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Combined operando spectroscopy with model-based experimental design to study the mechanism of catalytic surface reactions

Periodic Reporting for period 1 - OpeSpeKin (Combined operando spectroscopy with model-based experimental design to study the mechanism of catalytic surface reactions)

Période du rapport: 2019-10-01 au 2021-09-30

Selective catalytic reduction (SCR), also referred to as deNOx technology, is a commercialized catalytic reaction used in combustion engines to convert unwanted nitrogen oxides into a benign mixture of nitrogen gas and water. NOx gases are pollutants that pose a hazard to human respiratory function and environmental ecosystems. Almost 50 % of NOx emissions originate from combustion engines used in transport, with a further 20 % arising from the stationary production of energy such as thermal power plants and industrial boilers. To address this, SCR, which has been shown to reduce NOx emissions by as much as 95 %, is currently used in diesel engine cars and trucks to meet emissions standards in Europe (Euro6), US (EPA Tier 2 and 3) and Southeast Asia. SCR occurs when NOx gases are flowed through an appropriately selected catalyst in the presence of a reductant such as ammonia. Small pore Cu-zeolites have been identified as excellent catalysts in the SCR reaction, being lauded for their unrivalled NOx conversion. Additionally, their strong performance in the highly topical low-temperature SCR (LT-SCR) region (i.e. ≤350 °C), and durability following hydrothermal ageing make them excellent candidates for vehicular SCR, and the current catalysts of choice for commercialized SCR in heavy goods diesel vehicles. However, an issue that is becoming increasingly recognized is the unwanted formation of N2O during SCR where small pore Cu- and Fe- zeolites are used as catalysts. Although historically exempt from emissions regulations due to their believed lack of toxicity, harmful indications following long-term exposure and a global warming potential almost 300 times that of CO2 mean that N2O will inevitably become the subject of increasingly exacting legislation beyond emissions standards. While pronounced formation of N2O has been experimentally observed previously, the mechanism by which this occurs is not completely understood and it is likely there is some degree of catalyst dependency. The widely reported disparity in N2O production based on framework type implies that not just the structure but the availability of certain copper co-ordination environments is crucially important in the formation of nitrate species at low temperatures. This research aims to explore in greater depth the relationship between the copper environment of the Cu-zeolite catalyst and its propensity for N2O formation.
Intial period:
To achieve our aim, we compared Cu-SSZ-13 samples prepared by two different methods to induce the formation of different copper species within the framework: the first synthesized via a standard wet ion exchange method known to produce an efficient SCR catalyst material, and the second prepared by a solid state incorporation method. In particular, the presence of copper aluminate species, which has previously been associated with the deactivation of Cu-zeolites, is considered as a site for preferential nitrate formation. To demonstrate this correlation, the samples were studied using an ambitious combination of two synchrotron techniques at the Diamond Light Source. The hard X-ray nanoprobe on beamline I14 was used to generate X-ray fluorescence (XRF) and X-ray absorption near edge structure (XANES) maps, providing detailed characterization of the catalysts. The multimode infrared imaging and micro-spectroscopy (MIRIAM) beam on B22 was then used to identify vibrational modes of nitrate components under operando conditions. Furthermore, density functional theory (DFT) using a quantum mechanics/molecular mechanics (QM/MM) approach was employed to elucidate the reaction pathway for formation of N2O over Cu-SSZ-13 catalyst.
Final period- overview of the results:
Our resutls provide another perspective to elucidate the role of Cu speciation in NH3-SCR reaction. The data implied that catalyst performance in a reaction might be affected by the synthesis process. Spectroscopic techniques, including operando XANES, UV-Vis, Raman provided indirect evidence for the Cu speciation by detecting changes in the Cu coordination environment. While the XANES indicates the presence of a linear copper species such as CuAlO2, the data generated is not able to comment on what this species might ‘look’ like or how it might be distributed. Given that both samples have been subjected to standard high temperature activations and gas experiments, it is expected that the Cu(I) species must be stable, making a compound such as an aluminate phase one such possibility. Based on the higher production rate of N2O, this would also indicate that the species should be readily accessible. By exclusion, a crystalline phase might not constitute a large enough surface area to be considered ‘readily accessible’, meaning it may instead be a disperse species occurrent throughout the sample. Operando IR micro-spectroscopy pointed to the early formation of a new intermediate along with consumption of nitrate species and adsorption of NH3 on acid sites; attention should be paid to the Cu local environment that is closely correlated with the evolution of N2O. According to DFT calculation results both the adsorbed NH4+ and NO3- ions can be associated with CuAlO2 with a similar energy barrier and facilitate the generation of intermediate [(Al2O4Cu)HNO3, NH3] species during the NH3-SCR reaction. This may also explain why the presence of a linear copper species such as CuAlO2 significantly influences the local environment of Cu species towards N2O formation.
Exploitation and dissemination:
This research has been presented in national and international conferences, invited talk and seminars:
1- 6th UK Catalysis Conference, Loughborough, UK.
2- ICEC, 11th International Conference on Environmental Catalysis, UK.
3- UCL Chemistry seminars, UK
4- UK Catalysis Hub seminars, UK
5- Posters for Scientific Advisory Board, Research Complex at Harwell, UK.
Invited talk:
1- Chemical Engineering Lectures, The University of Edinburgh, UK.
2- Diamonds Seminar, Diamond Light Source, UK.
Our study shows that presence of alternative linear copper species which is not limited to the CZ-SSE catalyst. These results are part of an ongoing body of research and represent the first stages of characterization. Nonetheless, detailed understanding of Cu species will help establish more reliable guidelines for developing the NH3-SCR catalysts or even develop new concepts for optimizing the NH3-SCR process with Cu species as active sites.
Societal and economic impact:
This research addresses the key societal challenges of climate change and environmental degradation which necessitates the development of environmentally renewable energy technology. The results generated in this project is of interest to industrial party as it seeks to develop a strategy to increase the efficiency of catalytic processes. In the longer term this project could improve NH3-SCR catalytic processes preventing formation of highly undesired N2O. As such the research could have an important economic impact as well as a societal one in long and short term.
Proposed mechanism for formation of N2O over Cu-SSZ-13 zeolite