## 4 Selecting a Suitable Metal Oxide
### Durability  
Sorbents require a high mechanical and chemical strength such that they can sustain high operating hours. The main mechanical strength issue with metal oxide sorbents is attrition which is more significant in fluidized bed reactors compared with fixed beds reactors [75]. Also, some metal oxides experience sintering at high temperatures which leads to changes in the surface and pore structure [76]. Such changes can have a significant impact on the performance of the sorbent. Also, compounds other than H\({}_{2}\)S contained in the gas stream can interact with the solid oxide and affect the removal efficiency of the sorbent [77], so it is important that the metal oxide does not extensively participate in other reactions rather than reaction (Equation (1)). The most common solution to increase the chemical and mechanical resistance of the sorbent is to mix or dope it with other compounds such as titanium [61; 78].  
Table 4 presents an assessment of durability issues for different metal oxide together with remediation strategies. The metal oxides in Table 4 all have a favorable equilibrium and show reasonable fast kinetics.  
\begin{table}
\begin{tabular}{p{14.2 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**MO** & \multicolumn{2}{c}{**Durability Assessment**} & \multicolumn{1}{c}{**Remediation**} \\ \hline \multirow{4}{*}{CaO} & Not fully regenerable due to formation of calcium sulfate [79] & Regeneration is done by one step oxidation with O\({}_{2}\) followed by reduction with CO [80] or H\({}_{2}\)[81]. Core-shell pelletization leads to 10 cycles of constant \\  & 50\% capacity loss after 3 cycles due to sintering [80]. Slow kinetics at low temperatures, most effective at 880 \({}^{\circ}\)C [71]. Deep desulturization is not possible due to thermodynamic constraints [36]. Prone to attrition [80]. & Doping with Co [84], Mn [85] and Cu [28; 86] to increase stability and performance. Addition of Al\({}_{2}\)O\({}_{3}\)[87] or supporting on TiO\({}_{2}\) can improve the stability [61]. Stean regeneration prevents the sulfate formation to a good extend [88; 89]. \\ \hline \multirow{4}{*}{Fe2O3} & Reducing environment leads to spongy iron formation that reacts very slowly with H\({}_{2}\)S [67] and instead reacts very fast with a carbon source (e.g., CO or CO\({}_{2}\)) to form iron carbide [61] or catalyzes the Boudouard reaction and leads to choke formation [90]. & Using SiO\({}_{2}\), TiO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\) as binders improve the durability [78]. \\ \hline \multirow{4}{*}{CuO} & (a) Copper oxides reduce to elemental copper in a reducing environment like syngas and at high temperatures. Elemental copper is one order of magnitude less active than copper oxides in desulturization [27]. During sulfidation of copper oxide, sintering of the dens sulfide layer at the outer surface of sorbent particles reduces the sorbent utilization [91]. It has a favorable equilibrium therefore deep desulturization is achievable. \\ \hline \multirow{4}{*}{MnO} & Does not have a favorable thermodynamics. Stable even at high temperatures. Prone to form manganese sulfate which is difficult to regenerate. & Doping with chromium oxide [92] and manganese [93; 94; 95; 96]. Also addition of molybdenum [97], vanadium [96; 97; 98; 99] and cerium [100] can improve the sorbent performance. Dispersion on a support such as \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)[101], SiO\({}_{2}\)[102], and ZrO\({}_{2}\)[103]. \\ \hline \hline \end{tabular}
\end{table}
Table 4: Durability issues of different metal oxides and possible remediations.