The recent explosive growth in research on catalysis by supported single metal atoms proves the scientific interest in this new frontier of heterogeneous catalysis. A supported single-atom catalyst (SAC) contains only isolated individual atoms dispersed on, and/or coordinated with, the surface atoms of an appropriate support. SACs not only maximize the atom efficiency of expensive metals but also provide an alternative strategy to tune the activity and selectivity of a catalytic reaction. When single metal atoms are strongly anchored onto high-surface-area supports, SACs offer a great potential to significantly transform the field of heterogeneous catalysis, which has been critical to enabling many important technologies. In this Perspective, I discuss the most recent advances in preparing, characterizing, and catalytically testing SACs with a focus on correlating the structural perspective of the anchored single metal atoms to the observed catalytic performances. The grand challenge to successfully devel...

Catalysis by Supported Single Metal Atoms

III. Formation of Carbon Clusters 2317 IV. Thermodynamic Properties 2319 V. C2 2319 A. Structure and Spectroscopy 2319 B. C2 2320 C. C2 2321 VI. C3 2321 A. Early Experimental Work 2321 B. Mid-IR and Far-IR Spectroscopy 2321 C. Electronic Spectroscopy 2323 1. The Σu + Ground State 2323 2. The AΠu State 2324 3. The Σu + State 2324 4. The aΠu and bΠg States 2325 5. Other Band Systems 2326 D. C3 in the Interstellar Medium 2326 E. C3 2326 F. C3 2327 VII. C4 2327 A. Theory 2327 B. Experiment 2328 1. The Rhombic Isomer 2328 2. The Linear Isomer 2328 C. Excited Electronic States 2330 D. C4 2331 E. C4 2332 VIII. C5 2332 A. Theory 2332 B. Experiment 2332 C. Excited Electronic States 2334 D. C5 2334 E. C5 2335 IX. C6 2335 A. Theory 2335 B. Experiment 2336 1. The Cyclic Isomer 2336 2. The Linear Isomer 2336 C. Excited Electronic States 2337 D. C6 2338 E. C6 2338 X. C7 2338 A. Theory 2338 B. Experiment 2338 C. Excited Electronic States 2340 D. C7 2340 E. C7 2340 XI. C8 2341 A. Theory 2341 B. Experiment 2342 1. The Cyclic Isomer 2342 2. The Linear Isomer 2342 C. Excited Electronic States 2342 D. C8 2342 E. C8 2342 XII. C9 2343 A. Theory 2343 B. Experiment 2343 C. Excited Electronic States 2345 D. C9 2345 E. C9 2345 XIII. C1

http://www.cchem.berkeley.edu/rjsgrp/publications/papers/1998/200_orden_1998.pdf

Small carbon clusters: spectroscopy, structure, and energetics.

State of the Art and Challenges of the ab Initio Theory of Intermolecular Interactions.

The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988.In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials.Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research.Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments.The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

Research opportunities on clusters and cluster-assembled materials—A Department of Energy, Council on Materials Science Panel Report

The measurement of electronic spectra of supersonically cooled molecules and clusters is a widely used approach for addressing many problems in chemistry. The most established techniques for making such measurements are laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI), and both have been employed very successfully in a large number of studies. However, both methods often fail for systems containing more than a few atoms, due to rapid internal conversion, predissociation, or other dynamical processes. Even for small systems, the vibronic band intensities are often contaminated by intramolecular relaxation dynamics; in such cases, these techniques cannot be used for reliable intensity measurements. For clusters that exhibit rapid photofragmentation, depletion spectroscopy can be employed quite effectively to measure their vibronic structure, but again, dynamic effects complicate the interpretation of spectra. The same considerations apply to other types of “action” spectroscopy. It would often be preferable to measure the electronic spectra of molecules and clusters in direct absorption, as this approach is the most straightforward and accurate means of determining absolute vibronic band intensities and for accessing states that are invisible to LIF or REMPI. The problem, of course, is that direct absorption methods are generally orders of magnitude less sensitive than the “action” techniques and are, therefore, difficult to apply to transient species, such as clusters or radicals. In this review, we describe a relatively new direct absorption technique that we have developed for measuring the electronic spectra of jet-cooled molecules and clusters with both high sensitivity and high spectral resolution. The method is based on measurement of the time rate of decay of a pulse of light trapped in a high reflectance optical cavity; we call it cavity ringdown laser absorption spectroscopy (CRLAS). In practice, pulsed laser light is injected into an optical cavity that is formed by a pair of highly reflective (R > 99.9%) mirrors. The small amount of light that is now trapped inside the cavity reflects back and forth between the two mirrors, with a small fraction (∼1 R) transmitting through each mirror with each pass. The resultant transmission of the circulating light is monitored at the output mirror as a function of time and allows the decay time of the cavity to be determined. A simple picture of the cavity decay event for the case where the laser pulse is temporally shorter than the cavity round trip transit time is presented in Figure 1. In this case, the intensity envelope of these discrete transmitted pulses exhibits a simple exponential decay. The time required for the cavity to decay to 1/e of the initial output pulse is called the “cavity ringdown” time. Determination of the ringdown time allows the absolute single pass transmission coefficient of the cavity to be determined with high accuracy, given the mirror spacing. The apparatus is converted to a sensitive absorption spectrometer simply by placing an absorbing medium between the two mirrors and recording the frequency dependent ringdown time of the cavity. Ideally, the ringdown time is a function of only the mirror reflectivities, cavity dimensions, and sample absorption. Absolute absorption intensities are obtained by subtracting the base-line transmission of the cavity, which is determined when the laser wavelength is off-resonance with all molecular transitions. † IBM Predoctoral Fellow. Current address: Sandia National Laboratories, M/S 9055, Livermore, CA 94551-0969. ‡ Los Gatos Research. 25 Chem. Rev. 1997, 97, 25−51

