•Surface characterization on nanoparticles with aloof beam EELS.•Use of aloof beam EELS to minimize radiation damage.•Vibrational EELS as a tool to probe nanoparticle surfaces.•Detection of bandgap ...states in oxide nanoparticles.•Detection of surface layers and it relevance to catalysts.
In many materials systems, electron beam effects may substantially alter and destroy the structure of interest during observation. This is often true for the surface structures of catalytic nanoparticles where the functionality is associated with thin surface layers which are easily destroyed. The potential application of using aloof beam electron energy-loss spectroscopy as a non-destructive nanoscale surface characterization tool is discussed. Recent developments in monochromators make vibration and valence loss EELS possible in the electron microscope. The delocalization associated with these signals allows spectra to be acquired when the electron beam is position 2nm or more away from the particle surface. This eliminates knock-on damage and significantly reduces ionization damage. Theoretical and experimental results are employed to explore the potential strengths and weaknesses of monochromated aloof beam EELS for surface analysis. The approach is most favored for surface layers on insulators because the bandgap lowers the background for detection of the vibrational signal and bandgap states. Guided light modes and relativistic effects can complicate the interpretation of the spectra. The effects are suppressed at lower accelerating voltages and particle size especially for low refractive index materials.
Abstract
Reducible oxides are widely used catalyst supports that can increase oxidation reaction rates by transferring lattice oxygen at the metal-support interface. There are many outstanding ...questions regarding the atomic-scale dynamic meta-stability (i.e., fluxional behavior) of the interface during catalysis. Here, we employ aberration-corrected
operando
electron microscopy to visualize the structural dynamics occurring at and near Pt/CeO
2
interfaces during CO oxidation. We show that the catalytic turnover frequency correlates with fluxional behavior that (a) destabilizes the supported Pt particle, (b) marks an enhanced rate of oxygen vacancy creation and annihilation, and (c) leads to increased strain and reduction in the CeO
2
support surface. Overall, the results implicate the interfacial Pt-O-Ce bonds anchoring the Pt to the support as being involved also in the catalytically-driven oxygen transfer process, and they suggest that oxygen reduction takes place on the highly reduced CeO
2
surface before migrating to the interfacial perimeter for reaction with CO.
Heterogeneous catalysis is a chemical process performed at a solid–gas or solid–liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the ...surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.
Oxide-supported noble metal catalysts have been extensively studied for decades for the water gas shift (WGS) reaction, a catalytic transformation central to a host of large volume processes that ...variously utilize or produce hydrogen. There remains considerable uncertainty as to how the specific features of the active metal-support interfacial bonding-perhaps most importantly the temporal dynamic changes occurring therein-serve to enable high activity and selectivity. Here we report the dynamic characteristics of a Pt/CeO
system at the atomic level for the WGS reaction and specifically reveal the synergistic effects of metal-support bonding at the perimeter region. We find that the perimeter Pt
- O vacancy-Ce
sites are formed in the active structure, transformed at working temperatures and their appearance regulates the adsorbate behaviors. We find that the dynamic nature of this site is a key mechanistic step for the WGS reaction.
Au/CeO2 catalysts are highly active for low-temperature CO oxidation and water–gas shift reaction, but they deactivate rapidly because of sintering of gold nanoparticles, linked to the collapse or ...restructuring of the gold–ceria interfacial perimeters. To date, a detailed atomic-level insight into the restructuring of the active gold–ceria interfaces is still lacking. Here, we report that gold particles of 2–4 nm size, strongly anchored onto rod-shaped CeO2, are not only highly active but also distinctively stable under realistic reaction conditions. Environmental transmission electron microscopy analyses identified that the gold nanoparticles, in response to alternating oxidizing and reducing atmospheres, changed their shapes but did not sinter at temperatures up to 573 K. This finding offers a new strategy to stabilize gold nanoparticles on ceria by engineering the gold–ceria interfacial structure, which could be extended to other oxide-supported metal nanocatalysts.
We characterize electrical conductivity, microstructure, nano-scale grain boundary structure and chemistry of ceria electrolytes with nominal compositions of Gd sub(0.2)Ce sub(0.8)O sub(2- delta ) ...(GDC) and Gd sub(0.11)Pr sub(0.04)Ce sub(0.85)O sub(2- delta ) (GPDC). The electrolytes are fabricated using mixed oxide nanopowders synthesized by spray drying. AC impedance spectroscopy was performed from 150 degreesC to 700 degreesC in air to determine grain-interior electrical conductivity. Grain-boundary conductivity was determined below 300 degreesC. The grain-interior conductivity of the GPDC was higher than that of GDC by as much as 10 times, depending on the temperature. The GPDC specific grain-boundary conductivity was measured to be approximately 100 times higher than that of GDC. Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) confirmed the grain-to-grain compositional uniformity of both materials following heat treatments. Grain boundaries were free of glassy intergranular phases; dopant concentration and Ce oxidation state were found to vary significantly near grain boundaries. Boundary core composition was estimated from STEM EELS to be Gd sub(0.62)Ce sub(0.38)O sub(2- delta ), and Gd sub(0.29)Pr sub(0.16)Ce sub(0.55)O sub(2- delta ) in GDC and GPDC, respectively. Pr segregation to grain boundaries in the GPDC is hypothesized to enhance conductivity by both decreasing oxygen vacancy migration energy, and inducing mixed ionic-electronic conductivity in the near-boundary region.
In graphitic carbon nitrides, (photo)catalytic functionality is underpinned by the effect that residual hydrogen content, manifesting in amine (N–H x ) defects, has on its optoelectronic properties. ...Therefore, a detailed understanding of the variation in the local structure of graphitic carbon nitrides is key for understanding structure–activity relationships. Here, we apply aloof-beam vibrational electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM) to locally detect variations in hydrogen content in two different layered carbon nitrides with nanometer resolution. Through low dose rate TEM, we obtain atomically resolved images from crystalline and disordered carbon nitrides. By employing an aloof-beam configuration in a monochromated STEM, radiation damage can be dramatically reduced, yielding vibrational spectra from carbon nitrides to be assessed on 10’s of nanometer length scales. We find that in disordered graphitic carbon nitrides the relative amine content can vary locally up to 27%. Cyano (CN) defects originating from uncondensed precursor are also revealed by probing small volumes, which cannot be detected by infrared absorption or Raman scattering spectroscopies. The utility of this technique is realized for heterogeneous soft materials, such as disordered graphitic carbon nitrides, in which methods to probe catalytically active sites remain elusive.
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