The Origin of the Catalytic Activity of a Metal Hydride in CO2 Reduction.

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoHx in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time-of-flight secondary ion mass spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites.

Dimensional and Structural Control of Silica Aerogel Membranes for Miniaturized Motionless Gas Pumps.

With growing public interest in portable electronics such as micro fuel cells, micro gas total analysis systems, and portable medical devices, the need for miniaturized air pumps with minimal electrical power consumption is on the rise. Thus, the development and downsizing of next-generation thermal transpiration gas pumps has been investigated intensively during the last decades. Such a system relies on a mesoporous membrane that generates a thermomolecular pressure gradient under the action of an applied temperature bias. However, the development of highly miniaturized active membrane materials with tailored porosity and optimized pumping performance remains a major challenge. Here we report a systematic study on the manufacturing of aerogel membranes using an optimized, minimal-shrinkage sol-gel process, leading to low thermal conductivity and high air conductance. This combination of properties results in superior performance for miniaturized thermomolecular air pump applications. The engineering of such aerogel membranes, which implies pore structure control and chemical surface modification, requires both chemical processing know-how and a detailed understanding of the influence of the material properties on the spatial flow rate density. Optimal pumping performance was found for devices with integrated membranes with a density of 0.062 g cm(-3) and an average pore size of 142.0 nm. Benchmarking of such low-density hydrophobic active aerogel membranes gave an air flow rate density of 3.85 sccm·cm(-2) at an operating temperature of 400 °C. Such a silica aerogel membrane based system has shown more than 50% higher pumping performance when compared to conventional transpiration pump membrane materials as well as the ability to withstand higher operating temperatures (up to 440 °C). This study highlights new perspectives for the development of miniaturized thermal transpiration air pumps while offering insights into the fundamentals of molecular pumping in three-dimensional open-mesoporous materials.

Synchrotron high energy X-ray methods coupled to phase sensitive analysis to characterize aging of solid catalysts with enhanced sensitivity.

X-ray absorption spectroscopy and X-ray diffraction are suitable probes of the chemical state of a catalyst under working conditions but are limited to bulk information. Here we show in two case studies related to hydrothermal aging and chemical modification of model automotive catalysts that enhanced detailed information of structural changes can be obtained when the two methods are combined with a concentration modulated excitation (cME) approach and phase sensitive detection (PSD). The catalysts are subject to a modulation experiment consisting of the periodic variation of the gas feed composition to the catalyst and the time-resolved data are additionally treated by PSD. In the case of a 2 wt% Rh/Al2O3 catalyst, a very small fraction (ca. 2%) of Rh remaining exposed at the alumina surface after hydrothermal aging at 1273 K can be detected by PSD. This Rh is sensitive to the red-ox oscillations of the experiment and is likely responsible for the observed catalytic activity and selectivity during NO reduction by CO. In the case of a 1.6 wt% Pd/Al2O3-Ce(1-x)Zr(x)O2 catalyst, preliminary results of cME-XRD demonstrate that access to the kinetics of the whole material at work can be obtained. Both the red-ox processes involving the oxygen storage support and the Pd component can be followed with great precision. PSD enables the differentiation between Pd deposited on Al2O3 or on Ce(1-x)Zr(x)O2. Modification of the catalyst by phosphorous clearly induces loss of the structural dynamics required for oxygen storage capacity that is provided by the Ce(4+)/Ce(3+) pair. The two case studies demonstrate that detailed kinetics of subtle changes can be uncovered by the combination of in situ X-ray absorption and high energy diffraction methods with PSD.

Perovskite-supported palladium for methane oxidation - structure-activity relationships.

Palladium is the precious metal of choice for methane oxidation and perovskite-type oxides offer the possibility to stabilize it as PdO, considered crucial for catalytic activity. Pd can adopt different oxidation and coordination states when associated with perovskite-type oxides. Here, we review our work on the effect of perovskite composition on the oxidation and coordination states of Pd and its influence on catalytic activity for methane oxidation in the case of typical Mn, Fe and Co perovskite-based oxidation catalysts. Especially X-ray absorption near edge structure (XANES) spectroscopy is shown to be crucial to fingerprint the different coordination states of Pd. Pd substitutes Fe and Co in the octahedral sites but without modifying catalytic activity with respect to the Pd-free perovskite. On LaMnO(3) palladium is predominantly exposed at the surface thus bestowing catalytic activity for methane oxidation. However, the occupancy of B-cation sites of the perovskite structure by Pd can be exploited to cyclically activate Pd and to protect it from particle growth. This is explicitly demonstrated for La(Fe, Pd)O(3), where catalytic activity for methane oxidation is enhanced under oscillating redox conditions at 500 °C, therefore paving the way to the practical application in three-way catalysts for stoichiometric natural gas engines.