The remarkable promotional effect of SO₂ on Pb-poisoned V₂O₅-WO₃/TiO₂ catalysts: An in-depth experimental and theoretical study

Currently, Pb poisoning of heterogeneous catalysts is considered to be a key area of interest in research involved with industrial NO_x reduction. As such, a series of Pb-poisoned V₂O₅-WO₃/TiO₂ catalysts were prepared by a wet impregnation method and the influence of SO₂ on the performance of these poisoned catalysts for NO_x reduction was assessed both experimentally and using theoretical calculations. As expected, the incorporation of Pb in these materials resulted in a significant reduction in their catalytic performance. The conversion of NO_x over the Pb-V₂O₅-WO₃/TiO₂ catalyst increased from approximately 50% to 90% in presence of SO₂ (2000 ppm) at 350 °C. It was postulated that in the absence of SO₂, Pb reacts with surface V-OH species, which ultimately results in the destruction of Brønsted acid sites; considered to be crucial for the catalytic conversion of NO_x. In the presence of SO₂ however, enhanced catalytic activity was observed which was suggested to be a result of the formation of additional Brønsted sites (S-OH) via a surface bidentate sulfate intermediate species. The formation of these species was attributed to the interaction of Pb with SO₂ and O₂ on the surface of the catalyst. Density functional theory (DFT) calculations based on a monolayer V model on TiO₂ (0 0 1) showed that SO₂ absorbed selectively onto Pb sites rather than V or Ti oxides. It was subsequently determined that NH₃ absorption proceeds through the formation of Pb-N species with Pb atom and H-O with SO₂. We believe that the present work provides new insights into the design and application of SCR catalysts with specific relevance for application in flue gas streams which contain high quantities of Pb content.