Efficient CO Oxidation by 50-Facet Cu2O Nanocrystals Coated with

Subscriber access provided by University of Newcastle, Australia

Article

Efficient CO Oxidation by 50-Facet Cu2O Nanocrystals Coated with CuO Nanoparticles Ahmad M. Harzandi, Jitendra N. Tiwari, Ho Sik Lee, Him Chan Jeon, Woo Jong Cho, Geunsik Lee, Jaeyoon Baik, Ja-Hun Kwak, and Kwang S. Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13843 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 21

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 Applied Materials & Interfaces

Efficient CO Oxidation by 50-Facet Cu2O Nanocrystals Coated with CuO Nanoparticles Ahmad M. Harzandi1,‡, Jitendra N. Tiwari1,‡ ,*, Ho Sik Lee1,‡, Himchan Jeon2, Woo Jong Cho1, Geunsik Lee1, Jaeyoon Baik3, Ja Hun Kwak2,* and Kwang S. Kim1,* 1

Center for Superfunctional Materials, Department of Chemistry, Ulsan National Institute of

Science and Technology (UNIST), Ulsan 44919, Korea 2

Department of Chemical Engineering, School of Energy and Chemical Engineering, UNIST,

Ulsan 44919, Korea 3

Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 37673,

Korea

ABSTRACT: As carbon monoxide oxidation is widely used for various chemical processes (such as methanol synthesis and water–gas shift reactions H2O + CO ⇄ CO2 + H2) as well as in industry, it is essential to develop highly energy efficient, inexpensive, and eco-friendly catalysts for CO oxidation. Here we report green synthesis of ~10 nm-sized CuO nanoparticles (NPs) aggregated on ~400 nm-sized 50-facet-Cu2O polyhedral nanocrystals. This CuO-NPs/50-facetCu2O shows remarkable CO oxidation reactivity with very high specific CO oxidation activity (4.5 ߤmolCO m-2s-1 at 130 °C) and near-complete 99.5% CO conversion efficiency at ~175 °C. This outstanding catalytic performance by CuO NPs over the pristine multi-faceted-Cu2O nanocrystals is attributed to the surface oxygen defects present in CuO NPs which facilitate

ACS Paragon Plus Environment

1


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 21

binding of CO and O2 on their surfaces. This new material opens up new possibilities of replacing the usage of expensive CO oxidation materials.

KEYWORDS: 50-facet-Cu2O nanocrystals, CuO nanoparticles, CO oxidation, Oxygen vacancy, Turnover frequency.

1. INTRODUCTION

CO exhaust emissions in industrial processes and vehicle combustions seriously affect health of humans and animals.1-3 Expensive catalysts such as noble metals have shown high catalytic activity toward CO oxidation.4-11 The problems of these catalysts are their limited supply, high costs,13-16 and in particular high thermal activation required for CO oxidation.6,12 Therefore, the near perfect conversion of CO into CO2 at low temperatures by low-cost catalysts is in great demand. On the Cu surface, in contrast to the 4d metal surfaces, oxygens can form surface oxide structures and the most thermodynamically stable bulk oxide phase. This phase is considered as a catalytic active phase in relevant industrial reactions.17 In addition, copper oxide tends to easily change the valence state by capturing or releasing surface lattice oxygens.18 What is more important is that among various types of CO catalytic converters, cheap cuprous oxide (Cu2O) shows excellent catalytic activity in CO conversion19 at ~350 °C.20-22 However, this CO conversion temperature is high at the industrial level. High-index polyhedral 50-facet-Cu2O microcrystals of ~2.5 µm size show better specific CO oxidation activity18,23 at 240 °C. Yet, this temperature is still high, which limits practical applications. In addition, simple controllable “green” methods to synthesize Cu2O nanocrystals having high-index facets have been considered


2

Page 3 of 21

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


to be a highly challenging subject for their applications to CO oxidation.24,25 Here, we synthesize nano-sized polyhedral 50-facet-Cu2O crystals (1; Figure 1a) which show much better performance due to larger surface area than the micro-sized crystals.24 Then, in the absence of surfactant and template we fabricate the material of CuO-NPs/50-facet-Cu2O (2; Figure 1b) for which CuO-NPs are coated on the surface of 1. In comparison with 1 as well as 6, 8, 18, and 26 facets Cu2O nanocrystals, 2 displays far outstanding activity toward CO oxidation in CO/O2/He gas mixtures at temperatures lower than 175 °C with the conversion efficiency of higher than 99.5 %. It also exhibits a substantial specific CO oxidation activity at 130 °C.

