C?H Activation of Methane to Formaldehyde on Ce1?xZrxO2 Thin Films: A Step to Bridge the Material Gap

Article abstract of DOI:10.1002/cctc.201600670

Ce_1?xZr_xO₂ (CZ) thin films were prepared by a combination of sol–gel and spin-coating methods and were evaluated for the C?H activation of methane by using a molecular beam set-up with the aim to bridge the material gap. The C?H activation of methane begins at 950 K, and a Ce-rich CZ composition displays a high selectivity (4–12 %) to the partially oxidised product, formaldehyde. A 10–12 % selectivity towards HCHO with 1.6 % methane conversion was observed with methane-rich CH₄/O₂ reactant compositions at 1050 K. Short contact times, prevalent under molecular beam conditions, could be a possible reason for HCHO formation. Although combustion products were observed instantly upon shining the mixture of reactants on CZ surfaces, up to 20 s delay was observed before formaldehyde generation, which indicates that the oxygen vacancy migration contributes to the rate-determining step and the diffusion-controlled nature of the reaction. A burst in HCHO generation at the point of molecular beam opening, after beam-closed conditions, suggests that the diffusion of oxygen vacancies to the surface is the reason for HCHO formation. Kinetics results also indicate the necessity of reduction sites for HCHO generation.

Full text of DOI:10.1002/cctc.201600670

Accepted Article
Title: C-H Activation of Methane to Formaldehyde on Ce1-xZrxO2 Thin
films : A step to bridge the material gap
Authors: Anjani Dubey, SAdhu Kolekar, and Chinnakonda
Subramanian Gopinath
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To be cited as: ChemCatChem 10.1002/cctc.201600670
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Entry for the Table of Contents
FULL PAPER
Kiss me momentarily and often
C-H activation of CH4 to HCHO
was explored under technically
relevant conditions on porous Ce-
rich Ce1-xZrxO2 thin films with an
aim to address the material gap
and mechanistic aspects.
Migration of electron rich oxygen
vacancy sites to the surface is
responsible for HCHO formation.
Low contact time and fast
Anjani Dubey, Sadhu K. Kolekar, and
Chinnakonda S. Gopinath*
Page No. – Page No.
C-H Activation of Methane to
Formaldehyde on Ce1-xZrxO2 Thin
films : A step to bridge the
material gap
excursion between CH4 and O2
rich compositions could improve
the HCHO selectivity further.
1
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C-H Activation of Methane to Formaldehyde on Ce1-xZrxO2 Thin films : A
step to bridge the material gap
Anjani Dubey,[a] Sadhu K. Kolekar[a ]and Chinnakonda S. Gopinath[a,b]*
[a]Catalysis Division, National Chemical Laboratory, Dr.Homi Bhabha Road, Pune 411 008,
India
[b]Center of Excellence on Surface Science, National Chemical Laboratory, Dr.Homi Bhabha
Road, Pune 411 008, India.
Email: cs.gopinath@ncl.res.in; Ph: 91-20-2590 2043; http://academic.ncl.res.in/csgopinath
Abstract: Ce1-xZrxO2 (ceria-zirconia - CZ) thin films were prepared by a combination of sol-gel
and spin coating method and evaluated for C-H activation of methane in a molecular beam setup
towards bridging the material gap. C-H activation of methane begins at 950 K, and Ce-rich CZ
composition displays high selectivity (4-12 %) to partially oxidized product, formaldehyde.10-12
% selectivity towards HCHO with 1.6 % methane conversion was observed with methane-rich
CH4:O2 reactants compositions at 1050 K. Low contact time conditions, prevalent under
molecular beam experimental conditions, could be a possible reason for HCHO formation. While
combustion products were observed instantly upon shining reactants mixture on CZ surfaces, up
to 20 s delay was observed to begin formaldehyde generation underscoring the oxygen vacancy
migration predominantly contributes to the rate determining step and diffusion controlled nature.
