Martine Duclos

Philippe Lacomme

Celine Lambert

Bruno Pereira

Libo Ren

Gerard Fleury

Hervé Ovigneur

Thibault Deschamps

Nicole Fearnbach

Jeremy Vanhelst

Is physical fitness associated with the type of attended school? A cross

Keywords: cardiorespiratory fitness, muscular fitness, adolescents, health, academic achievement, type of schools

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

IS PHYSICAL FITNESS ASSOCIATED WITH THE TYPE OF ATTENDED SCHOOL? A CROSS-SECTIONAL ANALYSIS AMONG 20.000 ADOLESCENTS

Martine Duclos a,b,c , Philippe Lacomme d , Celine Lambert e , Bruno Pereira e , Libo Ren f , Gerard Fleury d , Hervé Ovigneur g , Thibault Deschamps g , Nicole Fearnbach h , Jérémy Vanhelst k,l , Jean-François Toussaint i , David Thivel b,j*

INTRODUCTION

Physical fitness is a modifiable factor that may influence health. Systematic reviews concluded that physical fitness (both cardiorespiratory fitness (CRF) and muscular fitness) is considered one of the most important health markers, as well as a predictor of morbidity and mortality for cardiovascular disease (CVD) and for all causes in adults [START_REF] Myers | Exercise capacity and mortality among men referred for exercise testing[END_REF][START_REF] Lee | Mortality trends in the general population: the importance of cardiorespiratory fitness[END_REF][START_REF] Leong | Prognostic value of grip strength: findings from the Prospective Urban Rural Epidemiology (PURE) study[END_REF] . In children and adolescents, a recent systematic review showed that higher physical fitness, especially CRF, during childhood and adolescence was associated with lower BMI, lower waist circumference, lower body fatness and a lower prevalence of metabolic syndrome later in life [START_REF] Mintjens | Cardiorespiratory Fitness in Childhood and Adolescence Affects Future Cardiovascular Risk Factors: A Systematic Review of Longitudinal Studies[END_REF] . In addition, a recent review including the entire spectrum of school-aged children, ranging from 6 to 18 years, showed a consistent and positive association between CRF and academic achievement [START_REF] Marques | Associations between selfreported fitness and self-rated health, life-satisfaction and health-related quality of life among adolescents[END_REF] . Similar results were obtained in another review that included both cross-sectional and longitudinal studies [START_REF] Santana | Physical fitness and academic performance in youth: A systematic review[END_REF] . Muscle mass and strength are also important for overall health. A prospective cohort study of 38,588 Swedish men aged 18 years showed an association between higher muscle strength and lower risk for future CVD events, independent of well-known risk factors such as low CRF, smoking and BMI until late middle age (up to 40 years of follow-up) [START_REF] Timpka | Muscle strength in adolescent men and risk of cardiovascular disease events and mortality in middle age: a prospective cohort study[END_REF] . An increased risk of middle age CVD mortality was also reported in men with low muscle strength, independent of common risk factors [START_REF] Timpka | Muscle strength in adolescent men and risk of cardiovascular disease events and mortality in middle age: a prospective cohort study[END_REF] .

Globally, in children and adolescents, overall physical fitness has been found associated with several factors such as gender, weight status, accommodation's neighborhood and geographical location for instance, but also with level of parental education and with the family socioeconomic status [START_REF] Rodrigues | Environmental and Sociodemographic Factors Associated with 6-10-Year-Old Children's School Travel in Urban and Non-urban Settings[END_REF][START_REF] Quinn | Associations Between Neighborhood Social Cohesion and Physical Activity in the United States, National Health Interview Survey[END_REF][START_REF] Silva | Insufficient levels of physical activity of adolescents associated with sociodemographic, environmental and school factors[END_REF][START_REF] Janssen | Cardiorespiratory fitness, muscular fitness and mental health in older adolescents: A multi-level cross-sectional analysis[END_REF] . Similarly, there is a clear association between the parents' socioeconomic level, their academic curriculum and their implication into their kids' education, with their children's academic achievement [START_REF] Zhang | Family socio-economic status and children's academic achievement: The different roles of parental academic involvement and subjective social mobility[END_REF][START_REF] Guo | Gender Differences in How Family Income and Parental Education Relate to Reading Achievement in China: The Mediating Role of Parental Expectation and Parental Involvement[END_REF] . This particularly important since education and income are associated with mortality [START_REF] Martikainen | Income differences in life expectancy: the changing contribution of harmful consumption of alcohol and smoking[END_REF] as well as with other health outcomes later in life [START_REF] Van Ourti | The effect of income growth and inequality on health inequality: Theory and empirical evidence from the European Panel[END_REF] .

