**1. Introduction**

Biomass is one of the most abundant renewable resources on the earth. Bio-oil, produced from biomass, is recognized as a green feedstock for the production of chemicals and fuels [1,2]. Bio-oil has the advantages of a higher energy density than the original biomass, secure storage and transportation, and flexible use. However, the much higher oxygen content in bio-oils results in lower heating value and poorer stability compared to crude oil, which makes it difficult to use directly as engine fuels or even oil additives without further upgrading [3,4]. Therefore, the study of upgrading bio-oils has attracted much attention in recent years due to environmental and sustainable concerns. One of the promising routines to upgrading bio-oils is catalytic hydrodeoxygenation (HDO) [4,5]. However, the conventional HDO process needs excessive hydrogen to maintain high hydrogen pressure of 7–20 MPa [6,7], which inevitably increases the storage and transportation costs and safety risks of hydrogen. Although the HDO process is useful, it is unfavorable for the production of fuels [8–10] and estimated to need approximately 0.11 kg H2/L oil [9] by a techno-economic analyses. Therefore, reducing external hydrogen supply, such as using in-situ generated hydrogen, might be one of the augmented approaches for improving the economic feasibility [11]. More importantly, compared with the conventional HDO process, the in-situ hydrodeoxygenation (in-situ HDO) process is more flexible without the complicated equipment and suited for the distributed upgrading of bio-oils [12], which helps to expand its industrial application.

In recent years, the in-situ HDO has become a trend in the bio-oil upgrading research [12–15]. Fisk et al. [12] reported that the Pt/Al2O3 catalyst had high activity for the in-situ HDO of a model bio-oil, and after upgrading the oxygen content of the model oil decreased from 41.4 wt% to 2.8 wt%. Other noble metal catalysts such as Pd/AC [13], Ru/MCM-41 [16] and Pd/C [11] also showed high activity for the in-situ HDO of some model components in bio-oils. Feng et al. [17] reported that the in-situ HDO process could not only reduce the usage of external hydrogen but also significantly improve the yield of target products in the bio-oil upgrading. Xiang et al. [18] used Raney Ni and a Pd/Al2O3 catalyst in the in-situ hydrogenation of phenol, o-cresol, and p-tert-butylphenol. They found that the activity of the in-situ generated hydrogen from aqueous-phase reforming (APR) was di fferent from that of H2 gas used in the hydrogenation of bio-oil. For example, the hydrogen generated from APR favored the production of cyclohexanone, while the hydrogen from H2 gas favored the generation of cyclohexanol in the hydrogenation reaction of phenol.

Ni-based catalysts also showed excellent catalytic activity for the upgrading of bio-oils [14,19–21]. Putra et al. [14] reported phenol conversion could increase to 86.75% when in-situ glycerol aqueous reforming and phenol hydrogenation over Raney Ni catalyst. Xu et al. [19] used Raney Ni for the in-situ HDO of phenol and found more than 64% phenol could be converted to cyclohexanol at 220 ◦C. However, they had low selectivity for some oxygen-free products. The combination with acidic sites could help to increase its deoxygenation ratio of bio-oil. Wang et al. [22] found the total deoxygenation ratio of bio-oil could reach 99.6% over the ZrNi/Ir-ZSM-5 and Pd/C catalysts through in-situ HDO by using methanol as a liquid hydrogen donor. When using Raney Ni and Nafion/SiO2 catalysts in the in-situ hydrodeoxygenation of bio-derived phenol, the phenol conversion and the total yield of cyclohexane can reach respectively 100% and 87%, respectively [23]. Feng et al. [15] reported that the catalysts of Raney Ni and HZSM-5 also yielded 70–90% cyclohexanes and hydrocarbons in the in-situ hydrodeoxygenation of biomass-derived phenolic compounds. Besides, the production of hydrogen obtained from aqueous-phase reforming of liquid hydrogen donors also had a significant e ffect on the in-situ HDO of bio-oils. Zeng et al. [16] observed that although methanol, ethanol, formic acid, and acetic acid could generate hydrogen, the product distributions of bio-oils after upgrading were very di fferent due to their di fferent productivity of hydrogen. Hence, another important strategy is to improve the productivity of hydrogen in the in-situ HDO of bio-oils. Many researchers have focused on the e ffect of liquid hydrogen donors [11,16,18] or the ratio of the hydrogen donors to phenols [13,14,22–24] on the hydrogen availability. Unfortunately, few works have reported the e ffect of the multifunctional catalysts on hydrogen production in this process. Raney Ni [14] and the monometallic Ni catalyst [24] have an excellent hydrogen yield for the oxygenates aqueous-phase reforming. However, a considerable amount of work [25–30] has demonstrated that the bimetallic or trimetallic catalysts have positive effects on hydrogen production in the oxygenates reforming and inhibit the sintering of the active phase due to their excellent dispersion and electronic properties. Also, bimetallic catalysts such as Ni-Cu/Al2O3 [31], Ni-Fe/Al2O3 [32,33], Ni-Fe/MCSs [34], and Ni-Co/HZSM-5 [35] had excellent catalytic activity for converting bio-derived phenols into hydrocarbons by the conventional HDO with external hydrogen gas. Recently, Zhang et al. [36] prepared the Cu-Ni/ZrO2 catalysts for in-situ HDO of oleic acid to heptadecane and found the bimetallic catalysts had higher activities compared with the monometallic Ni catalysts because of the synergistic e ffect between Ni and Cu alloy. These results indicated that the use of the bimetallic or trimetallic catalysts could improve both the hydrogen availability and the deoxygenation ratio of bio-oil in the in-situ HDO process.

In this study, a trimetallic Ni-Cu-Co/Al2O3 catalyst was prepared and applied in the in-situ hydrodeoxygenation of phenol. Phenol derivatives comprise up 30 wt% of bio-oil [37], and therefore, phenol is suitable as a probe molecular to understand the primary pathways of in-situ HDO reaction. The various reaction conditions such as temperature, N2 pressure, reaction time, the molar ratio of water/methanol, and liquid hydrogen donors on the in-situ HDO reaction were studied. The recyclability of the Ni-Cu-Co/Al2O3 catalyst was also tested. Furthermore, the relationships of the structure and catalytic activity of the Ni-Cu-Co/Al2O3 catalyst were studied by X-ray powder di ffraction (XRD), BET analysis, temperature programmed reduction (TPR), scanning electron microscope (SEM), and transmission electron microscope (TEM), and the synergistic effects of Ni with Co and Cu on the in-situ hydrodeoxygenation of phenol was discovered.

#### **2. Results and Discussion**
