A highly active Ag/alumina catalytic converter for continuous HC-SCR during lean-burn conditions: from laboratory to full-scale vehicle tests

Topics in Catalysis - A common-rail diesel vehicle was equipped with a full-scale Ag/alumina catalytic converter. The converter consisted of several Ag/alumina bricks, with free space in between...     

Thermally and chemically induced structural transformations of Keggin-type heteropoly acid catalysts

Raman characterization revealed that the Keggin anion structure of H₄PVMo₁₁O₄₀ is inherently unstable upon heat treatment and loss of water. Vanadyl and molybdenyl species are expelled from the Keggin cage and defective Keggin structures are formed. These defective structures further disintegrate to presumably Mo₃O₁₃ triads of the former Keggin. These Keggin fragments oligomerize at later stages to molybdenum oxygen clusters comparable to hepta- or octamolybdates. The final disintegration and structural reorganization product is MoO₃. This disintegration and recondensation process seems to be strongly affected by the heating rate and hence the presence of water in the sample. Only partial expulsion of V occurred under moderate dehydration conditions. The absence of water during heat treatments stabilizes the intermediate defective structures. Raman spectroscopy proved that free polyacids are unstable under catalytic partial oxidation conditions. Therefore, it can be suggested that intact Keggin anions are not the active species within an operating partial oxidation catalyst. From this Raman spectroscopy study it may be inferred that the structurally reorganized intermediates are relevant for the catalytic action. The Raman investigations of the HPA decomposition additionally revealed a dependency of the decomposition process on the reactive atmosphere and the presence of Cs. The presence of Cs led to a partial stabilization of the structural disintegration products of PVMo₁₁ and to the formation of the thermodynamically stable, but catalytically inactive Cs₃-salt. Cs also inhibited the condensation of MoO₃-type oxides. O₂ present in the gas phase also led to stabilization of the structural reorganization intermediates. Importantly, the presence of water did not lead to a stabilization of the intact Keggin structure. In contrast, hydrolysis of the Keggin anions seemed to be enhanced compared to the water-free situation. This observation is of high importance because water is added to the feed in industrial partial oxidation reactions. Hence, under industrial conditions, HPA-derived catalysts are inherently unstable and cannot contain intact Keggin anions at their active surface. Catalytic partial oxidation conditions even led to a more pronounced structural reorganization and amorphous suboxides of the MoO_3−x type seemed to be formed. Hence, heteropolyacids have to be understood only as defined molecular precursor compound.     

A Highly Active Ag/Alumina Catalytic Converter for Continuous HC-SCR During Lean-Burn Conditions: From Laboratory to Full-Scale Vehicle Tests

A common-rail diesel vehicle was equipped with a full-scale Ag/alumina catalytic converter. The converter consisted of several Ag/alumina bricks, with free space in between each brick to fulfil important gas phase reactions. An oxidation catalyst was placed at the end of the converter to remove formed CO and unburned HC. High conversion levels of NO _x , around 60%, were recorded at several speeds and loads using additional HC (diesel) injection corresponding to 2–5% fuel penalty.     

Single crystal and powder diffraction studies of the structure of heteropolymolybdophosphoric acid catalysts

Single-crystal structures were obtained of several vanadium-substituted forms (with coexistent Cu ions) of heteropolyacid catalysts prepared via hydrothermal synthesis. All V-containing species are of the inverse Keggin type; excessive addition of Cu reverts the structure back to the normal Keggin form. Structural details of the two poly-anions indicate qualitatively different bonding of the bridging oxygen atoms active in catalysis. From the single-crystal data reference powder patterns of the two base structures were simulated and experimentally verified. Failure to observe these patterns in clear form in conventional diffraction experiments is traced back to rapid changes in the state of hydration of the materials, with time constants comparable to acquisition times of powder diffraction data. Thermal dehydration leads to intermediate structures different from those subjected to dehydration in air. The anhydride of PVMo₁₁ is stable up to 670 K.     

The mechanism of the synthesis in connection with assignments for a solid reaction cycle of the HPA catalyst during catalytic reactions

The mechanisms of the oxide syntheses to PMo₁₂ and PVMo₁₁ were enlightened by in situ UV/VIS and ³¹P-NMR. Different solution stabilities of the products were discovered. After several crystalline phase transformations of the solid at higher temperatures (> 573 K) spectroscopic assignments for defect HPA formation were determined. Water supports reversible restructuring into the intact Keggin structure. As a consequence we propose a model concept for a solid repair cycle of the catalyst.