Mechanistic studies of atomic layer deposition on oxidation catalysts - AlO(x)and PO(x)deposition

摘要

Atomic layer deposition is a rising technique for catalyst synthesis and modification. Typically, the focus of ALD in catalysis is on supported metal nanoparticles. Here, the authors give mechanistic insights into the ALD of oxides on redox active catalysts by a combination ofin situanalytics, such as XPS, DRIFTS and gravimetric measurements. Phosphorus oxide and aluminum oxide were deposited on divanadium pentoxide powder in a fixed bed reactor. In contrast to the generally accepted concepts, the first half cycle does not proceed over surface hydroxyl groups but involves redox chemistry between the precursor and the vanadium atoms, as shown by(31)P-SSNMR and XPS. For PO(x)deposition, a temperature step from 150 degrees C in the first half cycle to 450 degrees C in the second half cycle is needed to obtain linear mass gain per cycle as the remaining ligands are combusted and reduced vanadium atoms are reoxidized. Homogeneous deposition was confirmed by STEM-EDX and XRD showing no additional phases, despite performing up to 10 ALD cycles. Even the well-known process of alumina ALD confirms the involvement of reduction-oxidation reactions between the ALD precursor and the substrate V2O5. However, redox chemistry can be suppressed for alumina ALD at low temperatures of 50 degrees C. Therefore, this study shows that ALD on oxidation catalysts is complex and thus the developed ALD processes are unusual compared to ALD on typical supports, such as SiO(2)or Al2O3.