2. EXPERIMENTAL SECTION

2.1. Synthesis of 1 (polyhedral 50 facets Cu2O nanocrystals) and other 26 and 18 facets Cu2O nanocrystals. In a typical synthesis, in a three-neck round bottom flask, a mixture of 1 mmol CuSO4 in 60 mL DI water was poured and then heated at 65°C with stirring. After temperature reached at 65°C, 20 mL of 10 M solution of potassium hydroxide was added. Then, the solution changed to supersaturated blue color for a short time (~2 second). Using the blue solution, a 10 mL of 0.08 M ascorbic acid (as a reducing agent), was added (to reduce the Cu2+ ion to Cu+) with stirring for 10 minutes. The observed color of the resultant solution was yellow, indicating the formation of 1. The product was centrifuged at 8000 rpm to remove debris and washed five times with DI water. Finally, the target product was dried at 25°C. Other Cu2O nanocrystals, 26 and 18 facets, were prepared with the same concentration of ascorbic acid (0.08M) but with different concentrations of 6 and 2 M of KOH solution, respectively.


3


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 21

2.2. Synthesis of 8 facets Cu2O. In this case, the whole procedure was the same as those for 18 facets Cu2O nanocrystals except using of 10 ߤl NH3 solution before pouring KOH solution.

2.3. Synthesis of 6 facets Cu2O. The procedure for synthesis of 1 was employed to prepare the 6 facets Cu2O sample except for the differences in concentrations, KOH pouring method, and ascorbic acid solution. To obtain the 6 facets Cu2O sample, 0.8 M KOH was poured drop-wise; then, after 10 min (observed black solution), 0.08M of ascorbic acid was slowly added with constant stirring of the solution.

2.4. Synthesis of 2 (1 with CuO-NPs coated on the surface). The whole procedure was similar to that for 1 except for concentrations of ascorbic acid. The concentration of ascorbic acid used for synthesizing 2 is 0.1 M.

2.5. Characterization. The X-ray diffraction (XRD) patterns obtained from a high power XRay diffractometer (Rigaku, Japan) were analyzed to study the structures of the samples. The products morphology was characterized by scanning electron microscopy (SEM; JEOL, FEG-XL 30S) and transmission electron microscopy (TEM; 2100F, JEOL, Japan). Focused ion beam (FIB) with Ga+ source was used for cross-section processing and sample preparation for TEM (FEI Helios NanoLab 450). Histograms of the particle size distributions for all types of samples


4

Page 5 of 21

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


were made by counting 100 particles in the SEM image in order to obtain the range of particle diameters. The structural information was obtained from XPS spectroscopy (Thermo Fisher, UK). Gauss–Lorentzian peak shapes were used to fit XPS spectra with the nonlinear least square fit program. By using UV-visible spectroscopy (Shimadzu UV-2401PC spectrophotometer), in the absorbance spectrum the alteration in absorption band maximum (λmax = 464 nm) was recorded and the adsorption and degradation rates of the dyes were monitored. The near-edge Xray absorption fine-structure (NEXAFS) spectra (O K-edge, Cu L3,2-edge) were collected in total electron-yield mode using a Pohang Accelerator Lab beam line. 2.6. CO oxidation measurements. We measured CO oxidation activity with the temperature programmed reaction method using 50 mg of all Cu2O nanocrystals catalysts and the fixed temperature method (130oC) using a different amount of catalysts to control the CO conversion less than 10 %, in a packed bed reactor (quartz tube reactor). Prior to catalytic measurement, all catalysts were treated by He (flow-rate: 60 ml/min) for ~30 minute at 20 oC. A feed gas of 3% O2 and 4% CO in He (total flow-rate: 60 ml/min) were used to measure the activity. Temperature programmed reaction test was carried out from 20 to 300 oC by ramping at a heating rate of 5 o

