Differential Reactivity and Structure of Mono- and Dialkoxides: The Reactions of Ethylene Glycol on Mo(110)

Modifier Effect in Silica-Supported FePO4 and Fe-Mo-O Catalysts for Propylene Glycol Oxidation

Currently, catalytic processing of biorenewable raw materials into valuable products attracts more and more attention. In the present work, silica-supported FePO₄ and Fe-Mo-O catalysts are prepared, their phase composition, and catalytic properties are studied in the process of selective oxidation of propylene glycol into valuable mono- and bicarbonyl compounds, namely, hydroxyacetone and methylglyoxal. A comparative analysis of the main routes of propylene glycol adsorption with its subsequent oxidative conversion into carbonyl products is carried out. The DFT calculations show that in the presence of adsorbed oxygen atom, the introduction of the phosphate moiety to the Fe-containing site strengthens the alcohol adsorption on the catalyst surface with the formation of the 1,2-propanedioxy (–OCH(CH₃)CH₂O–) intermediate at the active site. The introduction of the molybdenum moiety to the Fe-containing site in the presence of the adsorbed oxygen atom is also energetically favorable, however, the interaction energy is found by 100 kJ/mol higher compared to the case with phosphate moiety that leads to an increase in the propylene glycol conversion while maintaining high selectivity towards C₃ products. The catalytic properties of the synthesized iron-containing catalysts are experimentally compared with those of Ag/SiO₂ sample. The synthesized FePO₄/SiO₂ and Fe-Mo-O/SiO₂ catalysts are not inferior to the silver-containing catalyst and provide ~70% selectivity towards C₃ products, while the main part of propylene glycol is converted into methylglyoxal in contrast to the Ag/SiO₂ catalyst featuring the selective transformation of only the secondary C-OH group in the substrate molecule under the studied conditions with the formation of hydroxyacetone. Thus, supported Fe-Mo-O/SiO₂ catalysts are promising for the selective oxidation of polyatomic alcohols under low-temperature conditions.

Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts

The one-step reaction of glycerol with hydrogen to form propene selectively is a particularly challenging catalytic pathway that has not yet been explored thoroughly. Molybdena-based catalysts are active and selective to C-O bond scission; propene is the only product in the gas phase under the standard reaction conditions, and further hydrogenation to propane is impeded. Within this context, this work focuses on the exploration of the reaction pathways and the investigation of various parameters that affect the catalytic performance, such as the role of hydrogen on the product distribution and the effect of the catalyst pretreatment step. Under a hydrogen atmosphere, propene is produced primarily via 2-propenol, whereas under an inert atmosphere propanal and glycerol dissociation products are formed mainly. The reaction most likely proceeds through a reverse Mars-van Krevelen mechanism as partially reduced Mo species drive the reaction to the formation of the desired product.

Modeling the surface chemistry of biomass model compounds on oxygen-covered Rh(100)

This study provides a fundamental insight about how biomass-related compounds interact with metal surfaces in aqueous medium.

In situ PM-IRRAS of a glassy carbon electrode/deep eutectic solvent interface

Firstin situPM-IRRAS studies of a carbon electrode/deep eutectic solvent interface show ad- and desorption of electrolyte components.

Site-specific imaging of elemental steps in dehydration of diols on TiO(2)(110)

Scanning tunneling microscopy is employed to follow elemental steps in conversion of ethylene glycol and 1,3-propylene glycol on partially reduced TiO2(110) as a function of temperature. Mechanistic details about the observed processes are corroborated by density functional theory calculations. The use of these two diol reactants allows us to compare and contrast the chemistries of two functionally similar molecules with different steric constraints, thereby allowing us to understand how molecular geometry may influence the observed chemical reactivity. We find that both glycols initially adsorb on Ti sites, where a dynamic equilibrium between molecularly bound and deprotonated species is observed. As the diols start to diffuse along the Ti rows above 230 K, they irreversibly dissociate upon encountering bridging oxygen vacancies. Surprisingly, two dissociation pathways, one via O-H and the other via C-O bond scission, are observed. Theoretical calculations suggest that the differences in the C-O/O-H bond breaking processes are the result of steric factors enforced upon the diols by the second Ti-bound OH group. Above ∼400 K, a new stable intermediate centered on the bridging oxygen (Ob) row is observed. Combined experimental and theoretical evidence shows that this intermediate is most likely a new dioxo species. Further annealing leads to sequential C-Ob bond cleavage and alkene desorption above ∼500 K. Simulations demonstrate that the sequential C-Ob bond breaking process follows a homolytic diradical pathway, with the first C-Ob bond breaking event accompanied with a nonadiabatic electron transfer within the TiO2(110) substrate.

