Experimental adsorption data is typically analyzed within the context of a Langmuir adsorption model. Such an analysis can yield values for adsorption entropies which are far too large based on a comparison with predictions from statistical mechanics. This is due to the fact that the Langmuir model does not always adequately replicate the dynamics of the adsorbate, and thus other prototypical models incorporating different dynamical assumptions should be considered. We extend prior work on adsorption entropies by providing a framework for the interpretation of adsorbate dynamics based on a comparison of experimentally determined adsorption entropies, saturation coverages, and enthalpies to those that are predicted for prototypical models of surface dynamics. Models that have been considered in this work include various Langmuir-type adsorption and 2D gas models. We demonstrate that a two-dimensional gas can display a low apparent saturation coverage when adsorption is facilitated by a subset of surface sites. Additionally, we consider a Langmuir replacement reaction and show that a large apparent adsorption entropy can arise from such a situation. Finally, we discuss how the choice of a model affects the range of plausible values for the adsorption entropy and equilibrium constant. Adsorption of acetone on Degussa P25 TiO2 is used as an illustrative example.

Photoinduced conversion of surface-bound species on titania nanotubes that were first oxidized and then reduced (Ti–NT–O2–H2) and on platinized titania nanotubes subjected to oxidation and reduction (Pt–Ti–NT–O2–H2) has been investigated by means of in situ FTIR spectroscopy. Bidentate and monodentate carbonates as well as bicarbonates and carboxylates are formed subsequent to exposure of both Ti–NT–O2–H2 and Pt–Ti–NT–O2–H2 to CO2. Formic acid was only observed on Pt–Ti–NT–O2–H2. UV illumination of the nanotubes led to an increase in the number of surface-bound species as a result of the further reaction with gas-phase CO2 with a greater increase in surface species on Ti–NT–O2–H2 than on Pt–Ti–NT–O2–H2. The underlying basis of the photoinduced increase in adsorbed species is discussed for both types of nanotubes. Photoinduced reactions of surface species also take place and are remarkably different on the two types of nanotubes. UV illumination of Ti–NT–O2–H2 converts bidentate carbonates and bicarbonates to monodentate carbonates and carboxylates. There are less, and different, photoinduced reactions of surface species on Pt–Ti–NT–O2–H2: bicarbonates and monodentate carbonates convert to bidentate carbonates on the platinized titania nanotubes, and there is no obvious reaction involving carboxylates and formic acid upon irradiation of the platinized nanotubes. These differences in reactive behavior are discussed in the context of platinum acting as an efficient trap for photoelectrons which mitigates against reduction of Ti4+ to Ti3+, stabilizes holes, and alters the surface photochemistry taking place on the two different types of nanotubes. Photoinduced holes play an important role in photochemistry via oxidation of “structural water” and concomitant production of undercoordinated titania sites.

The pathways for the formation of 5-hydroxymethylfurfural (HMF) by dehydration of d-fructose and for the formation of levulinic acid and formic acid from HMF by rehydration were investigated by in situ13C and 1H NMR using both unlabeled and 13C-labeled fructose. Water or DMSO was used as the solvent with Amberlyst 70, PO43–/niobic acid, or sulfuric acid as catalysts. Only HMF is observed using NMR for fructose dehydration in DMSO with any of the three catalysts or without a catalyst. For each system, results with 13C-labeled fructose indicate that the first carbon (C-1) or sixth carbon (C-6) of fructose maps onto the corresponding carbons of HMF. For fructose dehydration in H2O with a PO43–/niobic acid catalyst, in addition to HMF, furfural was observed as a product. However, we show that furfural is not a reaction product deriving from HMF under our conditions. Rather our data indicate that there is a parallel reaction pathway open to fructose when the reaction takes place in H2O with a PO43–/niobic acid catalyst. The corresponding 13C-labeled results show that the first carbon in fructose maps onto the first carbon (aldehyde carbon) in furfural. Using 13C-enriched HMF formed from dehydration of 13C-labeled fructose in DMSO or H2O, we investigated the pathway for HMF rehydration to levulinic and formic acid. The data in different solvents and with different catalysts are consistent with a common mechanism for HMF rehydration, which results in the C-1 and C-6 carbon of HMF being transformed to the carbon of formic acid and methyl carbon (C-5) of levulinic acid, respectively.Keywords: fructose dehydration; HMF rehydration; in situ NMR spectroscopy; isotope labeling studies;

