Tailored femtosecond laser pulses from a computer-controlled pulse shaper were used to optimize the branching ratios of different organometallic photodissociation reaction channels. The optimization procedure is based on the feedback from reaction product quantities in a learning evolutionary algorithm that iteratively improves the phase of the applied femtosecond laser pulse. In the case of CpFe(CO)2Cl, it is shown that two different bond-cleaving reactions can be selected, resulting in chemically different products. At least in this case, the method works automatically and finds optimal solutions without previous knowledge of the molecular system and the experimental environment.
Catalysis by single isolated atoms of precious metals has attracted much recent interest, as it promises the ultimate in atom efficiency. Most previous reports are on reducible oxide supports. Here we show that isolated palladium atoms can be catalytically active on industrially relevant g-alumina supports. The addition of lanthanum oxide to the alumina, long known for its ability to improve alumina stability, is found to also help in the stabilization of isolated palladium atoms. Aberration-corrected scanning transmission electron microscopy and operando X-ray absorption spectroscopy confirm the presence of intermingled palladium and lanthanum on the g-alumina surface. Carbon monoxide oxidation reactivity measurements show onset of catalytic activity at 40°C. The catalyst activity can be regenerated by oxidation at 700°C in air. The high-temperature stability and regenerability of these ionic palladium species make this catalyst system of potential interest for low-temperature exhaust treatment catalysts.
[1] Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of subduction zone thermal models is the viscosity of the mantle wedge. The introduction of an olivine rheology, with appropriate stress and temperature dependence, focuses flow into the tip of the mantle wedge. This leads to a temperature increase of a few hundred degrees in the wedge and the top of the slab as compared to the isoviscous case. Sensitivity tests show that this conclusion is robust under a variety of subduction zone parameters. The new high-resolution finite element models are used to reevaluate the thermal structure of the Honshu and Cascadia subduction zones using the more realistic olivine rheology. For Honshu, the model predicts slabmantle interface temperatures of $800°C beneath the volcanic front. Deeper parts of the subducting oceanic crust and upper mantle remain relative cool (e.g., the subducted Moho beneath the volcanic front is $400°C) because of the rapid subduction of old Pacific lithosphere. High interface temperatures are consistent with Th and Be data, indicating sediment melting, whereas cooler crustal temperatures are consistent with boron evidence, indicating lower temperature dehydration. The strong temperature gradient at the top of the subducting plate may thus reconcile conflicting estimates of slab temperature in subduction zones that are characterized by rapid subduction of mature oceanic lithosphere. Low temperatures persist to great depth in the shallow upper mantle of the subducting plate, which allows for water transport in the form of hydrous minerals to the deep mantle. The thermal structure of the Cascadia subduction zone is markedly different because of the age of the incoming lithosphere and low subduction speed. In this case, shallow dehydration and melting of the subducting crust and lithosphere is predicted.Components: 9085 words, 13 figures.
First-principles DFT computations are performed to explain the origin and the mechanism of oxygen reduction reaction (ORR) on Co-N(x) (x = 2, 4) based self-assembled carbon supported electrocatalysts in alkaline and acidic media. The results show that the formation of graphitic Co-N(4) defect is energetically more favorable than the formation of graphitic Co-N(2) defect. Furthermore graphitic Co-N(4) defects are predicted to be stable at all potentials (U = 0-1.23 V) in the present study while Co-N(2) defects are predicted to be unstable at high potentials. Therefore the Co-N(4) defect is predicted to be the dominant in-plane graphitic defect in Co-N(x)/C electrocatalysts. O(2) chemisorbs to Co-N(4) and Co-N(2) defects indicating that both defect motifs are active for the reduction of O(2) to peroxide. However, the weak interaction between peroxide and Co-N(4) defect shows that this defect does not promote complete ORR and a second site for the reduction of peroxide is required, supporting a 2 × 2e(-) dual site ORR mechanism independent of pH of the electrolyte. In contrast, the much stronger interaction between peroxide and Co-N(2) defect supports a 2 × 2e(-) single site ORR mechanism in alkaline and acidic media.
Our results validate the use of independent DFT predicted BE shifts for defect identification and constraining ambient pressure XPS observations for Me-Nx moieties in pyrolyzed carbon based ORR electrocatalysts. This supports the understanding of such catalysts as vacancy-and-substitution defects in a graphene-like matrix.
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