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Advanced Composites and Hybrid Materials

, Volume 2, Issue 3, pp 501–519 | Cite as

Ambient temperature complete oxidation of carbon monoxide using hopcalite catalysts for fire escape mask applications

  • Subhashish DeyEmail author
  • Ganesh Chandra Dhal
  • Devendra Mohan
  • Ram Prasad
Review

Abstract

Carbon monoxide (CO) is one of the most poisonous gases present in the atmosphere. It also called the silent killer of twenty-first century. CO is produced into the environment by incomplete combustion of carbon containing compounds. It causes lots of people die every year including the firefighters. The main aim of this work to find out the literature study of standard respiratory escape masks for ambient temperature CO oxidation purposes. The research under concern is applicable for developing respiratory protection systems for military, mining, and space devices. There are many catalysts which are active for this process under different conditions. Among these catalysts, the hopcalite (CuMnOx) is one of the best-known catalysts for low-temperature CO oxidation. It is a low-cost, easily available, and highly stable catalyst. The hopcalite catalyst is active for a longer time and would be tolerant of moisture and impurities in reacting gases. The catalyst surface and reacting gases forever play a key role in catalytic reactions. Hopcalite is an ideal catalyst for use in next-generation respiratory protection devices. Although there are numerous research papers present on this topic until now no one review are present for demanding this issue. So there is a space in this area; it has been made an attempt to seal this hole by this review.

Graphical abstract

Summary

• To develop a standard fire escape masks for protection of respiratory systems in military, mining, and firefighting, etc.

Keywords

Carbon monoxide Hopcalite catalyst Combustion Escape mask and respiratory 

1 Introduction

Carbon monoxide (CO) is a toxic gas particularly to human beings and in general, to all life forms that respire [1]. CO is also called the carbonous oxide; it is a colorless, odorless, tasteless, and non-irritating gas, which makes it very difficult for humans to detect [2]. As a result, the CO has been called “the unnoticed poison of the 21st century” and “the silent killer,” because it gives no clear warning to its victims that they were at risk [3, 4]. CO poisoning is the most common type of fatal air poisoning in many countries [5]. Inhaling even relatively small amounts of CO can lead to hypoxic injury and neurological damage. It not only affects human beings but also vegetation by interfering with plant respiration and nitrogen fixation [6, 7]. The health effects of CO explored are fatigue, nuisance, seizure, coma, fatality, nausea, vomiting, and death also. It combined with hemoglobin present in blood cells and converted into carboxy-hemoglobin (CoHb), which reduces the oxygen carrying capacity of a human body. CO causes about 2100 deaths per year and about 10,000 physical injuries in the USA [8, 9]. It can be lethal at a concentration of more than 400 ppm. Depending upon cases, respiratory can contain as high as 3600 ppm of CO. Endogenously produced CO is now known to play a main role in cytoprotection against damage of body tissues [10]. Other places where it is possible to get poisoned by CO are ship boiler rooms due to defective ventilation, respiratory below deck in and out, manufacture of power plants, coal mining, certain classes of copper mining, wherever explosives are being used in enclosed spaces, leaky flues, exhaust gases from explosive engines, and places where coal respiratory are employed in case of improper ventilation [11].

Huge amounts of CO are emitted in the world (≈ 1.18 billion tons in 2018), mainly from transportation, power plants, industrial, and domestic activities. An estimate has shown that the vehicular exhaust contributes about 64% of the CO pollution in the developed countries. Forest respiratory and building respiratory also release a large quantity of CO in the atmosphere. The global concentrations of CO range between 0.06 and 0.14 mg/m3 [12, 13]. A catalytic converter is an emission control device that converts more toxic pollutants present in the exhaust converted into less toxic by catalytic reactions. It is also applications in housing, CO detectors, automotive air cleaning technologies, gas masks for respiratory fighters, and mining industry [14]. The performances of catalytic converter is highly depending upon the types of catalysts was used. In presence of catalyst, the rate of chemical reaction was increased; it acts like an agent that reduces the activation energy of the reactions [15].

The catalyst presence in a catalytic converter first was a reduction catalyst and second was an oxidation catalyst. In both conditions, the catalysts consist of a ceramic monolith structure and coated with metal support [16]. In the presence of catalyst, the rate of chemical reaction was increased; it acts like an agent that reduces the activation energy of reactions. The base metals (Cu, Mn, Co, Cr, Ni, Fe, etc.), noble metals (Pt, Pd, Rh, Au, etc.), and metal oxide (Cu2O, CeO2, ZnO, ZrO2, TiO2, etc.) are broadly used as a catalyst in the catalytic converter [17, 18]. The noble metal catalysts have a high activity and thermal stability. It was challenging to scale up for commercial applications due to their poor reproducibility, high cost, and deactivate easily. The effectiveness of catalytic converter is also depending upon the temperature [19]. As compared to other catalysts, the hopcalite (CuMnOx) is one of the oldest known catalysts for low-temperature CO oxidation. It was attracted much attention because of its low cost, high catalytic activity and moisture resistance. In 1920, Bray, Lamb, and Frazer discovered that various oxide mixtures of Cu, Mn, Ag, and Co recognized as a group of catalysts identified as a hopcalite (CuMnOx). Jones and Taylor confirm the catalytic properties of such a system called hopcalite in the year 1923. It can well catalyze the oxidation of dry CO even at room temperature [20, 21]. A literature study reported that hopcalite catalyst is more active in spinel CuMn2O4 form has occurred [22]. The Cu oxide is found weakly active for CO oxidation, but in conjunction with Mn oxide in appropriate proportions, some very active catalyst system was generated. The high catalytic activity of CuMnOx catalyst in CO oxidation could be attributed to the resonance system Cu2+ + Mn3+ ⇆ Cu+ + Mn4+ and high adsorption of CO onto Cu2+/Mn4+ and O2 onto Cu+/Mn3+ [23, 24].

The additional of Cu into MnOx improved its catalytic activity for CO oxidation. The oxygen species associated with Cu in CuMnOx catalyst are very active and may be dominated by the low-temperature catalytic oxidation of CO as shown in Fig. 1. In characterization points to increase the activity of lattice oxygen associated with Cu species as well as the mobility of lattice oxygen from Mn species [25]. Addition of various dopants into the CuMnOx catalyst enhances their performance for CO oxidation. A lot of attention has been made to modification of hopcalite catalyst to eliminate its faults of moisture deactivation and lower activity. The doping of transition metal into the CuMnOx catalysts could tune the oxygen mobility and reduced the ability of catalysts, thus improving the catalytic activity [26, 27]. By adding low levels of promoters into the CuMnOx catalysts can display much higher activity for CO oxidation as compared to the un-promoted CuMnOx catalysts [28]. The inclusion of these dopants can modify the physical, chemical, and catalytic properties of CuMnOx catalyst while having little effects on the nanostructure. The dopants are highly affected by the size, morphology, and surface area of catalysts [29, 30]. The addition of Au or Pt into the CuMnOx catalyst not only improved its catalytic activity but also prevents the deactivation of catalyst. It provides active sites on the catalyst surfaces for a potential variation of the reaction mechanism [31, 32].
Fig. 1

Physical structure of CuMnOx catalyst and their CO adsorption property

The CuMnOx catalyst is most active in the amorphous state, but they lost their activity at a temperature above 500 °C, where crystallization phase of the CuMnOx catalyst has been occurred. When materials heat treated under an oxygen-containing atmosphere formed the CuMnOx spinel phase, while oxygen-free atmospheres resulted in the reduction of Cu phases to generate Cu metal and Mn oxide phases [33]. Due date, there are various methods has been applied to prepare the CuMnOx catalysts, including co-precipitation, sol-gel, reduction and pyrolysis method, etc. Among these methods, the co-precipitation can be produced by highly active CuMnOx catalyst for CO oxidation [34]. Other preparation parameters include pH and temperature and (Cu:Mn) molar ratio is extremely influenced on the performance of catalyst. The effect of preparation conditions and calcination strategy has highly influence on the catalytic activity of CuMnOx catalyst for CO oxidation [35, 36].