C/min. CO oxidation activities were collected from extrapolation of time-on-stream

measurements at 130 oC. A gas chromatograph (Agilent 7820A) equipped with capillary-column and thermal conductivity detector was used to measure the concentrations of all reactants and products.

3. RESULTS AND DISCUSSION


5


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 21

3.1. Mechanism of synthesis. The synthetic procedure for 2 has been provided in the Experimental Section. After heating the CuSO4 solution to 65 °C, KOH solution was added to form a blue, supersaturated solution for a short time (Figure. S1b). This blue solution contained an immense number of growing units. As a result, the uniform size of 2 was obtained after adding the ascorbic acid as a reducing agent (Figure. S1c). Periodically alternately stacked layers of atoms represent the structure of Cu2O crystal. The {100} and {110} facets were terminated by the ‘–O’ and ‘–O–Cu’.26 In the {111} facet, a ‘Cu’ layer is between two layers of ‘O’ atoms, which constructs one period of three atom-layers.26 The three atom-layers can be observed as if they are on the same surface due to its short distance between two neighboring layers. The {111} facet is perpendicular to each of two ‘Cu’ atoms with a dangling bond. The positive charge of the {111} facet is due to the large proportion of the subjected Cu atoms exposed to dangling bonds.23,27 In the synthesis, increasing the concentration of OH− anions in KOH solution passivated the {100}, {110}, {111} and {211} facets via electrostatic interactions, leading to down-turn of the growth rate along these facets and forming 6, 18, 26 and 50 facets nanocrystals (Figures S2, S3, S5, S6, S8).23,27 The surface energy of a facet is strongly related to the crystal growth rate. Upon adding a slightly strong reducing agent, the excess ions helped CuO-NPs aggregate on the surface of 1, which resulted in the formation of 2 (Figures. 1, S4).

3.2. Physico-chemical property. Magnified SEM image displays that 1 has the 50-facet polyhedral structure with 400-500 nm diameter (Figure 1a), while 2 has extra ~10 nm-sized CuO-NPs coated on the surface of 1 (Figure 1b). The SEM and TEM images of both 1 and 2


6

Page 7 of 21

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


match perfectly the outlines of 50-facet solid polyhedral model in various orientations (Supporting Information Figure S2). These magnified TEM images of 2 show the presence of NPs on the surface of 1 (Figure 1c-d). The high resolution TEM (HRTEM) images and the selected area electron-diffraction patterns indicate that lattice spacings in yellow area are ~0.275, ~0.196, ~0.252, ~0.186 and ~0.232 nm which match well with the {110}, {11-2}, {11-1}, {202}, and {111} planes of CuO, respectively (Figure 1e). Also the HRTEM image of inner side of 2 shows that 2 is single crystalline (Figure 1f), where the lattice spacings in the selected region are ~0.25, ~0.21, ~0.17 and ~0.30 nm, which match well with the {111}, {200}, {211} and {110} facets of Cu2O, respectively (Figures 1g-i). The sharp peaks in the XRD patterns of 1 and 2 show the absence and presence of CuO-NPs, respectively, on the surface of cubic structure of Cu2O (Figure S9a-b). We note that 1 which has {211} high index facets is well coated by CuO NPs, but other polyhedral nanocrystals which have low index facets are hardly coated by CuO NPS. Since the surface atoms on high index facets are in general energetically less stable than those on low index facets, we thus expect that such less stable surface atoms tend to bind other small NPs. Once small NPs begin to bind to atoms on high-index facet surfaces, they act as nucleation center for additional aggregation of small NPs on other neighboring facets, resulting in formation of 2 by covering small CuO-NPs on 1. The analysis of the NEXAFS near the O-K-edge and Cu-L3,2 -edge of both 1 and 2 as compared with the references of bulk CuO and Cu2O has been made to examine the changes in structure and composition. The O K-edge spectra in Figure 2a display three major peaks for 2 (529.9, 534.3, 538.5 eV) and two peaks for 1 (531.9, 537 eV). The intense peak at 529.9 eV arises from the 1s→3eg transition of CuO-NPs (Cu2+), since the 3eg orbital hybridized with O-2p and Cu-


