Calvin H. Bartholomew

Mechanisms of catalyst deactivation

The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts.

Carbon Deposition in Steam Reforming and Methanation

Abstract Deactivation of supported metal catalysts by carbon or coke formation is a problem of serious magnitude in steam reforming, methanation, and other important catalytic processes. Its causes are generally threefold: (1) fouling of the metal surface, (2) blockage of catalysts pores and voids, and/or (3) actual physical disintegration of the catalyst support. Since loss of catalytic activity and physical destruction of the catalyst by carbon deposits can occur rapidly (within hours or days) under unfavorable conditions, understanding and control of these effects are of major technological and economical importance.

The stoichiometries of H2 and CO adsorptions on cobalt: effects of support and preparation

Catalysts composed of unsupported cobalt and cobalt supported on silica, alumina, titania, magnesia, and carbon were prepared by thermal decomposition, impregnation, precipitation, and evaporative deposition. These catalysts were characterized by high temperature (323–423 K) hydrogen adsorption and conventional 298 K adsorptions of hydrogen and carbon monoxide. Total surface areas of unsupported cobalt catalysts were measured by BET. Metal crystallite sizes were determined independently from XRD, TEM, and hydrogen adsorption. Extents of reduction were measured by O2 titration at 673 K. Hydrogen adsorption on cobalt is activated and reversible; extents of activation and reversibility vary with support, metal loading, and preparation. The hydrogen adsorption stoichiometry is 1.0 hydrogen atom per surface cobalt atom, if total adsorption at the temperature of maximum uptake is considered. The adsorption of CO is nonactivated and reversible; the stoichiometry for irreversible adsorption varies from 0.4 to 2.3 molecules of CO per surface cobalt atom, depending upon support, metal loading, and preparation. Cobalt dispersion and extent of reduction also vary greatly with support, metal loading, and method of preparation. Cobalt/carbon catalysts prepared by evaporative deposition have unusually high dispersions relative to other cobalt catalysts. Hydrogen adsorption is recommended as the most convenient, reliable technique for measurement of cobalt crystallite size in CoAl2O3, CoSiO2, and CoC catalysts.

Effects of support and dispersion on the CO hydrogenation activity/selectivity properties of cobalt

Abstract Specific activities and selectivities of unsupported cobalt and cobalt supported on alumina, silica, titania, carbon, and magnesia carriers for CO hydrogenation were measured in a single-pass differential reactor at low conversions, 1 atm, and 175–350 °C. The results indicate that specific activity and selectivity of cobalt vary with support, dispersion, metal loading, and preparation method. The order of decreasing CO hydrogenation activity at 1 atm and 225 °C for catalysts containing 3 wt% cobalt is Co TiO 2 , Co SiO 2 , Co Al 2 O 3 , Co C , and Co MgO . The specific activity of cobalt decreases significantly with increasing dispersion. Product selectivity is best correlated with dispersion and extent of reduction i.e., the molecular weight of hydrocarbon products is lower and the CO 2 H 2 O ratio is higher for catalysts having higher dispersions and lower extents of reduction. This effect may be due to stable oxides in the well-dispersed, poorly reduced catalysts, which catalyze the water-gas-shift reaction thereby increasing the H 2 CO ratio at the surface. In the Co Al 2 O 3 system, activity and selectivity for high molecular weight hydrocarbons increase very significantly with increasing cobalt loading. A 15% Co Al 2 O 3 is 20 times more active than 3% Co Al 2 O 3 ; moreover, 86 wt% of its hydrocarbon fraction is in the C 5 –C 12 (gasoline) range compared to 18 wt% for 3% Co Al 2 O 3 .

Chemistry of nickel-alumina catalysts

Abstract A study of the chemistry involved in preparation, pretreatment, and reduction of Ni Al 2 O 3 catalysts was carried out using hydrogen chemisorption, thermal gravimetric analysis, and chromatographic analysis. Data showing effects on nickel surface area of calcination, heating rate during reduction, nickel loading, and passivation are presented and discussed. The results indicate that controlled decomposition of alumina-supported nickel nitrate in hydrogen atmosphere (without prior calcination) tends to maximize nickel surface area, dispersion, and reduction to nickel metal. Nickel surface area and the fraction of nickel reduced to the metallic state both increase as the hydrogen space velocity during reduction is increased. Upon increasing the reduction temperature from 300 to 500 °C, the percentage reduction to nickel metal is increased from 40 to 100% whereas nickel surface area changes relatively little.

