The influence of calcination temperature on hydrotalcite structure 

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Chemical utilization of carbon dioxide

The growing concerns about global climate change and increasing social awareness of the environmental problems have created a need for more sustainable development. Thus, our society needs to face new challenges, such as mitigation of climate change, preservation of the environment, usage of renewable energy and replacement of fossil fuels. The realization of these challenges requires new breakthrough solutions in order to be successfully addressed. There is no doubt that carbon dioxide (CO2) is a common factor in these great challenges. The increasing emissions of this greenhouse gases (GHG) are of large concern and therefore nowadays a huge effort is dedicated to reduce emissions of GHG, especially carbon dioxide, which contributed in total to ca. 75% of 49 GtCO2eq (in 2010) GHG emissions (Fig. 3.1 A) [1].

CO2 emissions

Carbon dioxide emissions have been constantly growing worldwide since pre-industrial era reaching the level of 35.9 Gt in 2014 [2]. This caused the increase of CO2 concentration from ca. 280 ppm (parts per million) in the mid-1800s to 397 ppm in 2014, with an average growth of 2 ppm/year in the last 10 years [3]. More than 60% of anthropogenic greenhouse gas emissions, for which over 90% is associated with CO2, are coming from energy sector. Between 1971 and 2013 an increase by 150% in global total primary energy supply (TPSE) has been observed, which is mainly caused by worldwide economic growth and development. Although for last few decades a huge development of renewable and nuclear energy sources was observed (which are considered non-emitting sources of energy), the world energy supply was relatively unchanged over past 42 years and fossil fuels still account for ca. 82% (in 2013) of world primary energy supply [3]. Therefore, carbon dioxide emissions are strongly associated with the combustion of fossil fuels (Fig. 3.1 B). Two fuels which accounted for the highest CO2 emissions are coal and oil. Till early 2000s the shares of oil in global CO2 emissions was exceeding those from coal. The situation changed in the beginning of 2000s, due to the higher consumption of coal by developing countries, such as India and China, where energy-intensive industrial processes are growing rapidly and large coal reserves are present.
Power plants, petrochemical and chemical industry and cement industry are currently considered as the main sources of carbon dioxide emissions [4]. However, electricity and heat generation accounted for 42% of global CO2 emissions in 2013, making it the highest emitting sector (Fig. 3.1 C) [3]. Such high emissions from energy and heat generation are associated with high consumption of coal, which has the highest carbon content per unit of energy released with respect to other fossil fuels. This trend is foreseen to be the same for the coming years, as many countries, such as Australia, China, India, Poland and South Africa produce over two thirds of electricity and heat from the combustion of fossil fuels [3].
The CO2 emissions of energy production by country or region are dependent on the geo-political situation, economy, type of fuel and energy mix. However, it is important to underline that top 10 emitting countries (China, USA, India, Russia, Japan, Germany, Korea, Canada, Iran and Saudi Arabia) account for two thirds of global CO2 emissions [2, 3, 6]. The region of highest CO2 emissions is Asia (mainly China and India) (Fig. 3.1 D). In 2014, China increased its CO2 emissions only by 0.9% with respect to 2013, which was the lowest annual increase observed in the last 10 years. The United States (second biggest emitter of CO2) also showed increase of CO2 emissions by 0.9% in 2014, which is lower than in the previous 2 years and was associated with a decrease in coal-fired power generation and increased consumption of natural gas. In 2014, the European Union continued to decrease emissions of CO2 and due to the decrease in fossil-fuel consumption for power generation and lower demand for space heating, the EU-28 decreased the total CO2 emissions by 5.4% [6].

