Unpromoted Catalysts

Although less selective, Pt is normally the metal of choice, especially when supported on graphite. Because selectivity has been found to be structure-sensitive, several catalysts, prepared by different methods, should be tested. In a study of graphite supported Pt catalysts [16], large faceted metal particles (3-6 nm) proved to be most selective (98 % selectivity at 50 % conversion). 

Promoters might be added to a PGM catalyst by the manufacturer, or directly to the reaction mixture. Each route has its benefits. In cinnamaldehyde hydrogenation the promoter is added to inhibit olefin hydrogenation during reduction of the carbonyl function. This has been achieved by addition of bases such as KOH or NaOH at 5-10% concentration [17], or more often by addition of other electropositive metals that are reduced on the catalyst surface in situ upon admission of hydrogen into the reactor. 

It is important to optimize the amount of modifier metal added (too much will mask the catalytic metal) 0.005-0.03 mol% relative to the PGM is a reasonable place to start. 

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The reaction is exothermic and so to avoid serious temperature excursions the reactor consists of a bundle of narrow tubes, each a few centimeters in diameter, surrounded by a heat transfer medium. The catalyst consists of relatively large silver particles on an inert a-Al203 support. The surface area is below 1 m g". Promoters such as potassium and chlorine help to boost the selectivity from typically 60% for the unpromoted catalysts to around 90%, at ethylene conversion levels of the order of50%. 

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

In many cases, those promoters which seem to act by virtue of a combination of their chemical affinities with those of the main catalysts do not merely increase the activity of the unpromoted catalyst, but they also cause the catalytic reaction to proceed in a more specified direction. The application of promoters to guide reactions selectively toward the formation of desired product is, from a practical viewpoint, often more important than the achievement of an overall acceleration of the catalytic process. 

More than 20 catalyst samples were prepared, pretreated and tested during the present work. Their characteristics, averaged over all samples at various stages are shown in Table 1. Pretreatment reduces the A1 content of the unpromoted catalyst to near zero accompanied by a large fall in surface area. The Cr-promoted catalyst retains significant A1 after pretreatment and the loss in surface area is proportionately much less. The Cr203 content is not changed. With both catalysts, subsequent use has little effect on properties beyond that of pretreatment. 

Figure 6 shows a plot of this relationship as a function of k2/ki together with horizontal lines corresponding to the maximum values attained by Yheg during reaction over the unpromoted catalyst (-0.53) and for the promoted catalyst (-0.43). These values correspond to R = 0.41 and 0.71 respectively. Table 4 lists the values for ki and k2 obtained for the two catalysts on this basis. The CH2OH groups in DEA appear to be about twice as active as those in HEG (but see below). 

More recently, Koizumi et al. observed that Mn has an additional beneficial effect in unsupported Fe-based F-T catalysts. These authors studied the sulfur resistance of Mn-Fe catalysts and they observed superior catalysts stabilities, especially when the catalysts were pre-reduced in CO. This group also used IR spectroscopy in combination with CO as a probe molecule to compare Fe and Mn-Fe catalysts. It was found that the addition of Mn led to the appearance of several well-resolved bands upon CO adsorption. The appearance of the bands arising from bridged-bonded CO on Fe indicated that the size of the Fe particles were clearly larger than in the case of the unpromoted catalysts. They attributed the decreased reactivity towards H2S to the observed increase in Fe particle size. 

The use of precursor synthesis techniques as described above is driven by the fact that decomposition of the precursor material into the catalyst often results in catalysts that have activity or selectivity superior to that of preferred products. The amine thiomolybdate decomposition described above results in MoS2 catalysts with surface areas exceeding several hundred square meters per gram (27). The HDS activity increases correspondingly and unpromoted catalysts have activities approaching those of promoted systems. 

The products distribution as a function of time is illustrated in Fig. 2 for the unpromoted catalyst derived from the Ni Al alloy. 

