Plastic
waste is currently generated at a rate approaching 400
Mt year–1. The amount of plastics accumulating in
the environment is growing rapidly, yet our understanding of its persistence
is very limited. This Perspective summarizes the existing literature
on environmental degradation rates and pathways for the major types
of thermoplastic polymers. A metric to harmonize disparate types of
measurements, the specific surface degradation rate (SSDR), is implemented
and used to extrapolate half-lives. SSDR values cover a very wide
range, with some of the variability arising due to degradation studies
conducted in different natural environments. SSDRs for high density
polyethylene (HDPE) in the marine environment range from practically
0 to approximately 11 μm year–1. This approach
yields a number of interesting insights. Using a mean SSDR for HDPE
in the marine environment, linear extrapolation leads to estimated
half-lives ranging from 58 years (bottles) to 1200 years (pipes).
For example, SSDRs for HDPE and polylactic acid (PLA) are surprisingly
similar in the marine environment, although PLA degrades approximately
20 times faster than HDPE on land. Our study highlights the need for
better experimental studies under well-defined reaction conditions,
standardized reporting of rates, and methods to simulate polymer degradation
using.
Our
civilization relies on synthetic polymers for all aspects of
modern life; yet, inefficient recycling and extremely slow environmental
degradation of plastics are causing increasing concern about their
widespread use. After a single use, many of these materials are currently
treated as waste, underutilizing their inherent chemical and energy
value. In this study, energy-rich polyethylene (PE) macromolecules
are catalytically transformed into value-added products by hydrogenolysis
using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by atomic layer deposition. Pt/SrTiO3 completely converts PE (Mn =
8000–158,000 Da) or a single-use plastic bag (Mn = 31,000 Da) into high-quality liquid products, such
as lubricants and waxes, characterized by a narrow distribution of
oligomeric chains, at 170 psi H2 and 300 °C under
solvent-free conditions for reaction durations up to 96 h. The binding
of PE onto the catalyst surface contributes to the number averaged
molecular weight (Mn) and the narrow polydispersity
(Đ) of the final liquid product. Solid-state
nuclear magnetic resonance of 13C-enriched PE adsorption
studies and density functional theory computations suggest that PE
adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concentrations of undercoordinated
Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons.
The current scale of plastics production and the accompanying waste disposal problems represent a largely untapped opportunity for chemical upcycling. Tandem catalytic conversion by platinum supported on γ-alumina converts various polyethylene grades in high yields (up to 80 weight percent) to low-molecular-weight liquid/wax products, in the absence of added solvent or molecular hydrogen, with little production of light gases. The major components are valuable long-chain alkylaromatics and alkylnaphthenes (average ~C30, dispersity Ð = 1.1). Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280°C. This approach demonstrates how waste polyolefins can be a viable feedstock for the generation of molecular hydrocarbon products.
Atomically precise copper nanoclusters (NCs) are of immense interest for a variety of applications, but have remained elusive. Herein we report the isolation of a copper NC, [Cu 25 H 22 (PPh 3 ) 12 ]Cl (1), from the reaction of Cu(OAc) and CuCl with Ph 2 SiH 2 , in the presence of PPh 3 . Complex 1 has been fully characterized, including analysis by X-ray crystallography, XANES, and XPS. In the solid state, complex 1 is constructed around a Cu 13 centered-icosahedron and formally features partial Cu(0) character. XANES of 1 reveals a Cu K-edge at 8979.6 eV, intermediate between the edge energies of Cu (0) and Cu(I), confirming our oxidation state assignment. This assignment is further corroborated by determination of the Auger parameter for 1, which also falls between those recorded for Cu(0) and Cu(I).
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This
Perspective illustrates how the presence of internal and external
electric fields can affect catalytic activity and selectivity, with
a focus on heterogeneous catalysts. Specifically, experimental investigations
of the electric field influence on catalyst selectivity in pulsed
field mass desorption microscopes, scanning tunneling microscopes,
probe–bed–probe reactors, continuous-circuit reactors,
and capacitor reactors are described. Through these examples, we show
how the electric field, whether externally applied or intrinsically
present, can affect the behavior of a wide number of materials relevant
to catalysis. We review some of the theoretical methods that have
been used to elucidate the influence of external electric fields on
catalytic reactions, as well as the application of such methods to
selective methane activation. In doing so, we illustrate the breadth
of possibilities in field-assisted catalysis.
Benchmarking is a community-based and (preferably) community-driven activity involving consensusbased decisions on how to make reproducible, fair, and relevant assessments. In catalysis science, important catalyst performance metrics include activity, selectivity, and the deactivation profile, which enable comparisons between new and standard catalysts. Benchmarking also requires careful documentation, archiving, and sharing of methods and measurements, to ensure that the full value of research data can be realized. Beyond these goals, benchmarking presents unique opportunities to advance and accelerate understanding of complex reaction systems by combining and comparing experimental information from multiple, in situ and operando techniques with theoretical insights derived from calculations characterizing model systems. This Perspective describes the origins and uses of benchmarking and its applications in computational catalysis, heterogeneous catalysis, molecular catalysis, and electrocatalysis. It also discusses opportunities and challenges for future developments in these fields.
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