The stoichiometry of hydrogen and carbon monoxide chemisorption on alumina- and silica-supported nickel

Adsorption stoichiometries of H2 at 298 K and CO at 190–298 K on alumina- and silica-supported Ni catalysts prepared by impregnation and precipitation techniques were investigated. In the case of alumina-supported nickel catalysts the metal loading was varied from 0.5 to 23 wt% in order to study the effects of metal-support interactions. NiSiO2 catalysts were prepared by both impregnation and precipitation techniques to determine effects of catalyst preparation. Room-temperature H2 adsorption on alumina- and silica-supported nickel occurs with a stoichiometry of one hydrogen atom per surface nickel atom as determined by chemisorption, X-ray diffraction, and electron microscopy. CO adsorption is considerably more complex, the stoichiometry of which varies with equilibration pressure, temperature, metal crystallite size, and metal loading. Formation of nickel carbonyl and substantial amounts of chemical and physical adsorption of CO on the support provide additional complications.

Sulfur Poisoning of Metals

Publisher Summary This chapter provides an overview of the sulfur poisoning of metals. Sulfur apparently bonds so strongly to metal surfaces that marked activity reduction occurs at extremely low gas-phase concentrations of sulfur-containing compounds. In commercial practice, the life of supported metal catalysts may be reduced to only a few months or weeks in the presence of only ppm quantities of sulfur contaminants in the feed. Because of the essentially irreversible adsorption of sulfur compounds on metals, regeneration is usually impossible or impractical. This chapter integrates available information on the interaction of sulfur with metal surfaces with that of poisoning studies to provide a more complete picture of sulfur poisoning and of the mechanism. The chapter stresses the importance of experimental techniques that provide definitive, fundamental information regarding sulfur adsorption and poisoning.

Support and crystallite size effects in CO hydrogenation on nickel

Adsorption and CO hydrogenation activity/selectivity properties of well-defined NiSiO2, NiAl2O3, and NiTiO2 catalysts representing wide ranges of dispersion and nickel concentration were investigated. CO and H2 adsorption uptakes were determined for all of the catalysts. The extent of reduction to the metal was also determined for all catalysts by oxygen titration or nickel carbonyl extraction. Specific activities for COH2 synthesis were measured for each of the catalysts at 500–550 K and 140 kPa. Effects of strong metal-support interactions are evident in NiTiO2, in well-dispersed NiAl2O3, and to a lesser extent in very well-dispersed NiSiO2 from (i) changes in the nature and stoichiometry of CO and H2 adsorption and (ii) significant changes in activity and selectivity properties for COH2 synthesis with changes in metal dispersion, support, preparation technique, and catalyst pretreatment. Both COH adsorption ratio and selectivity to C2+ hydrocarbons increase with increasing metal dispersion in NiSiO2 and NiAl2O3 systems, suggesting that metal-support interactions affect selectivity by changing the relative abundance of adsorbed CO and H2 during reaction.

Hydrogenation of CO2 on group VIII metals. IV: Specific activities and selectivities of silica-supported Co, Fe, and Ru

Specific intrinsic rates and product distributions for CO2 hydrogenation on Co/SiO2, Fe/SiO2, and Ru/SiO2 were determined at 450–650 K, 140–1030 kPa, and a range of space velocities. Specific activities in the form of turnover frequencies for CO2 hydrogenation decrease in the order Co/SiO2, Ru/SiO2, Ni/SiO2, Fe/SiO2. The order of decreasing selectivity for methane is Ru/SiO2, Co/SiO2, Ni/SiO2, Fe/SiO2. High levels of CO are produced in CO2 hydrogenation on Fe/SiO2, Co/SiO2, and Ni/SiO2 while significant fractions of C2+ hydrocarbons (about 1 order of magnitude lower than typical methane fractions) are produced on Fe/SiO2 and Co/SiO2 catalysts.

Hydrogenation of CO2 on group VIII metals: II. Kinetics and mechanism of CO2 hydrogenation on nickel

The rate of CO2 hydrogenation on Ni/SiO2 was measured as a function of H2 and CO2 partial pressures at 500–600 K, 140 kPa, and 30,000–90,000 h−1. The data show that the rate of CO2 hydrogenation is moderately dependent on CO2 and H2 concentrations at low partial pressures but essentially concentration independent at high partial pressures. Under most typical reaction conditions CO is observed as a product of the reaction at levels determined by quasi-equilibrium between surface and gas phase CO species. Addition of CO to the reactants above this equilibrium level causes a significant decrease in the rate of CO2 hydrogenation apparently as a result of product inhibition. Reaction orders and the true activation energy are quite temperature dependent indicating that a simple power law rate expression provides an inadequate fit of the data. Indeed, the kinetic results are consistent with a complex Langmuir-Hinshelwood mechanism involving dissociative adsorption of CO2 to CO and atomic oxygen followed by hydrogenation of CO via a carbon intermediate to methane.