ABSTRACT
State-resolved measurements on clean Ni(100) and Ni(111) surfaces quantify the reactivity of CH4 excited to v = 3 of the nu4 bend vibration. A comparison with prior data reveals that 3nu4 is significantly less effective than the nu3 C-H stretch at promoting dissociative chemisorption, even though 3nu4 contains 30% more energy. These results contradict statistical theories of gas-surface reactivity, provide clear evidence for vibrational mode specificity in a gas-surface reaction, and point to a central role for C-H stretching motion along the reaction path to dissociative chemisorption.
Subject(s)
Carbon/chemistry , Methane/chemistry , Models, Chemical , Nickel/chemistry , Carbon/analysis , Computer Simulation , Hydrogen Bonding , Methane/analysis , Molecular Conformation , Nickel/analysis , VibrationABSTRACT
State-resolved gas-surface reactivity measurements revealed that vibrational excitation of nu3 (the antisymmetric C-H stretch) activates methane dissociation more efficiently than does translational energy. Methane molecules in the vibrational ground state require 45 kilojoules per mole (kJ/mol) of translational energy to attain the same reactivity enhancement provided by 36 kJ/mol of nu3 excitation. This result contradicts a key assumption underlying statistical theories of gas-surface reactivity and provides direct experimental evidence of the central role that vibrational energy can play in activating gas-surface reactions.
ABSTRACT
The development, optimization, and application of a novel method for arsenic speciation based on capillary gas-liquid chromatography with simultaneous quadrupole ion-trap mass spectrometric (MS) detection and pulsed flame photometric detection (PFPD) is described. The method couples the sensitive arsenic-selectivity of PFPD with the structure elucidation capability of molecular MS detection for the determination of trace levels of unknown organoarsenicals in complex matrices. The conditions that affect the PFPD response in the presence of interfering species were optimized using the sequential Simplex algorithm for three key factors: gate delay (18.3 ms), gate width (9.1 ms), and combustion gas composition (16.6 ml/min H2). Complete discrimination in the PFPD of the arsenic signal from interfering S-, C-, and OH-emitting species that are problematic in existing methods was achieved. Additionally, a revised interpretation of our previously reported mechanism [J. Chromatogr. A 807 (1998) 253] for the dithiol derivatization and subsequent GC-MS determination of dimethylarsinic acid is presented.
