key: cord-0846543-zh45hsjr
authors: Alahmari, Ahmed A.; Khan, Anas A.; Elganainy, Ahmed; Almohammadi, Emad L.; Hakawi, Ahmed M.; Assiri, Abdullah M.; Jokhdar, Hani A.
title: Epidemiological and clinical features of COVID-19 patients in Saudi Arabia
date: 2021-01-16
journal: J Infect Public Health
DOI: 10.1016/j.jiph.2021.01.003
sha: 6dfc2a7b9e6df16399e82709215342c49c2f4962
doc_id: 846543
cord_uid: zh45hsjr

BACKGROUND: The aim of this study is to describe the clinical and demographic characteristics of COVID-19 patients, and the risk factors associated with death in Saudi Arabia to serve as a reference to further understand this pandemic and to help in the future decisions and control of this global crisis. METHODS: This multicenter, retrospective, observational, cross-sectional study was conducted on 240,474 patients with confirmed COVID-19 in Saudi Arabia. Data was collected retrospectively through the Health Electronic Surveillance Network at the Ministry of Health. Patients were classified based on their outcome as recovered, dead, or active with no definite outcome. We must specify the date period. RESULTS: As of 20th of June 2020, 79.7% of COVID-19 cases were young and middle-aged, ranging between 20–59 years. There was evidently a difference in the sex ratio, where males constituted 71.7% of cases. The majority were non-Saudi nationals, representing 54.7% of cases. Furthermore, the contraction of COVID-19 was travel-related in 45.1% of cases. Signs and symptoms were reported in 63% of cases, the most common of which were fever; 85.2%, and cough; 85%. Deaths occurred more frequently in patients 40−49 years, 50−59 years, and 60−69 years, representing 19.2%, 27.9%, and 21.3% of deaths, respectively. Additionally, the case fatality rate (CFR) was higher in older age-groups, reaching 10.1% in those ≥80 years. Moreover, the CFR of males was higher than that of females, with 0.95% and 0.62%, respectively. As for nationality, Saudis had a CFR of 0.46% versus 1.19% in non-Saudis. CONCLUSION: The total number of positive COVID-19 cases detected constitute 0.7% of the Saudi population to date. Older age, non-Saudi nationalities, being male, travelling outside Saudi Arabia, and the presence of symptoms, as opposed to being asymptomatic were considered risk factors and found to be significantly more associated with death in patients with COVID-19.

The catalytic reforming of methane via CO2 utilization, the so-called friendly dry reforming of methane (DRM), is a well-known process for the production of synthesis gas with a suitable H2/CO ratio to be used as feedstock in various chemical processes including the production of liquid fuels via Fischer-Tropsch synthesis, oxygenates such as methanol and dimethyl ether (DME), and so on [1, 2] . The DRM process represents an inviting alternative to the more common wet reforming of methane, the so-called steam reforming of methane (SRM) for industrially producing utilization technologies (CCU). Indeed, it allows to convert two potent greenhouse gases such as CO2 and CH4 in a useful and valuable feed stock and concomitantly replacing most costly H2O with CO2 reactant in methane reforming reactions. Furthermore, compared to depleted crude oil, the reservoirs for methane are still significantly large from natural gas, shale gas, methane hydrates, and so on, therefore the DRM is becoming a very interesting topic.

The i) CO2 reforming of methane proceeds according to the reaction below [5] : 2 + 4 ↔ 2 2 + 2 ∆ 298 0 = +247 −1 (1) which is extremely endothermic in character and reversible reaction, and for high methane conversions high temperatures are employed increasing so cost processing. Some secondary reactions can take place as follows [5] :

ii) Reverse water gas shift (RWGS) reaction: 2 + 2 ↔ + 2 ∆ 298 0 = +41.1 −1 (2) iii) Boudouard reaction: (3) which is favored at low temperature and high pressure; iv) Methane decomposition: (4) which is favored at high temperature and low pressure.

The simultaneously occurrence of RWGS reaction (Eq.2) during the DRM reaction (Eq.1) might bring to a H2/CO ratio lower than unity owing to higher CO2 conversion respect to that of CH4.

