Alumina-supported Rhodium Catalysts Research Articles

Controlled oxygen chemisorption on an alumina supported rhodium catalyst. The formation of a new metal-metal oxide interface determined with EXAFS

An alumina-supported rhodium catalyst has been studied with EXAFS. After reduction and evacuation, oxygen was admitted at 100 and 300 K. EXAFS spectra of the catalyst after oxygen admission at 100 K indicated the beginning of oxidation. At 300 K only a small part of the rhodium particles remained metallic and this metallic 'kernel' was partly covered with rhodium oxide. In the rhodium metal to rhodium oxide interface the same 2.7-A Rho-0' distances are present as in the metal-support interface. A model is presented that explains the observed formation of the rhodium metal-rhodium oxide interface.

Carbon monoxide isotopic exchange reaction over rhodium/alumina: nondissociative exchange on metallic rhodium sites

The isotopic exchange reaction /sup 12/C/sup 18/O + /sup 13/C/sup 16/O ..-->.. /sup 12/C/sup 16/O + /sup 13/C/sup 18/O has been studied over alumina-supported rhodium catalysts between 300 and 470 K. For a 2.2 wt % Rh/Al/sub 2/O/sub 3/ catalyst containing both metallic Rh/sub x//sup 0/ sites and Rh/sup I/ sites, the onset of the CO isotopic exchange reaction is observed at 373 K, and an activation energy of 10.8 kcal/times/mol/sup /minus/1/ is estimated. In the same temperature range, no CO disproportionation/dissociation reactions are observed. The rate of isotopic exchange per Rh site over 0.2% Rh/Al/sub 2/O/sub 3/ is found to be slower by a factor of 7 than that over 2.2% Rh/Al/sub 2/O/sub 3/, due to the low relative population of metallic Rh sites compared to Rh/sup I/ sites on the 0.2% Rh/Al/sub 2/O/sub 3/ surfaces. It is proposed that the CO isotopic exchange proceeds via a nondissociative mechanism on the metallic Rh sites of the Rh catalysts and that the exchange reaction will not occur on Rh/sup I/ sites. Mass spectrometric, chemisorption, and IR spectroscopic measurements have been employed.

Effects of cerium addition on CO oxidation kinetics over alumina-supported rhodium catalysts

The kinetics of CO oxidation in a strongly oxidizing environment (i.e., P O 2 ⪢ P CO) over a low-loaded Rh/Al 2O 3 catalyst are not significantly affected by the presence of cerium. Under moderately oxidizing or net-reducing conditions, on the other hand, the addition of sufficient amounts of cerum oxides (≥2 wt% Ce) to the Rh/Al 2O 3 catalyst was found to cause the following changes in CO oxidation kinetics: suppression of the CO inhibition effect, decreased sensitivity of the reaction rate to gas-phase O 2 concentration, and decreased apparent activation energy. These cerium-induced changes in the kinetics lead to enhancement of CO oxidation activity and can be rationalized on the basis of a mechanism involving CO 2 formation via a reaction between CO adsorbed on Rh and surface oxygen derived from the neighboring ceria particles. The effects of Ce addition on the CO oxidation kinetics were also independent of whether the Ce was deposited before or after the Rh.

Dehydrogenation of Cyclohexane on Alumina-Supported Rhodium Catalysts. Effect of Oxidation-Reduction Treatment on the Catalytic Activity

Abstract The cyclohexane dehydrogenation and H2 chemisorption on Rh/Al2O3 catalysts have been studied as a function of reduction temperature (at 373–773 K), preceded by O2 treatment at 673 K. The catalytic activity increased by a factor of 5 after high-temperature reduction at 773 K, while there was almost no change in the H2 chemisorption capacity with varying the reduction temperature.

The effect of chlorine in the hydrogenation of carbon monoxide to oxygenated products at elevated pressure on Rh and Ir on SiO 2 and Al 2O 3

The catalytic behaviour of silica- and alumina-supported rhodium and iridium catalysts in synthesis gas reaction at elevated pressures was investigated. Temperature programmed reduction and hydrogen chemisorption measurements were used to characterize the catalysts. Rhodium was more active than iridium and had a better selectivity to higher hydrocarbons and C 2-oxygenates. For rhodium on silica high oxo-selectivities were obtained (40%), while on chlorine containing alumina this selectivity was rather low. When a chlorine-free metal precursor was used or when special pretreatments were applied to a RhCl 3/Al 2O 3 catalyst, oxo-selectivities of rhodium on alumina were also rather high (30%)

Chemistry of acetylene exposed to alumina and alumina-supported rhodium catalysts

On utilise la spectrometrie de masse et la spectrometrie Raman pour obtenir des informations sur la chimie de l'acetylene exposee a une catalyse par l'alumine, l'alumine activee thermiquement et le rhodium sur support alumine. On montre qu'on obtient des resultats similaires avec les trois catalyseurs. Des especes C n H m de masse elevee apparaissent dans les spectres de masse et sont caracteristiques des polymeres presentant des liaisons simples, doubles et triples

