Nanocrystals of MgO and CaO have been prepared by a modified aerogel/hypercritical drying/dehydration method. For nanocrystalline MgO (AP-MgO) surface areas ranged from 250 to 500 m2/g, whereas for AP-CaO 100−160 m2/g. These materials have been compared with more conventional (CP) microcrystalline samples of lower surface area with regard to (1) morphology (AP-samples (autoclave preparation) are tiny polyhedral crystallites, while CP-samples (conventional preparation) are larger, hexagonal platelets and cubes); (2) residual surface OH (AP-samples have less acidic OH, which are more isolated from each other; (3) acid gas adsorption (AP-samples adsorb more SO2 and CO2 at low pressures and room temperature and prefer monodentate rather than bidentate adsorption modes, but at higher pressures CP-samples adsorb more SO2 and HCl apparently due to the formation of more well ordered multilayers); (4) destructive adsorption of organophosphorus compounds and chlorocarbons (AP-samples are superior due to higher surf...

Nanocrystals as Stoichiometric Reagents with Unique Surface Chemistry

The electrochemical reduction of nitrogen to ammonia on Au-based catalysts showed a reasonably high Coulombic efficiency. The pathway of this promising reaction, however, is not clear partially due to the lack of information on reaction intermediates. Herein, surface-enhanced infrared absorption spectroscopy (SEIRAS) was employed to study the reaction mechanisms of nitrogen reduction on an Au thin film for the first time. During the nitrogen reduction, the N2Hy species was detected with bands at 1453 (H-N-H bending), 1298 (-NH2 wagging), and 1109 cm-1 (N-N stretching) at potentials below 0 V against reversible hydrogen electrode. This result indicates that the nitrogen reduction reaction on Au surfaces follows an associative mechanism, and the N≡N bond in N2 tends to break simultaneously with the hydrogen addition. By comparison, no absorption band associated with N was observed on Pt surfaces under the same reaction condition. This result is consistent with the low efficiency of nitrogen reduction on Pt due to the much faster kinetics of hydrogen evolution reaction.

A Spectroscopic Study on the Nitrogen Electrochemical Reduction Reaction on Gold and Platinum Surfaces.

Aerogels, which are nanoscale mesoporous materials of low density
and high surface area, have been well studied as thermal insulators, heterogeneous
catalysts, and novel particle or radiation detectors. Now, electrically conducting
oxide aerogels are being explored as new materials in electrochemistry and
for their innate ability to amplify the nature of the surfaces of technologically
relevant conducting oxides in batteries, ultracapacitors, and fuel cells.
Recent results are reviewed in which the mixed electron- and cation-conducting
transition metal oxides of vanadium, molybdenum, ruthenium, and manganese
have been prepared as low density, highly porous, and high surface area aerogels
and then studied as charge-storage electrode materials. These materials challenge
the standard ways in which electrochemically active oxides are conceived,
studied, and used.

Electrically conductive oxide aerogels: newmaterials in electrochemistry

The oxidation of hydrogen-terminated porous silicon surfaces produced by electrochemical etching has been studied using transmission FTIR spectroscopy. The surface is passivated to oxidation by surface hydrogen below about 523 K. Above this temperature as hydrogen depletion occurs by H2 evolution, Si surface dangling bond sites, capable of O2 dissociation, are involved in initiating the first stage of oxidation. Two reactions are observed. The first, O insertion into Si−Si back-bonds, leads to −OySiHx surface species which exhibit frequency shifts to the blue compared to parent SiHx stretching modes. In addition, Si−O−Si modes are also observed to form. The second reaction involves oxygen atom insertion into Si−H bonds to produce isolated Si−OH surface species.

FTIR Study of the Oxidation of Porous Silicon

In Situ Infrared Study of Methanol Synthesis from H2/CO over Cu/SiO2and Cu/ZrO2/SiO2

Reaction of Methanol with Porous Silicon

The dehydroxylation of SiO{sub 2} and adsorption and thermal decomposition of CH{sub 3}OH on dehydroxylated SiO{sub 2} prepared at extreme temperatures up to 1475 K has been studied using transmission infrared spectroscopy. Formation of Si-O Si linkages (as evidenced by 888 and 908 cm{sup -1} bands) is directly related to the loss of the isolated SiO-H band at 3748 cm{sup -1} during SiO{sub 2} dehydroxylation to produce H{sub 2}O{sub 1}. High-resolution (0.75 cm{sup -1}) studies of the isolated SiO-H band show sharpening and a slight 4 cm{sup -1} shift to higher frequency with decreasing -OH coverage and may be explained by site inhomogeneity effects. Adsorption of CH{sub 3}OH on dehydroxylated SiO{sub 2} occurs first by reaction with SiOSi sites, followed by exchange with SiOH groups at elevated temperatures leading to SiOCH{sub 3} surfaces species. CH{sub 3}{sup 18}OH adsorption studies demonstrated that adsorption on SiOSi sites occurs by cleavage of the methanol O-H bond forming SiOH and Si{sup 18}OCH{sub 3}. Thermal decomposition of SiOCH{sub 3} groups creates SiH{sub x} (x = 1,2) species and prevents formation of the SiOSi sites indicative of the production of free radical sites at the surface. At temperatures above 1300 K, SiH{sub x} decomposition leads tomore » SiOSi site formation. 40 refs., 11 figs.« less

Active sites on SiO2 : role in CH3OH decomposition

Transmission infrared spectroscopy was used to monitor CCl x (a) intermediate surface species formed during the decomposition of CCl 4 on high surface area calcium oxide. Carbon tetrachloride was adsorbed on calcium oxide at 113 K During CCl 4 desorption, additional infrared bands at 801, 787, 775, 765, and 751 cm -1 were observed in the temperature range 110-250 K. These bands, which disappeared in the temperature range 150-250 K, were attributed to the formation and depletion of CCl x (a) species as C-Cl bonds are broken and Ca-Cl bonds are formed on the CaO surface. Carbon tetrachloride ( 13 C labeled) was employed to prove that the infrared bands of the surface intermediates were due to C-Cl vibrational modes. To compare carbon tetrachloride decomposition on different types of calcium oxides, both autoclave prepared (nanoscale particles) and conventionally prepared CaO were investigated.

John A. Glass

Papers

FTIR Study of the Oxidation of Porous Silicon

Reaction of Methanol with Porous Silicon

Active sites on SiO2 : role in CH3OH decomposition

Decomposition of CCl4 on CaO

Reaction of atomic hydrogen with hydrogenated porous silicon — detection of precursor to silane formation