Meanwhile the two side reactions like the CO disproportion reaction (Eq.3) and the methane decomposition reaction (Eq.4), other than to consume reactants, may provoke coke formation when non-noble metal catalysts are employed [6, 7] . The prominent catalyst till remains nickel because it is cheap and worldwide compared to noble metal catalysts. However, the main drawback in the use of this kind of catalyst in reforming reactions is the deactivation due to the nickel carbide formation with the catalytically formed carbon compounds [8, 9] . Furthermore, the catalytic activity decay might also occur by the loss of Ni active particles due to severe sintering at high temperatures (Tammann temperature of 518°C) [10] . Therefore, the development of an active and selective J o u r n a l P r e -p r o o f catalyst for the efficient production of syngas at low temperature is highly desirable from a point of view of process safety and energy saving [11] .

The resistance of the nickel catalyst to carbon deposition during working life has generally been encouraged by adequately choosing the support, adding promoters, or selecting suitable preparation methods [12] [13] [14] . Regarding the nature of support it can play an important role in DRM reaction as the interface between metal and support significantly improves the stabilization of Ni particles, participates in the adsorption and activation of the CH4 and CO2 reagents on the active phase of the metal and reduces the formation of residual carbon [15] . Indeed, the CH4 and CO2 reactants does not react directly but trough the following two most accredited mechanisms [16] : i) CH4 activation on metal active phase via a series of CH4 dehydrogenation steps that ultimately leads to the formation of atomic hydrogen and carbon on the surface catalyst; and ii) CO2 adsorption and activation on support that directly decomposes to CO and active O* contributing to oxidize the adsorbed C and/or CHx to form CO and H.

Various types of supports such as Al2O3, MgO, La2O3, CeO2, and TiO2 were widely employed in heterogeneous catalysts production for DRM process owing to their capacity of adsorbing CO2 molecules via carbonates or formates formation [17, 18, 19] . Because the acidic/basic properties of the support will influence the adsorption and activation of CO2, modifications by adding alkaline earth metal oxides might increase the Lewis base strength of catalyst inducing higher thermal stability [8, 20] . It was reported that by coating alumina support with CaO, MgO, and BaO the CO2 chemisorption ability of the catalyst was noticeably improved [21] [22] [23] [24] . This enhanced CO2 adsorption capacity on catalyst surface promoted by alkaline earth oxides allowed to produce more CO and O 2-, whereas the O 2species through the oxygen mobility inside the support might speed up the oxidation of accumulated carbon during DRM, so suppressing severe carbon built up [6] . Therefore, a greater amount of adsorbed CO2 nearby the Ni metal surface will depress the CH4 decomposition and/or CO disproportionation parasite reactions;

consequently there will be a delay in carbon formation due to a more rapid gasification of the surface C atoms during CO2-reforming over nickel-based catalysts promoted by basic metal oxides [25] . MgO promoter was extensively studied as it stabilizes the Ni crystallites in Ni/Al2O3 by avoiding NiAl2O4 formation, enhancing coke resistance, and ameliorating the CO2 adsorption due to more basic sites on catalyst surface [26] [27] [28] . However, CaO-promoted nickel catalysts are recognized to improve thermal stability and resistance to coke during the reforming processes, mainly due to the greater chemisorption capacity of CO2 and the lower interaction of Ni with CaO compared to MgO, which increased reducibility [29, 30] . Recently, a DFT study suggested the cooperation between Ni particles and CaO promoter allowing to increase CO2 activation within the J o u r n a l P r e -p r o o f interface channel of metal-support by attracting more CO2 on CaO sorbent in DRM reaction [31] .

More recently, a Ni-CaO/supported catalyst was proposed to realize the capture and conversion of CO2 in one integrated chemical process by the calcium-looping CO2 capture and the CH4 dry reforming reaction [32] .

Among the most studied catalytic supports with basic properties, those based on mixed calcium and aluminum oxides represent a class of materials with moderate oxygen storage capacities and mobility [33, 34] . Oxidic supports can enhance catalytic activity as more molecular oxygen dissociation into reactive oxygen (O*) can facilitate adsorption and activation of CO2 molecules by carbonates formation, which can then react with CHx fragments adsorbed on metal active sites within the metal-support interface to produce H2 and CO [35] [36] [37] . Furthermore, they are inexpensive, non-toxic and can be synthesized under mild preparation conditions. The basic mixed calcium aluminate oxide supports include: CaAl4O7, CaAl2O4, Ca12Al14O33 (mayenite) [38] [39] [40] .

Mayenite employed as carrier offers high resistance to carbon deposition owing to high oxygen mobility, ionic conductivity, and oxidative catalytic properties thanks to the special free oxygen restored structure [41] . Furthermore, the Ca12Al14O33 ceramic containing Ospecies might facilitate C-H bond activation of CH4 molecule and/or absorb CO2 molecule to form carbonate able to oxidize the carbon deposited on surface catalyst and appreciably minimize coke accumulation.

