Chemical reactions at silica surfaces

Morrow, B. A.; McFarlan, Andrew

doi:10.1016/0022-3093(90)90191-n

Cited by 151 publications

(132 citation statements)

References 39 publications

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“…It is now agreed that the relative contribution of the geminals on the silica's surface is relatively small to the total number of silanol groups. Morrow [43] observed that a small quantity of silicon atoms on the silica's surface (less than 5% of the total concentration of silanols) treated at 700 ∘ C carried out geminal silanols; while most of the silicon atoms Figure 11: Representation of different silanol groups (isolated, silanediol and Silanetriol silanols), H-bonded silanols (single and silanediol) alongside siloxanes bridges on the surface of colloidal silica [42] carried single hydroxyl groups being either free or vicinal silanols.…”

Section: Silica Surface Structure (Physisorbed Water Silanol Groups mentioning

confidence: 99%

A review of Fischer Tropsch synthesis process, mechanism, surface chemistry and catalyst formulation

Mahmoudi

Doustdar

et al. 2017

Biofuels Engineering

165

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For more than half a century, Fischer-Tropsch synthesis (FTS) of liquid hydrocarbons was a technology of great potential for the indirect liquefaction of solid or gaseous carbon-based energy sources (Coal-To-Liquid (CTL) and Gas-To-Liquid (GTL)) into liquid transportable fuels. In contrast with the past, nowadays transport fuels are mainly produced from crude oil and there is not considerable diversity in their variety. Due to some limitations in the first generation bio-fuels, the Second-Generation Biofuels (SGB)' technology was developed to perform the Biomass-To-Liquid (BTL) process. The BTL is a well-known multi-step process to convert the carbonaceous feedstock (biomass) into liquid fuels via FTS technology. This paper presents a brief history of FTS technology used to convert coal into liquid hydrocarbons; the significance of bioenergy and SGB are discussed as well. The paper covers the characteristics of biomass, which is used as feedstock in the BTL process. Different mechanisms in the FTS process to describe carbon monoxide hydrogenation as well as surface polymerization reaction are discussed widely in this paper. The discussed mechanisms consist of carbide, COinsertion and the hydroxycarbene mechanism. The surface chemistry of silica support is discussed. Silanol functional groups in silicon chemistry are explained extensively. The catalyst formulation in the Fischer Tropsch (F-T) process as well as F-T reaction engineering is discussed. In addition, the most common catalysts are introduced and the current reactor technologies in the F-T indirect liquefaction process are considered. process overview IntroductionThe over-reliance of the world's nations on conventional fossil fuels puts our planet in peril. The continuity of the current situation will result in the rise of a combined average temperature over global land and ocean surfaces by 5 ∘ C in 2100, bringing a rise in sea levels, food and water shortages and an increase in extreme weather events. The global warming, caused by humans, is one of the biggest threats to our future well-being [1]. In addition, oil reserves are limited and these reserves are decreasing dramatically. This reduction alongside the other relevant economic factors affects the world's oil prices. The need to run engines with the new generation of liquid fuels is inevitable. The investigations by the US Energy Information Administration (EIA) published in 2013 expressed a 56 percent increase in the world's energy consumption by the year 2040. Total world energy demand will have risen to 865 EJ (exajoule) by this year. The total world energy consumption was reported as 553 EJ in 2010. The outlook indicates that renewable energy is one of the fastest-growing energy sources in the world; where its usage increases 2.5 percent per year. Despite increasing success in the renewable energies, it is predicted that the fossil fuels will supply al-

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Section: Silica Surface Structure (Physisorbed Water Silanol Groups mentioning

confidence: 99%

A review of Fischer Tropsch synthesis process, mechanism, surface chemistry and catalyst formulation

Mahmoudi

Doustdar

et al. 2017

Biofuels Engineering

165

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“…Thus, the preparation of highly dispersed metal oxides on silica by either impregnation or chemical vapor deposition often involves a highly reactive H-sequestering precursor, such as V-alkoxides, which will readily react with the surface hydroxyls of the silica support. 12,19,20 The aqueous impregnation method with NH 4 VO 3 as the precursor usually results in a low dispersion (<5 wt % V 2 O 5 ). 10,13,14,16,18 On the other hand, the CVD method with V-ethoxide as the precursor may result in thin overlayers of V 2 O 5 on SiO 2 with polymerized V-O-V bonds.…”

Section: Introductionmentioning

confidence: 99%

In Situ Spectroscopic Investigation of Molecular Structures of Highly Dispersed Vanadium Oxide on Silica under Various Conditions

et al. 1998

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The molecularly dispersed V 2 O 5 /SiO 2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of V-isopropoxide. The experimental maximum dispersion of surface vanadium oxide species on SiO 2 was achieved at ∼12 wt % V 2 O 5 (∼2.6 V atoms/nm 2 ). The surface structures of the molecularly dispersed V 2 O 5 /SiO 2 samples under various conditions were extensively investigated by in situ Raman, UVvis-NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO 4 species are present on the silica surface up to monolayer coverage. Interestingly, the three-member siloxane rings on the silica surface appear to be the most favorable sites for anchoring the isolated, three-legged (SiO) 3 VdO species. Hydration dramatically changes the molecular structure of the surface vanadium oxide species. The specific structure of the hydrated surface vanadium oxide species is dependent on the degree of hydration. The molecular structure of the fully hydrated vanadium oxide species closely resembles V 2 O 5 ‚nH 2 O gels, rather than V 2 O 5 crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highly distorted square-pyramidal VO 5 connected by V-OH-V bridges, which are stabilized on the silica surface by the sixth neighbor of Si-OH hydroxyls via Si-OH‚‚‚V hydrogen bonds. In analogy to the hydration process, alcoholysis occurs during methanol chemisorption, and similar molecular structures are proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica.

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“…Extensive characterization experiments under dehydrated conditions have also been conducted by means of in situ IR, UV-Vis DRS, XPS, XANES, and 27 Al NMR, 4j 1 Use of Oxide Ligands in Designing Catalytic Active Sites which provide additional molecular information about the supported MO x phases on SiO 2 [77][78][79][80]. The supported 5% TiO 2 /SiO 2 and 5% ZrO 2 /SiO 2 samples consist of 100% dispersed surface TiO x and ZrO x species on the SiO 2 support.…”

Section: Molecular Structural Determination Of Supported Metal Oxide mentioning

confidence: 99%

“…These supported metal oxide catalysts consist of highly dispersed surface metal oxide species -the catalytic active sites -anchored to the underlying oxide support. Over the decades, there have been many successful methods of controlling and anchoring the molecularly dispersed metal oxide phase by chemical vapor deposition (CVD) [26][27][28], incipient wetness impregnation [29][30][31][32][33][34], and precipitation [35][36][37]. The surface coverage of the metal oxide overlayer can vary from below to above monolayer coverage or the maximum dispersion limit, that is, maximum metal oxide dispersion before formation of crystalline metal oxide nanoparticles (NPs).…”

Section: Introductionmentioning

confidence: 99%