Bare support

Porosity and Pore Size. The same methods used to determine the porosity and pore si2e distribution of the support generally can be used for the catalyst. However, the values found for the catalyst usually ate different from those of the bare support. Porosity could be increased if a part of the support is leached away during preparation of the catalyst, or, more likely, porosity will be decreased because catalytic materials deposited on the support win occupy a part of the support s pore volume. 

The acidic properties of the bare supports were studied by IRS method using CO adsorption at 77 K. The IR spectra were measured on a Shimadzu FTIR-8300 spectrometer over a range of 700-6000 cm with a resolution of 4 cm. Before spectra registration, sample of the supports powder was pressed in wafer (p = 0.007-0.016 g/cm ) and treated in vacuum (450°C, 1 hr., < 10 Torr). 

Fig.l Selectivities of the main isomerization products during the /3-pinene HDS on the bare supports (200°C, 1 atm). ir a-pinene dipentene camphene. 

As the permeance and permselectivity measurements show, it is possible to prepare high-quality doped silica membranes with excellent properties. Moreover it was possible to perform permeance and permselectivity measurements at temperatures up to 600°C on flat membranes. To the author s knowledge these are the first reliable measurements ever performed on flat membranes at such a high temperature. A more detailed discussion of the permeance and permselectivity results follows. It must however be noted that the relatively low hydrogen permeances obtained for the described membranes were at least partly due to the used AKP-30 supports, which had a bare-support hydrogen permeance of-8 10 mol/m sPa. 

Figure 9 Adsorption process of NO on Pd particles supported on MgO(l 00). (a) Global adsorption probability as a function of surface temperature and for various particle sizes (from Ref. [89]). (b) Schematic representation of die elementary processes in die molecular adsorption of NO on supported Pd particles (1) quasi-elastic redection on die bare support, (2) physisorption-diffusion-desorption from the bare support, (3) direct chemisorption on die Pd particles, (4) NO chemisorption on the Pd particles via a precursor physisorbed state on die bare support. Xs is die mean diffusion length of die NO molecules on the support and p is die width of die collection zone around die Pd particles.

Let us explore the electronic stracture of the bare supported island [Ptbuik-XsJ-Pts in some more detail. The oibital resolved d-DOS of the Pts island supported on CosPt (1), Co (2), Pt (3), Ni (4) and Fe (5) reveals that the i-band orbitals without z-component,... 

If the TPR profiles for the NM/Ce02 catalysts and the bare support, also included in Figure 4.3, are compared, a common high temperature feature centred at 1090 K may be noted. This peak is generally interpreted as due to the bulk reduction of ceria (61, and references there in). In agreement with several earlier studies (73,110,283), the position of this peak does not seem to be modified by the presence of any supported metal. This observation is typically interpreted in terms of a kinetic model (205) which assumes that the high temperature reduction process is controlled by the slow bulk difhision of the oxygen vacancies created at the surface of the oxide. 

On a sample pre-calcined at 673 K, further flash-heated under vacuum at 800 K, and finally treated with CO at 200 K, the subsequent TPD run showed that 63% of the preadsorbed CO was actually desorbed as CO2 (149). This fraction was much larger than that observed in a parallel study on the bare support. On repeating the experiment in a cyclic manner, successive but decreasing amounts of CO2 were observed. Conversely, on the catalyst reduced with H2 (Pic J0 Torr) at 773 K (5 min), prior CO adsorption, CO was practically the only desorption product The initial behaviour of the catalyst could be restored by subsequent oxygen treatment at 373 iC (149). Figure 4.11 summarizes the TPD-MS diagrams recorded after the two latter experiments. 

Describes a procedure for measuring the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just barely support flaming combustion of a material initially at room temperature. Covers various forms of plastics, including fQms and cellular plastics. 

Catalyst deactivation in hydrodemetallisation (HDM) is caused by the interaction of the metal deposits with the original active phase ( active site poisoning ) and the loss of pore volume due to the obstruction of catalyst pores i pore plugging) (1). However, metal deposits also have an auto-catalytic effect on the hydrodemetallisation reaction, thus active site generation may occur in low active phase loaded or bare support catalyst systems. 

