Abstract
In this study, a Pt/anodized aluminum oxide (AAO) catalyst was prepared by the anodization of an Al alloy (Al6082, 97.5% Al), followed by the incorporation of Pt via an incipient wet impregnation method. Then, the Pt/AAO catalyst was evaluated for autocatalytic hydrogen recombination. The Pt/AAO catalyst’s morphological characteristics were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average Pt particle size was determined to be 3.0 ± 0.6 nm. This Pt/AAO catalyst was tested for the combustion of lean hydrogen (0.5–4 vol% H2 in the air) in a recombiner section testing station. The thermal distribution throughout the catalytic surface was investigated at 3 vol% hydrogen (H2) using an infrared camera. The Al/AAO system had a high thermal conductivity, which prevents the formation of hotspots (areas where localized surface temperature is higher than an average temperature across the entire catalyst surface). In turn, the Pt stability was enhanced during catalytic hydrogen combustion (CHC). A temperature gradient over 70 mm of the Pt/AAO catalyst was 23 °C and 42 °C for catalysts with uniform and nonuniform (worst-case scenario) Pt distributions. The commercial computational fluid dynamics (CFD) code STAR-CCM+ was used to compare the experimentally observed and numerically simulated thermal distribution of the Pt/AAO catalyst. The effect of the initial H2 volume fraction on the combustion temperature and conversion of H2 was investigated. The activation energy for CHC on the Pt/AAO catalyst was 19.2 kJ/mol. Prolonged CHC was performed to assess the durability (reactive metal stability and catalytic activity) of the Pt/AAO catalyst. A stable combustion temperature of 162.8 ± 8.0 °C was maintained over 530 h of CHC. To confirm that Pt aggregation was avoided, the Pt particle size and distribution were determined by TEM before and after prolonged CHC.
Highlights
In nuclear power plants (NPPs) and the mining industry, passive autocatalytic recombiners (PARs) are considered hydrogen (H2) control systems for emergency gas removal based on their recombination with ambient oxygen
The metallic Al core was preserved during the anodization procedure, and it acted as a thermal conduit during catalytic hydrogen combustion (CHC) reactions, affording a Pt/anodized aluminum oxide (AAO) catalyst with high thermal conductivity
To simulate a nonideal (“worst-case”) scenario, Pt/AAO catalysts were prepared to contain hotspots. They were formed by nonuniform Pt distribution and had a temperature gradient of 42 ◦C over 70 mm of the catalyst surface
Summary
In nuclear power plants (NPPs) and the mining industry, passive autocatalytic recombiners (PARs) are considered hydrogen (H2) control systems for emergency gas removal based on their recombination with ambient oxygen. Results of studies have shown that the thermal distribution over the catalytic surface plays an important role in risk mitigation related to the PAR autoignition. In a study by Du Preez et al, aggregation of Pt catalyst occurred on the SSM support during CHC at 420–520 ◦C This occurrence was ascribed to the fact that the SSM surface allows significant Pt mobility at high temperatures. These authors demonstrated that calcination of the SSM support at 500–1000 ◦C resulted in the stabilization of Pt particles, effectively preventing their aggregation during CHC [25]. A numerical model was created in STAR-CCM+ in combination with the chemistry tools of CHEMKIN to investigate temperature distribution throughout the Pt/AAO catalyst and to compare it with the experimental data [29]
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