M.J.D. Low

Reactions of gaseous pollutants with solids. V. Infrared study of the sorption of NO on CaO

A Fourier transform spectrometer was used to record spectra of the surface layer formed on CaO by sorbed NO, and during the conversion and destruction of the layer by degassing and treatments with oxygen. NO was taken up at pressures in the low micro-Torr range, most of the surface layer being relatively firmly bound. Numerous infrared bands were observed, pointing to the existence of at least a dozen distinct surface species. Tentative band assignments suggest the formation of NO−, NO2−>, NO3−), NO+, and monodentate and bidentate nitrato complexes. At least six other species are unidentified, so that the mechanisms cannot be considered. The extreme complexity of the system and the numerous surface species found with CaO lead to the speculation that on other adsorbents, on which quite few NOx species have been found, there may also exist numerous NOx species.

Reactive silica: XV. Some properties of solids prepared by the reaction of trimethylaluminum with silica

Abstract Infrared spectrocopic studies confirm earlier work that when trimethylaluminum (TMA) reacts with silica, the surface is dehydroxylated and Si-CH 3 and A1-CH 3 species are formed; H 2 O hydrolizes the latter. H 2 O partially hydroxylated the surface; two types of adsorbed water formed, absorbing at 1670 and 1640 cm −1 . Degassing the TMA-treated samples caused them to become partially activated, but deactivation occurred readily. Degassed samples could be partially reduced with H 2 , resulting in the formation of silanols and species thought to be Al + HO − and Al-H.

Infrared radiometry of thermal transients on surfaces: An approach to the study of thermal effects caused by gas-solid interactions

A radiometer was used to study the sorption of O2 on SiO2-supported Ni. The measurements are based on the premise that the energy produced by the gas-solid interaction heats the surface of the solid, the radiation emitted by the heated surface then being detected. Rapid thermal transients were observed. The method and some preliminary results are discussed.

Reactive silica: VII. Chemisorption and pyrolysis procedures in a flow system

Abstract The vacuum pyrolysis of surface SiOCH 3 groups leads to the formation of SiH 2 and silanol groups which disappear on degassing. The silica then becomes remarkably active. To check if the reaction was unique to SiOCH 3 , a variety of reagents were tested using a flow system and monitoring the reactions by infrared spectroscopy. Most alcohols and simple esters became chemisorbed as silyl alkyl ethers. In general, with the exception of benzyl alcohol, a reagent was effective in producing SiH 2 if, upon chemisorption, methoxy groups were formed directly or if the adsorbed species itself contained methoxy groups. The results suggest that the initial reaction of a reagent with surface silanol s involved a mechanism in which an initial hydrogen bonding led to the formation of a surface-stabilized ion pair which could then readily suffer a direct displacement; the postulated mechanism can account for the observed order of chemisorption. The pyrolysis of alkoxy groups is thought to involve the homolytic cleavage of the OC bond to generate a siloxy radical which would then react further with desorbed material and/or with neighboring alkoxy groups. With methoxy, the SiH 2 and silanols were formed because the siloxy radical was too far from neighboring methoxy groups for interaction to occur. With alkoxy groups other than methoxy, however, the siloxy radical was within bonding distance of the alkoxy, and direct interaction was possible, surface silanols being generated.

The reaction of ammonia with SiOSiHCl2 monolayers on silica

Abstract High surface area silica was completely dehydroxylated by reaction with HSiCl 3 . The SiOSiHCl 2 (I) monolayer was then exposed to NH 3 at 22 °C, the surface reactions being followed by infrared spectroscopy using the perturbation of the SiH band as probe of the otherwise unobservable reactions occurring at the SiCl bond. NH 3 was taken up at quite low pressures to form a mixture of surface species. I reacted to form a species II which was then converted to a species III. Degassing at 22 °C reversed the processes, almost all I being re-formed. Successive sorption-degassing cycles caused the formation or at least three other species. The spectra suggest that reversible ammonolysis of I occurred, the SiH mode being perturbed by the reversible formation of SiNH 2 ··· NH 4 Cl. The complex results suggest that reactions on halogenated surfaces are more involved than has so far been suspected.

Reactive silica. XVII: The nature of the reaction center

Abstract All of the various experimental data concerning the properties of reactive silica (RS) were reexamined and reevaluated. The assignments of two bands of surface silane species appearing in infrared spectra of RS were found to be wrong and had to be revised. Although none of the assignments of other bands of surface species were affected, this revision, along with certain unsatisfactory features concerning the activation mechanism and RS reactivity, made the model of the RS center untenable. That center was a dual one, incorporating two spatially separated silicons, each of which acted as a reaction site. The reaction site now proposed is also a dual one, as demanded by the various data, but is a single silicon which acts as the dual site. The entire RS center consists of two 6-membered rings, each consisting of three siloxane bridges, joined at and incorporating the dual-site silicon. The latter is in octahedral rather than the usual tetrahedral configuration. The new model is consistent with all experimental observations concerning the formation and reactions of RS.

Reactive silica: VIII. Methoxylation of silica using trimethoxymethane☆

Abstract Trimethoxymethane (TMM) sorption by severely and partially dehydroxylated, as well as fully deuterated silicas, was followed by infrared spectroscopic techniques. TMM is physically adsorbed at 25 °, interacting with surface silanols to yield a hydroxyl shift of 350 cm−1. The weakly adsorbed TMM can be removed at 25 °. Near 300 ° the reaction SiOH + HC(OCH3)3 → SiOCH3 + CH3OOCH + CH3OH becomes dominant, and all accessible surface hydroxyls are readily removed in the 300–500 ° range. Above 500 ° the thermal decomposition of TMM is dominant. These decomposition products, as well as hydrolysis products, play a minor role in modifying silica surfaces in the 25–275 ° range, most effects being caused by methanol contaminant.

Reactive silica: XIII. Activation of silica by pyrolizing chemisorbed HSiCl3☆

Abstract A new method of activating silica consists of chemisorbing HSiCl 3 and then degassing above 600 °C. Infrared spectra show that surface silane groups are eliminated, and the surface is and remains completely dehydroxylated. The formation of the centers responsible for the activity probably involves the simultaneous elimination of SiCl and SiH 2 groups, leading to a rearrangement of the surface.

Reactive silica: VI. Evidence for the existence of a dual reaction center involving ṠiȮ pairs☆

Abstract Extensive work has pointed to the existence on the surface of reactive silica [Morterra and Low, J. Catal. 28 , 265 (1973)] of a dual reaction center having the properties expected of a pair of closely spaced Si sites associated with two anomalously reactive oxygen atoms; that center could be converted to another apparently containing SiȮ pairs. Evidence based on some of the reactions of these centers is now used to postulate the existence of a third center consisting of an SiȮṠi pair.

Reactive silica: XVI. Reaction with cyclopropane, benzene, and toluene☆

Abstract Infrared spectroscopy was used to follow the sorptions of relatively stable adsorbates by reactive silica. Dissociative chemisorption occurred in each case. With cyclopropane, the ring was disrupted and at least three distinct surface species were formed. The main surface product resulting from cyclopropane sorption was also formed when propene was adsorbed. Ring disruption also occurred with benzene, leading to the formation of silanols and some coke-like hydrocarbons, but surface aryls were not observed. However, two surface aryl species were formed with toluene.