The preceding observation that surface oxygen is not only critical for the removal of hydrogen adatoms but also to suppress decarbonylation of selox products over metallic palladium is in excellent agreement with an in situ ATR-IR study of cinnamyl alcohol selox over Pd/Al2O3 [148].

In related earlier investigations employing aqueous electrochemical protocols, the same researchers postulated that oxidative dehydrogena­tion of alcohols requires PGM catalysts in a reduced state, hypothesis­ing that ‘over-oxidation’ was responsible for deactivation of palladium selox catalysts [69]. A subsequent operando X-ray absorption spectros­copy (XAS) study by Grunwaldt et al. [150], bearing remarkable simi­larity to an earlier study to the author of this review [149], evidenced in situ reduction of oxidised palladium in an as-prepared Pd/Al2O3 catalyst during cinnamyl alcohol oxidation within a continuous flow fixed-bed reactor. Unfortunately the reaction kinetics were not measured in paral­lel to explore the impact of palladium reduction, however, a follow-up study of 1-phenylethanol selox employing the same reactor configura­tion (and oxygen-deficient conditions) evidenced a strong interplay be­tween selox conversion/selectivity and palladium oxidation state [151].

It was concluded that metallic Pd was the catalytically active species, an assertion re-affirmed in subsequent in situ ATR-IR/XAS measurements of benzyl [152-154] and cinnamyl alcohol [155] selox in toluene and under supercritical CO2, respectively, wherein the C=O stretching intensity was assumed to track alcohol conversion. It is interesting to note that the in­troduction of oxygen to the reactant feed in these infrared studies dramati­cally improved alcohol conversion/aldehyde production (Fig. 12), which was attributed to hydrogen abstraction from the catalyst surface [156, 157] rather than to a change in palladium oxidation state. In contrast to their liquid phase experiments, high pressure XANES and EXAFS measure­ments of Pd/Al2O3 catalysed benzyl alcohol selox under supercritical CO2 led Grunwaldt and Baiker to conclude that maximum activity arose from particles mainly oxidised in the surface/shelfedge [48].

FIGURE 12: Impact of oxygen on the selective oxidation of (top left) cinnamyl alcohol; (bottom left) 1-phenylethanol; and (right) 2-octanol. Adapted from references [148, 151, 154] with permission from Elsevier

In a parallel research programme, the author’s group systematically characterised the physicochemical properties of palladium nanoparticles as a function of size over non-reducible supports to quantify structure — function relations in allylic alcohol selox [133-137, 158, 159]. The com­bination of XPS and XAS measurements revealed that freshly prepared alumina [134, 137] and silica [135, 158] supported nanoparticles are prone to oxidation as their diameter falls below ~4 nm, with the fraction of PdO proportional to the support surface area and interconnectivity.

Complementary kinetic analyses uncovered a direct correlation between the surface PdO content and activity/TOFs towards cinnamyl and crotyl alcohol selox [134, 137]. Operando liquid phase XAS of Pd/C and Pd/ Al2O3-SBA-15 catalysts during cinnamyl alcohol selox evidenced in situ reduction of PdO (Fig. 13), however, by virtue of simultaneously measur­ing the rate of alcohol selox, Lee et al. were able to prove that this oxide ^ metal structural transition was accompanied by coincident deactivation. Together these findings strongly implicate a (surface) PdO active phase, consistent with surface science predictions that metallic palladium favours aldehyde decarbonylation and consequent self-poisoning by CO and or­ganic residues [143, 160], akin to that reported during fatty acid decarbox­ylation over Pd/MCF [161].

To conclusively establish whether oxide or metal is responsible for alcohol selox catalysed by dispersed palladium nanoparticles, a multi­dimensional spectroscopic investigation of vapour phase crotyl alcohol selox was undertaken (since XAS is an averaging technique a complete understanding of catalyst operation requires multiple analytical techniques [162-164]). Synchronous, time-resolved DRIFTS/MS/XAS measure­ments of supported and colloidal palladium were performed in a bespoke environmental cell [165] to simultaneously interrogate adsorbates on the catalyst surface, Pd oxidation state and reactivity under transient condi­tions in the absence of competitive solvent effects [166, 167]. Under mild reaction temperatures, palladium nanoparticles were partially oxidised, and unperturbed by exposure to sequential alcohol or oxygen pulses (Fig. 14). Crotonaldehyde formed immediately upon contact of crotyl alcohol with the oxide surface, but only desorbed upon oxygen co-adsorption. Higher reaction temperatures induced PdO reduction in response to crotyl alcohol exposure, mirroring that observed during liquid phase selox, how­ever, this reduction could be fully reversed by subsequent oxygen expo­sure. Such reactant-induced restructuring was exhibited by all palladium nanoparticles, but the magnitude was inversely proportional to particle size [168]. These dynamic measurements decoupled the relative reactivity of palladium oxide from metal revealing that PdO favoured crotyl alco­hol selox to crotonaldehyde and crotonic acid, whereas metallic palladium drove secondary decarbonylation to propene and CO in accordance with surface science predictions [143].

FIGURE 14: (Left) Cartoon of operando DRIFTS/MS/XAS reaction cell and resulting temperature dependent behaviour of Pd oxidation state and associated reactivity towards crotyl alcohol oxidation over a Pd/meso-Al2O3 catalyst—only selective oxidation over surface PdO occurs at 80 °C, whereas crotonaldehyde decarbonylation and combustion dominate over Pd metal at 250 °C; (top right) relationship between Pd oxidation derived in situ and crotyl alcohol conversion; (bottom right) summary of reaction-induced redox processes in Pd-catalysed crotyl alcohol selox. Adapted with permission from references [166, 168]. Copyright 2011 and 2012 American Chemical Society

Recent ambient pressure XPS investigations of crotyl alcohol/O2 gas mixtures over metallic and oxidised Pd(111) single crystal surfaces con­firmed that only two-dimensional Pd5O4 and three-dimensional PdOx surfaces were capable of crotonaldehyde production (Fig. 15) [169]. However, even under oxygen-rich conditions, on-stream reduction of the Pd5O4 monolayer oxide occurred >70 °C accompanied by surface poisoning by hydrocarbon residues. In contrast, PdOx multilayers were capable of sustained catalytic turnover of crotyl alcohol to crotonalde — hyde, conclusively proving surface palladium oxide as the active phase in allylic alcohol selox.

FIGURE 16: Comparative activity of Pd nanoparticles dispersed over amorphous, 2D non­interconnected SBA-15 and 3D interconnected SBA-16 and KIT-6 mesoporous silicas in the selective aerobic oxidation of crotyl alcohol. Adapted from reference [135]. Copyright 2011 American Chemical Society