In France, pupils from the 7th to the 9th grade with low academic performance are often encouraged to follow a technical curriculum in technical schools, which are of shorter duration and predominantly include manual and practical teaching. Since, the life expectancy of blue collar workers is 13 years shorter than that of white collar workers, most of these students are therefore at risk for reduced life expectancy [START_REF] Blanpain | L'espérance de vie par niveau socio-économique[END_REF] . Although there is a growing body of literature linking physical activity and CRF with academic achievement in children and adolescents, there is, to our knowledge, no study that has examined the association between physical fitness and the type of school attended. Properly identifying whether the type of school attended is associated with fitness impairments might help develop appropriate educational strategies from adolescence to potentially counteract the differences observed between individuals working in different occupational domains.

METHODS AND METHODS

Participants

Participants were recruited between April 2009 and March 2016 during scholar events, in eight sub-regions of the French former Rhône-Alpes region. These events were organized on a voluntary basis at the request of municipal councilors or directors of institutions. Different types of institutions were included: higher education institutions (school type 1), general high schools (school type 2), professional-oriented high schools (school type 3), agricultural high schools (school type 4), vocational training centers (school type 5) and rural vocational training centers (school type 6). When an institution volunteered, all classes were involved.

The objective was to assess the physical capacity of students and to improve their knowledge regarding the preventive role of sports participation and physical activities. Data were collected in accordance with the Helsinki principles for human research as requested by the ethical authorities (CNIL number 1232206).

Procedure

The Diagnoform © Tonic physical test, developed by the Rhône-Alpes Athletic Federation and previously shown to be valid and reproducible [START_REF] Mouraby | Reliability and validity of Diagnoform fitness test procedure[END_REF] , was used to assess physical fitness of a French population (n=31,349) in 22 regions of France, offering an important monitoring tool in the general population [START_REF] Nassif | Monitoring fitness levels and detecting implications for health in a French population: an observational study[END_REF] . For the current analysis, we used the results of the Diagnoform in a subsample of

Instruments

Description of tests (35-40 min workshop)

For better understanding of the Diagnoform © results, the name of each test indicates the main related physical capacity. Each test composing the battery is described below and its reliability and validity indicated, as previously detailed [START_REF] Mouraby | Fiabilité et validation de la batterie de tests physiques Diagnoform[END_REF] .

1. Cardiorespiratory fitness was evaluated using the non-progressive 20 m shuttle run

(

RESULTS

The Table 2 presents the results from a multivariate analysis considering gender, age and BMI for the total population and then age and BMI in boys and girls separately (The results from a univariate analysis are proposed in the supplementary Table S2). Average physical fitness score (QPC) was 47.3±16.1 out of 100 (girls: 35.4±10.8, boys: 57.3±12.6). Significant differences were found between the type of school and physical fitness (Table 2), with a decreasing value of QPC from higher education institutions to more technical schools in both genders. Among girls, QPC was significantly higher in higher education institutions (43.8±13.5) and in general high schools (37.1±10.0) compared with all other types of schools (p<0.001). Among boys, QPC was significantly higher in higher education institutions (71.2±8.8) than in all other types of schools where the average QPC were lower than 60 (p<0.001). In higher education institutions and general high schools, girls presented better performances for all physical tests. Similar results were found for boys, except for strength/speed.

Results regarding the reliability of the Diagnoform © tests battery reliability are presented as supplementary files (Table S1 and Figure S1).

DISCUSSION

The aim of this study was to investigate whether the type of school attended by French adolescents (high schools vs technical or training school) might be associated with their physical fitness level. According to our results, boys and girls of higher educational institutions show higher physical fitness compared with their peers from technical schools. As every component of physical fitness has prognostic value for present and future health, students of technical or training schools appear then at higher risk for future chronic diseases, calling for specific interventions to improve their fitness level.

In France, depending on academic results, students are oriented either towards technical or academics schools. Numerous factors are involved in the capacity to reach high academic performance and high diplomas. Among them, academic results, intellectual capacities, health status and socio-economic status are well recognized factors [START_REF] Tejero-Gonzalez | Physical activity and cognition in adolescents: A systematic review[END_REF] . But for the last couple of years, CRF has emerged as another potential contributing factor [START_REF]Physical Activity Guidelines Advisory Committee Scientific Report[END_REF] .

Our results show a difference in physical fitness between the types of academic schools in both genders. In higher education institutions and general high schools, girls presented better performances for all physical tests than in technical schools. Similar results were found for boys (except strength/speed). In other words, boys and girls of higher educational institutions have higher physical fitness (CRF, musculo-skeletal, motor fitness, coordination) than their peers from technical schools.

