Opposite Effects of Co and Cu Dopants on the Catalytic Activities of Birnessite MnO2 Catalyst for Low-Temperature Formaldehyde Oxidation

Defect engineering is an effective strategy to enhance the activity of catalysts for various applications. Herein, it was demonstrated that in addition to enhancing surface properties via doping, t...


INTRODUCTION
Volatile organic compounds (VOCs) constitute one of the main sources of air pollutants that pose grievous challenges to human health. [1][2] Various removal techniques have been devised to eliminate VOCs from the environment, which include adsorption, photocatalysis and catalytic oxidation. [3][4] Amongst these pollutants, formaldehyde (HCHO) is of great interest, because it is one of the most commonly found pollutants in the indoor environment. Its removal is pertinent to improving human health and indoor air quality, as exposure to HCHO can cause adverse health effects and nasopharyngeal cancer in extreme cases. 5 Catalytic oxidation is a promising technique that could effectively mineralize HCHO into harmless products: CO2 and H2O, and completely remove it from the indoor environment. [5][6] Recent attention has focused on the development of cost-effective transition metal based catalysts for various applications in catalysis. Structured manganese oxide (MnO2) catalysts have been a subject immense research focus as promising cost-effective, active and environmentally friendly alternatives. 5,7 Amongst the structured manganese oxide catalysts, birnessite (δ-MnO2) has received a special attention due to its unique properties. Important characteristics of δ-MnO2 include its layered structure, mixed oxidation state and charge-neutralizing cations situated in the interlayer spaces. This allows possibilities for further structural modification to enhance its properties and catalytic activity. Various strategies devised, such as defect engineering, have enhanced the properties and catalytic activity of δ-MnO2 catalysts for various applications including oxidation reactions. Structural defects could be induced through doping or the incorporation of metal cations into the lattice framework of the host material. [8][9] The partial substitution of Mn 4+ by dopants within the [MnO6] octahedral framework of δ-MnO2 could lead to the generation of oxygen vacancies and structural lattice distortion. 10 These defective sites serve as centers for the adsorption and activation of molecules, and in particular, the generation of active oxygen species, through the activation of water and/or molecular oxygen, which were shown to actively participate in most oxidation reactions, particularly HCHO oxidation. 7,[11][12] Additionally, dopants can improve the catalytic activity of δ-MnO2 by enhancing its redox properties and lattice oxygen mobility and reactivity through the reduction of charge transfer resistance. [13][14] Due to their similar ionic radii, Cobalt (Co) was shown to effectively incorporate into the octahedra of δ-MnO2 by substituting Mn 4+ leading to the creation oxygen vacancies. 10 Elmaci et al. 13 explored various transition metals as dopants for δ-MnO2 and demonstrated that, compared to the pristine δ-MnO2, Co-doped δ-MnO2 exhibited enhanced catalytic activity and chemical stability for water oxidation reaction. This is due to the substitution of Mn 4+ by Co 3+ in the octahedral framework of δ-MnO2 leading to an increase in the ration of Mn 3+ /Mn 4+ . Similar observations were reported elsewhere for Co doped δ-MnO2 for water oxidation catalysis. 15 Yin et al. 16 observed that the substitution of Mn 4+ by Co 3+ resulted in the negative charge of the δ-MnO2 layers and an increase in the concentration of hydroxyl groups, which accounted for its improved capacity for the adsorption of lead and arsenite from water. The formation of Co-Mn-O and Cu-Mn-O bridges in the case of Co 17 and Cu 18 doped MnO2 respectively, was shown to improve the mobility and activity of lattice oxygen, which enhances catalytic activity for the preferential oxidation of CO. In another report, Wang et al. 19 investigated the modification of δ-MnO2 for CO oxidation with various transition metals. They showed that Copper (Cu) doped δ-MnO2 exhibited improved low temperature activity compared to the pristine catalyst. The substitution of Mn by the dopants facilitated the generation of oxygen vacancies and the mobility of lattice oxygen leading to the generation of more active surface oxygen species. Similar observations were reported for Cu doped δ-MnO2 for toluene 20 and benzene 21 oxidation, with significantly lower temperature activities compared to the unmodified δ-MnO2, owing to the formation of oxygen vacancies and its resultant effect on low-temperature reducibility and lattice oxygen reactivity. Doping Cu into the octahedra framework of δ-MnO2 was also shown to reduce the electron transfer resistance of δ-MnO2, thereby enhancing its catalytic activity and stability for oxygen reduction reaction. 14 The above works have demonstrated the prospects of Co and Cu as suitable dopants for enhancing the properties and catalytic activity of δ-MnO2 for HCHO oxidation, as defective sites were demonstrated to improve the redox properties of Mn, and facilitate the activation and mobility of oxygen into surface active species, which in turn enhances catalytic activity for HCHO. 8,[22][23] Besides the enhancement of catalysts' properties induced by doping, the influence of dopants on the surface reaction of intermediates is another critical parameter that influences catalytic activity.
