1 Introduction

The catalytic converter applications in an automobile vehicle were used for reducing the poisonous gases emissions from an internal combustion engine. The catalysts which exist in a catalytic converter were a reduction catalyst and an oxidation catalyst [1, 2]. In all situations, the catalysts exist in a ceramic monolith structure and covered with metal support. The catalytic reaction is the reaction between catalyst surfaces and other gases present in the vehicle exhaust [3]. The activity of catalytic converter is strongly depending on the different types of catalysts were applied. In the presence of catalyst, the rate of chemical reaction was improved; it acts like an agent that decreases the activation energy of the reactions. The transition metals, noble metals, and metal oxide are broadly applied as catalyst in a catalytic converter [4, 5].

Carbon monoxide (CO) is very dangerous gas and also represents as the silent killer. CO is produced into the atmosphere by partial oxidation of carbon-containing compounds. It is a neutral and nonirritating gas which makes it very difficult for humans to identify [6, 7]. The low-temperature catalytic conversion of CO is highly important process for life support in enclosed atmospheres such as submarines and spacecraft. Due to the rising cost of noble metals and outstanding performances in oxide preparation, catalytic conversion of CO over transition metal oxide catalysts is currently of much interest, still if several oxides are recognized to present an outstanding performance since the start of the twentieth century [8]. The transition metal oxides (TMOs) constructions, buildings, designing, and structures have been represented as one of the most important useful elements in the area of catalysis, fuel cells, energy storage, air pollution control, and so on. These materials structured units frequently possess certain physical and chemical properties that are especially helpful for their efficient applications [9, 10].

The TMOs catalysts are novel types of mixed oxide catalyst whose exterior surface area chemistry can be considered in the similar situation as effective oxide catalysts [11]. There are different synthesis methods that have been developed for the production of TMOs catalyst with distinctive structure and higher activity [12]. It has a cheaper cost, environmentally friendly, and simply available catalyst for ambient temperature CO conversion. The TMOs catalysts are synthesized via various methods and had been reported to often produce some small/nanostructures with attractive shapes, such as spherical, pyramids, laminated-cube, and dumbbell structures [13, 14]. The mixed transition metal oxide (MTMOs) catalysts observed from various transition metals should be the new types of candidates due to their prospective application in catalysis and energy exchange [15]. The current research investigated that the structure, characterization, activity, and applications of various transition metal oxides oxide catalysts for CO oxidation [16]. It is very complex species making with oxygen, during the presence of cations; therefore, the characterizations of supported transition metal oxides catalysts are very difficult. The transition metal oxides have superior potential for reduction of CO in the atmosphere so that it is mostly used in a catalytic converter [17, 18].

Transition metal oxides represents huge structural differences due to their capacity to produced phases of different metal to oxygen ratios showing several steady oxidation states of the metal ions. The performances of most active transition metal oxides, such as Co3O4, CuOx, Ni2O4, ZnOx, or MnOx, are highly dependent on the reaction conditions [19]. Transition metal oxides exhibit high crystallographic anisotropy which may explain the various catalytic properties for various uncovered crystal faces. The surface structures mainly depend upon various strong metal-oxygen bonds at different surface planes [20]. The superior metal-oxygen bond is made more crucial by its function in CO oxidation. The real metal-oxygen stoichiometry and defect structure participate a significant role in partial oxidation reactions [21, 22].

Transition metal oxides contain a large amount of surface structures which involve the surface energy of compounds and affects their chemical properties. It contained oxygen atoms bound to transition metals [23]. The surface metal oxides are highly influenced by the coordination of metal cation and oxygen anion, which modify the catalytic activity of these compounds. The structural defects in TMO are highly influence by their catalytic performances. The location of surface ions will change from the massive structure [24]. In all the transition metal oxides, Mn2O3, CuO, ZnO, TiO2, Fe2O3, NiO, and Co3O4 show better catalytic performance in CO oxidation [25]. The oxidation state of MnOx is highly affected when applied with MnOx as a support. The certain oxidation state of Mn represents to perform as a reducing agent or oxidizing agent. The oxygen-containing ability in the crystalline lattice and capacity of Mn to create oxides with different oxidation states are its catalytic property [26]. The TiO2 has more oxygen-containing capacity with superior oxygen adsorption and oxide reduction rates, which can be applied to decrease the amount of CO in the exhaust gases. The titanium oxide can represent superior performances per unit surface region than the individual one of noble metal catalysts [27]. The Co3O4 is a unique spinel-structure transition metal oxide catalyst with applications in various fields. The Co3O4 as catalysts of environmental implication includes complete oxidation for CO and HC oxidation and NO and SO2 reduction at a low temperature [28].

Cobalt-based catalyst is sometimes applied in place of noble metals due to its lower cost, high availability, and more activity for complete conversion of CO. Co3O4 is the strongly active catalyst in TMO for CO oxidation and represents unusual activity and stability under low temperature in dry conditions [29, 30]. Zinc oxide (ZnO) catalyzes more oxygen available for CO oxidation process. ZnO acts as a reducible oxide support, improving the catalytic performances via metal-support interface and/or better spreading of active metal components [31]. An important property of zinc is its ability to stabilize the catalyst against deactivation due to more thermal constancy and/or superior distribution of active metal. In place of using ZnO individual as the catalyst for CO oxidation, more scientists focus on changing ZnO with various ions to improve their performances and thermal stability [32]. Copper oxide is originally lesser active for CO conversion, but in combination with Mn-oxide in certain proportions, several different active catalytic systems were produced [33]. CuMnOx catalyst is more active in the amorphous state still at room temperature where crystallization of the spinel CuMn2O4 phase has occurred. In high oxygen-rich environmental conditions, the affirmation of oxidation of CuO and reduction of MnO phases in the oxygen-deficient conditions occur, to create residual Cu2O and Mn2+/3+ oxide phases [34, 35].

