Abatement of formaldehyde with photocatalytic and catalytic oxidation: a review

Abstract: Formaldehyde is one of the vital chemicals produced by industries, transports, and domestic products. Formaldehyde emissions adversely affect human health and it is well known for causing irritation and nasal tumors. The major aim of the modern indoor formaldehyde control study is inviewof energycapacity, product selectivity, security, and durability for efficient removal of formaldehyde. The two important methods to control this harmful chemical in the indoor environments are photocatalytic oxidation and catalytic oxidationwith noblemetals and transitionmetal oxides. By harmonizing different traditional photocatalytic and catalytic oxidation technologies that have been evolved already, here we give a review of previously developed efforts to degrade indoor formaldehyde. The major concern in this article is based on getting the degradation of formaldehyde at ambient temperature. In this article, different aspects of these two methods with their merits and demerits are discussed. The possible effects of operating parameters like preparation methods, support, the effect of light intensity in photocatalytic oxidation, relative humidity, etc. have been discussed comprehensively.


Introduction
Most of the people spend their time at shopping center, home, car and office, etc. Sometimes, there are more pollution levels indoors compared to outdoors. The common indoor air pollutants are volatile organic compounds (VOCs), carbon oxides (CO and CO 2 ), nitrogen oxides (NO x ), and particulate matter etc. The possible ways of emission of VOC are from chemical industrial plants, transportation, painting, electroplating, etc. (Wang, Ang, and Tade 2007). There are several origins inside the indoor which contaminate the air. Due to the adverse effect of these contaminants a term "Sick building syndrome (SBS)" has used for the buildings. Filtration and proper ventilation are commonly used for the purpose of the solution to indoor air pollutants as per the typical technology. But hazardous effects arise due to the microbiological and VOC contamination of air (Goswami 1999a).
So, for restraining pollution inside the building, several techniques have used like air exchange rate and the recent one is by using air purifiers. These air purifiers generally use the filters to remove the particulate matters or sorption material to adsorb odors or gases. But these techniques are not much efficient because they can't eliminate them completely (Zhao and Yang 2003). VOCs e.g. benzene, formaldehyde (HCHO), xylene, and toluene are always discharged in both outdoor and indoor environments through various methods. Long-time divestment to contaminated surroundings though having a very few ppm of VOCs may source of long term sicknesses (Tang et al. 2006a). As per the various health agencies, VOCs are measured as a carcinogenic chemical for human being and longtime contact leads to diseases such as asthma and caused decrement in functioning of pulmonary part. Also, long time expose to formaldehyde can leads to nasopharyngeal cancer. These VOCs have a very close relationship with sick building syndrome which is an important term used by occupants to represent reduced comfort or health. Fresh painting indoor may produce diseases like asthma (Norbäck, Edling, and Wieslander 1994). A little amount of HCHO can be hazardous to human being in case of long term aspects which produces serious health problems like nasal tumors, skin irritation, mucous membranes irritation of eyes etc. (Li et al. 2016a). In addition, the release of VOCs to the environment can form ozone layer depletion, urban smog in the greenhouse effect and the stratosphere (Liang, Li, and Jin 2012). The cumulative investigation is needed to overwhelmed hazards by VOCs to human healthiness and our environment has caused fast appreciation of toxic aspects of VOCs throughout the past two decades (Goode 1985). It is significant to change injurious volatile organic compounds (VOCs) into the environment-sociable compounds. Various methods have been proposed for reduction of VOCs like physical adsorption (Domingo-Garcia et al. 1999;Matsuo et al. 2008), plasma technology (Liang et al. 2010), plant absorption Terelak et al. 2005), photocatalysis (Akbarzadeh et al. 2010;Fu, Zhang, and Li 2011;Photong and Boonamnuayvitaya 2009), and catalytic oxidation (Hu et al. 2015;Sekine 2002;Tang et al. 2006b).
Inside airstream can contain other hazardous contamination like microorganisms (Goswami 1999a). A combination of electrostatic precipitation and photocatalytic oxidation (PCO) has been found as an interactive effect to destroy contamination of air by the microorganism. Removal of microorganism can be done through the electrostatic precipitation. It brings microorganism on the surface coated with photocatalyst (Goswami 1999b). Unlike many other contaminants in indoors which can satisfactorily have diminished by many processes applied, formaldehyde can't be correctly measured just due to its higher vapor pressure i.e. 3883 mm Hg at room temperature i.e. 25°C and relatively very low boiling point temperature i.e. −19.3°C (Pei and Zhang 2011). Subsequently catalytic oxidation process needed decreased temperature for degradation of HCHO, the current investigation is grounded on a catalyst for the complete oxidation at environmental conditions. The possible main sources of HCHO emissions are industries, building materials, and transportation (vehicles running by mainly diesel engine) (Singh and Chauhan 2016). Conventional methods, like using filters to remove the contaminants or by sorption materials (like granular activated carbon) to absorb various VOCs are the methods that can only decrease concentration of contaminants in environments. These methods can only alter the stage of the contaminants relatively than the removing them entirely. For HCHO degradation various methods have been reported including (a) chemisorption (b) biological/botanical filtration (c) thermal and non-thermal catalytic oxidation, (d) plasma decomposition with or without using catalyst, and (e) photocatalytic oxidation (PCO).
Plasma cluster ions generated by plasma disintegration process also has the capacity to degrade the HCHO. Generation of plasma is simply through electronic and photoionization the targeting factor through plasma process are density of ion, processing timing, and arrangement of chemicals (Ding et al. 2006). Byproducts and small amount of HCHO are the restriction of plasma technology. Plasma catalyst degradation of HCHO is sometimes problematic due to over formation of ozone (Yuan et al. 2020). In current time botanical filtration is also investigated for degradation of HCHO (Wu et al. 2003). For adsorption of HCHO, biomass bricks are also analyzed and have given an alternate option for diminishment (Zhang et al. 2019). Modified photocatalytic oxidation (PCO) is also a good removal technique for HCHO. Traditional UV light has replaced by visible light in latest PCO for the diminishing HCHO. For diminishment of HCHO, the most suitable photocatalyst TiO 2 with varying absorption of noble metal (Pt, Au, Pd, Ag, etc.) and oxides of transition metal are generally used. Zhu and Wu (2015) has used Pt/TiO 2 photocatalyst for abatement of HCHO.
In this review, there is a thorough description of the photocatalytic oxidation and thermal and non-thermal catalytic oxidation for the degradation of HCHO due to their merits among all mentioned process.

Photocatalytic oxidation process
A typical PCO process consists of light energy companionship of water vapor. Water vapors are used for production of necessary hydroxyl radicals to diminish the VOCs and microbes (Goswami 2003). Water vapour and carbon dioxide (CO 2 ) are the harmless byproducts of degradation of VOCs using PCO process with semiconductor (Tompkins 2001). The aforementioned ability of PCO attained focus which generally destroys the hydrocarbons with the use of TiO 2 photocatalyst. For regulation of CO 2 i.e., regulated emission there is a carbon dioxide cycle available in nature to balance the concentration of CO 2 in the environment but for the VOCs, there is no such cycle available (Singh and Chauhan 2016). A patent has been filed for disinfecting the indoor air from the clothes, curtain, coverings, etc. and used it for producing the pure air of heating, ventilating, and air conditioning systems (Goswami 2008).
This invention is based on the PCO process in that it uses lightening the surface of photocatalyst with the origin of photons. The photocatalytic oxidation process combines water, oxygen, and VOCs on the catalyst surface to produce H 2 O and CO 2 under light radiation. Researchers have used nano Titania (TiO 2 ) widely as a catalyst which is activated by light energy. Titania has been proven as effective photocatalyst. Photocatalysis technology using TiO 2 has sufficient potential for the purification of organic pollutants and wastewater (Hashimoto, Irie, and Fujishima 2005;Hoffmann et al. 1995;Wu et al. 2003). There are further several photocatalysts which have been used for degradation of HCHO which are ZnO, Al 2 O 3 , ZrO 2 , WO 3 , SnO 2 , CeO 2 , ZnS, Fe 2 O 3 and CdS (Hoffmann et al. 1995). But the most popular photocatalyst used for oxidation of formaldehyde into harmless gases is TiO 2 and ZnO. TiO 2 is the readily available, relatively inexpensive, chemically stable element. Holes generated by photo-oxidation process are highly oxidizing in nature (Jin et al. 2007). Fe 2 O 3 -TiO 2 supported with Pt doping is also an excellent catalyst for the decomposition of formaldehyde (Yang et al. 2000). TiO 2 doped with Ag metal also shows enhanced photocatalytic activity for the devastation of bacterial spores as a comparison to typical photocatalysis (Vohra et al. 2005). Arranging the photocatalysts into the active catalysts between Ti, Zn, and tungsten; the result shows that: TiO 2 (anatase) < ZnO < WO 3 . There is generally two crystal form of TiO 2 available: anatase and rutile. The difference between these two structures is of band gaps. The band gaps of anatase and rutile phases are 3.23 and 3.02 ev respectively.