Nitric oxide reduction by methane over [formula omitted] catalysts

The mechanism of nitric oxide reduction with methane has been investigated over an alumina-supported rhodium catalyst. A series of kinetic studies were performed using initial rate data obtained in a recirculation reactor. Between 300 and 400 °C NO elimination is initially fast over a reduced catalyst, but the reaction rate rapidly decreases due to oxidation of the catalyst surface. The decomposition is apparently a noncatalytic stoichiometric reaction between nitric oxide and surface rhodium atoms. The initial rate of disappearance of NO is adequately described by a dualsite Langmuir-Hinshelwood expression. In presence of reducing agents such as CO or CH 4, oxygen is effectively removed as CO 2 (plus H 2O). In the reduction of NO with CH 4, the initial rate of NO disappearance fits the following empirical rate expression Rate = Ae − E RT (P No ) −0.63(P CH 4 ) where A = 3.57 × 10 3 NO Rh s · sec · ( N m 2 ) 0.37 and E = 77 kJ/mole. A deuterium isotope effect of 1.9 is observed in the reduction of NO with mixtures of CH 4 and CD 4. This, along with the linear rate dependence on CH 4 partial pressure, indicates that the dissociative adsorption of CH 4 is the rate limiting step of the reaction. An experiment run with a 15NO, N 2O, and CH 4 mixture indicated that N 2O is not an exclusive gas phase intermediate in the pathway to N 2 formation from NO. However, all these results are consistent with N 2O being a true surface intermediate. A reaction mechanism is proposed for NO reduction by methane. It is based on the assumption that two adsorbed NO molecules disproportionate to (N 2O) a + (O) a. Adsorbed (N 2O) a either desorbs as N 2O or decomposes to N 2 and (O) a. The role of the reductant is to remove the strongly adsorbed (O) a and to keep the catalyst in an active reduced state for NO reaction.

Rhodium-catalysed hydrogenation of allene as revealed by [14C]propylene and [14C]carbon monoxide tracer studies

The low-pressure hydrogenation of allene has been studied over alumina-supported rhodium catalysts. During a series of hydrogenation reactions the activity of the catalyst progressively decreases to a steady-state value and thereafter remains constant. The reaction proceeds in two distinct stages. During the first stage the selectivity for the formation of propylene is ca. 95%.

Decomposition of Methanol over Alumina Supported Rhodium Catalysts

The decomposition of methanol was investigated over a variety of alumina (Al2O3) supported rhodium (Rh) catalysts. Decomposition as well as dehydration took place in the reaction of methanol over Rh/Al2O3 catalyst. Pyridine adsorption during the reaction revealed that Rh was active sites for the decomposition, and Al2O3 was those for the dehydration.When base metals were added to Rh/Al2O3, the selectivity for the decomposition increased with lowering electronegativity of metal ions added. Thus, alkali metals such as potassium (K) gave the highest selectivity for the decomposition. Further addition of some base metals to Rh-K/Al2O3 selectively promoted the decomposition of methanol.

The influence of particle size on the catalytic properties of alumina-supported rhodium catalysts

The catalytic activities of rhodium for benzene hydrogenation and cyclopentane hydrogenolysis have been investigated as a function of the state of dispersion. The specific surface area of rhodium was varied by changing the conditions of preparation: metal content, rhodium salt, and temperature of reduction. The dispersion of rhodium was determined by measurements of hydrogen and oxygen chemisorption, by H 2 titration of preadsorbed oxygen, and by direct examination of the distribution of particle sizes by electron microscopy. The stoichiometries of adsorption are found to correspond to Rh H and Rh O, whatever the dispersion of rhodium. The turnover number for hydrogenation is approximately constant and independent of dispersion in the range of 20 to 90% dispersion. An eightfold increase in the turnover number for hydrogenolysis is found when the dispersion decreases from 95 to 20%. The selectivity of hydrogenolysis also changes. The particle size effect is attributed to the formation of dense crystallographic planes at the surface of the large particles.

Radiochemical studies of chemisorption and catalysis: XII. Studies of the adsorption of [ 14C]acetylene, [ 14C]ethylene and [ 14C]carbon monoxide on silica- and alumina-supported rhodium catalysts

The interaction of 14C-labeled acetylene, ethylene and carbon monoxide with rhodium supported on silica or alumina catalysts has been studied at room temperature at pressures of a few Torr. Adsorptions of acetylene, carbon monoxide and ethylene were performed separately: adsorptions of each of these gases followed by another were also studied. The behavior of the system was observed by counting of surface radioactivity. Acetylene gave adsorption isotherms with two well-defined regions, one of which, the primary region, was steeply sloping. [ 14C]CO as a probe for metal sites showed that few were left after adsorption of acetylene. CO-precoverage of the metal surface showed that the extent of secondary adsorption of acetylene was unchanged and that this adsorption was probably associated with the support. C 2H 2 had the ability to displace about 40% of the adsorbed CO. This study is to be taken in conjunction with Part XIII of the series, in which mass spectrometry was used to elucidate the nature of the reactions occurring.

RATES OF ADSORPTION OF HYDROGEN ON PALLADIUM AND ON RHODIUM

A study of the effects of temperature and initial gas pressure on the kinetics of chemisorption of hydrogen on alumina-supported palladium and rhodium catalysts revealed, for both gas–solid systems, that the adsorption proceeded via three distinct and consecutive kinetic stages. Each stage could be described by the Elovich equation. The temporal range of existence of each kinetic stage was temperature- and pressure-sensitive, low initial pressures and high temperatures favoring early appearance of each stage. On increasing the temperatures from 0° to 400 °C, the amounts of hydrogen adsorbed on both solids decrease. Over that temperature range the rates of adsorption on Pd decrease, while those on Rh increase, with increasing temperature. The general effect of increasing the initial gas pressure over the range 10–60°mm Hg is to increase both rates and extents of adsorption.