National Centre for Catalysis Research ELECTROCHEMISTRY FOR A CLEANER ENVIRONMENT M. HELEN Research...
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Transcript of National Centre for Catalysis Research ELECTROCHEMISTRY FOR A CLEANER ENVIRONMENT M. HELEN Research...
National Centre for Catalysis Research
ELECTROCHEMISTRY FOR A CLEANER ENVIRONMENT
M. HELENResearch Scholar
Pollution is the action of environmental contamination with
man-made waste
Environment and Pollution
With a rapidly growing world population and an increasing number of reports on detrimental effects on the environment, its protection has become a major issue
The strategies for environmental protection in industry generally include processes for waste treatment as well as development of new processes or products which have no or less harmful effects on the environment
Electrochemistry has important roles to play in both types of strategies
Electrochemical processes can be used for recovery or treatment of effluents from industrial or municipal plants. Industrial electrochemistry has undergone a development towards cleaner processes and more environmentally friendly products
Electrochemical sensors are effective and inexpensive devices for environmental monitoring of an increasing range of toxic substances
A big and important class of environmental problems can be found in the energy and transportation sectors
Electrochemistry offers unique ways to generate pure electric power at high efficiency in fuel cells or to store it in batteries
Possibilities offered by Electrochemistry
Electrochemistry, with its unique ability to oxidize or reduce compounds at a well-controlled electrode potential and by just
adding (at the anode) or withdrawing (at the cathode) electrons, offers many interesting possibilities in environmental engineering
Reaction Eo
F2 + 2e- ---> 2F- +2.87
Co3+ + e- ---> Co2+ +1.80
Cl2 + 2e- ---> 2Cl- +1.36
O2 + 4H+ + 4e- ---> 2H2O +1.23
NO3- + 4H+ + 3e- ---> NO + 2H2O +0.96
Ag+ + e- ---> Ag +0.80
Fe3+ + e- ---> Fe2+ +0.77
I2 + 2e- ---> 2I- +0.54
Cu+ + e- ---> Cu +0.52
Cu2+ + 2e- ---> Cu +0.34
Cu2+ + e- ---> Cu+ +0.15
2H+ + 2e- ---> H2 0.00
Fe3+ + 3e- ---> Fe -0.04
Sn2+ + 2e- ---> Sn -0.14
Ni2+ + 2e- ---> Ni -0.25
Co2+ + 2e- ---> Co -0.29
Fe2+ + 2e- ---> Fe -0.41
Zn2+ + 2e- ---> Zn -0.76
2H2O + 2e- ---> H2(g) + 2OH- -0.83
V2+ + 2e- ---> V -1.18
Mn2+ + 2e- ---> Mn -1.18
Al3+ + 3e- ---> Al -1.66
Reaction Eo
Na+ + e- = Na -2.71
Li+ + e- = Li -3.04
Standard Reduction Potentials (in Volts), 25oCS
tron
g ox
idiz
ing
agen
ts
Strong reducing
agents
Electrochemical processes for waste treatment
In both types of electrode processes, the operating conditions must be carefully controlled in order to avoid side reactions.
In aqueous solutions, which are most often used, the side reactions are mainly oxygen evolution at the anode and hydrogen evolution at the cathode.
These side reactions lower the current efficiency thereby increasing the operating costs, and may disturb the process because of vigorous gas evolution or pH changes at the electrodes.
Not only can the two electrodes of the electrochemical cell be used in purification processes, but the ion-selective membranes that are often placed between the electrodes to have a selective transfer of only anions or cations can also.
New electrodialytic processes using such membranes have been developed, which can solve a variety of environmental problems.
Anodic processes To oxidize organic pollutants to harmless products To remove toxic compounds from flue gases
Cathodic processes To remove heavy metal ions from waste water solutions
Cyanide Poisoning
Electroplating - Zinc, copper, cadmium, silver, gold, brass and nickel are commonly plated using cyanide solutions
Cyanide solutions - intrinsic cleaning ability - effective in keeping metals in solution during the
plating process - to obtain very finely grained metal deposits
Cyanide - harms the brain and heart, and may cause coma and death
Electrochemical process
Anode (graphite or stainless steel) : cyanide is oxidized
CN- + 20H- CNO- + H20 + 2e-
Cathode: metal deposition with hydrogen evolution as a side reaction.