The history of respiratory escape masks began in 1853. The apparatus was developed by Prof. Schwann as part of the academic competition at Academie Royale Belgique. The machine looked similar to those in use today. In late 1800s, Fleuss apparatus was developed in England by Prof. H.A. Fleuss. In 1903, an early type of Draeger apparatus appeared in Germany. In the USA, mining companies were the initial ones to recognize the importance of breathing apparatus. Philadelphia & Reading Coal & Iron Co. of Pennsylvania and Anaconda Copper Mining Co. of Montana were the first 3 ones to provide breathing machines as a rescue operation in their mines in 1907. The USA investigated the cause of casualties in mining operations, as a part of which the Bureau of Mines came into being in 1910. The bureau took up the development of breathing apparatus [11]. After World War I (1914), Bureau of Mines carried on investigations to develop gas mask that would be suitable for industrial uses where poisonous gases could be sometimes encountered. The very first escape mask category that bureau of mines came up with was the “Universal Gas Mask.”

The modern gas mask was developed in 1943 by the British. It was made of plastic and rubber-like material that greatly reduced the weight and bulk, compared to World War I gas masks and fitted the user’s face more snugly and comfortably. The main improvement was replacing the separate filter canister connected with a hose by a filter canister screwed on the side of the gas mask that could be replaced easily. Also, it had replaceable plastic lenses, much helping vision. In 1984, Iran received gas masks from the Republic of Korea and East Germanys. In April 1988, Iran started domestic production of gas masks by the Iran Yasa factories. For the year 2000, researchers need to pay more attention to the development of gas masks which can be used for longer intervals of time amidst the battlefield conditions. Reusable, biodegradable, user-friendly masks (Fig. 2) with enhanced protective potential against blister agents need to be developed to avoid disposal problems.
Fig. 2

Respiratory escape masks

Design improvisations shall be made (2005) to overcome problems related to fogging of lenses, leaking of air, breathing resistance, and ergonomics. Carbons with enhanced capacity to react with both persistent and non-persistent chemical warfare agents shall be made available for the use in canisters. Advanced mechanical filters in 2010 composed of nanofibrous filter medium incorporated with nanomaterials, with low breathing resistance and higher CO removal efficiencies, 99.99%, can be achieved and these filters can be incorporated into the advanced canisters. Since that early time, there have been significant advances in gas mask technology, particularly in the area of new filtration aids. In addition, masks have been made more comfortable and tighter fitting with modern plastics and silicone rubber compounds. Today, air-purifying respirator (APR) are used to filter many undesirable airborne substances, including toxic industrial fumes, vaporized paint, particulate pollution, and some gases used in chemical warfare. These masks are produced in several styles, some that cover only the mouth and nose and others that cover the entire face, including the eyes. They may be designed for military as well as industrial use but, even though the two types are similar in design, the military masks must meet different standards than those used in industry. This article will focus on manufacture of the full face type of mask used for industrial and firefighting applications. The catalyst developed for application in universal gas masks can be very effective to remove CO from the breathing air by incorporating it into the gas mask or breathing equipment. The catalyst particles loading inside the cartridge of the gas mask are packed bed formed. In the presence of moisture, the catalyst deactivates easily. Hence, these masks have excess quantities of catalyst. The escape mask incorporating materials should be lighter in weight and better in performance. The catalysts can be incorporated into the respiratory escape mask to convert CO to CO2 thereby reducing the problem of physical injury [37, 38, 39, 40, 41].

2 Chemistry of carbon monoxide

Carbon monoxide consists of one carbon atom and one oxygen atom, associated a triple bond which consists of two covalent bonds as well as one dative covalent bond. It is the simplest oxo carbon. In coordination complexes, the CO ligand is called carbonyl. CO is a molecule with three resonance structures or Lewis structures in Fig. 3. It can explain the exceptional adsorption properties and reactivity of this molecule on oxide and metal surfaces. CO is one of the strongest diatomic molecules and a weak electron donor. In the structure with three covalent bonds, the octet rule is satisfied, but the electropositive carbon has a negative formal charge [42].
Fig. 3

Resonance structure of CO

The structure with two covalent bonds would be consistent with very small dipole moment of the molecule if the bonds were non-polar. The structure of one covalent bond expresses the greater electro negativity of oxygen and calculated net atomic charges. None of them do accurately meet the actual electronic structure. Calculations with natural bond orbital’s as shown in Fig. 3 that the structure with a triple bond is the most important. This is in accordance with other theoretical and experimental studies which show that despite the greater electro negativity of oxygen that dipole moment points from the more negative carbon end to the more positive oxygen end [43, 44].

3 Application of respiratory escape masks for protection from irrespirable gases

A respirator intended to be used only for emergency exit. In a building on respiratory situation, escape respirators with appropriate purifying materials in the canister of the mask are used for escape purposes. Mask is used by the firefighters and other workers in areas where heat, flame, and airborne particulate hazards exist and a self-contained breathing apparatus is impractical or impossible [44]. Respiratory fighting masks the hot shield is extreme heat and flame resistant respirator housing exclusively designed for the half face respirator. It has been designed by fire extinguishers specifically for wild land respiratory fighting but has found application in other industries and mining area for respiration purposes.

The universal gas mask did not supply oxygen and it was one of gas purification type [45, 46]. Gas masks contained following different adsorbents and catalysts for gas purification purposes:
  1. I.

    Activated charcoal for removing organic vapors

     
  2. II.

    Caustic soda or pumice stones for acid gas removal

     
  3. III.

    Fused calcium chloride to remove water vapor so that catalyst (susceptible to poisoning) does not deactivate

     
  4. IV.

    Hopcalite catalyst for removing carbon monoxide by direct oxidation

     
  5. V.

    Silica gel for removing ammonia

     
  6. VI.

    Filters of cotton wool to eliminate suspended particulate matter such as dust and mists

     
As seen in Fig. 4, this canister does not have the silica gel layer for ammonia removal. Instead of silica, the activated charcoal was also impregnated with copper sulfate to remove ammonia. Copper sulfate reacts with ammonia to form ammonium copper sulfate. The canister rested on wearer’s chest with a hose connected to the face piece. The total volume of the canister was 1720–1750 cc. A modern mask is usually constructed of an elastic polymer in various sizes. It is fitted with various adjustable straps which may be tightened to safe a good fit. Crucially, it is connected to a filter cartridge near the mouth either directly, or via a flexible hose. The protection will wear off over time. Filters will clog up, substrates for absorption will fill up, and reactive filters will run out of reactive substance. Thus, the user only has protection for a limited time, then after either replaces the filter device in the mask or uses a new mask [47, 48].
Fig. 4

Schematic diagram of all service gas masks

3.1 Types of respiratory protection system

OSHA (Occupational Safety and Health Administration) Respiratory Protection Standard defines the following different classes of respiratory protection systems.