In youth, physical fitness is considered as an important marker of health and is now considered one of the most important health indicators throughout the lifespan [START_REF] Ortega | Physical fitness in childhood and adolescence: a powerful marker of health[END_REF] . For children and adolescents, higher CRF is associated with a healthier cardiovascular profile [START_REF] Ruiz | Cardiorespiratory fitness and ideal cardiovascular health in European adolescents[END_REF] and a healthier body composition [START_REF] Ortega | Physical fitness in childhood and adolescence: a powerful marker of health[END_REF][START_REF] Moliner-Urdiales | Association of objectively assessed physical activity with total and central body fat in Spanish adolescents; the HELENA Study[END_REF] later in life and improvements in CRF are associated with reduced mortality risk in adults [START_REF] Lee | Mortality trends in the general population: the importance of cardiorespiratory fitness[END_REF] . Muscular fitness has been increasingly recognized in the prevention of chronic disease and has been inversely associated with established and emerging CVD risk factors in adolescents [START_REF] Steene-Johannessen | Low muscle fitness is associated with metabolic risk in youth[END_REF][START_REF] Artero | Muscular and cardiorespiratory fitness are independently associated with metabolic risk in adolescents: the HELENA study[END_REF] .

While about half of the variance in CRF is considered to be attributable to heritable factors [START_REF] Bouchard | Familial aggregation of VO2 max response to exercise training: results from the HERITAGE family study[END_REF] , the other half is influenced by several factors, including body fatness, growth, sexual maturation, age, sex, health status, yet its principal modifiable determinants are moderate-tovigorous PA (MVPA) and sedentary behavior [START_REF] Teran-Garcia | Genes, exercise, growth, and the sedentary, obese child[END_REF] . Although we controlled for several potential confounders such as age, growth, pubertal status and health status, it cannot be excluded that other confounders such as genetics and body composition differed, despite similar BMI.

We hypothesized that sedentary activity time would be lower in technical schools since students are involved in manual classes that often imply a standing position (passive or active), while higher academic classes usually lead students to accumulate a higher total sitting time (during classes and homework hours). Structured PA such as extracurricular sports was recorded and was not significantly different between types of schools. However, habitual levels of PA were not recorded. It is noteworthy that previous studies have reported low levels of PA at this age, with PA among older adolescents matching those of 60-year-old individuals, providing strong evidence that adolescence represents a high-risk time period for physical inactivity [START_REF] Varma | Re-evaluating the effect of age on physical activity over the lifespan[END_REF] . In addition, several studies have reported little to no effect of family wealth on PA levels [START_REF] Jimenez Pavon | Socioeconomic status influences physical fitness in European adolescents independently of body fat and physical activity: the HELENA study[END_REF][START_REF] Costa-Tutusaus | Relationship between Healthy Lifestyle and Sociodemographic Factors in Adolescents in Catalonia: Application of VISA-TEEN Questionnaire[END_REF] . This is contrary to findings with physical fitness, as stressed in the HELENA study, where a strong positive association between socioeconomic status and physical fitness was reported in European adolescents (15.0±1.3yr) independently of body fat and objectively assessed PA [START_REF] De Cocker | Self-reported physical activity in European adolescents: results from the HELENA (Healthy Lifestyle in Europe by Nutrition in Adolescence) study[END_REF] . Although the socio-economic status and income were not measured in the present study, the level of education is considered as a socio-economic marker. At a population level, manual occupation is associated with lower socio-economic status compared to intellectual occupation.

School types 3 to 6 will lead to manual professions (blue-collar workers of both urban and rural environment and agriculture). In France, life expectancy of blue collar workers is 13 years shorter than that of white collar workers [START_REF] Blanpain | L'espérance de vie par niveau socio-économique[END_REF] . These results come from associations with working conditions, health behavior (including PA and nutrition) and health care [START_REF] Van Ourti | The effect of income growth and inequality on health inequality: Theory and empirical evidence from the European Panel[END_REF] . As physical fitness is associated with health outcomes [START_REF] Ross | Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign: A Scientific Statement From the American Heart Association[END_REF] and as our results show for the first time that a difference already exists between 16-year-old students of academic and technical schools, it is necessary to develop preventive strategies such as the promotion of PA in these youth before they start their professional lives. Importantly, sedentary behavior is already known to increase during adulthood, and PA habits and CRF in youth track into adulthood [START_REF] Biddle | Tracking of sedentary behaviours of young people: a systematic review[END_REF][START_REF] Telama | Tracking of physical activity from childhood to adulthood: a review[END_REF] . Therefore, designing specific interventions targeting physical fitness in technical school students are warranted.

Finally, the high intra-class correlation coefficients (ICC) values highlighted great withinschool correlations between participants. Therefore, for more efficient physical fitness promotion, school-specific PA programs should be designed.

We usually consider mortality, or the presence of chronic disease, to be a standard, objective health indicator [START_REF] Hoffmann | The impact of increasing income inequalities on educational inequalities in mortality -An analysis of six European countries[END_REF] . Alternative health measures such as CRF or muscle strength may be better predictors of health status and targets for the prevention of chronic disease [START_REF] Ross | Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign: A Scientific Statement From the American Heart Association[END_REF] . These indicators improve quickly with regular PA.

