Fig. 1. Common deactivation profile

Fig. 1. Common deactivation profile for cobalt catalysts in FTS. Adapted from reference [4].
Fig. 1) is associated with irreversible deactivation and has therefore operational significance. This change in deactivation rate with time on stream suggests that the actual cause of deactivation is not the result of one, but a combination of several phenomena.
The proposed mechanisms of catalyst deactivation include poi- soning, sintering, surface carbon formation, carbidization, cobalt re-oxidation, cobalt–support mixed compound formation, surface reconstruction and mechanical deactivation through attrition. The Fischer–Tropsch catalysts are usually very sensitive to poisoning and purification of the synthesis gas is therefore an important part of the process, particularly for processes using coal and biomass as feedstocks [5]. The loss of activity is also related to process condi- tions such as temperature, pressure, conversion, partial pressures of synthesis gas and steam and the type of reactor (fixed-bed or slurry). Hence, reproduction of a realistic FT environment in deac- tivation studies is fundamental.
The study of catalyst deactivation is mainly a characterization oriented problem. Spent catalyst has to be characterized and com- pared with its activated counterpart. A main challenge for studying catalyst deactivation in FTS is the fact that the catalyst is embedded in wax after use. The wax limits the range of technique that can be applied for characterization of the spent catalysts. In addition, the sensitivity of the active phase against air hampers the handling of the dewaxed catalysts. Deactivation is an inevitable phenomenon in FTS although catalytic systems show different behaviour. Regen- eration is therefore also an important topic.

1. Causes of catalyst deactivation in Fischer–Tropsch synthesis

In the following paragraphs the main mechanisms of catalyst deactivation in FTS are discussed.

Poisoning

Sulphur compounds
Sulphur is a known poison for metals since it adsorbs strongly on catalytically active sites. The consequences of this strong bond- ing are usually a physical blocking of the sites and possibly the electronic modification of neighbouring atoms [6]. For cobalt FT catalysts poisoning by sulphur appears to be more a geomet- ric blockage of sites than an electronic modification. It has been reported that one sulphur atom adsorbed on a Co/Al2O3 catalyst poisons more than two cobalt atoms [7]. Sulphur is usually present in the feed and is therefore considered as a potential cause of deacti- vation. Raw synthesis gas derived from biomass or coal will usually contain significant amounts of sulphur, whereas sulphur usually is removed from natural gas before the reformer section. Sulphur may also stem from corrosion inhibitors which are occasionally added.
In any case, there is a possibility that traces of sulphur can reach the FTS reactor, typically during operational upsets. Thus, already at the early stages of the development of the FT technology, the effect of sulphur in different molecular forms was studied [8]. An upper limit of sulphur concentration in the feed was proposed already by Fischer (1–2 mg/m3) [9]. However, these limits are decreased and usually kept below 0.02 mg/m3 in today’s applications [10].
The oil crisis in the 70 s raised crude oil prices and hence renewed the interest in FTS mainly using natural gas as feedstock. This resulted in increased research and process development in the field of cobalt based FTS. Madon and Seaw summarized in 1977 the literature concerning the effect of sulphur [9]. In particular they presented several studies carried out for more than four decades dealing with sulphur effects on different FT catalysts. For cobalt catalysts the results seemed to agree that sulphur, added in low
concentrations in the forms of H S and CS , has a promotional effect