It is well known, even from old literature data (ref. 1) that the presence of metal promotors like molybdenum and chromium in Raney-nickel catalysts increases their activity in hydrogenation reactions. Recently Court et al (ref. 2) reported that Mo, Or and Fe-promoted Raney-nickel catalysts are more active for glucose hydrogenation than unpromoted catalysts. However the effects of metal promotors on the catalytic activity after repeated recycling of the catalyst have not been studied so far. Indeed, catalysts used in industrial operation are recycled many times, stability is then an essential criterion for their selection. From a more fundamental standpoint, the various causes of Raney-nickel deactivation have not been established. This work was intended to address two essential questions pertinent to the stability of Raney-nickel in glucose hydrogenation namely what are the respective activity losses experienced by unpromoted or by molybdenum, chromium and iron-promoted catalysts after recycling and what are the causes for their deactivation ... 

Now the influence of water or ammonia on copper catalysts is being investigated. Previously A. BAIKER and coll, have shown that ammonia could modify the catalytic properties of copper catalysts used in the amination of alcohols (9). These authors noticed the formation of copper nitride after NH3 exposure at a temperature of about 300°C which is the reaction temperature of our study. The first results that we obtained in our study showed that both H2O and NH3 decrease significantly the copper dispersion in unpromoted catalysts and that this modification is less significant when Ca or Mn are added to the Cu-Cr catalyst. We are now studying what are the superfical modifications consecutive to the addition of promoters or/and water and ammonia. 

N-methylation with methanol. These results can be obtained without increasing the partial hydrogen pressure as was observed for unpromoted catalysts. On the other hand we noticed that these compounds don t modify the metallic area but decrease the reducibility which means that copper oxide and chromium (VI) oxide are only partially reduced. Moreover as the highly adsorbed hydrogen is also inhibited and as these catalysts are more stable in the presence of H2O or NH3 than unpromoted catalysts, one can also deduce that one of the important roles of the hydrogen during the reaction is to prevent the modification of catalysts or/and the amination reaction by ammonia and water. 

The best over-all results, where the unpromoted catalyst is improved in every aspect, are obtained for the addition of Sn, Ge and Mn. Because of practical considerations we selected the Sn-promoted type to look for further improvements. 

The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et al. is also reflected in the work of Tops0e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe304 carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203) have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the lead-promoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96). For the reverse CO shift reaction over supported europium (176), the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section III, A, 4. 

Fig. 5, Test for second-order deactivation of alkaline earth metal—promoted and unpromoted catalysts-...

The catalyst was prepared from 2 g of 40% Ni-Al alloy by the procedure for the T-4 catalyst each time before use. T-4 unpromoted catalyst T-4/Pt the catalyst platinized during leaching process with 0.05 g of chloroplatinic acid (0.0185 g Pt) T-4/Pt (Delepine-Horeau) T-4 Raney Ni platinized with 0.05 g of chloroplatinic acid by the method of Delepine and Horeau (Ref. 66). 

Besson et al. studied the hydrogenation of valeronitrile over Raney Ni, prepared from chromium- and molybdenum-doped Ni2Al3 alloys, in cyclohexane at 90°C and 1.6 MPa H2.30 Chromium was found to be an effective promoter for initial activity and for the selectivity to primary amine (83-85%, compared to 79.2% with unpromoted catalyst), whereas the addition of molybdenum was not effective. 

Bej and Rao (186-190) conducted a detailed investigation of molybdenum- and cerium-promoted vanadium phosphate catalysts. They foimd an increase in the selectivities of these catalysts as a result of incorporation of the promoters, albeit with slight decreases in activity. They attributed the improved selectivity to a role of the promoters in preventing overoxidation of the MA to carbon oxides. They also found that the promoted catalysts could withstand more severe reaction conditions than the unpromoted catalyst, and this property was also attributed to the formation of less carbon oxides, which can poison the catalyst. 

Zazhigalov and coworkers [143] investigated cobalt-doped VPO catalysts prepared by co-precipitation and impregnation methods. The performance of catalysts prepared by both methods was increased, compared to the unpromoted catalyst The cobalt is thought to be present as cobalt phosphate, which is considered to stabilize excess phosphorus at the surface, which has previously been found to be an important feature of active catalysts (Section 12.2). 

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