Chemical quenching of positronium in OMC/SBA-15, OMC@SBA-15 and CuO@SBA-15 catalysts

Abstract

Owing to highly ordered two-dimensional hexagonal structure, large surface area, variable pore size, high thermal stability and especially the electron delocalization energy determined by its frame structure, SBA-15 catalysts have received more and more researchers’ attention. By using the structure-directing agent of P123 and the silicon source of TEOS, we synthesize ordered mesoporous silica SBA-15. At the same time, ordered mesoporous carbon OMC is succefully synthesized with the template of SBA-15. The small angle X-ray diffraction, high resolution transmission electron microscopy and N₂ adsorption-desorption measurements are conducted to verify the highly ordered pore structure and relatively high specific surface area of SBA-15 and OMC, and their average pore radius are about 7.5 nm and 3.3 nm, respectively. Positron lifetime spectrum of SBA-15 catalyst is composed of two longer lifetimes and two shorter lifetimes: two longer lifetimes <inline-formula><tex-math id="M11">\begin{document}$ {\tau }_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M11.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M12">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M12.png"/></alternatives></inline-formula> are the annihilation in micropore and large pore of positronium (Ps), are 7.5 ns and 106 ns. However, there is nearly no longer lifetime component in OMC, which indicates that there might exist the quenching or inhibiting of positronium by carbon material. To verify this guess, we synthesize the catalysts of OMC/SBA-15, OMC@SBA-15 and CuO@SBA-15 by the solid state reaction and the impregnation filling method. With the increasing of OMC and CuO content, both the o-Ps lifetime <inline-formula><tex-math id="M13">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M13.png"/></alternatives></inline-formula> and its intensity <inline-formula><tex-math id="M14">\begin{document}$ {I}_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M14.png"/></alternatives></inline-formula> of these three compounds decrease. The annihilation rate of o-Ps lifetime varying with OMC and CuO content can be better fitted by one or two straight lines, The values of reaction rate constant K in OMC/SBA-15, OMC@SBA-15 and CuO@SBA-15 are <inline-formula><tex-math id="M15">\begin{document}$(2.39\pm $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15.png"/></alternatives></inline-formula><inline-formula><tex-math id="M15-1">\begin{document}$ 0.44)\times {10}^{7}~{\mathrm{s}}^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M15-1.png"/></alternatives></inline-formula>/<inline-formula><tex-math id="M16">\begin{document}$(6.65\pm 0.94)\times {10}^{6}~{\mathrm{s}}^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M16.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M16.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M17">\begin{document}$(2.28\pm 0.19)\times {10}^{7}~{\mathrm{s}}^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M17.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M17.png"/></alternatives></inline-formula>, and <inline-formula><tex-math id="M18">\begin{document}$(8.76\pm 0.47)\times {10}^{6}~{\mathrm{s}}^{-1},$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M18.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M18.png"/></alternatives></inline-formula> respectively. Therefore, our results indicate that there are quenching effect and inhibition effect among the carbon, the CuO and the positronium, which lead <inline-formula><tex-math id="M19">\begin{document}$ {\tau }_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M19.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M20">\begin{document}$ {I}_{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211814_M20.png"/></alternatives></inline-formula>to decrease, and positronium is also a probe for detecting the pore structure of porous material.