Various calcium-aluminum oxide system as supports were used in dry reforming of methane [42, 43] . Despite their low specific surface areas, which does not allow high metal catalyst dispersion, the coke deposited on catalyst surface during reforming reactions was found to be negligible.

In the present paper the 15wt.%Ni/CaO-Ca12Al14O33 catalyst was applied in the green synthesis gas production via DRM process. Typically, the catalyst was obtained by reduction of NiO/CaCO3-Ca3Al2O6 catalyst, which was prepared by wet impregnation/calcination of a mixed calcium-aluminum oxide (CAO) ceramic support with nickel nitrate solution as described in detail in our previous work [44] . The present catalyst is highly promising and feasible for CO2 capture and utilization through its valorization by DRM process.

J o u r n a l P r e -p r o o f A home-made mixed calcium-aluminum-oxide (CAO), prepared by a wet chemical method as described in [40] , was used as support. 

The Ni/CAO catalyst was prepared by Ni-impregnation method as described in [44] . [46] , where DXRD is the crystallite size in nm,  the radiation wavelength (Cu K radiation 0.15406 nm),  the bandwidth at half-height, and  is the diffraction peak angle.

The specific surface area, total pore volume and pore size distribution of samples were determined from N2 adsorption-desorption isotherms at -196 °C using a Micromeritics ASAP 2020 adsorption apparatus through the Brunauer-Emmett-Teller (BET) calculation methods [47] . Typically, 0. over the H2 required for complete reduction of Ni loading considering the following reduction reaction: NiO+H2Ni°+H2O. After TPR measurement chemisorption of hydrogen was followed by H2-TPD (temperature programmed desorption) analysis. The dispersion of the reduced nickel was defined as ratio of 2(area of the TPD peak)/(area of the TPR peak), whereas an estimation of the Ni° crystal size was given by inverse of the dispersion data (1/D) [48] . The runs were carried out at least in duplicate.

Temperature programmed desorption of CO2 was conducted in the same experimental apparatus used for H2-TPR analysis. Specifically, about 0.05 g of fresh calcined catalyst were in situ reduced at 700°C for 0.5 h under 10%H2-Ar (99.99%, Air Liquid, France) at 0.05 L min -1 flow rate and cooled down to 50°C at the same atmosphere. The reduced sample was then fluxed with Ar for 0.5

h. Subsequently, CO2-chemisorption was performed at room temperature under 10%CO2-Ar (99.99%, Air Liquid, France) at 0.03 L min -1 flow rate for 1 h and followed by further Ar fluxing to sweep out the physically adsorbed CO2 on the sample surface. The temperature was then raised up to 850°C at 10°C min -1 heating rate under Ar flow (0.03 L min -1 ). The desorbed CO2 was continuously detected by TCD. The outlet gas from the microreactor was also monitored on-line by FTIR (Gasmet DX 4000).

The morphology of catalysts was observed by Vega 3 (Tescan) scanning electron microscope equipped with Element Energy Dispersive Spectroscopy (EDS) System EDAX. Prior to analysis, the powdered sample was dropped on aluminum stubs using a conducting double stick carbon tape.

The DRM catalytic tests were conducted in a continuous flow reaction system at atmospheric pressure in the same apparatus used for H2-TPR and CO2-TPD analyses. Typically, 0.05 g of fresh 

In Fig.4 are showed the N2 adsorption/desorption isotherms, the BJH desorption pore-size distribution (PSD) and the cumulative particle size distribution curves (inset) for the two catalysts and corresponding supports. The textural properties of all samples are listed in Table 1 . According to the IUPAC classification [55] all these isotherms belonged to Type IV, which denoted mesopores materials. The N2-isotherms of calcined and reduced Ni/CAO catalysts present a hysteresis loop of type H1 at relative pressures range of p/p0=0.6-1.0 (Fig.4a) , which are associated with filling and empty of disordered porous solid materials. From the PSD curves a bimodal distribution was exhibited for both the CAO support and calcined Ni/CAO catalyst. For CAO support one small peak located around at 4 nm and another large peak located in the mesopore-macropore region 10-200 nm. By incorporation of nickel species on CAO support and activation by calcination a larger and wider distribution of pore size occurred with two large overlapped peaks located in the region 3-200 nm, suggesting some interaction of NiO with the support. After H2-reduction step, the Ni/CAO catalyst showed unimodal distribution of pore size with one large peak centered at 60 nm.

