Monolithic catalysts active phase, deposition

In principle, deposition of an active phase (metal and/or oxide) on a monolithic catalyst support can be carried out in a manner similar to that used to prepare a t) ical catalyst. However, the large dimension of a monolith can easily enhance problems of nonhomogeneous deposition. For example, if in the preparation of conventional catalyst particles the active phase would be deposited at the external surface of the support, the result would be an egg-shell-t) e catalyst, which for many processes can be advantageous. However, if this pattern of deposition were applied to a monolithic support, it could result in a monolith with only the outer charmels of the structure having a significant catalytic activity, resulting in a dramatically poor catalytic reactor. The critical steps in the s)mthesis process are the deposition and drying steps, which are discussed separately below. Calcination, reduction, etc. for monolith catalysts are not different from those used to manufacture t) ical catalysts, and these steps are therefore not discussed here. 

Similar to normal catalyst synthesis whereby ion exchange methods can result in egg-shell structures, in the preparation of monolith catalysts the majority of the metal can be deposited at the entrance of the monolith. Egg-shell structures can be attractive for catalyst particles, but for monoliths, analogous uneven distributions of the active phase are a disaster. Fortunately, extensive literature is available describing ion-exchange procedures for conventional catalysts that yield homogeneous metal distributions. This literature can be used as a guide for preparing satisfactory monolithic catalysts. 

Impregnation is one of the most used techniques to incorporate an active phase in a support. It can also be used to deposit active phase to a monolith [85]. Usually, a high-surface-area monolith is dried, evacuated, and dipped in a solution containing a precursor of the active phase. After drying and calcination a monolithic catalyst is obtained. Often, an activation step is necessary to convert the precursor of the active phase into the active phase, e.g., the transformation of a metal oxide in the corresponding metal or metal sulfide. Monolithic catalysts with complex compositions of active phases can be prepared by sequential impregnations with suitable solutions or with a conunon solution containing various precursors of the components. 

An alternative method to deposit the oxidic layer or precursors of the active phase is precipitation or coprecipitation. This is widely used in conventional catalyst manufacture. An advantage is that a high loading of the active phase can be reached. As in monolithic reactions, catalyst loading is a point of concern. It is not surprising that precipitation methods are often applied in monolithic catalyst synthesis. 

As the deposited oxide layer is well mixed, strong interaction between the oxides is expected, leading often to mechanically strong materials, but pretreatment procedures can be hindered. For instance, in the preparation of a metal-based catalyst, a reduced reducibility of the precursor is often encountered, and, as a result, a reduced availability of the catalytically active phase is encountered. Moreover, under strongly acidic or basic conditions, some support materials, e.g., alumina, may be dissolved, as mentioned before. Furthermore, the adhesion of the precipitated layer with the monolith substrate is often a point of concern, especially during drying and heat treatment. 

Sometimes, due to a low melting point of a salt, e.g., when a nitrate is chosen as the precursor of the active phase, homogeneous distribution is difficult to obtain. Then, the deposition precipitation method might be appreciated [91-93]. This method is illustrated for the synthesis of a nickel monolithic catalyst. 

By the sol-gel method, an oxidic layer together with the precursor of the active phase sometimes can be deposited simultaneously [58,59,70]. The resulting monolithic catalyst will contain both the washcoated oxide layer and the precursor of the active phase. 

In principle, the same method can be applied to a monolith substrate to obtain a zeolite-on-monolith catalyst. Of course, this method is not limited to zeolite as the active phase. It is not difficult to envisage that many other active phases can be deposited in this way. 

The methods proposed in the literature to do so, e.g. spin-coating [9], thermal evaporation [10], chemical vapor deposition [11], flash evaporation [12], laser deposition [13] and r.f. reactive sputtering [14], are rather scarce and complex. Moreover, they are often more dedicated to the deposition of active phase on flat and/or monolithic supports (to produce model catalysts for surface science purposes) than on powder supports. These methods thus usually only allow the production of samples at a small scale, so that they are often inadequate for the production of pulverulent real catalysts in large amounts. 

One of the difficulties to study this catalyst is the possible influence of the poisons deposited on the active phases during the test and originated from the gasohne or motor oil components such as Si, Ca, P, Zn, S. .. [11,12]. In particular, the TPR study may become totally erroneous if additional reducible compounds are present. To take into account this influence and to evidence an eventual aging gradient along the axis of the monolith, three samples were selected after the test, at the inlet, in the middle and at the outlet of the monohth. 

For successful application of carbon-coated monolithic catalysts, the deposition of active phase in the walls of a monolithic substrate should be prevented. To prevent deposition of ruthenium in the wall (1) substrates with nonporous walls can be used, or (2) the cordierite monolithic substrates can be modified with a-AEOs, blocking the macroporosity of the cordierite and rounding the channel cross section to enable a more uniform thickness of the carbon coating layer. Alternatively, ACM monoliths or integral carbon monoliths with very thin walls having a characteristic diffusion length similar to the activated carbon slurry catalysts can be employed. 

The supports were specially manufactured by CTI Company they are cylindrical honeycomb-type ceramic monoliths 50 mm diameter and 100 mm length, i.e. two orders of magnitude larger than the lab-scale catalysts (Figure 8-a, b) [10]. For aU samples, the active phase is deposited after specific wash-coating procedures to increase the specific surface area of these supports from 0.5 m g for the as-received monolith to 22 m g ... 

Conventional procedures to prepare catalysts, however, can not be simply applied to monolith-based catalysts. The wash-coating procedure focuses on the quality of the layer deposited on the monohth to increase the specific surface area and to ensure a better dispersion of the metallic phase two wash-coating procedures have been used but only one is described in this paper. The deposition of active phase like platinum, iridium or rhodium on a wash-coated monohth is carried out in a similar manner as with pellet snpports nsing impregnation with an excess of impregnation solution. The concentration of active phase precnrsor in the impregnation solution takes the porous volume of the wash-coat layer [3], the solubility limit and the desired final metal loading into account. 

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