Lean-Burn Natural Gas Engines: Challenges and Concepts for an Efficient Exhaust Gas Aftertreatment System

High engine efficiency, comparably low pollutant emissions, and advantageous carbon dioxide emissions make lean-burn natural gas engines an attractive alternative compared to conventional diesel or gasoline engines. However, incomplete combustion in natural gas engines results in emission of small amounts of methane, which has a strong global warming potential and consequently makes an efficient exhaust gas aftertreatment system imperative. Palladium-based catalysts are considered as most effective in low temperature methane conversion, but they suffer from inhibition by the combustion product water and from poisoning by sulfur species that are typically present in the gas stream. Rational design of the catalytic converter combined with recent advances in catalyst operation and process control, particularly short rich periods for catalyst regeneration, allow optimism that these hurdles can be overcome. The availability of a durable and highly efficient exhaust gas aftertreatment system can promote the widespread use of lean-burn natural gas engines, which could be a key step towards reducing mankind’s carbon footprint

Heterogeneous chemical reactions—A cornerstone in emission reduction of local pollutants and greenhouse gases

The current state and challenges of advanced experimental and modeling methods for a better understanding of heterogeneous chemical reactions are discussed using examples from developing and future technologies in the area of emission reduction of local pollutants and greenhouse gases. In situ and operando experimental techniques using laser and X-ray absorption spectroscopy, for instance, are able to resolve spatial and temporal concentration and temperature profiles in the near-wall gas phase, the interphase and inside the solid bulk. They have been exploited for a better understanding of the interaction of chemical reactions and transport processes. The experimental elucidation of chemical conversion on the microscopic scale leads to elementary step-like surface reaction mechanisms. The microkinetic description of gas-surface reactions is still challenging due to the complex influence of the modification of the solid material itself on the microscopic scale during the chemical reaction, which is caused by intrinsic materials’ modifications due to adsorbed species and temperature variations. Furthermore, transient inlet and boundary conditions on the reactor scale have a strong impact on the material and reaction rate. In addition to thermochemical reactions, an additional complexity comes into play with electrochemical ones. This paper will discuss heterogeneous chemical reactions in the light of emerging technologies such as emission control of natural gas and hydrogen fueled engines, use of CO$_{2}$ in chemical (methanation, dry reforming) and steel industry (off-gas reforming), hydrogen production by pyrolysis of methane, small-scale ammonia synthesis and use, and recyclable carbon-free energy carriers. Hence, this article will also reveal a new playground and the potential of methods, know-how, and skills of the combustion community to significantly contribute to the solution of climate-change relevant challenges

Selective Catalytic Reduction with Hydrogen for Exhaust gas after-treatment of Hydrogen Combustion Engines

In this work, two palladium-based catalysts with either ZSM-5 or Zeolite Y as support material are tested for their performance in selective catalytic reduction of NOx with hydrogen (H$_2$-SCR). The ligh-toff measurements in synthetic exhaust gas mixtures typical for hydrogen combustion engines are supplemented by detailed catalyst characterization comprising N$_2$ physisorption, X-ray powder diffraction (XRD), hydrogen temperature programmed reduction (H$_2$-TPR) and ammonia temperature programmed desorption (NH$_3$-TPD). Introducing 10% or 20% TiO2 into the catalyst formulations reduced the surface area and the number of acidic sites for both catalysts, however, more severely for the Zeolite Y-supported catalysts. The higher reducibility of the Pd particles that was uncovered by H$_2$-TPR resulted in an improved catalytic performance during the light-off measurements and substantially boosted NO conversion. Upon exposition to humid exhaust gas, the ZSM-5-supported catalysts showed a significant drop in performance, whereas the Zeolite Y-supported catalyst kept the high levels of conversion while shifting the selectivity from N$_2$O more toward NH$_3$ and N$_2$. The 1%Pd/20%TiO$_2$/HY catalyst subject to this work outperforms one of the most active and selective benchmark catalyst formulations, 1%Pd/5%V$_2$O$_5$/20%TiO$_2$-Al$_2$O$_3$, making Zeolite Y a promising support material for H$_2$-SCR catalyst formulations that allow efficient and selective NOx-removal from exhaust gases originating from hydrogen-fueled engines

Effects of hydrothermal aging on co and no oxidation activity over monometallic and bimetallic pt‐pd catalysts