From the results of Table 1 the SBET and total pore volume of calcined Ni/CAO were substantially improved likely due to the structural rearrangements occurring within the structure of CAO support during impregnation/calcination steps. Newsworthy, the SBET and total pore volume of reduced Ni/CAO catalyst were noticeably enhanced owing to the structurally re-transformation of Ni/CAO, which was accompanied with in situ formation of CaO that has greater porosity compared to parent CaCO3 [56] . In Fig.4 (b) the N2-isotherms of calcined/reduced Ni/ALO catalysts and the commercially available -Al2O3 support showed high steepness of H1 hysteresis loop at higher relative pressures range (p/p0=0.6-1.0). From the corresponding PSD curves uniform distribution of pore size was seen with rather wide peaks centered at pore diameter around to 40, 20, and 30 nm, for ALO support, calcined and reduced Ni/ALO catalysts, respectively. After nickel loading the J o u r n a l P r e -p r o o f SBET dropped, whereas total pore volumes and average pore size accordingly increased due to diminishing of pores [57] . 700°C, which are associated to the reduction of exposed NiO species with different interaction with the CAO support. The first weak reduction peak at 330°C was attributable to reduction of free NiO particles on the CAO carrier surface [58] . The second reduction peak at 490°C was due to reduction of dispersed Ni 2+ ions moderately bound to carrier surface, whereas the large part of H2 consumption occurring at 650°C was due to the reduction of NiOx that strongly interact with the support. The reduction proportion values were 4, 38, and 58%, respectively. As already discussed before the highly basic properties of CAO ceramic support could allow strongly interactions with the acidic nickel nitrate solution during impregnation/calcination steps [44] . It is well to keep in mind that under H2-reducing condition the evolved CO2 from calcium carbonate decomposition was transformed into CO and/or CH4 gases [59] . 

The basicity of the as-prepared CAO support and two freshly reduced Ni/CAO and Ni/ALO catalysts was investigated using CO2 as acidic probe gas, that interacting with surface basic sites through absorption is then released from the support/catalyst surface at different temperatures according to the strength of the Lewis basic sites [62, 63] . The CO2-TPD profiles of all samples are showed in Fig.6 along with the deconvolution of the CO2-TPD profiles using Gaussian-type function and peak fitting (Origin Pro 8 software). Curve A of Fig. 6 shows the CO2-TPD profile of CAO support, which was obviously supposed to have basic properties. Indeed, two well defined CO2 desorption peaks at high temperature of 510 and 735°C were observed, indicating strong basic sites related to interactions of CO2 with cation-anion Ca 2+ -O 2pair [64] . After Ni-impregnation on the CAO support and calcination/reduction steps, new basic sites were clearly evidenced on the CO2-TPD profile of freshly reduced Ni/CAO catalyst (curve B of Fig. 6 ). This was probably connected to in situ formation of CaO promoter during reduction step which enhanced basic properties [65, 66] . Five deconvoluted CO2 desorbed peaks with varied intensities and distributed over a wide range of temperature between 50 and 800°C were identified. The CO2 desorption peak centered at low temperature of 160°C was usually ascribed to weak basicity due to CO2 physisorption on surface catalyst. The CO2 desorption peak at 330°C was due to moderate basic sites such as bicarbonate species formed from the interaction between CO2 and basic surface hydroxyl groups, whereas the CO2 peaks at 510, 630 and 750°C were generated from strong basic sites like unidentate carbonates [67] . Curve C of Fig.6 shows the CO2-TPD desorption profile of freshly reduced Ni/ALO catalyst. There were two very low-intense deconvoluted CO2 peaks at 400 and 620°C of moderate-strong basic character. Table 2 summarizes the results of CO2-TPD analysis such as the maximum temperature of each deconvoluted peak and its integrated area (10 -3 mol CO2

per gram of sample) as well the total basicity. These results clearly show the scarce ability of Ni/ALO catalyst to adsorb CO2 on catalyst surface, whereas the high capacity of Ni/CAO catalyst to absorb CO2 on strong basic sites at high temperature can lead to high DRM reaction rates and prevent carbon deposition [68] [69] [70] . morphology broken into small nanosphere particles of a few dozens nanometers in size and bonded through a necking porous structure (Fig.7B) . The growth of spherical crystallites of small size could be due to sustained evolution of CO2 gas from decarbonation of CaCO3 that promotes formation of porous structure and rough surface. This is consistent with the BET results of increase of surface area after reduction step. The SEM-EDS element mapping results of freshly reduced Ni/CAO reported in Fig.7C showed that nickel particles are evenly distributed throughout the calciumaluminum oxide framework without no substantial enrichment of Ni metallic. The representative SEM image of calcine Ni/ALO indicated a morphology formed by agglomerates of large sizes composed by particles of small sizes (Fig. 8A ). After reduction, the morphology seems to be no changed (Fig. 8B) . From SEM-EDS reported in Fig.8C , it can be concluded that metallic Ni particles are evenly distributed on the -Al2O3 surface and no aggregation of nickel particles is observed. 

