Catalyst support was prepared by anodization of the 200 × 200 × 0.4 mm plate-type aluminum plate (1060) in a 0.4 wt% oxalic acid solution for 10 hours at 20˚C with the current density of 25 A/m². Then the catalyst was calcined at 350˚C for 1 hour in muffle, in order to the residual oxalic acid could be decomposed in the pore structure of the anodic alumite film. To increase the BET surface area of the support, immerse the plate in deionized water at 80˚C for 1 - 2 hours, namely Hot Water Treatment (HWT) [29]. After HWT, the plate was naturally dried at room temperature for 6 hours, the resulting plate was denoted as a-AlOOH. The a-AlOOH support was calcined at 250˚C for 4 h and then was immersed in nickel nitrate solution (Ni(NO₃)₂), subsequently washed with deionized water and dried in ambient conditions for 12 hours, denoted as Ni_x/ a-AlOOH.

Ni_x/a-AlOOH support was immersed into the mixed solution of 0.1 - 0.8 g/L H₂PtCl₆∙6H₂O under magnetic stirring for 4 h to achieve the nominal weight of Pt equals to 1.2 wt%, then dried in ambient conditions for 12 hours. Then the catalyst was immersed into NaBH₄ solution (0.01 mol/L) for 1h. Finally, the catalyst was washed with deionized water and dried in ambient conditions for 12 hours. The obtained catalyst was denoted as Pt/Ni_x/a-AlOOH.

The surface morphology of catalysts was characterized by a scanning electron microscopy (SEM) (JSM-6360LV, JEOL). The Brunauer-Emmett-Teller (BET) surface area (S_BET) determined by a multipoint BET method, and pore structure of catalysts were obtained from nitrogen adsorption data in the relative pressure P/P₀ range of 0.0 - 1.0 measured by using a Micromeritics ASAP 2020-M nitrogen adsorption apparatus (USA). The single-point pore volume (Vp) and pore size distributions (PSD) were determined by using desorption data by the Barret-Joyner-Halender (BJH) method.

X-ray powder diffraction (XRD) patterns of the different supports and as-prepared samples were recorded on a Rigaku D/MAX 2550 VB/PC X-ray diffractometer (Japan) using copper-anode radiation operated at 30 kV and 40 mA. The data was collected over the 2θ angle ranging from 10˚ to 80˚ with a scan step of 0.02˚. Transmission electron microscope (TEM) was characterized on a JEM-2100HR TEM (JEOL, Japan). Thermal gravimetric analysis (TGA) of the samples was performed on a WRT-3P TG equipment with a humidity controller. To determine the temperatures at which water molecules were lost as samples were heated. Samples (~10 mg) were heated in N₂ flow (40 cm³∙min⁻¹) at a heating rate of 10˚C∙min⁻¹ from 25˚C to 650˚C. XPS measurements were analyzed on an XSAM-800 apparatus (Kratos, UK), equipped with Al Kα X-ray with 1486.6 eV as the excitation source, and all the binding energies were calibrated respect to the graphite C 1s peak at 284.8 eV.

The catalytic activity evaluation for the catalytic oxidation of HCHO under atmospheric pressure was performed in a fixed-bed quartz flow reactor (i.d = 10 mm) as shown in Figure 1. Air from a compressor flowed into a bubbler filled with HCHO solution (HCHO: 35%) in a water bath maintained at 30˚C and then mixed with the HCHO stream evaporated by heating. The inlet concentration of HCHO was controlled by adjusting the bath temperature and the air flow rate. The feed gas composition was 180 ppm of HCHO. The total flow rate was 220 mL/min, corresponding to a gas hourly space velocity (GHSV) of 15,000 h⁻¹. The plate-type aluminum monolithic catalyst was cut into 2 - 4 mm². Then, approximately 0.3 g catalyst mixed with 1 g quartz sands (20 - 40 plate, Yonghua Co., Ltd.) was packed into the quartz tube reactor. The left and right sides of the reactor were filled with quartz fibers. The catalytic activity tests were conducted at the temperature range from 15˚C to 140˚C. The inlet and outlet gas CO₂ were monitored using a gas chromatograph equipped with a flame ionization detector (FID) and methanizer. Before the gas analysis, the CO₂ was quantitatively converted into methane under H₂ present. A PORAPAK-Q column was used to separate the CO₂ and CO gas. The CO₂ was determined by residence time. The temperature of the ionization detector and the column were 120˚C and 90˚C. The HCHO conversion was calculated using the degree of HCHO consumption.

The removal percentage of HCHO was calculated as follows:

HCHO   convers ( % ) = n HCHO,0 − n HCHO,0 n HCHO,0 × 100 %

where the n_HCHO,0 and n_HCHO,t represent the concentration of HCHO at the inlet and outlet of the reactor.