The iron oxide (Fe2O3) catalyst showed a good CO oxidation activity, even at room temperature and it is a potential substitute for precious metal catalysts system. The isolated tetrahedral coordinated Fe(3+) centers have a more catalytic performance in CO oxidation. The existence of Fe2+ or Fe3+ ion has been recognized as individual one of the major factors governing the structure property of Fe-containing products [36]. The superior activity of iron nanoparticles in CO oxidation was associated with a smaller particle size, higher exterior area, and closely occupied surface coordination unsaturated sites. The iron phase species formed during the activation depends on the time of exposure to the reactant feed, mixture of the feed, and pretreatment conditions [37,38,39]. The noble metals are highly applied in an automobile vehicle as a catalytic converter. In the noble metal catalysts, rhodium is used as a reduction catalyst, palladium (Pd) is used as an oxidation catalyst, and platinum (Pt) is used for both the reduction and oxidation catalyst. The Pt-, Pd-, and Rh-based catalysts are mostly used for ambient temperature CO oxidation process [39,40,41].

Gold (Au) is very active for ambient-temperature oxidations of CO if dispersed on suitable metal oxides and complex oxides. Gold supported on reducible oxides represents to catalyze the CO conversion well at the low temperatures over 0 °C. The addition of gold into the catalyst improved their catalytic performances for CO oxidation [42]. It has been reported for high thermal stability mostly due to the substantially reduced gold-gold nanoparticle interactions. The introduction of gold will add new active sites into the catalysts and also increases the reducibility of the catalyst. An increase of Au0/Au+3 ratio has also been reported when increasing the calcination temperature. An excess of Au0 is detrimental for CO oxidation [43]. Silver (AgO, Ag2O, Ag2O3)-based catalyst is represented as an attractive metal oxide catalysts and more activity and thermal stability for ambient temperature CO conversion. The performances of silver catalysts are mostly dependent upon their surface structure and composition [44]. The activity of AgO catalyst is most commonly as a consequence of the occurrence of different Ag–O interactions. The surface and subsurface oxygen atoms are represented to be the active sites for Ag-based catalysts in CO oxidation reactions. The oxygen pretreatment at high temperature results in the formation of subsurface oxygen and activates silver catalysts Ag0 as an active species was observed to increase the catalytic activity at a low temperature [45].

Nickel is the best catalyst for the reaction of virtue of its life, high activity, and selectivity toward CO oxidation at a cheaper cost. The high performances of Ni were associated to the existence of both intra- and inter-particle porosity of the catalysts and high Ni metal particles dispersion on the catalyst surfaces. The NiO has a bimodal pore structure; this represents huge performances for CO oxidation [46]. This bimodal pore structure will provide more favorable pore size for the chemisorptions of reactants. The potential of chemisorptions on nickel has transitional binding energies; therefore, it has an intermediate value of the heat adsorption. It is a lower cost, active, and best catalyst to be produced economically. The performance of catalysts also depends upon the calcination conditions of precursors and subsequent pretreatment of the catalysts [47]. This review paper mainly focused on the low-cost transition metal catalysts. The influence of particle size on catalytic activity has been subjected of continuous interest due to its importance from both primary and practical viewpoints [48]. Future studies will focus on the development and utilization of transition metal catalysts for high catalyst selectivity. The catalytic activity of various transition metal catalysts in different compositions in CO oxidation reactions has been discussed in this review paper.

2 Chemistry of carbon monoxide

Carbon monoxide contains single C atom, and a single O atom consisted of a triple bond that has two covalent bonds and one dative covalent bond. This is the easiest oxo-carbon. In coordination complexes, the CO ligand is called carbonyl. CO is a molecule with three resonance structures or Lewis structure. It can clarify the outstanding 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 was satisfied, but the electropositive carbon has a negative charge [44]. Calculations with usual bond orbital exposed that the structure through the triple bond is the most significant. This is following other theoretical and experimental studies that represent the superior electro-negativity of oxygen dipole moment points from the negative carbon end to the positive oxygen end [49].

3 Property and structure of various transition metal oxide catalysts

The transition metal oxides are composed of oxygen atoms bound to transition metals. It is frequently applied for their catalytic performances and semi-conductive properties. TMO have a broad range of surface structures which influence the surface energy of these compounds and affects their physical-chemical properties. The surface metal oxides are also influenced by the allocation of metal cation and oxygen anion, which influences the catalytic activity of these compounds [46, 47]. The structural defects in TMO are highly affected by their catalytic properties. The surface structure of transition metal oxides catalysts is bulk crystalline in nature. The oxide crystal structure is depending upon on close-pack arrangement of oxygen anions, with metal cations covering interstitial sites. The close-packed arrays, such as face-centered-cubic (fcc) and hexagonal close-packed (hcp), have both octahedral and tetrahedral interstices. In mono-oxide, many TMO (TiO and NiO) have rocksalt structure. In rocksalt structure, the octahedral sites have cations in an oxygen anion fcc array [44]. The majority of TMO (MO2; M = Ti, Cr, V, Mn, Zr, Pd, etc.) have the rutile structure. The rutile structure is produced by filling half of the octahedral sites with cations of the hcp oxygen anion array. In trioxides, few TMO can reach to the +6 oxidation state. The mixture of two or more metals in an oxide form produces a huge number of structures [47,48,49]. The ternary oxide may be influenced by changing the proportions of two components and their (SrV2O6 and Sr2VO4) oxidation states. The perovskite structure is commonly observed in transition metal oxides produced with one big (A) and one small cation (B). In this structure, a cubic array of B cations with the A cations occupying the center of cube and oxide atoms are located at the center of 12 edges of plain cube [50]. The bond between oxygen and transition metal as shown in Fig. 1 is strongly affected by the coordination of metal cations and oxygen anions as well as the filling of metal in d-orbitals. The surface coordination is illicit by the face that is exposed in the surface reduction [51].