The catalytic activities of the pre-reduced Ni/CAO and Ni/ALO catalysts were evaluated in DRM processes to yield synthesis gas (H2+CO) using CH4:CO2:N2 (1:1:2) feed gas and 120 L h -1 gcat -1 WHSV in the range temperature from 600 to 800°C under ambient pressure. In Fig. 9 [71] . The catalytic activities expressed as turnover frequency (TOF) based on CH4 conversions after 1 h of operation and H2-TPD data as a function of temperature were reported in Table 3 . As expected, the TOFCH4 values increased with the increase of temperature regardless of the catalyst studied and were comparable because both the catalyst owned almost the same degree of Ni dispersion. Tab.3

The H2 and CO selectivity tested during DRM over Ni/CAO and Ni/ALO catalysts vs temperature along with the H2/CO molar ratio are showed in Fig. 10 [16] . The H2/CO molar ratio was close to unity (theoretical value of H2 and CO in equimolar composition) remaining quite constant in the entire range of temperature studied for the Ni/ALO catalyst. Conversely, for the Ni/CAO catalyst this molar ratio remained constant between 600 and 700°C, whereas noticeably increased at 800°C. It is probable that the minor difference between the H2 and CO yields at that temperature to be due to the suppression of side reaction reverse WGS.

The long-term stability of the catalysts was evaluated for a certain period of reaction time on stream (12 h) under the previously used reaction conditions. The results of CH4 and CO2

conversions as a function of time on stream at a given temperature (600-800°C) for both in-situ reduced Ni/CAO and Ni/ALO catalysts are illustrated in Fig. 11 . The CH4/CO2 conversion ratio is also reported as a function of time. From the results reported in Fig. 11 it is evident that high values of reactants conversion were quickly reached during the initial operation period. Both catalysts showed good stability over time at temperature of 800°C maintaining high reactants conversions J o u r n a l P r e -p r o o f and CH4/CO2 conversion ratio higher than one. When the temperature dropped down (600-700°C), the trend of CO2 conversions was reversed being the CO2 conversions values higher than that of CH4 owing to the secondary RWGS reaction instauration. As consequence the CH4/CO2 conversion ratio were between 0.90-0.95. A slightly deterioration of catalytic performances in terms of reactants conversions on time was observed for the Ni/ALO catalyst at low operating temperatures.

The rate of reactants conversions on Ni/ALO catalyst remained stable over time, but gradually decreased from 88 to 81% for CH4 and from 89 to 82% for CO2 in the last hours of operation time probably due to excessive accumulations of carbon rather that Ni° sintering as will be discussed in the next paragraph. Conversely, the novel Ni/CAO catalyst showed a very good stable catalytic activity for producing synthesis gas during 12 h of operation at all working temperatures, whereas no deactivation of the catalyst was observed. As it is well-known CO2 reforming of methane reaction includes the adsorption and dissociation of CO2 over surface catalyst, which are affected by the acid-base properties of the support [72] . Indeed, as CO2 is an acid gas the interaction with the surface catalyst need of basic sites to be neutralized. As discussed above, the strong basic character of Ni/CAO catalyst greatly allowed adsorption of CO2 molecules on surface catalyst followed by dissociation in CO and reactive O*, which is able to gasify carbon deposit by methane decomposition (Eq. 4) or Boudouard reaction (Eq. 3). 

Nickel-based catalysts are known to undergo to deactivation during catalytic hydrocarbon reforming reactions at high temperature owing to the metal sintering effect, which reduces active metallic surface area. The formation of Ni3C phase by the occurrence of reactions described in Eq.

