Ensemble control

The ensemble control plays a role also in catalytic reforming on platinum or bimetallic catalysv. A good catalyst should have low activity for hydrogenolysis resulting in production of lower alkanes, and it should have a slow build-up of carbon overlayers to maintain stable activity for isomerization, dehydrogenation and dehydrocyciiration ... 

It is well-established that hydrogenolysis is a nore demanding reaction than the other reactions. Therefore one should expect the effect of ensemble control by means of chemisorbed sulfur, as observed in sulfur passivated steam reforming. This was demonstrated by Kayes et a1 who observed an optimum sulfur level in terms of catalyst activity and product selectivity. However, rather high contents of sulfur of about 1000 ppm were required. This is not surprising in view of the chemisorption equilibrium HjS/Pt compared to HjS/Ni as illustrated in Table 1. 

With the introduction of Pt/Re catalysts, it is possible to achieve the ensemble control with much smaller sulfur addition. The su1fur-free Pt/Re catalyst by itself has a higher relative activity for hydrogenolysis than a platinum catalyst. However, this is changed when sulfur Is present In the feed. Kughes has described the first observations in a pilot plant. The catalyst produced more methane than any other that had been tested, and the run would probably have been aborted if it had not been an ordinary catalyst screening test. However, after the first and second weeks on stream, the selectivity improved and finally became similar to that of a fresh platinum/ alumina catalyst and as the run continued, the catalyst proved to be more stable than any previous catalyst tested. These results were ascribed to the presence of sulfur in the feed and could be obtained even with very low sulfur contents, l.S ppm. ... 

Studies by Sachtler et al have indicated that the ensemble control may also retard the formation of carbonaceous overlayers. Also, it was suggested that the chemisorption of sulfur on rhenium may enrich the surfaces of the platinum particles with rhenium. 

In conclusion by using rhenium as adsorbent instead of platinum, it is possible to achieve the ensemble control by sulfur passivation, at sulfur levels comparable to those applied for nickel and much lower than that which would have been required on a non-alloyed Pt-catalyst. A similar ensemble effect is achieved by alloying alone on Pt-Sn catalysts ... 

With these assumptions, it is possible to analyze the data of Cull is et a1 in terms of ensemble control . 

The examples have shown that it is possible to explain promotion by poisoning of metal catalysts by ensemble control. It nay give ideas for the design of more selective catalysts exposing almost two-dimensional shape selectivity. 

During its in-depth review, the committee observed differences in the types and completeness of entries made in JACADS and TOCDF operating logs (deactivation furnace system, demilitarization protective ensemble, control room. 

The SPARG (Sulfur Passivated Reforming) principle (ref. 3) allows operation below the carbon limit curve. It was demonstrated (ref. 4) that carbon-free operation could be obtained above a certain sulfur coverage at conditions which would otherwise result in carbon formation. Sulfur passivated reforming as practiced in the Topsoe SPARG process (ref. 5) solves the problem of carbon formation by "ensemble control" which means that the sites for carbon formation are blocked while sufficient sites for the reforming reactions are maintained. This effect is obtained by adding sulfur to the process feed. 

In the following, examples of the ensemble control by means of adsorbed poisons are discussed with the the emphasis on steam reforming of methane on sulfur passivated nickel catalysts. The conclusions for ensemble control will be compared with data for catalytic reforming on PtRe(S) catalysts and for the impact of chlorine on partial oxidation of methane on Pd-catalysts. 

C carbon limit with ensemble control or noble metals. Reproduced with the permission... 

The retarded dissociation of the hydrocarbon was the explanation in a series of studies of the impact of a number of oxides such as La, Ce, Ti, Mo, W [70] [80] [101]. For a PtyTi02 catalyst it was concluded that TiOx layers on the platinum surface were suppressing carbon formation, probably by ensemble control. 

Ensemble control is also involved in carbon-fiee steam reforming on a sulphur passivated catalyst [390] (Section 5.5). Ensemble control was also reported for the addition of Bi [500] or B [526] to nickel and for bimetallic catalysts such as Ni,Au [50], Pt,Re [367], Pt,Sn [474], and Ni,Sn [217] [247] [431] [456] [545]. Alloying nickel with copper [49] [16] can also decrease the rate of carbon formation, but it is not possible to achieve die same high surface coverage with copper as with sulphur and gold, because copper and nickel forms a bulk alloy with a fixed surface concentration of copper over a wide range as alloy composition. 

The ensemble control allows operation at (H2O + C02)/CH4 ratios close to stoichiometry. Operation on mixtures of CO2 and methane without steam is also possible except for the steam required for prereforming of the higher hydrocarbons in the feed. No steam was needed when operating on pure methane (CO2/CH4 = 1.2) in a laboratory reactor. Conversion of CO2 to CO - by reaction (4) and by the reverse shift reaction (reaction(2)) -of up to 75% may be achieved. 

Hochberg LR, Sermya MD, Friehs GM, Mukand JA, Saleh M, Caplan AH, et al. (2006) Neuronal ensemble control of prosthetic devices by a human with tetraplegia. Nature. 442 164-171. 

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