Figure 2(a) and Figure 2(b) show the surface morphology and structure of a-AlOOH and Pt_1.15/Ni_3.1/a-AlOOH catalyst by SEM. It can be seen that the surface of the catalyst is like honeycomb. The surface of the catalyst changes a little before and after the loading, probably because of the interaction between Pt or Ni and the carrier, indicating that Pt distributed on the surface of the catalyst evenly. This is the same with the specific surface area and pore width in the BET characterization results.

Table 1 shows that with the increasing of Ni content, S_BET of the catalyst gradually decreased, but they are all a little bigger than those without Ni, which might be due to the loading of Ni, or because the strong acidity of the Ni(NO₃)₂

Notes: ^aResults of ICP-AES analysis.

(pH = 2.0) corroded the carrier. This may facilitate the formation of surface reacting sites.

Figure 3 shows the catalytic activities of a-AlOOH, Ni/a-AlOOH Pt/a-AlOOH and Pt/Ni_x/a-AlOOH. Figure 3(a) shows that a-AlOOH can oxidize HCHO at 20˚C with CO₂ conversion of 10%, and 100% at 250˚C. While the HCHO complete oxidation over Ni_3.1/a-AlOOH is at 120˚C, and the HCHO conversion at 15˚C is 20%, which is higher than that of a-AlOOH. This may be because the presence of Ni in the form of NiOOH enriches the number of surface active hydroxyl groups of the catalyst and provides more active sites.

Meanwhile, Figure 3(b) shows that the catalyst Pt/Ni_x/a-AlOOH has higher oxidation performance than Ni/a-AlOOH and Pt/a-AlOOH. And the catalytic activity of Pt/Ni_x/a-AlOOH increased with the content of Ni before the Ni loading was 3.1 wt%. For the catalyst Pt_1.15/Ni_3.1/a-AlOOH, the HCHO conversion rate was 81% at 15˚C and the complete decomposition temperature of HCHO was 40˚C. While the corresponding for the catalysts Ni/a-AlOOH and Pt/a-AlOOH are 20%, 120˚C and 56%, 80˚C. This may be attributed to the loading of Ni not only enriches the number of surface active hydroxyl groups of

the catalyst, but also increases the content of Pt⁰, which is more conducive to converting formaldehyde. Figure 3(c) shows the activities of different catalysts at 15˚C. It can be seen that the catalyst shows the best activity when the Ni content was 3.1 wt%. It can be seen that even at low temperature Pt_1.15/Ni_3.1/a-AlOOH exhibits the activity of 80%, while 55% over Pt_1.2/a-AlOOH.

Figure 4 displays the XRD patterns of Pt/Nix/a-AlOOH. The diffraction peaks at 14.3˚, 28.2˚, 38.4˚, 49.3˚, 55.2˚, 65.0˚ and 71.7˚ correspond to structured a-AlOOH (a-AlOOH, JCPDS 21-1307) for all catalysts. The diffraction curves of Pt/Ni0.8/a-AlOOH and structured a-AlOOH are similar, which may be because of the low content of Ni and Pt or the small particle size of Ni and Pt. There are diffraction peaks at 12.8˚, 25.9˚ and 37.9˚ and 43.2˚ for Pt/Ni_2.2/a-AlOOH, Pt/Ni_3.1/a-AlOOH and Pt/Ni_4.5/a-AlOOH, which is the characteristic peak of NiOOH (Nickel oxide hydroxide, JCPDS 06-0075), and they correspond {003}, {006}, {101} and {105} faces. Among them, the first diffraction peak of these three samples is obviously different from structured a-AlOOH, possibly because of the influence of Ni diffraction peak. The formation of NiOOH species enables Pt/Nix/a-AlOOH to have two different hydroxyl groups on the surface in addition to water, increasing the content of hydroxyl groups on the surface of the catalyst, thus improving the efficiency of catalytic oxidation of HCHO.

Figure5 shows the microscopic morphology of Pt/Ni_3.1/a-AlOOH was observed by TEM and HRTEM. Figure5(a) shows the microstructure of Pt/Ni_3.1/a-AlOOH. Some tiny Pt and Ni were loaded on these nanorods, which were attributed to the shape of the plate-like a-AlOOH body after hydration. By further enlarging the magnification, it can be seen that Pt particles have a good dispersion on the catalyst, as shown in Figure5(b). The black spots in the Figureare Pt particles. It can be seen from Figure5(b). that Pt particle size distributed between 0.5 and 7.5 nm, while the main particle size is mainly concentrated in the range of 2.75 - 3.75 nm. Combined with XRD testing, the well dispersion of Pt on the catalyst

may be due to the impregnation of Ni(NO₃)₂, which may be because of abundant surface hydroxyl. Furthermore, HRTEM analysis was carried out to investigate the facets difference of the catalysts, as shown in Figure 5(c). The presence of the Pt_1.15/Ni_3.1/a-AlOOH indicated that the lattice spacing values parallel to the top and om facets are ca. 0.211 which corresponds to the {101} planes of the crystallized NiOOH. The presence of NiOOH increases the content of hydroxyl on the surface of the catalyst, which is conducive to adsorption and conversion of formaldehyde.