3.1.1 Filtering face piece (dust mask)

A negative pressure particulate respirator with a filter as an important part of theface-piece or with the complete face-piece composed of filtering medium.

3.1.2 Air-purifying respirator (APR)

It is a respirator with an air-purifying filter, cartridge, or canister that removes specific air pollutions by passing ambient air throughout the air-purifying element.
  1. a.

    Powered Air-Purifying Respirator (PAPR) is an air-purifying respirator that uses a blower to force the ambient air passing through the air-purifying elements to the inlet covering.

     

3.1.3 Atmosphere-supplying respirator (ASR)

  1. a.

    Self-contained Breathing Apparatus (SCBA) is an atmosphere-supplying respirator for which the inhalation air source is designed to be approved by the user. These are used in oxygen deficiency type of environments or any situation that is IDLH (Immediately Dangerous to Life or Health) or become IDLH or any environment that has unknown environment both in terms of levels and types of contaminants.

     
  2. b.

    Supplied Air Respirators is an atmosphere-supplying respirator for which the source of breathing air is not designed to be carried by the user. It is also called airline respirator [49].

     

3.1.4 Escape only respirator

A respirator mask should be an active minimum for 15 min while removing 5150 ppm CO down to less than 200 ppm instantaneous at room temperature as well as 0 °C, and 90% relative humidity of the feed. Gas masks and air-purifying respirators, in general, are regulated by the Code of Federal Regulations (CFR). Over the last 80 years, the basic technology of gas masks has been tested frequently and so is not likely to modify in the future. The challenges for the APR industry will develop products for special purposes, such as infant respirators or masks for persons with head wounds and other disabling injuries [50]. The potential of these products also relies on improvement in the material sciences, which allows the formation of smaller, more lightweight products. These and other improvements in materials will result in new generations of respiratory devices for industrial use, as well as for medical, mining, respiratory fighting, and military applications. The escape masks consist of the following components (Fig. 5).
  1. A.

    Hood

     
  2. B.

    Visor

     
  3. C.

    Half mask

     
  4. D.

    Canister

     
  5. E.

    Particulate filter

     
  6. F.

    Exhalation valve

     
Fig. 5

Schematic of respiratory escape masks

To develop a suitable material for respiratory protection (Table 1) that would effectively remove CO before inhalation, the escape mask incorporating the material should be lighter in weight and better in performance [51, 52]. The breathing problem offered by the material should be particularly very low for easiness of utilizing. Micro fibrous entrapped oxidation catalysts offer assurance for research into this arena. This catalyst can be incorporated into the respiratory escape mask to convert CO into CO2 thereby reducing the risk of physical injury [53, 54].
Table 1

Description features of escape respiration masks

Attribute

Modern respiration masks

Old respiration masks

Area

73 cm2

71 cm2

Total catalyst loading

182 g

(C = 58 g, mol. sieve = 40 g, hopcalite = 84 g)

173 g

(C = 56 g, mol. sieve = 34 g, hopcalite = 83 g)

Total bed depth

4.5 cm

4.5 cm

Pressure drop through the bed

> 40 mm of H2O

> 40 mm of H2O

Time

35 min

19 min

The perfect respirator features for confined space rescue would have the following attributes:
  1. i.

    Unlimited air supply

     
  2. ii.

    Very low weight and bulk

     
  3. iii.

    Be self-contained and have no airline to drag behind the wearer

     
  4. iv.

    A breathing attachment for the victim (remember the unlimited air supply part)

     
  5. v.

    Provision for excellent visibility and communications while wearing the face piece

     
  6. vi.

    The reality in confined space rescue respiratory protection is much different. Currently, there is not one, all-around, all-purpose respirator that meets all of the needs for confined space entry and rescue.

     
  7. vii.

    In many instances, a supplied air respirator with an acceptable escape provision (15 to 20 min air supply minimum) is the safest for confined space entry and rescue.

     

3.1.5 Respirator mask characterization

The characteristics of respiratory mask mainly include the micro fibrous bed. The micro fibrous materials can be layered every millimeter with different particle and fiber size. Smaller catalyst particles can ignite the reaction by increasing the temperature locally since the heat capacity is very low. If front part of the bed can be packed with extremely small particle size layer, then increasing the particle size gradually, following advantages, can be realized:
  1. i.

    Front of the bed ignites hence drives the reaction faster due to the temperature rise.

     
  2. ii.
    Back part of the bed, having larger particle size structure, can offer resistance to deactivation. The top layer can be deactivated quickly but the inner layer of catalyst still remains active. Thus, layering of particles can help not only this reaction, but any reaction that has tendency to show multiplicity behavior. A qualitative reaction model could be proposed with the help of surface chemistry. There are two general types of respiratory protective equipment (RPE), based on the principle by which protection is provided to the user. The two types are following (Fig. 6):
    1. 1.

      Respirators (filtering equipment): filter, gas filter, combined filter, filtering half mask.

       
    2. 2.

      Breathing apparatus (isolating equipment): self-contained breathing apparatus (open-circuit and closed circuit), compressed line breathing apparatus.

       
     
Fig. 6

Characterization of respirator mask

Respirators are designed (Table 2) to filter out or clean contaminated air from the workplace atmosphere before it is inhaled by the respirator wearer. It is used in atmospheres with oxygen deficiency (concentration of oxygen is below 19%) or where the concentration of unknown contaminants has not been evaluated. Fire standards have been developed by many organizations. Many fire standards involve the testing and evaluation of the ignition, burning, or combustion characteristics of certain materials. Following are specifics extracted from the testing procedures highlighting the necessities to insure life safety in the firefighting. Firefighting employers are responsible for providing a safe and healthful work environment for their employees.
Table 2

Standard of respiratory escape masks

Operating variable/parameter

Range

Catalyst activity time

15, 30, 45, and/or 60 min

Pressure drop

70 mm of water (maximum permissible)

Inlet CO concentration

1200–10,000 ppm

Face velocity

6.88–21.5 cm/s

Relative humidity

> 90%

Bed depth

4 mm

Reaction mixture

Carbon monoxide, air, moisture

Temperature

25 °C

Pressure

1 atm.

Breathing apparatuses deliver breathable air from an independent source (compressed air vessels, compressed line) to the user. Breathing apparatuses are designed to use in atmosphere with oxygen deficiency (concentration of oxygen is below 19%). Both types of RPEs are available with a range of different face pieces, i.e.:
  • Tight-fitting face pieces (filtering face pieces, half and full-face masks) that rely on a good fit-seal between the mask and wearer’s face.