The mechanism of temperature-programmed desorption (TPD) of nitric acid chemisorbed on BaNa–Y was studied over the temperature range from 200 to 400 °C, in the presence and absence of CO. Nitric acid dissociates to form H+ and NO3− when chemisorbed on BaNa–Y. The results of these experiments are consistent with H+ and NO3− either reacting directly to produce OH and NO2 or recombining to produce HNO3, which is desorbed and rapidly decomposes within the zeolite pores to OH and NO2. The kinetics and stoichiometry suggest that the hydroxyl radicals produced react with CO and NO2 to form CO2 + H and NO + HO2, respectively. The H atoms thus formed react with OH in preference to NO2, a change in mechanism consistent with literature rate constants and the expectation that the zeolite pore walls act as a third body for the reaction of H with OH. Finally, OH may react with NO2 to form HO2, which can undergo further reactions to form O2, H2O, and/or H2. No reaction between CO and NO3 or CO and surface-bound NO3− was observed.

Nitromethane (NM) is a very efficient reductant for converting NO2 to N2 over Ag/Y: Between 140 °C and 400 °C, the N2 yield is close to 100%. This high N2 yield results from the ability of Ag/Y to effectively catalyze the reaction between NM and NO2. This high catalytic activity of Ag/Y is minimally affected by surface bound CN−, NC−, or acetate, all of which are stable at temperatures below ∼300 °C. At T ≥ 400 °C, there is a reaction path that yields N2 from NM even in the absence of NO2. However even at 400 °C, under typical deNOx conditions, most N2 molecules are formed as a result of the reaction of NM and NO2.

The kinetics of alkyl group migration in RMn(CO)5 complexes ( R=CH3, C2H5 and C3H7) were studied. Isomers of CH3Mn(CO)5 with an agostic structure, an η1 structure, and an η2 structure were found to be local minima on the system's potential energy surface. Transition states for the inter-conversion of these species were also located. The activation free energy for this migration reaction was compared with experimental data and provides insights into the important steps in the overall reaction mechanism.The kinetics of alkyl group migration in RMn(CO)5 complexes (R=CH3, C2H5 and C3H7), isomers of CH3Mn(CO)5, and transition states for interconversion of these species were studied using density functional theory. The activation free energy for this migration reaction was compared with experimental data and provides insights into the important steps in the overall reaction mechanism.

•Three intermediates identified in the acid catalyzed dehydration of Fructose to HMF.•Experimentally identified intermediates correspond to calculated low energy species.•Explicit involvement of DMSO solvent demonstrated in intermediate formation.•Intermediates’ structures independent of the catalyst used.We report on a combined experimental and theoretical study of the acid catalyzed dehydration of d-fructose in dimethylsulfoxide (DMSO) using; Amberlyst 70, PO43−/niobic acid, and sulfuric acid as catalysts. The reaction has been studied and intermediates characterized using; 13C, 1H, and 17O NMR, and high resolution electrospray ionization mass spectrometry (HR ESI–MS). High level G4MP2 theory calculations are used to understand the thermodynamic landscape for the reaction mechanism in DMSO. We have experimentally identified two key intermediates in the dehydration of fructose to form HMF that were also identified, using theory, as local minima on the potential surface for reaction. A third intermediate, a species capable of undergoing keto–enol tautomerism, was also experimentally detected. However, it was not possible to experimentally distinguish between the keto and the enol forms. These data with different catalysts are consistent with common intermediates along the reaction pathway from fructose to HMF in DMSO. The role of oxygen in producing acidic species in reactions carried out in DMSO in presence of air is also discussed.

