.5Pt-Ox, 1Pd-Ox, 0.5Pt-1Pd-Ox, 1Pt-1Pd-Ox, and 2Pt-1Pd-Ox catalysts, 10% O2/He (prepared by mixing 99.99% O2 and 99.999% He, Linde) was introduced into the infrared cell at 0.83 cm3 s-1 to react with pre-adsorbed CO and then the catalyst samples were heated to 373 K at 0.083 K s-1. The temperature was then held isothermally at 373 K for 10-30 min before the catalyst samples were cooled down to ambient temperature. The infrared spectra were measured at 296 K, before the introduction of 10% O2/He as well as after 10 min or 15 min of in situ CO oxidation reactions between the CO* and O2, respectively. After 15 min of CO*-O2 reactions, no variation in peak positions and intensities was detected, therefore the reaction was completed. 2.3. Steady-State Catalytic Rate Measurements. Catalyst powders (< 75 μm particle size) were diluted with SiO2 diluent (Grace, LC150A, 550 m2 g−1 surface area, 1.23 cm3 g-1 pore volume, < 75 μm particle size), which was pre-treated in stagnant room air in a muffle furnace by heating to 973 K at 0.083 K s-1 and holding isothermally for 4 h, at intrapellet SiO2-to-catalyst mass ratios () of 99. The resulting physical mixtures were pressed into a pellet using a die (Carver, 31 mm ID) and 7 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 35

hydraulic press (Specac) at 130 MPa for 10 min. The pellet was sieved to retain aggregates with particle diameters between 125 and 180 µm. These diluted catalyst particles were further mixed at an interpellet quartz-to-catalyst mass ratios () of 10899 with quartz sand of the similar size range (Sigma-Aldrich, CAS#60676-86-0, purum p.a., 125-180 µm particle size), which was treated in stagnant air by heating at 0.083 K s-1 to and holding isothermally at 973 K for 4 h. The catalyst and diluent mixtures were held on a quartz frit within a micro-catalytic plug-flow reactor (quartz, 8.1 mm ID) equipped with a K-type thermocouple placed at the center, in both the axial and radial directions, of the packed catalyst bed. Potential heat and mass transport limitations in this reactor configuration at these dilution levels were ruled out with the Koros-Nowak criterion,39 as measured and confirmed in previous studies.40,41 Using this reactor configuration, CH4-O2 reaction rates were measured in the kinetically controlled regime under fuel-lean conditions at 698-773 K. In addition, the reactor was operated under differential conditions (< 5% CH4 conversion). Prior to CH4-O2 rate measurements, each catalyst sample was pre-treated in situ by heating to 773 K at 0.083 K s-1 in flowing 25% O2/He (prepared by mixing 99.99% O2 and 99.999% He, Linde) at 20000-26700 cm3 gcat-1 s-1. During rate measurements, the reactant gas mixtures of CH4 (Linde, certified standard, 5% CH4/Ar), O2 (Linde, 99.99%), and He (Linde, 99.999%) were metered with thermal mass flow controllers (Brooks, SLA5850). Chemical compositions of the reactant and product streams were quantified with a Hayesep D packed column connected to a micro-methanizer equipped with a flame ionization detector (FID) in a gas chromatograph (SRI, 8610C). After CH4-O2 reactions for 25 h, methane conversions on all catalysts became stable, with a variation of < 2.9% over the entire duration of the eight-hour rate measurements. CH4 turnover rates were determined using the CO2 concentrations in the reactor effluent stream after achieving a stable reactivity, together with the number of exposed metal atoms measured from oxygen chemisorption experiments at 313 K. Table 1. Catalyst Metal Compositions, Notations, and Treatment Conditions for the Series of Monometallic Pt and Pd and Bimetallic Pt-Pd Catalysts Catalyst synthesis Catalyst compositiona

Notationb

Treatment temperature (K) Air (4 h)

H2 (1 h)

O2 uptakesc

Infrared study Average cluster diameter (nm)

Pre-treatment atmosphere (573 K)

0.92 wt% Pt

0.5Pt-Rd

973

923

313

7.3

101 kPa H2

0.92 wt% Pt

0.5Pt-Ox

973

923

773

7.3

20 kPa O2d

1 wt% Pt

0.55Pt-S-Rd

-

873

313

2.0

101 kPa H2

1 wt% Pd

1Pd-Rd

673

773

313

7.7

101 kPa H2

1 wt% Pd

1Pd-Ox

673

773

773

7.7

20 kPa O2d


Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

ACS Catalysis

0.92 wt% Pt-1 wt% Pd

0.5Pt-1Pd-Rd

673

923

313

7.2

101 kPa H2


0.5Pt-1Pd-Ox

673

923

773

7.2

20 kPa O2d


1Pt-1Pd-Rd

673

943

313

6.9

101 kPa H2


1Pt-1Pd-Ox

673

943

773

6.9

20 kPa O2d


1Pt-1Pd-T-Rd

673

873

313

6.3

-


1Pt-1Pd-T-Ox

673

873

773

6.3

-


2Pt-1Pd-Rd

673

965

313

6.7

101 kPa H2


2Pt-1Pd-Ox

673

965

773

6.7

20 kPa O2d

Nominal weight loading, supported on Al2O3. b nPt-mPd: n-to-m denotes the Pt-to-Pd atomic ratio in the catalyst. c Temperatures of

oxygen uptake measurements, carried out after treatments in air and H2 at temperatures described in the previous two columns. d Balanced with He.

Table 2. Oxygen Contents, Expressed in Terms of Atomic Oxygen-to-metal (Os/M), Total Oxygen-to-metal (Ot/M), and Total Oxygen-to-Pd (Ot/Pd) Ratios on Bimetallic Pt-Pd Catalysts and Monometallic Pt and Pd Catalysts

a

Oxygen contents during exposure to O2 from 0.001 to 25 kPa at 773 K Ot/Ma Ot/Pd Ot/Ma Ot/Pd (0.1 kPa O2) (25 kPa O2) (0.1 kPa O2) (25 kPa O2)

Catalyst

Os/Ma (13 kPa O2, 313 K)b

0.5Pt-Rd

0.153

0.207

-

0.290

-

1Pd-Rd

0.144

0.235

0.235

1.041

1.041

0.5Pt-1Pd-Rd

0.156

0.155

0.232

0.531

0.799

1Pt-1Pd-Rd

0.162

0.215

0.431

0.435

0.870

2Pt-1Pd-Rd

0.168

0.211

0.633

0.434

1.300

M denotes total metal; M = Pt+Pd for bimetallic catalysts and Pt or Pd for monometallic catalysts. b Oxygen chemisorption uptakes.

3. RESULTS AND DISCUSSION 3.1. Methane Conversion Rates and C-H Activation Barriers during CH4-O2 Reactions on Bimetallic PtPd and Monometallic Pt and Pd Catalysts. CH4 turnover rates (𝑟𝐶𝐻4, 𝑀, per exposed metal site) during steady-state CH4-O2 reactions on bimetallic Pt-Pd and monometallic Pt and Pd catalysts are first-order in CH4 (0.1-2 kPa) and zero-order in O2 (10-50 kPa) between 733-803 K, as established in previous studies.3,4,42 The mechanistic reason for these rate dependencies is obvious, as the first C-H activation in methane limits the catalytic turnovers, occurred on monometallic Pt0 cluster surfaces saturated with chemisorbed oxygen adatoms3 or on monometallic PdO crystallites without any lattice oxygen vacancies,4 expected at these high oxygen chemical potentials relevant to the treatment of exhaust gas under fuel-


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 35

lean conditions. Under these conditions, the first-order rate constant (𝑘𝐶𝐻4, 𝑀, M denotes metal identity; M = Pt+Pd, Pt, or Pd) is 𝑘𝐶𝐻4, 𝑀 =

𝑟𝐶𝐻4, 𝑀 𝑃𝐶𝐻4

[

= 𝐴𝑀 𝑒𝑥𝑝

― 𝐸C ― 𝐻, 𝑅𝑇

]

𝑀

(3)

where 𝑟𝐶𝐻4, 𝑀 is the methane turnover rate (per exposed metal site), 𝑃𝐶𝐻4 is the methane pressure, and 𝐴𝑀 and 𝐸C ― 𝐻,

𝑀

are the pre-exponential factor and activation energy, respectively. The first-order rate constant 𝑘𝐶𝐻4, 𝑀 is

also the elementary C-H activation rate constant, because it reflects the probabilities of reactive methane precursors to undergo successful C-H bond cleavage. 𝐸C ― 𝐻,

𝑀

equals the C-H activation energy, also the

activation enthalpy required to evolve the C-H activated complex from methane reactant. Figure 1 shows the first-order rate constants on the array of bimetallic Pt-Pd catalysts with different Pt-to-Pd atomic ratios (0.5Pt-1Pd-Ox, 1Pt-1Pd-Ox, and 2Pt-1Pd-Ox), monometallic Pt catalyst (0.5Pt-Ox), and monometallic Pd catalyst (1Pd-Ox) as a function of inverse temperature (698-773 K), after the controlled oxidative treatment at 773 K following the method described in Section 2.2.1. These bimetallic and monometallic catalysts have similar average metal/alloy cluster diameters, ranging from 6.7 to 7.7 nm. During CH4-O2 catalysis, CO remains undetected, therefore CH4 reacts exclusively via the combustion reaction. Table 3 summarizes the activation barriers and pre-exponential factors together with the first-order rate constant values at 698 K for these catalysts. For bimetallic Pt-Pd catalysts, the first-order rate constants decrease with increasing Pt-to-Pd atomic ratio over the entire temperature range [e.g., 𝑘𝐶𝐻4, 𝑃𝑡 ― 𝑃𝑑 = 3.73, 2.08, and 0.92 molCH4 (g-atom-Msurface s kPa)-1 on the 0.5Pt-1Pd-Ox, 1Pt-1Pd-Ox, 2Pt-1Pd-Ox catalysts, respectively, at 698 K]. Their apparent barriers remain similar at 35-43 kJ mol-1, irrespective of their Pt-to-Pd atomic ratios, much smaller than their monometallic counterparts and other reported examples in the literature.3,4,9-11 This result suggests that the controlled oxidative treatment carried out on these bimetallic catalysts creates active site structures that are highly effective for activating the C-H bonds in methane. Since the activation energies remain nearly the same, the active site structures on this series of catalysts are likely identical, but their site densities differ. Over the same temperature range, first-order rate constants for bimetallic Pt-Pd catalysts are much larger than that for the monometallic Pt catalyst. For example, the rate constant for the bimetallic catalyst (0.5Pt-1Pd-Ox) is 65 times higher than that for the monometallic Pt at 698 K, although these two catalysts have the same Pt weight contents (0.92 wt%). Among the bimetallic catalysts, only the one with a Pt-to-Pd atomic ratio of 0.5 (0.5Pt-1PdOx) exhibits higher turnover rates than the monometallic Pd catalyst, occurred at temperatures below 758 K. These results suggest that Pt does promote the methane turnover, but as its content increases, turnover rates, when normalized by all exposed metal atoms, decrease and become lower than the monometallic Pd catalyst. At first 10 ACS Paragon Plus Environment

Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

glance, this result may suggest a change in the types of active sites, perhaps the surface compositions of the alloy clusters, but this explanation remains inconsistent with the fact that all Pt-Pd catalysts exhibit near identical activation barriers (35-43 kJ mol-1) that are exceptionally low for activating the C-H bond in methane. The barriers for bimetallic Pt-Pd catalysts are extremely low for any catalytic CH4-O2 reactions and definitely much lower than those for the C-H activation on chemisorbed oxygen pairs (O*-O*) on O* covered Pt clusters (155 kJ mol-1),3 on chemisorbed oxygen-Pt metal pairs (O*-*) on Pt clusters (144 kJ mol-1)3, and on Pd2+-O2- pairs on PdO clusters (64 kJ mol-1),4 suggesting that the active site structures formed on these bimetallic catalysts are much more effective than any of the oxygen-oxygen, oxygen-metal, and oxygen anion-metal cation pairs for CH4 activation, previously found on monometallic Pt or Pd clusters and other Group VIII metal/alloy catalysts.2-4,9-11 The much higher rate constants for the 0.5Pt-1Pd-Ox catalyst than the monometallic 0.5Pt-Ox and 1Pd-Ox catalysts indicate that highly reactive sites present exclusively in the bimetallic catalysts. The identical barriers (35-43 kJ mol-1) across the series of bimetallic Pt-Pd catalysts further suggest that the identities of the active site are the same. The different pre-exponential factors and, on two of the Pt-Pd catalysts, the lower rates give indications that there are multiple types of sites, some of which are largely inactive for methane oxidation turnovers. When the fraction of highly effective sites varies with the Pt-to-Pd atomic ratio, the measured reaction rate constant values differ among the catalyst series, simply because these first-order rate constants in Figure 1 reflect the ensemble averaged reactivities normalized by all exposed metal atoms. Recent studies have shown that the Al2O3 substrates may disperse Pt as single atoms, interacting strongly with the underlying Al2O3 substrates.43,44 In addition, as a more oxophilic metal than Pt, Pd is known to undergo bulk oxidation.4,45 These previous studies and our findings on the monometallic Pt and Pd systems, taken together, have suggested that (i) a new type of highly reactive sites is created under the controlled oxidative treatment and (ii) the site densities of these highly effective sites vary across the series of bimetallic catalysts. Our goal is to probe and understand the structures of these highly active sites and then to create more of these sites at reduced metal loadings. With this in mind, we next interrogate the structural and compositional changes of the metal/alloy clusters contained within the bimetallic catalysts under different conditions, starting from characterizing them after treatments in a reductive environment in Sections 3.2-3.3 and then in an oxidative environment in Sections 3.4-3.6.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 35

Figure 1. First-order rate constants of CH4-O2 reactions [𝑘𝐶𝐻4, 𝑀 in Equation (3), 1 kPa CH4, 20 kPa O2, 20000-26700 cm3 gcat-1 s-1] on the 0.5Pt-Ox ( ■ , 𝑘𝐶𝐻4, 𝑃𝑡), 1Pd-Ox ( ● , 𝑘𝐶𝐻4, 𝑃𝑑), 0.5Pt-1Pd-Ox ( ▲ , 𝑘𝐶𝐻4, 𝑃𝑡 ― 𝑃𝑑), 1Pt-1Pd-Ox ( ◆ ,

𝑘𝐶𝐻4, 𝑃𝑡 ― 𝑃𝑑), and 2Pt-1Pd (▼, 𝑘𝐶𝐻4, 𝑃𝑡 ― 𝑃𝑑) catalysts. Table 3. First-order Rate Constants, C-H Activation Barriers, Pre-exponential Factors, and Activation Entropies of CH4-O2 Reactions First-order rate constant (698 K)a Catalyst

[𝑘𝐶𝐻4, 𝑀, molCH4 (gatom-Msurface s

C-H activation barrier (kJ mol-1)

Pre-exponential factor (kPa-1 s-1)

kPa)-1]b

Activation entropy (J mol-1 K-1)

0.5Pt-Ox

0.0567

65±4

4.58×103

-103

1Pd-Ox

2.42

64±1

1.54×105

-74

0.5Pt-1Pd-Ox

3.73

35±1

1.57×103

-112

1Pt-1Pd-Ox

2.08

40±1

2.07×103

-109

2Pt-1Pd-Ox

0.92

43±1

1.47×103

-112

a

Defined in Equation 3. b M denotes metal identity; M = Pt+Pd for bimetallic catalysts and Pt or Pd for monometallic catalysts.

3.2. Structure and Morphology of Metal Clusters in Bimetallic Pt-Pd Catalysts after Reductive Treatments. 12 ACS Paragon Plus Environment

Page 13 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 2a shows a representative spherical aberration-corrected HAADF image of the bimetallic Pt-Pd catalyst, taken after reductive treatments at 873 K (1Pt-1Pd-T-Rd, treatment details in Table 1). This micrograph, together with other HAADF micrographs taken on the same catalyst over a wide magnification scale of 500 kX-4.5 MX, as provided in Figure S1, shows clusters of metal or alloy at two distinct diameter ranges of 0.8-3.0 nm and 5.117.0 nm, respectively, determined by correlating the diameters to measured areas (Equation 2, Section 2.2.2). Figures 2b-2d show micrographs on the same catalyst, but taken at much higher magnifications at ~9 MX. Collectively, these micrographs reveal metal or alloy in three distinct morphologies of: (i) single atoms (labeled I, Figure 2b), (ii) small clusters [0.8-3.0 nm, labeled Ⅱ(i), Ⅱ(ii), Ⅱ(iii), and Ⅱ(iv), selected clusters in Figures 2b-2c], and (iii) large clusters (labeled III, 5.1-17.0 nm, a representative cluster in Figure 2d), respectively. Figure 3a shows these three types of morphologies after reductive treatments, denoted as Type-ⅠRd, Type-ⅡRd, and Type-ⅢRd, respectively. The bright spots (Ⅰ) in Figure 2b are atomically dispersed metals (Type-ⅠRd), which can be either Pt or Pd. A previous extended X-ray absorption fine structure (XAFS) study on a monometallic Pt/SiO2 catalyst treated in H2 at 523 K shows an average Pt coordination number of 4.2, which corresponds to an average cluster diameter of ~0.4 nm.46 This value, however, remains much smaller than that observed in their HAADF micrographs of 1.6 nm. The study has attributed the inconsistency between average Pt cluster diameters obtained from the different techniques to the presence of ultra-small clusters (0-1 nm diameter) and single atoms (without coordinating to another metal atom); these atoms are undetectable in this previous study that uses conventional HAADF-STEM, due to its resolution limit of ~1 nm. A similar XAFS study of a Pd/Al2O3 catalyst, after treatment in H2 at 373 K, shows an average Pd coordination number of 9, which corresponds to an average cluster diameter of 1.8 nm; this value is also smaller than that of 2.5 nm derived from the transmission electron microscopic (TEM) analyses.47 Results from these previous studies on monometallic catalysts suggest the presence of isolated Pt and Pd single atoms, and thus these atoms may co-exist with the alloy clusters in the bimetallic catalysts. HAADF-STEM studies, by itself, could not determine their chemical identities. Energy dispersive X-ray spectroscopy (EDS), due to the low metal contents in these selected areas and its resolution limit of 1 nm, is also unable to conclusively provide their chemical compositions. In addition to single metal atoms, HAADF micrographs in Figures 2b-2c also show smaller clusters (< 2.0 nm diameter, Type-ⅡRd) in irregular shapes of pyramidal, crescentic, truncated triangle, and ovoid [Ⅱ(i), Ⅱ(ii), Ⅱ(iii), and Ⅱ(iv), respectively]. Assuming an ideal face-centered cubic (fcc) packing and hemispherical shape, each of these small clusters contains ~18-936 metal atoms, determined from previously derived correlations.48 For selected clusters [e.g., Ⅱ(iii) in Figure 2c, 1.7 nm diameter], the detected d-spacing of 2.30 ± 0.05 Å and dihedral angle of 70° could correspond to (111) planes of either Pt, Pd, or Pt-Pd alloy. This method, by itself, 13 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 35

does not allow an unambiguous differentiation in the clusters’ chemical compositions, because the lattice fringes for monometallic Pt(111) and Pd(111) at 2.265 and 2.246 Å, respectively,49 and for the Pt-Pd alloy at ∼2.25 Å50 are all within the accuracy of the instrument (± 0.05 Å). The lattice fringe of 2.30 ± 0.05 Å is closer to that of Pt(111), suggesting that these small clusters may be Pt. Aside of these atomically dispersed metals and small metal clusters, a major fraction of the clusters has larger diameters in the range of 5.1-17.0 nm (Type-ⅢRd). Figure 2d shows a representative single cluster (Ⅲ) from this fraction. This cluster has a diameter of 6.9 nm and d-spacing of 2.25 Å ± 0.05 Å, which lies between those for Pt(111) and Pd(111) facets49 and in agreement with the expected value for Pt-Pd alloy (∼2.25 Å).50 This result, by itself, confirms that the cluster is in its metallic state, because this d-spacing is inconsistent with any of the crystallographic planes of PdO (JCPDS No. 46-1211) and PtO (JCPDS No. 83-1997). It, however, does not provide further information on the exact chemical compositions of the cluster. We next interrogate the surface chemical compositions of these various types of metal atoms and clusters with FTIR using CO as the probe molecule. In contrast to the HAADF-STEM analyses, FTIR is a surface sensitive technique that probes all exposed metal atoms and is much more sensitive towards single atoms and small clusters than the larger clusters, because the former two expose large numbers of under-coordinated surface atoms, and these surface atoms bind to CO much more strongly than the more coordinatively saturated atoms prevalent on the exposed facets of large clusters.


