Kinetically Limited CO Adsorption: Spill‐Over as a Highly Effective Adsorption Pathway on Bimetallic Surfaces

Abstract

We demonstrate that the (local) adsorbed carbon monoxide, COad , coverage on the Pt-free areas of bimetallic Pt/Ru(0001) surfaces (a Ru(0001) substrate partly covered by Pt monolayer islands) can be increased to ∼0.80 monolayers (ML), well above the established saturation COad coverage of 0.68 ML, even under ultrahigh vacuum conditions by using spill-over of CO adsorbed on the Pt islands to the Ru areas as an highly effective adsorption channel. The apparent COad saturation coverage of 0.68 ML on pure Ru(0001) is identified as due to kinetic limitations, hindering further uptake from the gas phase, rather than being caused by thermodynamic reasons. This spill-over mechanism is proposed to be a general phenomenon for adsorption on bimetallic surfaces.

Interaction of Coadsorbed CO and Deuterium on a Bimetallic, Pt Monolayer Island Modified Ru(0001) Surface

“…Considering our above explanation of this D 2 desorption peak, where we attributed this to displacement of D ad due to thermally activated spillover of CO ad from Pt monolayer sites to Ru(0001) sites, the present results must mean that the CO ad spillover is rather slow at 150 K, leaving enough CO ad on Pt monolayer sites that can spillover to the Ru(0001) areas during subsequent desorption to create the 200 K peak. This agrees well with the IR data in Figure , which also indicated that CO ad spillover on the mixed adlayer covered Pt 0.3‑ML /Ru(0001) surface occurs mainly in the temperature range 180–200 K. In contrast, for a pure CO adlayer CO ad migration was found to be activated at about 150 K. , …”

“…Here it should be noted that a similar spillover process of CO adsorbed on Pt monolayer island modified Ru(0001) surfaces was reported before in TPD measurements recorded after lowtemperature CO adsorption (90 K). 31,32 In that case, spillover of CO ad was activated at ∼150 K, and it was found to result in significant modifications of the CO TPD spectra. It should further be noted that for adsorption at 90−100 K deuterium desorption from the Pt monolayer islands starts right at the adsorption temperature, which means that additional deuterium moved to these sites will desorb instantaneously above a critical D ad coverage.…”

“…It is important to note, however, that the resulting TPD peaks do not reflect the distribution of the CO ad molecules directly after adsorption at 90 K but the situation at the respective desorption temperature, which can be influenced by adsorbate migration in the course of the TPD run. 31,32 This point will be addressed in more detail below, together with the IR results.…”

“…Considering the higher adsorption energies of CO on the Ru and Pt sites compared to those of deuterium, this result agrees with expectations. In addition, the mobility of CO ad on the surface at 90 K is too low to enable site exchange processes (see refs and ). Smaller effects caused by the onset of CO ad mobility during the heating ramp in the TPD experiments will be discussed in section .…”

“…Furthermore, these studies revealed a distinct kinetic effect in CO adsorption. 31,32 While for Ru(0001) CO adsorption is well-known to saturate at 0.68 monolayers (ML), it was found that spillover of CO adsorbed on the Pt monolayer islands to the Ru(0001) areas may increase the local CO ad density in these areas up to a maximum value above 0.8 ML. Spillover is kinetically hindered, however, at low temperatures and activated only at ∼150 K, e.g., during a thermal desorption experiment, which causes distinct modifications in the CO TPD spectra recorded on these bimetallic Pt/Ru(0001) surfaces.…”