Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis. Their distinct catalytic properties are often hypothesized to result from the large fraction of under-coordinated surface atoms. Here, we show that size-preselected Pt(8-10) clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over by-products. Quantum chemical calculations indicate that under-coordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes.
Dimethyl carbonate (DMC) is considered an option for meeting the
oxygenate specifications
on gasoline and as a means of converting natural gas to a liquid
transportation fuel. In this
report, the fuel characteristics and known chemical synthesis schemes
for DMC are reviewed.
Three production schemes have a commercial track record, while
others are still under
development. The older of the three commercially proven schemes is
undesirable because it
employs phosgene. The other two commercially proven schemes have a
complex mixture of
advantages and disadvantages with regard to the synthesis chemistry and
are reviewed in greater
detail. One other commercially viable production scheme that
involves coproduction of either
ethylene or propylene glycol is also reviewed. This scheme is
still in the development stage and
would require a commitment to coproduce the glycol from ethylene or
propylene. The authors
are not aware of any refiner that either has blended or is blending DMC
into gasoline for
commercial use.
The chemistry of vanadium has seen
remarkable activity in the past
50 years. In the present review, reactions catalyzed by homogeneous
and supported vanadium complexes from 2008 to 2018 are summarized
and discussed. Particular attention is given to mechanistic and kinetics
studies of vanadium-catalyzed reactions including oxidations of alkanes,
alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species,
as well as oxidative C–C and C–O bond cleavage, carbon–carbon
bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation,
dehydrogenation, ring-opening metathesis polymerization, and oxo/imido
heterometathesis. Additionally, insights into heterogeneous vanadium
catalysis are provided when parallels can be drawn from the homogeneous
literature.
Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2 O3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.
Platinum atomic layer deposition (ALD) using MeCpPtMe 3 was employed to prepare high loadings of uniformsized, 1-2 nm Pt nanoparticles on high surface area Al 2 O 3 , TiO 2 , and SrTiO 3 supports. X-ray absorption fine structure was utilized to monitor the changes in the Pt species during each step of the synthesis. The temperature, precursor exposure time, treatment gas, and number of ALD cycles were found to affect the Pt particle size and density. Lower-temperature MeCpPtMe 3 adsorption yielded smaller particles due to reduced thermal decomposition. A 300°C air treatment of the adsorbed MeCpPtMe 3 leads to PtO. In subsequent ALD cycles, the MeCpPtMe 3 reduces the PtO to metallic Pt in the ratio of one precursor molecule per PtO. A 200°C H 2 treatment of the adsorbed MeCpPtMe 3 leads to the formation of 1-2 nm, metallic Pt nanoparticles. During subsequent ALD cycles, MeCpPtMe 3 adsorbs on the support, which, upon reduction, yields additional Pt nanoparticles with a minimal increase in size of the previously formed nanoparticles. The catalysts produced by ALD had identical water-gas shift reaction rates and reaction kinetics to those of Pt catalysts prepared by standard solution methods. ALD synthesis of catalytic nanoparticles is an attractive method for preparing novel model and practical catalysts.
Pd–Ag
alloy catalysts with very dilute amounts of Pd were
synthesized. EXAFS results demonstrated that when the concentration
of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated
single atoms in Ag nanoparticles. The activity for the hydrogenation
of acrolein was improved by the presence of these isolated Pd atoms
due to the creation of sites with lower activation energy for H2 dissociation. In addition, for the same particle size, the
0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than
their monometallic counterparts, suggesting that the Pd atom may act
as a site for the favorable bonding of the acrolein molecule for facile
hydrogenation of the aldehyde functionality.
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