Reduction of CO2 emissions – policies

It is important to understand global driving factors of CO2 emissions in order to find effective solutions for reduction of GHG emissions. The growing world population and demand for energy, as well as practically unchanged carbon intensity of the energy mix, caused drastic increase of CO2 emissions during last two decades. It is well known that the high increase of emissions of greenhouse gases to the atmosphere in the last 150 years was caused mainly by well-developed countries, which emitted high amounts of GHG during industrial era. These countries in general apply today a greenhouse gas emissions reduction policies, which are resulting in decreasing emissions. However, at the same time, currently developing countries emit enormous amounts of GHG. This is mainly caused by differences in economic, demographic and technological levels. Thus, in order for the world to develop in the sustainable way, the efforts must be undertaken by all countries.
The first international agreement which forced reduction of GHG emissions was Kyoto Protocol linked to the United Nations Framework Convention on Climate Change, adopted in 1997 and entered into force in 2005. In general, Kyoto Protocol stated that industrialised countries were required to reduce GHG emissions (CO2, CH4, N2O, HFCs, PFCs, SF6) on average by 5% against 1990 levels during the years 2008-2012 (first commitment period). The specific levels of reduction differed for each participating country depending on political and economic situation, i.e. targets for Poland, France, Germany and European Union were equal respectively, -6, 0, -21 and -8% [7]. The second commitment period (years 2013-2020) requires to reduce GHG emissions by at least 18% with the respect to 1990 levels. In order to bring Kyoto’s Protocol second commitment period into force, it requires ratification by two-thirds of participating countries (144 countries). Till 1st October 2015 only 49 countries have ratified Kyoto’s Protocol second commitment period [3, 7]. The fact that not all countries have ratified Kyoto Protocol, and some of the biggest emitters did not participate in it (United States), requires that new international agreements are established.
In December 2015, in Paris, during COP21 (United Nations Conference on Climate Change) a new international climate agreement was finalised which will be applied from 2020. This agreement assumes the participation of both developed and developing countries. The goal is to limit global temperature increase to less than 2°C above industrial levels, which will be realised by reduction of GHG emissions [8].
European Union has been applying different GHG emission reduction policies for several years now, which resulted in the decrease of total CO2 emissions by 0.4, 1.4 and 5.4%, respectively in 2012, 2013 and 2014 [6]. These achievements were reached thanks to the implementation of 20/20/20 policy which sets the following targets for year 2020 [9]: (i) reduction of greenhouse gas emissions by 20%, (ii) the share of renewable energy at the level of at least 20%, and (iii) improvements in energy efficiency by 20%. In 2014 a new policy was accepted for the period of 2020-2030, which set new targets for 2030 year: (i) the reduction of GHG emissions by 40% with respect to 1990 level, (ii) at least 27% share of renewable energy consumption, and (iii) at least 27% energy saving compared with business-as-usual scenario [9, 10]. These policies frameworks are applied in order to develop low-carbon economy and meet EU long-term targets till 2050, which assume, among others, the reduction of GHG emissions by 80-95% as compared to 1900 levels [11].

Solutions for the reduction of CO2 emissions

The generally accepted solution for reducing CO2 emissions into the atmosphere involves the implementation of three strategies [12]: (1) a reduction in energy consumption, (2) a change in what we consume, or (3) a change of our attitude towards the resources and waste. Currently, the most developed strategies are (1) and (2). These two strategies are resulting in lower carbon consumption by development of technologies with higher efficiency, the decrease in consumption of energy per capita and replacement of fossil fuel-based energy sources by renewables, such as wind, solar, biomass etc. However, there is a huge potential in changing our attitude towards greatly produced waste – carbon dioxide. The implementation of carbon dioxide utilization processes is a key element to sustainable development, as strategies (1) and (2) have a limited capacity, and, as it is predicted, fossil fuels will still be our main source of energy in coming decades. The reduction of carbon dioxide can be realized either by carbon capture and storage (CCS) technologies or via utilization of carbon dioxide as a chemical feedstock – CCU (Carbon Capture and Utilization). These two approaches are complementary, and while CCS technologies are aiming at capturing and subsequently storing huge quantities of carbon dioxide, the chemical utilization of CO2 aims at generating added-value products. Moreover, most technologies, which are currently developed as future CO2 utilization processes, require pure streams of CO2. Thus, implementation of both solutions CCS and chemical utilization of CO2 is required.