Page 15 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 2. Spherical aberration-corrected HAADF micrographs of the 1Pt-1Pd-T-Rd catalyst, which show (a) a representative region of high-magnification acquisitions, (b) single atoms (labeled I) and small clusters [< 2.0 nm, labeled Ⅱ (i)], (c) small clusters [< 2.0 nm, labeled Ⅱ (ii), Ⅱ (iii), and Ⅱ (iv)], and (d) a large cluster (6.9 nm, labeled Ⅲ).


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

Figure 3. Schematic illustration of the inhomogeneity, compositions, and morphologies of nanostructures in the Pt-Pd/Al2O3 catalysts, showing the dynamics of metal/alloy clusters under (a) reductive atmospheres (e.g., H2, 873-965 K) and (b) oxidative atmospheres (e.g., 10-25% O2/He, 773 K) (orange: Pt0; teal: Pd0; blue: Pdδ+; purple: O*, red: O2-).

3.3. Infrared Studies of CO Adsorption on Bimetallic Pt-Pd and Monometallic Pt and Pd Catalysts after Reductive Treatments in Hydrogen. Figures 4a-4c show the infrared absorption spectra for irreversible CO adsorption on the series of bimetallic Pt-Pd catalysts (0.5Pt-1Pd-Rd, 1Pt-1Pd-Rd, and 2Pt-1Pd-Rd), monometallic Pt catalyst (0.5Pt-Rd), and monometallic Pd catalyst (1Pd-Rd), respectively, obtained after exposure to 2 kPa CO, followed by purging in He to remove the gas phase CO at 296 K. Before the measurements, the samples were pre-treated in situ in flowing H2 and purging in He at 573 K. These catalysts contain metal/alloy clusters of similar ensemble averaged diameters, between 6.7 and 7.7 nm. These spectra show absorption bands expected for C≡O stretching vibrations in 2000-2105 and 1720-2000 cm-1 assigned to chemisorbed CO in linear and in bridged or three-fold configurations on transition metal surfaces, respectively.51 For bimetallic Pt-Pd catalysts (Figure 4a), the main absorption band for linearly adsorbed CO centers at 2065 cm-1. The lack of any absorption features above 2105 cm-1 suggests that metal clusters retain their metallic state, because cationic Pt2+, Pt4+, Pd+, and Pd2+ ions would lead the C≡O stretching bands to appear at higher wavenumbers of 2138, 2186, 2110, and 2144 cm-1, respectively,52,53 than those of their respective metallic atoms. The higher wavenumbers are a result of stronger C≡O bond strengths on cationic centers, caused by a lower extent of back-donation of electron density from their d-band to the CO π* antibonding molecular orbital.54 On reduced, monometallic Pt clusters (0.9-19.0 nm average diameter), previous studies have attributed the different C≡O 16 ACS Paragon Plus Environment

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

stretching wavenumbers, detected in Figure 4b, to changing Pt coordination. Wavenumbers decrease from 2097, 2085, 2070, to 2050 cm-1, as the coordination number of Pt0 that binds the CO decreases from 9, 8, 7, to less than 7, respectively.43,54-58 Coordinatively unsaturated Pt atoms tend to back donate larger portions of their d-electron densities to the CO π* antibonding molecular orbitals, weakening the C≡O bond strength and reducing the wavenumber to a larger extent.54 With respect to reduced, monometallic Pd clusters (1.0-15.0 nm average diameter), previous infrared spectroscopic studies of CO adsorption have shown the asymmetric absorption centered at 2075 cm-1 (Figure 4c), as a result of the superposition between bands for adsorbed CO on terrace (2080 cm-1) and under-coordinated (2060 cm-1) Pd0 sites.52,59-61 Comparing the absorption features between monometallic and bimetallic catalysts, the main absorption band centered at 2065 cm-1 for the Pt-Pd catalysts may reflect the linear binding of CO on Pt0 and Pd0 sites on metal/alloy cluster surfaces. The absorption spectra for the bimetallic Pt-Pd catalysts in Figure 4a show CO absorption bands in bridged or three-fold configurations at 1990 and 1820 cm-1. On reduced, monometallic Pt clusters (Figure 4b), the bridgebound CO leads to a band at 1820 cm-1.55,57 In contrast, the bridge-bound CO on reduced, monometallic Pd clusters (Figure 4c) gives a strong absorption feature at 1990 cm-1 at near monolayer coverages, and this feature shifts to 1950 cm-1 as the coverages decrease.52,61 The adsorbed CO on three-fold hollow Pd0 sites contributes to the shoulder at 1895 cm-1.52,61 On bimetallic catalysts, the absorption features at 1820 cm-1 and 1990 cm-1 (Figure 4a) reflect the bridge-bound CO on Pt0 sites and Pd0 sites, respectively. The relative intensities of these bands infer the relative amounts of Pt ensemble sites to Pd ensemble sites on cluster surfaces and, in turn, the potential enrichment of a specific metal on the cluster surfaces. In situ oxidation of these chemisorbed CO species, by reacting them with O2, allows the differentiation among the various adsorbed CO species and, in turn, the types and reactivities of the metal sites. For monometallic Pt clusters (0.5Pt-Rd catalyst) pre-covered with CO, introducing 10 kPa O2 at 373 K leads to near complete disappearance of all CO absorption bands ranging from 2000 to 2100 cm-1 (Figure 4d), because the adsorbed CO molecules convert readily to CO2 and desorb from Pt0 cluster surfaces. The remaining CO absorption band at 2105 cm-1 becomes a single, discernable peak. This peak at 2105 cm-1 was previously identified as the fingerprint of strongly bound CO on Pt single atoms binding to O*.43,62 It remains small, when comparing to those between 2000 and 2100 cm-1, detectable before the O2 introduction, apparently because only a very small fraction of Pt atoms exists as atomically dispersed metals. In the contrasting case of monometallic Pd clusters (1Pd-Rd catalyst), O2 reactions with the pre-adsorbed CO under identical conditions cause the CO absorption bands to disappear completely (Figure 4e). This disappearance, together with the absence of any absorption feature at 2045 cm-1, suggests that Pd single atoms do not exist, as reported previously, that Pd single atoms agglomerate at above 510 K on MgO,63,64 which stabilizes and disperses Pd atoms even better than Al2O3.65 Comparing to the monometallic Pd catalyst (773 K), the bimetallic catalysts were treated at much higher temperatures (923-965 K) during their 17 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

syntheses (Table 1), as they exhibit better resistance towards sintering, therefore higher treatment temperatures are required for their clusters to attain similar average diameters with that of the monometallic Pd catalyst. The lack of single atoms in the monometallic Pd catalyst, together with the higher thermal treatment temperatures for the Pt-Pd catalysts, has led us to rule out the presence of Pd single atoms in the bimetallic catalysts. Thus, the isolated single atoms (Type-ⅠRd) observed in the HAADF micrograph for the bimetallic Pt-Pd catalyst (Figure 2b) must be Pt and not Pd. Deconvolution of the CO absorption spectra in Figures 4a-4c with a linear combination of Voigt curves (details in Supporting Information, Section S2) based on the assigned features for Pt and Pd sites in Table S1 provides semi-quantitative analyses on the surface compositions of these metal/alloy clusters. Figure S2a shows an example of the deconvoluted spectra from the 0.5Pt-1Pd-Rd catalyst. Figure 5a shows the integrated band intensities for bridged CO adsorption on Pd0 (1990 cm-1) and Pt0 (1820 cm-1) ensemble sites, obtained from the deconvoluted spectra, as a function of Pt-to-Pd atomic ratio. These bands, together with those for linearly adsorbed CO on Pd0 (2060 and 2080 cm-1) and Pt0 (2050, 2070, 2085, and 2097 cm-1) sites, give the linear-tobridged intensity ratios, shown as a function of Pt-to-Pd atomic ratio in Figure 5b. Despite their similar ensemble averaged cluster diameters (6.7-7.7 nm) and Pd weight contents (1 wt%), the band intensity for bridged CO adsorption on Pd0 is larger for the bimetallic Pt-Pd catalysts than the monometallic Pd catalyst (Figure 5a); it actually increases drastically with increasing Pt-to-Pd atomic ratio. In contrast, the linear-to-bridged intensity ratio varies only slightly from 0.79 to 1.09 and remains largely insensitive with increasing Pt-to-Pd atomic ratio (Figure 5b). Taken together, these results indicate that (i) increasing the Pt content increases the number of surface Pd0 ensemble sites available for CO adsorption and (ii) Pt incorporation does not reduce the size of Pd atom ensemble to any significant extent. This can occur for this series of catalysts with the same Pd loadings, when Pd atoms remain on and cover the bimetallic cluster surfaces; in this case, any increase in the Pt content, while keeping the average cluster diameter constant, leads the Pt to reside inside the Pt-Pd cluster core, thereby increasing the number density of the Pt-Pd clusters on the support and, in turn, in the number of surface Pd0 ensemble sites and in CO adsorption on such site per unit mass of the catalyst sample. In other words, the surfaces of the Pt-Pd alloy clusters are enriched with Pd, in agreement with the expected structure predicted from thermodynamics—Pd surfaces have much lower surface free energies than Pt surfaces [e.g., 1.90 vs. 2.48 J m-2, on (100) surfaces],20 thus Pd atoms tend to remain on the cluster surfaces in order to minimize the overall free energies of the entire clusters. The band intensity for bridged CO adsorption on Pt0 (1820 cm-1) in Figure 5a for the bimetallic Pt-Pd catalyst (0.5Pt-1Pd-Rd) is also larger than the monometallic Pt catalyst, despite their same Pt weight contents (0.92 wt%). This absorption band feature does not originate from the Pt-Pd alloy clusters, as these clusters contain 18 ACS Paragon Plus Environment

Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

predominantly Pd on their surfaces. The larger amount of Pt0 ensemble sites in bimetallic Pt-Pd catalysts reflects the presence of Pt clusters, segregated from the Pt-Pd alloy clusters. The linear-to-bridged intensity ratios for CO adsorption on Pt0 sites decrease with increasing Pt-to-Pd atomic ratio, suggesting that the Pt clusters increase in their size (Figure 5b). To further interrogate the size of these Pt clusters, we carry out infrared absorption studies on another monometallic Pt catalyst (0.55Pt-S-Rd, treatment details in Table 1), which contains clusters with a much smaller average diameter (2.0 nm). On this catalyst, the linear-to-bridged intensity ratio for CO adsorption, as shown in Figure S3, is similar with that of the 2Pt-1Pd-Rd catalyst (Figure 5b). This similar linear-to-bridged intensity ratio between the 0.55Pt-S-Rd and 2Pt-1Pd-Rd catalysts, which contain 1 wt% and 3.67 wt% Pt, respectively, has led us to propose that the average Pt cluster diameters in both catalysts are ~2 nm. Taken this finding, together with the similar linear-to-bridged intensity ratio for CO adsorption on Pd0 and the evidence of small metal clusters detected under the spherical aberration-corrected HAADF-STEM in Figures 2a2d, we conclude that the small metal clusters (Type-ⅡRd) are predominantly Pt clusters, segregated from the large metal clusters (Type-ⅢRd), which consist of a Pt-Pd alloy bulk with Pd-rich surfaces. As the Pt atom ensemble exists exclusively on Type-ⅡRd, the marked increase of Pt0 ensemble sites with increasing Pt-to-Pd atomic ratio suggests that, as the Pt loading increases, excess Pt atoms are unable to form an alloy with the Pd, instead they begin to aggregate, forming Type-ⅡRd on Al2O3 support. In addition, the integrated CO absorption band intensity on Pt0 ensemble sites increases 5.9 times, whereas that on Pd0 ensemble sites only increases 3.0 times when the Pt-to-Pd atomic ratio increases from 0.5 to 2 (Figure 5a). The much more significant increase of Pt0 ensemble sites than Pd0 ensemble sites evidently illustrates that larger Pt incorporation into the bimetallic catalysts increases the proportion of Pt0 sites.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

Figure 4. Infrared absorption spectra of irreversibly adsorbed CO (2 kPa CO, 296 K) on the (a) 0.5Pt-1Pd-Rd, 1Pt-1Pd-Rd, and 2Pt-1Pd-Rd, (b) 0.5Pt-Rd, and (c) 1Pd-Rd catalysts. Time-dependent infrared absorption spectra (296 K) taken after (i) 0 min, (ii) 10 min, and (iii) 15 min of in situ CO*-O2 reactions at 10 kPa O2 and 373 K on the (d) 0.5Pt-Rd and (e) 1Pd-Rd catalysts (pre-saturation of CO* at 2 kPa CO and 296 K).


Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 5. Variations of (a) the integrated intensities of CO absorption bands at 1820 cm-1 (■) and 1990 cm-1 (●) and (b) the linear-to-bridged ratios of the integrated intensities of CO absorption bands for Pt0 sites (■) and Pd0 sites (●), derived from deconvolution of the spectra in Figures 4a-4c, shown as a function of Pt-to-Pd atomic ratio.

3.4. Infrared Studies of CO Adsorption on Bimetallic Pt-Pd and Monometallic Pt and Pd Catalysts after Oxidative Treatment. Figures 6a-6c show the infrared spectra of irreversibly adsorbed CO at 296 K on the series of bimetallic Pt-Pd (0.5Pt-1Pd-Ox, 1Pt-1Pd-Ox, and 2Pt-1Pd-Ox) and monometallic Pt (0.5Pt-Ox) and Pd (1Pd-Ox) catalysts, after the controlled oxidative treatment at 773 K described in Section 2.2.1. On the bimetallic catalysts, C≡O stretching vibrational frequencies at the higher wavenumbers (2105-2250 cm-1) begin to emerge after the oxidative treatment and appear together with those at the lower wavenumbers (1720-2105 cm-1). These bands (> 2105 cm-1) are the results of CO adsorption on cationic sites;43,52,61 their appearance suggests that some of the exposed metal atoms become oxidized during the controlled oxidative treatment. On the monometallic Pd catalyst, the same oxidative treatment also leads to absorption bands at 2105-2250 cm-1 (Figure 6c), assigned previously to Pd+ (2110 cm-1) and Pd2+ (2144 cm-1) cations on oxidized Pd clusters (1.4-15.0 nm).52,61 The shift of the main band position for linearly adsorbed CO from 2075 to 2103 cm-1 after the oxidative treatment (Figures 6c vs. 4c) indicates that Pd0 (2060 and 2080 cm-1) has undergone oxidation, forming cationic Pd+ and Pd2+ species. For the contrary case of monometallic Pt catalyst, the oxidative treatment does not result in any absorption feature above 2105 cm-1 (Figure 6b). After the oxidative treatment, the band at 2097 cm-1, which assigns to well-coordinated Pt0 sites, remains at the similar position and intensity, when comparing to that of the reduced catalyst (Figures 6b vs. 4b). This band becomes the main absorption feature, as the bands for CO adsorption on under-coordinated Pt0 sites (2000-2070 cm-1) disappear almost completely, because undercoordinated Pt sites bind strongly to oxygen and, as a result, are unable to interact with CO at 296 K.66 Based on the above findings on the monometallic catalysts, we conclude that the absorption above 2105 cm-1 on the bimetallic Pt-Pd catalysts reflects the CO adsorption on cationic Pd (Figure 6a), and thus a portion of exposed Pd atoms has undergone oxidation. The band at 1985 cm-1 is evident for all Pt-Pd catalysts and that at 1820 cm-1 is evident only for the catalyst with the highest Pt-to-Pd atomic ratio (2Pt-1Pd-Ox). The former reflects bridged CO adsorption on Pd+ and Pd0 sites, since the bands associated with these sites are known to appear in this region (1973 for Pd+ and 1990 cm-1 for Pd0),52,61 as confirmed independently from treating the monometallic Pd catalyst in oxygen (Figure 6c) and hydrogen (Figure 4c), respectively. The latter reflects bridged CO adsorption on Pt0, as verified separately with the monometallic Pt catalyst, after the same oxidative treatment (Figure 6b). The much higher intensity of this band for the 2Pt-1Pd-Ox catalyst is consistent with the conclusion from Section 3.3—after treating in hydrogen, 21 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

this catalyst contains a much higher density of Type-ⅡRd with a larger average diameter than those of other bimetallic catalysts, leading to a larger number of well-coordinated Pt sites. Oxygen adatoms bind weakly on these well-coordinated sites and, as a result, CO molecules are able to displace the oxygen and adsorb on these sites.33,66 In situ oxidation of these oxygen treated Pt-Pd catalysts, after CO adsorption, at 10 kPa O2 and 373 K removes all C≡O stretching vibrations completely, except for that associated with the Pt single atoms (denoted as TypeⅠOx, Figure 3b) at 2105 cm-1 (Figures 6d-6f). The intensities of this band actually increase, when comparing to the counterpart samples treated in hydrogen for both the bimetallic Pt-Pd (Figures 6d-6f vs. 4a) and monometallic Pt catalysts (Figures 6g vs. 4d). These intensity increases at 2105 cm-1 suggest that the oxidative treatment increases the surface density of Type-ⅠOx, most likely through an oxidative redispersion event, during which Pt atoms migrate from the metal clusters to interact directly with Al2O3 support, as previously shown on monometallic Pt clusters with TEM and X-ray absorption spectroscopy (XAS).23,44,67 In the contrasting case of monometallic Pd, the in situ oxidation (10 kPa O2, 373 K) on CO covered clusters results in a near complete disappearance of all absorption bands (Figure 6h). Since neither the monometallic Pd nor bimetallic Pt-Pd catalysts shows the residual absorption intensity previously ascribed to Pd single atoms at 2045 cm-1,63,68 we conclude that Pd single atoms do not present. Spectral deconvolution of Figures 6a-6c using linearly combined Voigt curves with the band assignments shown in Table S1 gives the integrated band intensities for adsorbed CO on metallic Pt and Pd sites as well as on cationic Pd sites (Pdδ+, including Pd+ and Pd2+), which allows for a detailed, semi-quantitative analysis of the various sites present on the oxygen treated clusters. Figure S2b shows an example of the deconvoluted spectra from the 0.5Pt-1Pd-Ox catalyst. Figure 7a summarizes the spectral deconvolution results, showing the total integrated band intensity for CO adsorption on Pdδ+ sites (1973, 2110, and 2144 cm-1), individual band intensities for bridged CO adsorption on an ensemble of Pd0 (1990 cm-1) and Pt0 (1820 cm-1) sites, and the band intensity for CO adsorption on Type-ⅠOx (2105 cm-1) as a function of Pt-to-Pd atomic ratio. First, on the reference monometallic Pd clusters, the band for adsorbed CO on Pd0 ensemble sites is largely undetected, whereas those on Pdδ+ sites are significant (Figures 7a-7b), an indication that nearly all Pd0 atoms convert to Pd cations. Second, comparing between the Pt-Pd catalyst (0.5Pt-1Pd-Ox) and monometallic Pd catalyst, the total band intensity associated with Pdδ+ sites is much larger for the bimetallic than monometallic catalyst, which suggests that, upon the oxidative treatment, metallic Pd on Type-ⅢRd surfaces undergoes oxidation and resides on the surfaces, while Pt remains in the cluster core, forming a core-shell structure depicted as Type-ⅣOx in Figure 3b. This structure with a Pt-rich core and a PdO shell is thermodynamically more stable than other alternatives (e.g., Pt shell structures), as PdO has a much lower surface free energy than Pt [e.g., 0.53 vs. 2.48 J m-2, on (100) surfaces].20,69 22 ACS Paragon Plus Environment

Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Third, as Pt-to-Pd atomic ratio increases, the band intensity related to Pdδ+ sites decreases and that to Pd0 ensemble sites concomitantly increases, suggesting that Pt incorporation into Type-ⅣOx clusters decreases the fraction of Pdδ+ cationic species. Finally, within the cationic Pd species, the fraction of Pd2+ decreases and that of Pd+ concomitantly increases (Figure 7b). The Pd+-to-Pdδ+ intensity ratio is 0.45 on the reference monometallic Pd catalyst. Pt incorporation, which increases the Pt-to-Pd atomic ratio from 0.5, 1, to 2, enlarges this ratio from 0.48, 0.62, to 0.68, indicating that Pt promotes the partial reduction of Pd. After the oxidative treatment, the small Pt0 clusters are covered with chemisorbed oxygen (denoted as TypeⅡOx, Figure 3b). As Pt-to-Pd atomic ratio increases, the band intensity for bridged CO adsorption on these Pt0 clusters at 1820 cm-1 increases (Figure 7a), because both the average diameter and number density of these clusters increase. Similar trends are found for linear CO adsorption on Type-ⅠOx at 2105 cm-1, because the oxidative treatment redisperses Type-ⅡOx. Despite their same Pt contents, the band intensity for CO adsorption on TypeⅠOx is much smaller for the Pt-Pd (0.5Pt-1Pd-Ox) than the Pt (0.5Pt-Ox) catalysts, indicating that the bimetallic catalysts contain a lower surface density of Type-ⅠOx. This is expected, because the majority of Pt atoms resides inside Type-ⅣOx and thus remains covered by PdO layers.