A burst in HCHO generation at the point of molecular beam open, after every beam close
conditions, underscores the diffusion of oxygen vacancies to the surface which is the reason for
HCHO formation. Kinetic results also indicate the necessity of reduction sites for HCHO
generation.
Keywords: Heterogeneous catalysis, contact time, diffusion control, C-H activation, material
gap.
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1. Introduction:
Selective conversion of methane to oxygenates, such as formaldehyde, is important to exploit
natural gas reserves. Formaldehyde is the simplest aldehyde which is an important precursor for
many useful chemical compounds. The direct and selective oxidation of methane to oxygenates
is very difficult process as it requires high temperature (~1000 K and above) for the activation of
C-H bond.[1] In the past few decades many efforts have been made to prepare active catalysts for
methane to oxygenates with molecular oxygen.[2] Despite the serious efforts, low yield and
selectivity was observed.[3] Direct oxidation of methane to formaldehyde is possibly more
efficient due to heat integration, and certainly a simpler method than steam reforming of methane
to syngas process, which is the initial step for production of formaldehyde.[4] Activation of C-H
bond occurs on ceria-zirconia (CZ) catalyst with an activation energy of 439 kJ/mol. HCHO
selectivity is very low as it is thermodynamically unstable and it further oxidizes to CO2 at high
temperatures, (Eqns. 1 and 2), which leads to low selectivity.[5,6]
CH4 + O2
HCHO + H2O
CO2 + H2O
∆Ho298 = ‒319kJmol-1
∆Ho298 = ‒571kJmol-1
(1)
(2)
HCHO + O2
One of the main problems is the high contact time prevalent in the fixed bed reactors
being widely employed for methane oxidation. It is generally around 1 s for methane oxidation in
fixed bed reactors.[1-2] Probably the best activity reported so far is with monomeric or isolated
vanadium supported on mesoporous silica by Nguyen et al[7] at very high GHSV (80000 to
740000 L/kgcat h) or low contact time with methane conversion up to 6 % and HCHO selectivity
between 57-93 %. It is also well known that changes in contact time influence the selectivity to a
large extent.[8-11] Molecular beam offers the single collision condition of reactants on catalyst
surfaces and hence offers to decrease the contact time by few orders of magnitude.
Methane partial oxidation to formaldehyde with high selectivity at low methane
conversion has been reported with MoO3, V2O5, Nb2O5.[7-11] Michalkiewiczet al.[9] employed
polymorphic niobium (V) oxides (H-Nb2O5 and M-Nb2O5) as a catalyst and evaluated methane
conversion to formaldehyde between 693 and 1023 K; 78 % formaldehyde selectivity was
observed at 0.2 % methane conversion at a contact time of 0.9 s and 1000 K. More than 0.2 %
methane conversion decreases the selectivity exponentially. Nedyalkovaet al.[11] synthesized Fe-
doped CZ fluorite catalyst and found that low temperature partial oxidation of methane to
formaldehyde occurs between 573 and 873 K; while HCHO selectivity was observed to be 20 to
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55 % below 673 K, methane conversion is less than 0.5 %. When methane conversion increases
to 6 % at ≥773 K HCHO selectivity decreases to 2-4. Banares et. al[12] carried out partial
oxidation of methane on silica supported transition metal (V, Mo, W and Re) oxide and found
that 0.8 wt % MoOx/SiO2 shows the maximum activity with 13.9 % methane conversion. Highly
monodispersed vanadium oxide[13] and molybdenum oxide[14] on high surface area support shows
the better activity for methane oxidation to formaldehyde. Zhang et. al.[15] claimed that NO2
promotes the partial oxidation of methane and lowers the reaction temperature by 300 K (from
1173 to 873 K). They also claim that NO2 favours the formation of oxidation product at the
expense of C2 hydrocarbon. Weng et. al.[16] has carried out partial oxidation of methane to
formaldehyde on Mo/Sn/P silica supported catalyst in temperature regime of 848 to 948 K. They
achieved 2 to 5.8 % methane conversion with 91.9 to 30.6 % HCHO selectivity; selectivity
decreases with an increase in reaction temperature. Indeed, above discussion underscores the
complexity of the problem; however, low CH4 conversion generally favours HCHO formation.