Temperature - 50-90 ºCCurrent density - 500 A m-2
High concentration of cyanide (> 1 M)
Low concentration in outlet stream
Chemical method
CN- CNO- using hypochlorite (NaClO) as an oxidizing agent
Chlorine in Drinking Water
Effective disinfectant - destroy many of the bacteria in your drinking water
Chlorinated hydrocarbons - chlorine reacts with decomposed plant and animal materials
Chlorine - irritant to the eyes, skin, the upper respiratory tract, and the lungs
- solvents and disinfectants, bleaching in the pulp and paper industry (chlorinated phenols)
Combustion membrane separation, adsorption on activated carbon and
stripping, chemical oxidation with air, ozone or other oxidants Chemical reduction techniques, such as catalytic dehalogenation
with hydrogen or other reducing agents Biological techniques using special microorganisms or enzymes Electrochemical dechlorination - either anodically or cathodically
Dechlorination
First step
chlorine is substituted by hydroxyl radicals formed from water
Subsequent steps
further oxidation yields quinone, which decomposes into maleicacid, oxalic acid (primarily) and carbon dioxide.
Oxygen and significant amounts of ozone were formed as by-products at the anode
Riskchloride ions formed may be oxidized to hypochlorite, which can then form chlorinated organic compounds
p-chlorophenol and pentachlorophenol - anodically on lead dioxide
Cathodic Electrochemical Dechlorination
RCl + H+ + 2e- RH + C1-
Electrode material - thin graphite/carbon fibres high specific surface area and high overpotential for the competing hydrogen
evolution reaction
Waste waters containing heavy metal ions - generated in metallurgical and electroplating industries and in the manufacture of printed circuit boards.
Conventional purification - uses hydroxide precipitation gives voluminous metal hydroxide sludge that has to be disposed of
Complexed metal ions in alkaline solutions-hydroxide precipitation is not a viable method
Cathodic removal of heavy metal ions attractive alternative processMetal can be recovered in its pure metallic form
Removal of heavy metal ions
Cu, Hg, Zn, Cr and Cd
Anode - three-dimensional electrode or just a planar electrode for e.g. oxygen evolution
Cathode - graphite particles, expanded metal, metal wool & graphite fibres
Three-dimensional electrodes -offer both a high specific surface area and high mass transport rate conditions.
Metal concentration can be reduced from 100 to 0.1 ppm at a residence time of a few minutes
Operational costs are favourable compared with classical waste water treatment systems
the space required by the process is low
deposited metal in the cathode may be recovered as a concentrated solution by chemical dissolution
Electrodialytic processes
Splitting of sodium sulfate solutions into sodium hydroxide and sulfuric acid solutions
Aqueous streams containing e.g. NaCl and Na2S04 - chemical processing operations
Flue gas scrubbing Metal pickling Fermentation Rayon manufacture
Large scale brackish and seawater desalination and salt production.
Small and medium scale drinking water production (e.g., towns & villages, construction & military camps, hotels & hospitals)
Water reuse (e.g., industrial laundry wastewater, produced water from oil/gas production, metals industry fluids)
Applications
Electrochemical remediation of soils
Restoration of contaminated soils
Anode -oxygen evolution
H20 1/2 02 + 2H+ + 2e-
Cathode- hydrogen evolution
H20 + 2e- H2 + 20H-
Ions will move – due to migration, diffusion and convection
Heavy metal ions - move to the cathode
Organic compounds - by means of the electroosmotic flow
I step : Absorption of the gaseous species in a liquid
II step: Electrochemical conversion of them to less harmful products
Chemically
Bromine as a mediator to oxidize SO2
SO2 + Br2 + 2H20 H2S04 + 2HBr
Electrochemically
Bromine is regenerated
2 HBr Br2 + H2
H2S H2 + 1/2 S2
Electrochemical gas purification
Reduction of chlorine to chloride Oxidation of nitrous oxides to nitric acid Sulfur dioxide to sulfuric acid
The reduction or oxidation - directly at the electrode or indirectly via a redox mediator
Electrochemical power sources for cleaner electrical energy
Thermal combustion of fossil fuels in power plants and vehicles is a major environmental problem in modern society
The immediate damage of air pollution has been estimated to cost about three times more than the fossil fuels themselves
The most important gaseous impurities in the flue gas from electricity generation plants are C02, NO, SO2 and dust particles
C02 is a major contributor to the greenhouse effect and NO, contributes to the acidification of water and soil, eutrophication, and the formation of smog
Global warming and climate change
Road traffic alone generates more than 50 % of the total emissions of nitrogen oxides, carbon monoxide and hydrocarbons.
