Emma Adams

Ammonia formation over supported platinum and palladium catalysts

We report experimental results for the formation of ammonia from nitric oxide and hydrogen, and from nitric oxide, water and carbon monoxide over silica, alumina and titania supported platinum and palladium catalysts. Temperature programmed reaction experiments in gas flow reactor show a considerable formation of ammonia in the temperature range 200-450 C, which is suppressed by the presence of excess oxygen. However, oxygen sweep experiments show that for the titania supported catalysts minor amounts of oxygen promotes the ammonia formation at low temperatures. In situ DRIFT spectroscopy measurements indicate that cyanate species on the support play an important role in the ammonia formation mechanism. This work shows that alumina supported palladium is a promising system for passive selective catalytic reduction applications, exhibiting low-temperature activity during the water-gas-shift assisted ammonia formation reaction. Conversely, titania supported samples are less active for ammonia formation as a result of the poor thermal stability of the titania support.

Spatially and temporally resolved gas distributions around heterogeneous catalysts using infrared planar laser-induced fluorescence

Visualizing and measuring the gas distribution in close proximity to a working catalyst is crucial for understanding how the catalytic activity depends on the structure of the catalyst. However, existing methods are not able to fully determine the gas distribution during a catalytic process. Here we report on how the distribution of a gas during a catalytic reaction can be imaged in situ with high spatial (400 μm) and temporal (15 μs) resolution using infrared planar laser-induced fluorescence. The technique is demonstrated by monitoring, in real-time, the distribution of carbon dioxide during catalytic oxidation of carbon monoxide above powder catalysts. Furthermore, we demonstrate the versatility and potential of the technique in catalysis research by providing a proof-of-principle demonstration of how the activity of several catalysts can be measured simultaneously, either in the same reactor chamber, or in parallel, in different reactor tubes.

Study of methane oxidation over alumina supported Pd–Pt catalysts using operando DRIFTS/MS and in situ XAS techniques

Graphical Abstract Abstract Methane oxidation over Pd–Pt/ model catalysts calcined at three different conditions is investigated using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, and in situ X-ray absorption spectroscopy while cycling the feed gas stoichiometry between lean (net-oxidising) and rich (net-reducing) conditions. When calcined in air, alloy Pd–Pt nanoparticles are present only on catalysts subjected to elevated temperature () whereas calcination at lower temperature (500 ) leads to segregated Pt and Pd nanoparticles on the support. Here, we show that the alloy Pd–Pt nanoparticles undergo reversible changes in surface structure and composition during transient methane oxidation exposing a PdO surface during lean conditions and a metallic Pd–Pt surface (Pd enriched) under rich conditions. Alloyed particles seem more active for methane oxidation than their monometallic counterparts and, furthermore, an increased activity for methane oxidation is clearly observed under lean conditions when PdO has developed on the surface, analogous to monometallic Pd catalysts. Upon introducing rich conditions, partial oxidation of methane dominates over total oxidation forming adsorbed carbonyls on the noble metal particles. The carbonyl spectra for the three samples show clear differences originating from different surfaces exposed by alloyed vs. non-alloyed particles. The kinetics of the noble metal oxidation and reduction processes as well as carbonyl formation during transient methane oxidation are discussed.

Comparison of Lab Versus Engine Tests In the Development of a Highly Efficient Ammonia Formation Catalyst for a Passive SCR System

Commercial three way catalysts have limited capacity towards reducing NOx in the presence of excessive oxygen. This prevents lean-burn combustion concepts from meeting legislative emission standards. A solution towards decreasing NOx emissions in the presence of excess air is the use of a passive-SCR system. Under rich conditions ammonia is formed over an ammonia formation catalyst, the ammonia is stored in the SCR and in its turn reacts with the NOx under lean engine conditions. Here up-scaled Pt/Al2O3 and Pd/Al2O3 catalysts as well as a commercially Pd-Rh based three-way catalyst (TWC) are evaluated using both engine and further lab-scale tests. The purpose of these tests is to compare the ammonia production for the various catalysts under various lambda values and temperatures by means of engine and lab scale tests. The Pd/Al2O3 showed little sensitivity to temperature both under engine and lab scale experiments. The Pt/Al2O3 was affected to a large extend by temperature for both test methods. The TWC showed stable production during the engine measurements while under lab tests an increased temperature resulted in a lower ammonia yield. Differences between the engine and lab scale tests are mainly due to catalyst temperatures, space velocity, CO poisoning and uncertainties in the composition of the engines exhaust gas. Both Pt/Al2O3 and Pd/Al2O3 form ammonia although the former generate higher amounts at high temperature but are believed to suffer from CO poisoning at low temperatures.