(778d) 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.