Noble metal catalysts deactivation

Novel Regeneration Method for Deactivated Noble Metal Catalysts... 

In CO2 reforming, most of the reported research has been focused on non-noble metal catalysts, particularly nickel, because nickel has activity and selectivity comparable to those of noble metals, at much less cost. However, thermodynamic investigations indicated that the nickel-containing catalysts are prone to carbon deposition in CO2 reforming, resulting in catalyst deactivation (5). Therefore, an important challenge is to increase the resistance of nickel-containing catalysts to deactivation by carbon deposition. 

Supported noble metal catalysts (Pt, Pd, Ag, Rh, Ni, etc.) are an important class of catalysts. Depositing noble metals on high-area oxide supports (alumina, silica, zeolites) disperses the metal over the surface so that nearly every metal atom is on the surface. A critical property of supported catalysts is that they have high dispersion (fraction of atoms on the surface), and this is a strong function of support, method of preparation, and treatment conditions. Since noble metals are very expensive, this reduces the cost of catalyst. It is fairly common to have situations where the noble metals in a catalyst cost more than 100,000 in a typical reactor. Fortunately, these metals can usually be recovered and recycled when the catalyst has become deactivated and needs to be replaced. 

The selective catalytic oxidation over highly dispersed noble metal catalysts in aqueous media is gaining interest (1-5). One of the main problems consists of the deactivation of the catalyst. The role of oxygen is recognized as being crucial in this matter (6,7). 

Carbon formation is also observed on noble metal catalysts, with substantially different behavior on Pt versus Pd. For example. Figure 13 shows that Pt deactivated much more rapidly than Pd in the ATR of n-Cg. ... 

Studies of the deactivation of ATR catalysts show that the sulfur present in conventional fuels is responsible for rapid deactivation of both Ni-based and noble metal catalysts. At some conditions, sulfur appears to selectively poison the sites responsible for the SR reaction(s). 

Noble Metal Catalysts. Noble metal-based catalysts have been widely used in reforming reactions, and are logical choices for ATR. Results of reaction studies generally suggest that these catalysts are comparable in activity to Ni-based catalysts, but they appear to be somewhat more resistant to deactivation. 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

There is no evidence in vehicle operation that the oxidation activity of noble metal catalysts suffers from poisoning by SOz (24, 28, 84), although Hunter claims (43) that Pt can be poisoned below 900°F. In contrast, severe deactivation of base metal catalysts has been observed in many instances. 

In certain instances of poisoning, especially in the case of base metal catalysts, the deactivation can be simply explained by the formation of new bulk solid phases between the base metal and the poison. Examples are the formation of lead vanadates (14) in vanadia catalysts, or of sulfates in copper-chromite and other base metal catalysts (81). These catalyti-cally inactive phases are identifiable by X-ray diffraction. Often, the conditions under which deactivation occurs coincide with the conditions of stability of these inert phases. Thus, a base metal catalyst, deactivated as a sulfate, can be reactivated by bringing it to conditions where the sulfate becomes thermodynamically unstable (45). In noble metal catalysts the interaction is assumed to be, in general, confined to the surface, although bulk interactions have also been postulated. 

The deoxo reaction, performing the reduction of dioxygen with hydrogen, usually catalysed by a noble metal catalyst, was also reported to occur with NaY encapsulated complexes of Cu(embelin) (18) and 2-aminobenzimidazole (19).[134] The Cu(embelin) complex entrapped in NaY is a stable catalyst, that showing enhanced activity compared with the homogeneous case and may be reused many times, the corresponding benzimidazole complex is deactivated rapidly. 

The main causes of the deactivation of diesel catalysts are poisoning by lubrication oil additives (phosphorus), and by SOx, and the hydrothermal instability. The SCR by HC is less sensitive to SOx than the NO decomposition. The Cu-based catalysts are slightly inhibited by water vapor and SOx, and suffer deactivation at elevated temperature. Noble metal catalysts such as Pt-MFI undergo low deactivation under practical conditions, are active at temperatures below 573 K but the major and undesired reduction product is N20 (56). 

To summarise the results concerning the study of reversibility of metal-support interaction states, we could first state that the classic reoxidation treatment at 773 K does not allow the recovery of the NM/Ce02 catalysts from the decorated or alloyed states. The noble metal/ceria phase separation may only be achieved upon reoxidation at temperatures well above 773 K. This observation represents an additional major difference between titania and ceria supported noble metal catalysts. Moreover, the likely regeneration of NM/CcOi catalysts reduced at 773 K by reoxidation at 773 K would actually prove, in good agreement with earlier HREM studies on the reduced catalysts (117,194), that the observed deactivation effects are not due to decoration or alloying phenomena, rather consisting of purely electronic effects (105). 

Steam reforming of small organic molecules, to facilitate indirect electrochemical oxidation via H2, involves some thermodynamic inefficiency as well as formation, usually, of some CO in the H2 produced. Special catalysts for the fuel-cell oxidation of the H2 thus formed are then necessary, namely, catalysts that can effect dissociative adsorption of H from H2 in the presence of small but significant concentrations of CO in the H2. In recent years, such catalysts have been engineered (95) that allow oxidation of H2 at rates of several amperes per square centimeter in the presence of traces of CO. Similarly, a variety of modified noble metal catalysts have been developed that allow CH3OH oxidation to proceed with improved performance with respect to avoidance of self-deactivation behavior. Doping of Pt by Sn02 or Ru has been effective in this direction (96. 97). 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

Noble metal catalysts are known for their propensity to poisoning by chlorine released in the reaction. For enlightening the deactivation process we studied selected samples at the Pd Lm-and the Cl K-edges. Due to the low photon energy (2.S-3.2 keV) these experiments cannot be carried out in situ. The amount of chlorine incorporated in the different samples can easily be calculated from the edge jump of the chlorine K-edge. Table 1 summarises the results of four different samples for 15 minutes or one hour on stream and after deactivation. 

The primary cause for deactivation in current reforming catalysts, such as supported nickel, is the formation of carbon on the catalyst during reaction [20]. However, when postreaction samples of P-M02C and a-WC were examined by HRTEM, no observable carbon deposition had occurred on the catalyst surface during the reaction. In addition, activity studies demonstrated that M02C had a methane dry reforming activity similar to an active supported noble metal catalyst, namely 5% Ir/Al203 [27]. 

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