Chemisorption suppression

Another approach to the chemisorption-overlayer question is to determine the degree of coverage with Auger spectroscsopy and compare this with the amount of chemisorption suppression. Two such studies have been reported and have reached opposite conclusions. In one case, varying amounts of Ti were deposited onto Pt and then oxidized to TiOx. H2 and CO adsorptions were found to... 

The SMSI induced chemisorption suppression could be reversed in these thin film samples by heating 1n oxygen. This fact is demonstrated in Fig. 4 where the left hand panel contains CO TPD results from the previously encapsulated samples after oxidation (875 K, P ... 

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... 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

Figure B3.2.12. Schematic illustration of geometries used in the simulation of the chemisorption of a diatomic molecule on a surface (the third dimension is suppressed). The molecule is shown on a surface simulated by (A) a semi-infinite crystal, (B) a slab and an embedding region, (C) a slab with two-dimensional periodicity, (D) a slab in a siipercell geometry and (E) a cluster.

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... 

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

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 ... 

In conclusion, we have seen that an applied held has the ability to strongly affect the chemisorption process. One trend, clearly observable in both Fig. 7.6(b) and Fig. 7.7(b), is that the sign of F determines whether or not Aq is enhanced by the presence of the held, i.e., Aq is increased (decreased) when F is positive (negative). More variable is the dependence of AE on F, due to the variability in the existence (or not) of Absolutions, which, when they do occur, represent the more stable interaction. Consequently, the presence of the held may either enhance or suppress the chemisorption process. 

The suppression of H2 adsorption on Ni/A.C. suggests that a unique interaction with activated carbon exists. The large chemisorption of CO and its TPR spectrum indicate the formation of subcarbonyl. 

Bi does not adsorb hydrogen, thus a Bi/Pt coverage can be calculated from the hydrogen chemisorption data. It is seen in Figure 2 that there is an excellent correlation between the Bi-coverage of Pt and the rate of 1-phenylethanol oxidation. It seems that the hydrogen chemisorption ability of Pt or the size of active sites ensembles has to be minimized to avoid deactivation. There are indications in the literature that the suppression of hydrogen sorption on a Pt electrode can eliminate the poison formation (20). 

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