Fig. 1
figure 1

Molecular structure of transitional metal oxides in perovskite catalysts

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The catalytic properties of perovskite-type oxides mainly depend on the nature of A and B ions and their valence state. The nature of these ions also influences the stability of perovskite phase. Many TMOs are highly active for CO oxidation without utilizing noble metal [52]. The two key factors usually establish the efficiency of TMOs for CO oxidation, reducibility, and oxygen mobility. In these reactions, the CO reduces a dynamic metal center. The oxygen mobility in the transition metal oxides lattice, as shown in the Fig. 2, corresponds to improved oxygen vacancies at the exterior surface to adsorb active oxygen and extra reactive lattice oxygen [53].

Fig. 2
figure 2

Oxygen position in various transition meal oxide catalysts

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The character of cobalt oxides mainly in the proportion of Co2+, Co3+, and Co4+ was mostly applied for CO oxidation. In all the presence of cobalt oxide, the most active form would be Co3O4 in which cobalt is observed in two valence states (+2 and +3). The crystal structure of Co3O4 as shown in Fig. 3 is a perfect spinel structure in which Co2+ cations occupy one-eighth of the tetrahedral sites while Co3+ cations occupy half of the octahedral sites. The spinel Co3O4 is also the most stable oxide [52].

Fig. 3
figure 3

Formation of various cobalt nanoparticles

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In comparison to Co3O4, the unsupported CuO have been rarely used in CO oxidation. This is necessary due to the distinctive structure of Co3O4, which contains both Co2+ and Co3+, whereas CuO or Cu2O have only either Cu2+ or Cu+ ions [53]. However, CuO are not highly stable as shown in the Fig. 4 and oxidation state of Cu may fluctuate in the course of reaction. Depending on the CO/O2 ratio and temperature, it very much depend on Cu2+ and Cu+ that coexist, despite of the first materials. The outstanding behavior of Cu2O in CO oxidation at room temperature observed in detail is the O2 chemisorptions on the cuprous oxide.

Fig. 4
figure 4

Structure of Cu and O atoms in copper oxide catalysts

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The manganese oxide nanoparticles are prepared by self-assembly in a single pot reaction in ambient conditions with interesting morphologies and high surface areas. Manganese oxide catalysts are steady in the presence of Mn2O3, MnO2, and Mn3O4 phases which were mostly dependent on the calcination temperature between 200 and 300 °C [54]. The labile oxidation state represents Mn to act either as a reducing agent or oxidizing agent. The oxygen contains ability in the crystalline lattice and capacity of Mn to produce oxides with different oxidation states determines its catalytic property [55]. The structure of manganese oxide catalysts is shown in Fig. 5 and represents huge amount of necessary work dedicated to instruct the role played by each element and nature of active sites [56].

Fig. 5
figure 5

Structure of different manganese oxide catalysts

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The isolated tetrahedral coordinated Fe(3+) centers and phases of FeMnO2 are recognized as structural applications for high catalytic performances in the CO oxidation. Iron is usually played as the active metal in the catalyst for industrial scale due to their economically acceptable cost. The oxide of Fe2+ as shown in Fig. 6 was consisting of (FeO and FeO2) and oxides of Fe2+ and Fe3+ were consisting of (Fe3O4, Fe4O5, Fe5O6, Fe5O7, Fe25O32, and Fe13O19) oxides. The oxide of Fe3+ was consisting of Fe2O3. The metallic iron (Fe0) peaks present in the iron oxide precursor before calcination [57].

Fig. 6
figure 6

Structure of iron oxide catalysts

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The nickel oxide (NiO) also represents an active phase for CO oxidation when deposited on the catalyst surfaces. The NiO was produced and highly dispersed on catalyst surface, resulting in the production of rich surface Ni2+ ions. The Ni2+ ions moderately replace Mn3+/Mn4+ ions to produced a Ni–Mn solid solution and creating surface oxygen vacancies, which are crucial for CO oxidation. The Ni and Mn have same electronic configurations, which possible expected results in the Ni–Mn composite oxides capable to synergistic catalytic effect [46]. The oxidation of metallic alloys are shown in the Fig. 7 containing Ni, which can be expected to yield strong interaction of Ni and support oxides with high dispersion. The sub and full monolayer of oxygen chemisorbed over Ni surfaces play no significant role in low-temperature CO oxidation process but NiO can catalyze the low-temperature CO oxidation when NiO was saturated by O2. The NiO cubic phase was thermo stable and stop changing phase has occurred [58].

Fig. 7
figure 7

Structure of nickel oxide catalysts

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Titania is sometimes used for CO conversion processes although can be an outstanding support for certain metals, mainly gold. The highly dispersed Ir/TiO2 catalysts represent huge catalytic activity for CO oxidation even at room temperature, similar to that of size-selected supported Au nanoparticles [27]. Titanium dioxide is a naturally occurring oxide of titanium (TiO2). TiO2 represents in nature as the recognized raw materials like rutile, anatase, and brookite. Molten TiO2 has a simple structure in which each Ti is coordinated to, on average, about 5 oxygen atoms and crystalline produced in which Ti coordinates to 6 oxygen atoms [59].

Nanosized TiO2 is shown in the Fig. 8, the small size represents high catalytic activity. This activity is apparently distinct at the {001} planes and even though the {101} planes are thermodynamically further stable structure. The TiO2 in thin film and nanoparticle form is probable applied in energy production [27,28,29]. The TiO2 was applied as different support materials for heterogeneous catalyst due to the consequence of its huge textural area stabilizing the catalysts in its mesopores structure. Among all the transition metal oxide additives, zinc oxide (ZnO) was used as promoter to increase the catalytic performances of gold catalysts for partial oxidation of methanol and CO oxidation. The Au-ZnO/Al2O3 catalyst is further active than the Au/Al2O3 catalyst, where smaller particles led to higher activities [32].

