Chemisorption suppression platinum

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. 

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). 

The titania supported platinum systems showed complete suppression in the hydrogen chemisorption unlike the system involving the mixed oxide support. 

In general terms, the chemisorption of H2 on the platinum metals supported on Ti02 always decreases as the temperature of reduction is raised. However, the extent of the suppression of chemisorption and the minimum temperature required to achieve a significant effect is less clear. It certainly seems possible that a suppression may be detected after reduction at temperatures as low as 473 K, which if correct has profound implications regarding the nature of the SMSI. It is also worth noting that there is some evidence of a trend, in that the tendency of a metal to enter the SMSI state seems to decrease as one moves from Group Vlll(i) to Group Vlll(ii). There is some evidence that Re is even more susceptible to interaction with titania. - ... 

Ti02-supported Ni has been studied in great detail, and the fairly low H/M ratios have been taken as evidence for the existence of SMSI. However, the results for Ti02-supported Ni differ in two important respects from those for the platinum metals on Ti02. First, there is no information available to show that for Ni/Ti02 there is a suppression of chemisorption when the reduction temperature is increased by a small amount (say from 600 to 750 K). Second, Ni dispersions of 5-10% are not necessarily very low. Thus, it is well known that Ni (and Fe and Co) tend to be poorly dispersed even on a variety of supports where no SMSI is suspected. When one considers that the Degussa P-25 Ti02 which is almost universally used for this work has a surface area of only 50 m g , as compared with perhaps 300 m ... 

In contrast to the platinum metals, there is no clear evidence from chemisorption experiments that there is a suppression of the amount of adsorption of H2 or CO on Ni supported on Ti02. This may also be true for Co and Fe, providing in the latter case that allowance is made for some iron being lost into the support. 

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