PA and sedentary behaviors are difficult to measure with reliability using self-reported questionnaires, as they are prone to overestimation biases [START_REF] Prince | A comparison of direct versus self-report measures for assessing physical activity in adults: a systematic review[END_REF] and have a low to moderate correlation with objective measures of PA [START_REF] Prince | A comparison of direct versus self-report measures for assessing physical activity in adults: a systematic review[END_REF][START_REF] Steene-Johannessen | Are Self-report Measures Able to Define Individuals as Physically Active or Inactive?[END_REF] . Accelerometers devices remain however expensive and require specific scientific and technical skills. Moreover, most commercial devices are not reproducible, valid, or accurate 39,40 . By contrast, physical fitness is an objective and accurate measure, representing usual PA. Moreover, as stated by Van der Velde 41 , the relative importance of high PA for cardiometabolic health should consider the mediating role of CRF, explaining 73% of the association between moderate-to-vigorous PA and metabolic risk. In the present study, the high correlation between QPC and Agility, strength/speed and endurance/strength in both girls and boys, and the fact that these three tests decrease progressively from higher education institutions to more technical schools in both genders suggest that these three physical fitness components may be good proxies of overall physical fitness in large segments of population.

The present study has important strengths, including the large sample size and the careful harmonization of anthropometric and fitness assessment techniques throughout the study, including demonstrated reliability [START_REF] Nassif | Monitoring fitness levels and detecting implications for health in a French population: an observational study[END_REF] . However, our results have to be considered in light of some limitations. First, it was limited to the Rhône-Alpes region and might not be nationally representative. However, due to the homogeneity of educational program in France it is unlikely that the results will be different between regions. The absence of measures of habitual PA level could interfere with the results but, as discussed, CRF is more accurate than PA questionnaires and the effect of CRF on health are independent of the effects of PA.

Previous studies also show that at this age, adolescent PA has steadily declined [START_REF] Varma | Re-evaluating the effect of age on physical activity over the lifespan[END_REF] . Although gender, BMI and age are considered in the present analysis, other factors could have impacted our results and should have been considered, such as the socio-economical status of the participants, their family culture toward PA, their potential implication in extra-school regular physical activities, the school infrastructures or geographical location, among others. This has to be considered here as a limitation of the present analysis. Importantly, it would have been also interesting to have information regarding the participants' level of physical fitness before their integration in these particular school types, which was unfortunately not possible in the present study.

As any cross-sectional study, we cannot state the direction of the interaction and it must be noted that the adolescents' physical fitness level is not the primary or sole determinant of school academic status in high school students. According to a recent review from the U.S.

Department of Health and Human Services 21 insufficient evidence is available to determine whether a relationship exists between moderate-to-vigorous PA and cognition in adolescents ages 14 to 18 years (although "all studies reported positive results, but the difficulty to conclude mainly relies on the insufficient number of studies and the difficulties to reliably study cognition" [START_REF] Ortega | Physical fitness in childhood and adolescence: a powerful marker of health[END_REF] ).

CONCLUSION

The main finding of this study is that physical fitness is associated with the type of school that young students attend; 16 year old girls and boys of higher educational institutions have higher physical fitness (CRF, musculo-skeletal, motor fitness, coordination) than their peers from technical schools. As every component of physical fitness has prognostic value for present and future health, students of technical or training schools represent a group at higher risk of future chronic diseases. Specific attention is required regarding the evaluation and development of physical fitness in students attending technical schools, especially since physical fitness remains modifiable and since its components improve quickly with regular PA. We suggest the use of these three physical tests to establish physical fitness systematically in all provided education for students at the levels of 15-16 years. Future prospective studies should focus on evaluating in technical schools the potential benefits of improving physical fitness for future health of the professionals they produce.

FILIBERTO and Philippe COLLARD (Rhône-Alpes League of Athletism) for their full support.

Funding: this research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors

  All resultsrelated to the reliability of the Diagnoform © test battery are presented as a supplementary file.Because observations from the same cluster (= institution) are usually more similar to each other than observations from different clusters, comparisons between the six school-types were performed with linear mixed models to take into account the dependencies among cluster members. The ratio of the between-cluster variance to the total variance, called "intraclass correlation coefficient" (ICC), was calculated to measure how individuals within a cluster are similar to one another. Analyzes covering the entire sample were adjusted for sex, age and BMI; and those relating to the samples of girls and boys were adjusted for age and BMI.

	maximum distance reached during a 3 min run). This test has been validated against the 20-m
	shuttle run reference test from Leger et al (1988) (r = 0.78; P =0 .001) and showed a good
	reliability (0.84). The adolescent ran back and forth on a straight 20 m line for 3 min. The
	supervisor counted the number of times the adolescent went back and forth and translated it to
	a measurement, expressed in meters.