7


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 21

3d28 is not completely filled due to unfilled 3d orbital. The 1s→eg transition of Cu2O is responsible for the broad peak at ~531.9 eV,29 which appeared due to the creation of unfilled core-hole in the process of X-ray absorption.30 In both samples the hybridized states of O (2p,3p) and Cu (4s,4p) can be responsible for the remaining peaks at higher energy region.28,29,31 The strong peak at ~932.9 eV in the Cu L-edge spectrum of 1 arises from the 2p63d10→2p53d104s1 transition, while the peaks at ~ 948.9 and ~953.1 eV appear from 2p corehole spin-orbit interaction.32 Two intense peaks at 930.5 (Cu L3) and 950 eV (Cu L2) in the Cu L3,2-edge spectrum of 2 (Figure 2b) correspond to the 2p to 3d transition. The interaction of 2p spin-orbit core-hole can lead to the splitting of L3 and L2.32 The XPS spectra of O 1s in 2 indicates 0.4 eV energy down-shift as compared to those of 1, which is attributed to the surface CuO NPs. Also the presence of oxygen defects in the matrix of metal oxide can be responsible for the peak at 531.5 eV 33 (Figures 2c-d). The ratio (0.96) of peak areas of 529.9 eV: 531.5 eV in 2 is lower than the ratio (1.16) of peak areas of 530.3 eV: 531.5 eV in 1. This indicates the existence of high oxygen defect concentration, which is supported by photo-catalysis degradation of dye (methyl orange) analysis34 (Figure S16). We show the temperature dependent durability by checking the crystal morphology and structure using SEM and XRD at different temperatures. The crystal morphology changes as temperature increases over 150 ºC (the image shown below), however the Cu2O phase in the crystal structure shown in XRD pattern below is stable even after the temperature increases over 350 ºC around which the CuO nanoparticles are desorbed from the Cu2O crystals (Figure S20) 3.3. Catalytic CO oxidation. The catalytic CO oxidation of all nanocrystals is examined in a reaction flow with gas mixture of 3%-O2/4.0%-CO/He (Figures 3a-c). For comparison of the


8

Page 9 of 21

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


catalytic performances, specific CO oxidation activities are calculated from the surface area of Cu2O catalysts and initial reaction rates. To avoid the exothermic reaction effect of CO oxidation, conversion is controlled to less than 10 % by using appropriate catalyst loading for each cubic, octahedral, beveled cube, rhombicuboctahedron, 1 and 2 (Figures 3a-b, S17a, Table S3). 2 exhibits remarkable specific CO oxidation activity (4.5 ߤmolCO m-2s-1 at 130 °C) compared with 6, 8, 18, 26 and 50 facets nanocrystals. This value is much higher than the previously reported data and commercial CuO NPs (as compared with Cu, CuO, and Cu2O, Tables S3, S4). CO conversion tests were also performed on similar loads of different types of nanocrystals. 2 is much more active than 1 as well as 6, 8, 18, and 26 facets nanocrystals (Figures 3b-c, S17b, Table S4) and reaches the CO conversion of more than 90% at 160 °C (99.5% at 175 °C) which is very close to DOE target35 (90% at 150 °C). The SEM, TEM, and HRTEM images after CO oxidation display that the whole surface of 2 is still well reactive without significant changes in surface morphology (Figures 3e-f). Also the high-resolution XPS spectra and XRD patterns of 2 before and after CO oxidation are very similar (Figure S9, S10). The same study on 1 and other different faceted nanocrystals also showed similar XPS spectra and XRD patterns before and after CO oxidation (Figures S5-S9, S11-S15). The HRTEM images and fast Fourier Transform (FFT) patterns confirm that the single crystal structure of 50-facet-Cu2O remains apparently intact after CO oxidation (Figure 3g). 1 also exhibits no significant change in morphology before and after CO oxidation (Figure S3a,d). The CO oxidation could be affected by a local structural change due to weak adsorption of CO or O2 which cannot be easily seen by high energy HRTEM. The rapid catalytic rate of 1 is likely to be related to high-index {211} facets exhibiting high chemical activity.18 In consideration of Mars-van Krevelen redox mechanism (Figure 3d):1


