One of the greatest ills of rapid industrialization has been the release of toxic pollutants into the surrounding biosphere, with often disastrous consequences for humans. Several industrial processes, such as the manufacturing and printing of chemicals, as well as facilities such as power plants, emit volatile organic compounds (VOCs) known to be carcinogenic and which raise a significant environmental problem requiring a solution. Traditionally, VOCs are controlled through a process called “catalytic oxidation,” in which they are converted to benign materials in the presence of noble metal nanoparticles (eg, gold, silver, and platinum). However, the process is expensive and requires fine tuning of the characteristics of the nanoparticles. Thus, a catalytic process which does not require noble metal catalysts is highly desirable. While transition metals and their oxides are a possible alternative, they require complex syntheses and precise control of chemical composition.
So, can we do better than that? It turns out we can! A team of scientists led by Professor Takashi Shirai of the Nagoya Institute of Technology (NITech), Japan, reported complete catalytic decomposition of VOCs using an inorganic compound called ‘hydroxyapatite’ (HAp), a natural form of the mineral calcium phosphate which makes up most of the human bone structure. “HAp is composed of elements abundant in nature, is non-toxic and exhibits high biocompatibility. Our results have thus opened up a new possibility for the design of inexpensive and noble metal-free catalysts for the control of VOCs”, explains the professor. Shirai.
In a new study published in Scientific Reports, Professor Shirai and his colleague Yunzi Xin of NITech now go one step further by tailoring the “active surface” of HAp using a mechanochemical treatment under ambient conditions that leads to oxidation. Highly efficient VOC catalytic converter with 100% conversion to harmless compounds! Specifically, they mixed the initial HAp with ceramic balls in a container and performed “planetary ball milling” at room temperature and pressure. This essentially altered the chemical structure of HAp and allowed its selective customization by simply changing the size of the ball.
By using different sizes of beads (3, 10 and 15mm) to systematically vary the morphology, crystallinity, surface defects / oxygen deficiency, acidity / basicity and VOC affinity of HAps, scientists carried out their characterization using various techniques such as scanning. electron microscopy, X-ray powder diffraction, Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, electron spin resonance analysis, surface acidity / basicity assessment, and diffuse reflectance infrared Fourier transform spectroscopy.
They observed a predominance of oxygen vacancy formation in the PO43- (triple charged PO4) site with an enhanced base site population caused by selective mechanochemical activation of the c-plane (plane perpendicular to the axis of symmetry) of the hexagonal crystal HAp and attributes it to the excellent catalytic conversion of VOCs to CO2/CO.
In addition, they found that HAps treated with 3mm beads exhibited higher catalytic activity than that treated with 10 and 15mm beads, even though larger beads caused more defects and basicity. Examining the surface absorption of a VOC, ethyl acetate, scientists attributed this anomaly to the inhibited absorption of ethyl acetate in HAp treated with larger beads, leading to catalysis deleted.
The results have excited scientists about the future prospects of HAps. “We hope that our catalyst will make a significant contribution to controlling VOCs and cleaning up the environment around the world over the next decade, achieving the sustainable goals of clean air and water, affordable energy and climate action, ”comments delighted Professor Shirai.
Indeed, it is a big step forward towards a society more respectful of the environment.
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