2. Coordination/Endurance. This standardized motor coordination-endurance test provides both a qualitative and quantitative evaluation of the student's motor competence. Adolescents performed a maximal number of sprint hops over a cross following a predefined sequence for a session of 30 seconds. Only successful sequences were counted. A good reliability was also found for this component (0.81).

3: Agility/Coordination. It refers to the participant's ability to perform five consecutive strides (total length in centimeters for the five consecutive strides). The reliability for this test was excellent (0.90). 4: Lower muscular strength was assessed by the standing broad jump test. The adolescent jumped as far as possible with legs together. Results was recorded in cm. This test showed a good reliability (0.84). 5: Lower muscular endurance. A repeated squat jump test measured quadriceps strength, endurance and fatigue. The subjects had to perform five consecutive jumps with legs together, and results were recorded in cm. 6: Speed/Agility. A motor coordination test recording the minimal time needed to complete the 4x10 m shuttle run. Time was recorded in second. An excellent reliability was found for the 4x10m shuttle run in adolescent (0.90). 7: Speed. The anaerobic capacity is measured by running a 30m linear course as fast as possible and results were recorded in seconds. This test showed a good reliability (0.85).

8: Upper muscular endurance. Subjects were asked to perform push-ups (muscular strength and endurance) with knees on the ground, lowering body to a 90° elbow angle and then back up to the initial position (number of maximal push-ups without break). A good reliability was found for this test (0.81).

The results of each test were analyzed in absolute value and also transformed to a normalized score from 1 (poor) to 20 (excellent). The mean of the normalized values across the 8 tests was calculated to create the Quotient of Physical Condition (QPC), a global indicator of individual physical fitness.

Data analysis

Statistical analysis was performed using Stata software (version 13; StataCorp, College Station, Texas, USA) and R3.3.1 (http://cran.r-project.org/). All tests were two-sided, with a Type I error set at 0.05 and all analyses were performed on the whole population and by sex.

Continuous variables were expressed as mean ± standard and categorical variables as frequencies and associated percentages. Internal consistency was assessed using Cronbach's alpha, item-total correlation and item-rest correlation (correlation between an item and the scale formed by all other items). Cronbach's alphas were calculated for all the scales and by removing each item, one after the other. Associations between physical test items and QPC were assessed with Pearson's or Spearman's correlation coefficient according to statistical distribution (the Gaussian distribution was verified by the Shapiro-Wilk test).

Table 1 : Descriptive analysis Total (n=20,228) Girls (n=9,196) Boys (n=11,032) Characteristics of students

 1 Disclosure of interest. The authors report no conflict of interest Authors' contribution. PL, CL, BP, LR, GF, HO and TD designed the study; PL, CL, BP, LR, GF, HO and TD collected the data; PL, CL, BP, LR, GF, HO and TD were in charge of the data management; MD, CL, BP and TD analyzed the data; MD, CL, BP, NF, JFT and TD wrote the manuscript; MD, PL, CL, BP, LR, GF, HO, TD, NF, JFT and TD reviewed and cowrote the manuscript. All the authors read and approved the final version of the manuscript.39.Wallen MP, Gomersall SR, Keating SE, Wisloff U, Coombes JS. Accuracy of Heart Rate Watches: Implications for Weight Management. PloS One. 2016;11(5):e0154420. 40.Guidoux R, Duclos M, Fleury G, Lacomme P, Lamaudière N, Saboul D, et al. The eMouveRecherche application competes with research devices to evaluate energy expenditure, physical activity and still time in free-living conditions. J Biomed Inform.

	2017;69:128-34.
	41.	van der Velde J, Schaper NC, Stehouwer CDA, van der Kallen CJH, Sep SJS, Schram
	MT, et al. Which is more important for cardiometabolic health: sedentary time, higher
	intensity physical activity or cardiorespiratory fitness? The Maastricht Study. Diabetologia
	2018;61(12):2561-9.

TABLES

Data are presented as frequencies (associated percentages) or as mean ± standard deviation. BMI: body mass index.

