**2. Catalysts for hydrogen gas production by hydrothermal gasification processes**

Raney nickel and platinum-based catalysts are common catalysts that have been used for hydrogen gas production in various processes including hydrothermal gasification methods [2, 3]. Despite Raney nickel catalysts exhibit better activity than precious metals, the use of these catalysts in hydrothermal gasification processes is not the best because of the following reasons: **(a)** Raney nickel deactivates easily in hydrothermal conditions. The studies showed that oxidation of Raney-Ni surface caused the deactivation. The nickel metal particle size in Raney-Ni catalysts became larger, and surface area was significantly decreased during deactivation. To prevent oxidation and deactivation of Raney nickel catalysts, the gasification environment should be reductive as possible to keep Raney-Ni in metallic form and extend its activity [4, 5]. **(b)** Raney nickel catalysts contain significant amounts of hydrogen gas. Because of this, these catalysts can easily ignite when exposed to oxygen. To prevent ignition, Raney nickel catalysts are kept under nonreactive inert environments and stored in water or in a suitable organic solvent such as cyclohexane, ethanol, and dioxane [4]. **(c)** Nickel is not stable under high carbon-containing reaction environments [5]. Therefore, it is hard to keep nickel in its metallic form under organic-rich hydrothermal conditions.

On the other hand, precious metals also show high activities in the reformation of oxygenated hydrocarbons. Since these metals are expensive, they are widely used in the supported form on activated carbon, alumina, titanium dioxide, silica, etc., for recycling [2]. Dispersion of small metal particles on the support with high specific surface area is considered more advantageous. The decrease in the particle size of metals on the support could enhance metal-support interaction and increase the activity of the catalyst for hydrogen production [6]. Large metal particles could cause more severe carbon formation and deposition on the catalyst surface [7].

### **3. Supported metal catalysts**

Dispersion of metals on the support, resistance of metal particles to sintering, and the accessibility of active sites to reactants are highly affected by physical and chemical properties of the catalyst support [8]. Numerous studies showed that catalyst support could significantly affect the catalytic performance of metal catalysts in the various reactions including hydrothermal processes [9, 10]. The catalyst support provides a physical surface for the dispersion of metal particles and affects the catalytic activity [11]. The catalytic activities of the catalysts can be increased by the selection of a novel supportive material for better catalytic action (more active sites)

and excellent mechanical strength and high surface area. The supportive materials can be chemically modified to increase surface area and porosity and create specific functional groups on the surface.

### **3.1 Carbon materials as supportive materials for supported precious metal catalysts**

Carbon materials have been recognized as the most active supports for aqueousphase reforming of biomass-derived compounds for hydrogen-rich gas production [2]. The interaction between active metal component and support plays an important role in the catalytic reactions. Therefore, catalytic properties of supported catalysts depend on the combination of the type of metal and supportive material. The catalytic activity of these catalysts for hydrogen production has been significantly improved with recent research activities; however, they are still not good as Raney nickel catalysts. The activity of the supported precious metal catalysts reduce because of aggregation or poisoning of metal particles in reaction environment or coke deposition on active surfaces of the catalysts [12]. Leaching of the metal particles from support is another problem that causes decay of catalyst activity. Increasing the catalytic activity and stability of the catalysts is a challenge for high-yielding hydrogen gas production.

The porous structure and surface chemistry of activated carbon as a support material highly affect the activity of the catalyst. Porous carbon materials are of interest in many applications due to their high surface area and physicochemical properties. Those carbon materials can be categorized according to their pore sizes as microporous (pore size <2 nm), mesoporous (2 nm < pore size <50 nm), and/or macroporous (pore size >50 nm) [13]. Mesoporous carbons have large surface areas and are rich in oxygen-containing functional groups in the surfaces. They can enhance the affinity between the substrate molecules and the catalyst through abundant functional groups and large uniform pores. Those materials may offer great advantages over other carbon materials owing to their well-controlled pore structures in the mesopores, which might be favorable to the transportation of large molecules such as polysaccharides released from cellulose and hemicellulose structures in the biomass.