Figure 6. Infrared absorption spectra of irreversibly adsorbed CO (2 kPa CO, 296 K) on the (a) 0.5Pt-1Pd-Ox, 1Pt-1Pd-Ox, and 2Pt-1Pd-Ox, (b) 0.5Pt-Ox, and (c) 1Pd-Ox catalysts. Time-dependent infrared absorption spectra (296 K) taken after 0


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

min and 15 min of in situ CO*-O2 reactions at 10 kPa O2 and 373 K on the (d) 0.5Pt-1Pd-Ox, (e) 1Pt-1Pd-Ox, (f) 2Pt-1PdOx, (g) 0.5Pt-Ox, and (h) 1Pd-Ox catalysts (pre-saturation of CO* at 2 kPa CO and 296 K).

Figure 7. Variations of (a) the integrated intensities of CO absorption bands at 1820 cm-1 (■), 1990 cm-1 (●) and 2105 cm1

(★) as well as the sum of the integrated band intensities at 1973 cm-1, 2110 cm-1, and 2144 cm-1 (▲) and (b) the integrated

intensities of CO absorption bands at 1990 cm-1 (red) and 2144 cm-1 (blue) and the sum of the integrated band intensities at 1973 cm-1 and 2110 cm-1 (wathet), derived from deconvolution of the spectra in Figures 6a-6h, shown as a function of Ptto-Pd atomic ratio.

3.5. Oxygen Contents in Bimetallic Pt-Pd and Monometallic Pt and Pd Catalysts. Contacting metal/alloy clusters to oxidative atmosphere at elevated temperatures leads to oxygen dissolution.20,23 At a low temperature of 313 K, O2 dissolution into the cluster bulk is kinetically restricted, therefore controlled, isothermal volumetric oxygen uptakes at this temperature on pre-reduced catalysts selectively titrate the number of chemisorbed oxygen adatoms on cluster surfaces (𝑂𝑠∗ ).3,4 In contrast, oxygen uptakes at a higher temperature relevant to CH4-O2 catalysis (773 K) give the total number of oxygen contained within the clusters (𝑂𝑡), which includes oxygen at cluster surfaces and that dissolve into the cluster bulk (𝑂𝑏): 𝑂𝑡 = 𝑂𝑠∗ + 𝑂𝑏

(5)

Figure 8a shows the total oxygen contents in the metal/alloy clusters, expressed in terms of the Ot-to-M atomic ratio (where M denotes total metal) for bimetallic Pt-Pd and monometallic Pt and Pd catalysts, as a function of 24 ACS Paragon Plus Environment

Page 25 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

the contacting O2 pressure at 773 K. For the 0.5Pt-1Pd-Rd catalyst, the Ot-to-M atomic ratio initially increases sharply to 0.155, even below 0.1 kPa O2. Since this ratio equals that expected for monolayer oxygen coverage (0.156, Table 2, determined from oxygen chemisorption at 313 K), the oxygen coverage is 0.99 ML O*. At the same O2 pressure range, the Ot-to-M atomic ratios for 1Pt-1Pd-Rd and 2Pt-1Pd-Rd catalysts, which contain more Pt than the 0.5Pt-1Pd-Rd catalyst, are 0.215 and 0.211, exceeding the monolayer coverage values of 0.162 and 0.168, respectively. These Ot-to-M atomic ratios suggest that the oxygen coverages are 1.32 and 1.26 ML on 1Pt1Pd-Rd and 2Pt-1Pd-Rd catalysts, respectively. At the same condition (0.1 kPa O2, 773 K), the Ot-to-M atomic ratio for monometallic Pt catalyst (0.5Pt-Rd) is 0.207, which also exceeds the monolayer coverage (0.153), but is well below the stoichiometries required for bulk oxidation to PtO and PtO2. Because the Gibbs free energies for bulk phase transition from Pt to PtO and PtO2 are positive (7.8 and 28.1 kJ mol-1 at 773 K, respectively),70 which predict unfavorable bulk Pt oxidation, the Ot-to-M atomic ratio of 0.207 (1.35 ML) suggests that Pt clusters retain their metallic bulk but their surfaces are covered with chemisorbed oxygen and sub-surface layers contain oxygen atoms. The Ot-to-M atomic ratio for the monometallic Pd catalyst (1Pd-Rd) reaches 0.235, which corresponds to 1.63 ML of oxygen. The PdO formation reaction is given by: 𝑃𝑑 + 0.5𝑂2↔𝑃𝑑𝑂

(6)

When the reaction in Equation (6) is at equilibrium, the Gibbs free energy change (𝛥𝐺) relates to the Gibbs free energy of PdO formation (𝛥𝑟𝐺𝛩𝑚) and equilibrium constant (𝐾𝑒𝑞) via: 𝛥𝐺 = 𝛥𝑟𝐺𝛩𝑚 + 𝑅𝑇𝑙𝑛𝐾𝑒𝑞 = 0

(7)

For this case, 𝐾𝑒𝑞 relates to the O2 pressure via: 𝐾𝑒𝑞 =

𝑃𝛩

0.5

(8)

[𝑂2]0.5

where 𝑃𝛩 is the ambient, reference pressure (101.325 kPa). Equations (7-8), taken together, give the relationship between the O2 pressure and 𝛥𝑟𝐺𝛩𝑚 via:

[𝑂2] =

𝑃𝛩

[ (

𝑒𝑥𝑝 ―

2𝛥𝑟𝐺𝛩𝑚 𝑅𝑇

)]

(9)

At 773 K, the Gibbs free energy for PdO formation (𝛥𝑟𝐺𝛩𝑚) is -72.6 kJ mol-1.45 Substituting 101.325 kPa, 773 K, and -72.6 kJ mol-1 for 𝑃𝛩, T, and 𝛥𝑟𝐺𝛩𝑚 in Equation (9), the O2 pressure for Pd-to-PdO phase transition is 1.56× 25 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

10-8 kPa. At this temperature and the O2 pressure range between 0.001 and 25 kPa, PdO is the thermodynamically stable phase. The measured Ot-to-M atomic ratio of 0.235 (1.63 ML oxygen) at 0.1 kPa O2 indicates that a portion of near-surface Pd atoms has converted to form a PdO shell covering the underneath Pd0 core, but the bulk oxidation of Pd is kinetically restricted at such a low O2 pressure. After reductive treatments, the Pt-Pd bimetallic catalysts contain predominantly small Pt clusters (Type-ⅡRd) and large Pt-Pd alloy clusters (Type-ⅢRd), as shown and concluded in Sections 3.2-3.3. The total oxygen contents at 0.1 kPa O2 and elevated temperature (773 K) thus reflect the sum of oxygen from (i) the chemisorbed oxygen on the surfaces of Type-ⅡRd and Type-ⅢRd, (ii) sub-surface oxygen in Type-ⅡRd, and (iii) oxygen in the PdO shell of Type-ⅢRd. At 0.1 kPa O2 pressure and 773 K, the oxygen uptakes on the 0.5Pt-1Pd-Rd catalyst are smaller than those on the monometallic Pt and Pd catalysts. The smaller oxygen uptakes are expected, as Pt incorporation into Type-ⅢRd clusters reduces their affinity to oxygen, as illustrated by the infrared studies (Section 3.4). The 1Pt-1Pd-Rd and 2Pt-1Pd-Rd catalysts, which contain clusters with even lower affinities to oxygen, show larger Ot-to-M atomic ratios at 0.1 kPa O2 pressure. These larger ratios are most likely caused by increases in the surface density of Type-ⅡRd, which bind to more O*, in agreement with findings from the infrared studies (Section 3.4). Next, we examine the oxygen uptakes above 0.1 kPa O2. For the bimetallic Pt-Pd catalysts, Ot-to-M atomic ratios increase continuously with increasing O2 pressure and reach relatively constant values when O2 pressures increase above 17 kPa. For the monometallic Pt catalyst, the Ot-to-M atomic ratio increases only slightly from 0.207 to 0.290, with 53% of the oxygen contents remain on the cluster surfaces and the rest in the sub-surfaces for O2 pressures higher than 2 kPa. For the monometallic Pd catalyst, the Ot-to-M atomic ratio increases markedly to and remain constant at 1.041, the expected stoichiometry of bulk PdO. Thus, at this higher O2 pressure range, the increase in oxygen contents in the bimetallic catalysts reflects both the increases in sub-surface oxygen of Type-ⅡRd and bulk Pd oxidation of Type-ⅢRd. Figure 8b expresses the total oxygen contents in terms of Ot-to-Pd atomic ratio rather than the Ot-to-M atomic ratio. At ~25 kPa O2 pressure, the 0.5Pt-1Pd-Ox catalyst shows an Ot-to-Pd atomic ratio of 0.799 (Table 2), suggesting that a large portion of Pd has undergone oxidation, leaving a small portion in the metallic state contained in Type-ⅣOx. Although the 1Pt-1Pd-Ox and 2Pt-1Pd-Ox catalysts contain more reduced Pd, as a result of Pt incorporation, these catalysts actually exhibit larger Ot-to-Pd atomic ratios of 0.870 and 1.300, respectively, at ~25 kPa O2. These larger ratios are caused by the increase in the number density of Type-ⅡOx.


Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 8. Oxygen contents in the (a) 0.5Pt-Rd (■), 1Pd-Rd (●), 0.5Pt-1Pd-Rd (▲), 1Pt-1Pd-Rd (◆), and 2Pt-1Pd-Rd (▼) catalysts, expressed in terms of the total oxygen-to-metal atomic ratio (Ot/M), and (b) 1Pd-Rd (●), 0.5Pt-1Pd-Rd (▲), 1Pt1Pd-Rd ( ◆ ), and 2Pt-1Pd-Rd (▼) catalysts, expressed in terms of the total oxygen-to-Pd atomic ratio (Ot/Pd), plotted against the O2 pressure during oxygen uptakes at 773 K.