  • Loose-fitting face pieces (hoods, helmets) rely on enough air being provided to prevent the contaminant leaking into the face piece as the wearer breathes and moves about. They are used only with powered respirators or with suitable breathing apparatuses. General classification of the respiratory protective equipment is as follows:
    1. 1.
      Respirators filter out contamination from the air in the workplace before it is inhaled by the user:
      1. a.
        Filtering respirators:
        • Filtering face pieces

        • Half mask with filter(s)

        • Full-face mask with filter(s)

         
      2. b.
        Powered/assisted respirators:
        • Powered hoods and helmets with filter(s)

        • Powered-assisted half mask with filter(s)

        • Powered-assisted full-face mask with filter(s)

         
       
    2. 2.
      Breathing apparatuses provide uncontaminated breathable air from independent source:
      1. a.
        Compressed airline breathing apparatuses:
        • Constant flow with any type of face piece

        • Negative demand half or full-face mask

        • Positive demand half or full-face mask

         
      2. b.
        Indented line self-contained breathing apparatuses:
        • Open-circuit negative demand full-face mask

        • Open-circuit positive demand full-face mask

        • Closed-circuit full-face mask demand

         
       

4 Catalysts for CO oxidation

Catalytic oxidation of CO at ambient temperature has considerable more interest due to its importance in human safety in mines, deep sea diving, space exploration, masks for the crater of the volcano as well as in many other applications such as indoor air cleaning, CO sensors and in minimizing the CO build-up in CO2 lasers. In addition, CO oxidation is an elementary step in other important industrial processes such as water-gas shift reaction and production of methanol. The ability to oxidize CO at low temperatures is important in many applications [55]. The use of a low-temperature active catalyst can lower the emission during the cold start of a car, and volatile organic compounds in exhaust gases from stationary sources might be treated without the need of preheating the gas. Catalytic CO oxidation to CO2 is the most viable option for CO removal at ambient conditions. At ambient temperature (T < 50 °C), catalytic CO oxidation is difficult to sustain due to the competing non-linear influences of strong adsorption and self-poisoning (which preferentially accumulates CO on the surface) versus surface oxidation kinetics which removes CO from the surface [56]. The activity, selectivity, and stability of the catalyst are a fundamental step for improving the catalytic reaction process between the catalyst and reaction gases. The effectiveness of catalytic converter is also depending upon the temperature [57, 58].

4.1 Groups of catalysts for respiratory escape mask applications

In the exhaust gases, CO proves the most perilous so that more attention has been focused on catalytic control of CO gas. A wide variety of catalysts are used in respiratory masks, which initiates the oxidation of CO. The reaction temperature and kinetic reaction rate depend much upon the chemistry of catalyst [38]. The catalytic performance is strongly influenced by the oxygen coordination around their surfaces. The low price and high performance catalysts may have a high perspective to find its appliance to the catalytic reaction [39]. The catalysts are used in escape masks for various applications which are discussed below:
  1. I.

    Hopcalite

     
  2. II.

    Perovskites

     
  3. III.

    Spinel

     
  4. IV.

    Monel

     
  5. V.

    Pyrochlores

     
  6. VI.

    Hydrotalcite

     

These catalysts have been found to be active at low temperature but deactivate rapidly. The deactivation is attributed to carbonate formation and consumption of surface oxygen that makes the particular site lose its activity. The catalyst could be effectively regenerated by light irradiation [59]. The effect of preparation conditions including metal ions concentration, aging time, pH, drying temperature, and calcination temperature is highly effective on the activity of catalyst. The various parameters, which highly affected the activity of resulting catalyst, include calcination temperature, reduction temperature, and reduction time, etc. [60, 61]. The calcination temperature has a strong influence on the chemical composition and physico-chemical properties of the resulting catalyst. Using different supports in the catalyst improved their performance and reduced their cost for CO oxidation. The promoters are the substances that increase the activity of catalyst; they are creating an ideal condition of the catalyst and even enhance the life of catalyst by saving them from poison [62, 63].

4.2 Application of hopcalite catalyst in respiratory escape masks

The hopcalite catalysts have long been used in the removal of environmentally damaging gases over a wide range of temperatures, from ambient through to high temperature. Such catalysts can effectively catalyze the oxidation of dry CO even at room temperature and also active for CO oxidation at low temperature − 20 °C but it fails in respiratory escape masks operating at the higher temperatures due to sintering. The activity of hopcalite catalyst is strongly depending upon the nature of metal ion concentration and their allocation in the crystal lattice [64]. The Mn-based catalysts are very active for CO oxidation and needed tetravalent Mn but, Mn (IV) compounds are usually unstable at the high temperature; therefore, the addition of Cu increased their stability and activity also in respiratory escape masks [65]. The presence of MnO2 in a highly amorphous form is an assertion for a high surface area for contacting with the Cu atoms, which was a prerequisite for the high activity of a hopcalite catalyst [66]. Hopcalite catalyst is highly choice for respiratory protection/e.g. (for respiratory fighter, industry workers, and scuba divers, etc.) (Fig. 7).
Fig. 7

AUROliteTM vs. hopcalite and PtPd/SnO2 technology in escape masks

The higher activity, selectivity, and stability of hopcalite catalysts are the basis of their increasing use in various reactions of respiratory escape masks in firefighting and other fields. It is also use for applications in air purification devices for respiratory protection in aircraft, mining, space labs, closed room burning activities, military and in industrialized emission control [67]. There are fewer synthesis steps that are required for product recovery or separation. The commercial variety of hopcalite catalyst has been synthesized by a solid-state reaction between an active component of CuO and MnO2. The hopcalite catalyst is highly used in the designing of respiratory safety masks for human respiration safety purposes. The addition of various promoters like Co, Fe, Au, Ni, Ce, and Ag enhances the activity of hopcalite catalyst [68, 69, 70]. A lot of interest has been made to modification of hopcalite catalyst to remove its faults of moisture deactivation and lower activity. The preparation of catalysts by other methods including anti-solvent precipitation method, the sol-gel method is reported to give better CO conversion than commercial hopcalite. The addition of noble metals into the hopcalite catalyst not only improved its activity but also reduces the deactivation of catalyst. Hopcalite catalyst must be used in respiratory safety masks due to their large bed sizes. Further, these catalysts deactivate and have short service lifetimes of 15–30 min. Therefore, it gives rise to the need of catalyst for use in gas masks that is highly active and tolerant to moisture that comprise a long active life. Catalysts consisting of gold (Au) nanoparticles (NPs) supported on transition metal oxide carriers such as Al2O3, TiO2, CuMnOx, and FeOx have been shown to highly active for CO oxidation under varying environmental conditions. The use of Au in hopcalite catalyst maintained their stability against moisture at CO oxidation in respiratory devices using the respiratory safety mask catalysts. It is applicable in next-generation respiratory protection devices [59].