The oxygen-containing functional groups on the surface of carbons significantly influenced their performance in catalytic reactions [14]. Many different activating agents such as strong or weak acids and bases or oxidants (H2SO4, H3PO4, HNO3, CH3COOH, NH3, KOH, H2O2, O3, etc.) can be used in this process to introduce oxygen-containing functional groups on the carbon surface [15, 16]. The abundant oxygen groups in the supports facilitate the access of oxygenated biomass components to the surface of the catalyst; the open pore structure permits the molecules (solubilized biomass components) to effectively diffuse to the active sites and reacts with metal catalyst particles.

*Hierarchical porous carbons (HPCs):* HPCs are carbon structures with macropores and interconnected meso- and micropores. Porous carbons with large specific surface area and pore volume provide multiple sites for interfacial reaction, molecule retention, and catalytic activity. However, surface area itself is not sufficient for exhibiting exceptional performance because narrow and internal pores may not provide access for some reactants to react.

HPCs structured with micro-, meso-, and macropores can provide multi-active sites in the catalytic conversion processes and better accessibility for the feeds that are composed of different molecular weight organic compounds such as lignocellulosic

biomass hydrolysates. The mass transport of different sized biomass compounds can be easily facilitated by the macropores and micro−/mesopores that provide access to the metal catalysts deposited in HPC. Currently, HPC preparation methods are expensive and complex. Synthesis of HPCs by utilization of abundant and widely available waste materials can provide opportunity to develop such materials in a sustainable way for many applications including hydrogen production. Direct carbonization of biomass or any solid organic wastes for the synthesis of HPCs is not applicable because of heterogeneity and uncontrollability in morphology and pore structure of the resulted carbon products. It is almost impossible to produce same carbon materials continuously from direct carbonization of nonuniform solid precursors.

Hierarchical structures with uniform and controllable pore sizes can be prepared by carbonization of different precursors using various templates. Low-density HPCs are prepared from various carbon nanomaterials such as graphene, and the resulted HPCs are aerogel-type carbon materials with moderate surface areas and different sized micro−/mesopore structures [17]. Unfortunately, all these and other existing methods have various drawbacks such as using nonrenewable precursors or templates, long synthesis period (e.g., solvent exchange and supercritical drying in the case of aerogels), involving multiple steps that make the process more costly and time-consuming, etc. [18].

*Carbon aerogels* are another class of hierarchical porous carbon materials with remarkable physicochemical properties, including low density, large surface area, highly porous structure, and good chemical stability. They can be prepared in different forms (monoliths, beads, powders, or thin films). Different types of carbon aerogels with unique characteristics are suitable for different applications, and some carbon aerogels can exhibit promising catalytic performance in various reactions because of their well-developed and controlled porous structure and high specific surface area.

The most common carbon aerogels are prepared by sol–gel polymerization of resorcinol and formaldehyde mixtures followed with supercritical drying and carbonization that were first prepared by Pekala [19]. In addition to resorcinol-formaldehyde, various monomers can be used to prepare these materials, including melamine-formaldehyde, phenol-formaldehyde, cresol-formaldehyde, phenol-furfural, and some polymers such as polystyrenes and polyurethanes [20–23].

A wide spectrum of different carbon aerogel materials with unique properties can be prepared depending on synthesis and processing conditions. Synthesis conditions of organic aerogel (e.g., resorcinol/formaldehyde ratio, catalysts, and pH), curing and drying methods, and carbonization conditions determine surface area, pore volume, and pore size distribution of final carbon aerogel [24].

Supercritical carbon dioxide and freeze-drying are preferable methods to dry the organic gel while retaining skeletal pore structure during drying stage. During carbonization, dried aerogel is heated under inert atmosphere to obtain carbon-rich structure by removing oxygen and hydrogen functionalities.

