Microwave effects in heterogeneous catalysis: Application to gas-solid reactions for hydrogen production

Abstract

Due to the quest for more efficient production processes both from the energy and selectivity point of view, microwave irradiation has attracted significant scientific attention over the last three decades, as an alternative means of chemical activation. Over this period, striking process benefits, such as higher conversions and selectivities and/or a shorter reaction times, compared to the respective conventionally heated processes have been reported. The aim of this work is to investigate the influence of microwave energy on heterogeneous gas-solid catalytic reactions. As example process, the steam reforming of methanol and the water-gas shift reaction were selected. In a first step, the interaction of microwaves with different catalysts was investigated in a non-reactive environment, followed by investigation of the microwave effects on the reactions themselves. Comparison of the microwave- and electrically heated processes was performed in terms of conversion, selectivity and energy efficiency of the reactor. Contrary to other works in the literature, a two-dimensional temperature map along the centre plane of the reactor was recorded with both heating modes. The study of interaction of microwaves with the solid state catalysts revealed that the heating rate, the maximum temperature at constant power, and the heat distribution inside the catalyst bed strongly depend on the catalytic support morphology, the metal loading and the particle size of the catalyst. Moreover, the experiments proved that even small catalytic samples (~2g) experience non-uniform heat distribution inside their volume when exposed to a well-defined, mono-mode type of microwave field. These temperature gradients, although sometimes being severe, are undetectable by the commonly employed in microwave chemistry infra-red temperature sensors. These types of sensors are often built-in in the microwave applicator and serve as benchmark for the power control unit, which adjusts the power delivered into the microwave cavity. Therefore, a fibre optic based direct temperature measurement was selected as more accurate method in further stages of the research. The investigation of methanol steam reforming reaction was performed with employment of two catalysts - PdZnO/Al2O3 and CuZnO/Al2O3 – at an average reaction temperature ranging between 190oC – 250oC and 170oC – 230oC, respectively. In order to account for possible temperature gradients occurring across the catalytic bed, multi-point temperature mapping was implemented. The experiments revealed that at corresponding thermal conditions, the feed conversion in the microwave-heated process is significantly higher than in the electrically-heated process, regardless of the employed catalyst. However, the product distribution remained unaffected. Comparison of the reactor energy efficiency demonstrated that the MW-assisted process exhibits higher reactor energy efficiency than the corresponding electrically heated process for both catalysts and over the range of the studied reaction temperatures. This entails that a given conversion can be achieved with lower net heat input to the reactor under the microwave heating mode and thus indirectly confirms the selective microwave heating principle (microscale hot spot formation). Pre-conditioning of the catalyst in the presence of the microwave field prior to the reaction did not affect the reaction performance. The catalyst surface investigation showed no difference in the morphology of the catalyst used either between the microwave and the conventionally heated process, or between the preconditioned and the non-preconditioned samples. Consequently, specific non-thermal microwave effects were excluded as justification for the enhancement of the reactor performance. In order to confirm the thermal nature of the microwave effects observed in the methanol steam reforming reaction, a mildly exothermic process, a water-gas shift reaction, was investigated at the latest stage of the research. Contrary to steam reforming, the water-gas shift reaction did not exhibit significant enhancement neither in terms of conversion nor in terms of reactor energy efficiency. This is because a significant part of the net heat input to the reactor comes from the heat of reaction; therefore, the heat input from microwave irradiation and the associated local overheating of active sites diminishes. Consequently, the microwave effect is not pronounced. Based on the experimental experience obtained and the theoretical knowledge regarding the shortcomings of the available microwave types of equipment, an alternative reactor design, based on travelling microwave fields, is proposed for application to heterogeneous gas-solid catalytic reactions. The new concept may enable uniform spatial heating, improved electromagnetic energy utilization and electromagnetic field spatial localization (i.e. on the catalytic reactor walls).