Working principle
When TiO 2 is irradiated by light, electron and hole pairs are generated in the bulk particle. While producing holes, electrons gain energy to leave from valence band to conduction band with crossing the band gap. The holes in valence band have great oxidative power and due to this, it makes hydroxyl radicals with reaction of absorbed hydroxide ions. This formation process is accountable for photo oxidation of HCHO. But alone TiO 2 has the restriction in case of application because of requirement of UV light and lesser solar radiation coming to Earth surface. Although this restriction has been eliminated by expansion of optical retaliation of TiO 2 up to visible light from UV. These UV lights are harmful to the human body. Decomposition through irradiation of light on catalyst surface is shown in Figure 1.
Electrons and holes permit oxidation and reduction on the photocatalyst surface (Ohtani 2008). The corresponding equation of generation of holes and electron due to irradiation of photon can be written as: In above reaction process, holes are responsible for oxidation process while electrons for reduction which is also shown in Figure 2.
Reduction reaction : The role of hydroxyl radical (OH*) is important as formaldehyde gas is passed through the photocatalytic oxidation device, it oxidizes the gaseous HCHO into harmless CO 2 and H 2 O. These hydroxyl radical generally derived from oxidation of water vapor or adsorbed hydroxyl anion.
Different activity during the PCO has various advantages (Ollis 2000); -It is recognized as the safe process which commonly uses the anatase TiO 2 as a photocatalyst, an n-type semiconductor found also in some toothpaste and pharmaceutical suspensions. -During the oxidation process, final source of oxygen is molecular oxygen which is a safer oxidant as compared to hydrogen peroxide or ozone etc. -The photocatalysis process generally requires room temperature. -As mentioned above hydroxyl radicals are photogenerated in the surface of Titania which is simpler process since there are many mechanisms available for the oxidation process.
UV-Vis light is responsible for the creation of electronholes pairs in the photocatalyst during PCO of VOCs.

UV responsive photocatalysts
Emphasize on different types of defects in TiO 2 like surface oxygen vacancies, Ti interstitials and oxygen atoms for high impact on the photo-dissociation of HCHO. These defects have a high impact on the thermal excitation of oxygenated organic compounds on TiO 2 (Benz et al. 2009;Bowker and Bennett 2009;Landis et al. 2012;Li and Diebold 2009;Lu, Linsebigler, and Yates 1994;Wang et al. 2003;). Due to wide energy bandgap titanium oxide activates only through ultraviolet (UV) region. TiO 2 is activated through the irradiation of photon (hν) on the surface with ultra-band energy from the UV light source. Formation of electrons and holes caused oxidation of absorbed toxic organic pollutants and form CO 2 and H 2 O (Zou et al. 2006). All the literature tills now had shown that the surface of noble metal sides has the most active site for the oxidation process of HCHO (Kecskés, Raskó, and Kiss 2004). In different semiconductor materials, ZnO has bandgap relatively equal to TiO 2 (Xu and Schoonen 2000) and the origin of ZnO is quite wide. But UV activation of ZnO was not easy till suitable morphological differences invented. It is quite understood that defects influenced the photocatalysis (Gupta, Barick, and Bahadur 2011;Shi et al. 2011). Therefore, by improving the morphologies and the defects in ZnO enhance the photocatalytic properties. UV activation of ZnO enhances by etching the sample with Argon (Ar) (Liao et al. 2013 (Liang, Li, and Jin 2012). The reason for increasing photocatalytic oxidation due to doping is increase of chemisorbed oxygen at the surface of TiO 2 . It creates an imbalance charge vacancy on the catalyst surface.

Visible light responsive photocatalysts
Since UV sources are power consuming, costly, and hazardous to health, visible sources are readily available, inexpensive and energy saving. To enhance the spectrum, range of photocatalyst from UV light to visible light; three methods have been used: transition metal doping into TiO 2 ; doping of nitrogen into TiO 2 and with sensitizing dyes. Transition metal ions like Mn, Fe, V, Cr, Co, Ni or Cu increases the spectrum region into the visible light region (Zang et al. 2000). Presence of metals like Li + , Ce 3+ , Co 3+ , Zn 2+ , Cd 2+ , Cr 3+ , Fe 3+ , Al 3+ , Mn 2+ , and Pt sufficiently change the photocatalytic activity of TiO 2 . Different nitrogen-doped TiO 2 were used as photodegradation of gaseous HCHO (Zang et al. 2000). In the case of visible light irradiation TiO 2−X N X has better photocatalytic activity than TiO 2 . The addition of ZrO 2 intoTiO 2−X N X possesses higher porosity, higher surface area, and enhanced thermal stability rather than TiO 2−X N X samples . A very few literatures have quoted the effect of noble metal on photocatalyst. Noble metal doping into TiO 2 has a higher capability to transfer electrons. In the case of Pt-TiO 2 , Pt plays the role of core of core-shell Pt-TiO 2 which can increase the photo-activity for abatement of HCHO (Zhu and Wu 2015). To enhance the spectrum, range of photocatalyst from UV to visible range, gold (Au) nanoparticle support was used on TiO 2 because of its high visible light absorption (Tsukamoto et al. 2012;Wang and Astruc 2014;Yang et al. 2014). The preparation of Au nanocatalyst affects the catalytic activity process. Atmospheric pressure oxygen has strong activity for CO oxidation which activates the Au/TiO 2 nanocatalyst (Zhang et al. 2015a).
There is some literature available on silver ion (Ag + ) (Liu et al. 2003;Venkataraj et al. 2002) which suppressed the recombination of charge at the surface of TiO 2 . Doping of silver into TiO 2 reduces the anatase growth rate which ultimately results in low crystallinity of anatase structure with comparison to pure TiO 2 , the crystallite size of Ag-doped TiO 2 decreases. TiO 2 -Ag and TiO 2 -Ag-TEA permit higher degradation rates as compared to pure TiO 2 (Kocareva, Grozdanov, and Pejova 2001). The different state of silver Ag + , Ag 0 , and metal Ag at the surface of TiO 2 suppresses the recombination of electron-hole which leads to an increase the photocatalytic activity of TiO 2 (Ubolchonlakate, Sikong, and Tontai 2010). The mechanism involves; HCHO degradation by TiO 2 or TiO 2 composite can be written as: HCOOH + 2h + → CO 2 + 2H + Metal and non-metal doping are suitable techniques for improving the photo-activity of Titania. Doping metal atoms depressed the recombination of electrons and holes while doping nonmetal atoms can generally influence the different properties of titania such as lattice structure, band gap (Asahi et al. 2001;Khan, Al-Shahry, and Ingler 2002;Lin et al. 2007;Ohno et al. 2004). Photocatalyst doped with silver ion has been developed by Vohra et al. (2006) to devastate the airborne microbes. The result shows that photocatalyst doped with Ag can be used effectively for high concentrated contaminants. Zhao et al. (2004) have quoted that spectrum region and photocatalytic activity can be improved by doping with non-metal boron and metal oxide Ni 2 O 3 . Zhang et al. (Liu et al. 2008) have reported that strong absorption in the visible range for nitrogen nickel doped Titania. Absorption line is moved to lower energy region for the different proportion of nitrogen nickel doped titania such as N (0.010) Ni (0.015) TiO 2 (600), while no absorption for samples Ni (0.015) TiO 2 (600) and pure TiO 2 (600) have observed. The bandgap energies were 2.29, 2.30, 3.17 and 3.15 ev have been reported for sample N (0.010) Ni (0.015) TiO 2 (600), N (0.010) TiO 2 (600), pure TiO 2 (600) and Ni (0.015) TiO 2 (600).
It shows the decrease in the bandgap due to the mixing of nitrogen. Transition metal ions doping in TiO 2 inject high energy ions into the TiO 2 lattice during ion implantation techniques. Anpo, Tanahashi, and Kubokawa (1980), Anpo et al. (1985Anpo et al. ( , 2000, Yamashita et al. (1998) have experimented with the transition metal doping like V, Cr, and Fe on TiO 2 . Lam et al. (2007) has reported that in absence of Cr ion; even 1.2 mW/cm 2 visible intensity light could not activate the photo-activity of TiO 2 . After injection of Cr ion Cr/TiO 2 thin film gets activated for visible light. Heterostructured combination of waste zeolite and graphitic carbon nitride supported on TiO 2 has prepared by Liu and Lin (2019) under the market available LED light for diminishment of HCHO. The decomposition of formaldehyde in the range of UV and visible light at different wavelengths is given in Table 1.