Different metal-doped carbon aerogels have been developed including W-, Ru-, Co-, Ni-, Pd-, and Pt-carbon aerogels for various catalytic reaction, and these catalysts can show good activity in hydrogen production processes as well (**Figure 1**).

*Graphene-nanostructured carbon materials:* Graphene has unique physicochemical properties such as great stability, high surface area, electron mobility, thermal conductivity, and mechanical strength. Graphene is an excellent catalyst support for metal-supported type of catalysts because of its great stability and chemical inertness that are important parameters for increasing lifetime of the catalysts. Graphene has a

*Improving Hydrogen Production Yield in Hydrothermal Gasification Processes through Novel… DOI: http://dx.doi.org/10.5772/intechopen.106289*

**Figure 1.**

*Resorcinol-formaldehyde aerogel and its carbonized form (carbon aerogel).*

high theoretical surface area (approximately 2600 m<sup>2</sup> g−1). However, large extent of van der Waals interactions between surfaces lead to graphene sheets to stack on top of each other. Surface area between graphene layers becomes inaccessible depending on the degree of stacking that changes its catalytic activity.

Reactivity of the graphene support can be increased by different approaches. Chemical doping is important approach to tailor the property of graphene that can change its surface reactivity and increase its performance as catalyst support. Introduction heteroatoms, such as nitrogen, boron, phosphorus, or sulfur atoms, into the carbon lattice of graphene by chemical doping can change the electronic properties of graphene [25].

Carbon nanotubes (CNTs) are cylindrical molecules that consist of rolled-up typical graphene sheets (**Figure 2**). Different CNTs (different rolling up direction of graphene layers or single-walled and multi-walled carbon nanotubes) determine the mechanical, electrical, and structural properties of the nanotubes that affect their catalytic activities. Metal particles supported on CNT can poorly be affected from carbon monoxide poisoning than traditional catalyst systems [26], and for this reason CNTs are good candidates to be used as catalyst supports.

A reported study showed that Pt on a single-walled carbon nanotubes catalyst were better catalysts in terms of hydrogen production activity and selectivity than Pt on a multi-walled carbon nanotube for hydrothermal gasification of biomass hydrolysates. Since the biomass hydrolysates tested were composed of large carbohydrate molecules (consisting of two carbohydrate fractions with 69,800 and 25,400 Da), these compounds were unable to enter narrow graphene sheets of multi-walled carbon nanotubes to react with Pt metals deposited inside the graphene layers. On the other hand, when the simplest biomass model compound, glucose, was used as feed

**Figure 2.** *Chemical structures of graphene, single-walled, and multi-walled carbon nanotubes.*

solution, same catalytic activity was observed for both catalysts [27]. These results indicated that the catalytic activity of graphene-based carbon materials is strongly dependent on how the graphene sheet is shaped. Different shaped graphene-based structures with new properties could be promising supportive materials for Pt deposition, and resulting reforming catalysts could exhibit unique properties for hydrogen production (e.g., more active sites and suitable gaps between graphene sheets for the entrance of biomass molecules). The controlled graphene structure permits the small biomass molecules (oligo- and monosaccharides) to effectively diffuse the active sites and react with Pt particles to produce hydrogen gas.

Different graphene nanostructures can be prepared by growing graphene on presynthesized nanostructured metal templates by chemical vapor deposition and then etching away the metal to get a free-standing graphene nanostructure with novel properties to be used as catalyst support in hydrothermal gasification processes [28].

#### **3.2 Heteroatoms-doped carbons as catalyst supports for hydrogen production**

The introduction of heteroatom into carbon structure can change the physicochemical and electronic properties of the carbon material and enhance the catalytic functions [29]. Nitrogen atoms can create high positive charge distribution in the nearby C atoms due to its high electron withdrawing ability. Atomic sizes of nitrogen and carbon atoms are similar, and five available valence electrons in nitrogen can lead to the formation of valence bonds with C atoms and covalent bonding between N and C network of carbon results in more stable structure. Dual or multiple heteroatoms (B, N, P, S, F*, etc.*)-doped carbon catalysts enhance the activity of the catalysts [30, 31].