9


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

CO + OS → CO2 + ØV

(1)

O2 + 2ØV → 2OS

(2)

Page 10 of 21

where ØV is the surface oxygen vacancy, the superior specific catalytic rate of 2 could be related to the presence of high oxygen vacancies on surfaces of ultrafine 10 nm-sized CuO NPs.1

4. CONCLUSIONS To sum up, we successfully devised a novel, green, and low-cost method to achieve different facets/shapes of Cu2O nanocrystals (cubic, octahedral, beveled cube, rhombicuboctahedron, 1 and 2). The reaction is triggered between KOH and ascorbic acid in mild conditions without addition of any templates or surfactants. The as-obtained 2 is built up with spontaneously selfaggregated CuO-NPs on the surface of polyhedral 1. An excess of ascorbic acid plays a key role in covering ultrafine CuO-NPs on the polyhedral Cu2O crystals. 2 exhibits remarkable specific CO oxidation activity because of high surface oxygen vacancy. The surface oxygen defects of CuO assist remarkable catalytic performance of 2 over 1 and other polyhedral (6, 8, 18, and 26 facets) Cu2O nanocrystals. Overall, highly faceted Cu2O crystals show better CO conversion activity because of the less energetic stability of high indexed surfaces. Once the crystal size becomes smaller, the activity increases due to the increased surface area per catalyst weight. The ultrafine CuO NPs show very high CO conversion activity. Thus, extremely high catalytic activity for the CO conversion of 2 is due to its ability to have many ultrafine CuO NPs on the surface of 50-facet-Cu2O nanocrystals which was indebted from somewhat less stable {211} facets. Various nanocrystals


10

Page 11 of 21

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


facets which are useful for preparation of novel crystallographic structures offer a new avenue for fabricating new materials with various intriguing properties.

∎ AUTHOR INFORMATION Corresponding Authors * K.S.K. ([email protected]), J.H.K ([email protected]), and J.N.T. ([email protected]). Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ∎ ACKNOWLEDGMENTS This work was supported by NRF (National Honor Scientist Program: 2010-0020414) and KISTI (KSC-2015-C3-059, KSC-2015-C3-061). The NEXAFS experiments were performed in Pohang Accelerator Lab beamline. ∎ ASSOCIATED CONTENT *Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Supplementary experimental sections, additional characterization data of the 1, 2 and other Cu2O nanocrystals. TEM and SEM characterization data; XRD spectra; and XPS analyses.