Kinetics and mechanism involved
For progression of the photocatalysis, generally initial steps are as below: Ti 4+ + e − → Ti 3+ (10) The absorbed formaldehyde on the surface of catalyst will be oxidized by hydroxyl radicals and superoxide ions which are also termed as highly reactive components. During this process some byproducts would also have formed which will be converted in CO 2 by O • 2 or O • oxidation. Photo-degradation process for formaldehyde in the steps of oxidation and reduction can be inscribed as follows (Ding et al. 2006): Oxidation HCOOH + 2h + → CO 2 + 2H + Reduction     The above reactions illustrated that four h + is stoichiometrically required for complete oxidation of one HCHO molecule.

Kinetic experiments of photocatalytic oxidation
Extracting data from kinetic experiments is the prime factor for the optimum design of the reactor. An ample number of the test has been performed by Goswami, Trivedi, and Block (1997) by changing the influencing parameters like water vapor content, the intensity of light, air velocity, etc. over the TiO 2 photocatalyst for varying the reaction timing for the destruction of contaminants.

Light sources and intensity
In photocatalytic process recombination electron-hole pair and interstitial charge transfers are second-order and first-order processes respectively which show that while increasing the light intensity rate of recombination of electron and holes increases which ultimately lower the quantum efficiency . By literature it has been noted that reaction rate is directly proportional to light intensity for the low intensity of light and is however proportional to the square root of intensity of light under medium light intensity and totally independent of light intensity under high light intensity.

Effect of water vapor
Total photocatalytic decomposition of formaldehyde into CO 2 gets decreased due to the insufficient water vapor. 50% relative humidity along with sufficient stay time creates an efficient quantity of water molecules to cause inactivation of microorganisms (Goswami 1998). On the other hand, on surface of catalyst, the presence of excess water vapor can also reduce the reaction rate because excess water vapor can block active sites on the surface. The effect of water vapor on formaldehyde oxidation can analyze as: with an increase of water vapor from low to high, the oxidation rate will also increase proportionally but after that as water vapor will cross the optimum level the reaction rate immediately start decreasing. For the below, 10 ppmv concentration of formaldehyde with relative humidity ranges between 15 and 60% Langmuir-Hinshelwood suggests 1st order oxidation rate for formaldehyde (Zhao and Yang 2003). Up to 35% RH the decomposition of formaldehyde into CO 2 increases after that it will start diminishing hence 35% RH turns to be the optimal condition for degradation rate (Liang, Li, and Jin 2012).
In existence of water vapor, the increased reaction rate is observed due to the behavior of hydroxyl radicals as a hole trap to create surface occupied hydroxyl radicals. During photocatalysis hydroxyl radicals carried out two action, one is capturing of HCHO molecules and the second one is reducing the electron-hole recombination (Liang, Li, and Jin 2012).

Contaminant concentration
The concentration of reactants may also have an impact on the reaction rate similar to light intensity and relative humidity. Different reactant proportion alters the reaction rates. As per Noguchi et al. (1998), HCHO concentration above the 600 ppmv reduces reaction rates. Deng et al. (2017) have also given the range of initial concentration to determine the order of the reaction. According to him HCHO ranging from 35 to 107 ppm does not affect the conversion of formaldehyde. Langmuir-Hinshelwood model directly relates between contaminant concentration and reaction rates. Generally, the actual concentration of VOCs is lesser in normal indoor but for the investigation of the oxidation process, there is a need for higher concentration.

Photocatalytic reactors
Basically the surface of reactors layered with catalyst is used to degrade the VOCs. Flat plate and annular reactor are the basic structure for the evaluation of kinetic models. Other than this, for achieving greater surface area different models are design which ultimately improves performance and provide better radiation on catalyst surface. Commercially available examples are: monolith, packed bed and fluidized bed photo catalytic reactors. With some improvisation for increasing the catalyst area other reactors are corrugated plate (Passalía, Alfano, and Brandi 2012), multi plate, multi annular (Zazueta, Destaillats, and Puma 2013), foam packed bed (Ibhadon et al. 2007) and paper based immobilized TiO 2 reactors (Adjimi et al. 2014).
For greater air inlet, plate type and annular type reactors were not commercially appropriate. Though these types' reactors are only used for investigating different kinetic parameters. For appropriate design and greater conversion over unit mass of catalyst, packed bed reactors are usually available. This reactor also has the limitation of unit maintenance. For overcoming this fluidized bed reactors are the option with low pressure drop. Monolith reactors possess increased flow, low pressure drop and compact design. This has also limitation of quick diminishment of light intensity. This limitation can overcome with improvisation of optical fibers passing with each monolith (Boyjoo et al. 2017).
To overcome the different drawback of photo catalytic reactors several improvements have been done to increase the efficiency. For example: Qin et al. (2020) has used annular reactor to provide uniform illumination of catalyst and increased flux flow of the contaminants. This modified photocatlytic reactor is widely used in practical use. They have concluded that degradation efficiency of HCHO with foil catalyst annular photo reactor was 2.28 times greater than commercial TiO 2 powder catalysts.

