Robert L. Burwell

An infrared study of the physisorption and reaction of molybdenum hexacarbonyl on γ-alumina

Abstract The infrared spectra of molybdenum hexacarbonyl adsorbed on two types of γ-alumina have been determined after initial adsorption at 25°C and after activation in flowing helium at 100°C to produce Mo(CO)3ads. Nearly fully hydroxylated alumina (HA), partially dehydroxylated alumina (PDA, pretreated at 450–500°C), and nearly fully dehydroxylated alumina (DA, pretreated at 1000°C) were employed. Reaction between physisorbed Mo(CO)6 and the surface of alumina commences at 25°C or slightly above. It is proposed that surface O2− (σ-O−) or OH− (σ-OH) add to the carbon atom of one CO ligand with consequent labilization of other CO ligands for displacement by σ-O− or σ-OH. An infrared absorption band for Mo(CO)3ads was observed on HA, PDA, and DA at near 1580 cm−1 but only weakly with HA. On HA it is proposed that Mo(CO)3ads is primarily (σ-OH)3Mo(CO)3, and on DA, species like (σ-O)3Mo(CO)2(C00-σ). On PDA, (σ-O)3 would be replaced by (σ-O)2(σ-OH) or (σ-O)(σ-OH)2.

The reaction of nickel tetracarbonyl with γ-alumina

Abstract Nickel tetracarbonyl physisorbed from helium carrier onto partially dehydroxylated γ-alumina at 23–50°C rapidly reacts with surface sites to form a yellow complex, probably adsorbed nickel tricarbonyl. Carbon monoxide rapidly reverses this reaction. In flowing helium, the yellow complex loses further carbon monoxide to form a gray species in which CO Ni is about 1.4. Heating to 100°C reduces the ratio to 0.4. The gray species are small clusters of metallic nickel covered with carbon monoxide. Heating to about 450°C liberates additionally about 0.33 molecules of hydrogen, 0.1 of carbon monoxide, and 0.1 of carbon dioxide. This oxidation of nickel(0) by surface hydroxyl groups occurs even in the presence of hydrogen. Heating nickel carbonyl on highly dehydroxylated alumina liberates no hydrogen.

Isotopic exchange between deuterium and cycloalkane on palladium catalysts

Results of isotopic exchange between deuterium and the hydrocarbons, bicyclo[3.3.1]nonane, cis- and trans-1,2-dimethylcyclopentane, on palladium-alumina catalysts at about 50°C are reported. An intermediate common to the exchange of both dimethylcyclopentanes gives nearly completely exchanged cis- and trans-1,2-dimethylcyclopentane in the ratio of one to about four. In addition to this two-set exchange process, there is a one-set exchange process unaccompanied by epimerization in which nearly all of the hydrogen atoms in one set exchange. Other processes lead to the exchange of a smaller number of hydrogen atoms.The isotopic distribution patterns of exchanged bicyclononane are inconsistent with the occurrence of one-set exchange via alternation between monoadsorbed alkane and π-complexed olefin but support instead alternation between mono-adsorbed and di-adsorbed alkane. The mechanism of two-set exchange is considered in the light of these experiments. Several forms of the π-allyl mechanism and of the roll-over mechanism are consistent with observations.

The effect of coordinatively unsaturated surface Cr3+ on the adsorption of gases on chromia

Abstract Clean chromias of smaller and larger degrees of dehydroxylation, i.e., smaller and larger surface concentrations in Cr 3+ ( cus ), were prepared by activation in flowing helium to 315°C (microporous, amorphous) and to 520°C mesoporous, microcrystalline). These chromias were then exposed to argon, hydrogen, methane, or nitrogen at −160 to −78°C, flushed with helium, and the residual amount of adsorbate was determined after various times. From these data and from displacement experiments in which pulses of one gas were passed over chromia on which another gas was adsorbed, the relative strengths of adsorption at Cr 3+ ( cus ) are Ne ⪡ Ar ⪡ H 2 ⪡ CH 4 2 . Converting Cr 3+ ( cus ) to Cr 3+ CO by adsorption of carbon monoxide or to Cr 3+ OH − by rehydroxylation with water somewhat decreased the strength of adsorption of argon and greatly decreased those of nitrogen and methane. On microporous chromia, reduction in the strength of adsorption of nitrogen and methane was less for poisoning by carbon monoxide than it was for poisoning by water. It is suggested that this resulted from Cr 3+ CO projecting more into the micropores than Cr 3+ OH − .

Modified silica gels as selective adsorbents for sulphur dioxide

Silica gel modified by –CH2CH2CH2NH3X where X is chlorid, bromide, or iodide adsorbs sulphur dioxide selectively to form the halogenosulphinate ion SO2X–; the process reverses upon warming.

Catalysis and catalysts

The phenomenon in which a relatively small amount of foreign material, called a catalyst, speeds up …

Catalytic sites in the hydrogenolysis of cyclopropane on platinum

The hydrogenolysis of cyclopropane on Pt/SiO2 occurs on the actual platinum surface of the catalyst and not on any carbonaceous deposit.

Adsorption and inter-reaction of hydrogen and deuterium on chromias at low temperatures

Deuterium displaces hydrogen from an irreversible, molecular chemisorption on activated chromia at –196 °C without formation of hydrogen deuteride and with an isotope effect of 50 favouring adsorption of deuterium vs. hydrogen.