Dilute Pd-in-Au alloy catalysts are promising materials for selective hydrogenation catalysis. Extensive surface science studies have contributed mechanistic insight on the energetic aspect of hydrogen dissociation, migration and recombination on dilute alloy systems. Yet, translating these fundamental concepts to the kinetics and free energy of hydrogen dissociation on nanoparticle catalysts operating at ambient pressures and temperatures remains challenging. Here, the effect of the Pd concentration and Pd ensemble size on the catalytic activity, apparent activation energy and rate limiting process is addressed by combining experiment and theory. Experiments in a flow reactor show that a compositional change from 4 to 8 atm% Pd of the Pd-in-Au alloy catalyst leads to strong increase in activity, even exceeding the activity per Pd atom of monometallic Pd under the same conditions, albeit with an increase in apparent activation energy. First-principles calculations show that the rate and apparent activation enthalpy for HD exchange increase when increasing the Pd ensemble size from single Pd atoms to Pd trimers in a Au surface, suggesting that the ensemble size distribution shifts from mainly single Pd atoms on the 4 atm% Pd alloy to larger Pd ensembles of at least three atoms for the 8 atm% Pd/Au catalyst. The DFT studies also indicated that the rate-controlling process is different: H 2 (D 2 ) dissociation determines the rate for single atoms whereas recombination of adsorbed H and D determines the rate on Pd trimers, similar to bulk Pd. 2Both experiment and theory suggest that the increased reaction rate with increasing Pd content and ensemble size stems from an entropic driving force. Finally, our results support hydrogen migration between Pd sites via Au and indicate that the dilute alloy design prevents the formation of subsurface hydrogen, which is crucial in achieving high selectivity in hydrogenation catalysis.
Ethanol can be converted to heavy diesel ethers and jet fuel precursor olefins through sequential Guerbet coupling and dehydration.
Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multi-pronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge which increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogenous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step towards developing structure function relationships that inform the design of new catalysts.
The reaction pathway and products of cellulose supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) were investigated. Monoalcohols, diols, alcohol ethers, and methyl esters were produced from cellulose at 300 °C with a CuMgAl mixed metal-oxide catalyst. Timecourse experiments show that cellulose is rapidly solubilized and depolymerized within 1 h with C 2 −C 4 diols being intermediates. Experiments with glucose-13 C 6 show that methanol is incorporated in all liquid products accounting for approximately 30−40% of the carbon in these products. Experiments with model compounds (dihydroxyacetone, isosorbide, and 5-hydroxymethylfurfural) indicate that the reaction pathway for cellulose occurs primarily through retroaldol condensation of solubilized cellulose followed by recondensation with methanol. Methanol produces H 2 , CO, and CO 2 through reformation with 30% of the generated H 2 being incorporated into the liquid products. Analysis of the liquid products with Fourier transform ion cyclotron resonance MS (FT-ICR MS) measured C 7 −C 12 partially oxygenated species with 2−6 double bond equivalence which could not be detected via gas chromatography (GC). We conclude that the reaction pathway occurs through rapid solubilization and depolymerization of cellulose followed by retro-aldol condensation to C 2 −C 4 oxygenates. Retro-aldol condensation products undergo hydrodeoxygenation and extensive carbon−carbon coupling to produce C 2 −C 7 alcohols or other oxygenates.
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