Götz Veser

Experimental and theoretical investigation of H2 oxidation in a high-temperature catalytic microreactor

Abstract A sample and flexible quartz-glass-based microreactor design is presented for high-temperature catalytic gas-phase reactions. The reactor was tested with the platinum-catalysed hydrogen oxidation reaction, withstanding extremely high reaction temperatures in excess of 1000°C without any signs of degradation. Experimental results are compared to those from a previous, alternative microreactor configuration, indicating substantially reduced heat losses. No homogeneous flames or explosions are observed under any reaction conditions, indicating that homogeneous reactions can be very effectively suppressed in a microreaction channel. A theoretical analysis of the explosion limits in the homogeneous H 2 /O 2 -system confirms that reactors with characteristic dimensions in the sub-millimetre range become intrinsically safe at ambient pressure conditions. Furthermore, the analysis shows that the suppression of the explosive reaction behaviour in these microreactors can be traced to a kinetic quenching of the radical chain mechanism rather than a thermal quenching due to increased heat transfer rates.

Exceptional high-temperature stability through distillation-like self-stabilization in bimetallic nanoparticles

Metal nanoparticles with precisely controlled size and composition are highly attractive for heterogeneous catalysis. However, their poor thermal stability remains a major hurdle on the way towards application at realistic technical conditions. Recent progress in this area has focused on nanostructured oxides to stabilize embedded metal nanoparticles. Here, we report an alternative approach that relies on synthesizing bimetallic nanoparticles with precise compositional control to obtain improved high-temperature stability. We find that PtRh nanoparticles with sufficiently high Rh content survive extended calcination at temperatures up to approximately 850 degrees C without significant sintering. For lower Rh content, sacrificial self-stabilization of individual nanoparticles through a distillation-like process is observed: the low-melting-point metal (Pt) bleeds out and the increasing concentration of the high-melting-point metal (Rh) leads to re-stabilization of the remaining nanoparticle. This principle of thermal self-stabilization should be broadly applicable to the development of multi-metallic nanomaterials for a broad range of high-temperature applications.

Ignition in alkane oxidation on noble-metal catalysts

Abstract The ignition behavior in the oxidation of four simple alkanes (methane, ethane, propane and isobutane) with air on a platinum-foil catalyst, as well as that of ethane/air mixtures on four noble-metal foil catalysts (Pt, Pd, Rh, and Ir) was studied at atmospheric pressure over the entire range of fuel-to-air ratios. While, Pd showed the widest range of surface flammability, ignition temperatures for ethane/air mixtures were lowest on Pt. Both, Rh and Ir deactivated rapidly under fuel-lean conditions and ignited considerably higher than Pd and Pt. The surface ignition temperatures were found to correlate well with the C–H bond energy of the hydrocarbon and the metal-oxygen bond energy of the noble metal. A very simple analytical model was able to reproduce the dependence of surface ignition temperatures on fuel-to-air ratios, yielding apparent activation energies for the surface reactions and indicating an oxygen-covered surface before catalytic ignition due to strong site competition between the hydrocarbon and oxygen on the catalyst surface.

Modelling steady state and ignition during catalytic methane oxidation in a monolith reactor

A one-dimensional two-phase reactor model for the oxidation of methane to synthesis gas over platinum in a monolith reactor is presented. The model incorporates a detailed elementary step reaction mechanism for methane oxidation, which is verified against experimental data. A good quantitative agreement with steady-state experiments and qualitative agreement with ignition experiments is achieved. The importance of individual reaction steps, homogeneous side reactions, and main reactor parameters are investigated. Essential steps in the catalytic reaction mechanism as well as crucial reactor parameters are identified. It is shown that the reaction system is strongly dominated by competition for oxygen on the catalyst surface. Conclusions about optimal reactor configurations are discussed.

Regular and irregular spatial patterns in the catalytic reduction of NO with NH3 on Pt(100)

The catalytic reduction of NO with NH3 on a Pt(100) surface, which exhibits kinetic oscillations under isothermal conditions in the 10−6 mbar pressure range, has been studied by photoemission electron microscopy (PEEM) as a spatially resolved technique. Oscillations in the rate of product formation for N2 and H2O are observed between 425 and 450 K. During the rate oscillations, the surface reacts predominantly spatially uniformly. Towards the lowerT-boundary for oscillations, however, fluctuating adsorbate islands (diameter ≈ 10–50 μm) appear and one observes target patterns and rotating spirals. Below the lowerT-boundary for oscillations, the reaction rate is stationary, but with PEEM one observes a spatially chaotic pattern in which the surface is still oscillating locally. The transition from macroscopic rate oscillations to unsynchronized oscillatory behavior can be associated with the breakdown of long range synchronization via gas phase coupling. In the spatial patterns imaged by PEEM, one can clearly identify three distinct grey levels which undergo a cyclic transformation into each other via propagating reaction fronts. One can assign different mechanistic steps to these transformations, namely the lifting of the hex reconstruction through NO adsorption and the dissociation of NO on the 1x1 phase, decomposition of NH3 on the 1x1 Oad/NOadd phase, and the restoration of the hex surface.

