Zinc oxide poisoning reactions

High pressure processes P > 150 atm) are catalyzed by copper chromite catalysts. The most widely used process, however, is the low pressure methanol process that is conducted at 503—523 K, 5—10 MPa (50—100 atm), space velocities of 20, 000-60,000 h , and H2-to-CO ratios of 3. The reaction is catalyzed by a copper—zinc oxide catalyst using promoters such as alumina (31,32). This catalyst is more easily poisoned than the older copper chromite catalysts and requites the use of sulfiir-free synthesis gas. 

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. 

In the actual process (Figure 10-5), the natural gas feedstock must first be desulfurized in order to prevent catalyst poisoning or deactivation. The desulfurization step depends upon the nature of the sulfur-containing contaminants and can vary from the more simple ambient temperature adsorption of the sulfur-containing materials on activated charcoal to a more complex high-temperature reaction with zinc oxide to catalytic hydrogenation followed by zinc oxide treatment. 

As the nickel-containing catalysts used in the reforming reaction are sensitive to poisons, any sulfur compounds present in the hydrocarbon feedstock have to be removed by hydrodesulfurization, generally with a combination of cobalt - molybdenum and zinc oxide catalysts [413] - [415], (Eqs. 38, 39). In a few cases, especially with... 

Natural gas or light hydrocarbons that serve as feed gas in the synthesis of ammonia contain sulfur compounds that act as poisons for the nickel catalyst used in synthesis gas production. Hydrogen sulfide and mercaptans are the dominant sulfur species in natural gas, while the light hydrocarbons may contain higher boiling sulfur species. A fixed-bed reactor containing zinc oxide is often used to desulfurize the feed gas. The chemical reaction with hydrogen sulfide is... 

A typical layout of the steam-reforming section of a syngas plant with hydrocarbon feedstock is illustrated in Fig. 6. The first step is purification of the feedstock to remove sulfur so as to avoid poisoning of the downstream reformer catalysts. This is typically accomplished in a two-step process. In the first step, organic sulfur compoimds are converted into hydrogen sulfide by a hydrogenation catalyst. In the second step, H2S is absorbed by zinc oxide by the following reaction ... 

The mildly endothermic steam reforming of methanol is one of the reasons why methanol is finding favour with vehicle manufacturers as a possible fuel for FCVs. Little heat needs to be supplied to sustain the reaction, which will readily occur at modest temperatures (e.g. 250°C) over catalysts of mild activity such as copper supported on zinc oxide. Notice also that carbon monoxide does not feature as a principal product of methanol reforming. This makes the methanol reformate particularly suited to PEM fuel cells, where carbon monoxide, even at the ppm level, can cause substantial losses in performance because of poisoning of the platinum anode electrochemical catalyst. However, it is important to note that although carbon monoxide does not feature in reaction 8.7, this does not mean that it is not produced at all. The water-gas shift reaction of reaction 8.5 is reversible, and carbon monoxide in small quantities is produced. The result is that the carbon monoxide removal methods described in Section 8.4.9 are still needed with PEM fuel cells, though the CO levels are low enough for PAFC. 

Precipitated copper oxide/zinc oxide catalysts were more active for a range of reactions than zinc chromite but lost activity as the copper was poisoned by gaseous impurities in the synthesis gas. The two oxides were found to be mutually promoting in methanol synthesis because the mixture of very small crystallites was more active than the individual oxides. 

It has been concluded from experimental work with catalysts containing alumina that methanol forms from carbon dioxide and that the catalyst activity is proportional to the copper metal surface area. The presence of carbon dioxide in the gas increases the synthesis rate. The zinc oxide and alumina play little part in the actual reaction apart from stabihzing the reduced copper and protecting it from the effect of any poisons. On the other hand, with catalysts containing chromia, the carbon dioxide leads to a decrease in the reaction rate. ... 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

---