3.6. Chemical Compositions, Structural Dynamics, and Active Site Evolvements in Pt-Pd Catalysts. Figure 9a shows a HAADF image of a single, representative Type-ⅣOx cluster (9.8 nm diameter) of the bimetallic Pt-Pd catalyst (1Pt-1Pd-T-Ox) after an oxidative treatment at 773 K in incremental O2 pressures to ~25 kPa. The cluster (labeled Ⅳ) acquires a core-shell structure comprising two distinct phases—a crystalline, brighter inner core and an amorphous, darker shell. Figure 9b shows the intensity line scan profile through the center of this cluster from left to right, which covers the region highlighted in blue rectangle in Figure 9a. The central region of ~7 nm diameter exhibits a relatively uniform intensity, suggesting that the chemical compositions of the inner core remain essentially constant. At the core/shell interfaces (red arrows in Figures 9a and 9b), the intensity decreases sharply, indicating that the shell composes of a material with a lower atomic number than that of the core. The thickness of the shell, defined as the distance between the core/shell interfaces and the cluster surfaces (between the red and green arrows in Figures 9a and 9b), varies between 1 and 2 nm. The highly crystalline core has a d-spacing of 1.96 ± 0.05 Å, identical to (200) planes of either Pt at 1.961 Å or Pd at 1.945 Å,49 confirming that the core is metallic. Since the lattice mismatch between Pt and Pd is only 0.8%, which is within the measurement error, the absolute d-spacing value, by it self, does not allow us to further determine the chemical identity of this core. Fourier transforms of the HAADF image (Figure 9a) carried out on 27 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

the two separate regions of: (i) the central region in the core (magenta frame) and (ii) the edge region (orange frame) lead to the diffractograms in Figures 9c-9d. Projected along zone axes, the {200} diffraction spots are closer in the diffractogram from the central region than that from the edge region by 2-3 pixels (Figures 9c vs. 9d), suggesting a larger d-spacing at the center. This implies that the metallic core of the cluster predominantly consists of Pt, which agrees with the result that Pt does not undergo bulk oxidation during the oxidative treatment. The diffractogram from the edge region (Figure 9d) also shows a unique pair of spots reflecting a d-spacing of 3.27 Å, which does not match with any of the crystallographic planes of Pt0 or Pd0 metals. The d-spacing of these unique diffraction spots from the edge region matches the {111} planes of PdO (3.260 Å, JCPDS No. 46-1211). This result agrees well with the findings from infrared studies in Section 3.4, which suggest the formation of PdO shell covering an underlying Pt-rich core (Type-ⅣOx). This structure is also consistent with the significantly brighter Z-contrast on the core than the shell (Figure 9a), as Pt has a much larger atomic mass than PdO (195.1 vs. 122.4). The aberration-corrected HAADF-STEM studies, semi-quantitative analyses of the CO absorption bands in the infrared absorption spectra, as well as the oxygen uptakes at temperatures relevant to CH4-O2 catalysis on the series of bimetallic Pt-Pd catalysts, taken together, confirm the co-existence of: (i) Pt single atoms (Type-ⅠOx), (ii) small Pt clusters with chemisorbed oxygen (~2 nm, Type-ⅡOx), and (iii) large clusters with a Pt-rich core and a PdO shell (> 5 nm, Type-ⅣOx). Since the first-order rate constants for CH4-O2 reactions on the monometallic Pt catalyst (0.5Pt-Ox), which contains only Type-ⅠOx and Type-ⅡOx, are one to two orders of magnitude smaller than those on Pt-Pd catalysts at 698-773 K (Figure 1), these nanostructures and their sites from Pt alone are relatively inactive, when comparing to Type-ⅣOx, for CH4-O2 catalysis. These results show that Type-ⅣOx clusters are highly effective for methane activation. On monometallic PdO clusters (1Pd-Ox catalyst), the surfaces contain electrophilic Pd cation-nucleophilic oxygen anion site pairs (Pd2+O2-) that catalyze the kinetically relevant, initial C-H cleavage of CH4 via the formation of a four-center transition state (H3Cδ−--Pd2+--Hδ+--O2-)⧧.4 When these sites form on top of a Pt-rich core and interact with the underneath Pt metal atoms, they become much more reactive, activating the C-H bond with an exceptionally low barrier of 35-43 kJ mol-1. An X-ray photoelectron spectroscopic (XPS) study of clusters with a Pt core and a Pd shell shows a much wider valence band, when comparing to that of monometallic Pd clusters, because of the incorporation of Pt valence electrons to the overall band structures.71 This wider valence band lowers the d-band center from the Fermi-level, thus there is less overlap between the d-states of the surface Pd atoms and the 2p-states of the adsorbed oxygen atoms, leading to much weaker oxygen binding on the cluster surfaces.72 Comparably, for the case of Type-ⅣOx, the surface O2- anions may bind much more weakly than those of the monometallic PdO clusters, and they are more nucleophilic and able to interact with the leaving H in CH4 effectively to form the 28

ACS Paragon Plus Environment

Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

highly stabilized four-center transition state (H3Cδ−--Pd2+--Hδ+--O2-)⧧,4 thereby leading to an extremely low C-H activation barrier (35-43 kJ mol-1). The activation entropy values, derived from the measured pre-exponential factors (𝐴𝑀) in Table 3, are much more negative on Type-ⅣOx than monometallic PdO clusters (ranging from 109.3 to -112.2 vs. -73.5 J mol-1 K-1), because of their more stable transition states. Last, the co-existence of the various nanostructures (Type-ⅠOx, Type-ⅡOx, and Type-ⅣOx) at different relative fractions, depending on the Pt-to-Pd atomic ratios, reconciles the different reactivity trends. At temperatures below 758 K, the first-order rate constants for the 0.5Pt-1Pd-Ox, the bimetallic catalyst with the smallest Pt-toPd atomic ratio, are higher than those on the monometallic Pd catalyst, but as the Pt-to-Pd atomic ratio increases (for 1Pt-1Pd-Ox and 2Pt-1Pd-Ox), their values become smaller. The increase in Pt-to-Pd atomic ratio causes commensurate increases in the proportion of inactive Pt sites, from both Type-ⅠOx and Type-ⅡOx, and promotes the partial reduction of the PdO shell on Type-ⅣOx. First, the presence of these inactive Pt sites decreases the overall CH4 conversion rates, because these rate values are averaged over all exposed metal sites. Second, the partial reduction of Pd leads to less nucleophilic O2- anions and, in turn, also to lower CH4 conversion rates. Together, these factors lower the first-order rate constants for the 1Pt-1Pd-Ox and 2Pt-1Pd-Ox catalysts, because the rate constants are an ensemble averaged kinetic property, normalized over all exposed sites. The complex compositions, structures, dynamics, quantities, and reactivities of nanostructures in the realistic bimetallic Pt-Pd catalysts captured here demonstrate that there is an optimal Pt-to-Pd atomic ratio, which is ~0.5, for creating the effective Pd2+-O2- sites on Type-ⅣOx and minimizing the formation of smaller, much less reactive Type-ⅡOx for CH4-O2 reactions.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Figure 9. (a) Spherical aberration-corrected HAADF micrograph of a representative cluster (9.8 nm, labeled Ⅳ) in the 1Pt-1PdT-Ox catalyst, (b) its corresponding intensity line scan profile across the center covering the region in blue rectangle from left to right in (a), as well as (c) the diffractogram from the center of the cluster [the region highlighted in the magenta frame in (a)] and (d) the diffractogram from the edge of the cluster [the region highlighted in the orange frame in (a)].

4. CONCLUSIONS


Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Rate measurements in the kinetically controlled regime under differential fuel-lean conditions at 698-773 K, together with aberration-corrected HAADF-STEM analyses, CO infrared absorption spectroscopic studies, and oxygen uptake-titration measurements were employed to elucidate the inherent structural inhomogeneity that leads to several types of nanostructures, each has its unique chemical composition, in bimetallic Pt-Pd catalysts (0.92-3.67 wt% Pt, 1 wt% Pd). These nanostructures undergo their respective dynamic transition in response to changing compositions of the contacting gas phase. After reductive treatments, these bimetallic catalysts contain (i) Pt single atoms, (ii) small Pt clusters (~2 nm), and (iii) large Pt-Pd alloy clusters (> 5 nm). Exposure of the catalysts to oxygen chemical potentials and temperatures comparable to those in CH4-O2 catalysis leads to redispersion of the small Pt clusters and thus to a concomitant increase in the number of Pt single atoms. The remaining Pt clusters retain their metallic bulk, while the large Pt-Pd alloy clusters undergo thermodynamically driven structural reconstruction. Specifically, oxygen dissolves into these large Pt-Pd alloy clusters and Pd as the more oxophilic metal migrates onto the cluster surfaces and becomes oxidized, because the free energy of PdO formation is largely negative and the surface free energy of PdO is lower than that of Pt. On the contrary, Pt retains its metallic state and remains inside the cluster bulk. This reconstruction leads to core-shell clusters with a Pt-rich core and a PdO shell. Residing on the core-shell cluster surfaces are Pd2+-O2- site pairs that interact with the underneath Pt-rich metallic core. These site pairs are highly effective in catalyzing the kinetically relevant C-H bond cleavage of methane via the formation of a highly stabilized four-center transition state (H3Cδ−--Pd2+-Hδ+--O2-)⧧ with an extremely low C-H activation barrier of 35-43 kJ mol-1, much lower than the monometallic Pt or PdO catalyst. The Pt single atoms and small Pt clusters, in contrast, are relatively inactive. The increase of Pt incorporation into the bimetallic Pt-Pd catalysts results in a larger proportion of inactive Pt sites as well as lower nucleophilicity of O2- anions in the Pd2+-O2- site pairs, and thus the rate constants for CH4-O2 catalysis, which reflect the ensemble averaged reactivity over all sites, decrease with increasing Pt-to-Pd atomic ratio. Findings on the diverse compositions, dynamics, quantities, and reactivities of nanostructures in the realistic, working bimetallic Pt-Pd catalysts offer an optimal Pt-to-Pd atomic ratio of ~0.5 for effective methane oxidation without using excess Pt. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. HAADF micrographs of the bimetallic Pt-Pd catalyst at different magnifications, deconvolution of the CO absorption spectra for bimetallic Pt-Pd catalysts and monometallic Pt and Pd catalysts after reductive or oxidative treatments, and infrared studies of CO adsorption on the monometallic Pt catalyst with 2.0 nm average cluster diameter after the reductive treatment. 31 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

ACKNOWLEDGEMENTS The STEM work was performed at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). This study was supported by Natural Sciences and Engineering Research Council of Canada (NSERC), Hitachi High-Technologies Canada Inc., Imperial Oil Limited, DCL International Inc., Canada Foundation for Innovation (CFI). H. Nie acknowledges Connaught International Scholarship for Doctoral Students.

