Catalysts promoters

Naphtha at one time was a more popular feed, and alkah-promoted catalysts were developed specifically for use with it. As of 1994 the price of naphtha in most Western countries is too high for a reformer feed, and natural gas represents the best economical feedstock. However, where natural gas is not available, propane, butane, or naphtha is preferentially selected over fuel oil or coal. 

The amount of promoter used can also be a variable in influencing the degree of CO combustion. A promoted catalyst system can be classified as fully or partially promoted. A partially promoted system is one in which an increase in promoter content results in a decrease in the dilute-dense AT... 

Can one further enhance the performance of this classically promoted Rh catalyst by using electrochemical promotion The promoted Rh catalyst, is, after all, already deposited on YSZ and one can directly examine what additional effect may have the application of an external voltage UWR ( 1 V) and the concomitant supply (+1 V) or removal (-1 V) of O2 to or from the promoted Rh surface. The result is shown in Fig. 2.3 with the curves labeled electrochemical promotion of a promoted catalyst . It is clear that positive potentials, i.e. supply of O2 to the catalyst surface, further enhances its performance. The light-off temperature is further decreased and the selectivity is further enhanced. Why This we will see in subsequent chapters when we examine the effect of catalyst potential UWR on the chemisorptive bond strength of various adsorbates, such as NO, N, CO and O. But the fact is that positive potentials (+1V) can further significantly enhance the performance of an already promoted catalyst. So one can electrochemically promote an already classically promoted catalyst. 

Why do negative potentials (UWr=-1 V) fail to further enhance to any significant extent catalyst performance of the promoted catalyst whereas the unpromoted Rh catalyst is electrochemically promoted with both positive and negative potentials (Fig. 2.3). The answer will become apparent in subsequent chapters In a broad sense negative potential application is equivalent to alkali supply on the catalyst surface. They both lead to a substantial decrease (up to 2-3 eV) in the catalyst work function, O, aquantity which as we will see, plays an important role in the description of promotion... 

Subsequently this classically promoted Rh(Na)/YSZ catalyst is subject to electrochemical promotion via application of positive (IV) and negative (-1V) overpotential. The classically promoted catalyst performance is further dramatically enhanced especially in terms of rN2, rN20 and SN2 (Figs. 2.3, 8.66 and 8.67) and particularly with positive overpotentials. The resulting pN2 and PN20 values are on the order of 10 in the temperature range 240° to 360°C. Figures 2.3, 8.66 and 8.67 demonstrate two important facts ... 

That a classically promoted catalyst can be further electrochemically promoted. 

Consequently the proven functional identity of classical promotion, electrochemical promotion and metal-support interactions should not lead the reader to pessimistic conclusions regarding the practical usefulness of electrochemical promotion. Operational differences exist between the three phenomena and it is very difficult to imagine how one can use metal-support interactions with conventional supports to promote an electrophilic reaction or how one can use classical promotion to generate the strongest electronegative promoter, O2, on a catalyst surface. Furthermore there is no reason to expect that a metal-support-interaction-promoted catalyst is at its best electrochemically promoted state. Thus the experimental problem of inducing electrochemical promotion on fully-dispersed catalysts remains an important one, as discussed in the next Chapter. 

A related approach is to interface an industrial promoted catalyst with a solid electrolyte (Fig. 12.2). In this case the bulk of the commercial catalyst must be conductive. This concept has been already demonstrated for the case of NH3 synthesis on Fe-based promoted commercial catalysts (BASF S6-10 RED)16 and for the case of SO2 oxidation on V2O5-K2S2O7 based catalysts (Haldor-Topsoe VK-58).17... 

The laboratory prototype of the Dinex electrochemically promoted catalyst unit is shown in Figure 12.12 and the assembled unit schematically in Fig. 12.13. It consists (Fig. 12.14) of a tubular bundle porous (ceramic foam) structure made of CeOa-GcfeOj (CGO) which is an O2" conductor with ionic conductivity significantly higher than YSZ at temperatures below 500°C... 

Furthermore, the application of the SOD membrane in a FT reaction has been investigated. The advantages of water removal in a FT reaction are threefold (i) reduction of H20-promoted catalyst deactivation, (ii) increased reactor productivity, and (iii) displaced water gas shift (WGS) equilibrium to enhance the conversion of CO2 to hydrocarbons [53]. Khajavi etal. report a mixture of H2O/H2 separation factors 10000 and water fluxes of 2.3 kg m h under... 

In our previous work [4, 5], results on the catalytic pyrolysis of R22 over Cu-promoted catalysts were reported. In this work, various metal fluoride catalysts were introduced to improve the relatively poor yield of TFE. 