Catalytic oxidation
Due to the formation of harmless CO 2 and H 2 O through highly effective total degradation of HCHO, photocatalytic oxidation was a demanding option for complete catalytic oxidation of HCHO. Various by-products which are harmful in nature like CO were found in the degradation of HCHO ). Another disadvantage that makes PCO insufficient is that catalytic surface gets deactivated by time. Moreover, a catalyst possesses high reactivity with the confirmation of restricted energy requirement for the process of catalytic oxidation. At an increased temperature i.e. thermal or at room temperature i.e. non-thermal, catalytic oxidation uses a catalyst for diminishment of VOCs. Hence, at ambient temperature catalytic oxidation possess assuring option for diminishment of VOCs emission since it converts VOCs into CO 2 (Zhang, Jiang, and Shangguan 2016). Catalytic oxidation also termed as a low temperature (293-673 K) degradation process and it is termed as powerful post remedial technology.
The major advantage of catalytic oxidation includes the formation of CO 2 and H 2 O from HCHO with no intermediate pollutants (Bai, Arandiyan, and Li 2013;Pei and Zhang 2011). General catalysts in catalytic oxidation process increase the chemical reaction by providing sufficient activation energy (E a ) which is shown in Figure 3. Transition metal oxides (Co, Ag, Ni, and Mn) (Chen et al. 2009;Imamura et al. 1994;Ma et al. 2014;Tang et al. 2006b;Wen et al. 2009;Xu et al. 2011) and supported noble metal (Pd, Rh, Pt and Au) (Arzamendi et al. 2009;Huang and Leung 2011a;Imamura et al. 1991;Kim, Park, and Hong 2011;Li et al. 2011;Sekine 2002;Zhang and He 2007;Zhang, He, and Tanaka 2005;Zhang, He, and Tanaka 2006;Zhang et al. 2012) catalysts are the established catalysts for the HCHO degradation at room temperature.

Noble metal catalysts
For degradation of HCHO, noble metals support effect the catalytic activity strongly. Many researchers have been proposed in support Pt, Pd, Au, Rh which is having strong catalytic activity and stability. Complete oxidation of formaldehyde can be achieved through noble metal. In Figure 1, it has been shown that activation energy requirement after noble support gets decreased and ambient temperature is sufficient for oxidation of HCHO. Among all noble metal catalysts, Pt support affects strongly and it is sufficient to do oxidation at ambient temperature. Generation of CO 2 and H 2 O on noble doped supports is shown in Figure 4.

Pt supported noble metal catalyst
Platinum has a very high dispersion degree over TiO 2 (Zhang, He, and Tanaka 2005). Zhang, He, and Tanaka (2005) have reported that Pt/TiO 2 catalyst has a 100% conversion of HCHO. Peng and Wang (2007) has reported that at 20°C HCHO conversion was only 14.3% but at 120°C the conversion becomes 97% even at 1% Pt loaded TiO 2 . The optimized use of Pt is a great concern till now since it is   highly expensive from other catalysts. A very low concentration of Pt even at 0.1 wt% at TiO 2 possesses excellent conversion of HCHO at ambient temperature (Huang and Leung 2011b).
A comparative catalytic activity has been shown with taking pure TiO 2 and Pt-TiO 2 is given in Table 2. The removal efficiency has been remarkably increased by using such catalyst. With the various pre-treatment of Pt, the valence state of Pt can be changed which ultimately influenced the HCHO removal efficiency ). MnO 2 have better catalytic efficiency rather than other transition metal oxide. Yu et al. (2011) has made catalyst Pt/MnO 2 to test their performance. They reported that catalytic activity of Pt loading on urchin (support) like MnO 2 vary in a fashion of 2 wt% Pt/ urchin like MnO 2 > 1 wt% Pt/urchin-like MnO 2 >> urchin-like MnO 2 . Preparation through colloidal deposition route Pt supported iron oxide (Pt/Fe 2 O 3 ) showed excellent catalytic activity and long duration stability over HCHO oxidation ). An et al. (2013) has prepared Pt/Fe 2 O 3 catalyst with several methods like co-precipitation, impregnation, and colloidal-deposition method. They found out that relatively higher catalytic activity for the colloidal deposition method (Pt/Fe 2 O 3 -CD) among different methods used. Due to its high dispersion degree, Pt could weaken the Fe-O bond presented at the Pt/FeO x interstitial and ultimately fasten the active oxygen HCHO oxidation. Extremely low Pt content was proved to be the dominant for catalytic oxidation of HCHO at ambient temperature, although practical aspect of this type of catalyst diminishes with the period of time i.e. not active.
Alkali metal ions (like Li + , K + and Na + ) have strong stabilization ability. Pt-O (OH) x -alkali-metal species have stabilized atomic dispersion on the surface of the catalyst and it became an efficient low surface catalyst. Zhang et al. (2012) have shown a low-temperature catalyst z% Na-1% Pt/TiO 2 with varying loadings of Na (z = 0, 1, 2) at a gas hourly space velocity (GHSV) of 12,000 h −1 . They found out that HCHO completely conversed at 15°C while in the case of 1% Na and 0% Na addition the conversion was 96 and 19% at the same temperature respectively. Without Pt, Na can be washed out from TiO 2 surface hence Pt only can have stabilized it on TiO 2 . These results can also be reported for Pt/SiO 2 with the addition of alkali ions (Zhai et al. 2010). Due to formation of Pt-(ONa) x species, Pt species doped by Na were oxidized more as compared to Pt/TiO 2 Peng and Wang 2007). The result proposed by Zhang, He, and Tanaka (2005) shows that the particle size of Pt is affected by the addition of Na as 10 (Na free), 4.8 (2% Na) and 6.8 Å (1% Na). Addition of alkali metal ion can suppress the Pt-Pt bond in presence of O 2 and deepens atomically dispersed Pt-O species on TiO 2 surface. Most of the work has been proposed on the support of TiO 2 Leung 2011a, 2011b;Li et al. 2011;Nie et al. 2013;Yu et al. 2011;Zhai et al. 2010;Zhang et al. 2012;Zhu et al. 2011). Different positive influence of alkali metal on Pt/ MnO 2 has been analyzed by Chen et al. (2014). To increase the catalytic activity of birnessite and cryptomelane by noble metal support, various studies have been done till now (Liu et al. 2012a;Yu et al. 2011). Chen et al. (2014) has investigated the effect of salts (Na 2 CO 3 , Na 2 SO 4 , and NaNO 3 ) over Pt/MnO 2 and found the increasing effect of anions in salt as the following manner: CO −2 3 > SO −2 4 > NO − 3 . HCHO conversion at 50°C over the Na 2 CO 3 supported catalyst was found 100%.
Since, residual anions (SO −2 4 , and NO − 3 ) were col lected on the surface of Pt/MnO 2 hence effective surface area gets reduced with Na 2 SO 4 and NaNO 3 which ultimately influence the physiochemical aspects of the catalysts ).
The advantages with using Na 2 CO 3 support is that CO −2 3 got burn at the higher temperature which ultimately exposed the higher specific surface area and pore volume for increasing catalytic degradation of HCHO oxidation. During practical uses generally all the catalysts adding noble metal which are structurally granular or powder possess great value of pressure drop. They also possess some engineering limitation like dust pollution, removal of nano catalysts with corresponding gases and so on. Hence, due to low pressure drops and forming quality, macroscopically fabricated catalysts are more considerable (Groppi and Tronconi 2005;Kołodziej and Łojewska 2005). Yu et al. (2013) has investigated the diminishment of HCHO over Au 0.5 Pt 0.5 /MnO 2 /cotton catalyst and found that 15 wt% Au 0.5 Pt 0.5 /MnO 2 /cotton possess best performance among 25 wt% Au 0.5 Pt 0.5 /MnO 2 / cotton, 10 wt% Au 0.5 Pt 0.5 /MnO 2 /cotton, and 6 wt% Au 0.5 Pt 0.5 / MnO 2 /cotton. They also reported that cotton surface has gully shape which increases the superficial area of cotton and it does not affect the decomposition of HCHO itself. Cotton fiber just promotes the adsorption of HCHO. Zhang and He (2007) has proposed a comparative data for the catalytic over TiO 2with 1 wt% noble metal supports (Pt, Rh, Pd, and Au). They resulted in the catalytic activity of Pt/TiO 2 >> Rh/TiO 2 > Pd/ TiO 2 > Au/TiO 2 >> TiO 2 . They also found that 1 wt% of Pt/TiO 2 catalyst could decompose the HCHO into CO 2 and H 2 O in a GHSVs of 50,000 h −1 even at ambient temperature. So, format species are potent surface variant on the Pt/TiO 2 and Rh/TiO 2 including with absorbed CO variant in Pt/TiO 2 . As in case of Pd/TiO 2 and Au/TiO 2 , formation of format species and its degradation into CO is not influencing in nature.