We investigated C-H activation of methane to HCHO on Ce1-xZrxO2 (CZ; x = 0.1 to 1)
thin films in a home-built molecular beam (MB) instrument (MBI).[17] Zr-introduction into the
ceria lattice improves the redox properties of CZ along with thermal stability, which prevents
agglomeration at high temperatures.[18-19] Brinker et al and Niemantsverdriet et al used different
methods to prepare thin films, with the aim to bridge the material gap.[20,21] Our earlier report on
porous CZ thin films addresses bridging the material and pressure gaps by correlating the oxygen
storage capacity (OSC) measured in MBI and CO oxidation activity measured on powder CZ
catalysts at ambient pressure.[22a] One to one correlation was observed between the above two
extreme conditions makes the method relevant to address material gap problems. Mn-doped CZ
films have been employed for methane to syngas conversion.[22b] From the present MBI study on
CZ films, it has been observed that high cerium content of CZ film increases partial oxidation
activity. C-H activation of methane to HCHO on Ce-rich CZ thin film starts at ≥950 K and
Ce0.9Zr0.1O2 shows partial oxidation of CH4 to HCHO at T ≥1000 K and its activity increases
with increase in temperature. Present article is focussed on bridging the material gap and related
aspects, rather than finding a new or improved catalyst for methane activation. Further, emphasis
is given to understand the reaction mechanism aspects through molecular beam approach.
2. Experimental section
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CZ (Ce1-xZrxO2) thin films were prepared by a combination of sol-gel and spin coating method
on Si wafer (MaTeck Germany, 99.9 % pure).[22-23] General characterisation about the films by x-
ray diffraction (XRD), Raman spectroscopy, surface profilometer, transmission electron
microscopy (TEM) are given in the supporting information (see SI-1) along with preparation
details. The custom built ambient pressure photoelectron spectrometer (Prevac, Poland) equipped
with VG Scienta R3000HP analyzer and monochromatic Al Kα x-ray (1486.6eV) was used to
record the core levels of different elements of CZ thin films.[24]
C-H activation experiments was performed in a 22 L capacity home-built MBI.[17,25] Base
pressure of the chamber was maintained in typical ultra high vacuum (UHV) around 2 x 10-10
Torr. MBI was equipped with a quadruple mass spectrometer (Pfeiffer, HiQuadTM, QMG 700
with QMA 410, 1-128 amu) to observe the change in partial pressure of reactants and products.
Molecular beam (MB) can be closed or open at will with the help of a shutter placed between the
MB doser and thin film. Elaborate details about MB generation and various parameter
calculations are available in ref. 25. CZ film spun coated on Si wafer has been placed in niobium
sample holder which can be resistively heated unto 1150 and cooled unto 125 K with liquid
nitrogen. Cr-Al (K-type) thermocouple was spot welded on the sample holder to measure
temperature of the assembly containing sample as well as sample holder. Selective oxidation of
methane to HCHO has been measured on CZ thin film under isothermal conditions at different
reaction temperatures. Between any two successive measurements, surface was regenerated by
simple oxygen treatment at 773 K for 20 min.