By combining scanning transmission electron microscopy, CO chemisorption, and energy dispersive X-ray spectroscopy with CO and NO oxidation light-off measurements we investigated deactivation phenomena of Pt/Al$_{2}$O$_{3}$, Pd/Al$_{2}$O$_{3}$, and Pt-Pd/Al$_{2}$O$_{3}$ model diesel oxidation catalysts during stepwise hydrothermal aging. Aging induces significant particle sintering that results in a decline of the catalytic activity for all catalyst formulations. While the initial aging step caused the most pronounced deactivation and sintering due to Ostwald ripening, the deactivation rates decline during further aging and the catalyst stabilizes at a low level of activity. Most importantly, we observed pronounced morphological changes for the bimetallic catalyst sample: hydrothermal aging at 750 °C causes a stepwise transformation of the Pt-Pd alloy via core-shell structures into inhomogeneous agglomerates of palladium and platinum. Our study shines a light on the aging behavior of noble metal catalysts under industrially relevant conditions and particularly underscores the highly complex transformation of bimetallic Pt-Pd diesel oxidation catalysts during hydrothermal treatmen

Composition Modulation over Three-Way Catalysts

Compared to conventional internal combustion engines, modern hybrid electric vehicles (HEVs) can save fuel under urban driving conditions, which results in lower CO2 emissions. However, frequent stops and restart of the HEV engine go along with periods of low exhaust gas temperatures and therefore cause a decline in pollutant conversion over the three-way catalyst (TWC) system that is typically used for exhaust gas after-treatment [1]. Composition modulation is reported to increase pollutant conversion over the TWC at low temperatures and to be beneficial for cold start performance [2]. In this regard, dithering frequency, amplitude, temperature, and space velocity are the most important parameters influencing the rate enhancement from composition modulation. A synthetic gas bench with fast switching valves is used to conduct a comprehensive parameter study on the influence of dithering parameters on the TWC performance. For this, monolithic catalysts with Pd/Al2O3 and a commercially relevant Ce-based Pd catalyst are prepared by incipient wetness impregnation and subsequent dip coating. The application of a square wave signal to the catalytic converter remains a challenging task due to axial dispersion in the setup periphery. To further examine the phenomena and predict the behaviour of the TWC under periodic conditions, a detailed kinetic model is under development. Literature suggests that the dithering effect can be described by kinetic models with detailed chemistry considering the interaction among species adsorbed on surface sites [3]. Under the assumption of an ideally backmixed reactor, initial modelling results exploiting a detailed microkinetic model for CO oxidation [4] show an increased average CO conversion for all frequencies around the light-off and a decrease for higher temperature (Figure 1). Furthermore, an increase of optimal frequency with increasing temperature and amplitude was observed for constant amplitude and temperature respectively, which is in line with experimental data from literature [2]. Using transient data for model development will provide valuable insights on surface phenomena that are responsible for the dithering effect on three-way catalysts and will ultimately allow for reducing pollutant emissions from HEVs. [1] Y. Huang, N. Surawski, B. Organ, J. Zhou, O. Tang and E. Chan, "Fuel consumption and emission performance under real driving: Comparison between hybrid and conventional vehicles", Sci. Total Environ. 659, 275-282 (2019). [2] P. Silveston, "Automotive exhaust catalysis: Is periodic operation beneficial?", Chem. Eng. Sci. 51, 2419-2426 (1996). [3] P. Kočí, M. Kubíček, M. Marek, "Multifunctional aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition", Chem. Eng. Res. Des. 82, 284-292 (2004). [4] D. Chan, S. Tischer, J. Heck, C. Diehm, O. Deutschmann, "Correlation between catalytic activity and catalytic surface area of a Pt/Al2O3 DOC: An experimental and microkinetic modelling study", Appl. Catal. B 156-157, 153-165 (2014)

MOx_x@VOx_x-Pd‐type Nanorods and Nanotubes as Catalysts for Selective Reduction of NO

Vanadium oxide (VO$_x$-Pd) nanotubes as well as VO$_x$-coated ZnO nanorods (ZnO@VO$_x$-Pd) and VO$_x$-coated, layered-titania nanotubes (l-TiO$_2$@VO$_x$-Pd) are decorated with Pd nanoparticles and evaluated for selective catalytic reduction with hydrogen (H$_2$-SCR) for the first time. The nanostructures exhibit lengths of 300 to 700 nm, diameters of 20–100 nm and, in the case of the nanotubes, an inner tube diameter of about 10 nm. Pd nanoparticles (14±5 nm) are well-dispersed over the respective nanorod/nanotube nanostructure. Structure and composition are characterized by SEM, TEM, EDXS with element mapping, XPS, FT-IR, XRD, and sorption analysis. Thermal analysis indicates the nanostructures to be thermally stabile up to 350 °C (VO$_x$), and 500 °C (ZnO@VO$_x$, l-TiO$_2$@VO$_x$). All catalysts are tested for their activity in regard of the selective catalytic reduction of NO with H$_2$, revealing a significant impact of the catalyst support on both activity and selectivity. Specifically, l-TiO$_2$@VO$_x$ nanotubes show promising properties with an activity up to 70 % and a selectivity up to 80 % N$_2$