It is well known that the surface accumulation of intermediates species grossly affect catalytic activity for reactions such as CO preferential oxidation, and their decomposition and desorption from the surface becomes critical. [24][25][26] The type of dopant employed for CO preferential oxidation, was shown to have a tremendous effect on the decomposition of adsorbed intermediates and catalytic activity. For instance, the presence of La, as a dopant, induced the surface accumulation of carbonate intermediates leading to the blockage of active sites and reduced catalytic activity, while a promotional effect was observed in the presence of Nd, for CO preferential oxidation over ceria. 25 Similar intermediates poisoning effect was observed on CO oxidation rate when Zr was used as a dopant in CuO/CeO2. 26 Despite recent research works that demonstrated δ-MnO2 to be an active and cost-effective transition metal catalyst for low-temperature HCHO oxidation, 5,7,11,27 only a few works were reported on its enhancement, via doping, which include Ce, 22  Herein, we demonstrated that besides the structure-activity effects of dopants, their influence on surface interaction with intermediates is another critical parameter that determines the catalytic

Catalytic Activity Evaluation. The catalytic activity of the catalysts for HCHO oxidation
within the temperature range of 30 to 120°C were evaluated. In a typical test, about 50 mg (40 -60 mesh) of catalyst was weighed and loaded into a fixed bed reactor (6 mm ID). 170 ppm of HCHO was generated by passing air over paraformaldehyde (97% Alfa Aesar) maintained at 30°C in a water bath. The total feed flowrate over the catalyst bed was maintained at 100 ml min -1 (120,000 ml •g -1 •h -1 ) with a relative humidity (RH) of ~ 50 %. With the aid of an Agilent 7890B GC fitted with an FID and a Polyarc universal carbon detector/reactor (Activated Research Company (ARC), US), the concentration of generated CO2 and unreacted HCHO in the outlet stream were simultaneously monitored online. CO2 was the only detected reaction product. The effect of space velocity on catalytic activity was investigated in the GHSV range of 120,000 to 400,000 ml•g -1 •h -1 . The stability and room-temperature oxidation of HCHO were investigated at 25°C in a dynamic mode at 60,000 ml g -1 •h -1 in the presence of ~ 10 ppm HCHO concentration (21%O/79%N2) for 72 hrs. Since CO2 was the reaction product, the conversion of HCHO (%) was calculated using the equation below: where CO2out and HCHOin represent the outlet concentrations of CO2 (ppm) and inlet concentration of HCHO (ppm) respectively. To determine the activation energy, the space velocity was varied from 240,000 to 400,000 ml•g -1 •h -1 to maintain the conversion below 20%. Birnessite MnO2 with 2D-layer is mainly composed of edge-sharing MnO6 octahedra with varying amounts of Mn 3+ /Mn 4+ , resulting to electrostatic charge imbalance and octadedra vacancies.
Positively charged alkali cations such as K + or Na + and water molecules are situated in the interlayer spaces, to provide charge balance, leading to an interlayer spacing of ~ 0.7 nm. 29  into nanospheres of about 110 nm. Though the nanospherical morphologies were retained even after doping with Co and Cu, a large degree of particle size reduction into smaller nanospheres of around 37.6 nm were observed in the case of Co, while large, dense and compact aggregated nanospheres with irregular shapes of about 147 nm in size were observed after Cu doping. This could be a result of the incorporation of Co into the lattice structure of birnessite leading to the inhibition of the growth of manganese crystals thereby resulting into smaller particle sizes, which is accompanied by increase in surface area, 22 as shown in Table 1. It was also reported elsewhere that the substitution of Mn ions by V [33][34] and Mo 35 in the lattice structure of of MnO2 leads to lattice deformation and inhibition of crystal growth, with the formation of smaller particle size. In the case of 0.05Cu-δ-MnO2, the larger particle sizes and particle agglomerations led to significant reduction in surface area as observed in the FE-SEM ( Figure 3g) and BET (Table 1) (Figure 3f and 3i). This could be attributable to structural defects, due to lattice distortions, 37-38 possibly resulting from incorporation of the dopants into the lattice structure of δ-MnO2. A higher degree of lattice distortion was observed on 0.05Co-δ-MnO2 compared to 0.05Cu-δ-MnO2, likely indicating a higher level of Co incorporation.  The elemental composition of the catalysts are presented in Table 1 The XPS results are summarized in Table 2 and depicted in Figure 4. corresponding to Mn 3+ and Mn 4+ respectively. 6,19,37 The splitting of Co 2p1/2 (795.12 eV) and Co 2p3/2 (780.1 eV) ( Figure S1) was found to be 15.02 eV, in agreement with the reported values for Co 3+ 16 .The presence of a shakeup peak at around 943 eV and a peak centered at around 933.6 eV ( Fig S2) indicates that the doped Cu exists in Cu 2+ state. [39][40] The observed decrease in the ratio of  water molecules. 11,19 As seen in Figure 4(b), and Table 2, the relative amount of surface active oxygen species increases after the addition of dopants. Relative to the pristine δ-MnO2, 0.05Co δ-MnO2 exhibited an increase in the content of surface active oxygen species compared to 0.05Cu δ-MnO2, in agreement with the decpnvolution results of Mn 2p3/2 ( Table 2) and HRTEM results.