3.0 Results and Discussion:
3.1 Textural and Spectral Characteristics of Ce1-xZrxO2 thin film:
X-ray diffraction results are shown in Fig. SI-2. CZ thin film on Si wafer is crystalline in
nature and from the diffraction pattern it was identified that the structure to be cubic fluorite and
as that of ceria. Surface profile and thickness of the film was measured by surface profilometer at
different positions. Thickness was observed to be in the nano-regime between 30 and 50 nm,
which is homogeneous and uniform from small to large area (Fig. 1). Surface profiles also show
some sporadic spikes, which is due to Si substrate and not from thin film.[22] Surface profile
measured separately on Si-substrate supports this. It has been found from surface profile analysis
that as the Ce content of CZ film increases porosity of CZ thin film also increases. In fact,
thickness and surface roughness appears to be similar (40 nm) for Ce0.9Zr0.1O2 composition.
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Fig. 1: Surface profiles and thickness of films
measured by profilometer for a) Ce0.9Zr0.1O2, c)
Ce0.5Zr0.5O2.Arrows indicating the Si-substrate
level, on which thin film was deposited. (b and d)
TEM image of Ce0.9Zr0.1O2and Ce0.5Zr0.5O2 thin
film, respectively. (e) Raman spectral results
display a marginal shift in F2g feature with
different CZ compositions.
TEM analysis also indicates the CZ films synthesized are porous in nature, which is also shown
in Fig. 1.[22,23] Porous nature of the film brings in the characteristic of real world catalyst, which
helps for diffusion of reactants and products.
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CeO2 catalyst exhibits a strong characteristic peak at 464 cm-1, which correspond to the
F2g vibration mode of cubic fluorite structure (Fig. 1e). This feature arises due to the symmetric
arrangement of O-atoms in the cubic fluorite structure with Fm3m space group. However, upon
introduction of 10 atom % Zr in ceria, a marginal peak shift was observed and narrow peak
appears at 461 cm-1; while it appears at 457 cm-1 and 460 cm-1 for 30 and 50 atom % Zr4+ ions
into ceria lattice, respectively. It was also observed that with increase in Zr4+content there is a
linear decrease in intensity of F2g peak was observed. This is mainly due to the symmetry
breaking in the ceria lattice due to the formation of CZ solid solution. Since Zr4+ ionic radii (84
pm) is lower than that of Ce4+ (94 pm), to minimize the structural stress larger size Ce3+ and
some oxygen vacancy (Ov) occurs; this leads to a lattice contraction (Fig. SI-2) which causes a
red shift in Raman results. No sign of ZrO2 feature was observed in Raman results suggesting the
solid solution nature of CZ. Sharp feature observed at 520 cm-1 is due to Si-substrate.
XPS measurements were made on CZ thin films and Fig. 2a shows the Ce 3d core level
spectra, which is complex in nature. Two sets of multiplets were observed which is symbolized
as v and u for 3d5/2 and 3d3/2spin-orbit doublets. A total of ten features appear due to various final
states and extensive literature reports are available.[26] Two prominent features of Ce3+, which
appears at 884.6 eV (v´) and 903.2 eV (u´) (indicated by dashed arrows), increases in intensity
and suggesting an increasing Ce3+ content with increasing Zr-content.[26] Fig. 2b shows the Zr
3dspectra of CZ thin film which shows two peaks at 182.9 and 185.3 eV, corresponds to 3d5/2
and 3d3/2 spin-orbit doublets.[27] As the Ce content of CZ film increases there is a minor shift
observed towards higher BE in Zr 3d spectra; indeed this indicates a change in electronic
environment of CZ surfaces upon Zr-introduction into the ceria lattice. Fig 2c shows O 1s spectra
of CZ thin film. The feature observed at 530.3 eV (532.6 eV) corresponds to lattice (hydroxyl)
oxygen of CZ surfaces. A careful look at the spectra reveals a narrowing of the valley between
the above two features with increasing Ce-content, suggesting the presence of third component.
