The rational design and optimisation of palladium selox catalysts require a microscopic understanding of the active catalytic species responsible for alcohol and oxygen activation, and the associated reaction pathway to the aldehyde/ketone products and any competing processes. A key char­acteristic of palladium is its ability to perform selox chemistry at tem­peratures between 60 and 160 °C and with ambient oxygen pressure [39, 142] via the widely accepted oxidative dehydrogenation route illustrated in Scheme 3 [39, 67]. Whether O-H or C-H scission of the a-carbon is the first chemical step remains a matter of debate, since the only fundamental studies over well-defined Pd(111) surfaces to date employed temperature — programmed XPS [143] and metastable de-excitation spectroscopy (MDS) [144] with temporal resolutions on the second ^ minute timescale, over which loss of both hydrogens appears coincident. However, temperature — programmed mass spectrometric [145] and vibrational [146] studies of unsaturated C1-C3 alcohols implicate O-H cleavage and attendant alkoxy formation over Pd single crystal surfaces as the first reaction step [142, 147]. It is generally held that the resultant hydrogen adatoms react with dissociatively absorbed oxygen to form water, which immediately desorbs at ambient temperature thereby shifting the equilibrium to carbonyl for­mation [39, 67]. Temperature-programmed XPS studies of crotyl alcohol adsorbed over clean Pd(111) [143] prove that oxidative dehydrogenation to crotonaldehyde occurs at temperatures as low as -60 °C (Fig. 11), with alcohol dehydration to butane only a minor pathway. These ultra-high vac­uum measurements also revealed that reactively formed crotonaldehyde undergoes a competing decarbonylation reaction over metallic palladium above 0°C yielding strongly bound CO and propylidene which may act as site-blockers poisoning subsequent catalytic selox cycles, coincident with evolution of propene into the gas phase. Unexpectedly, pre-adsorbed atomic oxygen switched-off undesired decarbonylation chemistry, pro­moting facile crotonaldehyde desorption.