Table 2 : Physical performance according to school types. School-type 1 School-type 2 School-type 3 School-type 4 School-type 5 School-type 6

 2 St 31.5 ± 14.2 23.2 ± 12.9 a 18.3 ± 11.6 ab 18.1 ± 11.8 ab -17.6 ± 11.2 ab

	Total	n=870	n=13,125	n=3,569	n=1,325	n=202	n=1,137
	QPC	61.3 ± 17.0	48.3 ± 15.2	43.6 ± 16.8 ab 42.6 ± 16.0 a	53.2 ± 11.8	42.2 ± 15.5 ab
	En	626 ± 86	565 ± 80 a	537 ± 99 ab	541 ± 84 a	588 ± 71 a	548 ± 83 a
	CoEn	34.6 ± 8.4	31.1 ± 7.7	28.1 ± 7.5 ab	29.0 ± 7.2 a	27.4 ± 7.9 a	28.2 ± 6.6 ab
	Co	1,067 ± 166	977 ± 157	946 ± 175 b	929 ± 172 b	1,051 ± 133	938 ± 159 b
	StSp	199 ± 40	176 ± 38	170 ± 42 b	169 ± 41	198 ± 36	170 ± 39 b
	EnSt	1,035 ± 201	889 ± 189	841 ± 213 ab	838 ± 205 a	1,000 ± 173	848 ± 182 ab
	SpCo	10.1 ± 1.3	11.0 ± 1.4	11.3 ± 1.7 ab	11.4 ± 1.4	10.9 ± 1.3	11.6 ± 1.4 ab
	Sp	4.4 ± 0.8	4.9 ± 0.8	5.1 ± 1.0	5.2 ± 0.9	5.0 ± 1.2	5.3 ± 0.9 abc
	St	50.3 ± 24.3	35.4 ± 21.0 a 30.5 ± 20.2 ab 29.4 ± 18.1 ab	42.8 ± 17.2	30.6 ± 20.1 ab
	Girls	n=314	n=6,237	n=1,647	n=553	n=0	n=445
	QPC	43.8 ± 13.5	37.1 ± 10.0 a 30.6 ± 10.4 ab 29.6 ± 10.1 ab	-	29.6 ± 9.7 ab
	En	553 ± 71	518 ± 64 a	475 ± 79 ab	487 ± 73 a	-	490 ± 69 ac
	CoEn	32.8 ± 8.3	30.1 ± 7.3	26.8 ± 7.0 ab	27.9 ± 6.2 ab	-	28.0 ± 6.4 ab
	Co	909 ± 131	875 ± 113	830 ± 118 ab	803 ± 117 ab	-	825 ± 116 ab
	StSp	160 ± 30	152 ± 29	142 ± 29 ab	137 ± 29	-	141 ± 29 ab
	EnSt	830 ± 142	760 ± 128 a	694 ± 138 ab	686 ± 133 ab	-	714 ± 127 abc
	SpCo	11.2 ± 1.1	11.7 ± 1.3	12.2 ± 1.6 ab	12.3 ± 1.3 a	-	12.4 ± 1.3 ab
	Sp	5.1 ± 0.7	5.2 ± 0.7	5.6 ± 1.0	5.6 ± 0.8	-	5.8 ± 0.9 abc
	Boys	n=556	n=6,888	n=1,922	n=772	n=202	n=692
	QPC	71.2 ± 8.8	58.3 ± 11.7	54.7 ± 12.8	51.9 ± 12.6 ab	53.2 ± 11.8 50.3 ± 12.9 abc

Acknowledgments:

The authors thank Pierre-Antoine Onfroy (LIMOS), Yoann Alvarez (LIMOS), Stéphane MARCHAND MAILLET and Rémi GOUBE (MGEN), Stéphanie


Céline Chizallet
email: celine.chizallet@ifpen.fr

Christian Schlaup

Emiliano Fonda

Xavier Carrier
email: xavier.carrier@sorbonne-universite.fr

Surface orientation dependent interaction of cobalt (II) precursors with alpha-alumina

ABSTRACT.

The performances of alumina-supported cobalt catalysts strongly depend on Co/alumina interactions, that are poorly understood. In the present work, α-alumina surfaces are used as models of polycrystalline support, to unravel their orientation dependent behavior by GI-EXAFS. The Co II / α-alumina interactions in the dried state are investigated, with a resolution of the parallel versus perpendicular deposit formation. DFT calculations shed light on the existence of specific grafting modes. EXAFS shows that Co(OH) 2 forms parallel to the R(1 02), A(11 0) and M(10 0) surfaces, whatever the contact time (from 1 hour to 6 days).

However, a specific behavior of the C(0001) surface is observed. Whereas after one hour, the same parallel Co(OH) 2 is observed, at longer times it disappears while species exhibiting long then short Co-Co distances appear. The latter is assigned to epitaxial grafting of Co II leading to a mixed phase, that is possible parallel to the C surface due to surface symmetry.

KEYWORDS.

