(388a) The Effect Pd Crystallite Size and Oxygen Vacancies on the Partial Oxidation of Methane on Pd/Al2O3 and Pd/TiO2

Advances
in hydraulic fracturing have led to a surplus of natural gas. Methane is the
main component of natural gas and, according the Environmental Protection
Agency, is a potent greenhouse gas and the second most prevalent emitted [1].
With the expected emergence of more methane regulations in the coming years, a
renewed interest in understanding methane activation for conversion to value
added products is underway. Partial oxidation of methane (POM) is an attractive
alternative to methane steam reforming due to it being mildly exothermic
(ΔH_reaction = -35.9 kJ mol^-1) and producing syngas
as reactants for the Fischer-Tropsch and methanol synthesis reactions [2,3].
POM has been extensively investigated on noble metals such as Rh, Pt, and Pd or
cheaper alternative such as Ni [4]. There is considerable debate as to whether
CO and H₂ are formed directly or indirectly through combustion to CO₂
and H₂ then reforming [2,4]. The literature suggests the mechanism
depends on the nature of the active sites [5,6].

In
this study, to gain further insight on these active sites, the rate of reaction
was related to Pd crystallite size, geometry and ease of oxygen vacancy
formation, to determine where on the catalyst methane was activated. Pd
supported on Al₂O₃ and rutile TiO₂ were investigated.
Pd/Al₂O₃ initially shows a decrease in the reaction rate with
increasing crystallite size but then after about 4 nm, an increase in rate is
noted with further particle size increase. This can be associated with the lack
of oxygen vacancies on Pd caused by stronger interactions with oxygen for
smaller particles [7]. A decline in the rate is seen on Pd/TiO₂ with
increasing crystallite size. Since TiO₂ is easily reducible, the
Pd-O bond is weaker allowing more oxygen vacancies at the edge of the Pd
particle. With oxygen vacancies predominately at the Pd-TiO₂
interface, the rate of methane consumption tends to scale with the particles
perimeter as suspected. The availability of these oxygen vacancies plays the
critical role in allowing methane to be activated on Pd.

References:

(1)   Methane and
Nitrous Oxide Emissions from Natural Sources; U.S. Environmental Protection
Agency; Washington, DC, 2010.

(2)  
Chin, Y.; Buda, C.; Neurock, M.; Iglesia, E.; J. Catal. 283
(2011) 10.

(3)  
Vellam L.D.; Villoria, J.A.; Specchia, S.; Mota, N.; Fierro, J.L.G.;
Specchia, V.; Catal. Today 171 (2011) 84.

(4)  
Rodrigues, L.M.T.S.; Silva, R.B.; Rocha, M.G.C.; Bargiela, P.; Noronha,
F.B.; Brandão, S.T.; Catal. Today 197 (2012) 137.

(5)  
Lu, Y.; Xue, C.; Yu, C.; Liu, Y.; Shen, S.; Appl. Catal. A: General 174
(1998) 121.

(6)   Looik, F.; Feus, J.W.;
J. Catal. 168 (1997) 154.

(7)  
Fujimoto,
K.; Ribeiro, F.; Avalos-Borja, M.; Iglesia, E.; J. Catal. 179 (1998)
431.