REFERENCES (1) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Catalysts for Combustion of Methane and Lower Alkanes. Appl. Catal. A-Gen. 2002, 234, 1-23. (2) Gélin, P.; Primet, M. Complete Oxidation of Methane at Low Temperature Over Noble Metal Based Catalysts: A Review. Appl. Catal. B-Environ. 2002, 39, 1-37. (3) Chin, Y.-H.; Buda, C.; Neurock, M.; Iglesia, E. Reactivity of Chemisorbed Oxygen Atoms and Their Catalytic Consequences during CH4–O2 Catalysis on Supported Pt Clusters. J. Am. Chem. Soc. 2011, 133, 15958-15978. (4) Chin, Y.-H.; Buda, C.; Neurock, M.; Iglesia, E. Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH4 Reactions on Pd Catalysts. J. Am. Chem. Soc. 2013, 135, 15425-15442. (5) Lu, Y.; Zhang, R.; Cao, B.; Ge, B.; Tao, F. F.; Shan, J.; Nguyen, L.; Bao, Z.; Wu, T.; Pote, J. W.; Wang, B.; Yu, F. Elucidating the Copper–Hägg Iron Carbide Synergistic Interactions for Selective CO Hydrogenation to Higher Alcohols. ACS Catal. 2017, 7, 5500-5512. (6) Chin, Y.-H.; King, D. L.; Roh, H.-S.; Wang, Y.; Heald, S. M. Structure and Reactivity Investigations on Supported Bimetallic AuNi Catalysts Used for Hydrocarbon Steam Reforming. J. Catal. 2006, 244, 153-162. (7) Schuur, E. A. G.; Abbott, B. Climate Change: High Risk of Permafrost Thaw. Nature 2011, 480, 32-33. (8) Shen, J.; Hayes, R. E.; Wu, X.; Semagina, N. 100° Temperature Reduction of Wet Methane Combustion: Highly Active Pd–Ni/Al2O3 Catalyst versus Pd/NiAl2O4. ACS Catal. 2015, 5, 2916-2920. (9) Junmei, W.; Enrique, I. Structural and Mechanistic Requirements for Methane Activation and Chemical Conversion on Supported Iridium Clusters. Angew. Chem. Int. Ed. 2004, 43, 3685-3688. (10) Santen, R. A. v.; Neurock, M.; Shetty, S. G. Reactivity Theory of Transition-Metal Surfaces: A Brønsted−Evans−Polanyi Linear Activation Energy−Free-Energy Analysis. Chem. Rev. 2010, 110, 2005-2048. (11) Pakulska, M. M.; Grgicak, C. M.; Giorgi, J. B. The Effect of Metal and Support Particle Size on NiO/CeO2 and NiO/ZrO2 Catalyst Activity in Complete Methane Oxidation. Appl. Catal. A-Gen. 2007, 332, 124-129. (12) Xin, Y.; Wang, H.; Law, C. K. Kinetics of Catalytic Oxidation of Methane, Ethane and Propane over Palladium Oxide. Combust. Flame 2014, 161, 1048-1054. (13) Yamamoto, H.; Uchida, H. Oxidation of Methane over Pt and Pd Supported on Alumina in Lean-burn Natural-gas Engine Exhaust. Catal. Today 1998, 45, 147-151. (14) Lapisardi, G.; Urfels, L.; Gélin, P.; Primet, M.; Kaddouri, A.; Garbowski, E.; Toppi, S.; Tena, E. Superior Catalytic Behaviour of Pt-doped Pd Catalysts in The Complete Oxidation of Methane at Low Temperature. Catal. Today 2006, 117, 564-568. (15) Castellazzi, P.; Groppi, G.; Forzatti, P. Effect of Pt/Pd Ratio on Catalytic Activity and Redox Behavior of Bimetallic Pt–Pd/Al2O3 Catalysts for CH4 Combustion. Appl. Catal. B-Environ. 2010, 95, 303-311. (16) Wilburn, M. S.; Epling, W. S. Sulfur Deactivation and Regeneration of Mono- and Bimetallic Pd-Pt Methane Oxidation Catalysts. Appl. Catal. B-Environ. 2017, 206, 589-598. 32 ACS Paragon Plus Environment

Page 33 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(17) Goodman, E. D.; Dai, S.; Yang, A.-C.; Wrasman, C. J.; Gallo, A.; Bare, S. R.; Hoffman, A. S.; Jaramillo, T. F.; Graham, G. W.; Pan, X.; Cargnello, M. Uniform Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability. ACS Catal. 2017, 7, 4372-4380. (18) Wilburn, M. S.; Epling, W. S. SO2 Adsorption and Desorption Characteristics of Bimetallic Pd-Pt Catalysts: Pd:Pt ratio dependency. Catal. Today 2019, 320, 11-19. (19) Porsgaard, S.; Merte, L. R.; Ono, L. K.; Behafarid, F.; Matos, J.; Helveg, S.; Salmeron, M.; Roldan Cuenya, B.; Besenbacher, F. Stability of Platinum Nanoparticles Supported on SiO2/Si(111): A High-Pressure X-ray Photoelectron Spectroscopy Study. ACS Nano 2012, 6, 10743-10749. (20) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Aksoy, F.; Aloni, S.; Altoe, V.; Alayoglu, S.; Renzas, J. R.; Tsung, C.-K.; Zhu, Z.; Liu, Z.; Salmeron, M.; Somorjai, G. A. Evolution of Structure and Chemistry of Bimetallic Nanoparticle Catalysts under Reaction Conditions. J. Am. Chem. Soc. 2010, 132, 8697-8703. (21) Nassiri, H.; Hayes, R. E.; Semagina, N. Stability of Pd-Pt Catalysts in Low-temperature Wet Methane Combustion: Metal Ratio and Particle Reconstruction. Chem. Eng. Sci. 2018, 186, 44-51. (22) Xu, J.; White, T.; Li, P.; He, C.; Yu, J.; Yuan, W.; Han, Y.-F. Biphasic Pd−Au Alloy Catalyst for LowTemperature CO Oxidation. J. Am. Chem. Soc. 2010, 132, 10398-10406. (23) Wang, T.; Schmidt, L. D. Intraparticle redispersion of Rh and Pt-Rh particles on SiO2 and Al2O3 by oxidation-reduction cycling. J. Catal. 1981, 70, 187-197. (24) Howe, J.; Chin, Y.-H.; Tu, W.; Yang, Y.; Shangguan, J.; Dogel, S.; Hoyle, D.; Reynolds, M.; Hosseinkhannazer, H.; Perovic, D. Visualization of Phase Segregation and Surface Reconstruction of Pt-Based Bi-metallic Clusters During in Situ Oxidation. Microsc. Microanal. 2016, 22, 734-735. (25) Gao, F.; Goodman, D. W. Pd–Au Bimetallic Catalysts: Understanding Alloy Effects from Planar Models and (Supported) Nanoparticles. Chem. Soc. Rev. 2012, 41, 8009-8020. (26) Beretta, A.; Donazzi, A.; Groppi, G.; Forzatti, P.; Dal Santo, V.; Sordelli, L.; De Grandi, V.; Psaro, R. Testing in Annular Micro-reactor and Characterization of Supported Rh Nanoparticles for the Catalytic Partial Oxidation of Methane: Effect of the Preparation Procedure. Appl. Catal. B-Environ. 2008, 83, 96-109. (27) Gremminger, A. T.; Pereira de Carvalho, H. W.; Popescu, R.; Grunwaldt, J.-D.; Deutschmann, O. Influence of Gas Composition on Activity and Durability of Bimetallic Pd-Pt/Al2O3 Catalysts for Total Oxidation of Methane. Catal. Today 2015, 258, 470-480. (28) Morlang, A.; Neuhausen, U.; Klementiev, K. V.; Schütze, F. W.; Miehe, G.; Fuess, H.; Lox, E. S. Bimetallic Pt/Pd Diesel Oxidation Catalysts: Structural Characterisation and Catalytic Behaviour. Appl. Catal. B-Environ. 2005, 60, 191-199. (29) Tabib Zadeh Adibi, P.; Zhdanov, V. P.; Langhammer, C.; Grönbeck, H. Transient Bimodal Particle Size Distributions during Pt Sintering on Alumina and Silica. J. Phys. Chem. C 2015, 119, 989-996. (30) Carrillo, C.; Johns, T. R.; Xiong, H.; DeLaRiva, A.; Challa, S. R.; Goeke, R. S.; Artyushkova, K.; Li, W.; Kim, C. H.; Datye, A. K. Trapping of Mobile Pt Species by PdO Nanoparticles under Oxidizing Conditions. J. Phys. Chem. Lett. 2014, 5, 2089-2093. (31) Carrillo, C.; DeLaRiva, A.; Xiong, H.; Peterson, E. J.; Spilde, M. N.; Kunwar, D.; Goeke, R. S.; Wiebenga, M.; Oh, S. H.; Qi, G.; Challa, S. R.; Datye, A. K. Regenerative Trapping: How Pd Improves the Durability of Pt Diesel Oxidation Catalysts. Appl. Catal. B-Environ. 2017, 218, 581-590. (32) Martinez-Arias, A.; Hungria, A. B.; Fernandez-Garcia, M.; Iglesias-Juez, A.; Anderson, J. A.; Conesa, J. C. Light-off Behaviour of PdO/γ-Al2O3 Catalysts for Stoichiometric CO–O2 and CO–O2–NO Reactions: A Combined Catalytic Activity–In Situ DRIFTS Study. J. Catal. 2004, 221, 85-92. (33) Allian, A. D.; Takanabe, K.; Fujdala, K. L.; Hao, X.; Truex, T. J.; Cai, J.; Buda, C.; Neurock, M.; Iglesia, E. Chemisorption of CO and Mechanism of CO Oxidation on Supported Platinum Nanoclusters. J. Am. Chem. Soc. 2011, 133, 4498-4517. (34) Stuve, E. M.; Madix, R. J.; Brundle, C. R. CO Oxidation on Pd(100): A Study of the Coadsorption of Oxygen and Carbon Monoxide. Surf. Sci. 1984, 146, 155-178. (35) Bergeret, G.; Gallezot, P. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, 2008.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