Visible light-induced degradation of ethylene glycol on nitrogen-doped TiO2 powders

The photocatalytic degradation processes of ethylene glycol (EG) during the UV or visible light irradiation of pure anatase and nitrogen (N)-doped TiO2 powders (TiO(2-x)N(x), x = 0, 0.002, 0.003, and 0.007) were investigated using time-resolved diffuse reflectance (TDR) and solid-state NMR spectroscopies. The TDR spectra and time traces observed for the charge carriers indicated that the scavenging of photogenerated holes (h+) by EG occurred during the 355-nm laser photolysis of the N-doped TiO2 powders, while no direct oxidation reaction of EG by h+ occurred during the 460-nm laser photolysis, although the charge carriers were sufficiently generated upon excitation. The solid-state magic-angle spinning (MAS) NMR measurements revealed that EG is preferentially chemisorbed on the surface of the N-doped TiO2 powders, in contrast to the pure TiO2, and degrades under visible light irradiation.

Thermal decomposition of HSCH2CH2OH on Cu(111): identification and adsorption geometry of surface intermediates

X-ray photoelectron spectroscopy has been employed to study the surface intermediates from the thermal decomposition of HSCH2CH2OH on Cu(111) at elevated temperatures. On the basis of the changes of the core-level binding energies of C, O, and S as a function of temperature, it is found that HSCH2CH2OH decomposes sequentially to form -SCH2CH2OH and -SCH2CH2O-. Theoretical calculations based on density functional theory for an unreconstructed one-layer copper surface suggest that -SCH2CH2OH is preferentially bonded at a 3-fold hollow site, with an adsorption energy lower than the cases at bridging and atop sites by 15.6 and 47.5 kcal x mol(-1), respectively. Other structural characteristics for the energy-optimized geometry includes the tilted C-S bond (14.1 degrees with respect to the surface normal), the C-C bond titled toward a bridging site, and the C-O bond pointed toward the surface. In the case of -SCH2CH2O- on Cu(111), the calculations suggest that the most probable geometry of the adsorbate has its S and O bonded at hollow and bridging sites, respectively. With respect to the surface normal, the angles of the S-C and O-C are 27.9 and 34.0 degrees.

Crotonaldehyde Formation from Decomposition of ICH2CH2OH on Powdered TiO2

Adsorption and reactions of 2-iodoethanol on TiO(2) have been studied by Fourier transform infrared spectroscopy. ICH(2)CH(2)OH possesses two reactive centers of C-I and C-OH. It is found that its decomposition leads to the formation of crotonaldehyde on TiO(2). A reaction sequence of ICH(2)CH(2)OH --> ICH(2)CH(2)O- --> CH(3)CHO --> CH(3)CH=CH-CHO is proposed. Although the decomposition routes of C(2)H(5)OH and C(2)H(5)I, both forming C(2)H(5)O- on TiO(2), suggest that -OCH(2)CH(2)O- may play a role in the crotonaldehyde formation, reaction of HOCH(2)CH(2)OH on TiO(2) shows that this is not the case. Adsorbed H(2)O is formed in the ICH(2)CH(2)OH decomposition on TiO(2); however, it is found that ICH=CH(2), possibly generated by ICH(2)CH(2)OH dehydration, is not important in the crotonaldehyde formation.

Reforming of Oxygenates for H2 Production: Correlating Reactivity of Ethylene Glycol and Ethanol on Pt(111) and Ni/Pt(111) with Surface d-Band Center

The dehydrogenation and decarbonylation of ethylene glycol and ethanol were studied using temperature programmed desorption (TPD) on Pt(111) and Ni/Pt(111) bimetallic surfaces, as probe reactions for the reforming of oxygenates for the production of H2 for fuel cells. Ethylene glycol reacted via dehydrogenation to form CO and H2, corresponding to the desired reforming reaction, and via total decomposition to produce C(ad), O(ad), and H2. Ethanol reacted by three reaction pathways, dehydrogenation, decarbonylation, and total decomposition, producing CO, H2, CH4, C(ad), and O(ad). Surfaces prepared by deposition of a monolayer of Ni on Pt(111) at 300 K, designated Ni-Pt-Pt(111), displayed increased reforming activity compared to Pt(111), subsurface monolayer Pt-Ni-Pt(111), and thick Ni/Pt(111). Reforming activity was correlated with the d-band center of the surfaces and displayed a linear trend for both ethylene glycol and ethanol, with activity increasing as the surface d-band center moved closer to the Fermi level. This trend was opposite to that previously observed for hydrogenation reactions, where increased activity occurred on subsurface monolayers as the d-band center shifted away from the Fermi level. Extrapolation of the correlation between activity and the surface d-band center of bimetallic systems may provide useful predictions for the selection and rational design of bimetallic catalysts for the reforming of oxygenates.