Titanium, chemisorption

Cremaschi P and Whitten J L 1987 The effect of hydrogen chemisorption on titanium surface bonding Theor. Chim. Acta. 72 485-96... 

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

Extraction process, 26 381 Extractive chemisorption, 37 80 Extra-framework titanium, 41 293 Extrinsic field effect, 27 26-48 catalysts for, 27 49 theory, 27 50, 51 Extrusion, 28 83... 

Quite recently it was shown that phosphonic esters, trimethylsilyl [124, 143] and alkyl esters [124,143, 145] could also be used to modify the surface of titanium or aluminum oxide in organic solvents at moderate temperatures. Unlike Si-O-C bonds, P-O-C bonds are not easily hydrolyzed, and their cleavage on an oxide surface was unexpected. Most probably, coordination of the phosphoryl oxygen to the surface assists the condensation by increasing the electrophilicity of the P atom, thus facilitating the condensation of P-0-R groups with surface hydroxyls (Scheme 7) [124]. The chemisorption of... 

Vanadium Catalysts Supported on Titanium (Anatase) Characterized by Ammonia and Low-Temperature Oxygen Chemisorption... 

The latest experimental evidence suggest the presence of titanyl groups (Ti=0) in the structure188 (Scheme 9.3). H202 is activated via chemisorption on these groups with the formation of a surface titanium peroxo complex (13). It may exist in the hydrated or open diradical form and initiates hydrogen abstraction. Rapid... 

As a consequence, incompletely coordinated titanium ions will be exposed. Type III sites would be able to re-form the original OH groups through chemisorption of water, whereas type IV would only coordinate molecular water. 

Ketones and nitriles are rather soft bases their coordination onto electron-deficient sites on oxides is, therefore, relatively weak. One may, however, expect an improved specificity of chemisorption due to their softness. Unfortunately, however, these substances very easily undergo chemical transformations at oxide surfaces. Thus, carboxylate structures are formed on adsorption of acetone on alumina (194, 245-247), titanium dioxide (194), and magnesium oxide (219, 248, 249). Besides, acetone is also coordinated onto Lewis acid sites. A surface enolate species has been suggested as an intermediate of the carboxylate formation (248, 249). However, hexafluoroacetone also leads to the formation of trifluoroacetate ions (219). The attack of a basic surface OH ion may, therefore, be envisaged as an alternative or competing reaction path ... 

The adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

Burch and Flambard (113) have recently studied the H2 chemisorption capacities and CO/H2 activities of Ni on titania catalysts. They attributed the enhancement of the catalytic activities for the CO/H2 reaction (after activation in H2 at 450°C) to an interfacial metal-support interaction (IFMSI). This interaction is between large particles of Ni and reduced titanium ions the Ti3+ is promoted by hydrogen spillover from Ni to the support, as pictured in Fig. 8. The IFMSI state differs from the SMSI state since hydrogen still chemisorbs in a normal way however, if the activation temperature is raised to 650°C, both the CO/H2 activity and the hydrogen chemisorption are suppressed. They define this condition as a total SMSI state. Between the temperature limits, they assumed a progressive transition from IFMSI to SMSI. Such an intermediate continuous sequence had been... 

On the other hand, catalytic effects of this surface titanium suboxide are not so clear. There is evidence in the literature (2) and in this Symposium (1JD that geometric effects are dominant in Pt. But in the case of Ni, the bonding of CO and H2 is altered (6, 11). Sachtler (12) suggested that the oxophilic nature of reduced titanium may weaken the C-0 bond upon CO chemisorption near such sites. If this is true, other oxophilic ions should show similar characteristics. In this paper, we report Auger and EELS studies of Ni/MnOx model catalysts prepared by vacuum evaporation of Ni onto an oxidized manganese foil and subsequent reduction at 500 K. 

The diffusion of an H atom over the very well dried, but not dehydroxylated, support surface can be described as hopping from one site to another, or alternatively as a synchronous hopping of a proton and an electron (25., 28). Each hopping consists of a simultaneous reduction of the receiving titanium cation and an oxidation of the leaving site, which may explain the similarity in activation energy for H2 chemisorption on and desorption from the support. 

Experimental results bear out these theoretical predictions. Work with Ti02 deposited on platinum foil (14, 15) indicates that under UHV conditions in the absence of oxygen little, if any segregation of titanium occurs, In agreement with the theoretical predictions (11). In contrast, an AES study (16) of stoichiometric PtjTi showed that the surface composition is close to the bulk composition, with consequent effects on H2 and CO chemisorption. 

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