Tin disulfide is attractive as a potential visible-light photocatalyst because its elemental components are cheap, abundant and environmentally benign. As a 2-dimensional semiconductor, SnS2 can undergo exfoliation to form atomic layer sheets that provide high surface areas of photoactive material. In order to facilitate the deployment of this exciting material in industrial processes and electrolytic cells, single crystals of phase pure SnS2 are synthesised and analysed with modern spectroscopic techniques to ascertain the values of relevant semiconductor properties. An electron affinity of 4.16 eV, ionisation potential of 6.44 eV and work function of 4.81 eV are found. The temperature dependent band gap is also reported for this material for the first time. We confirm the valence band is formed predominately by a mixture S 3p and Sn 5s, while the conduction band consists of a mixture of Sn 5s and 5p orbitals and comment on the agreement between experiment and theory for values of band gaps
A mixed oxide support containing Ce, Zr and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO2 as the oxidant. The activity of the support was compared with that of fully-formulated catalysts containing palladium. The Pd/CeZrAlOx material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h) which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH3 and CO2 it was found that the catalyst possessed both acidic and basic sites. In addition, temperature programmed reduction showed that palladium promoted both the reduction and re-oxidation of the support. When the role of CO2 was investigated in the absence of gas-phase oxidant, using a Temporal Analysis of Products (TAP) reactor, it was found that CO2 dissociates over the reduced catalyst leading to formation of CO and selective oxygen species. It is proposed that CO2 has the dual role of regenerating selective oxygen species, and shifting the equilibrium for alkane dehydrogenation by consuming H2 through the reverse water-gas-shift reaction. These two mechanistic functions have previously been considered to be mutually exclusive.
The in situ combination of electrochemistry
and shell-isolated
nanoparticle enhanced Raman spectroscopy (SHINERS) has been used for
the first time to investigate the surface structure sensitivity of
asymmetric catalytic hydrogenation at single-crystal Pt electrodes.
The adsorption and hydrogenation behavior of aqueous ethyl pyruvate
(EP) at a range of modified and unmodified Pt{hkl} electrodes was measured both by cyclic voltammetry and by recording
Raman spectra at hydrogen evolution potentials. Two primary surface
intermediates were observed, including the previously reported half-hydrogenation
state (HHS), formed by addition of a hydrogen atom to the keto carbonyl
group, as well as a new species identified as intact chemisorbed EP
bound in a μ2(C,O) configuration. The relative populations
of these two species were sensitive to the Pt surface structure; whereas
the μ2(C,O) EP adsorbate was dominant at pristine
Pt{111} and Pt{100}, the HHS was only observed at these electrodes
after the introduction of defects by electrochemical roughening. Intrinsically
defective Pt{110} and kinked Pt{321} and Pt{721} surfaces exhibited
behavior similar to that of electrochemically roughened basal surfaces,
indicating the requirement for low coordination sites for observation
of the HHS. Rationalization of the differing behaviors is given on
the basis of density functional theory (DFT) calculations, which indicate
that the μ2(C,O) EP adsorbate is considerably more
stable on basal {111} than on {221} stepped surfaces. A mechanism
is proposed in which the μ2(C,O)-bound species is
a precursor to the HHS but the rate of the first hydrogen atom addition
is slow, leading to a low steady-state population of the HHS at terrace
sites. The implications of this in the context of enantioselective
hydrogenation at chirally modified Pt are discussed.
The
utility of the surface reactivity observed for model systems
under ultrahigh vacuum for predicting the performance of catalytic
materials under ambient flow conditions is a highly debated topic
in heterogeneous catalysis. Herein we show that vast differences in
selectivity observed for methanol self-coupling across wide ranges
of temperature and reactant pressure can be accurately predicted utilizing
the kinetics and mechanism obtained from model studies on gold single
crystals in ultrahigh vacuum regressed to fit transient pulse responses
over nanoporous gold (Ag0.03Au0.97) at low pressures.
Specifically, microkinetic modeling of the complex sequence of elementary
steps governing this reaction predicts the dramatic effect of reactant
partial pressure on the product distribution and leads to conclusion
that the gas phase partial pressures of both reactants and the reaction
temperature determine the changes in selectivity to methyl formate
formation. Moreover, thorough analysis of the reaction network indicates
that the product distribution becomes increasingly insensitive to
kinetic effects at pressures approaching 1 bar, leading toward 100%
selectivity methyl formate. A rigorous kinetic sensitivity analysis
also demonstrates the complex interplay of the kinetics of the elementary
steps and the overall catalytic behavior.
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