**5. The deactivation of slurry bed catalyst**

Dealing with the catalyst deactivation in a slurry bed reactor is a challenging phenomenon [30]. The detailed mechanistic insights in realizing the origin of the deactivation are much more complex, for the simultaneous involvement of various factors. Where, the most common factors could be relied on the S and/or N2-containing compounds in the feed gas; the oxidation of the active metal (Co) components; strong metal-support interaction resulting into the hard to reduce species of silicates, aluminates; sintering, attrition, and carbon deposition on the nano-crystallites particles [31]. Since, these aforementioned factors of catalyst deactivation are mainly dependent on

the type of reactor to be employed, the nature of the support, and the partial pressure of H2O that can be further explained as follows.

#### **5.1 Sulfur and nitrogen containing poisons**

In carbon one chemical reactions, the poisons that may cause severe damage to the catalyst must be removed from the feed gas prior to the reaction, as these are independent of the catalyst nature to be used and operating conditions of the reactions. Since, the sensitive nature of cobalt-based catalysts to the sulfur content (<10 ppb) needs more attention of the additional gas cleaning steps for being expensive than Fe and also for its higher activity and a longer life time. In here, the different adsorption phenomena of the sulfur containing compounds (organic, inorganic) is more important for physically blocking the sites after being strongly adsorbed on the catalytic active sites, where one adsorbed S atom could deactivate the more than two Co atoms in a Co/Al2O3 catalyst. In a CO-rich syngas conversion to methanol reaction, the iron carbonyl, nickel carbonyl and carbonyl sulfide are severe catalyst poisons, which must be removed from the feed gas to avoid the catalyst deactivation [32]. Since, the removal of H2S as the probe molecule and the nitrogen compounds such as NH3, HCN (<50 ppb) are the crucial steps in gas purification, as they could lead to the severe damaging effect on the catalyst reducibility, activity, and selectivity.

#### **5.2 Sintering**

Besides the poisoning, the catalyst could also be deactivated due to the catalyst sintering at high temperature. Sintering is a common phenomenon of deactivation in metal catalysis, that is usually based on the minimization of surface energy of the crystallites. Sintering is usually accelerated by the high temperature and the partial pressure of water vapor; however, it can be controlled by interactions with the metal support. There are two empirical rules for the effect of temperature on highly dispersed metal crystallites: [1] When the temperature reaches 0.3Tm (called Hüttig temperature, Tm is the melting point), the migration of particles on the catalyst surface will occur [2]. When the temperature reaches 0.5Tm (Tammann temperature), the particle migration within the lattice phase will occur. For the methanol synthesis catalyst CuZnAl, the melting point of metallic Cu is 1358 K, and its Hüttig temperature is 407 K, and its Tammann temperature is 679 K; while the operation temperature of CuZnAl catalyst is generally at 480–553 K. Though the operation temperature is much lower than the Tammann temperature, it is higher than the Hüttig temperature, which inevitably resulted in the catalyst sintering and thus deactivated the catalyst. Lewnard et al. [33] investigated the stability test on CuZnAl catalyst, and obtained that the activation energy for catalyst deactivation is 91.3 kJ/mol. These activation energy data all demonstrate that catalyst sintering is an important reason for the deactivation of methanol synthesis catalyst in a gas-liquid-solid three-phase slurry bed.

#### **5.3 Carbon deposition**

Generally, the slurry bed reactor needs to be operated at the temperatures lower than 350°C due to the limitation of boiling point of mineral oil. Even at this low temperature, there are still some carbon deposited on the catalyst surface. For example, the result of CO methanation in a slurry-bed reactor over the Ni-Al2O3 catalyst [34] shows that, the catalytic activity of Ni-Al2O3 catalysts decreased slowly after a

reaction time of 450 h. The thermogravimetric analysis and microscopic morphology results show the carbon deposition on catalyst surface, which was attributed to the amorphous carbon. The carbon deposition occupies the surface of the catalyst and covers the active sites, resulting in a decrease in Ni metal surface area and thus reduce the methanation activity. The regeneration of the spent catalyst shows that the catalyst carbon deposit can be removed by calcination in the air, and the catalyst structure and catalytic performance can be recovered.

#### **5.4 Effects of water on catalyst structure**

The reoxidation of Co metal active sites usually occurs when the oxygen atom of CO is eliminated mainly as H2O (either from surface oxygen or OH species) during the FTS, where the influence of water contents in terms of its higher partial pressure than that of H2 and CO become more crucial at high CO conversion. Since, the effect of water contents on the catalyst deactivation is mainly adopted from the possible reoxidation of the surface depending on the operating conditions, presence of different promoters (Pt, Mn, Zn, Mg, etc.), nature of the support (Al2O3, SiO2, TiO2) to be used, dispersion of metal components, and on the size of the pores of the support. For the methanol synthesis from syngas in a slurry-bed reactor over Cu-based catalysts, the addition of water in the reaction shows that too much H2O accelerates the growth of grain and agglomeration of Cu-based catalyst, and a certain degree of carbon deposition, which leads to a fast catalyst deactivation, as shown in **Figure 7** [35].

For the syngas to DME in a slurry bed reactor, the methanol dehydrated and formed DME in the reaction, if the generated water cannot be removed in time, the pores of the catalyst will be blocked and the reaction performance will be affected [36]. If the partial pressure of CO2 in the feed gas is high, more ZnCO3 will be appeared on the surface of CuZnAl catalyst, which is due to that the solubility of ZnCO3 in water is several times higher than that of ZnO. Therefore, the leaching of Zn in the presence of water is one of the important reasons for the rapid deactivation of CuZnAl catalyst [37]. In addition, in the methanation reaction, the presence of water reacts with the γ-Al2O3 support of Ni-Al2O3 catalyst and thus formed AlOOH, which is one of the reasons for the deactivation of Ni-Al2O3 methanation catalyst.

**Figure 7.** *Effect of H2O on CO conversion for methanol synthesis [35].*

### **5.5 Attrition of catalyst particles**

One of the serious concerns of catalyst deactivation is the catalyst attrition by mixed particle fragmentation or surface abrasion, for breaking the particles into various fragments. It is a common phenomenon in fluidized or slurry bed reactors, where the interparticle collisions and bed-to-wall impacts cause the high mechanical stresses to catalyst bed particles, thus resulting the loss of valuable material, generation of fine particles, and the degradation of catalyst efficiency [38]. Usually, it is a time-dependent process that may systematically change with time, being more severe in the start of reaction and tends to a constant value of the nonsteady-state attrition rate with time. However, different strategies can be applied for increasing the attrition resistance and catalytic performance and decreasing the fraction of fines by spraydried Fe-based FTS catalysts in a stirred-tank slurry reactor.
