Axial Changes of Catalyst Structure and Temperature in a Fixed-Bed Microreactor During Noble Metal Catalysed Partial Oxidation of Methane

Abstract

The catalytic partial oxidation of methane (CPO) over flame-made 2.5%Rh-2.5%Pt/Al 2 O 3 and 2.5%Rh/Al 2 O 3 in 6%CH 4 /3%O 2 /He shows the potential of in situ studies using miniaturized fixed-bed reactors, the importance of spatially resolved studies and its combination with infrared thermography and on-line mass spectrometry. This experimental strategy allowed collecting data on the structure of the noble metal (oxidation state) and the temperature along the catalyst bed. The reaction was investigated in a fixed-bed quartz microreactor (1-1.5 mm diameter) following the catalytic performance by on-line gas mass spectrometry (MS). Above the ignition temperature of the catalytic partial oxidation of methane (310-330°C), a zone with oxidized noble metals was observed in the inlet region of the catalyst bed, accompanied by a characteristic hot spot (over-temperature up to 150°C), while reduced noble metal species became dominant towards the outlet of the bed. The position of both the gradient in oxidation state and the hot spot were strongly dependent on the furnace temperature and the gas flow (residence time). Heating as well as a higher flow rate caused a migration of the transition zone of the oxidation state/maximum in temperature towards the inlet. At the same time the hydrogen concentration in the reactor effluent increased. In contrast, at low temperatures a movement of the transition zone towards the outlet was observed at increasing flux, except if the self-heating by the exothermic methane oxidation was too strong. The results indicate that in the oxidized zone mainly combustion of methane occurs, whereas in the reduced part direct partial oxidation and reforming reactions prevail. The results demonstrate how spatially resolved spectroscopy can help in understanding catalytic reactions involving different reaction zones and gradients even in micro scale fixed-bed reactors.

Imaging Catalysts at Work: A Hierarchical Approach from the Macro‐ to the Meso‐ and Nano‐scale

“… Temperature profiles recorded using infrared thermography during the ignition of the catalytic partial oxidation of methane over a 2.5 % Rh/Al 2 O 3 catalyst, while increasing the oven temperature in 10 °C steps; insets show thermographic images recorded below and above the ignition temperature of 330 °C 26h…”

“…For in situ and spatially resolved UV/Vis/Raman measurements, a quartz reactor equipped with optical grade quartz windows has been used,44a while spatially resolved XAS has involved the use of quartz glass capillaries,48 as described earlier for integral XAS and XRD studies 9a. 49 In addition, both scanning X‐ray absorption microscopy using a microfocused X‐ray beam3c, 26h and full‐field X‐ray absorption microscopy using an X‐ray sensitive camera3c, 26h, 48, 50 have been used. In principle, the latter approach allows much faster acquisition of XANES data.…”

Operando spatially and time-resolved X-ray absorption spectroscopy and infrared thermography during oscillatory CO oxidation

“…The next obvious step would be the correlation of catalytic activity and structural studies in a spatially resolved and time-resolved manner, which appears to be particularly attractive for oscillatory reactions. In principle, the combination of spatially resolved XAS with IR thermography would allow the correlation of effects on the macroscopic scale (temperature distribution) with dynamics on the microscopic scale (e.g., oxidation state) [26][27][28] and thus significantly improve the development of structureperformance relationships.…”

Microfluidics and catalyst particles

“…With these techniques, as summarized in Table 4, the conversion and selectivity of a catalyst bed can be determined. 108,111,115,116,[123][124][125][126][127][128][129] The main advantage of MS and GC lies in their low limit of detection and broad range of sensitivity for multiple compounds. Unexpected products or molecules can be detected, since these in-line systems do not require tuning of the system in order to detect a specific molecule or compound.…”