Mihaylov Mihail, Hadjiivanov Konstantin, Knözinger Helmut
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
Phys Chem Chem Phys. 2006 Jan 21;8(3):407-17. doi: 10.1039/b511641d. Epub 2005 Nov 16.
Adsorption of IR probe molecules (CO and NO) has been used for characterization of a Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample. For better assignments of the results obtained 'blank' experiments with WO(3)-ZrO(2), Pt/WO(3)-ZrO(2) and Cr(2)O(3)/WO(3)-ZrO(2) were also performed. The Cr(2)O(3)/WO(3)-ZrO(2) sample was prepared via sulfate route and surface sulfates were detected on the activated sample. These species keep ca. 50% of their initial concentration even after reduction by hydrogen at 773 K. The amount of Lewis acid sites on the activated sample, as measured by CO adsorption, is relatively low and is associated mainly with coordiantively unsaturated (c.u.s.) Zr(4+) cations. Reduction of the sample generates Cr(3+) ions and the concentration of the Lewis acid sites continuously increases with the reduction temperature up to 773 K. At this reduction temperature, however, the strength of the acid sites decreases due to partial removal of the sulfates. While CO is not an efficient probe to discriminate between Zr(4+) and Cr(3+) sites, NO is selectively strongly adsorbed on Cr(3+) cations which allows a more precise monitoring of the evolution of the Cr(3+) sites during reduction. The activated Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample also shows presence of sulfates but these species are practically absent after reduction with hydrogen at 773 K. It was found that platinum promotes the reduction of chromium and tungsten species as well. CO adsorption on activated sample reveals the existence of cationic platinum. However, reduction with hydrogen even at room temperature generates metallic Pt. Its amount continuously increases with the reduction temperature up to 573 K and then, due to the SMSI effect and/or agglomeration, the amount of platinum accessible to adsorption decreases. Comparison with the results obtained with the Pt/WO(3)-ZrO(2) sample shows that chromium and/or sulfates stabilize the cationic form of platinum.
红外探针分子（CO和NO）的吸附已被用于表征Pt-Cr₂O₃/WO₃-ZrO₂样品。为了更好地分析所得结果，还对WO₃-ZrO₂、Pt/WO₃-ZrO₂和Cr₂O₃/WO₃-ZrO₂进行了“空白”实验。Cr₂O₃/WO₃-ZrO₂样品通过硫酸盐路线制备，在活化样品上检测到表面硫酸盐。即使在773K下用氢气还原后，这些物种仍保留约50%的初始浓度。通过CO吸附测量，活化样品上路易斯酸位点的数量相对较低，主要与配位不饱和（c.u.s.）Zr⁴⁺阳离子相关。样品的还原产生Cr³⁺离子，路易斯酸位点的浓度随着还原温度升高至773K而持续增加。然而，在这个还原温度下，由于部分硫酸盐的去除酸位点的强度降低。虽然CO不是区分Zr⁴⁺和Cr³⁺位点的有效探针，但NO选择性地强烈吸附在Cr³⁺阳离子上，这使得在还原过程中能够更精确地监测Cr³⁺位点的演变。活化的Pt-Cr₂O₃/WO₃-ZrO₂样品也显示出硫酸盐的存在，但在773K下用氢气还原后这些物种实际上不存在。发现铂也促进了铬和钨物种的还原。CO在活化样品上的吸附揭示了阳离子铂的存在。然而，即使在室温下用氢气还原也会生成金属Pt。其数量随着还原温度升高至573K而持续增加，然后由于SMSI效应和/或团聚，可用于吸附的铂数量减少。与Pt/WO₃-ZrO₂样品所得结果的比较表明，铬和/或硫酸盐稳定了铂的阳离子形式。