The only vehicles that are likely to meet “zero emission vehicles” demands are electric vehicles.
In order to meet these new regulations ‘The Big Three’, General Motors, Ford and Chrysler in the USA decided in January 1991 to form a consortium, The United States Advanced Battery Consortium (US ABC), for cooperation towards improved power sources for electric vehicles.
Specific energy/ W h kg-1
Peak specific power/W kg-1
Discharge cycles
US ABC 80-100 150-200 600
200 400 1000
Pb/PbO2 35-40 150-300 100-1000lithium-metal sulfide, lithium-polymer, lithium-ion batteries, metal hydride-nickel oxide, zinc-air and zinc-nickel oxide
Battery
Representation of a lead acid battery
Charging of the battery Discharging of the battery
Anode: Sponge metallic leadCathode: Lead dioxide (PbO2)Electrolyte: aqueous sulfuric acidApplications: Motive power in cars, trucks,
standby/backup systems
Batteries - used in all-electric vehicles or in hybrid vehicles that use also combustion engines for propulsion.
The batteries are then mainly used for acceleration and for citydriving, while the combustion engine gives a reasonable range.
In this application, a high peak power density of the battery is a major requirement, while the energy density, which determines the all-electric range, is less important compared to all-electric vehicles.
An interesting alternative, or complement, to batteries is elctrochemical capacitors (also called ultra capacitors or supercapacitors), which can give peak power densities greater than 1 kW kg-l while the energy density is only 2-10 % of that stored in a battery)
Battery vs Capacitors
Regenerative backing in hybrid vehicles,
cold engine starting, backup power systems and digital electronic devices
Applications
An electrochemical capacitor stores the electrical energy electrostatically by charging of the electrochemical double layer at the electrode/electrolyte interface
In some systems intermediates are adsorbed on the electrode surface or intercalated into the electrode material, which gives an additional so called pseudo-capacitance that may be 10 to 100 times higher than the double layer capacitance
In a finely porous electrode with a high specific surface area, fairly high amounts of electrical energy may be stored per unit volume or mass
Electrochemical capacitor
Research and development has been going on since the early 1990s to develop ultracapacitors using various types of carbon, doped conducting polymers, and metal oxides as electrode materials
The electrolyte may be aqueous, organic or a solid polymer
Ultracapacitors with aqueous electrolyte can store 1.5 W h kg-1 and deliver 1 kW kg-1, while the best values reported for devices using an organic electrolyte are 5-7 W h kg-l and 2 kW kg-1
Fuel Cells
Direct Energy Conversion vs Indirect Technology
Fuel Cell
ICEThermal Energy Mechanical Energy
Chemical Energy Electrical Energy
Applications
Submarine with fuel cell propulsion - German Navy
Monitoring of toxic compounds - Electrochemical sensors are convenient and effective devices
An electric signal can be related directly to the concentration of the compound being measured
Sensing pollutants - as potentiometric, amperometric or voltammetric sensors
Electrochemical sensors
These sensors measure the electrical potential of an electrode when no current is flowing. The signal is measured as the potential difference (voltage) between the working electrode and the reference electrode
The working electrode's potential must depend on the concentration of the analyte in the gas or solution phase
Ion-selective electrodes can be used to determine for example pH, fluoride and cyanide concentrations in water
The concentration of toxic gases such as sulfur and nitrogen oxides can also be determined with potentiometric sensors
Potentiometric Sensors
These sensors measure current at a fixed potential
The current is then proportional to the concentration of the measured species
The Clark electrode for measuring oxygen concentration is the classic example
Its general principle also works for toxic gases like CO, NO, NO*, SO2 and H2S
A sensor can be made selective by a suitable choice of electrode potential and electrode material
An array of such selective sensors can be built into one device for monitoring flue gases and other gas streams containing several toxic components
Amperometric sensors
Recent advances in photoelectrochemistry have led to new, interesting possibilities, both for treatment of pollutants and for conversion of solar energy from light to electricity
In the first case, suspensions of semiconductor particles can be used to harness the light with production of electrons and holes in the solid, which can destroy pollutants by means of reduction and oxidation, respectively
In this way, air or water containing organic, inorganic or microbiological pollutants can be effectively treated
Photoelectrochemical cells for electricity production offer a sustainable way to generate electricity, e.g. for charging batteries in electric vehicles