It is widely known that structural defects in the form of oxygen vacancies act as sites for the activation of molecular oxygen and or water molecules into active or defective surface oxides and also enhance the mobility of lattice oxygen. 8,11,42 This indicates that more Co 3+ is incorporated into the lattice of δ-MnO2 compared to Cu 2+ probably due to the coordination radius of the earlier being closer to that of Mn 4+ compared to the latter, 10   To understand the reduction behavior and further gain insight into the surface reducibility of the catalysts and the activity of the respective surface adsorbed oxygen and the effect of oxygen vacancies on oxygen mobility, the H2 TPR profiles of the catalysts were collected and depicted in Although both the pristine MnO2 and the doped catalysts exhibit similar reduction patterns, a slight shift of the reduction profile of the doped catalysts, to lower temperature, was observed compared to the pristine δ-MnO2, as shown in Figure 5. Noticeably, the peak attributed to the reduction of surface active oxygen species shifted to around 150°C in the doped catalysts from 170°C in the pristine δ-MnO2. Additionally, all the temperatures for the consecutive reduction of Mn from MnO2 to MnO (β, γ and δ) decreased after doping, as depicted in Figure 5 and in Table   3. This suggests that the presence of dopants within the framework of δ-MnO2 influences its reduction behaviour and mobility of lattice oxygen, likely due to electronic delocalization effect of the dopants 17-18 and the formation of oxygen defects. 19,37,42 This in agreement with the observed defect formation due to doping in the Raman, XPS and HRTEM results.    Table 3. The T50% for δ-MnO2, 0.05Co δ-MnO2 and 0.05Cu δ-MnO2 are 74, 63 and 85°C respectively, while the T90% are 87, 77 and 97°C respectively. A seen from Table 3, 0.05Co δ-MnO2 showed the highest reaction rates over followed by the pristine δ-  The increase in the surface concentration of the active oxygen species in the presence of dopants, particularly Co, suggests that the incorporation of dopants led to the generation of oxygen vacancies, which act as sites for the activation of molecular oxygen into active surface oxygen species. It is clear from the characterization results that the remarkable activity of 0.05Co δ-MnO2 is attributable to its abundant surface active oxygen species due to oxygen vacancies, and its relatively higher reducibility and mobility of lattice oxygen. Consumed surface active oxygen species could be regenerated by the interaction of structural defects with molecular oxygen and water molecules. [18][19] The presence of oxygen vacancies could also enhance the mobility and activity of lattice oxygen to the surface, while molecular oxygen is activated into lattice oxygen at the oxygen vacant sites, thereby sustaining the whole process. 22,47 Catalyst doping and or surface modification has been demonstrated to be an effective strategy   (Table S1). This observation is in agreement with previous works that showed that the oxidation of HCHO is not dependent on the surface area of the catalysts. 5,7,22,48 Next, DRIFTS analysis were conducted to understand if the type of dopant present has an impact on the surface reaction of δ-MnO2, in terms of intermediate generation and desorption and/or accumulation. This will allow a better understanding of the promotional or inhibition effects of dopants.