CO2 Capture Systems

Carbon dioxide capture is already, or will be, applied to large scale stationary sources of emissions, such as fossil fuel power plants, fuel processing plants and other industrial installations (iron and steel, cement and bulk chemicals production). The capture of CO2 from small and mobile sources (transportation, residential and commercial building sectors) would be rather difficult and more expensive than from large stationary sources, and thus currently capture systems from large scale sources are mainly developed. CO2 capture systems from installations combusting fossil fuels and biomass include the following types of capture (Fig. 3.2) [12]: (i) post-combustion, (ii) pre-combustion, (iii) oxyfuel combustion, and (iv) capture from industrial process streams.
In the post-combustion capture systems, the fossil fuel or biomass is combusted in air. Flue gases are passed through a separation equipment which captures CO2, and the remaining flue gas is discharged to atmosphere. The post-combustion capture system can be applied in fossil fuel-fired power plants.
A pre-combustion capture system involves a reaction of a fuel in oxygen or air, and/or steam, in order to obtain synthesis gas (mixture of H2 and CO) as the main product. Resulting carbon monoxide is further reacted with steam in water gas shift reaction (WGS) to produce H2 and CO2
[13]. CO2 is subsequently separated. In this way a hydrogen-rich fuel is obtained which can be used in many applications, e.g. gas turbines, engines, fuel cells, boilers or furnaces. IGCC plants (Integrated Gasification Combined Cycle) use syngas as a fuel and can apply pre-combustion capture system.
Oxyfuel combustion system assumes the combustion of fuel in the stream of pure oxygen instead of air. In this way, the produced flue gas consists mainly of CO2 and H2O. One of the drawbacks of this system is high temperature of flames, which is a result of combustion of fuel in pure oxygen [13]. However, a part of flue gases (H2O and CO2) can be recycled to the reactor in order to moderate combustion temperature. The second drawback is associated with high costs of oxygen separation from air.
The capture of CO2 from industrial processes can apply similar techniques as post-combustion, pre-combustion and oxyfuel combustion systems. This could be applied to such processes as purification of natural gas, production of hydrogen-rich synthesis gas for manufacture of ammonia, alcohols, liquid fuels, cement and steel production and fermentation processes for food and drink production [13].

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CO2 separation technologies

The methods of CO2 separation from flue gases, which are mainly applied in post-combustion capture system, as well as in capture from industrial processes, are based on physical and chemical processes, such as absorption, adsorption, membranes, cryogenic separation and chemical reactions (chemical looping) [13].
Chemical absorption process typically use amines e.g. MEA (Monoethanolamine). The stream of flue gases is bubbled through MEA solution, resulting in the formation of MEA carbamate. CO2 and MEA are regenerated by heating. The technique has some drawbacks, as it is highly energy intensive, has a low CO2 loading capacity and MEA is degraded by other components of flue gasses such as SO2, NO2, HCl, O2. Instead of MEA, other amines can be used e.g. diethanolamnie (DEA), methyldiethanolamine (MDEA), or other materials such as activated carbons. The absorption with aqueous ammonia solution is possible, if other flue gases components were oxidized (SO2 to SO3, NO to NO2), which results in less energy demanding process (40% reduction) [12]. Physical absorption techniques use in e.g. dimethyl ethers of poly(ethylene glycol).
Adsorption techniques usually apply solid materials, such as activated carbons, molecular sieves, polymers, templated silicas, materials with strong affinity for CO2 and with good adsorption/desorption capacity. Adsorption/desorption cycles are carried out by the change of pressure (PSA – pressure swing adsorption) or temperature (TPA – temperature swing adsorption). This processes are generally considered low energy intensive and cost effective [12], but its drawback is high amounts of adsorbent necessary connected with high volumes of flue gases in stationary power plants.
Other separation technique applies membranes which allow the penetration of a specific gas through them. The driving force in the membrane method is a difference in pressure, thus this technique can be applied for high pressure flue gases. The materials such as polymers, metals or ceramics found application as membranes in industrial processes to separate H2 from flue gases, CO2 or O2. The membrane techniques has not yet been applied for CO2 capture on a large scale, due to the problems with reliability and low cost required for CO2 capture [13].
Cryogenic distillation, which is e.g. applied for O2 separation from air, can be also used to separate CO2 from flue gases. The process requires condensation of gas to liquid by a series of compression, cooling and expansion steps, and subsequent distillation [13]. The drawback of this method is its high cost and high energy intensity. However, a high purity stream of CO2 can be obtained.
CLC (Chemical Looping Combustion) technologies can be also applied for CO2 separation [12]. They are relatively new methods which are currently being developed. CLC processes require the application of metal oxides in e.g. NiO, CuO, Fe2O3 or Mn2O3 (oxygen carrier). The metal oxide is circulating between two reactors containing air and fuel, respectively. In the air reactor the carrier is oxidized and undergoes subsequent reduction in the fuel reactor, resulting in fuel oxidation and production of H2O and CO2. The stream of flue gases containing water and CO2 is then dehydrated and compressed.

