Abstract
The catalytic oxidation of hydrocarbons and oxygenates used in the conversion of petroleum and biomass to chemicals and fuels results in formation of surface oxygen intermediates, namely chemisorbed oxygen (O*) and hydroxide (OH*) that can alter the reaction mechanism and control the overall catalytic activity and selectivity. The role of these intermediates and their influence on bond activation depends upon the nature of the metal that is used to carry out catalytic transformations, the specific reaction and the coverage of the adsorbate on the surface. For instance, O* or OH* can act as a Brønsted base reducing the barrier for O-H or C-H activation of alkanes or alcohols through H-abstraction mechanisms. Another example is when a bimetallic alloy is composed of a noble metal and an oxophilic metal, in which OH* bound to the oxophilic metal can act as a Brønsted acid that can catalyze dehydration and hydrogenolysis reactions.
On coinage (group IB) metals, the binding energy of O* or OH* is very weak compared to the binding energy on platinum group metals. Furthermore, the charge on the oxygen atom is more negative when O* or OH* is bound to coinage metals than when it’s bound to platinum group metals as there is significant charge transfer from the filled d-band to the oxygen. The weak binding energy and increased charge on the oxygen enable O* or OH* to act as a Brønsted base under a wide range of conditions, thus allowing for the H-abstraction of mildly acidic O-H bonds in alcohols and even non-acidic C-H bonds, such as those found in alkanes.
On platinum group metals, O* binds too strongly to the metal surface and is not basic enough, even at the bridge site, to help activate the C-H bonds of alkanes or alcohols. At higher coverages, the O* binding energy decreases, which leads to a greater basicity on the O* (reducing the barrier for O*-assisted C-H activation of methane). However, the O*-assisted activation does have a lower barrier for O-H activations, such as the initial activation of alcohols. For OH*-assisted R-H activation, the activation barriers are lower than the O*-assisted barriers for all metals except Cu and Ag.
During alcohol oxidation at high pH, the presence of hydroxide, in solution and adsorbed on the surface, can act as a base to catalyze the initial deprotonation of the alcohol, the hydride elimination of the alkoxide (on the surface) and the nucleophilic addition to the aldehyde, leading to the acid product. As the pH of the system decreases, the rate decreases until a point when a change in the mechanism occurs, in which direct C-H activation of the αC-H bond or O*-assisted O-H activation of the hydroxyl group takes over (depending on the coverage of O*).
During the hydrogenolysis of alcohols or cyclic ethers, noble metal catalysts (such as Rh or Pt) are promoted with partially reduced oxides of oxophilic metals (such as MoOx or ReO¬x). These partially reduced oxides hydroxylate in the presence of water, generating a OH* intermediate strongly bound to the oxophilic metal center, thus resulting in a Brønsted acid site. The acidity of such sites was studied for a wide range of alloys, to determine which combinations might produce acidic OH*. Further investigations studied the effect of the position and form of the alloy as well as the relationship between the acidity and the binding energy of O*, the covalent strength of the O-H bond and the binding energy of ammonia. The results indicate that as the binding energy of O* becomes stronger the strength of the O-H bond decreases thus increasing the acidity of the OH*. The binding energy of ammonia increases with increasing acidity non-linearly.
Through these studies, a set of rules can be established to determine the conditions for O* or OH* to act as a base or OH* as an acid when adsorbed on a metal surface (or alloy).