Table 3 shows the performance of the promoted-catalysts for the decomposition of methane to hydrogen at 5, 60, 120 and 180 min of time on stream. The results in Table 3 revealed that the activity of the parent catalyst and MnOx-doped catalyst remained almost constant until 120 min of time on stream. The activity of the other promoted-catalysts, on the other hand, decreased with an increase in the time on stream. The data for the CoO-doped catalyst and 20 mol%NiO/Ti02 could not be recorded at 120 min and 180 min, respectively because of the pressure build-up in the reactor. This finding indicates that adding MnOx enhances the stability and the resistibility of the NiO/Ti02 catalyst towards its deactivation. 

By modifying the catalyst with a so-called promoter (in this case vanadium oxide) it is possible to largely eliminate the intermediate. As Fig. 2.6 shows, the rate constant of the reaction from the hydroxylamine to the amine is much larger when the promoted catalyst is used, and thus the intermediate reacts instantaneously, resulting in a safer and environmentally friendlier process. 

Around 500 K, the catalyst consumes H2, as shown by the sharp peak, while simultaneously H2S and some additional H2O are produced, which indicates that the catalyst has taken up too much sulfur at lower temperatures, which is now released in the form of H2S. At higher temperatures, the catalyst continues to exchange oxygen for sulfur until all the molybdenum is present as M0S2. TPS has proven very useful in studying the sulfidation of M0O3 as well as Co and Ni promoted catalysts. 

The catalyst was reformulated by Alwin Mittasch, who synthesized some 2500 different catalysts and performed more than 6500 tests. They arrived at a triply promoted catalyst consisting of a fused iron catalyst, with AI2O3 and CaO as structural promoters and potassium as an electronic promoter. The process was first commercialized by BASF, with the first plant located in Oppau in Germany producing 30 tons per day in 1913. The plant initially produced ammonium sulfate fertilizer, but when the First World War broke out it was redesigned to produce nitrates for ammunition. The plant was expanded and in 1915 it produced the equivalent of 230 tons ammonium per day. 

All of the preparation procedures for the oxide promoted catalysts (T-O shared one common feature, heat-treatment of the oxide impregnated Ft on carbon catalysts in an inert atmosphere at elevated temperature, usually around 900 C. If an "alloy" phase of Ft with the metal of the metal oxide is formed by this heat-treatment, thermal reduction would have to occur with carbon as reducing agent, e.g. 

The Ru metal area was determined by volumetric H2 chemisorption in the quartz U-tube of an Autosorb 1-C set-up (Quantachrome) following the procedure described in ref. [16]. Prior to chemisorption, the catalysts were activated by passing 80 Nml/min high-purity synthesis gas (Pnj / Phj -1/3) from a connected feed system through the U-tube and heating to 673 K for alkali-promoted catalysts or to 773 K for alkali-free catalysts with a heating rate of 1 K/min. The BET area was measured by static N2 physisorption in the same set-up. 

Results of the H2 chemisorption measurements after NH3 synthesis based on H/Ru = 1/1. NHs synthesis was run at 773 K with Ru/MgO and RU/AI2OS, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assunung spherical particles. 

Previous reports on FMSZ catalysts have indicated that, in the absence of added H2, the isomerization activity exhibited a typical pattern when measured as a function of time on stream [8, 9], In all cases, the initial activity was very low, but as the reaction proceeded, the conversion slowly increased, reached a maximum, and then started to decrease. In a recent paper [7], we described the time evolution in terms of a simple mathematical model that includes induction and deactivation periods This model predicts the existence of two types of sites with different reactivity and stability. One type of site was responsible for most of the activity observed during the first few minutes on stream, but it rapidly deactivated. For the second type of site, both, the induction and deactivation processes, were significantly slower We proposed that the observed induction periods were due to the formation and accumulation of reaction intermediates that participate in the inter-molecular step described above. Here, we present new evidence to support this hypothesis for the particular case of Ni-promoted catalysts. 

Figure 2 Conversion - time curves on several Ni - promoted catalysts under n-C4Hio/He mixture. n-Butane conversion to isobutane as a function of time on stream over 0 4 g cat., at 150°C, n-C4Hio flow rate = 5.46 cmVmin, He flow rate = 10.4 cm7min. 3NiSZ(s) (squares) 2NiSZ(s) (triangles) INiSZ(s) (circles).

Carbonylation reactions encompass a diverse set of transformations used to synthesise many important high-value fine chemicals, synthetic intermediates and materials such as polycarbonates [36]. Palladium catalysts modified with PRj ligands facilitate these reactions. However, carbonylation often requires harsh conditions, especially for less reactive C-X bonds, thereby promoting catalyst degradation via P-C bond cleavage. The strength of the NHC bond may demonstrate the utility of... 

Figure 11.15. Promotional effect of different transition metals on initial NOx storage/reduction activity for 0.5Pt/7.5Ba/2.5 Promoter catalyst [90].

The diagnostic method can also be successfully used in the assessment of promoted catalysts. 

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