A review on exhaust gas after-treatment of lean-burn natural gas engines – From fundamentals to application

Modern lean-operated internal combustion engines running on natural gas, biogas or methane produced from wind or solar energy are highly fuel-efficient and can greatly contribute to securing energy supply, e.g. by mitigating fluctuations in the power grid. Although only comparably low emission levels form during combustion, a highly optimized emission control system is required that converts pollutants over a wide range of operation conditions. In this context, this review article pinpoints the main challenges during methane and formaldehyde oxidation as well as selective catalytic reduction of nitric oxides. The impact of catalyst formulation and operation conditions on catalytic activity and selectivity as well as the combination of several technologies for emission abatement is critically discussed. Additionally, recent experimental and theory-based progress and developments are assessed, allowing coverage of all time and length scales relevant in emission control, i.e. ranging from mechanistic and fundamental insights including atomic-level phenomena to full-scale applications

Microkinetic Modeling of the Oxidation of Methane Over PdO Catalysts—Towards a Better Understanding of the Water Inhibition Effect

Water, which is an intrinsic part of the exhaust gas of combustion engines, strongly inhibits the methane oxidation reaction over palladium oxide-based catalysts under lean conditions and leads to severe catalyst deactivation. In this combined experimental and modeling work, we approach this challenge with kinetic measurements in flow reactors and a microkinetic model, respectively. We propose a mechanism that takes the instantaneous impact of water on the noble metal particles into account. The dual site microkinetic model is based on the mean-field approximation and consists of 39 reversible surface reactions among 23 surface species, 15 related to Pd-sites, and eight associated with the oxide. A variable number of available catalytically active sites is used to describe light-off activity tests as well as spatially resolved concentration profiles. The total oxidation of methane is studied at atmospheric pressure, with space velocities of 160,000 h−1 in the temperature range of 500–800 K for mixtures of methane in the presence of excess oxygen and up to 15% water, which are typical conditions occurring in the exhaust of lean-operated natural gas engines. The new approach presented is also of interest for modeling catalytic reactors showing a dynamic behavior of the catalytically active particles in general

Formation of nitrous oxide over Pt-Pd oxidation catalysts: Secondary emissions by interaction of hydrocarbons and nitric oxide

The interaction of hydrocarbons (HC) and nitric oxide (NO) over noble metal catalysts for exhaust gas after-treatment of lean-operated combustion engines can lead to secondary emissions, namely the formation of nitrous oxide (N$_2$O), which is a strong greenhouse gas calling for N$_2$O reduction concepts. By means of a series of light-off tests over state-of-the-art Pt-Pd oxidation catalysts, this study identifies the most critical catalyst operation regimes that should be avoided in order to minimize N$_2$O levels. Especially unsaturated HCs react with NO to form significant amounts of N$_2$O between 150 °C and 350 °C; an increasing HC/NOx ratio generally promotes N$_2$O formation, whereas the NO oxidation reaction is increasingly inhibited. Since low space velocities and fast catalyst heating allow for minimizing N$_2$O levels, active heating of catalytic converters during cold start and phases of low exhaust gas temperatures may efficiently reduce the formation of N$_2$O in real-world applications

Spatially Resolved Measurements of HNCO Hydrolysis over SCR Catalysts

In order to understand deposit formation during urea selective catalytic reduction (SCR) resulting from isocyanic acid (HNCO) formation, the present study investigates the potential of HNCO hydrolysis by spatially resolved gas phase concentration profiles along a single catalyst channel of commercial Cu-zeolite and V-based SCR catalysts. The spatially resolved profiles, obtained in a special hot gas test rig via capillary technique, provide information on reaction rates of HNCO hydrolysis, NH$_3$ adsorption and NO conversion, hereby revealing a better performance of the standard V-based catalyst regarding the HNCO hydrolysis, which is attributed to the TiO$_2$ support