The photodissociation of N,N-dimethylnitrosamine at 355 nm: The effect of excited-state conformational changes on product vector correlations.

In a photodissociation experiment, the dynamics associated with creating reaction products with specific energies can be understood by a study of the product vector correlations. Upon excitation to the S1 state, N,N-dimethylnitrosamine (DMN) undergoes an excited-state geometry change from planar to pyramidal around the central N. The significant geometry change affects the vector correlations in the photoproducts. Using polarized lasers for 355 nm photodissociation of DMN and for NO photoproduct excitation in a velocity-mapped ion imaging apparatus reveals new vector correlation details among the parent transition dipole (µ), photofragment velocity (v), and photofragment angular momentum (j). The dissociation of DMN displays some µ-v correlation [ß02(20)=-0.2], little µ-j correlation [ß02(02)∼0], and, surprisingly, a v-j [ß00(22)] correlation that depends on the NO lambda doublet probed. The results point to the importance of the initial excited-state conformational change and uncover the presence of two photolysis channels.

Nickel-silver alloy electrocatalysts for hydrogen evolution and oxidation in an alkaline electrolyte.

The development of improved catalysts for the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in basic electrolytes remains a major technical obstacle to improved fuel cells, water electrolyzers, and other devices for electrochemical energy storage and conversion. Based on the free energy of adsorbed hydrogen intermediates, theory predicts that alloys of nickel and silver are active for these reactions. In this work, we synthesize binary nickel-silver bulk alloys across a range of compositions and show that nickel-silver alloys are indeed more active than pure nickel for hydrogen evolution and, possibly, hydrogen oxidation. To overcome the mutual insolubility of silver and nickel, we employ electron-beam physical vapor codeposition, a low-temperature synthetic route to metastable alloys. This method also produces flat and uniform films that facilitate the measurement of intrinsic catalytic activity with minimal variations in the surface area, ohmic contact, and pore transport. Rotating-disk-electrode measurements demonstrate that the hydrogen evolution activity per geometric area of the most active catalyst in this study, Ni0.75Ag0.25, is approximately twice that of pure nickel and has comparable stability and hydrogen oxidation activity. Our experimental results are supported by density functional theory calculations, which show that bulk alloying of Ni and Ag creates a variety of adsorption sites, some of which have near-optimal hydrogen binding energy.