(700f) A Generalized Thermodynamic Activity and Reactivity Relation of Metal and Oxide Clusters and Their Periodic Reactivity Trend for Methanol Oxidative Dehydrogenation

Catalytic oxidation of alkanol and oxygen on transition metal or metal-oxide clusters leads to aldehydes, esters, syngas, or combustion products (CO₂, H₂O) with rates and selectivities depended largely on metal identity, coverages of surface intermediates, bulk chemical state, and operating pressures. We report, in this contribution, the mechanistic synergy and a generalized relation between the reactivity for methanol oxidation dehydrogenation and the thermodynamic quantity at Pt, Ru, Pd, Ag cluster surfaces with metal or oxide bulk, after rigorous elimination of transport artifacts by extensive dilution at intra-pellet and bed scales. First-order rate coefficients for CH₃OH-O₂catalysis on Pt, Ru, Pd, and Ag clusters are a single-valued function of the operating oxidant-to-reductant ratio, irrespective of the metal identity and bulk chemical state. We show that the oxidant-to-reductant ratio is a surrogate of the oxygen chemical potential through kinetic coupling of the oxygen and methanol activation steps for the case of irreversible oxygen activation. The ratio, as a result, dictates the identity and coverages of reactive intermediates at cluster surfaces as well as the chemical state of the cluster bulk. The single-value functional dependence of rate coefficients on oxidant-to-reductant ratio provides the direct, macroscopic relation between kinetic properties (rate coefficients) and oxygen chemical potential at cluster surfaces.   

The rate coefficient values for methanol activation on Pt, Pd, and Ag cluster surfaces covered with reactive oxygen or on RuO₂ differ by five orders of magnitude in the decreasing reactivity order of Pt>Pd>Ru>Ag. These rate coefficients, when plotted against the oxygen binding energies, resemble a volcano type relation and contain the activation barrier and pre-exponential factor terms, which in the framework of transition state theory reflect the activation enthalpy and entropy required for the formation of O-H scission transition state of CH₃OH. The barriers on O* covered metal cluster surfaces (Ag: 77 kJ mol^-1; Pt: 84 kJ mol^-1; Pd 90 kJ mol^-1) increase as the O* binding strength on metal site increases, following a Brønsted-Evans-Polanyi type relation and then decrease on RuO₂ surfaces as the binding strength increases further. These reactivity trend with O* binding strength reflects that weakly bound O* are more effective as Brønsted base during the H abstraction step on oxygen covered Ag, Pt, and Pd surfaces and that there is a marked difference in active sites and O* reactivity between the RuO₂ and the O* covered metal surfaces. In addition to barriers, the reactivity difference among the metals is caused predominantly by the differences in the extent of CH₃O fragment stabilization at the CH₃OH activation transition state, as shown from the wide ranging activation entropy values (Pt=0±10, Pd=-18±10, Ag=-110±10, and RuO₂=-109±10 J mol^-1 K^-1). 

In summary, we report a generalized correlation between the first-order rate coefficient and the oxidant-to-reductant ratio and a direct connection of this ratio to the oxygen chemical potential at the metal or oxide cluster surfaces. The observed reactivity trend of Pt, Pt, and Ag metal and RuO₂ clusters reflects the reactivity of surface oxygen towards H abstraction and the extent of stabilization of the CH₃OH activation transition state.

Acknowledgement

We acknowledge Natural Sciences Council of Canada (NSERC) and Canadian Foundation for Innovation (CFI) for their financial supports.  

References

[1] WF. Tu and Y-H. Chin, J. Catal. 313 (2014) 55-69.

[2] WF. Tu and Y-H. Chin, Angew. Chem. Int. Edit., submitted.