(3) and Eq. (4) is also responsible for catalyst deactivation, which depends among others on metal catalyst size, support, morphology and so on [73] . The crystal structures of all the spent catalyst samples were examined by XRD analysis and the results are showed in Fig. 12 . The XRD patterns of the used Ni/CAO catalysts at different temperatures (Fig.12a) [74, 75] . Particularly, CaCO3 was recognized to be the active species in CO2-carbon gasification catalyzed by calcium compounds [69] . Similarly, for all the spent Ni/ALO catalysts (Fig.12b) The TG/DTG curves of all the spent catalysts are shown in Fig. 13 and the content of accumulated carbon during DRM process are reported in sample is shown in Fig. 13 [76] . The type of deposited carbon is closely connected to the combustion temperature of carbon residue, which corresponded to the maximum DTG peak temperature [77] . Generally, three types of carbon are formed and deposited on supported nickel catalyst surface, namely polymeric (C  ), filamentous (C  ), and graphitic (C  ) carbon [78] . C  is the most stable versus oxidation requiring high temperatures for carbon removal and closely related to catalyst deactivation [79] . C  is the most reactive and easily oxidable at low temperature, which can play an important role as intermediate in CO formation during DRM [80] .

With the increasing temperature the less reactive C  can transform in C  [81] . All the used Ni/CAO catalysts showed rather different DTG profiles (Fig. 13a) , indicating a pronounced difference in the combustion temperatures. It is worth to keep in mind that the obvious DTG peak located between 698-702°C in all the samples was originated from the CaCO3 thermal decomposition. The

Ni/CAO600 and Ni/CAO700 samples presented one small DTG peak at 430°C due to some easily oxidable carbonaceous species like C  . This deposited carbon compound is most likely in the proximity of nickel catalytic sites as it can be easily catalytically removed at low temperature [82] .Two further DTG peaks centered at 620°C for Ni/CAO600 and at 540°C for Ni/CAO700 might be due to combustion of whisker-type carbon nanotube like C  , which was reported to be oxidable in the temperature range 500-600°C [83] . In the air-combustion DTG curve of spent Ni/CAO800 sample solely the peak corresponding to calcium carbonate decomposition was observed.

Differently, solely one DTG peak centered between 500 and 700°C is observed for all the spent Ni/ALO catalysts, exception made for Ni/ALO700, which showed double overlapped peaks at 580 and 680°C (Fig.13b) , allowing not distinguishable characterization of type of deposited carbon in spent catalyst. To further get insights into the kind of deposited carbon on surface catalyst the CO2gasification through the temperature programmed reaction (TPRn) was conducted by dosing a mixture of 10%CO2-Ar over spent catalyst while the temperature rose from ambient to 850°C. It is well known that CO2 gasification reactivity is dependent on the type of carbon deposited [84] . In curves. From Fig. 14(a) it can be noted the CO2 peak release from calcium carbonate decomposition at 700°C for the spent Ni/CAO catalysts. It is worth to note that in Ni/CAO800 sample there was solely one evolved peak CO2-TPRn curve. The CO/CO2 profile for Ni/CAO600 sample shows one intense CO-evolved peaks at 427°C with a right-shoulder at 455°C. The proximity of the two peaks suggested the closer nature of the deposited carbon. It is known that C  is related to methane decomposition that might transform to C  at high temperature [85] . The CO-evolved peak at 583°C implies a different resistance to gasification of accumulated carbon, likely C  type carbon. From

Ni/CAO700 sample two less intense CO-evolved peaks at 441 and 559°C were likely generated by C  and C  type carbons gasification. The low gasification temperatures of deposited carbon might be due to the catalytic role of CaCO3 in the CO2-gasification of coke [86] . From Fig. 14 For the used Ni/ALO700 and Ni/ALO800 samples the catalyst surface was densely covered by encapsulating-type filamentous carbons network with large diameter (20-50 nm) and length ( Fig.15C and D) . These results are in accordance with the very high content of deposited carbon found by TG analysis (see Tab.5). The characteristic white points on carbon nanotubes might be ascribed to Ni° particles seating on the tip of the filamentous carbon [87] . According to the literature, the kind of deposited carbon and its growth rate were related to the small Ni° crystal size and strong interaction with support, which were able to slow down the growth rate of filamentous carbon [88] . The appearance morphology is typical of supported nickel catalysts, in which metallic nickel particles work as catalysts for the accretion of carbon filaments [89] . Based on the CO2-TPRn and SEM results it is likely that "active carbon" -type [90] , namely that participates to reaction path of CO formation, was mainly accumulated on the surface of spent Ni/CAO catalysts during DRM at low temperature because no appreciable nickel catalyst deactivation was found. However, the high J o u r n a l P r e -p r o o f carbon content deposited on the spent Ni/ALO catalysts might plug the reactor or collapse metal catalyst particles with consequent loss of catalytic activity [91] . 