on the catalysts. In particular, at low concentrations of sulphur, an increase in the catalysts lifetime and also the selectivity towards heavier hydrocarbons was observed. Further addition of sulphur compounds led to complete catalyst deactivation.
Studying sulphur poisoning by in situ methods is challenging. H2S, which is normally being used as a sulphur carrier, adsorbs strongly on metallic tubes. In addition, it is corrosive, toxic and flammable. The selection of sulphur carrier is important since there is a significant difference between adsorption phenomena of organic (e.g. C2H5SH) and inorganic (e.g. H2S) sulphur containing molecules. A proper sampling procedure is essential for the accu- racy of such studies, due to an expected intraparticle and reactor poisoning gradient (especially for plug flow reactors) [11,12].
Bartholomew and Bowman [13] studied the effect of sulphur by introducing 0.5–8 ppm H2S in the reactor feed through Teflon lines. For the silica-supported cobalt catalyst a decline in catalyst activity was observed for the entire range of sulphur content in the feed. The decline appeared to be more intense for concentrations between 0.5 and 2 ppm, while less for 5–6 ppm of H2S. A possible explanation for this trend was that at higher sulphur concentrations a surface sulphide of a different structure or multilayers of sulphide were created. Catalyst selectivity was also altered as a result of sulphur addition leading to increased production of heavier hydro- carbons (>C4). A possible reason for the increased selectivity to higher molecular weight products could be the selective adsorp- tion of the H2S on sites which normally adsorb hydrogen, resulting in a hydrogen deficient surface. Decreased water production, which is a result of the lower conversion, normally affects the product dis- tribution in the opposite direction. Chaffee et al. also studied in situ sulphur poisoning using H2S as the sulphur carrier in a fixed-bed reactor [14]. Commercial catalysts were used and the main focus of the study was on the effect of the H2/CO ratio on the catalyst deactivation behaviour. The result showed that for cobalt catalysts, H2 rich feeds appeared to be more sensitive to sulphur poisoning than lower H2/CO ratios. Furthermore, 300 ppm H2S in the feed had minor impact on the product selectivities, but in most cases it favoured the formation of methane and saturated products.
Due to the difficulty that arises when studying sulphur poison- ing by in situ techniques, the effect of sulphur poisoning has mainly been investigated by ex situ, “pre-sulphidation” procedures. Cov- ille and co-workers have studied extensively the effect of sulphur addition during catalyst preparation [15–17]. These studies also included the effect of additives such as boron and zinc, which act as sulphur sinks. Results from diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and temperature programmed reduction (TPR) that were carried out on TiO2 and SiO2 supported cobalt catalysts, showed that in the entire range of sulphur load- ing (100–2000 ppm) CO adsorption inhibition is being observed. In addition, in the range of 200–2000 ppm sulphur, an increase in the reduction temperature of the sulphided samples was detected
further studies of the catalyst activity by ir suggested that the sulphur loading had a promotional effect for concentrations lower than 200 ppm.

Kết quả (Việt) 1:

Fig. 1. Common deactivation profile for cobalt catalysts in FTS. Adapted from reference [4].Fig. 1) is associated with irreversible deactivation and has therefore operational significance. This change in deactivation rate with time on stream suggests that the actual cause of deactivation is not the result of one, but a combination of several phenomena.The proposed mechanisms of catalyst deactivation include poi- soning, sintering, surface carbon formation, carbidization, cobalt re-oxidation, cobalt–support mixed compound formation, surface reconstruction and mechanical deactivation through attrition. The Fischer–Tropsch catalysts are usually very sensitive to poisoning and purification of the synthesis gas is therefore an important part of the process, particularly for processes using coal and biomass as feedstocks [5]. The loss of activity is also related to process condi- tions such as temperature, pressure, conversion, partial pressures of synthesis gas and steam and the type of reactor (fixed-bed or slurry). Hence, reproduction of a realistic FT environment in deac- tivation studies is fundamental.Nghiên cứu vô hiệu hóa chất xúc tác là chủ yếu là một đặc tính theo định hướng vấn đề. Đã qua sử dụng chất xúc tác đã được đặc trưng và com - pared với đối tác kích hoạt của nó. Một thách thức chính cho việc học tập vô hiệu hóa chất xúc tác trong FTS là một thực tế là chất xúc tác nhúng trong sáp sau khi sử dụng. Sáp giới hạn phạm vi kỹ thuật có thể được áp dụng cho các đặc tính của chất xúc tác đã qua sử dụng. Ngoài ra, sự nhạy cảm của giai đoạn hoạt động chống lại máy cản trở việc xử lý các chất xúc tác dewaxed. Vô hiệu hóa là một hiện tượng không thể tránh khỏi trong FTS mặc dù xúc tác hệ thống hiển thị hành vi khác nhau. Regen-eration do đó cũng là một chủ đề quan trọng.1. nguyên nhân vô hiệu hóa chất xúc tác trong tổng hợp Fischer-TropschTrong đoạn văn sau đây các cơ chế chính của chất xúc tác vô hiệu hóa trong FTS được thảo luận. Ngộ độc Hợp chất lưu huỳnhSulphur is a known poison for metals since it adsorbs strongly on catalytically active sites. The consequences of this strong bond- ing are usually a physical blocking of the sites and possibly the electronic modification of neighbouring atoms [6]. For cobalt FT catalysts poisoning by sulphur appears to be more a geomet- ric blockage of sites than an electronic modification. It has been reported that one sulphur atom adsorbed on a Co/Al2O3 catalyst poisons more than two cobalt atoms [7]. Sulphur is usually present in the feed and is therefore considered as a potential cause of deacti- vation. Raw synthesis gas derived from biomass or coal will usually contain significant amounts of sulphur, whereas sulphur usually is removed from natural gas before the reformer section. Sulphur may also stem from corrosion inhibitors which are occasionally added.In any case, there is a possibility that traces of sulphur can reach the FTS reactor, typically during operational upsets. Thus, already at the early stages of the development of the FT technology, the effect of sulphur in different molecular forms was studied [8]. An upper limit of sulphur concentration in the feed was proposed already by Fischer (1–2 mg/m3) [9]. However, these limits are decreased and usually kept below 0.02 mg/m3 in today’s applications [10].The oil crisis in the 70 s raised crude oil prices and hence renewed the interest in FTS mainly using natural gas as feedstock. This resulted in increased research and process development in the field of cobalt based FTS. Madon and Seaw summarized in 1977 the literature concerning the effect of sulphur [9]. In particular they presented several studies carried out for more than four decades dealing with sulphur effects on different FT catalysts. For cobalt catalysts the results seemed to agree that sulphur, added in lowconcentrations in the forms of H S and CS , has a promotional effecton the catalysts. In particular, at low concentrations of sulphur, an increase in the catalysts lifetime and also the selectivity towards heavier hydrocarbons was observed. Further addition of sulphur compounds led to complete catalyst deactivation.Studying sulphur poisoning by in situ methods is challenging. H2S, which is normally being used as a sulphur carrier, adsorbs strongly on metallic tubes. In addition, it is corrosive, toxic and flammable. The selection of sulphur carrier is important since there is a significant difference between adsorption phenomena of organic (e.g. C2H5SH) and inorganic (e.g. H2S) sulphur containing molecules. A proper sampling procedure is essential for the accu- racy of such studies, due to an expected intraparticle and reactor poisoning gradient (especially for plug flow reactors) [11,12].Bartholomew and Bowman [13] studied the effect of sulphur by introducing 0.5–8 ppm H2S in the reactor feed through Teflon lines. For the silica-supported cobalt catalyst a decline in catalyst activity was observed for the entire range of sulphur content in the feed. The decline appeared to be more intense for concentrations between 0.5 and 2 ppm, while less for 5–6 ppm of H2S. A possible explanation for this trend was that at higher sulphur concentrations a surface sulphide of a different structure or multilayers of sulphide were created. Catalyst selectivity was also altered as a result of sulphur addition leading to increased production of heavier hydro- carbons (>C4). A possible reason for the increased selectivity to higher molecular weight products could be the selective adsorp- tion of the H2S on sites which normally adsorb hydrogen, resulting in a hydrogen deficient surface. Decreased water production, which is a result of the lower conversion, normally affects the product dis- tribution in the opposite direction. Chaffee et al. also studied in situ sulphur poisoning using H2S as the sulphur carrier in a fixed-bed reactor [14]. Commercial catalysts were used and the main focus of the study was on the effect of the H2/CO ratio on the catalyst deactivation behaviour. The result showed that for cobalt catalysts, H2 rich feeds appeared to be more sensitive to sulphur poisoning than lower H2/CO ratios. Furthermore, 300 ppm H2S in the feed had minor impact on the product selectivities, but in most cases it favoured the formation of methane and saturated products.Due to the difficulty that arises when studying sulphur poison- ing by in situ techniques, the effect of sulphur poisoning has mainly been investigated by ex situ, “pre-sulphidation” procedures. Cov- ille and co-workers have studied extensively the effect of sulphur addition during catalyst preparation [15–17]. These studies also included the effect of additives such as boron and zinc, which act as sulphur sinks. Results from diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and temperature programmed reduction (TPR) that were carried out on TiO2 and SiO2 supported cobalt catalysts, showed that in the entire range of sulphur load- ing (100–2000 ppm) CO adsorption inhibition is being observed. In addition, in the range of 200–2000 ppm sulphur, an increase in the reduction temperature of the sulphided samples was detected
further studies of the catalyst activity by ir suggested that the sulphur loading had a promotional effect for concentrations lower than 200 ppm.