Scanning mass spectrometer for quantitative reaction studies on catalytically active microstructures

Abstract

We describe an apparatus for spatially resolving scanning mass spectrometry which is able to measure the gas composition above catalytically active microstructures or arrays of these microstructures with a lateral resolution of better than 100 mum under reaction conditions and which allows us to quantitatively determine reaction rates on individual microstructures. Measurements of the three-dimensional gas composition at different vertical distances and separations between active structures allow the evaluation of gas phase mass transport effects. The system is based on a piezoelectrically driven positioning substage for controlled lateral and vertical positioning of the sample under a rigidly mounted capillary probe connecting to a mass spectrometer. Measurements can be performed at pressures in the range of <10(-2)-10 mbars and temperatures between room temperature and 450 degrees C. The performance of the setup is demonstrated using the CO oxidation reaction on Pt microstructures on Si with sizes between 100 and 300 mum and distances in the same order of magnitude, evaluating CO(2) formation and CO consumption above the microstructures. The rapidly decaying lateral resolution with increasing distance between sample and probe underlines the effects of (lateral) gas transport in the room between sample and probe. The reaction rates and apparent activation energy obtained from such measurements agree with previous data on extended surfaces, demonstrating the feasibility of determining absolute reaction rates on individual microstructures.

Nanostructured, mesoporous Au/TiO2 model catalysts – structure, stability and catalytic properties

“…The catalytic measurements were performed in a scanning mass spectrometer (SMS) system with a dedicated reaction chamber for reactions at pressures up to several mbar, and a separate second chamber containing a differentially pumped mass spectrometer (for details see [28–29]). The Au/TiO 2 samples were mounted on a heatable sample stage in the reaction chamber.…”

“…The flow restrictor ends in a flat Ti cap (cylindrical volume with inner diameter of 2.5 mm and height of 0.1 mm) to collect the gas species above a defined sample area. In this way, a much larger surface area of the underlying sample contributes to the measured signal than in the previous set-up [28], where the end of the capillary transformed into a tip with a constricted channel (inner diameter: 70 µm, outer diameter: 300 µm). The gas species are then guided towards the analysis chamber and, after leaving the quartz tube, directly into the ion source of a quadrupole mass spectrometer (QMS).…”

A convenient setup for laser-induced fluorescence imaging of both CO and CO2 during catalytic CO oxidation

“…(2)(3)(4)(5), where S LIF is the fluorescent intensity, η c is the collection efficiency, E is the laser energy, g is a function that describes the spectral overlap between the laser and the absorption spectral lineshape, f(T) is the Boltzmann distribution, σ 0 is the absorption cross section, N is the number density of the absorber, ϕ is the fluorescence quantum yield, Q i is the quenching rate, q i is the quenching rate coefficient, p i is the partial pressure for species i, σ i is the quenching cross section, and Eq. (3) assumes that the spontaneous emission rate is far less than the quenching rate.…”

“…These techniques have the benefit of measuring several species simultaneously, but they suffer from a time delay or poor temporal resolution, and are not capable to spatially resolve the gas composition around a sample. Although capillary sampling techniques can provide spatially resolved concentration profiles inside reactors [5], it cannot deliver two-dimensional measurements to follow dynamic changes in the gas phase on a sub-second scale, and the intrusive nature of the probe may introduce errors in data interpretation.As an in-situ and non-invasive gas detection technique with high spatial and temporal resolution, planar laser-induced fluorescence (PLIF) has been widely used in the combustion community for flame studies [6][7][8], but much less applied in the catalyst community [9]. In earlier studies during the 1990s, LIF has been used to study the OH formation close to a Pt catalyst during the H 2 oxidation [10][11][12][13][14], the distribution of OH desorbed from a Pt catalyst during catalytic water formation reaction [15], and the formaldehyde distribution above a platinum plate during catalytic combustion of methanol/air mixtures [16].…”

Product gas evolution above planar microstructured model catalysts—A combined scanning mass spectrometry, Monte Carlo, and Computational Fluid Dynamics study

“…21 The scanning mass spectrometer setup is largely identical to one described recently. 22 In contrast with other open designs, the probing capillary in our setup has only a sensing function and is not part of the reactor as, e.g., in the system presented by Johansson et al, 11,23 where the reactant gases are provided via a thin tube surrounding the probing capillary.…”