What is heat? Can heat capacities be negative?

In the absence of work, the exchange of heat of a sample of matter corresponds to the change of its internal energy, given by the kinetic energy of random translational motion of all its constituent atoms or molecules relative to the center of mass of the sample, plus the excitation of quantum states, such as vibration and rotation, and the energy of electrons in excess to their ground state. If the sample of matter is equilibrated it is described by Boltzmann’s statistical thermodynamics and characterized by a temperature T. Monotonic motion such as that of the stars of an expanding universe is work against gravity and represents the exchange of kinetic and potential energy, as described by the virial theorem, but not an exchange of heat. Heat and work are two distinct properties of thermodynamic systems. Temperature is defined for the radiative cosmic background and for individual stars, but for the ensemble of moving stars neither temperature, nor pressure, nor heat capacities are properly defined, and the application of thermodynamics is, therefore, not advised. For equilibrated atomic nanoclusters, in contrast, one may talk about negative heat capacities when kinetic energy is transformed into potential energy of expanding bonds.https://www.mdpi.com/journal/entropyam2024ChemistryNon

Symmetry and electronic properties of metallic nanoclusters

Spherical nanoclusters with countable member atoms and delocalized valence orbitals are superatoms with properties analogous to those of simple atoms. This is reflected, in particular, in their optical spectra and magnetic properties, in a similar sense to transition metal ions and complexes. Clusters can be of low-spin or high-spin with considerable contributions to magnetism by the large cluster orbital magnetic moment. Due to the large radius of the clusters, they can be diamagnetic with an unusually high diamagnetic susceptibility. Gold and platinum, which in the bulk are non-magnetic, show pronounced superparamagnetism associated with their high-spin nature, and the magnetic moment can be trapped in symmetry-breaking environments so that hysteresis pertains far beyond room temperature. A significant deviation from hydrogen-like orbitals results from the shape of the confining potential, which has the effect that the orbital quantum number ` is not limited to values less than the principal quantum number n.https://www.mdpi.com/journal/symmetryam2024ChemistryNon

Charge polarization at catalytic metal-support junctions : Part B : Theoretical modeling of Kelvin probe force microscopy

Existing models for the analysis of Kelvin probe microscopy experiments are extended and used to analyze the experimental electrical potential profiles for a Pt/TiO2 model nanoparticle. The derived model reproduces in detail the Kelvin probe image that reveals a characteristic ring-shaped negative charge zone at the surface around the particle: A planar negative charge zone at the surface of the support extends beyond the diameter of the Pt particle. It is compensated mostly by a planar layer of positive charges in the metal across the interface, and by a smaller number of positive charges at the metal-air interface. These latter charges determine the positive electrical potential of the metal particle, and they are likely responsible for the extent of the metal-support interaction in catalytic reactions.http://pubs.acs.org/journal/jpcafh2017-06-30hb2016Chemistr

Magnetic properties and the superatom character of 13-atom platinum nanoclusters

13-atom platinum nanoclusters have been synthesized quantitatively in the pores of the zeolites NaY and KL. They reveal highly interesting magnetic properties like high-spin states, a blocking temperature, and super-diamagnetism, depending heavily on the loading of chemisorbed hydrogen. Additionally, EPR active states are observed. All of these magnetic properties are understood best if one considers the near-spherical clusters as analogs of transition metal atoms with low-spin and high-spin states, and with delocalized molecular orbitals which have a structure similar to that of atomic orbitals. These clusters are, therefore, called superatoms, and it is their analogy with normal atoms which is in the focus of the present work, but further phenomena, like the observation of a magnetic blocking temperature and the possibility of superconductivity, are discussed.http://www.mdpi.com/journal/magnetochemistryhb201

Platinum-hydrogen vibrations and low energy electronic excitations of 13-atom Pt nanoclusters

Please read abstract in the article.http://www.rsc.org/pccp2015-08-28hb201

Charge polarization at catalytic metal-support junctions : Part A : Kelvin probe force microscopy results of noble metal nanoparticles

Metal oxide-supported nanoparticles of the platinum group metals Pt, Rh and Pd were studied at ambient temperature and atmosphere using Kelvin probe force microscopy. In all cases, the results reveal electron transfer from the metal to the oxide support which decreases in the order TiO2 > CeO2 >> Al2O3, leading to charge polarization at the Schottky type interfaces analogous to that of a parallel plane capacitor. This polarization cancels out to a large extent for the Kelvin signal. On top of this there is a much smaller number of positive charges at the outer catalyst particle surface, compensated by negative charges near the oxide surface. They show the same trend over the different oxides. These charges determine the constant electrical potential of the metal and are suggested to be the important component of the electronic catalytic metal-support interaction which are known to be much stronger for reducible than for non-reducible oxides.http://pubs.acs.org/journal/jpcafh2017-06-30hb2016Chemistr

