Catalysts from poisoning

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

With an H2S/H2 ratio in the gas of 1 ppb, the equilibrium surface coverage of nickel at 500°C is around 70%. This means that all sulfur in the feed is quantitatively adsorbed on the nickel catalyst of a prereformer. The result is not only the deactivation of the prereformer catalyst even at very low sulfur levels, but also the protection of downstream catalysts from poisoning. Sulfur uptake on the catalyst will initially take place as shell poisoning and because of pore diffusion restrictions, it may take years before sulfur reaches the center of the particle. ... 

The concept of zeolite membrane encapsulated catalyst (ZMEC) is an original way to use zeolite membranes in catalytic reactors [158,159]. The concept of coating catalyst particles with a selective zeolite layer can reveal useful for either increasing reactant selectivity (e.g. selective hydrogenation of linear molecules) or product selectivity (e.g. animation of methanol). It also protects the catalyst from poisoning. 

Therefore, how to enhance the catalytic efficiency of catalyst in water solution and prevent catalyst from poison are important factors in increasing the cmrent efficiency and stability of electrode in the electrochemical ammonia synthesis. The use of solid electrol de is one of ways to solve these problems. 

Typical reduction and operating conditions for industrial catalysts are described in the appropriate chapters. Despite efforts to protect the catalyst from poisons or maloperation, it is still possible for problems to affect the catalyst. A few typical examples are shown in Table 1.14. 

In many of the other processes that use base metal catalysts, irreversible poisoning of the catalyst occurs as a result of deposition of metal contaminants from the process feedstock onto the catalyst surface. These catalysts are not considered to be regenerable by ordinary techniques. 

Raw Material Purity Requirements. The oxygen process has four main raw materials ethylene, oxygen, organic chloride inhibitor, and cycle diluent. The purity requirements are estabHshed to protect the catalyst from damage due to poisons or thermal mnaway, and to prevent the accumulation of undesirable components in the recycle gas. The latter can lead to increased cycle purging, and consequently higher ethylene losses. 

Fig. 10. Catalyst macropores showing D noble metal sites and (a) narrowed micropores after exposure to high temperatures where H represents thermally damaged noble metal sites and (b) pore mouth plugging from poisons where A, if aUowed, diffuses in to be converted to B.

At low (>450° C) temperatures, the presence of these materials, particularly the oxides, leads to simple masking or fouling. In some cases, a catalyst that shows reduced activity beheved to be from poisoning may simply be masked, and activity can be rejuvenated by cleaning with aqueous leaching solutions (21). 

Other potential poisons include zinc, manganese, chlorine, and bromine. A number of metals may be deposited on the catalysts from engine erosion and wear, including copper, chromium, nickel, and iron. The mechanism of poisoning has been reviewed by Maxted (134) and by Butt (135). 

Arrhenius plots for poisoned and unpoisoned catalysts. [From AIChE J., 7 (211), 1961. Used with permission.]... 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

Stefanov and coworkers—deactivation pathways for industrial Cu/Cr/Zn catalysts. Stefanov and coworkers250 published an XPS study indicating that the Cu-Cr-Zn catalyst deactivates via two pathways in an industrial reactor-sintering and poisoning by chlorine adsorption, which caused a deactivation of the catalyst from... 

Carbon Monoxide The presence of CO in a H2-rich fuel has a significant effect on anode performance because CO affects Pt electrodes catalysts. The poisoning is reported to arise from the dual site replacement of one H2 molecule by two CO molecules on the R surface (40, 41). According to this model, the anodic oxidation current at a fixed overpotential, with (ico) and without (in2) CO present, is given as a function of CO coverage (0co) by Equation (5-11) ... 

Alberti et al. investigated the influence of relative humidity on proton conductivity and the thermal stability of Nafion 117 and compared their results with data they obtained for sulfonated poly(ether ether ketone) membranes over the broad, high temperature range 80—160 °C and RHs from 35 to 100%. The authors constructed a special cell used in conjunction with an impedance analyzer for this purpose. Data were collected at high temperatures within the context of reducing Pt catalyst CO poison-... 

Figure 12. Transient HCN yield over platinum catalysts. A and B represent one experiment (Exp. 1) in which a sulfur-poisoned catalyst was regenerated on admission of 1% O2 into the inlet gas mixture between times t, and C shows the resistance of the catalyst to poisoning by SO when oxygen is simultaneously present in the inlet gas mixture. At t only oxygen is removed from the inlet gas mixture. (See Ref. 16 for details.)...

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Titania as with the ceria, led to Pd promotion in terms of activity for nitrate degradation but yielded catalyst which exhibited poorer selectivity than bimetallic catalysts. However, unlike ceria based systems, these catalysts did not suifer from poisoning when CO2, was used as a pH buifer. Nitrate reduction over Pd/Ti02 catalysts was associated with partially reduced titania species which migrated over the Pd particles during the reductive pre-treatment. The latter also led to the formation of a Pd (3-hydride phase which was also thought to play a role in the reduction process. 

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