Direct observation of atomically-resolved silver species on a silver alumina catalyst active for selective catalytic reduction of nitrogen oxides

We characterize the size and state of the silver species in a 2 wt% silver alumina catalyst, which is highly active for the selective catalytic reduction of nitrogen oxides with ammonia or hydrocarbons as reductant. The silver alumina catalyst is prepared by a single-step sol-gel method and characterized with X-ray photoelectron and ultraviolet-visible spectroscopy, and high-resolution transmission electron microscopy. We show, for the first time, direct observations of atomically-resolved silver species and silver clusters on the silver alumina catalyst. The results determine the existence of these silver species on the alumina support, which corroborate previously reported indirect observations, and strengthen the hypothesis of small silver clusters as active sites for the selective catalytic reduction of nitrogen oxides

Efficient low temperature lean NOx reduction over Ag/Al2O3-A system approach

This study focuses on lean NOx reduction (LNR) by n-octane using silver-alumina based catalysts, with the addition of hydrogen. The work takes a system approach, where parameters such as temperature, reformate gas composition, fuel penalty and realistic monolith samples are considered. The LNR catalyst samples were prepared by impregnation and sol-gel methods and the NOx reduction performance was characterized by flow-reactor experiments, where realistic engine-out gas compositions were used. The hydrogen feed over the LNR catalyst samples was determined via data achieved by autothermal reforming experiments over a rhodium based catalyst, using real diesel as feedstock. The LNR catalyst samples generally show an enhanced NOx reduction when hydrogen is added to the gas feed. In particular, a 2 wt% silver-alumina sample with the addition of minute amounts of platinum, shows a high increase in NOx reduction when hydrogen is added to the feed. The addition of CO, a potential poison in the reaction and a by-product from the reforming, did not show any significant effect on the LNR catalyst performance at the conditions used. This is beneficial, since it renders a CO clean-up step in the reformer system unneeded. Ammonia formation is discussed in terms of a possible dual-SCR system. Finally, the fuel penalty for hydrogen production and hydrocarbon addition is taken into consideration. It is found that an addition of 1000 ppm H-2 leads to unacceptable fuel penalties. (C) 2011 Elsevier B.V. All rights reserved

On the performance of Ag/Al2O3 as a HC-SCR catalyst – influence of silver loading, morphology and nature of the reductant

This study focuses on the performance of Ag/Al2O3 catalysts for hydrocarbon selective catalytic reduction (HC-SCR) of NOx under lean conditions, using complex hydrocarbons as reductants. The aim is to elucidate the correlation towards the silver loading and morphology, with respect to the nature of the reductant. Ag/Al2O3 samples with either 2 or 6 wt% silver loading were prepared, using a sol–gel method including freeze-drying. The catalytic performance of the samples was evaluated by flow reactor experiments, with paraffins, olefins and aromatics of different nature as reductants. The physiochemical properties of the samples were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy, scanning transmission electron microscopy/high angle annular dark field imaging, X-ray photoelectron spectroscopy and N2-physisorption. The 2 wt% Ag/Al2O3 sample was found to be the most active catalyst in terms of NOx reduction. However, the results from the activity studies revealed that the decisive factor for high activity at low temperatures is not only connected to the silver loading per se. There is also a strong correlation between the silver loading and morphology (i.e. the ratio between low- and high- coordinated silver atoms) and the nature of the hydrocarbon, on the activity for NOx reduction. Calculated reaction rates over the low-coordinated step and high- coordinated terrace sites showed that the morphology of silver has a significant role in the HC-SCR reaction. For applications which include complex hydrocarbons as reductants (e.g. diesel), these issues need to be considered when designing highly active catalysts

Lean NOx reduction over silver-alumina catalysts - Aspects on synthesis, structure and activity

