Temperature-programmed hydrogenation

The coke formed on sulfided nickel catalysts was studied The TPHy profiles indicated that only a small fraction of the coke can be removed with H2, approximately 10%. The profile displays a small peak at 267°C, and a larger one at about 817°C. These results demonstrate the refractory nature of the carbon deposits toward hydrogen, and therefore, that there is little benefit in life characteristics to be gained by increasing the partial pressure of feed hydrogen. 

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These different types of carbon tend to have different reactivities toward gases such as hydrogen, oxygen, or steam. Hence, a relatively simple technique such as temperature-programmed hydrogenation or oxidation can be used to classify them. Table 4.2 summarizes different reactivities of carbon species toward hydrogen. 

Fig. 6. Temperature-programmed hydrogen release profiles of Pt/NaY (7.4 wt% Pt) calcined at (a) 360°C, (b) 450°C, and (c) 550°C, Solid lines, after reduction at 550°C dotted lines, after reduction at 400°C. The peaks at high temperatures for the dotted lines are attributed to reoxidation of Pt atoms by protons 150).

Sulfur desorption fiom the poisoned nickel catalysts was studied by temperature programmed hydrogenation (TPH). The poisoned catalyst beds were powdered to homogenize them. Approximately 0.5 g of powdered catalyst was placed in an atmospheric quartz tube reactor which had a thermocouple shield. The reaction gas was argon/hydrogen (70% Ar/30% Hj). The content of hydrogen in the gas was about the same as in the sulfurpoisoning tests. 

Figure 2. Temperature-programmed hydrogenation of sulfur from poisoned nickel catalysts (Al, A2 and C). Heating rate 20°C/min, gas atmosphere Ar/Hj.

In addition to the sintering phenomena, the carbidization of the nickel particles and the formation of external carbon deposits, already revealed by temperature programmed hydrogenation carried out after the methanation reaction [33], may also contribute to the overall deactivation process. Further experiments aimed at quantifying this type of deactivation are in progress. 

Fig. 3 Temperature-programmed hydrogenation spectra of carbon deposit on unpromoted and promoted nickel-alumina catalysts.

Temperature programmed hydrogenation of toluene was studied with the AMI-100 equipped with a quadrupole mass spectrometer (Balzers MSC 200 Thermo-Cube). A gas mixture of hydrogen, argon and toluene was fed into the reactor (a U-tube) in which the catalyst sample was loaded, and the temperature of the sample was raised linearly from 373 to 473 K. The exiting gas stream was analysed by the mass spectrometer utilising Quadstar 421 software. The conversion was calculated from the toluene molar flows. 

The reactivity of the deposited carbon was analysed by temperature programmed hydrogenation (TPH) and temperature programmed oxidation (TPO). The TPH experiments were carried out under a flow of H2 at 10 K min", from 298 to 1293 K and those of TPO were perfomed in a Shimadru TGA-50H under air stream. TEM studies were performed in order to determine the morphology of the deposited carbon on the coked samples. 

Figure 4 temperature programmed analysis of coked catalysts (A) Pd(0.9) (B) Pt(0.9) (1) temperature programmed hydrogenation (TPHy) (2) TPO after TPHy. 

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