4.3 Structural analysis of hopcalite catalyst

A structural analysis of hopcalite catalyst shows that the Cu2+ had displaced Mn3+ from the octahedral sites in the spinel and both the Cu2+ and Mn3+ have substantial Jahn-Teller stabilization energies, major deformation from the cubic symmetry would be expected. The effect of moisture has suggested that Mn3+ (as MnOOH) is transformed to Mn4+ which was assumed to be the active site for O2 adsorption. The highest activity occurs for Cu mole fraction of 0.5 coincidental and the maximum Mn3+/Mn4+ ratio. It has ascribed to the oxidation activity of CuMn2O4 to Mn4+ where:

$$ \mathrm{CO}+{\mathrm{Mn}}^{4+}\to {{\mathrm{CO}}^{+}}_{\mathrm{ads}}+{\mathrm{Mn}}^{3+} $$
(1)
The promotion of Cu has further been associated with the reduction of O2:
$$ 1/2{\mathrm{O}}_2+{\mathrm{Cu}}^{1+}\to {\mathrm{Cu}}^{2+}+{{\mathrm{O}}^{-}}_{\mathrm{ads}} $$
(2)
The oxidation occurs by the process:
$$ {{\mathrm{O}}^{-}}_{\mathrm{ads}}+{{\mathrm{CO}}^{+}}_{\mathrm{ads}}\to {\mathrm{CO}}_2 $$
(3)
The resonance reaction system brings the catalyst back to the active state:
$$ {\mathrm{Cu}}^{2+}+{\mathrm{Mn}}^{3+}\leftrightarrow {\mathrm{Cu}}^{1+}+{\mathrm{Mn}}^{4+} $$
(4)
These Eqs. (1–4) show that the deactivation of this CuMnOx catalyst system cannot be related to the oxidation state of Cu or Mn as long as the redox couple remains active. At lower temperatures, the cubic spinel phase of CuMnOx catalyst was found to contain a significant quantity of impurities. A cubic configuration could only start with excess of copper (x = 0.05) for Cu1 + XMn2–XO4. It indicated that tetrahedral Cu+ and octahedral Mn4+ offer the steadiest environment in copper magnetite spinels. The reaction between CuO and MnO2 or Mn2O3 was accompanied by electron transfer [60].
$$ \mathrm{CuO}+{\mathrm{Mn}}_2{\mathrm{O}}_3\to {\mathrm{Cu}}^{+}\left[{\mathrm{Mn}}^{3+}{\mathrm{Mn}}^{4+}\right]{{\mathrm{O}}_4}^{2-} $$
(5)
$$ {\mathrm{Cu}}^{2+}+{\mathrm{Mn}}^{3+}\to {\mathrm{Cu}}^{+}+{\mathrm{Mn}}^{4+} $$
(6)
The Cu+ preferred a tetrahedral site due to its d10 configuration, producing stable sp3 bonds. Whereas the two types of manganese cation, Mn3+ and Mn4+ that had 3d4 and 3d3 electronic configurations respectively, would be stabilized in octahedral sites through forming dsp2 and d2sp3 hybrid bonds. As the fraction of OH sites occupied by distorting cations (Mn3+) was reduced, the mutual interface between the indistinct OH, liable for parallel configuration, was also reduced, randomizing the orientation and preserving the cubic symmetry. Further discrepancies increase in the literature when considering the valances of Cu and Mn ions in CuMnOx catalyst [61]. The Cu+ in the presence of Mn4+ was the most viable environment in CuMnOx catalyst. The deficiency of deformation where distorting cation occupy a quarter of the OH sites, with the phenomena being observed in manganese containing a mixed of Mn3+/Mn4+ phase. With an increasing presence of Mn4+ in the crystals, the O2− could form three p-bond orbital and overlap with the three neighboring Mn3+/Mn4+ cations or form sp3 hybrid orbital (as in Cu2O) where Cu-O was covalent [62]. The structural analysis of CuMnOx catalyst for CO oxidation is shown in Fig. 8.
Fig. 8

Structural analysis of CuMnOx catalyst for CO oxidation

The Cu-O and Mn-O were intermediate between considered from the covalent and partially ionic radii. The ionization state of Cu in CuMn2O4 was different from that of Cu in other iso-amorphous compounds such as CuCr2O4 and CuFe2O4 where it was Cu2+. The Cr and Fe in these compounds are considered to be present as M3+, whereas in CuMn2O4, the consensus of findings points to Mn4+. The addition of metal cations to the standard CuMnOx has given rise to many interesting properties including increased intrinsic activity, surface area, and better stability of catalyst with increased usage [63].

4.4 Performance of various hopcalite catalysts for CO oxidation

Many hopcalite catalysts have been examined for their activity in CO oxidation. The performance of CuMnOx catalyst is great functions of various parameters such as feed composition, the degree of contact between CO and catalyst, preparation conditions, Cu:Mn molar ratio, and calcination temperature, etc. The redox behavior of Cu and Mn species is most viable to influence their activity in CO oxidation. The oxygen species associated with Cu in CuMnOx catalyst are very active and may be dominated by the low-temperature catalytic oxidation of CO [67]. In characterization points to increase the activity of lattice oxygen associated with Cu species as well as the mobility of lattice oxygen from Mn species. The catalytic oxidation of CO has performed in a series of quick steps, while the adsorption of CO proceeded quickly, therefore, limited desorption of CO2 [68, 92].

$$ \mathrm{Mn}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{Cu}\left(\mathrm{I}\mathrm{I}\right)\rightleftharpoons \mathrm{Cu}\left(\mathrm{I}\right)+\mathrm{Mn}\left(\mathrm{I}\mathrm{V}\right) $$
In most of the above cases, the oxidation of CO has been carried out at a low temperature. An amorphous CuMnOx catalyst with the highest measured for CO oxidation reactions and optimum molar ratio of (Cu:Mn) produced the highest catalytic activity, and also contained the largest quantity of highly active CuMn2O4 catalyst [69]. Hutching and co-workers envisioned a coupled of dehydration reaction between hydrated Cu and Mn oxides and also discussed the acid-base properties of reactants were used to support. CO is basic and O2 is acidic; it was expected that hydrated Cu oxide will dehydroxylate and hydrated Mn oxide will protonate to form moisture [55, 56, 57, 58]. This process would form a relatively unstable Cu3+ cation but the Mn3+ would remain unchanged. The excess electron on MnOO’ would be transferred to Cu3+ generating Cu2+ [70].
$$ \mathrm{CO}\to {\mathrm{Cu}}^{2+}\hbox{---} {\mathrm{Mn}}^{3+}\leftarrow {\mathrm{O}}_2 $$
(7)
$$ {\mathrm{O}}_2\to {\mathrm{Cu}}^{3+}\hbox{---} {\mathrm{Mn}}^{4+}\leftarrow \mathrm{CO} $$
(8)
These Eqs. (7–8) were supported by the fact that Cu2O and MnO2 are active catalysts for CO oxidation. The crystallographic properties are expected due to the presence of two types of Jahn-Teller ions, Mn3+ and Cu2+. The distribution of metal cations among the tetrahedral and octahedral sites, as well as the valances of Cu and Mn ions, is still weakly understood [71]. The light-off characteristic was representing the activity of catalysts with the increasing of temperature. The characteristic temperature T10, T50, and T100 corresponds to the initiation of oxidation, 50% conversion, and full conversion of CO, respectively. The increasing of temperature increases the specific surface area and pore volume of the catalyst; it causes an activity of the catalyst has been increased. The shape or morphology of the particles is also an important factor which influences the catalytic activity of metal catalysts. In crystalline shape, the exposed facets present on the catalyst surfaces improve the catalytic properties of metal crystals. The rate of CO conversion (Table 3) over bridged to linear species is a slow process, one mold observe slow chemisorptions in the high-pressure region. The process most likely involves transformations from structures which bind CO tightly to those of lower binding energy.
Table 3

