# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 3 Hydrogen Production by Catalytic Reforming of Pyrolysis Volatiles
### Slow Pyrolysis
#### 3.1.5 Catalyst Type  
Selecting the appropriate catalyst clearly has a significant role in designing the two-stage pyrolysis and reforming process, in order to achieve hydrogen-rich gas. Reviewing existing research articles shows that different types of metal-based catalysts such as Ni, Co, Pt, Fe, or Rh can be used for this process. [PERSON] et al. investigated the effects of different catalysts, including NiO/Al\({}_{2}\)O\({}_{3}\), NiO/CeO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\), and NiO/SiO\({}_{2}\). The obtained results showed that hydrogen productions of 44.4 vol%, 43.1 vol%, and 37.6 vol% were achieved by using NiO/Al\({}_{2}\)O\({}_{3}\), NiO/CeO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\), and NiO/SiO\({}_{2}\), respectively [79]. [PERSON] et al. also evaluated the effect of a 12 Ni/MnO\({}_{x}\)Al\({}_{2}\)O\({}_{3}\) catalyst on hydrogen production using a two-stage process. Their results demonstrated a 46.5 vol% hydrogen concentration [80]. The impacts of Ni/CaAlO\({}_{x}\) catalysts with various Ca/Al ratios on hydrogen production were assessed by [PERSON] et al. Based on their results, a maximum hydrogen yield of 15.57 mmol/g was achieved using a 1:2 ratio, and the lowest hydrogen production of 12.97 mmol/g was achieved by a 1:2 ratio of Ca/Al [64]. Moreover, [PERSON] et al. examined the effect of adding calcium to Mg-AlO\({}_{\rm x}\)-supported Ni catalysts. The results showed that adding calcium increased hydrogen production from 10.45 mmol/g to 18.2 mmol/g. Also, they evaluated the effect of using different calcium ratios on the two-stage process of reforming pyrolysis volatiles, and hydrogen production was maximized up to 21.4 mmol/g by using a 0.3 ratio of calcium [69]. From the analysis of the literature, it emerges that low-cost metal-based catalysts, e.g., Ni and Co catalysts, are the most used ones. Ni/Al\({}_{2}\)O\({}_{3}\) is one of the most commonly used catalysts in experimental studies. [PERSON] et al. [72] assessed the effect of a Ni/Al\({}_{2}\)O\({}_{3}\) catalyst on a two-stage process. Corncob was used as a feedstock, and the pyrolysis and catalytic reforming temperatures were set to 900 \({}^{\circ}\)C and 650 \({}^{\circ}\)C, respectively. The results of the catalytic steam reforming process showed a 51 mmol/g hydrogen yield. The impact of using Ni/Al\({}_{2}\)O\({}_{3}\) as a catalyst was evaluated for the dry and steam reforming of pyrolysis volatiles of sewage sludge at 900 \({}^{\circ}\)C. Based on the obtained results, using a Ni/Al\({}_{2}\)O\({}_{3}\) catalyst at 650 \({}^{\circ}\)C increased hydrogen production in the dry and steam reforming processes to 28 mmol/g and 53 mmol/g, respectively [75]. [PERSON] et al. [67] performed an experimental study to assess the pyrolysis and catalytic reforming process. This study used 1 g of raw wood sawdust and 0.5 g 10 wt% Ni/Al\({}_{2}\)O\({}_{3}\) as a catalyst to perform the experiment. The pyrolysis temperature range was considered between 300 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, and the reforming temperature was set to 800 \({}^{\circ}\)C. The obtained results demonstrated that the hydrogen yield increased up to 11.01 mmol/g at 600 \({}^{\circ}\)C.  
Moreover, other types of Ni-based catalysts, prepared by mixing nickel with other types of metals and non-metal materials, have been used in the literature to assess their effects on hydrogen yield in the two-stage process of reforming pyrolysis volatiles. The feasibility of using a novel catalyst by loading nickel on lignite char for the reforming of corncob pyrolysis volatiles was evaluated by [PERSON] et al. [76], considering 900 \({}^{\circ}\)C and 650 \({}^{\circ}\)C for the pyrolysis and reforming temperatures, respectively. The results of using this catalyst showed that the gas yield increased by 209.1% and reached up to 44.2 mmol/g. The hydrogen yield also increased from 4.1 up to 21.2 mmol/g. [PERSON] et al. [63] investigated the effects of using a 10 wt% Ni-dolomite catalyst on the two-stage pyrolysis and steam reforming process.