Catalyst passivation

VANADIUM DEACTIVATION UNDER SIMULATED CONDITIONS. The degree of catalyst deactivation was measured by comparing the activity of the catalyst or catalyst/passivator blend containing 5000 ppm V to that of the corresponding sample with no V added after similar treatment conditions (Table II). For the USY catalyst (catalyst A), steaming with VgO at 1450 F resulted in a 6855 decline in activitjr ... 

Modifiers have also been used to influence the selectivity of vapor-phase partial hydrogenations of benzene. The presence of ethylene glycol increased reaction selectivity with a ruthenium black catalyst from 7% to 41% while the turnover frequency (TOF) decreased from 31 to 3. Pyridine also increased selectivity in the short term, but prolonged use poisoned the catalyst. Passivating a ruthenium black catalyst with caprolactam not only stabilized the catalyst toward deactivation but also increased reaction selectivity from 7% to 20%. 

PAMAM itself was used as a multivalent macromolecular ligand, probably due to its multiple amino groups, in order to complex and immobilize metal ions, complexes, and nanoparticles with catalytic capabilities. Thus, Kawi and coworkers used PAMAM-on-silica and PAMAM-on-alumina templates to immobilize Rh(l) complexes as hydroformy-lation catalysts. Passivation of the silica OH sites outside the pores of SBA-15 silica resulted in a tighter binding of rhodium complexes inside the pores and led to a series of catalysts that displayed a positive dendritic effect up to the second PAMAM generation." Sreekumar and Krishnan used PAMAM on polystyrene to complex Mn(ll) precursors and catalyze the oxidation of secondary alcohols." " ... 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

At high metals levels, the coking characteristics of a cracking catalyst can be greatly increased that is, the ratio of contaminant coke to catalytic coke can be quite high. The effect of the contaminant metals on the coke response is affected not only by the level of metals but also by the type of catalyst and the use of a metals passivator. Catalysts, which contain effective metals traps to inhibit the contaminant effects, do produce much less contaminant coke than catalyst without metal traps. 

Passivate fresh catalyst prior to use or use prediluted catalyst... 

Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

A small amount of nickel in the FCC feed has a significant influence on the unit operation. In a clean gas oil operation, the hydrogen yield is about 40 standard cubic feet (scf) per barrel of feed (0.07 wi /r ). This is a manageable rate that most units can handle. If the nickel level increases to 1.5 ppm, the hydrogen yield increases up to 100 scf per barrel (0.17 wt%). Note that in a 50,000 barrel/day unit, this corresponds to a mere 16 pounds per day of nickel. Unless the catalyst addition rate is increased or the nickel in the feed is passivated (see Chapter 3), the feed rate or conversion may need to be reduced. The wet gas will become lean and may limit the pumping capacity of the wet gas compressor. 

It is usually more accurate to back-calculate the feed metals trom the equilibrium catalyst data than to analyze the feed regularly, l nickel will be a regular component of the feed, passivators are available. If nickel affects operation and margins, it is often beneficial to use antimony to passivate the nickel. This can be attractive if the nickel on the equilibrium catalyst is greater than 1,000 ppm. 

Consider reformulating the catalyst—custom formulations are available. Increasing rare-earth content can reduce the wet gas rate. Catalyst is usually selected for properties other than its ability to flow. However, if it does not flow, it is not going to work well. Catalyst physical properties should be compared with those of catalysts that have circulated well. Evaluate the economics of using metal passivation additives and other catalyst enhancing additives. 

Nickel in the feed is deposited on the surface of the catalyst, promoting undesirable dehydrogenation and condensation reactions. These nonselective reactions increase gas and coke production at the expense of gasoline and other valuable liquid products. The deleterious effects of nickel poisoning can be reduced by the use of antimony passivation. 

Commercial sodium sulfite is acceptable for food plants higher grades are used for pharmaceutical and electronic chip manufacturing, but in these higher grades the catalyst is usually changed to sodium erythorbate (at a level of 0.05 to 0.1%, which also provides lower pressure and temperature passivation). 

It is important to note that the selectivity of sulfur-passivated catalysts towards steam reforming is greatly enhanced because carbon formation is effectively suppressed. The decrease in activity can to largely be compensated for by selecting inherently more active catalysts and by operating at higher temperatures. Unfortun-... 

Bulk characterization of calcined precursors and reduced catalysts was carried out by X-ray diffractometry using Cu K radiation. Reduced catalysts were first passivated by exposure to N2O as described above. Line-broadening analysis was carried out on the Fe(llO) reflection to obtain the iron particle size. Overlap with the MgO(200) reflection limited its usefulness to the more highly-loaded catalysts. 

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