HPCs with heteroatom (e.g., nitrogen and sulfur)-doped carbon network are important carbon-based functional materials and have attracted great attentions because of their excellent properties [32]. Incorporation of nitrogen into the carbon improved the stability of precious metal particles by reducing sintering and leaching of the metals in aqueous-phase furfural hydrogenation reaction [33]. The studies showed that dual or multiple elements-doped carbon catalysts exhibited synergistic effects with enhanced activity in *oxygen reduction reaction* compared with mono heteroatom-doped carbon catalysts [31, 34], and similar effect can be expected for hydrothermal gasification reactions of biomass-derived carbons.

## **3.3 Increasing activity of the catalysts based on metals deposited on carbon supports**

Carbon-supported precious metal (e.g., platinum) catalysts are active catalysts for hydrothermal conversion of biomass-derived compounds to hydrogen [2]. For better hydrogen production yield, Pt particles deposited on the support should be nanosized, uniform, and well dispersed that highly depend on the type of support, deposition method for metals including solvents and other active chemicals used during deposition process, type of metal precursors, reduction method for metal precursors, drying and calcination treatments, etc.

Catalysts can be deactivated during gasification process due to strong adsorption of feed impurities, aggregation or poisoning of metal particles in reaction medium, or coke deposition on active surfaces of the catalysts [12]. These drawbacks can be partially eliminated or lowered by integrating non-precious metals in precious metals containing catalyst system. Catalytic activity of supported Pt catalyst considerably

*Improving Hydrogen Production Yield in Hydrothermal Gasification Processes through Novel… DOI: http://dx.doi.org/10.5772/intechopen.106289*

#### **Figure 3.**

*Comparison of activity of the catalysts prepared with different metals and combination of these metals with Pt for hydrothermal conversion of glucose to hydrogen-rich gas mixture [4].*

improved when some certain metals deposited on the support along with Pt. It was reported that the activity of Pt catalysts could be improved by the addition of transition metals that have C-C bond breaking ability (Co, Ni, Fe, Sn, etc.) to a supported Pt catalyst. For instance, the addition of Fe to Pt at a 1:1 Pt:Fe atomic ratio significantly increased hydrogen production yield and selectivity [35]. Addition of a non-expensive metal in precious catalysts can reduce oxidation of precious metal particles by diluting them with non-expensive metals, reduce metal sintering, and favor water-gas shift reaction [36].

Catalytic activity of alumina nanofibers supported Pt catalyst considerably improved when Ni was deposited on the support along with Pt [9]. Kunkes et al. reported that addition of Re to Pt on carbon catalyst enhanced the Pt dispersion and favored water-gas shift reaction [37]. Pt-Co and Pt-Ni catalysts have been reported to be highly active catalysts with high hydrogen selectivity and low coking tendency [7, 38]. Co-deposition of Sn with Pt on to Al2O3 support caused dilution of Pt particles with Sn particles that improved activity and selectivity of the catalyst [39].

A recent study showed that performance of Pt-only catalyst could be considerably enhanced by replacing some of Pt particles with different non-expensive metals for hydrothermal conversion of biomass compounds to hydrogen-rich gas mixture. As can be seen in **Figure 3**, the gas mixture produced from hydrothermal gasification of glucose was composed of hydrogen, carbon monoxide, and carbon dioxide. As expected, monometallic Pt catalyst resulted in the highest conversion with highest hydrogen yield as this metal is known to be best catalyst for hydrogen production. Incorporating W and Ni metals to Pt-only catalyst showed promising activity despite replacing half amount of Pt with Ni and W metals [4].

### **4. Core-shell-type catalysts alternative to supported metal catalysts**

Metal core-carbon shell catalysts have exhibited a great potential for various applications, and fabrication of these types of catalysts for specifically hydrogen gas production can be promising. Metal particles are encapsulated in hollow porous carbon shell or heteroatom-doped hollow porous carbon (shell) (**Figure 4**).

