Identification of Subnanometric Ag Species, Their Interaction with Supports and Role in Catalytic CO Oxidation

Abstract

Abstract:The nature and size of the real active species of nanoparticulated metal supported catalysts is still an unresolved question. The technique of choice to measure particle sizes at the nanoscale, HRTEM, has a practical limit of 1 nm. This work is aimed to identify the catalytic role of subnanometer species and methods to detect and characterize them. In this frame, we investigated the sensitivity to redox pretreatments of Ag/Fe/TiO 2 , Ag/Mg/TiO 2 and Ag/Ce/TiO 2 catalysts in CO oxidation. The joint application of HRTEM, SR-XRD, DRS, XPS, EXAFS and XANES methods indicated that most of the silver in all samples is in the form of Ag species with size <1 nm. The differences in catalytic properties and sensitivity to pretreatments, observed for the studied Ag catalysts, could not be explained taking into account only the Ag particles whose size distribution is measured by HRTEM, but may be explained by the presence of the subnanometer Ag species, undetectable by HRTEM, and their interaction with supports. This result highlights their role as active species and the need to take them into account to understand integrally the catalysis by supported nanometals.

Effect of the Metal Deposition Order on Structural, Electronic and Catalytic Properties of TiO2-Supported Bimetallic Au-Ag Catalysts in 1-Octanol Selective Oxidation

“…The observed effects can be explained as follows: in as-prepared samples, a significant part of both metals was in the ionic state, that is, Ag + , Au 3+ and Au + [39,40,63]. According to the previously reported results, single-charged ions are active sites of low-temperature oxidation of CO and alcohols, while Au 3+ was inactive in the studied processes [22,39,49,50].…”

“…This is due to the well-reported fact that, in some cases, only gold nanoparticles with sizes below 5 nm showed catalytic activity; the larger particles were simply "spectators" of the process [72][73][74][75][76]. Our previous studies have shown that the active particles are smaller than 2 nm, some of which were undetectable by TEM and XRD methods [63,73]. In addition, in bimetallic samples, it is impossible to discriminate TEM images of Au and Ag clusters.…”

“…The results presented represent one solution for resolving the peaks, and the true oxidation states may be dif- [39,40] (copyright 2004), with permission from Elsevier. Data for Ag/M/TiO 2 were adapted from [63].…”

“…AuAg AgAu ( Samples were studied after reduction in H2 at 300 °C for 1 h. Some previously reported XPS results of Ag-based samples are shown in (a) [63].…”

Special Issue: Coinage Metal (Copper, Silver, and Gold) Catalysis

“…The role of sub-nanometric Ag species (on Fe, Mg and Ce modified TiO 2 materials) on catalytic CO oxidation is studied by several techniques in [1]. The observed phenomena cannot be explained only considering the 1-13 nm Ag particle size distributions measured by HRTEM; the existence of Ag species with size < 1 nm, non-visible in HRTEM, and their interaction with the supports, needs also to be taken into account.…”