Table of contents :

1. Introduction 
2. Aim of this PhD thesis 
3 Chemical utilization of carbon dioxide 
3.1 CO2 emissions
3.1.1 Reduction of CO2 emissions – policies
3.2 Solutions for the reduction of CO2 emissions
3.2.1 CO2 Capture Systems
3.2.1.1 CO2 separation technologies
3.2.2 Applications of CO2
3.2.2.1 General application of CO2
3.2.2.2 Enhanced Fossil Fuel Recovery
3.2.2.3 Biological use of CO2
3.3 Utilization of CO2 as a chemical feedstock
3.3.1 Physicochemical properties of carbon dioxide – CO2 activation
3.3.2 Urea synthesis
3.3.3 Synthesis of salicylic acid
3.3.4 Synthesis of carbonates, cyclic carbonates and polycarbonates
3.3.5 Production of inorganic substances – mineralization
3.3.6 CO2 – gasification of coal
3.3.7 CO2 – Oxidative Dehydrogenation (ODH)
3.3.8. Production of synthesis gas
3.3.9 Hydrogenation of CO2
3.3.9.1. Methanol production from CO2
3.3.9.2 DME production
3.3.9.3 Methanation – production of CH4
3.3.9.4 Production of formic acid
3.3.9.5 Hydrocarbons synthesis
4 Dry Reforming of Methane 
4.1 Comparison of DRM with respect to other reforming processes
4.2 Thermodynamics of the DRM
4.2.1 Thermodynamics of side reactions
4.2.2 Optimal conditions for DRM reaction
4.3. Catalysts for dry reforming of methane
4.3.1 The role of active component
4.3.2 Nickel-based catalysts
4.3.2.1 The role of support
4.3.2.2 The role of promoters
4.3.2.3 Conclusions
4.4 Reaction Mechanism
4.5 Applications of DRM
4.5.1 Industrial experience with large scale reforming of CO2-rich gas
4.5.1.1 CALCOR process
4.5.1.2 SPARG process
4.5.2 Potential future applications of DRM reaction
4.5.2.1 Chemical Energy Transmission and/or Storage Systems (CETS)
4.5.2.2 DRM for production of chemicals
5 Hydrotalcites and hydrotalcite-derived materials 
5.1 Structure and composition
5.2 Properties
5.2.1 Thermal stability
5.2.2 Memory effect
5.2.3 Acid-base properties
5.2.4 Anion Exchange Capacity
5.3 Synthesis and modification of hydrotalcites
5.3.1 Co-precipitation
5.3.2 Urea method
5.3.3 Sol-Gel method
5.3.4 Salt-oxide method (Induced Hydrolysis)
5.3.5 Reconstruction
5.3.6 Anion-exchange
5.3.7 Hydrothermal treatment
5.4 Applications of hydrotalcite-like materials
5.4.1 Adsorption of CO2
5.4.2 Catalysis
5.5 Hydrotalcites in DRM
5.5.1 Ni/Mg/Al and Ni/Al hydrotalcite-derived catalysts
5.5.2 Ce promotion
5.5.3 Other promoters
5.5.2 Advantages of hydrotalcite-derived catalysts
6 Experimental 
6.1 Thermodynamic analysis of DRM process
6.2 Catalysts preparation
6.2.1 Co-precipitation
6.2.2 Reference catalysts
6.2.3 Method of nickel introduction
6.2.4 Influence of nickel loading in brucite-like layers
6.2.5 The influence of promoters (Ce and/or Zr)
6.2.6 Calcination
6.2.7 Catalyst activation
6.3 Characterization of the catalysts
6.3.1 X-Ray Diffraction (XRD)
6.3.2 Infrared spectroscopy
6.3.3 Low temperature N2 sorption
6.3.4 Elemental analysis
6.3.5 Temperature Programmed Reduction (H2-TPR)
6.3.6 Temperature Programmed Desorption of CO2 (CO2-TPD)
6.3.7 Scanning Electron Microscopy (SEM)
6.3.8 Transmission Electron Microscopy (TEM)