(36) Cargnello, M.; Jaén, J. J. D.; Garrido, J. C. H.; Bakhmutsky, K.; Montini, T.; Gámez, J. J. C.; Gorte, R. J.; Fornasiero, P. Exceptional Activity for Methane Combustion over Modular Pd@CeO2 Subunits on Functionalized Al2O3. Science 2012, 337, 713-717. (37) Niemantsverdriet, J. W. Spectroscopy in Catalysis: An Introduction; Wiley-VCH: Weinheim, 2007. (38) Goldstein, D. N.; McCormick, J. A.; George, S. M. Al2O3 Atomic Layer Deposition with Trimethylaluminum and Ozone Studied by in Situ Transmission FTIR Spectroscopy and Quadrupole Mass Spectrometry. J. Phys. Chem. C 2008, 112, 19530-19539. (39) Koros, R. M.; Nowak, E. A Diagnostic Test of the Kinetic Regime in a Packed Bed Reactor. J. Chem. Eng. Sci. 1967, 22, 470. (40) Ishikawa, A.; Neurock, M.; Iglesia, E. Structural Requirements and Reaction Pathways in Dimethyl Ether Combustion Catalyzed by Supported Pt Clusters. J. Am. Chem. Soc. 2007, 129, 13201-13212. (41) Chin, Y.-H.; Iglesia, E. Elementary Steps, the Role of Chemisorbed Oxygen, and the Effects of Cluster Size in Catalytic CH4–O2 Reactions on Palladium. J. Phys. Chem. C 2011, 115, 17845-17855. (42) Habibi, A. H.; Semagina, N.; Hayes, R. E. Kinetics of Low-Temperature Methane Oxidation over SiO2Encapsulated Bimetallic Pd–Pt Nanoparticles. Ind. Eng. Chem. Res. 2018, 57, 8160-8171. (43) Ding, K.; Gulec, A.; Johnson, A. M.; Schweitzer, N. M.; Stucky, G. D.; Marks, L. D.; Stair, P. C. Identification of Active Sites in CO Oxidation and Water-gas Shift over Supported Pt Catalysts. Science 2015, 350, 189-192. (44) Morgan, K.; Goguet, A.; Hardacre, C. Metal Redispersion Strategies for Recycling of Supported Metal Catalysts: A Perspective. ACS Catal. 2015, 5, 3430-3445. (45) Nell, J.; O'Neill, H. S. C. Gibbs Free Energy of Formation and Heat Capacity of PdO: A New Calibration of the Pd-PdO Buffer to High Temperatures and Pressures. Geochim. Cosmochim. Acta 1996, 60, 2487-2493. (46) Li, Y.; Zakharov, D.; Zhao, S.; Tappero, R.; Jung, U.; Elsen, A.; Baumann, P.; Nuzzo, R. G.; Stach, E. A.; Frenkel, A. I. Complex Structural Dynamics of Nanocatalysts Revealed in Operando Conditions by Correlated Imaging and Spectroscopy Probes. Nat. Commun. 2015, 6, 7583. (47) McCaulley, J. A. In-situ X-ray Absorption Spectroscopy Studies of Hydride and Carbide Formation in Supported Palladium Catalysts. J. Phys. Chem. 1993, 97, 10372-10379. (48) Van Hardeveld, R.; Hartog, F. The Statistics of Surface Atoms and Surface Sites on Metal Crystals. Surf. Sci. 1969, 15, 189-230. (49) Wyckoff, R. W. G.; Wyckoff, R. W. Crystal Structures; Interscience: New York, 1960. (50) Wu, J.; Shan, S.; Cronk, H.; Chang, F.; Kareem, H.; Zhao, Y.; Luo, J.; Petkov, V.; Zhong, C.-J. Understanding Composition-Dependent Synergy of PtPd Alloy Nanoparticles in Electrocatalytic Oxygen Reduction Reaction. J. Phys. Chem. C 2017, 121, 14128-14136. (51) Eischens, R. P.; Pliskin, W. A. The Infrared Spectra of Adsorbed Molecules. Adv. Catal. 1958, 10, 1-56. (52) Szanyi, J.; Kwak, J. H. Dissecting the Steps of CO2 Reduction: 2. The Interaction of CO and CO2 with Pd/γAl2O3: An in Situ FTIR Study. Phys. Chem. Chem. Phys. 2014, 16, 15126-15138. (53) Hadjiivanov, K. I. IR Study of CO and H2O Coadsorption on Pt+/TiO2 and Pt/TiO2 Samples. J. Chem. Soc., Faraday Trans. 1998, 94, 1901-1904. (54) Brandt, R. K.; Sorbello, R. S.; Greenler, R. G. Site-specific, Coupled-harmonic-oscillator Model of Carbon Monoxide Adsorbed on Extended, Single-crystal Surfaces and on Small Crystals of Platinum. Surf. Sci. 1992, 271, 605-615. (55) de Menorval, L.-C.; Chaqroune, A.; Coq, B.; Figueras, F. Characterization of Mono- and Bi-metallic Platinum Catalysts Using CO FTIR Spectroscopy Size Effects and Topological Segregation. J. Chem. Soc., Faraday Trans. 1997, 93, 3715-3720. (56) Kale, M. J.; Christopher, P. Utilizing Quantitative in Situ FTIR Spectroscopy To Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al2O3-Supported Pt Catalysts. ACS Catal. 2016, 6, 5599-5609. (57) Barth, R.; Pitchai, R.; Anderson, R. L.; Verykios, X. E. Thermal Desorption-infrared Study of Carbon Monoxide Adsorption by Alumina-supported Platinum. J. Catal. 1989, 116, 61-70. (58) Rothschild, W. G.; Yao, H. C.; Plummer, H. K. Surface Interaction in the Platinum-gamma-alumina system. V. Effects of Atmosphere and Fractal Topology on the Sintering of Platinum. Langmuir 1986, 2, 588-593. 34 ACS Paragon Plus Environment

Page 35 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(59) Dellwig, T.; Hartmann, J.; Libuda, J.; Meusel, I.; Rupprechter, G.; Unterhalt, H.; Freund, H. J. Complex Model Catalysts under UHV and High Pressure Conditions: CO Adsorption and Oxidation on Alumina-supported Pd Particles. J. Mol. Catal. A: Chem. 2000, 162, 51-66. (60) Wolter, K.; Seiferth, O.; Kuhlenbeck, H.; Bäumer, M.; Freund, H. J. Infrared Spectroscopic Investigation of CO Adsorbed on Pd Aggregates Deposited on an Alumina Model Support. Surf. Sci. 1998, 399, 190-198. (61) Tessier, D.; Rakai, A.; Bozon-Verduraz, F. Spectroscopic Study of the Interaction of Carbon Monoxide with Cationic and Metallic Palladium in Palladium-alumina Catalysts. J. Chem. Soc., Faraday Trans. 1992, 88, 741749. (62) Moses-DeBusk, M.; Yoon, M.; Allard, L. F.; Mullins, D. R.; Wu, Z.; Yang, X.; Veith, G.; Stocks, G. M.; Narula, C. K. CO Oxidation on Supported Single Pt Atoms: Experimental and ab Initio Density Functional Studies of CO Interaction with Pt Atom on θ-Al2O3(010) Surface. J. Am. Chem. Soc. 2013, 135, 12634-12645. (63) Abbet, S.; Heiz, U.; Häkkinen, H.; Landman, U. CO Oxidation on a Single Pd Atom Supported on Magnesia. Phys. Rev. Lett. 2001, 86, 5950-5953. (64) Abbet, S.; Sanchez, A.; Heiz, U.; Schneider, W. D.; Ferrari, A. M.; Pacchioni, G.; Rösch, N. Acetylene Cyclotrimerization on Supported Size-Selected Pdn Clusters (1 ≤ n ≤ 30): One Atom Is Enough! J. Am. Chem. Soc. 2000, 122, 3453-3457. (65) Aytam, H. P.; Akula, V.; Janmanchi, K.; Kamaraju, S. R. R.; Panja, K. R.; Gurram, K.; Niemantsverdriet, J. W. Characterization and Reactivity of Pd/MgO and Pd/γ-Al2O3 Catalysts in the Selective Hydrogenolysis of CCl2F2. J. Phys. Chem. B 2002, 106, 1024-1031. (66) Gracia, F. J.; Bollmann, L.; Wolf, E. E.; Miller, J. T.; Kropf, A. J. In Situ FTIR, EXAFS, and Activity Studies of the Effect of Crystallite Size on Silica-supported Pt Oxidation Catalysts. J. Catal. 2003, 220, 382-391. (67) Yasutaka, N.; Kazuhiko, D.; Yasuo, I.; Nobuyuki, T.; Toshitaka, T.; Naoyuki, H.; Gemma, G.; Sakura, P.; A., N. M.; Oji, K.; Hongying, J.; Hirofumi, S.; Shin'ichi, M. In Situ Redispersion of Platinum Autoexhaust Catalysts: An On‐Line Approach to Increasing Catalyst Lifetimes? Angew. Chem. Int. Ed. 2008, 47, 9303-9306. (68) Ortega, A.; Huffman, F. M.; Bradshaw, A. M. The Adsorption of CO on Pd(100) Studied by IR Reflection Absorption Spectroscopy. Surf. Sci. 1982, 119, 79-94. (69) Rogal, J.; Reuter, K.; Scheffler, M. Thermodynamic Stability of PdO Surfaces. Phys. Rev. B 2004, 69, 075421. (70) Hirohisa, S.; Toru, Y.; Nobuo, H.; Kazuya, Y.; Hideo, K.; Osamu, T. Oxidization Characteristics of Some Standard Platinum Resistance Thermometers. Jpn. J. Appl. Phys. 2008, 47, 8071-8076. (71) Gorzkowski, M. T.; Lewera, A. Probing the Limits of d-Band Center Theory: Electronic and Electrocatalytic Properties of Pd-Shell–Pt-Core Nanoparticles. J. Phys. Chem. C 2015, 119, 18389-18395. (72) Zhang, L.; Henkelman, G. Tuning the Oxygen Reduction Activity of Pd Shell Nanoparticles with Random Alloy Cores. J. Phys. Chem. C 2012, 116, 20860-20865.

For Table of Contents Only


Chemical and Structural Dynamics of Nanostructures in Bimetallic Pt

Recommend Documents