The catalytic performances of Ni/CaO-Ca12Al14O33 catalyst were compared to those of Ni/-Al2O3 through the synthesis gas production via DRM in the temperature range 600-800°C. Catalysts were synthesized by wet impregnation method at a fixed nickel loading ( 

There are no conflict of interest to disclose.

J o u r n a l P r e -p r o o f Table 1 . Textural properties of various supports and catalysts.

S BET a (m 2 g -1 )

Pore Volume Total basicity (10 -3 molg -1 )

As-prepared CAO support 

CO2-Reforming of Methane over Transition Metals

Review on dry reforming of methane, a potentially more environmentally-friendly approach to the increasing natural gas exploitation

Coke formation and minimization during steam reforming reactions

CO2 reforming of CH4

Production of Synthetic Gas-Reaction of Light Hydrocarbons with Steam and Carbon Dioxide

Progress in Synthesis of Highly Active and Stable Nickel-Based Catalysts for Carbon Dioxide Reforming of Methane

Co catalysts for the dry reforming of methane

Mechanisms of catalyst deactivation

A new model explaining carbon filament growth on nickel, iron, and Ni-Cu alloy catalysts

Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide

Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review

Effects of support modifiers on the catalytic performance of Ni/Al2O3 catalyst in CO2 reforming of methane

Effects of promoters and preparation procedures on reforming of methane with carbon dioxide over Ni/Al2O3 catalyst

Influence of the preparation method and the nature of the support on the stability of nickel catalysts

The role of catalyst support on the activity of nickel for reforming methane with CO2

Kinetic and mechanistic aspects for CO2 reforming of methane over Ni based catalysts

Catalytic activities and coking characteristics of oxides-supported Ni catalysts for CH4 reforming with carbon dioxide

Carbon and oxygen reaction pathways of CO2 reforming of methane over Ni/La2O3 and Ni/Al2O3 catalysts studied by isotopic tracing techniques

Catalytic performance and catalyst structure of nickel-magnesia catalysts for CO2 reforming of methane

Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression

Effect of basicity of metal doped ZrO2 supports on hydrogen production reactions

Suppression of carbon deposition in the CO2-reforming of CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst

Effect of alkaline earth promoters (MgO, CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al2O3 in dry reforming of methane

Characterization of Ca-promoted Ni/α-Al2O3 catalyst for CH4 reforming with CO2

Exploring, tuning, and exploiting the basicity of hydrotalcites for applications in heterogeneous catalysis

Dry reforming of methane over nickel catalysts supported on magnesium aluminate spinels

Carbon dioxide reforming of methane over ordered mesoporous NiO-MgO-Al2O3 composite oxides

Carbon dioxide reforming of methane on Ni-MgO-Al2O3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios

Insight into the role of CaO in coke-resistant over Ni-HMS catalysts for CO2 reforming of methane

Modifying alumina with CaO or MgO in supported Ni and Ni-Co catalysts and its effect on dry reforming of CH4

Cooperation of Ni and CaO at Interface for CO2 Reforming of CH4: A Combined Theoretical and Experimental Study

Calcium-looping reforming of methane realizes in situ CO2 utilization with improved energy efficiency

Mixed Electrical Conduction of Calcium Aluminates Synthesized by Polymeric Precursors

Electronic band structure and carrier effective mass in calcium aluminates

A crucial role of O2 − and O2 2− on mayenite structure for biomass tar steam reforming over Ni/Ca12Al14O33

Effect of addition of CaO on Ni/Al2O3 catalysts over CO2 reforming of methane

Development of new nickel-based catalyst for biomass tar steam reforming producing H2-rich syngas

Dry reforming reaction over nickel catalysts supported on nanocrystalline calcium aluminates with different CaO/Al2O3 ratios

Carbon dioxide reforming of methane over 5 wt.% nickel calcium aluminate catalysts-effect of preparation method

Preparation of nickel catalysts supported on CaO2Al2O3 for methane reforming with carbon dioxide

Steam pre-reforming of natural gas over nanostructured Ni/12CaO-7Al2O3 catalyst for hydrogen production: effect of support preparation method

Characterization of Carbonaceous Species Formed during Reforming of CH4 with CO2 over Ni/CaO-Al2O3, Catalysts Studied by Various Transient Techniques