GI-EXAFS, density functional theory, cobalt, alpha-alumina, grafting

INTRODUCTION

Cobalt-based catalysts are widely used at the industrial scale for many large-scale energyrelated processes. Among these, hydrotreating [START_REF] Raybaud | Catalysis by Transition Metal Sulphides, From Molecular Theory to Industrial Application[END_REF] and Fischer-Tropsch [START_REF] Khodakov | [END_REF]3] are the main Co consumers in heterogeneous catalysis. Oxygen evolution reactions [4,5] and many conversion reactions of short hydrocarbons also make use of cobalt catalysts [6][7][8]. In all cases, a careful control of the cobalt environment, particle size and dispersion is key in order to derive structure activity relationships needed for a rational development of improved catalytic formulations. As a matter of fact, the effect of cobalt particle size has been largely debated in Fischer-Tropsch (FT) where it was initially believed that FT is a structure insensitive reaction [START_REF] Khodakov | [END_REF]. However, de Jong and coworkers [9,10] later showed that activity and selectivity drop for Co particles smaller than 6-8 nm in model carbon-supported catalysts which was explained by the blocking of undercoordinated sites (edges/corners) as well as a low reactivity of terraces. Deactivation of FT catalysts is also strongly influenced by the Co particle size [11] and distribution [12]. For particles below about 5 nm, Co is reoxidized in reaction conditions which leads to the formation of an inactive surface cobalt aluminate.

A careful control of the cobalt particle size and Co environment can only be achieved through a molecular-scale understanding of the operation parameters during the preparation of the catalyst. In this respect, and in the frame of the present Special Issue, it has to be underlined that the work of Michel Che was pioneer in considering the interfacial chemistry at the solid/liquid interface as the key starting point for a rational design of heterogeneous catalysts [13]. As for Co-based catalysts, he showed that a systematic description of the Co speciation at the oxide/water interface during Co deposition is a crucial prerequisite to understand and control the formation of inactive cobalt aluminate species upon thermal treatment and Co particles upon reduction [14]. Related molecular-scale approaches by other groups were reviewed by Bourikas et al. [15] and the role of the support was also shown to be central for a full description of the Co sorption mechanism: silica for example was recognized to favor the formation of Co(OH) 2 -like precipitates while TiO 2 promotes the formation of surface complexes. Surface precipitates were also identified on -alumina but they were assigned to hydrotalcite-like Co/Al phases formed by alumina dissolution [16,17].

In a practical context, alumina supports are widely used despite the formation of these Co/Al phases and of the presence of poorly reducible species, thus in the present work, we focus our attention on alumina supported cobalt catalysts. For these systems, metal-support interactions where shown to have a direct role on metal distribution and in the end on the catalytic activity, selectivity and stability [18][19][20][21][22]. Understanding the interaction modes of the cobalt precursor with the surface of alumina is thus a prerequisite for the optimization of the performances.

Model approaches were also developed in order to better describe the role of the support on Co sorption, namely for alumina, using oriented single crystals of -alumina with the predominant C(0001) and R(1 02) orientations [23,24]. In both cases, EXAFS characterization revealed the formation of isolated surface complexes while no sign of surface precipitation was observed. Another type of model approach was provided by density functional theory (DFT) works, that appears to be a powerful approach to better define the coordination modes and strength of metallic compounds on inorganic surfaces, in particular in the case of cobalt supported on alumina [25][26][27]. Larmier et al. [25] studied the genesis of a surface Co(OH) 2 phase on two different surfaces of -alumina, the predominant (110) and (100) facets. The incipient formation of Co(OH) 2 is shown to be directed by a strong epitaxial relationship between the surface phase and the alumina facets.

This work aims at broadening the latter approaches on the support effect at the experimental and theoretical level by considering the adsorption mechanism of Co 2+ on four different -alumina surface orientations : A(11 0), C(0001), M(10 0) and R(1 02) that have been used before as surrogates for mimicking the reactivity of -alumina facets, the archetypal catalyst support but that is not available as single crystal wafers [28,29]. These previous works showed a support-dependent adsorption for Mo [28,29] and Ni [30] which needs to be evaluated for Co in order to assess a potential surface-dependent adsorption mechanism on alumina nanoparticles. Grazing-incidence EXAFS at the Co K-edge is used to probe the local structure of adsorbed Co at the dried state and these results are discussed in the light of periodic DFT calculations performed for each corresponding surface orientation. A surfaceorientation dependent behavior is observed, that opens perspectives in the control of the nature of the surface deposits as a function of the surface symmetry and hydroxyl group nature.

EXPERIMENTAL SECTION AND METHODS

Sample preparation

Electrolyte preparation

All electrolyte solutions were prepared using analysis grade chemicals and ultra-pure water (MilliQ®, Merck Millipore, 18.2 MΩ cm). In order to minimize intrusion of carbon dioxide, all electrolytes were deaerated with nitrogen for at least 1 hour before utilization.

Alumina sample preparations

Commercially available α-alumina single crystals (SurfaceNet, Germany) with four different orientations: A(11 0), C(0001), M(10 0) and R(1 02) with dimensions of Ø 50 × 0.5 mm (misorientation < 0.1°) were used for all measurements. Before utilization they were chemically cleaned and calcined. The chemical cleaning was conducted by immersing them subsequently into MilliQ water, diluted HNO 3 (pH = 2.0), MilliQ water; diluted NH 3 (pH = 9.0), and MilliQ water, each for 30 minutes, respectively.