Fig. 8
figure 8

Various structures of titanium oxide catalysts

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The zinc oxide crystallites are mainly found in dual forms hexagonal wurtzite and cubic zincblende. The wurtzite structure is highly stable at ambient conditions. The zinc-blended produces can be stabilized by increasing ZnO on substrates with cubic lattice structure. In these cases, the zinc and oxide centers are tetrahedral, the majority characteristic geometry for Zn(II). Nanostructures of ZnO as shown in Fig. 9 can be prepared in different morphologies including nanowires, nanorods, tetrapods, nanobelts, nanoflowers, nanoparticles, etc [60]. The novel metal catalysts are very helpful in CO oxidation processes. These catalysts are very dense, expensive relatively rare earth metal. The rate of CO dissociation will strongly depend on the particle size and geometry of surface assembly involved in the dissociation reaction. The CO adsorbs only on (111) and (001) surface planes as compared with stepped sites, where CO dissociates with low barrier. The energy barrier for CO dissociation is higher than the desorption energy for CO oxidation [61].

Fig. 9
figure 9

Structural formation of zinc oxide after calcination

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4 Chemisorptions of carbon monoxide over transition metal oxide catalysts

Several groups have been considered for CO conversion over different catalysts since they have significance in environmental protections. Catalytic performances for CO oxidation mostly depend upon the metal ion concentration on the surface and surface crystalline. Carbon dioxide is formed by the reaction of CO with oxygen adsorbed on the metal ions of outmost surface. A surface concentration of oxygen was monitored for subsequent reaction and partial pressures of reactants [62]. Co3O4 has perfect spinel structure in which one-eighth of the tetrahedral sites are covered by Co2+ cations while half of the octahedral sites are covered by Co3+ cations. Co3O4 is essentially active for CO oxidation even under a temperature of − 54 °C. The pure CoIII oxide was used in CO oxidation as a starting material and Co3O4 instinctively loses oxygen to give CoO.

High activity which originates from Co3O4 {110} facets has been described in Fig. 10 to the improvement of octahedral coordinated Co3+ cations which represent best CO chemisorptions properties and poorer barriers for reaction between adsorbed CO and oxygen. The adsorption of CO on the surface of cobalt oxide is demonstrated in Table 1 that Co ions mostly originate over the surface of Co3O4 {111} and Co3O4 {100} thin films or synthetically produced on the CoO {111} surface are very essential for chemical properties of the surface [61]. The Cu2O catalyst is highly active at low temperature and catalyst system composition of Cu2O nanoparticles supported on silica gel that exhibited exceptional activity toward CO oxidation. The oxygen species associated with Cu in the Cu2O catalyst are highly active and mostly dominated by the ambient conditions CO conversion. In oxygen-rich atmospheric conditions, the presence of reduced copper oxide to produce residual Cu2O oxide phase was noted. The Cu2O catalyst is very much active in amorphous state at ambient conditions but has been observed to lose their performances after exposition at high temperatures [63]. The CuO particles minor than (< 3 nm) are well isolated in the oxide matrix, to start with, and after the reaction. The advanced loading rate of CuO supports can lead to create bigger size nanoparticles; it is the main reason for lowering the activity. At the increasing temperature, the oxidation of CO proceeded fastest over Cu, followed by Cu+ and Cu2+, in order of rising oxidation state. High density of active sites on the catalyst exterior surfaces is present therefore its performances increase [63]. The reaction of Cu and CuO whereas a redox mechanism would preferentially be done over Cu2O which oscillates between Cu(II) and Cu(I) during the CO oxidation process. The Cu+ cations situated at the exterior surface of grain boundaries as shown in the Fig. 11 would be the most dynamic sites for CO conversion at low temperature as discussed in Table 2. The Cu2O catalyst is highly active in O2-rich than in O2-lean conditions [62, 63].

Fig. 10
figure 10

Structural bonding between cobalt oxide and carbon monoxide

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Table 1 The operating parameters and activity measurement of cobalt oxide catalysts for CO conversion
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Fig. 11
figure 11

Physical structure of CuMnOx catalyst and their CO adsorption property

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Table 2 The operating parameters and activity measurement of various copper oxide catalysts for CO oxidation
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The CO was chemisorb on six different sites of Cu2O catalyst surface and disperse fast over O sites. The reduced species of copper (Cu0, Cu+) are essential for better CO conversion but lower size copper particles could be lesser active than higher ones [64]. In all transition metal oxides, the Mn2O3 represents superior catalytic activity in CO oxidation. The reaction of Mn2O3 and O2 producing MnO2 is affected by oxygen concentration and temperature. The increasing oxygen concentration and lower temperature are more beneficial to the production of MnO2 [65].

The lower oxygen concentration and increasing temperatures are slower the generation of MnO2 and Mn2O3 as the reaction proceed. The manganese oxide catalysts as shown in the Fig. 12 have more activities per unit surface area than those of noble metal catalysts. The Mn2O3 applied to be the major active oxide, falling at the lowest temperature. The oxidation state of manganese continuously moves between Mn4+ and Mn3+ [66, 67]. The titanium oxide increased performances toward partial oxidation of CO, but had an opposite effect in the case of non-stoichiometric TiOx. The oxygen vacancies increased by the conductivity of TiOx film [27].