The operating parameters and activity measurement of various CuMnOx catalysts prepared by different methods for CO oxidation

Catalyst

Catalyst preparation method

Operating parameters

Remarks

References

Hopcalite catalysts and their derivatives

Prepared by co-precipitation method

CuMn2O4

Co-precipitation method

The 100-mg catalyst with feed gas consisted of 5% CO in He (5 mL/min) and O2 (50 mL/min) at temperature 20 °C and total GHSV was 33,000 h−1

CuMn2O4 (T100 = 20 °C for Time = 12 h)

[55]

CuMn2O4

Co-precipitation method

The 100-mg catalyst; usually CO (5% CO in He, 5 mL/min) and O2 (50 mL/min) and GHSV was 33,000/h

CuMn2O4 (T50 = 32 °C, T100 = 80 °C)

[56]

CuMn2O4

Co-precipitation method

The 100-mg catalyst, (0.45 vol% CO), at flow rate (20 mL/min) with CO (5% CO in He, 5 mL/min) and O2 (50 mL/min) and GHSV was 33,000 h−1

CuMn2O4 (T70 = 30 °C const., Time = 30 min)

[57]

CuMn2O4

Co-precipitation method

The 100-mg catalyst in the feed gas (0.25% CO, 5% O2 balance He) and entire gas flow rate was 60 mL/min

CuMn2O4 (Xco = 80%, at temp. 30 °C, Time = 10 h)

[58]

CuMnOx

Co-precipitation method

The 100-mg catalyst, with a gas mixture (1% CO, 99% dry air) was feed at a rate 80 mL/min

CuMnOx (T80 = 25 °C for time 10 min)

[72]

Au/CuMnOx

Co-precipitation method

The reaction conditions: 100 mg catalyst, total flow rate 22.5 mL/min, molar ratio (CO:O2:He = 1:89:10), reaction temperature 30 °C and a time on-line of 1000 min

Au/CuMnOx (T70 = 30 °C constant, Time = 1000 min)

[59]

Cu1.5Mn1.5O4

Co-precipitation method

The total flow rate was 1000 mL/min; (4 vol.% CO; 20 vol.% O2, balance He) and heating rate 12 °C min−1, with weight of catalyst was 100 mg, (SV being 310,000/h)

Cu1.5Mn1.5O4 (Ti = 75 °C T50 = 110 °C, T100 = 120 °C)

[60]

CuMnOx

Co-precipitation method

The 250-mg catalyst with specific velocity was 45,000 h−1, time 50 min and initial concentration of CO was about 3 × 10−5 to 5 × 10−5 mol L−1

CuMnOx (Ti = 40 °C T65 = 100 °C const.)

[61]

Cobalt promoted CuMnOx

Co-precipitation method

The 100-mg catalyst with 5000 vppm CO in air and GHSV velocity was 33,000 h−1

Cobalt promoted CuMnOx (Ti = 80 °C T50 = 120 °C, T65 = 160 °C const.)

[62]

CuMnOx

Co-precipitation method

The reaction conditions are 30 °C temperature, 5000 vppm CO in air, GHSV of 33,000 h−1 with the weight of catalyst was 100 mg

CuMnOx (Ti = 16 °C T50 = 30 °C const., Time = 90 min)

[25]

CuMnOx

Redox method

The 100-mg catalyst consisted of 1% CO, 2% O2, and 5% N2 in He, space velocity 35,000 mL h−1gcat−1

CuMnOx (T50 = 25 °C, T100 = 35 °C)

[28]

Au/CuMnOx

Co-precipitation method/deposition precipitation method

The Au/CuMnOx catalysts aged for 0.5 h at 25 °C temperature, 5000 vppm CO in air, GHSV = 12,000 h−1

Au/CuMnOx (T100 = 50 °C, Time = 20 min)

[63]

Mesoporous CuMnOx

Redox method

The 100-mg catalyst in presence of 1% CO, 1% O2, 60% H2 balanced N2 with a total flow rate of 50 mL/min and space velocity 35,000 mL/h gcat

Mesoporous CuMnOx (Ti = 4 °C,T50 = 12 °C, T100 = 25 °C)

[73]

CuMn2O4

Conventional precipitation method

The 50-mg catalyst at temperature 25 °C with space velocity 12,000 h−1, presence in a premixed cylinder (5000 ppm CO in air)

CuMn2O4 (T70 = 25 °C for 40 min)

[67]

CuMnOx

Co-precipitation method

The 200-mg catalyst with space velocity: 20,000 mL h−1gcat−1 and feed gas consisted of 1% CO, 20% O2, and 79% N2

CuMnOx (Ti = 0 °C T50 = 30 °C, T100 = 50 °C)

[68]

CuMnOx catalyst doping with Co3O4, CeO2, and AgO2

Co-precipitation method

The 0.7839-g catalyst in presence of 1% CO in air with a total flow rate of 60 mL/min

CuMnOx (T35 = 48 °C, Time = 120 min)

CuMnOx-Co3O4 (Ti = 20 °C, T50 = 30 °C, T80 = 54 °C, Time = 120 min)

CuMnOx-CeO2 (Ti = 20 °C, T50 = 46 °C, T100 = 65 °C, Time = 110 min)

CuMnOx-AgO2 (Ti = 20 °C, T50 = 30 °C, T100 = 45 °C with Time = 120 min)

[69]

CuMnOx

Co-precipitation method

The 200-mg catalyst with feed gas (1% CO-20% O2–79% N2) and SV was 20,000 mL g−1 h−1

CuMnOx (Ti = 13 °C, T50 = 30 °C, T100 = 70 °C)

[64]

Cu supported CeMnO2

Co-precipitation method

The 210-mg catalyst in the feed gas composition (2% CO, 2% O2 and 96% H2) with space velocity 20,000 h−1

Cu/CeO2 (Ti = 55 °C,T50 = 80 °C, T100 = 130 °C)

Cu/Ce0.9Mn0.1O2 (Ti = 30 °C,T50 = 65 °C, T100 = 120 °C)

[70]

CuMnOx addition SnO2

Co-precipitation method

The 100-mg catalyst with feed gas composition (1% CO, 21% O2 balanced N2), overall flow rate 30 mL/min and resultant space velocity was 18,000 mL/h/gcat

CuMnOx (Ti = 30 °C, T50 = 56 °C, T100 = 85 °C)

CuMnOx-10.6 wt.% SnO2 (Ti = 20 °C, T50 = 30 °C, T100 = 40 °C)

[71]

CuXMn3-X O4

Precipitation method

The 50-mg catalyst in presence of 0.8% CO, 20% O2 in Ar and heating rate 2 °C/min with a total flow rate of 1.63 mL/s

CuXMn3-XO4 (Ti = 25 °C, T50 = 50 °C, T100 = 80 °C)

[16]

Prepared by sol-gel synthesis method

CuMnOx

Sol–gel method

The 100-mg catalyst with space velocity 30,000 mL/g h at reaction temperature 60–120 °C with total flow rate 50 mL/min

CuMnOx (Ti = 60 °C, T30 = 120 °C const.)

