(630g) Methane Activation over Mn/Na2WO4/SiO2: The Balance of Surface Defects and Activated Oxygen Species

The
Mn/Na₂WO₄/SiO₂ catalyst system has been the
state-of-the-art for oxidative coupling of methane. Structure distorted WO₄^2- tetrahedron
stabilized by Na has been regarded as an active site for the hydrogen
abstraction from methane, an important first step in OCM [1,2]. The addition of
Mn could increase the gas phase oxygen activation and oxygen transport from Mn₂O₃ to
Na₂WO₄ in the catalyst system [3]. Oxygen vacancies
(surface defects) of WO_x may be hypothesized as the active site
for the CH₄ activation [4]. However, the literature contains
few reports regarding the correlation of the reactivity with oxygen vacancies
of Na₂WO₄ and activated oxygen species over Mn in
the Mn/Na₂WO₄/SiO₂ catalyst system for
OCM. In this study, combined Density Functional Theory (DFT) and experimental
techniques including Temporal Analysis of Products (TAP), steady state flow
reactor, high-temperature XRD, XPS, and Raman were applied to study the
structure related CH₄ reactivity and C₂ yield.

We performed a series of TAP pump-probe
experiments over different Mn/Na₂WO₄/SiO₂ catalyst
systems with various delay times between O­₂ pump molecules and CH₄
probe molecules at 800 ℃.   As shown in Figure 1A, CH₄ conversion
showed a maximum in CH₄ conversion at the delay time of 1s. The
catalyst can be thermally reduced above 700 ^oC due to the
transition to molecular Na₂WO₄ clusters as has been
found in both TPD and TGA experiments. The generated vacancies are believed to
be active sites for methane activation [4]. The longer delay time leads to the
production of more reduced catalyst. However, the CH₄ conversion
with a delay time of 2.0 s is lower than that with a delay time of 1.0 s, which
suggests the activated oxygen species on Mn also play a significant role for CH₄ conversion.
Based on the observed maximum in CH₄ conversion, we propose a mechanistic
scheme for methane activation shown in Figure 1B. Methane is activated by the
decomposition leading to the production of CH3ꞏ and a hydridic W-H bond
on the active W⁶⁺ site with no redox activity. Activated oxygen
species then kick off the hydridic H atoms with the formation of water and the regeneration
of active W⁶⁺ sites. Thus, a short time delay does not generate
significant active sites. With a time delay longer than 1.0 s, there are
less activated oxygen species, resulting in less regeneration of active sites.
 In summary, an optimum balance of surface defects and activated oxygen species
is beneficial for methane activation.

[1] J. Wu, S. Li, The Role of Distorted WO4 in the Oxidative
Coupling of Methane on Supported Tungsten Oxide Catalysts, The Journal of
Physical Chemistry, 99 (1995) 4566-4568.

[2] T.W. Elkins, H.E. Hagelin-Weaver, Characterization of Mn–Na₂WO₄/SiO₂ and
Mn–Na₂WO₄/MgO catalysts for the oxidative coupling of
methane, Applied Catalysis A: General, 497 (2015) 96-106.

[3] A. Malekzadeh, A. Khodadadi, M. Abedini, M. Amini, A.
Bahramian, A. Dalai, Correlation of electrical properties and performance of
OCM MO_x/Na₂WO₄/SiO₂ catalysts,
Catalysis Communications, 2 (2001) 241-247.

[4] X. Xu, F. Faglioni, W.A. Goddard, Methane activation by
transition-metal oxides, MO_x (M= Cr, Mo, W; x= 1, 2, 3), The
Journal of Physical Chemistry A, 106 (2002) 7171-7176.