The selective reduction of NOx with added oxygenates over BaNa/Y and Ag/Y zeolites and Ag/γ-Al2O3 takes place via complex reaction pathways with a number of common crucial intermediates. Acetate ions are formed by the oxidation of acetaldehyde over these catalysts. These acetate ions react with NO2 to form nitromethane which decomposes to HNCO via a dinitro-C1 intermediate. HNCO hydrolyzes to form NH3 which can react with HONO to form ammonium nitrite. This NH4NO2 efficiently decomposes to N2 and H2O at 100 °C, and at even lower temperatures in an acidic environment. The neutral surface species are expected to be in equilibrium with their ions. The rate-limiting step in these reaction sequences is the reaction of acetate ions to form nitromethane. When nitromethane is directly added to a NOx stream over Ag/Y, ∼100% conversion of NOx to N2 is achieved at temperatures as low as 140 °C. In such schemes, NO acts as a reductant of nitrate ions, ammonium nitrate and nitric acid. The benefits of isotopically labeled compounds in the elucidation of such reaction mechanisms and for providing insights into reaction dynamics are also discussed.

A multistep mechanism has been elucidated for the reduction of NOx in the presence of ethanol over silver-exchanged zeolite Y (Ag/Y). Ethanol reacts with O2 and/or NO2 to form acetaldehyde at temperatures as low as 200 °C. Surface acetate ions, formed from the oxidation of acetaldehyde, react with NO2 to yield nitromethane, a critical intermediate in subsequent deNOx chemistry. CN−, NC−, and NCO− are intermediates likely bound to silver ions. Both CN− and NC− are stable toward reaction under experimental conditions. A significant difference exists between the catalytic activities of Ag/Y and Ag/γ-Al2O3; oxidation of ethanol to acetate at low temperature is significantly faster over Ag/Y than over Ag/γ-Al2O3, and both NO2 and O2 are effective oxidants over Ag/Y. With Ag/Y, pretreatment with either O2 or H2 does not affect the yield of N2, which approaches 60% and remains constant for at least 5 h, making this catalyst promising for NOx reduction.

Ni-, Cu-, and Fe-loaded acidic and basic Y zeolites were synthesized, and their catalytic properties for oxidative dehydrogenation of ethane (ODHE) to ethylene were characterized. Acidic Ni-loaded Y zeolite exhibits an ethylene productivity of up to 1.08gC2H4gcat-1h-1 with a selectivity of ∼75%. Acidic Cu- and Fe-loaded Y zeolites have an ethylene productivity of up to 0.37gC2H4gcat-1h-1 and a selectivity of ∼50%. For the same metal, the acidity of the zeolite favors both ODHE productivity and ethylene selectivity. Extended X-ray absorption fine structure (EXAFS) studies show that Ni, present in particles on Ni/HY during the ODHE catalytic process, contains both Ni–Ni and Ni–O bonds, and that the ratio of oxidized Ni versus metallic Ni increases with the temperature. The insights these studies provide into the ODHE reaction mechanism are discussed.Metal-loaded zeolite Y catalyzed oxidative dehydrogenation of C2H6 to C2H4.Download high-res image (60KB)Download full-size image

The mechanism of temperature-programmed desorption (TPD) of nitric acid chemisorbed on BaNa–Y was studied over the temperature range from 200 to 400 °C, in the presence and absence of CO. Nitric acid dissociates to form H+ and NO3− when chemisorbed on BaNa–Y. The results of these experiments are consistent with H+ and NO3− either reacting directly to produce OH and NO2 or recombining to produce HNO3, which is desorbed and rapidly decomposes within the zeolite pores to OH and NO2. The kinetics and stoichiometry suggest that the hydroxyl radicals produced react with CO and NO2 to form CO2 + H and NO + HO2, respectively. The H atoms thus formed react with OH in preference to NO2, a change in mechanism consistent with literature rate constants and the expectation that the zeolite pore walls act as a third body for the reaction of H with OH. Finally, OH may react with NO2 to form HO2, which can undergo further reactions to form O2, H2O, and/or H2. No reaction between CO and NO3 or CO and surface-bound NO3− was observed.