Pd supported noble catalysts
Better thermal stability and comparatively low cost of Pd over precious metal catalysts (like Pt and Au dependent Metal catalyst (Peng and Wang )               catalysts) make it beneficial of use. But it requires a high temperature for oxidation. 1% of Pd nanoparticles can efficiently oxidize HCHO at ambient temperature (Vijayakrishnan et al. 1992). Huang et al. (2013a) have also found out that the deposition precipitation method is more capricious than the impregnation method in the case of Pd/ TiO 2 catalysts due to larger capacity for production and transfer of chemisorbed oxygen. The removal efficiency of HCHO with Pd wt% in the case of impregnation method shows the trend in the following manner: 8.5, 93.2, and 19% removal efficiency for 0.1, 0.5, and 1% Pd. Expensive cost of Pd limits the high Pd loading. Alkali metal as an electronic or textural conductor prefers as a catalyst for several catalytic processes including NO (Xu et al. 2014a(Xu et al. , 2014b and CO oxidation (Mirkelamoglu and Karakas 2006;Yentekakis et al. 1994). Zhang et al. (2014) has conducted a test on 2Na-Pd/TiO 2 catalyst at a GHSV of 95,000 h −1 . They found out that 100% HCHO decomposition into CO 2 and H 2 O at 25°C with a feed concentration of 140 ppm of HCHO.
Result proves that Na species act as an inductor and stabilizer for negatively charged and well-dispersed Pd variants that promotes water and chemisorbed oxygen's activation.
A series of alkali metal (K, Li, Na and Cs) doping have investigated on Pd/TiO 2 support by Li et al. (2016b). They resulted in K > Cs > Na > Li like the trend in the condition of 550 ppm HCHO and a GHSV of 640,000 h −1 over Pd/TiO 2 support. The highest Pd dispersion degree with active sites is showing by K-Pd/TiO 2 catalyst among all in ambient temperature HCHO oxidation. Li, Zhang, and He (2017) have reported the optimized value of loading at Pd/TiO 2 support is 2 wt% in a GHSV range of 80,000-190,000 h −1 and RH of 25-65%. They found an activity trend of 2.0 Na-Pd/TiO 2 > 1.0 Na-Pd/TiO 2 > 4.0 Na-Pd/TiO 2 > Pd/TiO 2 among the different loadings of sodium. Galvan et al. (Huang et al. 2013b) has reported that total combustion of HCHO had achieved at 220°C temperature over 18.2% Mn/ Al 2 O 3 catalyst. But due to the addition of 0.1% Pd this temperature gets reduced to 90°C (Álvarez-Galván et al. 2004). At the surface of Pd-Mn/Al 2 O 3 , several activities have involved for the decomposition of HCHO. The relative rates are PdO > MnO x >> Al 2 O 3 . Interstitial oxygen of the PdO cluster has high activity in oxidation of HCHO (Cordi and Falconer 1996).

Supported gold catalysts
Earlier gold was assumed as the poor heterogeneous catalyst due to its electronic structure. However, after the discovery of Haruta et al. (1989), highly dispersed particles of metallic gold permit co-oxidation at relatively low temperatures. This was just the initiation of the application of gold as the catalyst. Gold catalysts have been reported as active catalysts in hydrogenation of carbon oxide, in water gas shift reaction and most importantly in oxidation of VOCs (Delannoy et al. 2010;Dobrosz-Gómez, Kocemba, and Rynkowski 2008;Hosseini et al. 2012;Wang et al. 2011;Yi et al. 2010aYi et al. , 2010b. In spite of the high-temperature requirement for total oxidation of HCHO, Fe 2 O 3 -, CeO 2 -, and ZrO 2 -supported Au has been proved as an active catalyst (Hong et al. 2010;Li et al. 2007). Chen et al. (2013) has prepared Au/TiO 2 catalyst by two methods i.e. deposition precipitation (DP) with urea and with NaOH to examine the catalytic activity of Au/TiO 2 for decomposition of HCHO oxidation. They reported that Au/CeO 2 (DPU prepared) shows higher catalytic activity than Au/CeO 2 (prepared through DPN) due to higher active surface oxygen variant even at high GHSV of 143,000 h −1 . It has been illustrated that presence of Au weakens surface Ce-O bond (Scire et al. 2003;Serre et al. 1993). Li et al. (2008) have prepared a series of gold /iron-oxide catalysts for the decomposition of HCHO through co-precipitation method. They critically observed the decomposition of HCHO by varying the Au loading from 0.73, 2.52, 4.85 to 7.10 wt% on gold iron supported catalysts and found that 7.10 wt% of gold content showed highest catalytic activity.
They suggested that gold species exist in fractional charged gold atoms (Au δ+ where δ < 1) which is a transition state between cationic gold (Au + ) and metallic gold. Liu et al. (2012a) have fabricated 3-D ordered macro-porous (3DOM) Au/CeO 2 catalyst with a different process named colloidal crystal template process joined with a precursor complexion technique. They noticed that 3DOM Au/CeO 2 catalysts decomposes 30% HCHO at ambient temperature and 100% decomposition showed at ∼75°C for ∼0.56 wt% of Au. Mutually connected spherical voids promoting the less aggregation and fine dispersion of Au nanoparticles may credit for their improved catalytic activity for the 3DOM structure of Au/CeO 2 . Deactivation of 3DOM Au/ CeO 2 catalyst happened due to formation of carbonate and hydrocarbon at eon surface of 3DOM Au/CeO 2 . For enhancement of result, another test using 3DOM Au/ CeO 2 -Co 3 O 4 catalysts fabricated through precursor thermal decomposition assisted colloidal templating technique has carried out (Liu et al. 2012a). The result shows that catalyst 3DOM Au/CeO 2 -Co 3 O 4 acquired proper 3DOM shape with adjustable pore sizes. Their phase structures, compositions, and elemental valence state on the surface can be monitored by regulating the molar ratio (Ce/Co). With the help of thermal decomposition of Ce and Co oxalate precursor, the nanowalls pore size of 3-4 nm were made in 3DOM Au/CeO 2 -Co 3 O 4 . They achieved 100% decomposition of HCHO at ∼39°C for 3DOM Au/CeO 2 -Co 3 O 4 catalyst.

Supported silver catalyst
The different oxidation states of silver (Ag) permit it to use with different supports. Zhang et al. (2015b) has used different supports (TiO 2 , Al 2 O 3 , and CeO 2 ) for analyzing their catalytic activity doped with Ag. Results show that Ag/TiO 2 exhibit superior catalytic activity among them with 100% oxidation of HCHO at 95°C followed by 110, 125°C by Ag/Al 2 O 3 Ag/CeO 2 respectively. TiO 2 , Al 2 O 3 , and CeO 2 have approximately equal activation energy but high dispersion degree of silver and size factor over supports create a difference. Tang et al. (2006a) have used the mixed oxide (MnO x -CeO 2 ) doped with Ag to improve catalytic activity of mixed oxides. Result shows that due to oxygen transfer mechanism 100% oxidation of HCHO takes place at 373 K over Ag/MnO x -CeO 2 catalyst. To use the silver more effectively Bai and Li (2014) has prepared 3D-Ag/ Co 3 O 4 with addition of K + ions catalyst. Result shows that addition of Ag into 3D-Co 3 O 4 decreases the conversion temperature to 110°C in GHSV of 30,000 h −1 .

