PTQ Q2 2026 Issue

0 0.6 0.4 0.2 0.8 1.0 1.2 1.4 1.6

As poisoned Topsoe As Trap

NiAs bulk reference

11850

11860

11870

11880

11890

Reference spectrum indicates presence of NiAs

the active site of the catalyst. This preferential interaction explains the severe loss of hydrogenation and heteroatom removal activity observed even at very low arsenic exposure. At the molecular level, arsenic atoms coordinate strongly with sulphur (S) and Ni atoms at bond distances character - istic of surface interactions. The formation of stable As-S species and Ni-As bonds blocks access to active hydrogena - tion sites and disrupts the promoter functionality of nickel, resulting in a permanent loss of catalytic activity. These strong interactions explain why arsenic poisoning is irrevers - ible under typical hydrotreating regeneration conditions and why even trace arsenic concentrations can lead to significant performance loss over time. Arsenic trap development Nickel-containing arsenic guard catalysts have been installed in hundreds of commercial hydroprocessing units since the mid-1990s, providing protection of downstream catalysts against arsenic poisoning. The first dedicated arsenic trap established the foundation for arsenic removal in industrial applications. In 2006, another guard catalyst was introduced, with approximately 50% higher arsenic capacity compared to the earlier generation. This improvement was driven by an optimised formulation and pore architecture designed to enhance arsenic diffusion and active-phase utilisation. Continued development led to the TK-51 arsenic guard. Its design objective was not to increase total nickel content, but rather to maximise the effective nickel surface area accessible to arsenic species within the catalyst pore struc - ture. By optimising nickel dispersion and pore connectivity, TK-51 achieves higher volumetric arsenic uptake while maintaining mechanical integrity and stability under severe hydroprocessing conditions. The key innovation behind the newest arsenic traps lies in the engineering of diffusion-controlled arsenic uptake combined with enhanced accessible nickel surface area, as shown in Figure 3 . By optimising pore size distribution, internal connectivity, and nickel dispersion, these traps pro - mote deep arsenic penetration throughout the particle vol - ume rather than preferential capture at the external surface. In diffusion-limited systems, arsenic accumulates pre - dominantly near the external surface of the catalyst particle, Figure 2 Structural model of the hexagonally truncated NiMoS₂ slab with an As atom at the edge analysed by EXAFS spectroscopy

of organo-arsenic compounds (R-As-R′), which are ther - mally unstable under typical hydrotreating conditions. In the presence of hydrogen and sulphided catalyst surfaces, these compounds decompose to form highly reactive arsenic spe - cies that readily migrate into catalyst pore structures. Once formed, these arsenic species exhibit a strong affin - ity for sulphur-coordinated metal sites, leading to rapid and irreversible chemisorption on active catalyst surfaces. The stepwise mechanism can be described as follows: Figure 1 Hexagonally truncated Ni-MoS₂ slab with an As atom at the edge analysed by EXAFS spectroscopy. Atomic distance in Å (angstroms) u Decomposition of organo-arsenic species R-As-R′+H₂ → As ∗ + hydrocarbon fragments v Irreversible binding on sulphided Ni sites A s∗ +Ni-S edge → Ni-S-As w Progressive site blocking and deactivation Ni-Mo-S active As →⏞ Ni-Mo-S blocked On nickel-molybdenum (NiMo) catalysts, arsenic is com - monly detected as Ni-S-As species associated with the edges of Ni-Mo-S slabs, whereas on cobalt-molybdenum (CoMo) catalysts, it is often observed as a bulk As₂S₃-like phase. Figure 1 shows the atomic structure of a spent arsenic-poi - soned NiMoS₂ slab, as characterised by in-situ Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and in-situ fluorescence EXAFS. These are specialised techniques suitable for characterising low element concentrations and for determining the local coordination environment of arsenic species on sulphided hydroprocessing catalysts. The EXAFS examination shows that the arsenic prefera - bly associates with the nickel-promoted edge sites of the MoS₂ slabs and concurs with the EXAFS absorption spec - tra of arsenic-poisoned nickel catalysts shown in Figure 2 . The model shows the arsenic atoms located at the edges of hexagonal truncated NiMoS₂. In this position, arsenic blocks

PTQ Q2 2026