Ca₁₁E₃C₈ (E = Sn, Pb): New Complex Carbide Zintl Phases Grown from Ca/Li Flux

New carbide Zintl phases Ca₁₁E₃C₈ (E = Sn, Pb) were grown from reactions of carbon and heavy tetrelides in Ca/Li flux. They form with a new structure type in space group <i>P</i>2₁/<i>c</i> (<i>a</i> = 13.1877(9)­Å, <i>b</i> = 10.6915(7)­Å, <i>c</i> = 14.2148(9)­Å, β = 105.649(1)°, and <i>R</i>₁ = 0.019 for the Ca₁₁Sn₃C₈ analog). The structure features isolated E^4– anions as well as acetylide (C₂^2–) and allenylide (C₃^4–) anions; the vibrational modes of the carbide anions are observed in the Raman spectrum. The charge-balanced nature of these phases is confirmed by DOS calculations which indicate that the tin analog has a small band gap (<i>E</i>_g < 0.1 eV) and the lead analog has a pseudogap at the Fermi level. Reactions of these compounds with water produce acetylene and allene

Ca₁₁E₃C₈ (E = Sn, Pb): New Complex Carbide Zintl Phases Grown from Ca/Li Flux

New carbide Zintl phases Ca₁₁E₃C₈ (E = Sn, Pb) were grown from reactions of carbon and heavy tetrelides in Ca/Li flux. They form with a new structure type in space group <i>P</i>2₁/<i>c</i> (<i>a</i> = 13.1877(9)­Å, <i>b</i> = 10.6915(7)­Å, <i>c</i> = 14.2148(9)­Å, β = 105.649(1)°, and <i>R</i>₁ = 0.019 for the Ca₁₁Sn₃C₈ analog). The structure features isolated E^4– anions as well as acetylide (C₂^2–) and allenylide (C₃^4–) anions; the vibrational modes of the carbide anions are observed in the Raman spectrum. The charge-balanced nature of these phases is confirmed by DOS calculations which indicate that the tin analog has a small band gap (<i>E</i>_g < 0.1 eV) and the lead analog has a pseudogap at the Fermi level. Reactions of these compounds with water produce acetylene and allene

Reaction of Methane with Bulk Intermetallics Containing Iron Clusters Yields Carbon Nanotubes

Reaction of Methane with Bulk Intermetallics Containing Iron Clusters Yields Carbon Nanotube

Microwave-Specific Enhancement of the Carbon–Carbon Dioxide (Boudouard) Reaction

The Boudouard reaction, which is the reaction of carbon and carbon dioxide to produce carbon monoxide, represents a simple and straightforward method for the remediation of carbon dioxide in the environment through reduction: CO₂(g) + C(s) ⇌ 2CO. However, due to the large positive enthalpy, typically reported to be 172 kJ/mol under standard conditions at 298 K, the equilibrium does not favor CO production until temperatures >700 °C, when the entropic term, −<i>T</i>Δ<i>S</i>, begins to dominate and the free energy becomes negative. We have found that, under microwave irradiation to selectively heat the carbon, dramatically different thermodynamics for the reaction are observed. During kinetic studies of the reaction under conditions of flowing CO₂, the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction dropped from 183.3 kJ/mol at ∼1100 K to 33.4 kJ/mol at the same temperature under microwave irradiation. This changes the position of the equilibrium so that the temperature at which CO becomes the major product drops from 643 °C in the conventional thermal reaction to 213 °C in the microwave. The observed reduction in the apparent enthalpy of the microwave driven reaction, compared to what is determined for the thermal reaction from standard heats of formation, can be thought of as arising from additional energy being put into the carbon by the microwaves, effectively increasing its apparent standard enthalpy. Mechanistically, it is hypothesized that the enhanced reactivity arises from the interaction of CO₂ with the steady-state concentration of electron–hole pairs that are present at the surface of the carbon due to the space-charge mechanism, by which microwaves are known to heat carbon. Such a mechanism is unique to microwave-induced heating and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield energy savings in driving the general class of gas–carbon reactions

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO₂) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/SiO₂ was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of Cr^VI first to Cr^IV, then to Cr^II. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO₂) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/SiO₂ was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of Cr^VI first to Cr^IV, then to Cr^II. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization