AVS2012 Session SS+NS-TuA: Reactivity of Size and Shape Selected Nanoparticles

Tuesday, October 30, 2012 2:00 PM in Room 21
Tuesday Afternoon

Time Period TuA Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS2012 Schedule

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2:00 PM Invited SS+NS-TuA-1 2012 AVS Gaede-Langmuir Award Lecture: Surface Photochemistry on Compact Crystals and on Metal Nanoparticles
Dietrich Menzel (Fritz-Haber Institut, and Techn. Univ. Muenchen, Germany)

Adsorbing a molecule on a substrate changes its photochemistry. I shall briefly review characteristics of surface photochemistry, established mechanisms, and effects such as whether the substrate acts mainly as source or sink of electronic excitations of adsorbates, how long the latter survive, and how effects which influence their localization and delocalization influence the success rate of excitations. For laser excitation, linear and nonlinear response to excitations can occur.

Use of nanoparticles (MNPs) instead of bulk metals further changes surface photochemistry, mainly by changing the substrate optical excitations (e.g. the Mie plasmon of MNPs), and excitation lifetimes and efficiency (by confinement). This will be illustrated by data obtained in the past years in Berlin on NO dimers adsorbed on Ag NPs with varied size (2 to 10 nm) supported on thin alumina films on NiAl single crystals, laser-excited with 2 to 5 eV, with in situ comparison with Ag(111). The main channel is photodesorption of NO; conversion to N2O + O, and to NO(ad) stabilized by O also occur. Adsorption energies were characterized by TPD, cross sections (PCS) by photo-depletion, and desorbate energy distributions (translation, rotation, vibration) by TOF and REMPI analysis. Linear and nonlinear fluence dependencies of desorption signals have been found with ns and fs laser pulses, respectively. The main changes in NO photodesorption are found in the PCS which are strongly enhanced by plasmon excitation and more weakly by excitation confinement, and show clear size dependences interpreted by counteracting influences. The branching into the minor photoreaction channels is also changed at Ag NPs compared to Ag(111) which is due to varying PCS enhancement factors. The photochemical mechanism, however, as evidenced by state-resolved analysis of the desorbing NO molecules, remains the same – formation of transient negative ions by hot electrons in the substrate - for most of the investigated range (with an exception for high energy and small particles). With fs laser pulses further drastic PCS increases are found even at low fluences at the NPs but not at Ag(111). This nonlinear effect is explained by re-excitation of hot electrons confined in the NPs within a single laser pulse. But even here the individual dynamics stay the same. This action of NPs on the success probability of excitations with essentially unchanged dynamics appears to be the typical behavior for photochemistry on MNPs. Only in an unusual case (Xe/Ag NPs) we have seen a direct influence of plasmon excitation on desorption.

These findings may help in the understanding of photocatalysis on MNPs.

2:40 PM SS+NS-TuA-3 Photocatalytic Deposition of Au onto Ordered Linear Arrays of TiO2 Nanoparticles
James Taing, Alexandria Margarella, Yu Liu, John Hemminger (University of California Irvine)

TiO2 nanoparticles were decorated onto the step edges of highly oriented pyrolytic graphite (HOPG) via physical vapor deposition. Gold shells and nanoparticles were then grown on the TiO2 nanoparticles using a photoelectrochemical cell whereupon a photocatalytic reduction mechanism is verified by photocurrent measurements. Samples of TiO2 nanoparticles on HOPG, acting as a photoelectrode, were placed in a half-cell and immersed in either an electrolyte solution of 1.0 M NaCl or 1.0 M NaNO3. Bare HOPG, acting as a counter electrode, was placed in a second half-cell and immersed in the same electrolyte solution. The two half-cells were connected by a salt bridge and the electrodes by a picoammeter. Upon irradiation of the TiO2 nanoparticles by 365 nm UV light from a 200 W Hg lamp, photogenerated electrons produced a photocurrent. Subsequent to introducing 1 mL of 15 μM HAuCl4 into the cell containing the TiO2 nanoparticles, the photocurrent decreased as a result of the reduction of Au3+ to Au on TiO2. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray dispersive spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS) were used to characterize the morphology, crystal structure, and chemical identity of the nanoparticles. Images of TiO2 nanoparticles encapsulated in Au are included in the supplement.