Preserving cultural diversity in rural Africa using renewable energy

Ninety percent of the large interior, rural part of Africa is not an abundant user of fossil fuels and is not connected to an electricity grid. This limits habitability and leads to significant migration to larger cities in attempts to improve economic and social welfare, which happens at the cost of its rich cultural diversity by inevitable adaption and mixing of societies. A direct transition from a firewood to an off-grid renewable electricity age can mitigate this detrimental development. This perspective discusses the interdisciplinary requirements linking cultural, sociological, economic, and technical aspects for a transition to modern life without loss of valuable traditions. Photovoltaic power and wind energy can provide local affordable electricity in off-grid locations. Intermediate storage for day–night cycles is catered for by novel types of batteries. Purifying and recycling water, refrigerating food and medicine, and benefitting from contact with the world via electronic media permit a tremendous increase in living conditions and significantly lower the pressure of migration into cities. Access to energy is a fundamental requirement for the preservation of the rich cultural diversity with family and tribal bindings, local languages, traditions, and religions, and allows for a more moderate transition to a modern society.Open access funding enabled and organized by Projekt DEAL.https://onlinelibrary.wiley.com/journal/20566646hj2024ChemistrySDG-07:Affordable and clean energ

In command of non-equilibrium

The Second Law of Thermodynamics is well known for determining the direction of spontaneous processes in laboratory, life and universe. It is therefore often called the arrow of time. Less often discussed but just as important is the effect of kinetic barriers which intercept equilibration and preserve highly ordered, high energy non-equilibrium states. Examples of such states are many modern materials produced intentionally for technological applications. Furthermore, all living organisms fuelled directly in photosynthesis and those fuelled indirectly by living on high energy nutrition represent preserved non-equilibrium states. The formation of these states represents the local reversal of the arrow of time which only seemingly violates the Second Law. It has been known since the seminal work of Prigogine1 that the stabilisation of these states inevitably requires the dissipation of energy in the form of waste heat. It is this feature of waste heat dissipation following the input of energy that drives all process occurring at non-zero rate. Photosynthesis, replication of living organisms, self-assembly, crystal shape engineering and distillation have this principle in common with the well-known Carnot cycle in the heat engine. Drawing on this analogy, we subsume these essential and often sophisticated driven processes under the term machinery of life.http://www.rsc.org/journals-books-databases/about-journals/chem-soc-rev2017-05-30hb2016Chemistr

Aerobic one-step oxidation of benzene to phenol on copper exchanged HZSM5 zeolites : a mechanistic study

Various Cu/HZSM5-zeolites were prepared and their catalytic properties were investigated by productanalysis via GC/MS chromatography in order to trace down the mechanism of the gas phase one-stepoxidation of benzene to phenol with molecular oxygen. Comparison of Cu free and Cu containing zeolitesshowed that the activation of O2takes place at copper centers of the zeolite and high copper loadings leadto high yields of deep oxidation products (CO, CO2). No phenol was formed in the absence of Brønsted acidsites, i.e. on Cu/KZSM5, revealing the bifunctionality of the Cu/HZSM5 zeolite. The yields of the variousoxidation products and thus the selectivity toward phenol can be influenced by variation of the relativeO2concentration in the reaction mixture, indicating the possibility of a stoichiometric use of O2. The roleof the superoxide radical ion O2•−as a reactive intermediate is discussed and a radical ionic reactionmechanism is suggested.http://www.elsevier.com/locate /molcatahb201

Isotope dependence and quantum effects on atomic hydrogen diffusion in liquid water

Relative diffusion coefficients were determined in water for the D, H, and Mu isotopes of atomic hydrogen by measuring their diffusion-limited spin-exchange rate constants with Ni2+ as a function of temperature. H and D atoms were generated by pulse radiolysis of water and measured by time-resolved pulsed EPR. Mu atoms are detected by muonium spin resonance. To isolate the atomic mass effect from solvent isotope effect, we measured all three spin-exchange rates in 90% D2O. The diffusion depends on the atomic mass, demonstrating breakdown of Stokes−Einstein behavior. The diffusion can be understood using a combination of water “cavity diffusion” and “hopping” mechanisms, as has been proposed in the literature. The H/D isotope effect agrees with previous modeling using ring polymer molecular dynamics. The “quantum swelling” effect on muonium due to its larger de Broglie wavelength does not seem to slow its “hopping” diffusion as much as predicted in previous work. Quantum effects of both the atom mass and the water librations have been modeled using RPMD and a qTIP4P/f quantized flexible water model. These results suggest that the muonium diffusion is very sensitive to the Mu versus water potential used.Division of Chemical Sciences,Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533.http://pubs.acs.org/journal/jpcafh2016-12-30hb201