Acid strength and bifunctional catalytic behavior of alloys comprised of noble metals and oxophilic metal promoters

• Oxophilic metal oxide clusters promote noble metals and form Brønsted acid sites.
• Acid sites involve OH species bound to the partially oxidized promoter metal.
• The acidity of these sites depends on the structure and composition of the alloy.
• Activation barriers for ring opening of cyclic ethers correlate with the acidity.
• Deprotonation energies and NH3 adsorption energies provide two probes for acidity.

The promotion of metal catalysts with partially oxidized oxophilic MOx species, such as ReOx-promoted Rh, has been demonstrated to produce Brønsted acid sites that can promote hydrogenolysis of oxygenate intermediates such as those found in biomass-derived species. A wide variety of alloy compositions and structures are examined in this work to investigate strongly acidic promoters by using DFT-calculated deprotonation energies (DPE) as a measure of acid strength. Sites with the highest acid strength had DPE less than 1100 kJ mol−1, similar to DPE values of heteropolyacids or acid-containing zeolites, and were found on alloys composed of an oxophilic metal (such as Re or W) with a noble metal (such as Rh or Pt). NH3 adsorbs more strongly to sites with increasing acid strength and the activation barriers for acid-catalyzed ring opening of a furan ring decrease with increasing acid strength, which was also shown to be stronger for OH acid sites bound to multiple oxophilic metal atoms in a three-fold configuration rather than OH sites adsorbed in an atop configuration on one oxophilic metal, indicating that small MOx clusters may yield sites with the highest acid strength.

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