Portable device for generation of ultra-pure water vapor feeds.

A portable device for the generation of co-feeds of water vapor has been designed, constructed, and evaluated for flexible use as an add-on component to laboratory chemical reactors. The vapor is formed by catalytic oxidation of hydrogen, which benefits the formation of well-controlled minute concentrations of ultra-pure water. Analysis of the effluent stream by on-line mass spectrometry and Fourier transform infrared spectroscopy confirms that water vapor can be, with high precision, generated both rapidly and steadily over extended periods in the range of 100 ppm to 3 vol. % (limited by safety considerations) using a total flow of 100 to 1500 ml/min at normal temperature and pressure. Further, the device has been used complementary to a commercial water evaporator and mixing system to span water concentrations up to 12 vol. %. Finally, an operando diffuse reflective infrared Fourier transform spectroscopic measurement of palladium catalysed methane oxidation in the absence and presence of up to 1.0 vol. % water has been carried out to demonstrate the applicability of the device for co-feeding well-controlled low concentrations of water vapor to a common type of spectroscopic experiment. The possibilities of creating isotopically labeled water vapor as well as using tracer gases for dynamic experiments are discussed.

High-energy surface X-ray diffraction for fast surface structure determination.

Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields, such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction-based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a palladium surface in situ during catalytic carbon monoxide oxidation and follow dynamic restructuring of the surface with subsecond time resolution.

The Active Phase of Palladium during Methane Oxidation.

The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

Simulation of methane oxidation on Pt.

The authors present a generic model of CH4 oxidation on Pt with the emphasis on the role of surface-oxide formation. The latter process is treated in terms of the theory of first-order phase transitions. The corresponding Monte Carlo simulations indicate that the surface-oxide formation may result in stepwise features in the reaction kinetics. Specifically, with increasing CH4 pressure and/or decreasing O2 pressure, the model predicts a sharp transition from a low-reactive state with the surface completely covered by oxide to a high-reactive state with the surface covered by chemisorbed oxygen. In the former case, the reaction is first order in CH4 and zero order in O2. In the latter case, both reaction orders are positive. All these findings help in interpreting available experiments.