(778d) Catalytic Consequences of Reactive Intermediates During Partial Oxidation of Methanol On Pt Clusters

Chin, C. - Presenter, University of Toronto
Tu, W., University of Toronto

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

Weifeng Tu1,2 and Ya-Huei
(Cathy) Chin1,*

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

 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.  

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.    

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

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.

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



[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.