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Hydrogenation over gold catalysts: The interaction of gold with hydrogen

Gold is active in various hydrogenation reactions and often shows exceptional high chemo-selectivity when multiple functional groups are present. All hydrogenation reactions have in common, that the hydrogen molecules have to adsorb and dissociate before they react. Recent insights in the interaction of gold with hydrogen and simple reactions involving hydrogen are described.


Introduction
Gold is a fascinating element. It has captured the imagination as a valuable object in art and decoration. More recently, it has made a remarkable entrance in the field of catalysis. For many types of reactions, it shows a unique performance. Among the first people who recognized the promise of gold as a catalyst was David Thompson. His review written together with Geoffrey Bond in Catalysis Reviews -Science and Engineering (1) described the early works about gold catalysts and their structure. This review has inspired many researchers throughout the world and contributed to gold being one of the most-studied elements in the field of catalysis. The continued development in catalysis by gold is most apparent by the new review of catalysis by gold, which now appeared in the form of a book co-written by Geoffrey Bond, Catherine Louis, and David Thompson (2). Without the scientific contribution, inspiration and personal engagement and encouragement of David Thompson, the field would not be where it is at the moment. Most of our scientific conversations with David centered around the use of gold in hydrogenation reactions, which is by far less investigated and understood than oxidation reactions. We dedicate this work to him.

Interaction of gold and hydrogen
In hydrogenation reactions, chemisorption and dissociation of hydrogen are the essential initial steps. There are only a few studies that investigate the interaction of hydrogen with gold. On Au(110) (1 x 2) surfaces no hydrogen chemisorption was observed (61). Weak adsorption of a small amount of hydrogen was found on thin, unsintered gold films: At a pressure of 0.3 Pa and a temperature of 78 K, the hydrogen coverage was less than 0.015 (62). Hydrogen desorbed at around 125 K. The hydrogen desorption activation energy was estimated to be around 12 kJ/mol. Low coordinated gold atoms on the film surface were suggested to act as adsorption sites. Lin and Vannice (63) detected weak, reversible adsorption of hydrogen on 30 nm gold particles on TiO 2 at 300 and 473 K leading to a coverage of about 1% of the total amount of gold. As the hydrogen uptake was found to be greater at 473 K than at 300 K it was suggested that the adsorption of hydrogen is an activated process and very likely associated with hydrogen dissociation. The chemisorption of hydrogen on Al 2 O 3 supported gold particles with a mean cluster size of about 4 nm was examined by Jia and coworkers (21). They observed that about 14% of the surface atoms adsorbed hydrogen almost irreversible at 273 K. The hydrogen chemisorption properties of Au/Al 2 O 3 and Au/SiO 2 catalysts were investigated by Bus et al. (64) in a temperature range of 298 to 523 K. For the 1 to 1.5 nm supported gold clusters, H/M values (number of adsorbed hydrogen atoms per total number of metal atoms) of at least 0.1 and as high as 0.73 were determined. However only 10-30% of the total adsorbed hydrogen did not desorb while evacuating for 2 h at the analysis temperature. The shapes of the hydrogen adsorption isotherms indicated that hydrogen chemisorbs dissociatively. The hydrogen uptake increased or was constant with temperature. This was ascribed to the hydrogen chemisorption on gold being activated. The Au/Al 2 O 3 catalyst with the smallest particle size of about 1 nm exhibited the highest hydrogen uptake per surface atom. At this  Because the d-band is narrow it interacts with the adsorbate by forming bonding and antibonding density of states. Thus both the electron density and the shape of the filled and empty densities of states of the metal d-band change (66). The changes in the spectra in Fig. 1 result from the bonding of hydrogen to the surface gold particles, which alters the electronic structure. The changes in the gold spectra were small compared to those of platinum, because the interaction of hydrogen with gold is weak and the coverage is low.
In summary, hydrogen chemisorbs on gold, though weakly and in small amounts. For both the hydrogen uptake and the strength of adsorption there is a clear particle size effect. With decreasing particle size, increasing amounts of hydrogen are chemisorbed and an increasing fraction is adsorbed strongly. Thus it can be concluded, that the chemisorption is limited to low coordinated corner and edge atoms, which was recently confirmed by theory (50).

