A series of WO(3)/ZrO(2) catalysts with tungsten (W) loadings ranging from 0.5 to 11.4 wt% was prepared by incipient wetness impregnation on a preformed ZrO(2) support. The oxidic catalysts were characterized using XRD, Raman spectroscopy, XPS, ISS, and IR spectroscopy. XRD and Raman results showed that the ZrO(2) support was predominantly present in the monoclinic form. XPS and Raman measurements indicated the formation of increasing amounts of W interaction species for catalysts with W loadings up to 8.8 wt% WO(3). In addition to the W interaction species, bulk WO(3) was also observed for catalysts with W loadings > or = 3.0 wt% WO(3). Comparison of the XPS results with coverage measurements by ISS and CO adsorption suggests that the W surface phase is in the form of two-dimensional polymeric patches for catalysts with W loadings 3.0 < or = wt% WO(3) < or = 4.5. For catalysts with W loadings >4.5 wt% WO(3), the results indicated an additional build-up of a bilayer (or multilayer) polymeric W species. Analysis of the hydroxyl region of ZrO(2) by IR spectroscopy showed that initial additions of W occur on the high frequency hydroxyl group. A schematic for the structure of the catalysts has been proposed based on the above observations.
The desorption kinetics of a series of poly(ethylene glycol)-dimethyl ethers (PEG-DMEs, CH3(OCH2CH2)
n
OCH3, n = 1−22) adsorbed on graphite have been measured to study the detachment of flexible linear
heteropolymers from solid surfaces. Desorption rates were measured using temperature programmed
desorption (TPD) and indicate that the desorption of the PEG-DMEs from graphite can be described using
a first-order rate constant and a desorption barrier,
, that is independent of adsorbate coverage. The
average pre-exponent of the first-order desorption rate constant was measured to be v
av = 1018.8±0.7 s-1
and is roughly independent of the PEG-DME oligomer chain length. In addition the
scales
nonlinearly with the oligomer chain length and can be represented by the expression
= a + bN
γ
(where N = 3n + 3), with the exponent assuming a value of γ = 0.46 ± 0.01. A simple theory generates
an analytical expression for the measured values of
that accurately reproduces the measured
nonlinearity of
(N). The measured value of
is determined by the difference in the average energy
of the adsorbed state, 〈E〉, and the average energy of the transition state to desorption, 〈E
‡〉. The nonlinear
dependence of the
on chain length can be ascribed to conformational entropy in the adsorbed state.
For long oligomers entropy favors conformations of the molecule that are partially detached from the
surface at elevated temperatures and thus the average energy of the adsorbed state is quite different from
that of the minimum energy configuration.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.