Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.
Phys Chem Chem Phys. 2010 Jun 28;12(24):6520-31. doi: 10.1039/c000750a. Epub 2010 May 26.
The present study focuses on the characterization of the active sites for oxygen adsorption in both copper-free and copper-containing HZSM5 zeolites. FTIR, EPR, EXAFS and UV-Vis measurements offer insight into the initial state of the catalyst before oxygen adsorption. Both liquid and solid state ion exchanged samples contain a certain amount of Cu(ii) and Cu(i) ions in the alpha3, alpha4 and gamma6 position, their population ratio depending on the ion exchange temperature. They are accessible for interaction with the adsorbate, as the copper-oxygen spin exchange demonstrates. Both the sample magnetization and the EXAFS analysis indicate that 10-30% of the Cu(ii) exists in the form of oxygen bridged Cu-Cu pairs. UV-Vis measurements prove that two different antiferromagnetically coupled copper peroxide complexes are formed during the sample preparation process, the bis(mu-oxo)- and (mu-eta(2):eta(2)-peroxo) dimers. One of the complexes is susceptible to oxygen adsorption, which cleaves it irreversibly into two individual Cu(ii)-O(2)(-) units, while Cu(i) ions are oxidised to the same species. The Brønsted acid sites are also able to adsorb oxygen both at room and low temperatures. The presence of the different active sites may be an explanation for the high catalytic activity of the Cu/HZSM5 zeolite. The Brønsted sites near copper centers could protonate the peroxide complexes, leading to the in situ formation of hydrogen peroxide, a common oxidant. This peroxide would be a highly active species for catalytic reactions.
本研究重点研究了无铜和含铜 HZSM5 沸石中氧吸附活性位的特性。FTIR、EPR、EXAFS 和 UV-Vis 测量为氧吸附前催化剂的初始状态提供了深入了解。液体和固态离子交换样品在 alpha3、alpha4 和 gamma6 位置都含有一定量的 Cu(ii)和 Cu(i)离子，其浓度比取决于离子交换温度。它们可以与吸附质相互作用，因为铜-氧自旋交换证明了这一点。样品磁化和 EXAFS 分析都表明，10-30%的 Cu(ii)以氧桥接的 Cu-Cu 对形式存在。UV-Vis 测量证明，在样品制备过程中形成了两种不同的反铁磁耦合的铜过氧化物配合物，即双（mu-氧）-和（mu-eta(2):eta(2)-过氧）二聚体。其中一个配合物易于被氧吸附，这会不可逆地将其裂解成两个独立的 Cu(ii)-O(2)(-)单元，而 Cu(i)离子被氧化成相同的物种。Brønsted 酸位也能够在室温甚至低温下吸附氧。不同活性位的存在可能是 Cu/HZSM5 沸石高催化活性的原因之一。靠近铜中心的 Brønsted 位可以质子化过氧化物配合物，导致原位形成过氧化氢，这是一种常见的氧化剂。这种过氧化物将是催化反应的高活性物种。
