Catalytic Consequences of Reactive Intermediates During Partial Oxidation of Methanol On Pt Clusters

Catalytic Consequences of Reactive Intermediates during Partial Oxidation of Methanol on Pt Clusters

Weifeng Tu^1,2 and Ya-Huei (Cathy) Chin^1,*

¹ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada

 ² College of Power Engineering, Chongqing University, Chongqing, China

 (*) corresponding author: cathy.chin@utoronto.ca

 

Keywords: methanol oxidation, Pt, partial oxidation, alcohol, formaldehyde, chemisorbed oxygen, oxygen binding strengths      

 

 

Reactions of alkanol and oxygen over transition metal or metal-oxide catalysts lead to value added products of aldehydes and esters, syngas, or combustion products (CO₂, H₂O) at rates and selectivities that vary largely with the operating oxidant-to-alkanol ratios, because these ratios prescribe the chemical state of catalytic sites and the identity and coverages of reactive intermediates on Pt cluster surfaces during steady-state catalysis. The effects of surface coverages of the diverse intermediates and their relative abundances on supported Pt clusters are interpreted here in terms of oxygen and carbon virtual pressures, the rigorous surrogates of their respective chemical potentials, and connected to the rates and selectivities of methanol partial oxidation based on rate measurements at differential conditions and elimination of transport artifacts by extensive dilution within the intra-pellet and catalytic bed.  

CH₃OH-O₂ catalytic turnovers (O₂/CH₃OH=0.1-40; 353-473 K) on Pt clusters (1.5-10 nm) form CH₂O, CH₃OCHO, CO₂, and H₂O. The catalytic reactions do not form CO and H₂ at detectable limits under all conditions before the complete depletion of O₂ because O₂ reactions with CO and H₂ are much faster than with CH₃OH. The primary and secondary nature of the CH₂O, CH₃OCHO, and CO₂ product and the relative rates of their reactions with oxygen relative to those of CH₃OH were probed from CH₃OH-O₂, CH₂O-O₂, and CH₃OCHO-O₂ reactions at different oxidant-to-reductant ratios. The rates of CH₃OCHO oxidation were unimportant relative to the CH₃OH oxidation turnovers at Pt cluster surfaces occupied by chemisorbed oxygen (O*) and oxygenated intermediates (e.g. CH₃O*) to varying extents. In contrast, CH₂O oxidation rates became increasingly important relative to CH₃OH oxidation rates as Pt cluster surfaces became populated with chemisorbed oxygen atoms. These results suggest that chemisorbed oxygen atoms are effective for oxidizing CH₂O and CH₃OH and become more reactive towards CH₂O than CH₃OH as they are more weakly bound at high surface coverages.    

The individual rates of CH₂O, CH₃OCHO, and CO₂ formation from CH₃OH-O₂ reaction acquire diverse kinetic dependencies with O₂ and CH₃OH pressures that can be categorized in three kinetic regimes according to the operating O₂-to-CH₃OH ratio, because this ratio influences the oxygen and carbon chemical potential at Pt cluster surfaces and thus, determines the relative abundances of the various reactive species of chemisorbed oxygen, unoccupied metal site, and methanol derived intermediates on Pt cluster surfaces. The identity and coverages of the reactive intermediates define the kinetically relevant steps during methanol partial oxidation and, in turn, lead to complex kinetic dependencies for the formation of CH₂O, CH₃OCHO, and CO₂. At high O₂/CH₃OH ratios (> 7.5; 373 K), Pt cluster surfaces are saturated with reactive oxygen species; thus, CH₂O, CH₃OCHO, and CO₂ formation rates do not vary with O₂ pressure but increase with increasing CH₃OH pressure. As the O₂/CH₃OH ratio decreases to intermediate values between 0.5 and 7.5 (373 K), kinetic dependencies for all products undergo a drastic transition: HCHO and CO₂ formation rates are no longer proportional to CH₃OH but instead acquire a linear dependence on O₂ pressure and the CH₃OCHO formation rates increase more than linearly with O₂ pressure. Such kinetic transitions reflect a concomitant surface transition from O* saturation to uncovered of reactive intermediates, and from catalytic involvements of O* in CH₃OH oxidation to * in O₂ activation as the kinetically relevant step. As the O₂/CH₃OH ratio decreases further to below 0.5 (373 K), rates for CH₂O and CO₂ formation decrease with increasing CH₃OH pressure but the rate for CH₃OCHO is unaffected by CH₃OH pressure. All of the species formation rates acquire a much larger dependence on O₂ pressure with apparent reaction orders exceeding 1.5. These kinetic dependencies on CH₃OH and O₂ pressures are consistent with CH₃OH activation at vacancies on Pt cluster surfaces that are saturated with CH₃OH derived intermediates, prevalent only at the very low O₂/CH₃OH ratio.

The complex kinetic phenomena reported here are prescribed strictly by the operating O₂-to-CH₃OH ratio because the ratio determines the oxygen and carbon virtual pressures at the Pt cluster surfaces and in turn, the chemical states and identity and relative abundances of the surface intermediates during steady-state catalysis. Similar relations between the oxygen virtual pressure and the catalytic rate coefficients have been reported for alkane [1-2] and NO oxidation [3]. These relations appear to be ubiquitous for oxidation reaction on metal or metal-oxide clusters and govern the reaction path selectivities. <>Acknowledgements

We acknowledge China Scholarship Council for a Joint-Supervising Ph.D. program for Weifeng Tu and Natural Sciences Council of Canada (NSERC) and Canadian Foundation for Innovation (CFI) for their financial supports.  

 

References

 

[1] Y-H. Chin, C. Buda, M. Neurock, and E. Iglesia, J. Am. Chem. Soc. 133 (2011) 15958.

[2] M. Garcia-Dieguez, Y-H. Chin, and E. Iglesia, J. Catal. 285 (2012) 260.

[3] B.M. Weiss and E. Iglesia, J. Phys. Chem. C 113 (2009) 13331.