Chemical looping beyond combustion: production of synthesis gas via chemical looping partial oxidation of methane

The recent surge in natural gas reserves has revived interest in the development of novel processes to convert natural gas into valuable chemical feedstocks. In the present work, we are applying “chemical looping”, a technology that has found much attention as a clean combustion technology, towards selective partial oxidation of methane to produce synthesis gas (CLPOM). By tailoring the composition of NixFe1−x–CeO2 oxygen carriers and carefully controlling the supply of oxygen, i.e., the extent of the carrier reduction and oxidation in redox cycles, the reactivity and selectivity of these carriers for partial oxidation was optimized. Addition of a small amount of Ni to iron oxides allowed the combination of the high reactivity of Ni for methane activation with the good syngas selectivity of iron oxides. An optimized carrier with the composition of Ni0.12Fe0.88–CeO2 demonstrated excellent stability in multi-cycle CLPOM operation and high syngas yields with a H2:CO ratio of ∼2 and minimal carbon formation. Finally, a simplified fixed-bed reactor model was used to assess the thermal aspects of operating the process in a periodically operated fixed-bed reactor. We found that the process is highly sensitive to the degree of carrier utilization, but that maximum temperatures can be easily controlled in CLPOM via control of the active metal content and oxygen utilization in the carriers. Overall, chemical looping partial oxidation of methane emerges as an attractive alternative to conventional catalytic partial oxidation, enabling the use of low-cost transition metal oxides and air as oxidant, and resulting in inherently safe reactor operation by avoiding mixed methane/air streams.

Spatial pattern formation in the oscillatory NO+CO reaction on a Pt(100) surface and its vicinal orientations

Spatial pattern formation in the NO+CO reaction on a cylindrical Pt single crystal surface (axis parallel [001] direction) has been investigated using photoemission electron microscopy (PEEM) as an in‐situ method to image the lateral adsorbate distribution during the reaction with a resolution of ≊1 μm. The experiments were conducted in the 10−6 Torr range, between 380 and 430 K, under conditions where the (100) orientation exhibits oscillatory behavior. Of the different orientations of the [001] zone which are present on the surface only the orientational range between (100) and (310) was found to be very active in NO dissociation and hence in the surface reaction. A sharp phase boundary meandering between (210) and (410) parallel to the [001] direction separates the active from the inactive zone on the cylinder surface. In the active zone between (100) and (310) one finds propagating reaction fronts and complex spatiotemporal patterns. The velocity of the reaction fronts is strongly anisotropic with the...

Syngas formation by direct oxidation of methane: Reaction mechanisms and new reactor concepts

The reaction mechanism of direct catalytic oxidation of methane to syngas over a platinum catalyst under high temperature, short contact time conditions was studied with a detailed reactor and reaction model. Based on a detailed analysis of this mechanism, new integrated reactor concepts were deduced. Two concepts were studied in detail: a fixed bed reactor with integrated recuperative heat exchange, and a catalytic membrane reactor with distributed reactant feed. The reactor concepts are presented, and advantages and problems of the concepts are discussed.

Accurate Amorphous Silica Surface Models from First-Principles Thermodynamics of Surface Dehydroxylation

Accurate atomically detailed models of amorphous materials have been elusive to-date due to limitations in both experimental data and computational methods. We present an approach for constructing atomistic models of amorphous silica surfaces encountered in many industrial applications (such as catalytic support materials). We have used a combination of classical molecular modeling and density functional theory calculations to develop models having predictive capabilities. Our approach provides accurate surface models for a range of temperatures as measured by the thermodynamics of surface dehydroxylation. We find that a surprisingly small model of an amorphous silica surface can accurately represent the physics and chemistry of real surfaces as demonstrated by direct experimental validation using macroscopic measurements of the silanol number and type as a function of temperature. Beyond accurately predicting the experimentally observed trends in silanol numbers and types, the model also allows new insights into the dehydroxylation of amorphous silica surfaces. Our formalism is transferrable and provides an approach to generating accurate models of other amorphous materials.

Methane oxidation over noble metal gauzes: an LIF study

Abstract The contributions of homogeneous and heterogeneous reactions to high-temperature catalytic methane oxidation were studied over three different gauze catalysts (Pt, Pt-10%Rh, and Ni) using laser-induced fluorescence (LIF) spectroscopy to measure OH · concentrations in the boundary layers downstream of the gauzes. OH · concentrations were found to decrease in the order Ni > Pt-10%Rh > Pt, which could be correlated with the catalytic activity of the metals with Pt being the most active oxidation catalyst, followed by Pt-10%Rh, and then Ni, which is essentially inert in excess oxygen. The experimental LIF results were compared to one-dimensional reaction–diffusion simulations, confirming that the differences in the measured OH · concentrations can be explained by the different degrees of catalytic methane conversion by the three gauze catalysts. No evidence for a direct interaction of homogeneous and heterogeneous reaction pathways through the radical pool was observed. Rather, the heterogeneous and the homogeneous reactions appeared to be spatially decoupled.