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Cavity Ringdown Laser Absorption Spectroscopy: History, Development, and Application to Pulsed Molecular Beams.

The gas phase optical spectrum of jet‐cooled Pt2 has been investigated over the range of 11 300 to 26 300 cm−1 using resonant two‐photon ionization spectroscopy in combination with time‐of‐flight mass spectrometry. Numerous vibronic bands are observed. Analysis of the data gives the location of some 26 excited electronic states, which are characterized by the frequencies of their origin bands, vibrational frequencies, and anharmonicities. Variation of the second color in a two‐color resonant two‐photon ionization scheme has determined the ionization threshold of Pt2 to be 8.68±0.02 eV. The observation of the onset of predissociation, characterized by a sharp drop in excited state lifetime, places the dissociation energy of Pt2 at 3.14±0.02 eV. In combination with the Pt atomic ionization potential of 8.8±0.2 eV, these results give the bond strength of Pt+2 as D0(Pt−Pt+)=3.26±0.24 eV. The strength of the chemical bond in Pt2, as compared to Au2, demonstrates that there are significant 5d contributions to the chemical bonding in Pt2.

Resonant two‐photon ionization spectroscopy of jet‐cooled Pt2

Diatomic nickel–copper, NiCu, has been investigated by resonant two‐photon ionization spectroscopy in a jet‐cooled molecular beam Six band systems have been identified over the range 11 500–16 500 cm−1 The ground state of NiCu has been determined to be X 2Δ5/2, with ω‘e =27301±115 cm−1, ω″ex‘e =100±038 cm−1, and r″e =2233±0006 A This state derives from a strongly bound (205±010 eV) 3d10Cu3d9Ni 4sσ2 configuration Excited states observed in this work derive from the more weakly bound 3d10Cu3d8Ni 4sσ24sσ*1 configuration, and are characterized by smaller vibrational frequencies (191–208 cm−1) and a longer bond length (2351±0005 A) than the ground X 2Δ5/2 state

Spectroscopy and electronic structure of jet-cooled GaAs

Gas phase spectroscopic investigations of the jet‐cooled aluminum trimer are reported using the technique of resonant two‐photon ionization with mass spectrometric detection. A discrete band system in the 5200–6100 A region is observed, consisting of an extended vibrational progression in a single vibrational mode. In addition, an apparent continuum absorption is observed which gradually grows in toward shorter wavelengths. The apparent continuum exhibits a long lifetime, 24–35 μs, which is most unusual and indicates that the continuum arises from spectral congestion and not lifetime broadening. At 19 378 cm−1 both the discrete and the continuum absorptions terminate abruptly, indicating the onset of dissociation above this energy. Although it is not certain that dissociation above this energy leads to ground electronic state Al2, this measurement nevertheless places an upper limit on D0(Al2–Al) of 2.40 eV.

Spectroscopic studies of the jet‐cooled aluminum trimer

The reactivity of Nb clusters with nitrogen and deuterium was studied using a pulsed laser vporization source coupled to a fast flow reactor. The relative reactivity of the various clusters was determined by monitoring the depletion of the cluster ion signal as a function of reactant flow rate. A kinetic analysis of the chemisorption is given. The geometrical isomerism of the cluster ions and its relationship to reactivity is discussed.(AIP)

Evidence of structural isomerism in small niobium clusters

New multireference, configuration‐interaction potential energy surfaces are reported for the interaction of Ar with the CH radical in its ground (X 2Π) and second excited (B 2Σ−) electronic states. These potential energy surfaces are then used in an adiabatic analysis of the rovibronic levels of the ArCH(X) and ArCH(B) van der Waals complexes. A qualitative discussion of the expected features in the B←X electronic spectrum of ArCH is presented, and these are compared with the experimental spectrum reported earlier by Lemire et al. [J. Chem. Phys. 99, 91 (1993)].

Potential energy surfaces for the interaction of CH(X 2Π,B 2Σ−) with Ar and an assignment of the stretch‐bend levels of the ArCH(B) van der Waals molecule

George W. Lemire

Papers

Resonant two‐photon ionization spectroscopy of jet‐cooled Pt2

Spectroscopy and electronic structure of jet-cooled GaAs

Spectroscopic studies of the jet‐cooled aluminum trimer

Evidence of structural isomerism in small niobium clusters

Potential energy surfaces for the interaction of CH(X 2Π,B 2Σ−) with Ar and an assignment of the stretch‐bend levels of the ArCH(B) van der Waals molecule