Understanding how hydrotreating oxygen-containing compounds together with nitrogen-containing compounds affects the reactivity and selectivity is relevant for processing renewable feedstocks. In this work, competitive hydrodeoxygenation (HDO) and hydrodenitrogenation (HDN) reactions were studied by co-hydrotreating palmitic acid (C16 acid) and tetradecylamine (C14 amine) over a Pt/ZrO2 catalyst in a batch reactor. HDO proceeded faster than HDN in the studied system, and the deoxygenation reactions were found to have an inhibitory effect on HDN. Co-hydrotreating the C16 acid and the C14 amine expanded the reaction network from the individual HDO and HDN networks and changed the prevailing reaction pathways, initially in favor of oxygen removal. The formation of heavy secondary amides and amines through condensation reactions became increasingly favored as the share of C16 acid in the feed increased. For a given conversion level, the condensation product selectivity was observed to increase as the reaction temperature was decreased, whereas increasing the reaction temperature promoted the formation of the desired paraffins. This work described the ease of HDO compared to HDN, the role of condensation reactions in the co-hydrotreating reaction network, and the inhibitory effect on HDN thereof.

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Although it is experimentally difficult to observe, PdSO4 is considered to be the culprit for the reduced activity of SO2-poisoned methane oxidation catalysts. Density functional theory (DFT) predicts that the formation of bulk PdSO4 is unlikely, which explains the lack of X-ray diffraction (XRD) evidence for the PdSO4 phase. Instead, experimental observations support the idea of PdSO4 being formed on PdO as thin films. Our study found PdSO4(110) and PdSO4(111), corresponding to PdO(100) and PdO(101), respectively, to be the most likely surfaces to be found on a poisoned catalyst. On these sulfate surfaces, PdSO4(111) contains coordinatively unsaturated palladium, which enables catalytic activity. The first C–H dissociation of methane on PdSO4(111) was found to be rather accessible with an energy barrier varying between 0.74 and 0.87 eV, values similar to those reported for metallic Pd. However, the presence of hydroxyl groups increases the barrier height. Methane oxidation is also hindered by an exceptionally strong water adsorption of −1.45 eV on the PdSO4(111) surface, which causes site blocking. A significantly strong adsorption energy causes the combination of surface hydroxyl groups to form adsorbed water that is energetically favorable. The results provide a theoretical justification for the observation that SO2-poisoned PdSO4/Al2O3 catalysts produce proper methane conversion under dry conditions but perform poorly under wet feed.

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