11


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 21

∎ REFERENCES 1. Royer, S.; Duprez, D. Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides. ChemCatChem 2011, 3, 24–65. 2. Knözinger, E.; Kellersohn, A.; Langel, W.; Giersig, M. Cu2O Quantum-Dot Particles Prepared From Nanostructured Copper. Adv. Mater. 1995, 7, 652–655. 3. Prasad, R.;Singh, P. A Review on CO Oxidation Over Copper Chromite Catalyst Catal. Rev.: Sci. Eng. 2012, 54, 224–279. 4. Liu, K.; Wang, A.; Zhang, T. Recent Advances in Preferential Oxidation of CO Reaction Over Platinum Group Metal Catalysts. ACS Catal. 2012, 2, 1165–1178. 5. Vendelbo, S. B.; Elkjær, C.F.; Falsig, H.; Puspitasari, I.; Dona, P.; Mele, L.; Morana, B.; Nelissen, B.J.; Rijn, R.V.; Creemer, J.F.; Kooyman, P.J.; Helveg, S. Visualization of Oscillatory Behaviour of Pt Nanoparticles Catalysing CO Oxidation. Nat. Mater. 2014, 13, 884–890. 6. Peterson, E. J.; DeLaRiva, A.T.; Lin, S.; Johnson, R.S.; Guo, H.; Miller, J.T.; Kwak, J.H.; Peden, C.H.F.; Kiefer, B.; Allard, L.F.; Ribeiro, F.H.; Datye, A.K. Low-Temperature Carbon Monoxide Oxidation Catalyzed by Regenerable Atomically Dispersed Palladium on Alumina. Nat. Commun. 2014, 5, 4885. 7. Kwak, J. H.; Hu, J.; Mei, D.; Yi, Ch. W.; Kim, D. H.; Peden, C. H. F.; Allard, L. F.; Szanyi, J., Coordinatively Unsaturated Al3+ Centers as Binding Sites for Active Catalyst Phases of Platinum on γ-Al2O3. Science 2009, 325, 1670-1673.


12

Page 13 of 21

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


8.

Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skála, T.; Bruix, A.; Illas, F.; Prince, K. C.; Matolı´n, V.; Neyman, K. M.; Libuda, J. Support Nanostructure Boosts Oxygen Transfer to Catalytically Active Platinum Nanoparticles. Nat. Mater. 2011, 10, 310–315.

9. Quiao, B.; Wang, A.; Yang, X.; Allard, L.F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nat. Chem. 2011, 3, 634641. 10. Newton, M. A.; Ferri, D.; Smolentsev, G.; Marchionni, V.; Nachtegaal. M. RoomTemperature Carbon Monoxide Oxidation by Oxygen Over Pt/Al2O3 Mediated by Reactive Platinum Carbonates. Nat. Commun. 2015, 6, 8675. 11. Chen, G.; Zhao, Y.; Fu, G.; Duchesne, P. N.; Gu, L.; Zheng, Y.; Weng, X.; Chen, M.; Zhang, P.; Pao, Ch. W.; Lee, J. F.; Zheng, N. Interfacial Effects in Iron-Nickel Hydroxide–Platinum Nanoparticles Enhance Catalytic Oxidation. Science 2014, 344 495499. 12. Kašpar, J.; Fornasiero, P.; Hickey, N. Automotive Catalytic Converters: Current Status and Some Perspectives. Catal. Today 2003, 77, 419–449. 13. Kim, D. H.; Lim, M. S. Kinetics of Selective CO Oxidation in Hydrogen-Rich Mixtures on Pt/Alumina Catalysts. Appl. Catal. A 2002, 224, 27–38. 14. Avgouropoulos, G.; Ioannides, T.; Papadopoulou, Ch.; Batista, J.; Hocevar, S.; Matralis H.K. A Comparative Study of Pt/γ-Al2O3, Au/α-Fe2O3 and CuO–CeO2 Catalysts for the