This is attributed to those O-atoms that are closer to Ov sites. Introduction of Zr into ceria lattice
changes the stoichiometry of Ce4+/3+, which is responsible for Ov content.[28]
3.2 Molecular Beam Studies of Methane oxidation on Ce0.9Zr0.1O2thin film.
MB measurements were made for oxygen adsorption for three CZ compositions and the results
are shown in Fig. SI-3.[22] Generally, the rate of O2 adsorption increases with increase in
temperature for any CZ composition. Cerium-rich CZ film (Ce0.9Zr0.1O2) has shown high rate
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Fig. 2 XPS spectra of a) Ce 3d, b) Zr 3d and (c) O 1s core levels of Ce1-xZrxO2 thin film.
of oxygen adsorption and also high oxygen storage capacity (OSC). Detailed studies of oxygen
adsorption on CZ thin films have been reported in our earlier publication and readers are
requested to refer ref. 22. Partial oxidation of methane on CZ thin films have been measured and
it is found that Ce-rich thin film exhibits high activity and in correspondence with oxygen
adsorption characteristics. Partial oxidation of methane along with kinetic parameters have been
studied and discussed in detail, particularly for Ce0.9Zr0.1O2 composition.
Fig. 3 shows the first sign of methane oxidation at 950 K with CH4:O2 = 1:2 ratio on
Ce0.9Zr0.1O2 thin film; however, the same was not observed at 950 K with methane rich
compositions, such as CH4:O2 = 1:1 (see SI, Fig. SI-4). At 950 K, only combustion products
were observed with stoichiometric and more oxygen containing CH4 + O2 mixture. Although
oxygen adsorption was observed below 950 K, no methane oxidation/activation was observed
with any CZ compositions. At t = 25 s molecular beam of reactants mixture was allowed into the
UHV chamber with shutter in closed position; this prevents direct interaction of MB with the
surface of catalyst. Partial pressure of methane and oxygen increases immediately. At t = 45 s
shutter was opened which allows direct adsorption of reactants on the surface of the catalyst. O2
adsorption was observed immediately on shutter opening with sign of marked decrease in partial
pressure, which indicates the consumption of O2 during the course of reaction. No fast changes
in partial pressure for methane, unlike in the case of O2, was observed for shutter oscillation; this
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is mainly due to very small sticking coefficient of CH4.[29,30] However, a gradual and noticeable
change observed for adsorption of methane on shutter opening. Difference in partial pressures
between shutter open and close positions has been used for methane conversion calculations. A
gradual increase in partial pressure of H2O and CO2 was observed along with oxygen and
methane adsorption. CO2 (H2O) reaches the steady state at t = 155 s (105 s). Thus we observe
relatively slow CO2 formation kinetics, which directly indicates that reaction proceeds via
diffusion controlled process due to porous nature of CZ thin films. Being the smallest in size and
mass, H-atoms diffuse fast into the pores and reacts with lattice oxygen to form water molecules.
Fig. 3: Experimental kinetic data for oxidation of CH4with O2 in 1:2ratio at 950 K on
Ce0.9Zr0.1O2 thin film. Molecular beam oscillation was made for every 180 s. Only combustion
products were observed. Up and down arrow signs for shutter closing and opening, respectively,
are used in all figures.
At t = 240 s shutter was closed which prevents direct interaction of reactants on the
catalyst; however this leads to a decrease in partial pressure of CO2 and H2O and it reaches the
background level at t = 330 s. Similarly, reactants partial pressure reaches the original level
observed before shutter opening at the first instance. No other partially oxidized products were
observed, including HCHO. The oscillation of the molecular beam has been made several times
to reproduce the kinetic data. Further several parameters, like variation of reaction temperature,
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O2-rich to CH4-rich compositions has been studied and described below. All the reactions
reported in this communication have been reproduced at least three times on the same CZ
compositions within an error margin of 10 %. In fact this hints the quality of surface and thin
film remains the same.
