*2.3. Sulfur Uptake*

Differences between the various materials were also observed in their behavior during H2S adsorption. Figure 4a shows H2S breakthrough curves for LST-5Me catalysts during sulfur loading. H2S (50 ppm) was introduced to the reaction gas stream after 5 min equilibration time. Dosing H2S over a blank quartz reactor resulted in negligible retention time and 50 ppm were attained after ca. 5 min. Sulfur adsorption can be observed by the increased retention times when H2S is dosed over the catalyst bed. The longest retention time of around 8 min was recorded for LST-5Ni. The significant sulfur uptake of this sample (210 ppm by weight) is in line with its strong deactivation in terms of the catalytic activity after sulfur loading (Figure 3a). A retention time of around 4 min and 2 min was observed for LST-5Fe and LST-5Mn, respectively. H2S breakthrough of LST-5Mo and LST-5Cr was close to the blank experiment indicating low sulfur adsorption properties of these metals. This indicates that the ability of Mo as a sulfur scavenger was limited on these samples compared to the more conventionally applied single oxides MoO2 and MoO3, possibly due to the SrMoO4 phase formed during synthesis (Figure 1).

**Figure 4.** H2S breakthrough curves during H2S adsorption experiments on reduced (**a**) LST-5Me (Me = Mo, Mn, Cr, Fe, Ni) and (**b**) LSTN-5Me (Me = Mo, Mn, Cr and Fe). The start time of sulfur addition is indicated by the vertical dashed line.

Interestingly, LSTN-5Mo showed significant sulfur uptake exceeding the combined sulfur storage capabilities of LSTN and LST-5Mo. This can be regarded as an indication for close interaction between segregated Ni and Mo, which appeared to significantly change the sulfur uptake properties of the material. Retention times of LSTN-5Fe, LSTN-5Cr and LSTN-5Mo were approximately the sum of those of LSTN and their LST-5Me counterparts and can be therefore explained by the increased metal content on the sample surface. Sulfur uptake of LSTN was much lower than that of LST-5Ni (ca. 40 ppm compared to 210 ppm), which can be justified using the similar argumentation that when Ni is deposited on LST by impregnation, the Ni metal surface is higher compared to pre-reduced LSTN, due to partial Ni reduction in the latter. Although no obvious advantage of metal impregnation on LSTN could be observed from these H2S adsorption experiments because none of the metals reduced the H2S uptake of the sample, the increased sulfur tolerance in terms of the catalytic activity towards WGS exhibited by LSTN-5Fe (Figure 3b) could be an advantageous property. Therefore, the question arises whether it is possible to exploit the reversible segregation of metals from an LST-type host to produce both redox stable, as well as sulfur tolerant Ni/Fe catalysts.