13


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 21

Selective Oxidation of Carbon Monoxide in Excess Hydrogen. Catal. Today 2002, 75, 157–167. 15. Marino, F.; Descorme, C.; Duprez, D. Noble Metal Catalysts for the Preferential Oxidation of Carbon Monoxide in the Presence of Hydrogen (PROX). Appl. Catal. B 2004, 54, 59–66. 16. Miller, D.; Casalongue, H.S.; Bluhm, H.; Ogasawara, H.; Nilsson, A.; Kaya,.S. Different Reactivity of the Various Platinum Oxides and Chemisorbed Oxygen in CO Oxidation on Pt(111). J. Am. Chem. Soc. 2014, 136, 6340-6347. 17. Soon, A.; Todorova, M.; Delley, B.; Stampfl, C. Oxygen Adsorption and Stability of Surface Oxides on Cu (111): A First-Principles Investigation. Phys. Rev. B. 2006, 73, 165424-1-165424-12. 18. Leng, M.; Liu, M.; Zhang, Y.; Wang, Z.; Yu, C.; Yang, X.; Zhang, H.; Wang, C. Polyhedral 50-Facet Cu2O Microcrystals Partially Enclosed by {311} High-Index Planes: Synthesis and Enhanced Catalytic CO Oxidation Activity J. Am. Chem. Soc. 2012, 132, 17084–17087. 19. Huang, W. Oxide Nanocrystal Model Catalysts. Acc. Chem. Res. 2016, 49, 520–527. 20. Gawande, M. B.; Goswami, A.; Felpin, F. X.; Asefa, T.; Huang, X.; Silva, R.; Zou, X.; Zboril, R.; Varma, R. S. Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis. Chem. Rev. 2016, 116 3722–3811.


14

Page 15 of 21

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


21. Goncalves, R.V.; Wojcieszak, R.; Wender, H.; Dias, C. S. B.; Vono, L. L. R.; Eberhardt, D.; Teixeira, S. R.; Rossi, L. M. Easy Access to Metallic Copper Nanoparticles with High Activity and Stability for CO Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 7987– 7994. 22. Wang, W. W.; Du, P. P.; Zou, S. H.; He, H. Y.; Wang, R. X.; Jin, Z.; Shi, S.; Huang, Y. Y.; Si, R.; Song, Q. S.; Jia, C. J.; Yan, C. H. Highly Dispersed Copper Oxide Clusters as Active Species in Copper-Ceria Catalyst for Preferential Oxidation of Carbon Monoxide. ACS Catal. 2015, 5, 2088–2099. 23. Liang, Y.; Shang, L.; Bian, T.; Zhou, C.; Zhang, D.; Yu, H. Xu, H.; Shi, Z.; Zhang, T.; Wua, L. Z.; Tung, C. H. Shape-Controlled Synthesis of Polyhedral 50-facet Cu2O Microcrystals with High-Index Facets. CrystEngComm 2012, 14, 4431–4436. 24. Sun, S. D.; Yang, Z. M. Recent Advances in Tuning Crystal Facets of Polyhedral Cuprous Oxide Architectures. RSC Adv. 2014, 4, 3804–3822. 25. Sun, S. Recent Advances in Hybrid Cu2O-Based Heterogeneous Nanostructures. Nanoscale 2015, 7, 10850–10882. 26. Zhang, D.; Zhang, F. H.; Guo, L.; Zheng, K.; Han, X. D.; Zhang, Z. Delicate Control of Crystallographic Facet-Oriented Cu2O Nanocrystals and the Correlated Adsorption Ability. J. Mater. Chem. 2009, 19, 5220–5225. 27. Wang, X.; Jiao, S.; Wu, D.; Li, Q.; Zhou, J.; Jiang, K.; Xu, D. A Facile Strategy for Crystal Engineering of Cu2O Polyhedrons with High-Index Facets. CrystEngComm 2013, 15, 1849–1852.