[74]

CuMnOx

Sol–gel method

The (Cu/Mn = 1/2) at a flow rate: 50 mL/min with composition (1 vol% CO, 1 vol% O2, 60 vol% H2 and balance N2)

CuMnOx (Ti = 30 °C T50 = 80 °C, T100 = 120 °C)

[23]

Prepared by impregnation method

CuOx additives Mn2O3 and Cr2O3

Impregnation method

The 100-mg catalyst with a gas mixture (1% CO, 1% O2, 50% H2 and balanced N2) and overall flow rate was 60 mL/min and GHSV was 30,000 h−1

CuOx-Mn2O3 (Ti = 50 °C,T50 = 160 °C, T100 = 220 °C)

CuOx-Cr2O3 (Ti = 60 °C,T50 = 190 °C, T100 = 225 °C)

[75]

CuO/MnO2

Incipient wetness impregnation method

The 50-mg catalyst in presence of 1% CO, 99% dry air with a feed rate 20 mL/min, and space velocity was 24,000 mL/gcat/h

CuO (Ti = 140 °C,T50 = 200 °C, T100 = 415 °C)

MnO2 (Ti = 60 °C,T50 = 100 °C, T100 = 160 °C)

CuO/MnO2 (Ti = 30 °C,T50 = 60 °C, T100 = 100 °C)

[37]

Prepared by reactive grinding method

CuMnOx

Grinding and precipitation method

The 50-mg catalyst in presence of (5000 ppm CO in air) at a flow rate of 22.5 mL/min

CuMnOx (Ti = 120 °C, T50 = 190 °C, T100 = 240 °C)

[76]

CuMn2O4

Grinding ball mill method

The 50-mg catalyst with a flow rate of 21 mL/min and GHSV was 12,000 h−1 at temp. 25 °C, 5000 ppm CO in air

CuMn2O4 (XCO = 40% at temp. 25 °C, Time = 72 h)

[77]

Prepared by flame spray pyrolysis method

Cu1.5Mn1.5O4

Precipitation method and flame sprays pyrolysis

The 100-mg catalyst (0.67 vol.% CO, 66.00 vol.% N2 and 33.33 vol.% O2) at temp. 300 °C for 20 min and total flow rate was 100 mL/min

Cu1.5Mn1.5O4 nanoparticle (Ti = 25 °C, T50 = 50 °C, T100 = 110 °C)

[78]

CuO/Ce-MnO

Urea nitrate combustion method

The 500-mg catalyst in presence of 1vol. % CO in air with GHSV was 52,000 mL/(g h)

Ce-Mn-O (Ti = 225 °C, T50 = 270 °C, T70 = 350 °C const.)

CuO/Ce-Mn-O (Ti = 100 °C, T50 = 120 °C, T100 = 160 °C)

[79]

In the spinel CuMn2O4 catalyst containing more than one transition metal ion per unit, there are distribution and valence sites of the cations among both the tetrahedral (A sites) and octahedral (B sites) sub-lattices of the spinel structures. The electronic configuration of Cu(I) occupies in the CuMn2O4 spinel structure, the octahedral sites, where they are subjected to larger extra-atomic relaxation energy [74, 75]. In the CuMnOx catalyst presence of manganese oxide ability to absorb activate oxygen and subsequent, the inclusion of copper is to improve and stabilize active phases. The structural defects connected with the oxygen vacancies and Mn2O3 high distribution on the catalytic surface facilitates the catalyst reducibility [76]. A series of magnetic and structural investigations of CuMn2O4 lead to a number of researchers suggesting Cu and Mn be the most stable configuration of the mixed oxide. Both configurations have been supported by electronic measurements. Metal oxides and mixed metal oxide systems with spinel-like structure have received significant consideration because of their great electrical and magnetic properties. These properties are influenced by the nature of metal ions and their allocation in the crystal lattice of CuMnOx catalyst [77, 78].

4.5 Mechanism of CO oxidation over hopcalite catalyst

The efficiency of solid catalysts for reactions with stable molecules is depending upon the chemisorption processes. The chemisorption of the reacting gases is an important step, which increases the concentration of reactant on the catalyst surfaces which inducing the adsorbed molecules processing on high energy to be easy to get the chemical reactions. The discrete reaction mechanisms are steady with the observed kinetics. The first mechanism shows that the mostly accepted CO oxidation reaction on a CuMnOx catalyst surface involves O2 adsorption to form O2* precursors, which split on a vicinal vacancy [79]. In the second mechanism, O2 activation occurs via the kinetically applicable CO*-assisted O2 dissociation step without the specific concern of stable O2* precursors represented in Fig. 9. In the CO oxidation process, the oxygen is first adsorbed on the CuMnOx catalyst surface with the energy of activation. When the temperature is high in a significant amount so that the adsorption of oxygen reaches enough proportions, any CO passing over the catalyst surfaces either reacts directly with the adsorbed oxygen or else is initial adsorbed then reacts, after which the CO being produced was desorbed [68, 69, 70].
Fig. 9

Mechanism of CO oxidation over a hopcalite catalyst

The similar nature of CuMnOx catalyst was also synthesized by the redox method and could be one of the major factors contributing to their high catalytic activity. The co-precipitation method is also allowed for the preparation of amorphous catalysts with high surface areas and high catalytic activity [80]. A better tool for measuring the performance of hopcalite catalyst for CO oxidation reported the activation energy of the process. It is significant to develop the kinetic expressions for catalytic oxidation of CO also because they can be implemented into CFD models useful for optimization and reactor design [68, 69, 70, 71].

Chemical kinetics establishes the factors, which can influence the rate of reaction under concern and provide clarification for the measured value of rate and leads to the rate equations, which are valuable in reactor design [8]. The kinetics study of CuMnOx catalyst was shown in Fig. 10 that the highest activity and slowest deactivation rate in comparison to other catalysts. Early study indicates that the catalyst starting oxidized CO before is oxidized by air and this is an investigation of a Mars-van Krevelen-type mechanism, which has consequently found support. The conversion of CO by the Mars-van Krevelen mechanism would explain the relationship between easiness of catalyst activity and reducibility [80]. The oxidation of CO over CuMnOx catalyst could be described very well by the Langmuir-Hinshelwood model, in which molecularly adsorbed CO reacts with dissociative adsorbed oxygen in the first-order reaction [61, 81]. The amount of reactant consumed and product formed can be monitored as a function of surface composition of the catalyst (Table 4).
Fig. 10

Kinetics study of hopcalite catalysts for CO oxidation

Table 4

Activation energy for CO oxidation over CuMnOx catalysts

Catalyst

Activation energy (KJ/moL)

Temperature (°C)

Reference

CuMnOx

57

70

[61]

CuMnOx

64.08

90

[80]

CuMnOx

36.98

45

[81]

CuMnAgOx

30.65

35

[8]

CuMnAuOx

28.65

30

[63]

In contrast, a surface composition test, the catalyst composition does not modify significantly because many surface molecules are much lower than the number of surface active sites/species. The areal rate of a given catalyst for CO oxidation is defined as the moles of CO oxidized per unit surface area of catalysts per unit time [63].

