Silver catalyst calcined

Figure 2. Transient catalytic activity, selectivity, and surface oxygen activity of a freshly calcined silver catalyst during the first 40 h on stream. Catalyst is exposed to reactive gas mixture at t — 0, 410°C, Pet — 0.015 bar, and Po2 = 0.i0 for.

We modified the surface of the initial a-alumina support hy treating with CsOH solution followed by drying and calcination at 240 C in air. To remove Cs, we boiled the support for 5h and washed it with bidistilled water. To control the process, the Cs concentration was analyzed with SIMS (sensitivity towards Cs < 10 ). Using the modified support we managed to narrow considerably the Ag particle size distribution (Fig. 2b, 3) and increase the stability of the catalyst. The particle size deviation firom the average did not exceed 30-40 %. TEM stupes showed that the particle size distribution did not change after the catalyst calcination and treatment with reaction mixture. Probably, the restriction of the Ag particle mobility on die modified support results from the "point interactions of silver with a-Al203, which are hard to evidence by transmission electron microscopy. 

The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, Pt-Ir, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for a-alumina were evident. The low temperature required for calcination to the alpha-phase along with TEM results suggest that this material was formed as nano-phase, a-alumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

Catalyst was prepared by adding a silver nitrate solution to the spheres using incipient wetness impregnation, the moist pellets were dried at 120°C and calcined at 450 C. Impregnation of the foams followed the same procedure except the foam was rotated on rollers at 12 rpm during drying in a microwave oven for one minute to ensure uniform deposition throughout the foam. Composition and structure of the samples were confirmed by XRD. 

Silver-containing ceramic catalyst prepared by impregnating acid washed kaolin with silver nitrate solution after calcination in air at 1200 °C appeared to be highly active in the oxidation of methanol to formaldehyde. In this catalyst the silver was stabilized by the presence of [AIO4] in ionic form. Under the reaction conditions, the silver ions were partially reduced by the atomic hydrogen released from methanol and formed metallic particles. XPS and UV-Visible diffuse reflectance spectra (UV-Vis DRS) revealed that after the reaction about one-third of silver in the catalyst was still in ionic state. 

The same catalyst can be used in both air and oxygen processes. In the early catalysts about 10-15% silver was deposited on a low-surface-area support such as a-alumina or silicon carbide. The nature of the support is critical. Low surface area a-alumina is normally used. Any residual acidity in the support promotes the isomerisation of ethylene oxide to acetaldehyde, a key intermediate in the formation of caibon dioxide, and hence low selectivity. All y-alumina must be converted to a-alumina during calcination. Silver lactate solution plus an alkaline earth lactate could be used to coat the preformed support. Metallic silver formed as the catalyst was dried and then calcined. Suitable supports include small spheres or rings with a pore volume about 0.5 ml g". Up to 2% of an alkaline earth promoter such as barium may have been added to the catalyst. Further details are shown in Table 4.11,... 

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