Silica coatings metal colloids

In this paper, we present a study in which combustion catalysts based on silica-coated metal monoliths were prepared. The aim of this study was to prepare washcoated metal monoliths with controlled properties. The properties varied are specific surface area of the washcoat and washcoat thickness or washcoat loading. Furthermore, we discuss how the preparation procedure affects the resulting catalyst properties and related performance. We deposited washcoats based on colloidal silica sols. Colloidal silica sols give porous materials with rather narrow pore size distributions when dried and calcined. This gives us excellent control over the pore size distribution of the washcoat, as will be discussed. The technique presented here, allows deposition of washcoats with controllable thickness in one step, unlike techniques based on pure silica sols, reported elsewhere [7,8]. Washcoats were impregnated with paUadium salts to make active catalysts that were tested in methane combustion. The effects of the preparation procedure of the silica and of the impregnation procedure were studied using particulate catalysts. 

A sol-gel coating procedure for silica on metallic monoliths was developed by Zwinkels et al. (74), who used colloidal silica sols together with potassium waterglass. Various procedures resulted in washcoats 20-50 pm thick, with surface areas of 60-140 m /g. Coating with the colloidal silica without binder material resulted in a thickness of only a few micrometers. 

An experimental difficulty is coating a colloid particle with a metal homogenously. Electrochemical deposition onto an insulating surface is difficult. In some cases, adsorption of catalytic ions such as Pd(II), Ag(I) or Pt(II) can assist. An alternative is to controUably deposit small gold nanoparticles onto a larger silica or latex substrate. This can be done using LbL processes as outlined in the introduction. 

The excellent electron-transfer mediator properties of nanoparticles find special use in the different oxidation [126] and reduction [143,144] reactions catalyzed by noble metal colloids. Recently, Ung et al. [145] showed how Ag particles coated with a thin layer of silica act as redox catalysts, and how the control of the rate of the catalyzed hydrogen evolution reaction was possible by tuning the silica shell thickness. It was concluded that the shell acts as a size-selective membrane, which can be used to alter the chemical yields for competing catalytic reactions. This kind of tailoring of the catalyst properties opens up very interesting prospects in future catalyst planning. 

The outline of the paper is therefore as follows In section 1 we introduce the main interaction forces acting on colloidal particles, as well as the concept of nanostructured materials, in the form of 2D and 3D assemblies. We discuss the main stabilization techniques employed in the synthesis of nanoparticles in solution. Then we outline in section 2 the procedures involved in silica coating, and discuss its advantages as a general stabilization technique. Section 3 deals with the special properties of both metal and semiconductor nanoparticles, summarizing their... 

Silica coating of metal colloids presents the difficulty of a chemical mismatch between the core and the shell materials. Liz-Marzin and Philipse first approached the synthesis of Au Si02 particles through borohydride reduction of AuCU in the presence... 

The interest of the assembly of silica-coated gold nanoparticles relies on the possibility of controlling the particle volume fraction by means of the variation of the silica shell thickness, provided that the assembled films are close-packed. In closepacking conditions, the separation between metal cores is just twice the thickness of the coating shell, which can be controlled during the synthesis of the colloids. This system has been recently studied in detail by Ung et al.," and described here are the main results on the influence of metal nanoparticles volume fraction (through interparticle interactions) on the optical properties of the films. 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

The work of Larson et al. (62) represented the first detailed study to show agreement between AFM-derived diffuse layer potentials and ((-potentials obtained from traditional electrokinetic techniques. The AFM experimental data was satisfactorily fitted to the theory of McCormack et al. (46). The fitting parameters used, silica and alumina zeta-potentials, were independently determined for the same surfaces used in the AFM study using electrophoretic and streaming-potential measurements, respectively. This same system was later used by another research group (63). Hartley and coworkers 63 also compared dissimilar surface interactions with electrokinetic measurements, namely between a silica probe interacting with a polylysine coated mica flat (see Section III.B.). It is also possible to conduct measurements between a colloid probe and a metal or semiconductor surface whose electrochemical properties are controlled by the experimenter 164-66). In Ref. 64 Raiteri et al. studied the interactions between... 

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