4.6 Addition of gold in hopcalite catalysts

Pure gold has been considered for many years as almost catalytically inactive; however, recent works have clearly shown that gold is very active for low-temperature oxidation of CO, when it is highly dispersed over hopcalite catalyst. Since the pioneering work by Haruta, gold-based catalysts show the exceptional activity for low-temperature CO oxidation and good stability under moisture [82]. This surprising discovery, given that the long terms chemical inertia of gold, has completely changed the way scientists look at this magic metal (Fig. 11).
Fig. 11

Gold supporting hopcalite catalyst

The introduction of gold will introduce new active sites into the hopcalite catalysts that are associated with gold nanoparticles. It also increases the reducibility of catalyst significantly compared to unmodified hopcalite and most easily reduced catalyst was the most active, indicating that the liability of lattice oxygen was an important factor influencing activity. Recently, there has been incorporated a new type of catalysts 3 M nanogold having modified features and advanced activity [83, 84]. The nanogold catalyst has the following features:
  1. a.

    Efficient removal of carbon monoxide in high humidity

     
  2. b.

    Efficient removal of carbon monoxide below room temperature

     
  3. c.

    Effective in high and low concentrations of carbon monoxide

     

The activity of Au was divided into two broad categories: factors related to the gold particle itself and factors related to the oxide support. The size, thickness, and shape of gold particles all fall in the first category and their oxide support is considered a passive supporting material, the primary role being to provide sites for the nucleation and growth of gold nanoparticles [85]. One of the primary explanations for gold activity for CO oxidation is that there are under coordinated gold atoms, the prevalence of which is related to the size, thickness, or shape of the gold particles supporting over hopcalite catalyst [86].

4.7 Hopcalite catalyst deactivation

The activity and selectivity of hopcalite catalyst in escape masks are crucial for respiration protection. The longtime active catalyst is an essential requirement of the masks; it can be defined as the time over which the catalyst performance can be maintained. The catalyst deactivation and loss over time in respiration masks is trouble of huge and ongoing concern in the practice of catalytic process. The cause of hopcalite catalyst deactivation is mainly divided into three parts: chemically, mechanically, and thermally [71]. The lead, sulfur poisoning, carbon formation, and sintering is the main cause of catalyst deactivation (Fig. 12) in respiratory masks. The hopcalite catalyst present in respiratory masks is easily deactivated by a trace amount of moisture present in the catalyst. To reduce the deactivation of hopcalite catalyst present in respiratory masks added a small amount of Ag, Au, and Rh into the catalyst to increase the lifetime of catalyst. In addition, to increase the rate of CO oxidation, a further improvement of incorporating Au into the CuMnOx catalyst is that lower levels of deactivation are observed [83, 84, 85, 86].
Fig. 12

Deactivation of hopcalite catalyst

The chemical poisoning of hopcalite catalyst is also referring to the incomplete or complete deactivation by coverage to a wide range of chemical compounds. Supporting the hopcalite catalyst on alumina had the consequence of stabilizing the catalyst with respect to high-temperature heat treatment. The hopcalite catalyst is easily deactivated by the presence of moisture. The decreasing of hopcalite catalyst activity due to the accumulation of several tightly adsorbed fouling materials on the catalyst surface is a huge commercial importance since the rate of deactivation of hopcalite catalyst governs in many ways the economic design and process of respiration mask protection [87]. The deactivation is causing by a loss of catalytic surface area due to the crystalline growth of catalytic phase and collapse of the catalyst pore structure. The oxidative regeneration process is also able to reverse the major deactivation of catalyst. The regeneration of deactivated heterogeneous catalysts is highly depending on the chemical, economic, and environmental factors [88].

4.8 Development of hopcalite catalyst in respiratory protection masks

The main objective of this study to find out the suitable hopcalite catalyst application in respiratory protection masks by literature survey. The best combination of hopcalite catalyst corresponds to removal of 2500 ppm CO down to 130 ppm, for a flow rate of 30 LPM for minimum of 30 min. Hopcalite catalysts are highly advantageous for applications where high contacting efficiency is required. The preparation conditions of hopcalite catalyst for getting excellent catalytic activity are as follows: Cu/Mn molar ratio is 1/8, drying temperature of 110 °C, duration of drying 24 h, calcination at 300 °C for 2 h [89]. The optimum operating parameters for CO oxidation was 100 mg weight of catalyst at a flow rate of CO (1.5 mL/min) with the optimum particle size of 60 μ(micron) was used. The literature study shows that the precipitation and calcination conditions that were applied in the preparation procedure are crucially important. The influence of suitable amounts of promoters (Au/Ag/Ni/Co/Fe/Ce/Sn) in hopcalite catalysts improved the performance of low-temperature CO oxidation catalysts [90]. This body of literature can be further utilized to come up with a suitable model for promoted hopcalite catalyst. This model can also be helpful in predicting layering of various particle sizes. For a gamma alumina supported hopcalite catalyst, it is frequently desirable to determine the exposed metal area in distinction to the total surface area [91]. This can be achieved by measuring the uptake of a CO gas that is chemisorbed on the hopcalite catalyst but negligibly on the support, under conditions that allow monolayer coverage. A qualitative reaction model could be proposed with the help of surface chemistry of catalyst. A layer of 2-mm thick, when incorporated in a respiratory protection mask of 500 cm2 area, can provide protection from 2500 ppm CO for 29 min. Thermal behavior of reaction differs vastly in packed bed and diluted packed bed mode. The packaging issues need to be resolved as these materials may not be used for few years and they should still be active [27, 32].

5 Conclusions

The main reason for the losses of life in respiratory is not only the respiratory itself by poisoning caused the presence of CO. This work is applicable for developing respiratory protection systems for applications in military, mining, and space devices. There are various types of catalysts that have been investigated to remove CO from the breathing air by incorporating into the gas mask or breathing apparatus. The hopcalite catalyst is one of the most prominent transition metal oxide catalysts for low-temperature CO oxidation. The addition of appropriate promoters, supports, pretreatment, and advanced preparation methods would lead to improvements in the activity of hopcalite catalyst towards CO oxidation. Hopcalite catalysts are active for a longer time and would be tolerant of moisture and impurities in reacting gases. Addition of gold particle into the hopcalite catalysts improved its performance and reduces the deactivation of catalysts. The promoting and un-promoting hopcalite catalyst developed here can be very effective to remove CO from the breathing air by incorporating it into a gas mask or breathing apparatus. This review paper provides essential information about the synthesis of breathing masks for protection of a respiration system from CO gases.

Notes

Acknowledgments

The authors would like to express his gratitude to the Department of Civil Engineering and Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University) Varanasi, India, for their guidance and support.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Subhashish Dey
    • 1
    Email author
  • Ganesh Chandra Dhal
    • 1
  • Devendra Mohan
    • 1
  • Ram Prasad
    • 2
  1. 1.Department of Civil EngineeringIIT (BHU)VaranasiIndia
  2. 2.Department of Chemical Engineering and TechnologyIIT (BHU)VaranasiIndia

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