With semiconductor electrodes using dye sensitized nanocrystalline Ti02 films an efficiency of 12 % has been reported
Compared to conventional photovoltaic cells, this type of photoelectro-chemical cell is less expensive, since it uses inexpensive raw materials, is easily fabricated
Photoelectrochemical methodsPhotoanode
2h+ + H2O 2H+ + ½ O2
Cathode
2e- + 2H+ H2
A variety of selected electrochemical processes and devices for environmental protection have been presented
Some, but not all of them, have also been tested at pilot scale and some have reached commercialization
In some cases, it is only a matter of time, further development work (and investment) being required
In other cases, chemical or biological processes are preferred because they are competitive and do not require expertise in electrochemistry and electrochemical engineering
It may be expected that the number of electrochemical processes for treatment or prevention of pollution will increase in the future due to their specific advantages in a number of applications
A major beneficial impact of electrochemistry on the environment would be the future introduction of fuel cell or battery driven vehicles
In brief
Thank you
Delhi to observe Earth Hour every month
The Delhi government has proposed to hold an Earth Hour on the last working day of every month, urging residents to switch off non essential lights to save power on the lines of the recently held global campaign.
India is a signatory to the United Nations Educational, Scientific and Cultural Organization's (UNESCO) World Heritage Convention adopted in 1972. The main goal of the World Heritage Convention is to identify and protect monuments of great cultural and natural heritage throughout the world. In signing the Convention, a country pledges to conserve the World Heritage sites located in its own territory and protect its national heritage. The application for a site to be accepted as the World must come from the country itself. The application process includes submission of a plan detailing how the site is managed and the measures assuring its continued protection. In some cases, UNESCO identifies conditions to a country before accepting a site as a world heritage monument. For example, at the time Delphi was nominated by Greece, a plan was in the works to build an aluminum plant nearby. The Greek government was asked to find an alternative location for the plant, did so, and Delphi was accepted onto the World Heritage List. In other cases, such as the Giza Pyramids, UNESCO asks the country for remediation of potential threats. In 1995, the Pyramids were threatened by a highway project near Cairo which would have seriously damaged the monument. Negotiations with the Egyptian government resulted in a number of alternative solutions which replaced the disputed project. Ultimately, the treaty is not binding by the force of an ultra-national body but rather left to the discretion of the country. The Agra area currently has three world heritage sites: the Taj Mahal, Agra Fort and Fatehpur Sikri.
The Issue: Environmental pollution spurred by industry and automobiles has long been observed to be progressively destroying the Taj Mahal's white marble surface. Petitions of Indian environmentalists have led to a series of court challenges in the Indian Supreme Court and lower courts. The conflict has often pitted business and labor interests against environmentalists and preservationists as well as India's need to protect its cultural heritage versus its need to provide jobs for its citizens.
2. Description: Mark Twain once remarked the world is divided between two types of people: those who have seen the Taj Mahal and those who have not. The Taj is one of the most recognizable landmarks in the world and the image most associated with India. The Mughal emperor Shah Jahan erected the Taj Mahal at Agra as a mausoleum in memory of his beloved wife, Arjumarid Bano Begum; (popularly known as Mumtaz Mahal "favored of the court"), who died in A.D. 1630. Begun in 1632 AD, it took 20,000 men working every day over 22 years to complete. It is heralded by many as the greatest work of Mughal architecture. India has experienced exponential industrial growth in recent years. Increasingly, people have left villages for urban centers in order to try and find work. The result of this industrialization has often been overcrowded cities and dense pollution. Agra is no exception. It has been identified as a "pollution intensive zone" by the World Health Organization (WHO). It is estimated that the area around the Taj contains five times the amount of suspended particles (such as sulfur dioxide) that the Taj Mahal could handle without sustaining everlasting damage. India has been involved in a "greening" campaign particularly in regards to its national monuments. More recently, India has begun to try and attract more tourists: this has created a dilemma how to market its best Tourist attraction without causing significant damage to it in the process.