Fig. 12
figure 12

Chemisorptions of CO over manganese oxide catalysts

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The CO conversion activities over a series of TiO2-supported catalysts and activity order observed as Co/TiO2 > Cu/TiO2 > Mn/TiO2 > Ni/TiO2 > V/TiO2 > Cr/TiO2 > Fe/TiO2. The Co/TiO2 exhibits the best activity and 100% CO conversion, as shown in the Fig. 13, was obtained at temperatures below 15 °C [27, 59]. The Ir/TiO2 catalyst also shows good performance for CO oxidation. Iron is single one of the major active catalysts for CO oxidation. The moderate performances of Fe2O3 are more significant than the Fe3O4 catalysts. The performances of different Fe oxides catalysts were discussed in Table 3 as follows: Fe3O4 < Fe2O3. The Fe2O3 is most active iron oxide catalysts for CO conversion [68].

Fig. 13
figure 13

Catalytic activity of TiO2 catalysts for CO oxidation

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Table 3 The operating parameters and activity measurement of various iron oxide catalysts for CO oxidation
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The unsupported and supported Fe2O3, as shown in the Fig. 14, are highly active for CO conversion. The authors reported the following order of activity in CO conversion: Fe2O3/Al2O3 > Fe2O3/TiO2 ≈ Fe2O3 > FeSbO4 > FePO4 > Fe2(MoO4)3. A hexagonal close-packed structure in all of the Fe3+ ions in octahedral sites is noted. The octahedral sites are empty and can be packed by Fe2+ ions to produce magnetite Fe3O4. The Fe species can be stabilized in porous materials such as graphene or gamma-alumina [69]. After the Fe-C bond formation, a serious step of CO oxidation reaction participates one terminal Fe-O bond activation, and rate-determining step is O-O bond activation. In the presence of reduced cationic species of Fe, still metallic iron atoms would be very significant in the reaction mechanism, whereas over oxidized iron sites would be very active in the catalytic process [70]. Promotion of CuO/MnO2–Al2O3 by nickel may improve the catalytic performances and stability of CuMn–alumina catalysts. The NiO does not represent better activity in CO conversion as shown in the Fig. 15, compared to cobalt and copper oxides [58].

Fig. 14
figure 14

Chemisorptions of CO over iron nanoparticles

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Fig. 15
figure 15

Chemical bonding of carbon monoxide with NiO catalyst

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The activity of CuO is improved constantly by nickel accumulation resultants to a reduction in copper oxide dispersion. Although, a pure perovskite phase is obtained with crystal structure and cell parameter changed with nickel inclusion. The chemisorptions of CO over nickel at (111) faces were higher than copper and cobalt [46, 47]. The zinc oxide catalysts are important for the conversion of CO gas. The copper zinc oxides are highly active catalysts for the conversion of CO at ambient temperature. Pure ZnO is active at 200 °C in the catalysis oxidation of CO. Oxygen species appears to be adsorbed in an ionic as well as a non-ionic form, while CO and CO2 are adsorbed essentially in non-ionic forms [32].

Two surface sites, may be Zn+ ions, are shown in Fig. 16, necessary to attach O2 and CO2. Carbon dioxide is not an inhibitor in the reaction but common displacement of CO2 and O2 was shown. The kinetic study of CO oxidation shows that the O2 was adsorbed weakly and CO strongly on two different types sites of ZnO catalysts. The amount of ionic species O–(ads) nearly does not fluctuate on the exterior of ZnO, the order ½ with respect to oxygen is interpreted as participating in the reaction of non-ionic oxygen O(ads) [60].

Fig. 16
figure 16

Structural bonding of zinc oxide with carbon monoxide

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The electron exchange between Cu and Cr leads to the reserve of reduction in Cu-Cr catalysts and thus Cu is the major dynamic species for CO conversion. The catalysts performances were related with the production of various Cu-Cr species such as CuCr2O4 and Cu2Cr2O4. In Cu-Cr system, the dual types of active centers present on the catalyst surface [71]. The CO conversion rises with the increasing of Cr content in the active component. The rate of CO conversion passes all over highest at (15–30%) Cr content after which fast decreases. The involvement of Cu species with Cr species results to a superior CO conversion as shown in Fig. 17 because of the high synergistic effect in the Cu-Cr/γ–alumina catalysts. This catalyst is active when the metal particles are in metallic form, and agglomeration to bigger particles as well as oxidation decreases the activity. The noble metals (Pt, Pd, Rh, Ru, Ag, and Au) continuing price rise stimulated an increasing substitution by other low cost elements [72, 73]. The performance of supported catalyst is strongly affected by the volume and texture of metal species as discussed in Tables 4, 5, 6, 7, and 8. The performance of iridium species on supports for CO conversion can be measured on molecular concepts [74]. The iridium supported on TiO2 synthesized by the deposition-precipitation method was more active for CO oxidation than Ir/Al2O3 and Ir/Fe2O3 catalysts [75].

Fig. 17
figure 17

Structural arrangement of chromium oxide catalyst for CO chemisorptions

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Table 4 The operating parameters and activity measurement of various manganese oxide catalysts for CO oxidation
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Table 5 The operating parameters and activity measurement of various nickel oxide catalysts for CO oxidation
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Table 6 The operating parameters and activity measurement of various titanium oxide catalysts for CO oxidation
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Table 7 The operating parameters and activity measurement of various zinc oxide catalysts for CO oxidation
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Table 8 The operating parameters and activity measurement of various chromium oxide catalysts for CO oxidation
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To catalyze the conversion of dry CO over Ir/rutile, catalysts initiated their catalytic performances over 50 °C, while Au/rutile and Ir-Au/rutile catalysts were very active for this reaction under ambient temperature. The Ir/TiO2 is very active than Ir/SnO2, Ir/Al2O3, and Ir/Fe2O3 catalysts toward CO oxidation [76]. The small crystallites size depends on Ir and was highly constant due to IrO2 having the similar crystalline. The lanthanum perovskite catalysts showed that great catalytic performances for CO oxidation at the higher temperatures. Lanthanum is oxidized in the form of various La oxide catalysts with fine metallic particles of La produced in a radius of 1–3 nm [77]. The improvement of catalytic performances for conversion of CO over La catalysts, in association with La catalysts, could be a certain result due to mixing of La and Co in the small particles [78]. Alumina initially combine with Co3O4 can represent to the cobalt oxide as a superior thermal resistance can improve its performances. The Co3O4/γ-Al2O3 is very active catalyst for CO oxidation at low temperature. This single-atom catalyst has enormously more atomic efficiency and represents high activity and stability for CO conversion [79]. The CO conversion activity over distinct ferrites, say NiFe2O4, CoFe2O4, and CuFe2O4, is in the order NiFe2O4 < CoFe2O4 < CuFe2O4 which is in superior contact. The performances and selectivity of catalysts depend on the contact between individual components, for which a uniform distribution of components all through the material without definite separation is a pre-requisite [80].

