Size effect in the oxidation–reduction processes of platinum particles supported onto silicon dioxide

Abstract

Abstract-The interaction of the Pt/SiO 2 model catalysts as thin films on the surface of tantalum supports with a mixture of NO + O 2 (1 : 1) was studied by X ray photoelectron spectroscopy. The pressure of the reac tion mixture was varied from 6 to 64 mbar, and the temperature was varied from room temperature to 500°C. Two types of the catalysts, in which the Pt/Si atomic ratios were ~0.1 and ~0.3 (0.1 Pt/SiO 2 and 0.3 Pt/SiO 2 , respectively) according to the XPS data, were studied. In 0.1 Pt/SiO 2 , the particles of platinum predomi nantly had a size from 1 to 2.5 nm; a wide Pt particle size distribution in a range from 1 to 15 nm with a max imum at ~4 nm was characteristic of 0.3 Pt/SiO 2 . The interaction of all of the samples with NO + O 2 at room temperature led to the dissolution of oxygen atoms in the bulk of platinum metal particles. As the reaction temperature was increased, PtO x platinum oxide particles were formed: from small Pt particles in 0.1 Pt/SiO 2 at 300°C and from larger particles in 0.3 Pt/SiO 2 at 400-500°C. It was established that the reactivity of plat inum oxide particles toward hydrogen also depended on the particle size. The small particles of platinum oxide were converted into platinum metal under the action of hydrogen (16 mbar) at 300°C. The coarse par ticles of PtO x in the samples of 0.3 Pt/SiO 2 were reduced much more easily starting with room temperature.

The Effect of Pt Particle Size on the Oxidation of CO, C3H6, and NO Over Pt/Al2O3 for Diesel Exhaust Aftertreatment

“…Figure 6 shows that 391 particles smaller than 2 nm contain less terrace atoms, which is in good agreement with our observation that 392 both the activity and TOF are lower for catalysts with Pt particle size below 2 nm. Furthermore, small Pt 393 particles are also more easily oxidized than large particles [40,41,68,71] and, consequently, the reduction of 394 small Pt particles becomes harder. This is indicated by the 100-150 °C higher reduction temperature of oxidized 395…”

“…Pt particles of 1.5 nm in size compared to for Pt particles of 4 nm [40,71]. However, as oxygen always is 396 present under the typical conditions in a DOC, it is difficult to distinguish whether the lower activity is the result 397 from a change in chemistry due to oxide formation, or due to the loss of active sites, as both effects always will 398 occur simultaneously.…”

Model sulfur-resistant NSR catalysts: An XPS study of the interaction of BaO/TiO2-ZrO2 and Pt-BaO/TiO2-ZrO2 with NO2

“…7b, Table 2): the percentage of PtO 2 is lower, and the greater part of the platinum remains in the form of metal particles containing dissolved oxygen atoms. This finding is not surprising, because it is well known that larger plati num particles turn less readily into oxides [13,30], but, apparently, they dissolve oxygen rather readily [30].…”

О Распределении По Размерам Дисперсных Частиц Фрактальной Формы

“…-диапазон рассматриваемых стехиометрических чисел, и соответствует экспериментально полученным гистограммам для наночастиц [9,[39][40][41][42], а также трехмерных элементов кристаллической структуры [43,44]. Интересным также является то, что подобный вид имеют и гистограммы распределений по размерам пор в поликристалле [45].…”

“…Основным результатом предложенного описания является утверждение о том, что в общем случае нерегулярная форма частиц является наиболее характерной для дисперсных систем. Это проявляется при формировании частиц путем диспергирования и ансамблей зерен поликристаллического материала и может быть проиллюстрировано снимками, приведенными, например, в [9,17,22,39,40,42,47]. Отметим, что данная особенность подтверждается имеющимися результатами [1,48,49], свидетельствующими о нестабильности формы частиц малого объема, обладающих четкой огранкой.…”