REACTION MECHANISM
To further gain insights into the surface reaction mechanism of the HCHO oxidation and the influence of surface modification on the catalysts' reactivity, in situ DRIFTS analysis was conducted and the results are presented in Figure 8. In all the spectra, no peaks related to the C꞊O stretching band of surface adsorbed HCHO located at 1710cm -1 , was observed. 49  species. 6,11,48 Features attributed to bidentate carbonates (1653, 1240, and 1015-1053 cm -1 ) and monodentate carbonates (1470-1490, 1315-1331 cm 1 ) were also observed, 7,25,[51][52][53] while the bands at around 1620 cm -1 correspond to the bending vibration of adsorbed water molecules. [54][55] In the presence of Co, the DOM species (1161 cm -1 ) are quickly converted to formates intermediates, and exhibited the highest production rate of formates (1567 cm -1 ). Furthermore, the carbonate feature at around 1498 and 1231 cm -1 are only present as weak shoulder on 0.05Co-δ-MnO2, and the relative intensity of the carbonate peak at 1650 cm -1 is lower compared to other catalysts. This suggests that in the presence of Co, not only is HCHO quickly converted into its intermediate species but also are the intermeidates quickly desorbed from the surface -freeing up active sites, leading to high catalytic activity, in agreement with its high reaction rates and lower activation energy (Table 3). This could likely be related to the enhanced lattice oxygen mobility and generation of surface active oxygen species induced by surface defects due to the incorporation of Co into the lattice structure of δ-MnO2. Surface defective oxides and hydroxyl groups were reported to take part in the formation of formates intermediates and further oxidation of surface carbonates into CO2. 7,51 On the other hand, it is interesting to observe that the carbonates peak (1653 cm -1 ) on 0.05Cu δ-MnO2 nearly became the dominant peak, thereby indicating the surface accumulation of carbonates with time. Furthermore, additional weak feature appeared at around 840 cm -1 on 0.05Cu δ-MnO2, which is attributed to carbonate species. 35,53  To further ascertain this observation, the DRIFTS scans for 0.2Cu δ-MnO2 at room temperature are presented in Figure S7. Compared to other catalysts, a remarkable weakening in the intensities of all the absorbance peaks were observed for the RT scans, indicating a poor RT performance, in agreement with the activity test results ( Figure S4). Moreover, the peaks attributed to carbonates species (1020 and 1646 cm -1 ) dominated the entire spectra, which developed with time. The peaks related to formates appear very weak, notably, the asymmetric COO stretching peak of the formates (which is the dominant peak in other catalysts,) only appears as a weak shoulder band at 1580 cm -1 as depicted in Figure S7. This further supports the observed accumulation of carbonates on 0.05Cu δ-MnO2 ( Figure 8c) and could provide insights into the inhibitory effect of Cu on the activity of δ-MnO2, and the observed higher apparent activation compared to the pristine δ-MnO2.
A plausible explanation could be related to the direct surface coordination between Cu and carbonates leading to surface accumulation and blockage of catalytic active sites. This could lead to steric hindrance for the reaction of HCHO with active sites, and a slowdown in catalytic activity, as observed in the catalytic conversion of HCHO over all Cu modified δ-MnO2 catalysts ( Figure   S4). It has been demonstrated that the presence of some dopants ( La 25 and Zr 26 ) could induce surface accumulation of carbonates, leading to the blockage of active sites and reduced catalytic activity for CO preferential oxidation, [25][26] supposedly due to the thermal stability of the surface accumulated carbonates, 26 and hindrance of metal redox cycle and oxygen activation and mobility. 24 The high temperature DRIFTS results ( Figure S8) illustrated that, as the reaction increases, the intensity of the surface accumulated carbonates attenuated (1646 cm -1 ) while that of formates (asymmetric stretching of COO 1583 cm -1 ) progressively increased until it became the dominant feature at 120°C (Figure S8 and S9). This suggests that as the carbonates desorb from the surface, more active sites are exposed and HCHO is converted into formates (evident by the increase in intensity at 1583 cm -1 in Figure S8). The hindrance effect of carbonates accumulation, in the presence of Cu, could be seen in the catalyst's inability to generate formate below 60°C ( Figure S8). It is also noteworthy to mention that the oxidation of the carbonates into CO2 and the formation of formates, at elevated temperature, were accompanied by the consumption of surface hydroxyl groups ( Figure S8). This indicates the participation of surface hydroxyl species in the oxidation of carbonates into CO2.
In addition to surface hydroxyl groups, the main intermediates observed in the DRIFTS spectra  On the basis of the above observations, it is therefore rational to state that the dopantintermediate interaction is another critical parameter that affects catalytic activity. The presence of Cu in δ-MnO2 leads to surface accumulation of carbonates, which in turn led to partial blockage of surface active sites, and likely hinder surface redox cycle and oxygen activation and mobility. 24,26 This could explain why 0.05Cu δ-MnO2 exhibited reduced catalytic, evidenced by lower reaction rates and HCHO conversion, and higher activation energy, despite possessing improved redox properties and relatively enriched surface active oxygen species, compared to the pristine δ-MnO2. As such, for low-temperature oxidation of HCHO, desorption of carbonates from the catalysts' surface is a critical step to ensure availability of active sites for continuous reaction.
Therefore, in addition to improving catalyst properties via doping for HCHO oxidation, the influence of the dopants on intermediates desorption from the catalyst's surface, particularly for low reaction temperature oxidation, in which case, not enough energy is applied to drive the desorption, is another critical parameter to be considered in designing catalysts for practical abatement of HCHO from indoor air environment.

CONCLUSION
Co and Cu dopants were explored for enhancing the catalytic activity of δ-MnO2 for lowtemperature HCHO oxidation. While defect generation was found to be of promotional effect, results suggests that dopants-intermediates interaction is another critical parameter that influences

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.