Fig. 4 shows the experimental kinetic data for the oxidation of methane with CH4:O2 =
1:1composition at 1000 K on (a) Ce0.9Zr0.1O2, as well as (b) Ce0.7Zr0.3O2 compositions. On
opening the shutter at t = 55 s (dashed line, Fig. 4a), CO2 and H2O forms, which is similar to the
results discussed in Fig. 3. However, both H2O and CO2 reach the steady state at t = 200 and 125
s, respectively; this observation is similar to the combustion products evolution at 950 K,
however with relatively fast kinetics. Indeed, partial pressure of water initially overshoots the
steady state level indicating a fast diffusion of H-atoms to form water molecules at high
temperatures. Along with combustion products, interestingly, HCHO formation begins, albeit,
after a delay of 20 s at t = 75s. In fact, a sudden burst in HCHO was observed at the time of
shutter opening (t = 55 s), and the same subsides immediately. Spike in HCHO partial pressure
was reproducibly observed at the point of opening shutter and one such example is shown in the
inset of Fig. 4a. HCHO production reaches the steady state at t = 153 s, which is longer than that
for combustion products. This underscores the slow formation kinetics of HCHO and
predominantly contributes to the rate determining step for the overall reaction. In general, rate of
reaction increases with increase in reaction temperature, which can be easily identified by an
increase in consumption of O2, as well as larger amount of products formation, compared to the
reaction at 950 K. Selectivity and yield of HCHO is less compared to CO2, as the heat of
combustion of HCHO is highly exothermic than methane (Equn. 1 and 2).[6] While closing the
shutter at t = 240 s the partial pressure of all product i.e. CO2, H2O and HCHO decreases and it
attains the background level. CO2 takes only 10 s to reach the new steady state at t = 250 s, while
H2O and HCHO takes 65 s to reach steady state indicating that the delay in attaining the new
steady state for HCHO is probably due to its further oxidation into CO2 and H2O.
MB oscillation was made at least three times to ensure the kinetic data is reproducible
within the experimental error limit of 10 %. Every time the shutter was opened, importantly,
sudden burst in HCHO partial pressure was observed followed by HCHO production after about
20 s delay. We attribute this observation to the following: It is very likely that HCHO formation
requires catalytic sites that are rich with electrons, such as Ce3+ and associated Ov sites. Fully
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Fig. 4. Experimental kinetic data for selective oxidation of CH4 with O2 in 1:1 ratio on a)
Ce0.9Zr0.1O2 and b) Ce0.7Zr0.3O2 thin film at temperature T = 1000 K. The shutter operation i.e.
opening and closing of MB was made after every 180 s.
oxidized sites would lead to total oxidation products. After closing the shutter, due to the
unavailability of reactants directly, as well as due to high temperatures, CZ surface is likely to be
enriched with Ov andCe3+. This factor induces a sudden spurt in HCHO on opening the shutter.
Three points are worth highlighting about this observation. First is the direct conversion of
methane to HCHO, albeit in a short burst. Second is the availability of plenty of oxygen from the
direct beam of reactants makes the surface more oxidizing and hence combustion products
formation increases instantly. Oxygen diffusion into the subsurface and bulk brings in the Ov to
the surface and HCHO formation picks up gradually after a delay time of 20 s at 1000 K, and
reaches the steady state again. Third point to be noted is an increase in CO2 and H2O background
level pressure, while HCHO remains at the initial level, under beam off conditions after MB
oscillations, indicating the diffusion nature of the entire reaction. Fig. 4b shows the oxidation of
methane to combustion products at 1000 K on Ce0.7Zr0.3O2 composition, with no marginal sign of
HCHO formation. Similar results were observed for other Zr-rich CZ composition thin films.