**Figure 4.** *Core-shell-structured catalysts.*

Metal nanoparticles that are made of various combinations of precious metal (e.g., Pt) and/or inexpensive metals (Ni, Sn, Co, and W) in different compositions (e.g., mono and bimetallic) can be used as core materials. It is possible to improve the catalytic activity in this core-shell approach due to increased surface area and the closed interfacial interaction between the core and the shell. Reactants/products can transfer through penetrable shell structure. The shell prevents sintering of the metal nanoparticles in the core. Because of heterogeneous nature, the catalysts can be reused in the process after collecting from aqueous reaction medium by simple separation methods (centrifugation, filtration, etc.).

The encapsulation of metal nanoparticles in a penetrable shell catalyst has recently become a new strategy in the catalyst design area. The encapsulation of precious metal nanoparticles in a stable protective shell can enhance the activity and extent lifetime of the catalyst by maintaining and protecting the size and shape of the precious metal nanoparticles. This core-shell-type catalyst exhibits high stability, catalytic activity, and selectivity in various reactions [40, 41]. For an example, Pt nanoparticles-mesoporous silica core-shell catalyst was reported to have excellent stability for alkene hydrogenation [42].

In the core-shell type of catalysts, the metal nanoparticles are embedded in a protective matrix with channels, which can avoid aggregation or sintering of the metal nanoparticles in the core even at high temperatures and enables transfer of reactants/products through the channels [41]. The metal nanoparticles in the core are mainly responsible for the catalytic activity for a specific reaction. The channels in

*Improving Hydrogen Production Yield in Hydrothermal Gasification Processes through Novel… DOI: http://dx.doi.org/10.5772/intechopen.106289*

the shell give access to organic compounds to react with metal particles in the core. This protects metal particles from unwanted reactions in the reaction medium. Ikeda et al. developed such a catalyst design for platinum catalyst for hydrogenation of nitrobenzene in liquid phase [43]. The core-shell catalyst showed very high activity and exhibited almost same activity even after reused. Various porous materials have been used as shell materials to develop such catalysts for a wide range of applications (e.g., mesoporous silica, mesoporous carbon, metal oxides, titanium dioxide, and polymers) [44, 45].

Hollow porous carbon materials have unique properties, such as low density, controllable morphologies, available cavities, high surface areas, tunable porosity, and good chemical stabilities. The encapsulation of metal nanoparticles in a such selectively penetrable shell, hollow porous carbon, can result in novel catalysts for hydrothermal conversion technologies. Hollow porous materials not only have pores in their shell but also contain a hollow core and have higher porosity. Porous carbon shell with hollow structure can enhance the overall activity of the catalysts by increasing the accessibility of the reactants to the active phase. These nanostructures have emerged as an important class of carbon materials in many fields including energy, catalysis, and nanomedicine.

### **5. Conclusions and future trends**

Lack of economically feasible, highly active, and stable catalysts for hydrothermal conversion of biomass-derived compounds to hydrogen is a main challenge that impedes application of these technologies for large-scale hydrogen gas production systems. Choice of appropriate inexpensive metals with right combination of precious metal(s) could reduce catalyst cost and improve the catalytic activity of precious metal-based catalysts. However, the stability of these catalysts in an aqueous processing environment is an important issue that needs to be studied in detail.

Since protection of metal catalyst is a big challenge in hydrothermal condition in which organic-rich solutions/materials are used as feeds, the metal core-carbon shell catalysts could also be a solution. Hollow porous carbon materials can be synthesized in desirable morphologies, sizes, compositions, and pore structures depending on templating strategies used in the preparation steps. The physicochemical and electronic properties of the hollow porous carbon materials can be changed by the introduction of a heteroatom such as nitrogen into carbon shell that enhance the activity and stability of the catalyst.

Graphene-based carbons with controlled graphene nanostructures could show unique properties as catalyst support for metal particles and effectively diffuse the biomass compounds to the active sites and leave agglomeration byproducts and largesized contaminants in the solution.