15


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 21

28. de Groot, F. M.; Fuggle, J. C.; Ghijsen, J.; Sawatzky, G. A.; Petersen, H. Oxygen 1s XRay-Absorption Edges of Transition-Metal Oxides. Phys. Rev. B 1989, 40, 5715-5723. 29. Gurevich, A. B.; Bent, B. E.; Teplyakov, A.V.; Chen, J. G. A NEXAFS Investigation of the Formation and Decomposition of CuO and Cu2O Thin Films on Cu(100). Surf. Sci. 1999, 442, L971–L976. 30. Eren, B.; Heine, C.; Bluhm, H.; Somorjai, G. A.; Salmeron, M. Catalyst Chemical State during CO Oxidation Reaction on Cu(111) Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy and Near Edge X-ray Adsorption Fine Structure Spectroscopy. J. Am. Chem. Soc. 2015, 137, 11186−11190. 31. Grioni, M.; van Acker, J. F.; Czyžyk, M. T.; Fuggle, J. C. Unoccupied Electronic Structure and Core-Hole Effects in the X-Ray-Absorption Spectra of Cu2O. Phys. Rev. B 1992, 45, 3309-3318. 32. George, S. J.; Lowery, M. D.; Solomon, E. I.; Cramer, S. P. Copper L-Edge Spectral Studies: A Direct Experimental Probe of the Ground-State Covalency in the Blue Copper Site in Plastocyanin. J. Am. Chem. Soc. 1993, 115, 2968-2969. 33. Århammar, C.; Pietzsch, A.; Bock, N.; Holmström, E.; Araujo, C. M.; Gråsjö, J.; Zhao, S.; Green, S.; Peery, T.; Hennies, F.; Amerioun, S.; Föhlisch, A.; Schlappa, J.; Schmitt, T.; Strocov, V. N.; Niklasson, G. A.; Wallace, D. C.; Rubensson, J. E.; Johansson, B.; Ahujaa, R. Unveiling the Complex Electronic Structure of Amorphous Metal Oxides. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 6355–6360.


16

Page 17 of 21

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


34. Zhang, X.; Qin, J.; Xue, Y.; Yu, P.; Zhang, B.; Wang, L.; Liu, R. Effect of Aspect Ratio and Surface Defects on the Photocatalytic Activity of ZnO Nanorods. Sci. Rep. 2014, 4, 4596. 35. USDRIVE

Advanced

Combustion

and

Emission

Control

Roadmap,

www1.eere.energy.gov/vehiclesandfuels/pdfs/program/acec_roadmap_

http://

june2013.pdf

(2013).


17


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 21

Figure 1. CuO-NPs/50-facet-Cu2O nanocrystals. (a,b), High-magnification SEM images of 1 and 2, respectively (left figures), along with the ideal 50-facet solid model, where green, blue, violet, and red colors represent six {100}, eight {111}, twelve {110}, and twenty-four {211} facets, respectively (right figures). (c) TEM image of 2. (d) Typical TEM image of a small portion on the surface of 2 (see small NPs on the surface). (e) HRTEM image corresponding to the selected area in d, which shows the matching of the NPs lattice spacings with CuO index spacings and the matching of the lattice spacings of the nanocrystal surface with Cu2O index spacings. (f) HRTEM image of the inner side of 2 (sample prepared by FIB cross-sectional processing) (inset: corresponding diffraction pattern). (g-i), HRTEM images corresponding to regions 1-3 in f, which shows the matching with Cu2O index spacings.


18

Page 19 of 21

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


Figure 2. Chemical analysis for 1 and 2 before CO oxidation which are compared with bulk CuO and Cu2O. (a) O K –edge NEXAFS spectra. (b) Cu L3,2 –edge NEXAFS spectra. (c-d), Core level O 1s XPS of sample 1 (c) and 2 (d).


19


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 21

Figure 3. Catalytic performance and physical analysis. (a) Specific activity (in ߤmolCO m2 s-1) of CO oxidation at 130 oC for various facets Cu2O nanocrystals. (b) CO conversion rate and stability at 130 oC. (c) Temperature programmed reaction of CO oxidation. (d) Simplified Mars– van Krevelen redox mechanism model for CuxO (x =1,2) nanocrystals. (e) SEM image of 2 (inset: TEM image). (f) HRTEM image showing CuO-NPs on 50-facet-Cu2O after CO oxidation. (g) HRTEM image of the interface between CuO-NPs (yellow circles) and 50-facet-Cu2O (inset: FFT image of 1).


20

Page 21 of 21

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


Table Of Content (TOC) graphic

We report 10 nm-sized CuO nanoparticles coated on Cu2O 50-facet-nanocrystals, which show remarkable specific CO oxidation activity at 130 oC and achieve near-perfect CO conversion at only ~175 °C.


21

Efficient CO Oxidation by 50-Facet Cu2O Nanocrystals Coated with

Recommend Documents