5 Mechanism of CO oxidation over transitional metal oxide catalysts

The efficiency of transitional metal catalysts for reactions with CO molecules is strongly depending on the chemisorptions process. The chemisorptions of CO gases are significant step, which increses the concentration of reactant over the catalyst exterior surfaces which chemisorbed on the CO molecules applying on more energy to be simply obtain the chemical reactions. The distinct reaction mechanisms are stable with the observed kinetics. The initial mechanism represents that the highly accepted CO conversion reaction on catalyst exterior surface that participates the O2 adsorption to produced O2* precursors, which divide on a vicinal vacancy [81]. In the second mechanism, O2 activation done via the kinetically applicable with CO*-assisted O2 dissociation step lacking of the definite conditions of stable O2* precursors. In the CO conversion procedure, the oxygen was initially adsorbed on catalyst exterior surfaces with the energy of activation. The mechanism of transition metal catalysts for reaction with the CO molecule is shown in the Fig. 18. When the temperature is increase in a certain amount so that the adsorption of oxygen reaches on suitable proportions, therefore any CO passing over the catalyst surfaces either react directly adsorbed oxygen or initially adsorbed then reacts, after which the CO being produced was desorbed.

Fig. 18
figure 18

Mechanism of CO oxidation over transition metal catalyst

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The performance of transitional metal oxide catalysts for CO oxidation was measured on the activation energy of the procedure [82]. The activation energy results are very helpful for modeling and planning of catalytic converter. Chemical kinetics are applied on these factors, which can affect the rate of reaction under concern and shows clarification for the certain value of rate and leads to the rate equations, which are very valuable in the reactor design [83]. The catalyst initially oxidized CO before it is oxidized by air and this is an investigation by the Mars-van Krevelen-type mechanism, which has certainly found on the support. A Langmuir-Hinshelwood-type mechanism between adsorbed oxygen and CO has been applied, and it is not clear if operates completely or there is a combination of both.

The oxidation of CO by the Mars-van Krevelen mechanism would represents the correlation between Co3O4 catalyst activity and reducibility [79]. The conversion of CO over cobalt oxide catalyst could be discussed very well by the Langmuir-Hinshelwood model, in which molecularly chemisorbed CO reacts with dissociative adsorbed oxygen in the first-order reaction. Figure 19a represents the Langmuir-Hinshelwood mechanism for CO conversion on the Co3O4 metal surface. In the initial step of reaction, CO and O2 adsorb into the catalyst surface. Then after, the adsorbed O2 molecules separate into single O atoms [80]. The CO molecules and O atoms initiate to disperse on the exterior surface, and once a CO molecule and O atom combines each other, they recombine and produced CO2. In the next step, CO2 desorbs into the gas phase. The Eley-Rideal mechanism is shown in Fig. 19b, where the reactant molecules adsorbs initially into the Co3O4 catalyst surface and reaction was done when other reactant molecule combines it from the gas phase [84].

Fig. 19
figure 19

Illustration of a Langmuir-Hinshelwood mechanism, b Eley-Rideal mechanism, and c Mars-van Krevelen mechanism

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Finally, the Mars-van Krevelen mechanism is shown in Fig. 19c. Here the gas-phase molecules contact with the oxidized surface. In this procedure, CO contacts with the uppermost oxygen lattices and free the surface by producing CO2 [85]. In Fig. 19a and Fig. 19b, the blue spheres represent the Co3O4 atoms and Fig. 19c represents metal oxide film developed on the summit of substrate (gray atoms). The green spheres represent metal atoms. The red and black spheres in Fig. 19a, b, and c represent oxygen and carbon atoms, respectively. Thin oxide films developed on metal surfaces can be used for mechanism analysis. These thin oxide films should be thick enough to imitate the metal oxide as shown in Fig. 20 of the real oxide, but thin and thereby conductive adequate to allocate electron-based techniques to be applied.

Fig. 20
figure 20

Illustration of a Frank-van der Merwe, b Stranski-Krastanov, and c Vollmer-Weber

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Such thin cobalt oxide films are frequently formed by depositing and oxidizing a metal on an individual crystal surface. A catalytic reaction on the surface of catalyst and gas surface interface includes the observation of gas adsorption, dissociation, diffusion, and desorption. Gas adsorption as shown in Fig. 21 can be divided into dual categories depending on the strength of adsorbate-surface interface: (i) if adsorbate CO is combine to the catalyst surface by lose Van der Waals bonds, it represents to the adsorption process as physic-sorption and (ii) if actual chemical bonds are produced in the adsorbate CO on catalyst surface, it represents to the adsorption process as chemisorptions [86]. In the physic-sorption process, the adsorbates are captured at certain space from catalyst surface and no valence electrons are replaced with the surface. In comparison, the replacement of valence electrons has done among the catalyst surface and CO adsorbate for the chemisorptions process [87].

$$ {\mathrm{O}}_2+2\ast \to {2\mathrm{O}}_{\mathrm{ads}} $$
(1)
$$ \mathrm{CO}+\ast \to {\mathrm{CO}}_{\mathrm{ads}} $$
(2)
$$ {\mathrm{CO}}_{\mathrm{ads}}+{\mathrm{O}}_{\mathrm{ads}}\to {\mathrm{CO}}_2+2\ast $$
(3)
Fig. 21
figure 21

