Correlating Heat of Adsorption of CO to Reaction Selectivity: Geometric Effects vs Electronic Effects in Neopentane Isomerization over Pt and Pd Catalysts

Abstract

Silica-supported Pt and Pd nanoparticles from 1 to 10 nm in diameter were evaluated for neopentane conversion (hydrogenolysis and isomerization). Characterization of the catalysts was conducted utilizing scanning transmission electron microscopy (STEM), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, X-ray absorption spectroscopy (XAS), and isothermal calorimetry of CO adsorption to determine how geometric or electronic structure effects can explain changes in reactivity. Isomerization selectivity of Pt was much higher than Pd for all particle sizes. There is a pronounced effect of particle size on selectivity, with the highest isomerization selectivity achieved over catalysts containing the largest particle size for both Pt (57%) and Pd (26%) catalysts. For both Pd and Pt catalysts, DRIFTS showed a decrease in the ratio of linear-to-bridge bonded CO with particle size, while isothermal calorimetry of CO adsorption shows that both Pt and Pd enthalpies of adsorption decrease with increasing particle size. The isomerization selectivity was found to correlate inversely with the strength of CO adsorption for all catalysts suggesting that the chemisorption energy and not the particle size, coordination geometry, or ensemble size is the most important factor for increasing the isomerization selectivity.

Evidence for geometric effects in neopentane conversion on PdAu catalysts

“…The Pd-Pd bond distance in all of the catalysts is 2.80 Å, an expansion by 0.05 Å from the metallic bond distance of 2.75 Å. 12 This expansion in the Pd-Pd bond distance could be the result of strain due to the epitaxial growth of Pd over Au. 25 Differences in the coordination number between Pd-Pd and Pd-Au show the majority of the palladium atoms in this catalyst are bonded to other palladium, as would be expected for a core-shell configuration.…”

“…The monometallic Pd sample showed a single-point isomerization selectivity of almost 40% at low conversion (<1%), about a 15% increase from the selectivity found in previous work. 12 PdAu catalysts with lower Au content (0.05-0.20 wt.%) showed either no change or a slight decrease in isomerization selectivity whereas at a loading of 0.40 wt.% Au, the isomerization selectivity increased to almost 50% indicating a positive effect on selectivity.…”

“…Although the selectivity does not change drastically with conversion, the selectivities reported are those obtained by extrapolation to zero conversion. 12 Each flow rate was run for at least one hour to ensure steady-state conversion had been reached. An Agilent 6890 N gas chromatograph (using a …”

“…12 However, it is not clear if the relationship between isomerization selectivity and the electronic structure extends beyond monometallic systems. PdAu provides an interesting test case for electronic structure correlation since Au has previously been used as a modifier for improved selectivity in hydrocarbon reforming.…”

n-Hexane Hydrogenolysis Behavior of Alumina-Supported Palladium–Platinum Alloys

“…The weakening of the CO-metal bond implies also a weakening of the alkane-metal bond. Such parallelism was invoked by Childers et al [31], who even proposed a linear relationship between the energies of CO and hydrocarbon adsorption in the form: E CO,ads = 0.60E CH3,ads − 0.09 (energies in eV). It appears that the weakening of the alkane-metal bonding would lead to an increased internal rupture of n-hexane at the expense of its terminal demethylation.…”