Reaction mechanism and pathway for catalytic oxidation of formaldehyde
A generalized reaction mechanism is not illustrated because of characteristic of HCHO molecule and catalyst is the factors on which the legitimacy of every mechanism depends. For VOC s oxidation, there are possibly three mechanisms i.e., Eley-Rideal, Langmuir-Hinshelwood and Mars-Van Krevelan (Liotta 2010;Ordóñez et al. 2002).
As per the L-H mechanism, kinetics of reactions between the solid and gas phase can be quantitatively treated (Lam et al. 2007). According to this, the surface reaction between initial contaminants and oxidizer (absorbed molecules) is the basic process. In the E-R mechanism, the gas phase absorbed particle retaliates with contaminant particles. Further, in case of the MVK mechanism, it is twostage redox reaction mechanisms that involve the oxidation and reduction of VOC particles and oxygen. The redox states of the MVK mechanism are as follows. (a) First, catalysts will be oxidized by oxygen in gas phase to create an oxygen adsorption site (O). This (O) reacts with the absorbed particles or contaminants in the air. (b) Simultaneously in the 2nd step pollutant molecules will reduce the oxidized catalyst (Ding et al. 2006).
The following are the two steps where k o represents as the oxygen rate constant, k i represents as constant for surface reaction rate, and blank symbol () depicts an oxygen adsorption site.
Generally, MVK mechanism is broadly applied for HCHO oxidation over platinum catalysts (Liu et al. 2012a). Rapid formation of format species from the oxidation of HCHO is a major step for conversion into CO 2 and H 2 O is performed in two steps i.e.
In case of Pt/TiO 2 , at metal-support interface, after the formation of oxygen vacancies, oxygen absorbed on the active sites with packing the vacancy by oxygen molecule and coordinating with the Ti 4+ as an adatom (Alexeev et al. 2005). The formation of a high intensity of oxygen species greatly influences the oxidation reaction. For sodium-free Pt/TiO 2 catalysts, the reaction mechanism includes the formation of formate species which is purely degraded into CO species (Zhang, He, and Tanaka 2006). This formed CO species ultimately converted into CO 2 after exposure of the catalyst to O 2 and finally CO 2 leaves the surface. So the reaction mechanism includes the following steps for the decomposition of HCHO into CO 2 i.e.
In the decomposition process, rate demonstrative step is the conversion of formate into CO. Zhang et al. (2012) has studied reaction mechanism of decomposition of HCHO into CO 2 over Sodium doped Pt/TiO 2 catalyst with using in situ DRIFTS. They reported that on the surface of the catalyst, two types of surface (OH) groups were formed at 3695 and 3600 cm −1 . After 60 min, a complete reaction occurred between active OH groups and formate because of the disappearance of HCOO − species from the base.
This reaction gives a new preferable path for the decomposition of HCHO into CO 2 i.e. surface hydroxyls and formate reaction are preferred rather than degradation of formate into CO. Zhang, He, and Tanaka (2006) have also proposed two different paths for the decomposition process which are when deficiency of OH group occurs on the catalyst surface, the formation of surface CO starts from formate species.
But in the case of an abundant amount of OH groups present, decomposition of formate into CO 2 takes place.
A similar result has given by Zhang, He, and Tanaka (2006) that in case of Pt/TiO 2 and Rh/TiO 2 catalysts, effective surface species are formate species along with absorbed CO molecules on Pt/TiO 2 . Figure 5 order of the reaction for catalytic degradation of HCHO on the noble metal (Au, Pd, Rh, and Pt) catalyst supported with TiO 2 . Ma et al. (2011) has further defined the steps of catalytic reaction over Au/Co 3 O 4 -CeO 2 catalyst. The first step has defined for reaction is adsorption of HCHO on surface of Co 3 O 4 . •CHO species on surface forms due to adsorption of HCHO.
Now (C-H) formate is attacked by active surface oxygen to form bicarbonate (HCO − 3 ) species, and this bicarbonate (HCO − 3 ) will meet with H + to form carbonic acid. CO 2 will form due to dissociation of H 2 CO 3 which is shown in Figure 5. Liu et al. (2012a) has further explained the reaction mechanism of 3DOM Au/CeO 2 the catalyst for enhancing the catalytic oxidation of HCHO. They explained the reaction mechanism in two steps catalyzed with ionic Au 3+ and metallic Au 0 and suggested that higher catalytic performance has shown by ionic Au 3+ . Hereafter adsorption of HCHO takes place onto the CeO 2 surface. This process includes the formation of HCOOH and Au 0 with the transformation of active oxygen from Au 2 O 3 . Now formate species would be formed due to the interaction of CeO 2 and HCOOH. CO 2 would be produced with the complete oxidation of HCOOH. Chen et al. (2013) has elucidated the reaction intermediate using in situ DRIFTS for Au/CeO 2 catalysts. They also gave a similar reaction for decomposition of HCHO into CO 2 which involves four basic steps for the decomposition. The first major step includes the formation of formate species due to oxidation of HCHO in the deficiency of O 2 and/or H 2 O.
In water containing feed gas, generation of hydroxyl groups absorbed the HCHO and then these OH groups oxidize the reactant.
Now hereafter oxidation of equations (33) and (34) could form CO 2 on Au/CeO 2 catalyst. Tang et al. (2006b) has investigated the catalytic oxidation of HCHO over Ag/MnO x -CeO 2 catalysts. For oxidation purposes, he used CeO 2 which is famous for the oxygen equipped catalyst. They proposed the mechanism in a way that oxidation of HCHO is performed with the oxygen released due to the degradation of Ag 2 O. Further, the formation of Ag 2 O is continuously achieved through the reoxidation of Ag from MnO 2 and oxygen from CeO 2 regenerate the Mn 2 O 3 to MnO 2 . Simultaneously re-oxidation of CeO 2 from Ce 2 O 3 is formed through the oxygen of feed gas.

Metal oxide catalysts used in catalytic oxidation
After searching alternate to noble metal some metal oxide and transition metal oxides found as an active substitute (Reed and Ceder 2004). Due to different valence state and electronic configuration catalysts made through transition metal oxide possess strong catalytic activity for decomposition of volatile organic pollutants (Gluhoi, Bogdanchikova, and Nieuwenhuys 2005;Popova et al. 2010;Sinha et al. 2008;Wang and Li 2010). For VOCs oxidation, metal oxides play a strong role such as manganese, chromium, cobalt, copper, silver, iron, vanadium, and nickel. These are relatively cheaper catalysts with noble catalysts.

MnO x support
There are generally two forms available of MnO x i.e. cryptomelane and birnessite. It was investigated that MnO x shows a strong influence to decompose HCHO at low temperatures. Crptomelane type MnO 2 is chemically made of mixed-valence state of manganese (Mn 4+ , Mn 3+ and Mn 2+ ) and physically composed of big open layers or channels (Villegas et al. 2005). Tang et al. (Xingfu et al. 2006) have synthesized manganese oxide octahedral molecule sieve (OMS-2) nanorods for decomposition of HCHO formaldehyde at a lower temperature. They observed that 100% conversion at 353 K temperature with neglecting the intermediates such as CO and formic acid. For MnO x powder the temperature required for the degradation of MnO x was found at 373 K which ultimately proves the morphological impact. Chen et al. (2007) has conducted a test for the decomposition of HCHO with monodisperse manganese oxide honeycomb and hollow nanospheres.
Jingjing et al. (Pei, Han, and Lu 2015) has prepared a hopcatile catalyst (CuO-MnO 2 ) to examine the removal efficiency of indoor formaldehyde. They found that at room temperature (25°C, 50% RH, and 180 ppb concentration level indoor) removal efficiency was 30% which is a quite good removal performance at room temperature. The proposed reaction process over CuO/MnO 2 between oxygen and formaldehyde as follows: where K = reaction ppbm/s; K HCHO , K H 2 O , K O 2 , and K CO 2 are the absorption equilibrium constant respectively ppb −1 . A hybrid catalyst based on graphene-MnO 2 has been examined as a new catalyst for effective low-temperature oxidation of HCHO by Lu et al. (2016). This hybrid designed catalyst showed strong removal efficiency and indicating a much low 100% removal at 65°C temperature. They have resulted that the incorporation of graphene nanosheet exposes not only active surface but also promoted a large