The potential energy curves of gas adsorption process

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Depending on the nature of surface, the molecular chemisorptions can be done, or the molecules instinctively separate upon the adsorption. The distinction of oxygen molecules was done by the reaction between adsorbed oxygen atom and CO to CO2 is individual one of the accepted mechanisms for CO conversion. In this situation, the reaction rate was limited by the dissociation of O2. The molecular chemisorptions of CO can be done at the higher temperatures, which ensure that the appearance of reactive oxygen forms [88]. The O− ions are highly reactive and performances of superoxide are also increase, although much poorer as compared to O−. In oxygen species, the CO molecules from the gas phase can be oxidized. The concentration of adsorbed oxygen produced does not include to the concentration of transition metals, but its presence represents the development of these products [89, 90], where * represents a free site on the metal surface. The CO2 produced as shown in Fig. 22 is simply adsorbed and does not influence the rate significantly, since it is rapidly desorbed into the gas phase. No surface carbonate species are produced as intermediate in this reaction.

Fig. 22
figure 22

Mechanism for CO chemisorptions over perovskite catalysts

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The reaction rate will be proportional to the surface exposure of Oads and COads. The oxidation occurs in two steps. First, the CO is oxidized by exterior lattice oxygen in the metal oxide. An oxygen vacancy was produced, decreasing the nearest metal ions to a poorer oxidation state. In the second step, the exterior surface metal atoms are oxidized by the gas-phase oxygen [91]. The differences of activity and binding energy of transition metal catalysts were noted as a function of capacity factor for the series of catalysts.

CO is react with chemisorbed oxygen as shown in the Fig. 23 either adding on it from the gas phase or produced an adsorbed state subsequently to the adsorbed oxygen or on the peak of adsorbed oxygen. In reaction conditions, the rate was proportional to the O2 pressure and independent of CO pressure. The rate of CO oxidation was determined by following either the rate of production of CO2 or when the CO was captured and rate of disappearance of CO [92]. The reaction rate was found to be reduced when CO was introduced into the gas phase during the oxidation. The first-order reaction in CO can be done due to the reaction from as lower chemisorbed layer or although the reaction from the gas phase [93]. The amount of CO2 molecules chemisorbed corresponded to the amount of oxygen atoms pre-adsorbed on the catalyst surfaces. With the equal concentration of oxygen atoms in the gas phase over the surface, therefore, heterogeneous exchange reaction was taking places. The mechanisms of CO oxidation on the surfaces of catalysts are a top tactic in nature, thus responsible for the loss and uptake of bulk oxygen for the production and disappearance of vacancies as explained in these systems as attractive oxidation catalysts [94]. The incompatible oxygen stoichiometry of TMO structure is responsible for extraordinary performance of these materials [95]. For stable oxides, the rate-determining step will be the reaction between COads and Oads, while for a TMO that has an inferior M-O bond energy, the rate-determining step will be adsorption or dissociation of molecular oxygen on the metal surface. Accordingly, the CO oxidation processes to occur with high reaction rates if CO, adsorbed on the metal particle, interacts with oxygen adsorbed on an extremely reducible metal oxide support, with following dissociation at the metal–support boundary [96, 97]. The theoretical calculations suppose that the CO was oxidized by reaction with atomically adsorbed oxygen in a Langmuir-Hinshelwood mechanism. The Langmuir-Hinshelwood model has been explained in the CO oxidation mechanism taking place on transitional metal oxide catalysts [98]. In the Langmuir-Hinshelwood model, the CO conversion occurs on the exterior surface in the following steps are involved:

Fig. 23
figure 23

Kinetics study of CO adsorptions over transitional metal catalysts

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CO(g) → COads (4)

O2(g) → 2Oads (5)

COads + Oads → CO2(g) (6)

In this analysis, the function of substrate (such as Fe2O3, TiO2, NiO, Co3O4) is not constant with the small supported particles as represented in Fig. 24, but smooth to advance the adsorption and activation of oxygen. For non-reducible supports, such as SiO2 and Al2O3, the catalytic performances were much more strongly dependent on the transitional metal diffusion; in fact, the oxygen adsorption has done only the metal sites [99, 100].

Fig. 24
figure 24

Mechanism of CO oxidation over transition metal catalyst

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The CO conversion over unsupported transitional metal oxides takings according to a Langmuir-Hinshelwood mechanism was noted, in which the determining steps occur in the adsorbed phase. Both reactants are adsorbed [101]. The boundary of metal oxide and support is of particular interest since transition metal can act as a strong promoter to TMO for CO oxidation reaction. The goal of this work is to improve our understanding of the chemical process taking place on the transition metal catalysts [102].

6 Deactivation of transition metal catalysts

The performances and selectivity of transition metal catalysts in catalytic converter are essential for CO conversion reaction. The catalyst deactivation and failure over in catalytic performances create problem in the practice of catalytic process. The catalyst deactivation can be divided into six different types: (i) poisoning, (ii) thermal degradation, (iii) fouling, (iv) vapor compound formation, solid-solid and/or vapor-solid reactions, and (vi) crushing/abrasion [103]. The major reason of catalyst deactivation was divided into three parts: chemically, mechanically and thermally. The lead, sulfur poisoning, carbon production, and sintering are the major cause of catalyst deactivation. The dispersion of active phase rapidly decreases, which is one of the major reasons of catalyst deactivation as shown in Fig. 25 and Fig. 26.