3:00 PM SS+NS-TuA-4 Catalytic Activity of Gold-supported TiO2 Nanocrystals Towards Simple Alcohols
Denis Potapenko, Zhisheng Li, Yang Lou, Richard Osgood (Columbia University)
Titanium oxide is a versatile photocatalytic material and it has been the subject of much research throughout the last two decades. Nanostructuring is one approach for tailoring the properties of a catalyst. Previously we have developed a method of preparation of structurally homogenous TiO2 nanocrystals on Au(111) substrate through oxidation of Ti-Au surface alloy. In this work we explore catalytic properties of the nanocrystals through a series of temperature programmed desorption (TPD) studies with simple alcohols: ethanol and 2-propanol. Similarly to the single crystal TiO2 rutile(110) surface, TiO2 nanocrystals on Au(111) catalyze dehydrogenation of ethanol and 2-propanol into ethylene and propene. Dehydrogenation was observed in a wide range of temperatures from 400 to 550 K, which is lower than the temperature of the corresponding reaction on rutile(110). More interestingly, we have observed formation of acetone from 2-propanol on our TiO2/Au(111) surface at around 450 K; this reaction was not observed on rutile(110). The reactivity patterns of TiO2/Au(111) show strong dependence on geometry and structure of the nanocrystals.
3:20 PM BREAK
4:00 PM Invited SS+NS-TuA-7 Structure, Chemical State, and Reactivity Investigations of Size- and Shape-Selected Nanocatalysts under Operando Conditions
Beatriz Roldan Cuenya (University of Central Florida)

The rational design of the next-generation of catalysts requires detailed knowledge of the correlation between structure, chemical composition, and reactivity. Even though Pt and Pd are among the most industrially relevant and widely investigated nanocatalysts, their complex interaction with common reactants such as oxygen still provides many challenges to the scientific community. In this work, the relation between the structure and reactivity of nanocatalysts “at work” was obtained via X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, and mass spectrometry. Homogeneous size- and shape-selected metal nanoparticles (NPs) have been synthesized by means of diblock copolymer encapsulation.

The influence of the nanoparticle shape on the reactivity of Pt nanocatalysts on γ-Al2O3 will be described. Nanoparticles with similar size distributions (~0.8-1 nm) but with different shapes were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the NP surface and the onset reaction temperature was observed. Furthermore, platinum oxides were found to be the active species for the partial oxidation of 2-propanol, while the complete oxidation was catalyzed by oxygen-covered metallic Pt NPs.

The evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts during the in situ reduction of NO with H2 will also be discussed. Prior to the onset of the reaction, NO-induced redispersion of the Pd NPs over the ZrO2 support was observed, and Pdδ+ species detected. This process parallels the high production of N2O observed at the onset of the reaction (>120°C), while at higher temperatures (≥ 150°C) the selectivity shifts toward N2. Interestingly, concomitant with the onset of N2 production, the Pd atoms re-aggregate into large metallic Pd NPs, which were found to constitute the active phase for the H2-reduction of NO. The evolution of the oxidation state of Pd and Pt NPs during the oxidation of NO and the role of the NP size will also be presented.

Our findings highlight the decisive role of the nanoparticle structure and chemical state in catalytic reactions and the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.

4:40 PM Invited SS+NS-TuA-9 Particle Size, Support and Alloying Effects in Electrocatalysis: Relationships with Heterogeneous Catalysis
Brian Hayden (University of Southampton, UK)
High-Throughput Physical Vapour Deposition (HT-PVD) based on Molecular Beam Epitaxy methods1 has been used to synthesize libraries of catalysts which have subsequently been screened for their electrochemical activity and stability. A screening method is briefly described2 which has been applied to measurements on model supported metal nano-particle HT-PVD catalyst libraries.
 
Considerable effort has been made to find alternative supports for platinum based catalysts in order to improve the particle stability and improve the three-phase boundary in fuel cell applications. HT-PVD model catalyst methodology has been applied to the study of support and particle size effects in electrocatalysis.3 Experiments have demonstrated the potential for using a support such as titania to induce CO oxidation electro-catalytic activity in gold particles,4 with an optimum particle size observed at ca. 3nm (Figure). No induced activity is observed for carbon supports. The similarities with the low temperature oxidations exhibited by supported Au in heterogeneous catalytic are highlighted. Extending this methodology to supported platinum based catalysts, the effect of particle size is demonstrated in the reduction of oxygen for the model carbon supported platinum catalysts, highlighting the limitations of catalyst dispersion. Supporting platinum on titania can result is a strong poisoning of the oxygen reduction catalysis.5
 
The combination of ab-initio theory and electrocatalyst screening also provides a powerful combination in the search for precious metal alloy and non noble metal alloy catalysts. Examples are given for anode hydrogen oxidation (HOR) catalysts such as Pd based,6 and tungsten copper7 alloys.
 