Figure 6 shows the influence of the surface composition of a-AlOOH and Pt/Ni3.1/a-AlOOH catalyst on the catalytic performance of the catalysts. The two samples were compared by thermogravimetric (TG) analysis. The region (at ca. 87˚C - 93˚C) for a-AlOOH and the region (at ca. 90˚C - 100˚C) for Pt/Ni_3.1/a-AlOOH correspond to the weakly physisorbed water molecules, which can be easily removed below 100˚C. The region (ca.260˚C - 460˚C) for a-AlOOH and the region (at ca. 200˚C - 510˚C) for Pt/Ni_3.1/a-AlOOH are due to the loss of the chemisorbed molecules of structural hydroxyl groups over a-AlOOH, respectively. The overall weight loss of Pt/Ni_3.1/a-AlOOH was significantly higher than that of a-AlOOH, possibly because of the surface of Pt/Ni_3.1/a-AlOOH contained two kinds of substance: a-AlOOH and NiOOH. Besides, there is a third weightlessness peak at about 510˚C, which was due to the decomposition Ni₂O₃ into NiO. Because at 300˚C NiOOH may decompose into Ni₂O₃, then Ni₂O₃ was

thermal decomposed to form the third weightlessness peak. In summary, the surface of the catalyst doped with Ni had two different hydroxyl groups (Al-OH and Ni-OH), which enriched the surface active hydroxyl groups of the catalyst.

Figure 7 shows the XPS characterization of Pt_1.2/a-AlOOH and Pt_1.15/Ni_3.1/a-AlOOH to investigate the chemical valence states of Pt, O and Ni on the surface of the catalyst, and Table 2 concludes the obtained data. The deconvoluted O1s signal of Pt_1.2/a-AlOOH presents three peaks at 530.4 - 530.8, 531.5 - 531.8 and 532.9 - 533.1 eV, attributed to Al-O, Al-OH, and adsorbed water (Figure 7(a)), respectively. For comparison, the deconvolution of the O1s signal of Pt_1.15/Ni_3.1/a-AlOOH gives four peaks (Figure 7(c)). The values of the other three peaks are similar to those of the samples without Ni, while another one is about 532.1 eV, which is attributed to O 1s in NiOOH. This corresponds to the NiOOH crystals measured in XRD. In addition, the binding energy of oxygen Al-O in the lattice of Pt_1.15/Ni_3.1/a-AlOOH catalyst was shifted by 0.4 eV relative to Pt_1.2/a-AlOOH to the lower energy level, which may be due to the fact that NiOOH acts as an electron donor to transfer electrons to the carrier and Pt particles. By comparing the proportions of several kinds of oxygen, the percentage of lattice oxygen (Al-O) decreased from 41.5% to 18.5% after the addition of Ni, and the surface hydroxyl (−OH) increased from 36.4% to 72.8%, indicating that the NiOOH formed after the addition of Ni, increasing the surface hydroxyl content of the catalyst.

It can be seen from Figure 7(b) and Figure 7(d) that Pt can be divided into three valence states: P⁰, Pt²⁺ and Pt⁴⁺. Its peak binding energy is mainly about 314.5 - 314.6 eV, 316.5 - 317.3 eV and 317.9 - 318.7 eV. The results are summarized in Table 2. According to Table 2, the value of Pt⁰/(Pt⁰⁺ + Pt²⁺ + Pt⁴⁺) in Pt_1.15/Ni_3.1/a-AlOOH catalyst was 45.3%, while that in Pt_1.2/a-AlOOH was 27.2%. This may be because the addition of Ni makes it easier to reduce Pt. According

to our previous discussion, Pt⁰ plays an important role in the catalytic oxidation of HCHO. It is worth noting that the binding energy of Pt⁰ valence state turns to low energy level, while the binding energy of Pt²⁺ and Pt⁴⁺ turns to high energy level, which may be caused by the interaction between NiOOH, Pt and carrier. As the percentage of Pt⁰ state in Pt particles increases, Pt particles transfer electrons to nearby oxygen atoms to activate or promote the activation of adsorbed oxygen, thus improving the catalytic oxidation performance of the catalyst. In the Ni 2p spectra (Figure 7(e)), three peaks at 856.6 eV, 861.5 eV and 864.6 eV can be seen, which correspond to the typical NiOOH [40]. This is consistent with the characterization of XRD. Combined with the XPS analysis of O, it can be seen that the added nickel generates NiOOH, thus providing more surface hydroxyl and more adsorption sites for HCHO.