Gas phase contributions to the catalytic formation of HCN from CH₄and NH₃over Pt: An in situ study by molecular beam mass spectrometry with threshold ionization

Abstract

Molecular beam mass spectrometry has been used for an in situ study of the Pt-catalyzed formation of hydrocyanic acid from methane and ammonia. The goal was to identify transient gas phase intermediates which would indicate homogeneous contributions to the reaction mechanism. A catalytic wall reactor operated at 1300 1C, 1013 mbar, and 74% HCN yield was connected via a molecular beam interface with a quadrupole mass spectrometer, which allowed the measurement of ionization-and appearance potentials by electron impact. Shape and width of the electron energy spread function were determined by analyzing the ionization efficiency curve of helium; the experimental uncertainty of the measured threshold values was found to be 0.6 eV. By use of the threshold ionization technique it could be shown that methylamine (CH 3 NH 2 ) and methylenimine (CH 2Q NH) are present in the gas phase under reaction conditions. The measured threshold potentials at m/z ¼ 30 u (9.9 AE 0.6 eV) and m/z ¼ 29 u (10.6 AE 0.6 eV) were unambiguously assigned to the appearance potential of CNH 4 1 /CH 3 NH 2 and the ionization potential of CNH 3 1 /CH 2 NH, respectively. Both molecules dehydrogenate rapidly at reaction temperature to HCN so that they can be considered as true gas phase intermediates.

In situ generation of HCN for mass spectrometric studies

“…Subsequently, ion-exchange equilibria [8] and van't Hoff measurements [10] were used to determine anion-neutral binding energies of CN Ϫ and HCN containing clusters. Recent studies have utilized the reaction of CH 4 and NH 3 with Pt catalyst to generate HCN in a molecular beam apparatus [11].…”

“…Subsequently, ion-exchange equilibria [8] and van't Hoff measurements [10] were used to determine anion-neutral binding energies of CN Ϫ and HCN containing clusters. Recent studies have utilized the reaction of CH 4 and NH 3 with Pt catalyst to generate HCN in a molecular beam apparatus [11].In this work, we describe a simple in situ approach for generation of HCN for ion clustering studies. Hydrogen cyanide is formed by the neutral-neutral reaction of trimethylsilyl cyanide, Me 3 SiCN, with water in a flowing afterglow reactor.…”

New catalytic materials for the high-temperature synthesis of hydrocyanic acid from methane and ammonia by high-throughput approach

“…So far, it is assumed that the main reaction pathway of the HCN formation occurs via reaction of adsorbed CH x and NH x species [12,13]. Identified intermediates are CH 3 N 2 or CH 2 NH [12,14], which rapidly dehydrogenate to HCN at the applied reaction temperature. Additionally, the possibility of a C-N coupling reaction between atomic C ads and N ads species on Pt (1 1 1) surfaces was discussed [15].…”

C–N coupling in the gas-phase reactions of ammonia and [M(CH)]+ (M = Ni, Pd, Pt): a combined experimental/computational exercise

“…Mass-spectrometry-based experiments 3 have for example aided in the identification of CH 2 NH as a crucial intermediate in the Degussa process, 4 and its existence has been confirmed later by in situ photoionization experiments. 5 In the gas-phase model of the [Pt] + -mediated coupling of ammonia and methane, 4a,6 the generation of [Pt(CH 2 )] + from [Pt] + and methane constitutes the first step (eqn (1)).…”