Co oxide catalyst
Co 3 O 4 let strong catalytic performance for a different field. Bai, Arandiyan, and Li (2013) has performed a comparative study on nano-Co 3 O 4 , 3D-Co 3 O 4 and 2D-Co 3 O 4 for catalytic oxidation of HCHO at 400 ppm HCHO concentration and GHSV of 30,000 mL/(g h). At 130°C complete conversion of HCHO is getting through 3D-Co 3 O 4 in space velocity of 30,000 mL/(g h) and termed as best the catalyst among all. They found that 3D-Co 3 O 4 has best catalytic activity among 2D-Co 3 O 4 and nano-Co 3 O 4 . They found that the stability of HCHO conversion is also long when tested over 160 h. Ma et al. (2011) has evaluated the performance of Co 3 O 4 -CeO 2 and Au/Co 3 O 4 -CeO 2 which was made through the nano casting method. The evidence of the test result clearly shows the effect of morphology. Mesoporous nanocasted Co 3 O 4 catalyst yield CO 2 about 20.3% at 25°C in contrast to 2.5% CO 2 through Co 3 O 4 which were made through the precipitation method.

Al 2 O 3 catalyst
Due to high surface area of Al 2 O 3,  have used it for catalytic oxidation of formaldehyde at room temperature. The observation shows that no catalytic oxidation of formaldehyde takes place at room temperature in CO 2 , but at the catalyst surface adsorption takes place. When adsorption reached to saturation point at the catalyst surface, the intensity of HCHO decreased to zero. The time of conversion of formaldehyde to CO 2 decrease as 2100-700 and 400 min as the increase GHSV 10,000-30,000 and 50,000 h −1 . Li et al. (2014) has worked on the mixed oxide catalyst and investigated the catalytic activity for Mn x Ce 1−x O 2 (x varies like 0.3, 0.5, 0.7, and 0.9) for the removal of HCHO. They prepared Mn x Ce 1−x O 2 with different methods (pechini) from trend and observed that for x = 0.5, 100% removal efficiency has found at 270°C for GHSV of 10,000 h −1 . They arranged the result in the varying Mn fraction. The trend follows: T100 (100% completion at temperature) values are 285, 270, 295, and 310°C for x = 0.3, 0.5, 0.7, and 0.9 for catalyst Mn x Ce 1−x O 2 . The decomposition of formaldehyde due to different catalyst is given in Table 2 at room temperature.

Operating parameter for catalytic oxidation
For preparing strong influencing catalysts and increasing removal efficiency of HCHO, it is important to understand significance of the operating parameter of catalytic oxidation. Catalytic activity is directly affected through operating parameter hence optimized value is mandatory.

Effect of relative humidity
Atmospheric moisture promotes catalytic activity of HCHO oxidation. This has been verified through the diffuse reflectance infrared Fourier transform (DRIFT) that presence of moisture improves performance of the hydroxyl group. There have been found some contradictory reports that indoor moisture blocks the active site by adsorbing on them at low temperatures (Skrobot et al. 2003). The un stable condition for the catalyst is arisen due to presence of H 2 O 2 which helps in thorough loss of activity. In another literature, it has been found that presence of indoor moisture depreciated catalytic activity of TiO 2 supported gold catalyst (Bollinger and Vannice 1996). However, Kwon et al. (2015) have reported that in presence of water vapor excellent catalytic activity have been observed with the Pt loading of more than 0.5%. Even they reported that 100% Figure 6: Oxidation of manganese oxide on graphene support (Lu et al. 2016).
conversion of HCHO with the 0.1% platinum concentration.
In the absence of moisture, conversion decrease to 8% with 0.1% of platinum loading. The result was confirming by the DRIFTs that formation of hydroxyl group in presence of humidity improves results. This leads to the conversion of HCHO in formate even absence of atmospheric oxygen. Zhao et al. (2012) has investigated the impact of water vapor on reaction stability and conversion of HCHO into CO 2 over MnO x catalyst. Their report comes in the following form: for dryair flow approximate over 80% HCHO was converted into CO 2 by ozone and only 20% of CO 2 comes out. But for RH = 55%, ∼oxidation of HCHO into CO 2 reported 100% with enhanced reaction stability. This conversion was supported by formation of O 3 which thoroughly oxidized HCHO into CO 2 . A mechanistic study has been done by Huang et al. (2013b) for investigating the role of water vapor over Pd/TiO 2 catalyst. They reported that 0.1% Pd made by deposition precipitation method shows strong activeness over HCHO decomposition and stable in the highly humid air. Analysis also include dissociated hydroxyl radical from water vapour support th process of adsorption and oxygen movement over the Pd/TiO 2 catalyst. They resulted that for RH = 50%, 100% HCHO conversion was observed while it was limited to 78.5 and 54.5% for 25% RH and 0% RH respectively.

Effect of calcination temperature
Calcination temperature plays an influencing role in catalytic oxidation of HCHO. Tang et al. (2006a) has synthesized MnO x -CeO 2 catalyst at different calcinated temperature and resulted that below calcination temperature of 773 K due to making of MnO x -CeO 2 solid solution. No diffraction peak related to manganese oxide has been observed and at the calcinated temperature of 773 K due to the phase isolation between MnO 2 and CeO 2 , a clear diffraction peak observed for MnO 2 .

Effect of preparation method
Preparation of catalyst for catalytic oxidation involves different processes such as Co-precipitation, impregnation, colloidal deposition method, microemulsion, sol-gel, and modified co-precipitation. An et al. (2013) has prepared a catalyst Pt/Fe 2 O 3 for catalytic oxidation of HCHO using different methods which are co-precipitation, impregnation, and colloidal deposition methods which are listed in Table 2.
Result shows that enhanced dispersion of Pt causes weakening of Fe-O bond at interface of Pt/FeO x and hence improves the mobility of involved oxygen in oxidation of HCHO. Yu et al. (2011) have used high dispersion and small nano-sized Pt particles to improve the catalytic activity of Pt/MnO 2 for HCHO oxidation using reflux the method and hydrothermal method which is also shown in Table 2. Tang et al. (2008) has used impregnation technique for preparation Pt/MnO x -CeO 2 catalyst to decompose HCHO at room temperature. But An et al. (2013) has analyzed a different method to give a comparative view using co-precipitation (CP), impregnation (IMP), and colloidal deposition (CD). They resulted in higher catalytic activity for CD rather than CP and IMP method and proved through TEM result that at the surface of catalysts below 3 nm Pt particles were formed. Hence, it is clear that preparation method affects the structure of support and catalysts redox property. Tian et al. (2012) has used microemulsion process for birnessite type manganese oxides catalysts to increase the high surface area (upto 154 m 2 /g).

Effect of feed rate of HCHO
The performance of catalytic oxidation of HCHO varied with fluctuation in feed rate of HCHO. The reaction rate of catalytic oxidation is much dependent on the molar ratio of oxygen to HCHO. Phenomena have been illustrated that lower the concentration ultimately decrease the light off temperature for catalytic oxidation of VOCs (Veser, Ziauddin, and Schmidt 1999). Peng and Wang (2007) has experiment the performance of noble metal catalyst with different feed rate of 100, 240, 400 mg/m 3 and resulted 41.5, 33.1, and 22.7% conversion at ambient temperature for HCHO respectively which is described in Table 2. This result ultimately illustrated that with the decrease in feed rate of HCHO would increase catalytic activity at ambient temperature. Reaction rate is generally not affected through mass transfer if fast mass transfer or very slow reaction rate happened. Similar results have been also illustrated by Tang et al. (2008) that excellent HCHO decomposition is getting through low concentration of HCHO. Kim, Park, and Hong (2011) has given a similar applicability of feed rate while catalytic oxidation of HCHO. They evaluated a proper applicability of feed rate that if reaction rate will increase with increase in HCHO inlet feed and vice versa. The trend observed that by increasing feed concentration there is decrement in overall catalytic activity. As temperature perspective Pei, Han, and Lu (2015)

Effect of morphology
Morphology with exposed crystal plane on surface and crystallinity are crucial as a point of catalytic oxidation. Change in the morphology of oxide supported catalyst is in trend for regulating the catalytic activity (Feng et al. 2013;Zhou and Li 2012 Yu et al. (2013) have used different support MnO 2 /cotton possess strong catalytic activity for temperature below 100°C. Graphene-based MnO 2 supported catalyst has been synthesized by Lu et al. (2016) and revealed that the graphene nanosheet just reduces the activation energy of MnO 2 catalyst from 85.5 and 39.5 kJ/mol.

