Chemisorption suppression catalysts

Chemisorption suppression via a simple site blocking mechanism is not consistent with enhanced methanation rates observed for Pt/Ti02 catalysts in the SMSI state.(2-5) A simple loss of CO adsorption sites without a substantial change in the adsorption energy should result in a decrease in catalytic activity. It is likely then that... 

Pt. Methanation on Pt catalysts have been studied a number of times. Yang et al. [22] studied methanation on a 2 wt% Pt/Ti02 catalyst reduced at 225°C to avoid SMSI and chemisorption suppression. At the reaction temperature of 225°C and for H2/CO = 3, the TOFchem based on CO chemisorption was... 

To Illustrate the utility of the technique, we have addressed the question of the anomalous chemlsorptlve behavior of tltanla-supported group VIII metals reduced at high temperatures. The suppression of strong H2 chemisorption on these catalysts has been ascribed to a strong-metal-support Interaction (SMSI) ( ). It has also been found that the reaction activity and selectivity patterns of the catalysts are different In normal and SMSI states... 

Another important factor affecting carbon deposition is the catalyst surface basicity. In particular, it was demonstrated that carbon formation can be diminished or even suppressed when the metal is supported on a metal oxide carrier with a strong Lewis basicity [47]. This effect can be attributed to the fact that high Lewis basicity of the support enhances the C02 chemisorption on the catalyst surface resulting in the removal of carbon (by surface gasification reactions). According to Rostrup-Nielsen and Hansen [12], the amount of carbon deposited on the metal catalysts decreases in the following order ... 

An interesting explanation for Pt/TiOz catalysts in the SMSI state, which tends to suppress the chemisorption of H2, has been advanced by Chen and White (97). If Ti02 is reduced by H2 at 875°C before deposition of Pt, the SMSI state is also observed. If this reduced Ti02 is reoxidized before the deposition of Pt, the catalyst irreversibly adsorbs H2 at room temperature, since any Pt/Ti02 reduced at low temperatures and the SMSI state is not achieved. It may, therefore, be concluded that the nature of Ti ions should be correlated with the SMSI state. Further, Pt deposited on Ti203 or on TiO does not adsorb H2 (as in the SMSI state). High-temperature reduction of the... 

Burch and Flambard (113) have recently studied the H2 chemisorption capacities and CO/H2 activities of Ni on titania catalysts. They attributed the enhancement of the catalytic activities for the CO/H2 reaction (after activation in H2 at 450°C) to an interfacial metal-support interaction (IFMSI). This interaction is between large particles of Ni and reduced titanium ions the Ti3+ is promoted by hydrogen spillover from Ni to the support, as pictured in Fig. 8. The IFMSI state differs from the SMSI state since hydrogen still chemisorbs in a normal way however, if the activation temperature is raised to 650°C, both the CO/H2 activity and the hydrogen chemisorption are suppressed. They define this condition as a total SMSI state. Between the temperature limits, they assumed a progressive transition from IFMSI to SMSI. Such an intermediate continuous sequence had been... 

These methods suggested in the present form by Caunt83) rely on inhibition (retardation) effects of strong catalyst poisons on polymerization. Typical poisons potentially usable for this purpose are carbon oxides, carbonyl sulfide, carbon disulfide, acetylenes and dienes. All these substances exhibit a strong unsaturation they have either two double bonds or one triple bond. Most of the works devoted to application of the poisons to determination of active centers 10,63 83 102 1O7) confirm a complicated nature of their interaction with the catalytic systems. To determine the active centers correctly, it is necessary to recognize and — as much as practicable — suppress side processes, such as physical adsorption and chemisorption on non-propagative species, interaction with a cocatalyst, oligomerization and homopolymerization of the poison and its copolymerization with the main chain monomer. 

In accordance with the CO/M data summarized in Table 4.5, the apparent CO/M values, typically decrease with the reduction temperature. However, as already noted for the hydrogen adsorption, no drastic inhibition effects are generally observed. In good agreement with the volumetric data, most of the FTIR studies of CO chemisorbed on Pt/CcOj (130,131,133), Pd/CeO (64,78), and Rh/Ce02 (219) also show partial deactivation effects. An exception to this rather general observation is the case of a Pd/Ce02 catalyst, for which a complete suppression of the CO chemisorption capability (78) has been reported to occur. 

