Near-Surface Imaging of the Multicomponent Gas Phase above a Silver Catalyst during Partial Oxidation of Methanol

Abstract

Fundamental chemistry in heterogeneous catalysis is increasingly explored using operando techniques in order to address the pressure gap between ultrahigh vacuum studies and practical operating pressures. Because most operando experiments focus on the surface and surface-bound species, there is a knowledge gap of the near-surface gas phase and the fundamental information the properties of this region convey about catalytic mechanisms. We demonstrate in situ visualization and measurement of gas-phase species and temperature distributions in operando catalysis experiments using complementary near-surface optical and mass spectrometry techniques. The partial oxidation of methanol over a silver catalyst demonstrates the value of these diagnostic techniques at 600 Torr (800 mbar) pressure and temperatures from 150 to 410 °C. Planar laser-induced fluorescence provides two-dimensional images of the formaldehyde product distribution that show the development of the boundary layer above the catalyst under different flow conditions. Raman scattering imaging provides measurements of a wide range of major species, such as methanol, oxygen, nitrogen, formaldehyde, and water vapor. Near-surface molecular beam mass spectrometry enables simultaneous detection of all species using a gas sampling probe. Detection of gas-phase free radicals, such as CH3 and CH3O, and of minor products, such as acetaldehyde, dimethyl ether, and methyl formate, provides insights into catalytic mechanisms of the partial oxidation of methanol. The combination of these techniques provides a detailed picture of the coupling between the gas phase and surface in heterogeneous catalysis and enables parametric studies under different operating conditions, which will enhance our ability to constrain microkinetic models of heterogeneous catalysis.

Roadmap and Direction toward High-Performance MoS2 Hydrogen Evolution Catalysts

“…176 Furthermore, characterization should also focus on the nearsurface as well as the interface and trace the interplay between the bulk, near-surface, and interface. 177 Interfacial reactant and product diffusion and interfacial electric double-layer structures are not easy to track, and their influence on the catalytic performance was not known.…”

Near-Surface Gas-Phase Methoxymethanol Is Generated by Methanol Oxidation over Pd-Based Catalysts

“…Speciation in the Near-Surface Gas Phase during Oxidative Methanol Conversion . The details of the near-surface MBMS system employed here have been described in a recent publication . Briefly, through use of a small-aperture differentially pumped sampling nozzle, MBMS effectively “freezes” the local composition in the gas phase by rapid gas expansion, which transports near-surface species to the mass analyzer (a high-resolution reflectron time-of-flight mass spectrometer (RTOF-MS)).…”

Near-Surface Gas-Phase Methoxymethanol Is Generated by Methanol Oxidation over Pd-Based Catalysts

“…A schematic representation and details of the MBMS system used for near-surface species detection have been provided in previous publications. 1,2 Briefly, the instrument consists of a reactor chamber that is connected to a high-resolution reflectron time-of-flight mass spectrometer (RTOF-MS) via a quartz sampling probe. A molecular beam is obtained by a two-stage pumped vacuum system providing a sudden pressure drop from near-atmospheric pressure in the reactor to high vacuum conditions (10 −6 Torr) in the ionization chamber.…”

“…Conditions were described in a recent publication. 2 Catalysts were secured to boron nitride electric heaters in the reactor chamber. A thermocouple was attached to the surface of the film to record temperature.…”

“…Representative mass spectra between m/z = 55-65 recorded in the near-surface region above other catalysts considered in this report, in conditions similar to those presented in the main text. Ag is included for reference and is associated our previously published results 2.…”