Fig. 5a shows the experimental data for oxygen adsorption during the course of reaction
at 1000 K with various CH4:O2 ratio from 5:1 to 1:5. Figs. 3, 4 and 5a emphasize that oxygen
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was consumed predominantly under reaction conditions and irrespective of reactants
composition; this leads to an inevitable formation of the combustion products, especially with
O2-rich reaction mixture. Fig. 5b shows the rate of oxygen adsorption with various O2/CH4 ratios
at 1000 and 1050 K. Although oxygen adsorption increases with oxygen content, careful analysis
of Fig. 5b reveals a change in slope from methane-rich to oxygen-rich compositions. Rate of
oxygen adsorption is relatively small with CH4-rich compositions underscoring the hindrance for
oxygen adsorption; while it is the opposite trend with O2-rich compositions. Similar analysis
show methane conversion gradually changes from 0.9 (0.08 ML/s) to 1.6 % (0.15 ML/s) from
CH4-rich (O2:CH4 = 1:5) to O2-rich (O2:CH4 = 5:1) compositions, respectively, on Ce0.9Zr0.1O2.
Very likely, carbon or carbonaceous species on the catalyst surfaces due to methane-rich
compositions slow down the diffusion of oxygen atoms into the sub-surface and bulk; this is in
addition to the oxygen consumption for CO2 and H2O formation.
Fig. 6 shows the dependence of products evolution as a function of time and O2:CH4 ratio
at 1000 K. Evolution of all products was plotted in separate panels to minimize the complexity of
the reaction. Partial oxidation of methane has also been evaluated at 1050 K (results not shown),
and the same trend has been found as that of at 1000 K. Irrespective of reactants composition, all
products formation occurs, and the same increases form methane-rich to O2 rich compositions.
Especially, with oxygen-rich compositions above 1:1 of O2:CH4, the rate of products formation
seems to enhance linearly. Rate of CO2 and HCHO production shows less dependence on
CH4:O2 ratio for methane-rich compositions. Another important point to be noted is the decrease
in delay time between shutter opening and HCHO formation with oxygen rich reactants. In
contrast to the expectations of total combustion with O2-rich compositions, HCHO production at
high rate indicates the active nature of the surface. With more oxygen supply at high
temperatures, the oxygen diffusion to the bulk is also expected to be faster and this in turn
enhances the Ov diffusion to the surface, which favours high rate of HCHO formation. More
carbon species (C, CHx) deposition on the surface might prevent or decelerate Ov diffusion
towards the surface and hence low rate of HCHO occurs with methane-rich compositions;
nonetheless, CHx species on the surface makes it significantly reduced, especially with small
amount of O2 available in gas-phase. Partial (and total) oxidation of CH4 is facile in O2-rich
beam at high temperature, which accelerates the migration of Ov towards the surface and hence
the rate increases.[31]
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Fig. 5. a) Experimental data for O2 adsorption, and b) rate of oxygen adsorption during the course
of reaction with different O2/CH4 ratio at T = 1000 and 1050 K on Ce0.9Zr0.1O2.
Fig. 6. Products evolution as a function of time at 1000 K for different O2:CH4 ratios for a) CO2,
b) H2O, and c) HCHO.
Rate of formation of formaldehyde and combustion products has been calculated from
Fig. 6 under steady state conditions. Fig. 7 shows the rate of formation of products, (a) CO2, (b)
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H2O, and (c) HCHO, as a function of O2/CH4 ratio, at 1000 and 1050 K. Percent ratio of rate of
formation of HCHO to HCHO + CO2, which is selectivity of HCHO among carbon containing
products, is plotted in Fig. 7d. It has been found from Fig. 7 that the rate of formation of products
increases with increase in O2/CH4 ratio. There is a slow and steady increase unto O2/CH4 ratio of
2. A breakthrough in the overall rate is observed at a ratio of 3. Rate of formation of all products
increased 3-5 times that of methane rich composition. This trend is highly prominent at 1050 K.
Rate increase observed for all products underscores an enhancement in methane conversion also
to a similar extent. Percent selectivity of HCHO is an important factor, which is shown in Fig.