One of the main aspeets that determine the properties of a given catalyst is the nature of the interaction between the oxide support and the dispersed active metal. The influence of this interaction on the activity, selectivity and stability of the catalyst is determined by factors such as the preparation method, the atmosphere and temperature of the calcination and reduction stages (1,2), and the specific metal-support system studied. The latter is especially important for catalysts based on transition metals supported on partially reducible oxides (3). Such catalysts displaying strong metal-support interaction (SMSI) exhibit suppression of chemisorption of H2 and CO (4-6) when reduced at temperatures up to 773 K. 

O2 chemisorption. Whereas the chemisorption of hydrogen is suppressed in the SMSI state, chemisorption of oxygen still takes place after a metal-on-Ti<>2 catalyst has been reduced at high temperature. No information is available, however, whether there is a difference in O2 chemisorption in SMSI and normal state and whether the reduction temperature has any influence on this. Therefore we looked into the O2 chemisorption capacity of Rh/R-Ti<>2 and Rh/A-Ti(>2 catalysts at room temperature and 8.10 Pa after successive reductions and evacuations (1 h each) at temperatures of 200, 245, 280, 3S0 and 500 C. The results are presented in Figures 3A, B, C, 0 and E, respectively. To ensure that the starting conditions were the same in all cases, before each reduction-evacuation treatment and subsequent O2 chemisorption experiment the catalyst was oxidized at 150 C for O.S h because, as shown in the foregoing,... 

Chemical Measurements. All the catalysts in Table I showed a suppression in hydrogen chemisorption to varying extent. Such a suppression manifested itself either as an overestimation of crystallite size from chemisorption data or as a lower adsorption stoichiometry of H/Ni,, than what would be expected of a comparable Ni/Si02 catalyst. In addition, the suppression was more severe with increasing reduction temperature (8-10). As mentioned... 

The scope of the present paper is to emphasize that the interactions between support, metal and atmosphere are responsible for both the physical (size distribution, shape of the crystallites, wettability of the substrate by the crystallites and vice versa), the chemical and the catalytic (suppression of chemisorption, increased activity for methanation, etc.) manifestations of the supported metal catalysts. In the next section of the paper, a few experimental results concerning the behaviour of iron crystallites on alumina are presented to illustrate the role of the strong chemical interactions between the substrate and the compounds of the metal formed in the chemical atmosphere. Surface energetic considerations, similar to those already employed by the author (7,8), are then used to explain some of the observed phenomena. Subsequently, the Tauster effect is explained as a result of the migration, driven by strong interactions,... 

Surface thermodynamic considerations can be helpful in an understanding of the complex phenomena which occur in supported metal catalysts. Indeed, the physical and chemical interactions between metal, substrate and atmosphere lead to wetting and spreading phenomena (of the active catalyst over the substrate and of the substrate over the metal) which are relevant for the physical (sintering, redispersion) as well as chemical (suppression of chemisorption, modification of selectivity, enhanced activity) manifestations of supported metal catalysts. 

The dried samples were either reduced in flowing hydrogen (30 ml min ) at 450 °C using a ramp of 0.12 °C min or previously calcined in air at the same temperature, and then reduced in similar conditions. As C solid state MAS-NMR showed during the thermal treatment, the stabiliser ligand was decomposed leading to carbonaceous deposits. The H2-chemisorption was much suppressed, indicating that these carbonaceous deposits cover the colloid surface (Table 3). The same conclusion results from the textural measurements and catalytic activity data. Both reduced and calcined and reduced catalysts exhibited very small surface areas at least three times smaller than only dried catalysts. 

The authors have been studying the effect of alkaline addition on the catalytic activity of automotive three-way catalysts. We have found that the addition of Ba to Pd or platinum(Pt) three-way catalysts is effective for improvement of catalytic activity under reducing conditions, and that the suppression of hydrocarbon(HC) chemisorption on the catalysts by the addition of Ba allowed the catalytic reaction to proceed smoothly (16,17). 

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