## 3 Experimental
### Catalysts Characterization  
The thermal decomposition pattern and carbon content of nanocomposites were determined by TG/DTA using a Rigaku Thermo plus TG 8120 instrument (Tokyo, Japan). The analysis was performed at a heating rate of 10 \({}^{\circ}\)C min\({}^{-1}\) from 50 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C under dry air flow of 50 mL min\({}^{-1}\). The surface functional groups of materials were determined by XPS using an ESCA 1700R system (ULVAC-PHI, Inc., Kanagawa, Japan) with Al K\(\alpha\) radiation (1486.8 eV). The binding energy (BE) for the high-resolution C1s spectrum was calibrated by setting C1s at 284.6 eV. The curve-fitting of XPS spectra was performed with the OriginPro 8.5 software version 85E (OriginLab Corporation, Northampton, MA, USA). The structural information of the materials was obtained from powder XRD analysis using a Bruker D8 ADVANCE diffractometer (Bruker Corporation, Billerica, MA, USA) equipped with a Cu K\(\alpha\) radiation (\(\lambda\) = 1.5406 A), which was operated at voltage of 40 kV and current of 40 mA. The XRD patterns were recorded at room temperature over the 2\(\theta\) range of 5-80\({}^{\circ}\) with a scanning step of 0.02\({}^{\circ}\) and a count time of 1 s. The textural properties of the template-free materials were determined by N\({}_{2}\) physisorption measurement at \(-\)196 \({}^{\circ}\)C using a Micromeritics ASAP 2020 surface area and porosity analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). The sample was degassed at 150 \({}^{\circ}\)C for 2 h prior to the measurement. The specific surface area (\(S_{\rm BET}\)) was calculated from adsorption data in the relative pressure (\(P/P_{0}\)) range from 0.05 to 0.3 using the Brunauer-Emmett-Teller (BET) equation. The total pore volume (\(V_{\rm t}\)) was obtained from the adsorption branch at \(P/P_{0}\approx\) 0.99. The Barret-Joyner-Halenda (BJH) plot was applied to determine pore size (\(D_{\rm p}\)) using the adsorption data. The acidic properties of calcined catalysts were examined by NH\({}_{3}\)-TPD using a Micromeritics AutoChemII 2920 chemisorption analyzer (Georgia, USA). Prior to the analysis, the sample powder was pretreated at 500 \({}^{\circ}\)C for 60 min under a helium (He) flow (50 mL min\({}^{-1}\)). Subsequently, 10% NH\({}_{3}\) in He gas (10 mL min\({}^{-1}\)) was introduced to the sample at 50 \({}^{\circ}\)C for 30 min to allow the adsorption of NH\({}_{3}\). The physisorbed gas was flushed, and the sample was heated under a He flow (50 mL min\({}^{-1}\)) to 500 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C min\({}^{-1}\) to desorb NH\({}_{3}\). The total amount and distribution of acid sites were determined after deconvolution of TPD profiles using OriginPro 8.5 software version 85E (OriginLab Corporation, Northampton, MA, USA). The in situ Py-FTIR analysis was performed on a NICOLET iS10 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The powdery sample was pressed into a self-supporting disc and placed in a quartz cell equipped with CaF\({}_{2}\) windows. The sample disc was pretreated at 500 \({}^{\circ}\)C for 1 h under vacuum to remove some impurities. After cooling to 50 \({}^{\circ}\)C, pyridine vapor was introduced into the cell until the adsorption reached equilibrium, followed by evacuating at the same temperature for 15 min. The spectra were recorded for 96 scans. The corresponding bands were deconvoluted for better assignment using OriginPro 8.5 software version 85E (OriginLab Corporation, Northampton, MA, USA), and the content of both acid sites was quantified using extinction coefficients. The morphological study was performed by FE-SEM using a JEOL JSM-7610F instrument (JEOL Ltd., Tokyo, Japan) operating at 40 kV. The sample powder was dispersed on carbon tape, followed by gold coating. The FE-SEM images were recorded at magnification of 50,000\(\times\). TEM was used to investigate the dispersion of Nb crystallites.