7d. Not surprisingly, selectivity is found to be higher for the methane rich beam and it is found to
Fig. 7 Rate of formation of oxidation products, a) H2O, b) CO2, c) HCHO and d) HCHO/CO2
ratio at two different temperatures as a function of O2/CH4 ratio.
be the highest at 12.2 % for 5:1 ratio of CH4:O2 at 1050 K. With an increase in O2 content,
selectivity towards HCHO decreases. It was also found that with increase in temperature HCHO
selectivity increases up to O2/CH4 ratio of 2, thereafter selectivity decreases with increase in O2
rich compositions. Nevertheless, 12.2 % selectivity observed is better or comparable to the
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values reported for many of the reported catalyst systems.[8-9] However, compared to the vanadia-
based catalyst,[7,13] activity is lower with present CZ catalyst. Lin et al[32] observed HCHO
selectivity of 90 % at 873 K using MoO3/SiO2 catalyst with low CH4 conversion but as the
temperature increases to 983 K selectivity becomes zero; compared to this Ce0.9Zr0.1O2 thin film
shows a maximum selectivity of 12.2 %. However, considering the significantly large activity in
a burst of few seconds in the transient state (Fig. 4a inset, and Fig. 6c) followed by a relatively
slow HCHO kinetics, activity enhancement can be further manipulated by quick oscillations
between oxygen-rich and oxygen-lean reactant compositions. Oxygen-lean composition, due to
its reduction nature, is expected to migrate fast and more Ov to the surface, while allowing
methane adsorption leading to more HCHO. This would allow the HCHO generation selectivity,
at least, to double (20-25 %) by carefully maintaining the transient bursts. However, more
careful experiments are necessary to confirm this hypothesis.
It is also surprising that carbon monoxide was not observed in any of our experiment,
although it is very much expected. Possible reason could be its further oxidation to CO2 due to
porous nature of CZ. 100 % CO oxidation to CO2 was observed on Ce0.9Zr0.1O2 above 625 K.[22a]
To the best of our knowledge, complex reactions, such as methane activation has not been
measured on thin films of real-world catalysts in surface science setup such as MBI. Present
work underscores the direct conversion of methane to formaldehyde on reduced Ce3+ and oxygen
vacancy sites.
Conclusion. Partial oxidation of methane has been carried out on various compositions of ceria-
zirconia solid solution thin films in a home-built molecular beam setup. C-H activation of
methane generally takes place at temperatures higher than 950 K. Ceria rich thin film
Ce0.9Zr0.1O2 exhibits partial oxidation of methane to formaldehyde at 12 % selectivity and 1.6 %
methane conversion. Though rate of formation of formaldehyde increases with increase in
oxygen rich reactants compositions as well as at high temperatures, HCHO selectivity remains at
12 %. No formaldehyde formation was observed with Zr-rich CZ compositions. A careful
analysis of the results obtained suggests the reduction sites, such as oxygen vacancies, CHx, are
necessary for partially oxidized product. Beam oscillations carried out in the present set of
experiments also indicates the necessity to quickly alter the reactants composition between
methane-rich to oxygen-rich streams to maximize and manage the activity and selectivity. If
large amount of oxygen adsorption can be manipulated by quick excursion between O2-rich and
15
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10.1002/cctc.201600670
ChemCatChem
CH4-rich compositions, more partially oxidized product can be produced and more work is
suggested in this direction. Although diffusion is an integral part of catalysis phenomenon, such
material gap reduction attempts allow us to explore more on this. More work from around the
globe on similar lines will eventually lead to a better kinetic model for complicated reactions,
such as C-H activation of methane.
Supporting Information (SI) available: Ceria-zirconia thin films synthesis and characterization
(SI-1), X-ray diffraction (SI-2), oxygen adsorption on CZ thin films (SI-3), and methane
oxidation on Ce0.7Zr0.3O2 thin film at 950 K (SI-4) are available.
Acknowledgements
A.D. is thankful to the UGC, New Delhi for senior research fellowship. We thank BRNS
(2011/37C/36/BRNS) and SERB (SR/S1/PC-16/2012) for funding this research project.
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