**5. Mechanisms of Main Reactions**

#### *5.1. Partial Oxidation and Dry Reforming of Methane*

Mechanism of these reactions was studied by using such methods as SSITKA, kinetic transients and pulse techniques (pulse microcalorimetry, pulse studies in flow conditions including those carried out in vacuum systems called TAP) [19,72,82,87,101,102,108,109,111,112,134–137]. For majority of efficient catalysts based on mixed oxides with a high mobility and storage capacity of reactive oxygen species with supported Pt group metals and/or Ni mechanism can be described by so called bifunctional type, where molecules of oxidants are activated on support vacancies producing oxygen species (and CO in the case of CO2), while methane is activated on metal sites by C-H bond rupture (rate-limiting stage). These steps are conjugated by rapid transfer of surface oxygen species to the metal sites where they interact with CHx fragment transforming them into CO and H2. A typical feature of such redox scheme in the case of MDR is the same degree of methane conversion and syngas selectivity in pulses containing only CH4 or CH4 + CO2, as well as identical CO2 conversion into CO in mixed and CO2—containing pulses. Simplified scheme of methane dry reforming and sequence of elementary steps successfully applied for modeling of transient over Ni-Ru-Sm0.15Pr0.15Ce0.35Zr0.35O2 catalyst [72] are shown below:

Here →—denotes irreversible steps; ⇔—reversible steps; s—surface sites; Ols—lattice oxygen atoms; VOs—lattice oxygen vacancy, sm—surface metal sites

For catalysts comprised of Pt/doped CeZrO oxides direct route of CH4 partial oxidation into syngas, which generates CO and H2 even in the presence of oxygen, was reliably demonstrated in our studies. It is explained by stabilization of Pt cations due to metal-support interaction and fast migration of oxygen species activated on support to Pt. Moreover, Pt cations are less efficient in oxidation of CO and H2, while being more efficient in C-H bond activation in CH4.

Basic scheme of methane partial oxidation [109] is presented here:


Clearly this mechanism is impossible in the case of such supported metals as Ru, Pd, Ni, Co, etc., which can only combust methane, CO and H2 in the oxidized state. For these catalysts indirect scheme of methane partial oxidation is realized, in which all oxygen is consumed for methane combustion in the inlet part of the catalytic layer, while syngas is generated via steam and dry reforming of methane in the main part of catalytic layer where O2 is absent in the gas phase and supported metals are in the reduced state [82].

These unique features of Pt cations were also reflected in specificity of MDR relaxation after contact of oxidized Pt/PrCeZrO catalyst with reaction feed, where both CH4 and CO2 conversions decline with time-on-stream apparently caused by the catalyst progressing reduction [19,136,137]. Mathematical modeling using scheme of methane dry reforming mechanism given below allowed to describe such transients taking into account a high efficiency of Pt cations in CH4 activation, while CO2 transformation occurs via carbonates adsorbed on Ptn+–Pr4+–O oxidized sites:


Here [PtO] and [Pt] denote the oxidized and vacant Pt-centers, [PtCO3] is the carbonate complex, [Os] and [Vs] are the oxidized and vacant sites inside the lattice layer of Pt/PrSmCeZrO complex oxide composite,

Note that for catalysts with all other supported transition and precious metals in oxidized state catalytic activity in MDR is negligible and begins to increase to the steady-state value in the process of catalysts reduction by reaction mixture at sufficiently high temperatures.