6.3.9 Thermogravimetric measurements (TG)
6.4 Dry reforming of methane catalytic tests
7 Thermodynamic analysis 
7.1 The influence of temperature on equilibrium concentrations of reactants during DRM process
7.1.1 The effect of CH4/CO2/Ar feed gas composition on equilibrium concentrations of reactants during DRM process
7.2 The influence of pressure on reactant concentration during DRM
7.3 Thermodynamic limitations in tested reaction conditions
8.1 Composition, structural and textural properties of the hydrotalcite and hydrotalcite-derived materials
8.1.1 The influence of calcination temperature on hydrotalcite structure
8.2 The structural properties of the reduced catalysts
8.3 Reducibility of Ni species and acid-base properties
8.4 Activity and selectivity in DRM
8.4.1 Catalytic tests at temperature range 550-850°C
8.4.2 Low temperature DRM (isothermal tests at 550°C)
8.5 Conclusions
9 The effect of nickel content in brucite-like layers 
9.1 Physicochemical features of the hydrotalcite precursors and the derived catalysts
9.1.1 XRD analysis of synthesized hydrotalcites and calcined materials
9.1.2 FTIR experiments
9.1.3 Elemental analysis
9.1.4 Texture of the prepared catalysts
9.1.5 Reducibility of hydrotalcite-derived mixed oxides
9.1.6 Basicity of hydrotalcite-derived materials containing nickel in brucite-like layers
9.1.7 TEM and SEM analysis of calcined and reduced catalysts
9.1.8 XRD analysis of reduced catalysts
9.2 Activity, selectivity and stability of the hydrotalcite-derived catalysts during DRM runs
9.2.1 The characterization of the spent catalysts
9.2.2 On the influence of CH4 decomposition
9.2.3 On the effect of catalysts basicity on their catalytic performance in DRM
9.2.4 On the influence of temperature on catalytic activity of Ni/Mg/Al hydrotalcite-derived catalysts
9.2.5 On the effect of feed gas composition
9.2.6 Stability tests
9.3 Conclusions
10 Influence of promoters (Ce, Zr) 
10.1 Physico-chemical features of the Ce, Zr and CeZr doped HT-derived catalysts
10.1.1 XRD analysis of fresh, calcined and reduced hydrotalcite-derived catalysts
10.1.2 The elemental analysis of Ce and/or Zr promoted catalysts
10.1.3 FTIR experiments for Ce and/or Zr promoted hydrotalcites
10.1.4 Textural properties of Ce and/or Zr promoted hydrotalcite-derived catalysts
10.1.5 Reducibility of Ce and/or Zr promoted hydrotalcite derived catalysts
10.1.6 Basicity of Ce and/or Zr promoted hydrotalcite-derived catalysts
10.1.7 SEM and TEM analysis
10.2 Effect of Ce-promotion
10.2.1 Activity and selectivity in DRM at 550°C
10.2.2 The influence of the temperature
10.2.3 Characterization of the spent catalysts
10.2.4 On the effect of the feed gas composition
10.2.5 Stability tests for 24h
10.3 The comparison of Ce and Zr promoters
10.3.1 Activity and selectivity in DRM at 550°C
10.3.2 The influence of the reaction temperature
10.3.3 Characterization of the spent catalysts
10.3.4 Stability tests for 24h
10.4 Conclusions
11 Conclusions 
References 
Summary

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