The aim of this thesis is to increase the understanding of how to design highly active silver-alumina catalysts for selective catalytic reduction of NOx with hydrocarbons (HC-SCR) in oxygen excess at low temperatures (150-250 \ub0C). Silver-alumina catalysts have been prepared using different methods, including a new unique preparation method developed during the thesis project. The obtained catalysts have been thoroughly characterized by a range of methods, to determine the chemical and physical properties. The catalytic performance of the samples was determined using flow reactor experiments and mechanistic aspects of the HC-SCR reaction were elucidated by kinetic modeling and in-situ diffuse reflection infra-red Fourier transformed (DRIFT) spectroscopy.The results show that silver-alumina catalysts with highly dispersed silver can be obtained by the new preparation method based on freeze-drying of the silver-alumina gel, formed by the sol-gel method, presented in this thesis. However, the flow reactor experiments showed that to achieve a high NOx reduction activity at low temperatures, both the nature of the reducing agent and the silver loading of the catalyst must be considered. A delicate balance between the oxidative and reductive properties of the catalyst is required, which in turn is dependent on both the silver loading and the preparation method. This balance varies for different reducing agents, most likely due to the different terminal C-H bond strengths in the hydrocarbons. It is suggested that these two factors determine the rate of partial oxidation, required to open up the HC-SCR reaction path.A transient kinetic model for hydrogen assisted n-octane-SCR has been developed, using the reduction of stable surface nitrates by hydrogen as a key-step. The model is able to well reproduce changes in the gas feed, especially poisoning effects by higher NO concentrations in the feed. These results, together with in-situ DRIFT results showing the reduction of surface nitrates by hydrogen, strongly indicate that one important role of hydrogen is to suppress the self-poisoning of the active sites by reduction of surface nitrates

Lean NOx reduction over silver-alumina catalysts - Aspects on synthesis, structure and activity

The aim of this thesis is to increase the understanding of how to design highly active silver-alumina catalysts for selective catalytic reduction of NOx with hydrocarbons (HC-SCR) in oxygen excess at low temperatures (150-250 \ub0C). Silver-alumina catalysts have been prepared using different methods, including a new unique preparation method developed during the thesis project. The obtained catalysts have been thoroughly characterized by a range of methods, to determine the chemical and physical properties. The catalytic performance of the samples was determined using flow reactor experiments and mechanistic aspects of the HC-SCR reaction were elucidated by kinetic modeling and in-situ diffuse reflection infra-red Fourier transformed (DRIFT) spectroscopy.The results show that silver-alumina catalysts with highly dispersed silver can be obtained by the new preparation method based on freeze-drying of the silver-alumina gel, formed by the sol-gel method, presented in this thesis. However, the flow reactor experiments showed that to achieve a high NOx reduction activity at low temperatures, both the nature of the reducing agent and the silver loading of the catalyst must be considered. A delicate balance between the oxidative and reductive properties of the catalyst is required, which in turn is dependent on both the silver loading and the preparation method. This balance varies for different reducing agents, most likely due to the different terminal C-H bond strengths in the hydrocarbons. It is suggested that these two factors determine the rate of partial oxidation, required to open up the HC-SCR reaction path.A transient kinetic model for hydrogen assisted n-octane-SCR has been developed, using the reduction of stable surface nitrates by hydrogen as a key-step. The model is able to well reproduce changes in the gas feed, especially poisoning effects by higher NO concentrations in the feed. These results, together with in-situ DRIFT results showing the reduction of surface nitrates by hydrogen, strongly indicate that one important role of hydrogen is to suppress the self-poisoning of the active sites by reduction of surface nitrates

Silver Alumina Catalysts for Lean NOx Reduction - Influence of Preparation Methods and Type of Reductant, with Transient Kinetic Modelling