Effect of metal addition to support
To enhance catalytic oxidation of HCHO over catalyst and improve the reaction rate this parameter also has been taken into consideration via different researchers. For example, Zhang et al. (2012) have used alkali addition to find the stronger catalytic activity for Pt/TiO 2 catalyst at environment temperature and the effects are listed comprehensively in Table 2. These alkali metal ions (like K + , Na + , and Li + ) improve stability of catalysts by atomically dispersion. With different support of Pt/MnO 2 , Chen et al. (2014) have examined the behavior of alkali metal salts for enhanced HCHO oxidation. The different salt used by Chen involves Na 2 CO 3 , Na 2 SO 4 , and NaNO 3 which resulted that at a lower temperature of 50°C, 100% conversion of HCHO is getting through Na 2 CO 3 additional catalysts. Similar tests have also been conducted on Pd/TiO 2 catalysts by modification of sodium by Zhang et al. (2014) with different loadings of sodium ion. They gave a comprehensive result that 2 wt% Na addition into Pd/TiO 2 converts 100% HCHO even at room temperature of 25°C. The same results have been proposed by Li, Zhang, and He (2017) that due to addition 2 wt% Na catalytic surface produces abundant OH group which is ultimate increases the oxidation of HCHO.

Discussion on literature
In this paper, the methods of catalytic activation of HCHO at room temperature include two processes i.e. photocatalytic process and catalytic oxidation. With all the research main focus concentrated on photocatalyst and catalytic oxidation illustrated in this point of discussion. A photocatalytic oxidation process involves different processes during the decomposition of HCHO. These are: (1) flow of HCHO to the catalyst surface; (2) soaking of reactants on the catalyst surface; (3) irradiation of photon from light intensity; (4) redox reaction on catalyst surface; (5) flow of harmless product from surface, and (6) flow of product from the catalyst surface into the air. The important process in the PCO process is the absorption of the reactant, which have to oxidize. The adsorption process is an intermediate process which occurs before the photochemical reaction to start. So, adsorption of reactant is important to concern while PCO. Another term that can enhance the activity of PCO is the selection of a proper reactor which extensively includes an annular reactor, monolith reactor, fluidized bed reactor, and powder plate reactor. The structures of photocatalysts also influence the PCO of HCHO. Nanoparticles with zero dimensionally have a high specific surface area, while 1D fiber tubes possess the advantage of light scattering properties. Various intermediate has also been observed during the photocatalytic oxidation of HCHO which are HCOOH, CO, Ozone, etc. and these species were quickly observed by catalyst surface. They would convert into CO 2 otherwise they will find as the outcome. The photocatalytic performance of titania can be enhanced by addition of metal and nonmetal which suppressed recombination of electron and hole pairs. Applicability of PCO had been limited to UV activation since it extended to the visible light range (λ > 420 nm) with the addition of proper doping.
There are many disadvantages of PCO listed here which are: (1) concentration of HCHO in the indoor environment is less, for that PCO doesn't show efficient performance due to high film diffusional resistance; (2) it is very difficult to increase the photocatalytic activity of reactor per unit volume since space limitation is there when doped through noble metal on TiO 2 . As among all the noble metal catalysts and metal oxide catalysts, Pt shows good catalytic performance over oxidation of HCHO at room temperature. A small addition of Pt is enough for catalytic oxidation of formaldehyde but due to some limitation of high cost which makes Pt unused for practical application. Among various catalysts, Pd shows also good catalytic activity of different support like TiO 2 , Al 2 O 3 , MnO 2 , CeO 2 , and Zeolite for the oxidation of HCHO and is abundantly available in nature. Negatively charged Pd supported catalysts improve the catalytic oxidation. For enhanced catalytic oxidation of HCHO with improved structure and chemical state, Pd species done well. With the proper support of metal oxide and with different morphology Pd catalysts can easily oxidize HCHO in CO 2 , and H 2 O at even room temperature. For lowering the temperature for catalytic oxidation addition of alkali plays an effective factor also. This effect is discussed in Table 2. Another noble metal, gold also does an influencing catalytic oxidation with well-defined morphology such as: three-dimensionally ordered macroporous with nanowalls.
There are some disadvantages noticed as well except the cost is poorly resistant to poisoning. Other than this, manganese oxide exhibits good catalytic properties to the different oxidation states available. Manganese and ceria oxide supports are well in oxygen storage capacity, redox properties, and thermal resistance. Different morphological changes like pyrolusite, cryptomelane, and todorokite in the case of MnO x have a strong effect on reaction rate. The addition of noble metal over the transition metal oxide supported catalyst also improves the catalytic degradation of HCHO and makes the transition oxide catalysts preferable for the environmental temperature condition. The various structure has been used for enhancing the property of metal oxide such as CeO 2 nanorods, graphene support MnO x , manganese dioxide nano-sheets on cellulose fibers, nano-Co 3 O 4 , 3D-Co 3 O 4 and 2D-Co 3 O 4 .

Conclusion
In this article, the most popular techniques for formaldehyde degradation i.e. photocatalytic oxidation and catalytic oxidation have been discussed. In spite of availability of efficient catalysts, the challenging work concern over higher efficiency, optimum design, and low cost availability. Moreover, the catalytic degradation of HCHO through the catalytic oxidation with noble metal and transitional metals supported metal oxide is emphasized in this review paper. To do this catalytic oxidation process subdivided into two main section; noble metal and metal oxide catalysts which give further insights the dependency of relative humidity, calcination temperature, preparation method, feed rate, morphology, addition of metal. In recent years a lot of work has been carried out on lowering temperature of oxidation of HCHO to room temperature with noble metal-supported metal oxide and transitional metal oxide.
In various articles, different groups have worked on nanostructures, reduction properties, active surface area, etc. So, the recent work corresponding to the degradation of indoor HCHO has been focused on designing and finding effective support like graphene, cotton, ZnO. Current work also just focused on varying operating parameters like preparation method, calcination temperature, the oxidation state of metals, etc. The addition of alkali, rare Earth metal to the supported catalyst is still not cleared with transition metal oxide. Other degradation processes like thermocatalytic process are also used but possess the difficulty of conversion of HCHO into CO 2 at ambient condition. Advanced processes are using this process with modification in morphology, concentration etc. to completely degrade the HCHO from the indoor environment. Element like MnO 2 , without proper morphology and additives cannot enhance rate of degradation. The role of HCHO concentration, space velocity, and water vapor strongly affect the catalytic oxidation of HCHO. Long-time stability of catalyst and formation of the intermediate products are still a major concern for the practical application. In recent years, research work has been concentrated on complete catalytic oxidation of HCHO at ambient temperature. Some works are really appreciable in this context. But there is still a large gap of their practical uses either due to cost or intermediate product limitation. However, the development of effective low-cost catalyst formation is the biggest work that has to be done. The work on transition metal oxides like (MnO 2 , CeO 2 ) still needs a proper synthesis to work on room temperature of influencing the removal of formaldehyde.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission. Research funding: None declared. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.