Carbon dioxide reforming of methane over 5 wt.% Ni/CaO-Al2O3 catalyst

Novel synthesis of combined CaO-Ca12Al14O33-Ni sorbent-catalyst material for sorption enhanced steam reforming processes

TG-FTIR and kinetics of devolatilization of Sulcis coal

Bestimmung der Größe und der inneren Struktur von Kolloid-teilchen mittels Röntgenstrahlen

Adsorption by Powders and Porous Solids

CO2 reforming of CH4 over stabilized mesoporous Ni-CaO-ZrO2 composites

Thermal decomposition of nickel nitrate hexahydrate, Ni (NO3)2·6H2O, in comparison to Co(NO3)2·6H2O and Ca(NO3)2·4H2O

Assessment of the CaO-Al2O3 system

The role of intermediate calcium aluminate phases in solid state synthesis of mayenite (Ca12Al14O33)

Chemical preparation of the binary compounds in the calcia-alumina system by self-propagating combustion synthesis

Synthesis, experimental studies, and analysis of a new calcium-based carbon dioxide absorbent

Preparation and performance of Ni-based catalysts supported on Ca12Al14O33 for steam reforming of tar in coke oven gas

Reporting physisorption data for gas/solid systems with Special Reference to the Determination of Surface Area and Porosity

Calcination kinetics and surface area of dispersed limestone particles

Syngas production from methane dry reforming over Ni/Al2O3 catalyst

Chemistry of nickel-alumina catalysts

Methane formation by metal-catalyzed hydrogenation of solid calcium carbonate

Reforming of methane with carbon dioxide over Ni/Al2O3 catalysts: Effect of nickel precursor

Comparison of reducibility and stability of alumina-supported Ni catalysts prepared by impregnation and co-precipitation

Basicity and basic catalytic properties of zeolites

Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules

Effect of CaO Addition on Nickel Catalysts Supported on Alumina for Glycerol Steam Reforming

Catalytic steam reforming of complex gasified biomass tar model toward hydrogen over dolomite promoted nickel catalysts

Heterogeneous basic catalysis

Microcalorimetric study of the acidity and basicity of metal oxide surfaces

Effect of NiAl2O4 formation on Ni/Al2O3 stability during dry reforming of methane

Formation and Desorption of Oxygen Species in Nanoporous Crystal 12CaO7Al2O3

Understanding the role of surface basic sites of catalysts in CO2 activation in dry reforming of methane: a short review

Highly stable and active Nimesoporous alumina catalysts for dry reforming of methane

Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation

Progresses in the preparation of coke resistant Ni-based catalyst for steam and CO2 reforming of methane

Calcium carbonate precipitation for CO2 storage and utilization: a review of the carbonate crystallization and polymorphism

Methane formation by metal-catalyzed hydrogenation of solid calcium carbonate

A temperature-programmed reaction study of calcium-catalyzed carbon gasification

Characterization of Ca-promoted Co/AC catalyst for CO2-CH4 reforming to syngas production

Thermogravimetric determination of coke deposits on alumina-supported noble metal catalysts used as hydrodechlorination catalysts

A review on coke management during dry reforming of methane

Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide

Heterogeneous catalyst deactivation and regeneration: a review

Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts

Studies on Carbon Deposition in CO2 Reforming of CH4 over Nickel-Magnesia Solid Solution Catalysts

Reactivity of coke deposited on metal surfaces

Methane dry reforming with CO2: a study on surface carbon species

TPR and SIMS studies of CaCO3 catalyzed CO2 gasification of carbon

Impact of the structure and reactivity of nickel particles on the catalytic growth of carbon nanofibers

Structure and texture of filamentous carbons produced by methane decomposition on Ni and Ni-Cu catalysts

Catalytic growth of carbon filaments

Origin and reactivity of active and inactive carbon formed during DRM over Ni/Ce0. 38Zr0. 62O2-δ studied by transient isotopic techniques

Catalyst design for dry reforming of methane: Analysis review

Typical SEM images of freshly calcined Ni/CAO (A) catalyst, its reduced status (B), and SEM-EDS and elemental mapping of reduced Ni/CAO (C)

XRD patterns of spent Ni/CAO (a) and Ni/ALO (b) catalysts worked at different temperatures. Sh=sample holder. Contaminant from quartz wool: q= SiO2

CO/CO2 profiles during TPRn runs with CO2 for: (a) spent Ni/CAO600 (A), Ni/CAO700 (B) and Ni/CAO800 (C) catalysts; and (b) spent Ni/ALO600 (A)

Ni/ALO800 (C) catalysts