Ethene Adsorption, Dehydrogenation and Reaction with Pd(110):  Pd as a Carbon ‘Sponge'

Abstract

The interaction of ethene with the Pd(110) surface has been investigated, mainly with a view to understanding the dehydrogenation reactions of the molecule and mainly using a molecular beam reactor. Ethene adsorbs with a high probability over the temperature range 130 to 800 K with the low-coverage sticking probability dropping from 0.8 at 130 K to 0.35 at 800 K. The adsorption is of the precursor type, with a weakly held form of ethene being the intermediate between the gas phase and strong chemisorption. Dehydrogenation begins at approximately 300 K and is fast above 350 K. If adsorption is carried out at temperatures up to approximately 380 K, adsorption saturates after about 0.25 monolayer have adsorbed, but above approximately 450 K, adsorption continues at a high rate with continuous hydrogen evolution and C deposition onto the surface. It appears that, in the intermediate temperature range, the carbonaceous species formed is located in the top layer and thus interferes with adsorption, whereas the C goes subsurface above 450 K, the adsorption is almost unaffected, and the C signal is significantly attenuated in XPS. However, the deposited carbon can easily be removed again by reaction with oxygen, thus implying that the carbon remains in the selvedge, that is, in the immediate subsurface region probably consisting of a few atomic layers. No well-ordered structures are identified in either LEED or STM, though some evidence of a c(2 x 2) structure can be seen. The Pd surface, at least above 450 K, appears to act as a "sponge" for carbon atoms, and this effect is also seen for the adsorption of other hydrocarbons such as acetaldehyde and acetic acid.

Catalytic Oxidation of Ethene on Polycrystalline Palladium: Influence of the Oxidation State of the Surface

“…In order to prove this assumption we increased the carbon content of the surface and near-surface bulk regions by exposing the Pd surface to 6.0 mbar ethene at 493 K prior to reaction. At this temperature ethene decomposes and leaves a carbon-modified surface, but C atoms also migrate into surface near regions of the Pd bulk [15,16,9]. The reaction of 6 mbar ethene with 250 mbar oxygen was now repeated, starting from this 'PdC x ' surface as an initial state.…”

Kinetic and Spectroscopic Studies of Vinyl Acetate Synthesis Over Pd(100)

“…The more intense peak at 284.2 eV is assigned to a surface CH x (x=0-3) species formed from the dissociation of an ethylenic and/or acetate species. The features at 285.5 and 288.0 eV are assigned to CO and an acetate species, respectively [19][20][21][22].…”

“…Madix et al [25], have reported that the saturation coverage for CO on Pd(100) at room temperature is 0.53 ML. By comparing the peak area of the C 1s is [19] and acetic acid [26] to be approximately 1.5 ML. These authors report that of this 1.5, 0.5 ML is on the topmost surface while the remainder is interdiffused into the top several layers of the catalyst [18,19,26].…”

“…By comparing the peak area of the C 1s is [19] and acetic acid [26] to be approximately 1.5 ML. These authors report that of this 1.5, 0.5 ML is on the topmost surface while the remainder is interdiffused into the top several layers of the catalyst [18,19,26]. Figure 6a shows C 1s XPS spectra acquired at room temperature after reaction and after flashing the Pd(100) catalyst to various temperatures.…”

Migration of Carbon into Subsurface Layers of Rh(100): A DFT Study

“…Nevertheless, its existence was proven for the PtA C H T U N G T R E N N U N G (110) [5] and PdA C H T U N G T R E N N U N G (110) [6] surfaces. Theoretical work on subsurface carbon is relatively scarce as well.…”