References
1. S. Guerin and B. E. Hayden; J. Comb. Chem. 8 (2006) 66-73.
2. S. Guerin, B.E. Hayden, et.al.; J. Comb. Chem. 6 (2004) 149 - 158.
3. S. Guerin, B.E. Hayden, D. Pletcher, et.al.; J. Comb. Chem. 8 (2006) 791-798.
4. B.E. Hayden, D. Pletcher and J.-P. Suchsland; Angewandte Chemie Int. Ed. 46 (2007) 3530-3532.
5. B.E. Hayden, D. Pletcher, J.-P. Suchsland et.al.; Phys. Chem. Chem. Phys. 11 (2009) 1564-1570. ibid: Phys. Chem. Chem. Phys., 2009, 11, 9141–9148.
6. F. A. Al-Odail, A. Anastasopoulos, and B. E. Hayden; Phys. Chem. Chem. Phys. 12 (2010) 11398-11406. Ibid; Topics in Catalysis 54 (2011) 77-82.
7. A. Anastasopoulos, J. Blake, John and B.E. Hayden; J. Phys. Chem. C, 115 (2011) 19226-19230.
 
5:20 PM SS+NS-TuA-11 The Growth and Structures of Metal Nanoparticles on Ordered ZrO2(111) Surfaces
Yong Han, Shanwei Hu, Yonghe Pan, Jianbo Hou, Haibin Pan, Junfa Zhu (University of Science and Technology of China)
Metal nanoparticles supported on zirconia have attracted much attention in recent years owing to their variety of technological applications such as heterogeneous catalysis and gas sensor operation. In particular, as catalysts, the interface properties of metal/ZrO2 referring to the morphology, charge transfer, thermal stability and reactivity play crucial roles in determining their real applications. In this presentation, we report our recent studies on the growth, electronic structures and thermal stabilities of metal nanoparticles (Cu, Ag and Au) on well-defined ZrO2 thin films by synchrotron radiation photoemission spectroscopy (SRPES) together with scanning tunneling microcopy (STM) and low electron energy diffraction (LEED). The well-defined ZrO2(111) oxide thin films were epitaxially grown on Pt(111). It was found that the growth behavior of metals on ZrO2(111) strongly depends on the morphologies of oxide surfaces and the interfacial interactions between the metal deposits and the ZrO2(111) films. The binding energies of all three metal core-level peaks shift monotonically toward higher binding energy with decreasing the metal particle sizes. The contributions of initial and final state effects to the core level binding energy shifts are differentiated using the Auger parameters. At very low coverages, most likely Au forms Auδ-, while Ag remains the metallic state and Cu forms Cu+ on ZrO2(111).
5:40 PM SS+NS-TuA-12 Structure and Electronic Properties of Ni Nanoparticles Supported on Reducible CeO2(111) Thin Films
YingHui Zhou (Xiamen University, Republic of China); Jing Zhou (University of Wyoming)
Ceria-supported Ni nanoparticles have been of great interest as ethanol and methane reforming catalysts for hydrogen production in fuel cell applications. Recent studies have indicated that the catalytic reactivity of these ceria-supported Ni nanoparticles can be influenced by the redox properties of ceria as well as the synergistic effect between the two. To elucidate the nature of their activity, we studied Ni particles deposited on fully oxidized CeO2(111) and reduced CeO1.88(111) thin films using scanning tunneling microscopy and x-ray photoelectron spectroscopy at the fundamental level. Ceria thin films were grown in situ on Ru(0001) under ultrahigh vacuum conditions. Ni was vapor-deposited onto ceria thin films. At 300 K, metallic Ni is the only species present on the reduced ceria. However, a small amount of Ni is oxidized to Ni2+ on CeO2. Oxidation of Ni on CeO2 can be facilitated by annealing as well as by depositing Ni at 500 K. Scanning tunneling microscopy studies show that Ni forms two-dimensional particles on ceria at room temperature, which suggests a strong Ni-ceria interaction. The particles can agglomerate into large three-dimensional structures with further heating. The structure and electronic properties of Ni metal particles on ceria were further compared to those of bimetallic Ni-Au and Ni-Rh particles.
Time Period TuA Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS2012 Schedule