H/D exchange over gold
So far the interaction of gold and hydrogen has been highlighted. Now we take the next step and look at the simplest reaction involving hydrogen, namely the hydrogen-deuterium exchange reaction (Scheme 1). Nieuwenhuys et al. (67) demonstrated that Au/Al 2 O 3 catalysts which had a gold particle size smaller than 3 nm catalyzed the hydrogen-deuterium exchange reaction even at room temperature.  sites were identified, which could be assigned via DFT calculations. These results were then correlated to those obtained from H/D exchange experiments, and it was confirmed that among the different gold surface sites only low coordinated gold atoms at corners and edges, not directly bonded to oxygen, were active for hydrogen dissociation.

Hydrogenation of simple alkenes
Another simple reaction involving hydrogen is the hydrogenation of simple alkenes. Only a few studies are found dealing with gold`s ability to catalyze this reaction, probably because of its low activity.
In an early study, Bond et al. (13) investigated the hydrogenation of ethene with SiO 2 supported gold catalysts with gold loadings varying from 0.01 to 5.0 wt% at 453 K (Scheme 2). The rate of the ethene hydrogenation per unit weight of gold increased as the gold loading decreased. The least active catalyst was 5 wt% Au/SiO 2 which contained particles of around 23 nm. This suggests that the fraction of smaller gold particles increased with decreasing gold loading. A close relation between the rate of the ethene hydrogenation and that of hydrogen/ deuterium exchange was observed, reflecting the dependence of both rates on the hydrogen activation on the catalyst surface. Au/TiO 2 , prepared via micelle encapsulation which resulted in a mean particle size of 8 nm catalyzed the hydrogenation of propylene (Scheme 3) with a TOF of 0.04 s -1 (per surface atom) (12). Supported on other oxides such as ZnO, ZrO 2 and SiO 2 , prepared by the same method gold was not active, which was attributed to the formation of larger clusters. Gold nanoparticles supported on amorphous silica prepared by autoreduction of the gold precursor by the support were active in the liquid phase hydrogenation of cyclohexene at 353 K and 220 bar hydrogen pressure (10) (Scheme 4). The selectivity towards cyclohexane was 100%. However, the mean cluster sizes were large, ranging from 10 to 20 nm with broad size distributions. The smallest particles exhibited the highest catalytic activity. Bus et al. (45) investigated the activity of Au/Al 2 O 3 with a mean particle of 1 nm in the liquid phase hydrogenation of cyclohexene at 373 K and a hydrogen pressure of 80 bar (Scheme 4). The TOF per H adsorbing gold atom was 0.11 s -1 , which was two orders of magnitude lower than a platinum catalyst of similar size. The activity of the Au/Al 2 O 3 catalyst increased with increasing temperature, hydrogen pressure, and concentration of cyclohexene, suggesting positive order in both reactants. The TOF defined by the number of sites that adsorbs hydrogen was not influenced by the gold particle size. Upon reuse, no deactivation was observed.
In summary, gold is active in gas phase and liquid phase hydrogenation of simple alkenes, although much less than other transition metals such as platinum and palladium. This behavior can be attributed to the weak interaction of gold with both hydrogen and the C=C bond.

Conclusion
Gold activates hydrogen, provided that it is present as nano-sized particles, which are generally stabilized on a support. A strong particle size effect is observed: the smaller the particles are, the more hydrogen atoms adsorb on the gold, which has led to the conclusion that it is the edges and corners that adsorb the hydrogen. The role of the support in hydrogenation is much less understood. The adsorption is activated and dissociated, which is shown by the ability of gold catalysts to catalyze the exchange of hydrogen and deuterium and the hydrogenation of simple alkenes. The reactivity is significantly lower than supported metal catalysts that are typically used, which opens the possibility of reaching exceptionally high selectivity when multiple functional groups are present. Their combination provides insight into the structure and function of the catalytically active sites.