The objective of this work was to compare three different preparation methods for silver alumina catalysts for lean NOx reduction. Characteristics such as catalytic activity with propene and n-octane as reductants, chemical state of the silver and silver dispersion were analyzed. Furthermore, a global kinetic model for hydrogen assisted selective catalytic reduction of NOx with n-octane was developed for one of the prepared catalysts.Sample preparation was performed by the impregnation, thermally dried sol-gel and freeze-dried sol-gel methods. The latter is a new preparation method developed in this work. Characterization by XPS shows the presence of silver clusters on the impregnated and thermally dried sol-gel samples; however no such clusters are found on the freeze-dried sol-gel samples. Together with XRD and TEM analyses, the XPS results indicate that the silver is very finely dispersed throughout the alumina matrix in the freeze-dried sol-gel samples. This work has further shown that XPS enables detection of small silver clusters on the surface of silver alumina catalysts.The catalytic evaluation in flow-reactor shows high NOx reduction using propene as reductant for the freeze-dried sol-gel samples, however at elevated temperatures. The maximum NOx reduction, and also the corresponding temperature, is generally lower using n-octane as reductant. Moreover, the temperature window for NOx reduction is broader with n-octane compared to propene. The best catalytic performance with n-octane is found for a thermally dried sol-gel sample. The results from the flow reactor study imply that the ratio between silver clusters and oxidized silver species is crucial to achieve high NOx reduction. The kinetic model for H2 assisted n-octane-SCR of NOx was developed for the sample with the highest NOx reduction activity. The model was constructed from transient experimental data, with and without hydrogen feed, and it generally follows the changes due to these feed variations well. It is based on the removal of surface nitrates by hydrogen, also accounting for large temperature increases due to hydrogen combustion

Silver Alumina Catalysts for Lean NOx Reduction - Influence of Preparation Methods and Type of Reductant, with Transient Kinetic Modelling

The objective of this work was to compare three different preparation methods for silver alumina catalysts for lean NOx reduction. Characteristics such as catalytic activity with propene and n-octane as reductants, chemical state of the silver and silver dispersion were analyzed. Furthermore, a global kinetic model for hydrogen assisted selective catalytic reduction of NOx with n-octane was developed for one of the prepared catalysts.Sample preparation was performed by the impregnation, thermally dried sol-gel and freeze-dried sol-gel methods. The latter is a new preparation method developed in this work. Characterization by XPS shows the presence of silver clusters on the impregnated and thermally dried sol-gel samples; however no such clusters are found on the freeze-dried sol-gel samples. Together with XRD and TEM analyses, the XPS results indicate that the silver is very finely dispersed throughout the alumina matrix in the freeze-dried sol-gel samples. This work has further shown that XPS enables detection of small silver clusters on the surface of silver alumina catalysts.The catalytic evaluation in flow-reactor shows high NOx reduction using propene as reductant for the freeze-dried sol-gel samples, however at elevated temperatures. The maximum NOx reduction, and also the corresponding temperature, is generally lower using n-octane as reductant. Moreover, the temperature window for NOx reduction is broader with n-octane compared to propene. The best catalytic performance with n-octane is found for a thermally dried sol-gel sample. The results from the flow reactor study imply that the ratio between silver clusters and oxidized silver species is crucial to achieve high NOx reduction. The kinetic model for H2 assisted n-octane-SCR of NOx was developed for the sample with the highest NOx reduction activity. The model was constructed from transient experimental data, with and without hydrogen feed, and it generally follows the changes due to these feed variations well. It is based on the removal of surface nitrates by hydrogen, also accounting for large temperature increases due to hydrogen combustion

Ag-Al2O3 catalysts for lean NOx reduction - Influence of preparation method and reductant

Alumina supported silver catalysts for selective catalytic reduction of NOx with hydrocarbons under excess oxygen (HC-SCR) were prepared according to three different methods. These are impregnation (I) and two different sol-gel routes, via thermal drying (II) or freeze-drying (III). The latter method has not previously been reported for preparation of Ag-Al2O3 catalysts. The prepared samples were characterized by N2-sorption, TEM, XRD and XPS, and the catalytic properties were further investigated in flow reactor experiments. Two different hydrocarbons (propene and n-octane) were used separately as reductants for the SCR reaction. The TEM, XRD and XPS analyses confirmed that the sol-gel samples contain more dispersed silver of higher oxidation state than the corresponding impregnated sample. Further, it was shown that the freeze-dried sol-gel samples most likely contain well dispersed silver throughout the alumina matrix, mainly as oxidized silver. The XPS results also strongly indicate the presence of silver clusters on the surface of samples prepared by routes (I) and (II). The results from the flow reactor study imply that the ratio between silver clusters and oxidized silver species is crucial to achieve high NO reduction