Fig. 25
figure 25

Deactivation of transition metal catalysts

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Fig. 26
figure 26

Deactivation of transition metal catalysts by sintering and fouling

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The transition metal catalyst was easily deactivated by a trace amount of moisture present in the catalyst [104,105,106,107]. The substitutions of materials in transition metal catalysts should not affect their performance in reforming reaction; at the same time, these changes in the structure should keep their resistance to carbon deposition as well as to sulfur poisoning. The stability of transition metal catalyst could be recognized to the more movement of oxygen on the interface of mixed oxides [108,109,110]. To decrease the deactivation of transition metal catalyst, a certain amount of support materials like SiO2, TiO2, and Al2O3 was added into the transition metal catalyst and also improves the catalyst [111,112,113]. The deactivation trend of transition metal catalyst for the mixed stream oxidation suggests that the operating costs for this catalyst would rise rapidly with time on stream [76]. Chemical poisoning, coke formation, and dispersion of active phases are one of the main reasons for catalytic deactivation [114, 115].

The deactivation of transition metal catalyst decreases the surface area of catalysts if it exists in the surroundings. The deactivation is the main reason of failure of catalytic exterior surface area due to the crystalline development of catalytic phase. The main reason of poisoning is due to the high adsorption of feed impurities as shown in Fig. 27; therefore, poisoned catalysts are very tough to regenerate [116, 117].

Fig. 27
figure 27

Deposition of chemical poisoning on the catalysts surfaces

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Poisoning is highly affected by reactants, products, or impurities present on sites or else existing for the catalysis. Depending over the poisonous concentration, poisoning might be fast or slow; measured on the strong poison adsorption, poisoning is either reversible or irreversible. The current technologies shown for good understanding of deactivation mechanisms that will facilitate more efficient design and optimization of processes concerning deactivating catalysts were available [103,104,105,106].

7 Regeneration of transition metal catalysts

The different steps are applied in the regeneration of catalysts were represented in Fig. 28. The activity of reused catalysts mostly depends on catalysts chemical composition and recycling conditions. In general in a reaction cycle, the catalyst follows two main steps: (i) separation from the reaction mixture for catalyst collection and (ii) treatment for its regeneration. The catalysts are recovered by filtration using separation funnel, centrifugation, and sedimentation. Ideally, a catalyst should be reused as numerous times as possible without any treatment. The reusability of catalyst could be done by further treatment for regeneration prior to next use. The strategy for increasing the life and reusability of transition metal catalyst and preparation for catalyst removing with correspond to catalytic converters is noted [118, 119]. The mild drying/calcination in atmosphere increased temperatures to generate catalytically active oxides. In regeneration by the presence of robust washing and chemical treatments to remove channel blockages, deactivated catalyst metals, and poisons, followed by chemical treatments to restore active catalytic materials, normal regeneration conditions are considered to elimination of blockages, deactivated catalyst, poisons and restore active catalytic material [120, 121]. The mechanism of catalyst deactivation is more important to the applications of its regeneration process. An economical possibility to preparation of catalyst by decreasing the price of manufacture was shown, and for actual high-scale application, a significant characteristic feature of catalyst is its “reuse” property [122, 123].

Fig. 28
figure 28

Regeneration of transition metal catalysts

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The transition metal catalysts regeneration processes are applied for total removing of blinding layers, catalyst performances usually superior after regeneration, SO2 to SO3 conversion rate, mechanical constancy, and deactivation rate in all properties of regenerated catalyst are as better than the novel catalyst. The off-site regeneration processes are more sophisticated and demanding than on-site rejuvenation processes; it offer considerably more efficient cleaning and reconstitution of catalyst with complete improvement of activity—sometimes better than the fresh catalyst performances [124]. Rejuvenation showed only partial (up to 85%) recovery of the original activity. The fresh catalyst performs by removing the carbon deposits and returning the sintered transition metal catalyst particles close to the optimum size. The careful estimation of basic causes of TMO catalyst deactivation is significant catalytic system and represents how correct choice of regeneration conditions can improve the life of catalysts by redispersion of active metal. Sintering is most excellently removed by minimizing and controlling the temperature of reaction, even though the new developments have pay attention on encapsulating metal crystallites to remove the mobility, while remain allowing entrance for reactants and products [125,126,127]. Transition metal catalyst regeneration is possible in various situations in particular to recover activity failure due to fast coking or more term deactivation related with failure of active metal dispersion. Typically, the regeneration or rejuvenation strategies of transition metal catalysts are discussed by the procedure or financial requirement to obtain the certain process on running lengths [128,129,130,131]. Life cycle working strategies are essential when operational catalyst regeneration against replacement decisions. Rejuvenation treatments can improve the valuable life of transition metal catalysts. Selective catalytic reduction catalysts showed superior TMO catalysts rejuvenation practiced in a viable process. The regeneration of deactivated catalysts is mostly depending on the chemical, economical, and environmental factors [132,133,134].

8 Conclusions

Carbon monoxide is a poisonous gas which should be removed from exhaust gases and stationary sources. Catalytic reaction is broadly applied for measuring the catalytic performances of novel materials. The transition metal catalysts are highly prominent metal oxide catalysts for ambient temperature CO oxidation. The clean and reduced transition metal catalysts is needed rather it seems to be accepted that the adsorbed oxygen is required to bring about CO adsorption. The CO oxidation over transition metal catalysts to be associated in the presence of several forms of chemisorbs reactants, products, and intermediates on the surface. The Co3O4, Mn2O3, Cu2O, ZnO, NiO, Fe2O3, IrO2, and NiO catalysts are individual one of the most significant TMO catalysts for ambient temperature CO conversion. The accumulation of suitable promoters, supports, pretreatment, and superior preparation methods would result to improvement in the performances of transition metal catalyst toward CO oxidation. The relative ease of preparation, good thermal and chemical stability, and high activity of transition metal catalysts offer better activity for automobile vehicle pollution control applications. This review paper represents an important information about the pure and substituted transition metal catalysts for CO emissions control.