HREM/EDX analysis at several point on the spent catalyst showed that vanadium was present as isolated clusters on the bare support and in the surroimding of the active phase. 

All chemisorption experiments were single point measurements at 8.10 Pa. By measuring the adsorption isotherms from 10 -10 Pa for a few catalysts, it was checked that a relative comparison of the thus obtained chemisorption values was as justifiable as any other method based on other measuring points, or on extrapolation of measuring points to zero pressure, as advocated by Benson and Boudart (24). No corrections were made for chemisorption on the bare supports as such, because this was found negligible. 

The reaction mechanism has been described in ref, (23), The reaction is carried out at temperatures <-10°C where no dark thermal activity of the metal can be detected. It only occurs when near-UV light is admitted onto the solid, thus showing that the active phase is constituted by the support. However, the presence of a metal (Pt,Ni) is required to confer a catalytic character to the reaction since, otherwise, the reaction carried out on the bare support declines and stops after a certain time corresponding to an exhaustion of non-renewable active sites. These sites have been identified as deuterated hydroxyl groups since naked titania either dehydroxylated or pretreated in instead of is totally photo-inactive,... 

Figure 4.7. TPD-MS study of the H2 desorption from a Pt(7%)/Ce02 catalyst reduced at A) 473 K B) 773 K. After reduction, the samples were evacuated at 773 K (Ih) in a flow of He, and treated with flowing H2 (Ih) at 298 K (A1 and Bl) 473 K (A2 and B2) and 773 K (B3). Then, they were cooled to 191 K (solid/liquid acetone cold trap), and finally the TPD-MS diagrams were recorded in two steps from 191 K-298 K (free heating of the sample), and from 298 K upwards The reported diagrams correspond to the latter step. Trace C corresponds to the bare support reduced at 773 K (Ih) and further cooled to 191 K, always in a flow of H2. Catalyst prepared by impregnation from an aqueous solution of [Pt(NH3)4](OH)2 Support surface area 34 m g". Experimental TPD conditions amount of catalyst 200 mg He flow rate 60 cm. min Heating rate 10 K.min . Diagrams taken from (117).

However, some bare supports did seem to promote the formation of C02>... 

In Figure 4 1s shown a periodic table listing the catalysts Investigated and an Indication as to whether or not they produced greater or lesser amounts of C2s than the bare supports. Inspection of this figure shows... 

Inspection of the C2 and carbon dioxide yields of the various catalysts shows definite differences 1n the ratios of these two species. Unfortunately, the finding subsequent to these tests that the source of much of the carbon oxides was associated with the metal reactor wall makes 1t difficult to determine Inherent catalyst select1v1t1es. Subtraction of the amount of carbon oxides formed with Just bare support 1n the reactor 1s not warranted, since the rate of oxidation of C s compared to methane 1s not known. 

Reactor Apparatus. The reactor systems used for obtaining data on n-dodecane dehydrogenation over molybdena-alumina and platinum-alumina catalysts and the data on reaction of bare supports with -dodo-cene have been described previously (12). The reactors used for obtaining data on n-dodecane dehydrogenation over chromia-alumina and... 

The high acidity of molybdena-alumina composites appear to be derived mainly not from the intrinsic acidity of the alumina but from active sites on a surface phase containing molybdenum atoms. The amount of LB and DTA formation that is observed initially over molybdenum-alumina is at least 10 times greater than observed over the bare support when the latter reacts with 8% 1-dodecene. In this case, the bare support was alumina B listed in Table II. 

In this study, precursors of the active phase have been introduced by equilibrium adsorption of alumina as well as of functionalized alumina with Co2Moio(Co) based impregnating solutions in order to maximize CoMoS phase formation. Characterization and catalytic performances in toluene hydrogenation of catalysts prepared on both functionalized and bare supports are presented in this work. 

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