Surface chemistry of metal oxide materials: from metalorganic and organic reactions to gas sensing
Date
2015
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Metal oxide materials have been widely used in the fields of catalysis and gas
sensing fields. Due to its unique physical and chemical properties, zinc oxide (ZnO)
has attracted considerable attention. It is not only used as a catalyst for various surface
reactions but also as a catalytic support. Cu/ZnO is a commonly used industrial catalyst
for methanol synthesis. In chapter 3, to understand the surface chemistry between Cucontaining
species and ZnO substrate, we utilized the vacuum-based techniques to
produce a Cu/ZnO catalyst with molecular-level control, using a common chemical
vapor deposition precursor, copper hexafluoroacetylacetonate vinyl trimethyl silane,
Cu(hfac)(VTMS), to grow copper nanoparticles on commercial ZnO powder. This
deposition process is investigated by high-vacuum Fourier transform infrared (FT-IR)
spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron
microscopy (SEM). The growth is found to be promoted by exposing ZnO powder to
the gas-phase water, and the intensity of the hydroxyl groups stretching signatures in
infrared decrease after the powder is exposed to the copper deposition precursor.
Vibrational spectroscopy results support the reaction on both polar and (101 0)
surfaces of the powder, and XPS confirms that the copper deposition takes place and
identifies Cu+ species as the main copper species on the surface of ZnO powder. The
mechanism of the reaction includes the elimination of the hfac ligand that reacts with
surface hydrogen present in hydroxyl groups, and this surface-limited process stops
when the surface runs out of available hydrogen. SEM is used to visualize the
formation of copper-containing nanoparticles on ZnO(101 0) and ZnO(0001 ) surfaces
and defects. The mechanism for the initial stages of the deposition is proposed based on
the computational investigation consistent with the experimental results. With
successful growth of copper on the ZnO surface, the next issue to be addressed is how
to remove the surface contamination, which originates mainly from surface-bound hfac
ligands. Therefore two independent studies for the chemical and thermal stability of
hfac ligands on ZnO surface have been undertaken in order to understand how to
remove surface hfac ligands.
In chapter 4, a number of surface-bound species including ethoxy, acetoxy,
acetylacetate, or 1,1,1,5,5,5-hexafluoroacetylacetate (hfac) are generated on ZnO
powder and their displacement behavior was investigated. The displacement of the
surface species formed by these compounds on ZnO powder surfaces by a gas-phase
reagent is described by a model predicted by density functional theory and the strength
of binding of the second layer on top of the first is provided by the H of sublimation.
This simple model is tested by infrared spectroscopy following the adsorption of one
compound and its displacement by the other. A correlation between the enthalpic
driving force and the percentage of the displaced species observed experimentally is
found. Based on these results, it appears that the adsorbed hfac ligand is very tightly
bound and is not susceptible to displacement by other common organic compounds that
can also serve as surface–bound ligands. Thus, finding an appropriate displacement
agent for hfac removal from ZnO requires further research.
In chapter 5, the mechanisms of decomposition during thermal transformation
of three -diketones, acetylacetone (acacH), 1,1,1-trifluoroacetylacetone (tfacH), and
1,1,1,5,5,5-hexafluoroacetylacetone (hfacH), are analyzed on ZnO powder surface
using FT-IR, XPS, and density functional theory (DFT) computational investigation. The initial O-H dissociation leads to the formation of corresponding -diketonates in all
the cases investigated. These diketonates are important surface intermediates that can
be generated in a controlled manner in these experiments. The presence on the C-CF3
entity determines the preferred thermal decomposition pathways, as the C-C bond in
this group starts to react with a surface of ZnO around 400 K, followed by immediate
decomposition of the resulting CF3 group. Above 600 K, the presence of the CF3-
substituent leads to the formation of ketene-like structures observed by vibrational
spectroscopy. The reaction mechanisms examined with the help of DFT calculations
are correlated with vibrational signatures of the species produced and with the Fcontaining
species recorded by XPS. From this experiment, we know that surface hfac
species decomposes around 400 K. Thus, to avoid this decomposition pathway, thermal
treatment of the hfac-covered Cu/ZnO catalyst needs to adhere to exclusively low
temperature regimes, below this decomposition temperature. The thermal chemistry of
the hfac-covered Cu/ZnO is also examined by annealing to the desired temperatures.
The C-CF3 bond dissociation causes the formation of the ketenes as determined by FTIR,
but this takes place at lower surface temperatures. Due to the similarity of thermal
reactions of hfac on Cu/ZnO and on clean ZnO powder, we utilize time-of-flight
secondary ion mass spectrometry (ToF-SIMS) profiles for different annealing
temperatures. Surprisingly, surface bonding of hfac ligands does change following even
very mild annealing. For annealing temperatures around 350 K, the ToF-SIMS
investigation suggests that the intact hfac ligand is transferred from copper species to
the ZnO surface, making copper nanoparticles available for other surface processes.
This conclusion is supported by XPS, since the copper nanoparticles are found to be
oxidized without hfac protection (if the surface is annealed to 350 K). Our study
suggests that the nanoparticle is involved in this transmetalation process and adsorbed
hfac can spill over to the ZnO support material, making it a potentially very valuable
method to prepare Cu/ZnO catalytic systems.
In chapter 6, a gas sensor application of metal oxides is investigated with the
focus on the role of oxygen to avoid sulfur poisoning during the use of a commercial
SnO2-based H2S sensor. The commercial H2S sensor is first tested under low vacuum
conditions to understand the limitations of this type of sensor operating at different
oxygen concentrations. Then the H2S sensor is interrogated under high vacuum, as it is
exposed to 1×10 -7 Torr of H2S and is shown to be reactivated by oxygen exposure of at
least 1×10-4 Torr. These preliminary tests provide quantitative analysis for the oxygen
needed during the process of H2S sensing to maintain the appropriate sensor operation.
More detailed experiments are under way to propose an appropriate mechanism for the
sensor poisoning.