The subsequent calcination lasted 14 hours at a temperature of 700 °C in air in a muffle furnace.

Cobalt adsorption

Cobalt adsorption was performed by immersing the alumina single crystals into a Co(NO 3 ) 2 solution (c = 1•10 -3 mol•L -1 ) with a pH = 8.5 (adjusted with a 20•10 -3 mol•L -1 KOH solution). The choice of the base has little influence since no adsorbed potassium was found with XPS for all samples studied. After an adsorption time of 1 hour, 1 day, or 6 days, respectively, the samples were removed from the solution and excessive electrolyte was removed in a dry nitrogen stream at room temperature. The washed samples were immersed subsequently into MilliQ water for about 10 minutes and dried under nitrogen.

Quantification of adsorbed cobalt was conducted with XPS using the Co 2p signal (Figure S1), following the method detailed before [31]. Results are shown in Figure 1 It has to be noted that in our experimental conditions ([Co 2+ ] = 1•10 -3 mol•L -1 , initial pH = 8.5), no homogeneous precipitation of Co(OH) 2 was visually detected in the impregnation solution. Nevertheless, thermodynamic calculations reported by Towle et al [32] show that the solution is supersaturated with respect to Co(OH) 2 precipitation in our initial conditions but undersaturated at the final pH (about 7) of long-term experiments (6/7 days). No measurable Co sorption was observed for a final pH below 6.5.

XAS measurements

Grazing-Incidence XAS measurements (XANES and EXAFS) at the Co K-edge were conducted on the SAMBA beamline [START_REF] Briois | UVX 2010 -10e Colloque sur les Sources Cohérentes et Incohérentes UV[END_REF] of the Soleil synchrotron facility. The beamline was equipped with a Si220 sagittal bending monochromator and a pair of Pd coated mirrors set at an incidence of 6 mrad. Mirrors were used to vertically collimate X-ray beam (1 st mirror) and vertically focus radiation closer to sample (2 nd mirror placed after monochromator) and for harmonic rejection. Nitrogen/Helium filled ionization chambers were used for monitoring Xray flux, while X-ray fluorescence emitted by the sample was measured with a solid state energy resolved HPGe multi pixel detector (monolithic 36 pixels Canberra) coupled to XIA DxMap digital signal processors.

Samples have been loaded on a triple axis goniometer to align their surface parallel to the incident radiation electric field or perpendicular, while keeping the ability to rotate them and aligning at close to grazing incidence. Measurements have been optimized in total reflection at an incidence angle of 0.25°. Data have been measured in fluorescence mode.

The setup was specially designed for the experiment for short adsorption times (1 hour) that were conducted in-situ (i.e. on the beamline) : up to two plastic Beakers containing different solutions (cobalt solutions or water) were put on a translator. The sample could be automatically immersed and removed from any solution at a speed of 0.1mm/s ensuring that any drops were left on the surface. Thus measurements could be remotely sequenced at each preparation step (adsorption and washing) and at different incidence angle between two extreme light polarization orientations (parallel and perpendicular to the surface). The fluorescence detector was able to vertically translate to avoid shadowing from the sample when in parallel orientation (sample nearly parallel to ground) and avoid too grazing emergence position. For each polarization multiple spectra were acquired and averaged.

Incidence angles were optimized for maximum intensity and adjusted to avoid substrate Bragg reflections that considerably complicate data acquisition.

EXAFS data were processed using the Demeter software package with FEFF6 theoretical standards [34].

DFT calculations

Methods

Periodic density functional theory (DFT) calculations were performed using a planewave method as implemented in the Vienna Ab initio Simulation Package [35,36]. The exchange-correlation functional was treated within the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof [37] (PBE). The D2 correction of Grimme [38,39] was applied to account for long-range dispersion interactions. The electronion interaction was described by the projector augmented wave (PAW) scheme [40] with an energy cutoff of 400 eV. Spin-polarized calculations were performed (cobalt (II): d 7 electronic configuration) and used the interpolation formula of Vosko, Wilk and Nusair [41]. Following the analysis of experimental UV-visible spectra [42] (according to Tanabe-Sugano diagrams), cobalt atoms were considered in their high-spin state (quartet). With several complexes in the cell, the spin coupling was taken as ferromagnetic. It was indeed shown that in β-Co(OH) 2 , a ferromagnetic coupling exists in the layers, while it is antiferromagnetic between those layers [43]. Gaussian smearing with σ = 0.05 eV was used. The criterion for the convergence of the self-consistent cycles was set to 10 -6 eV. Geometry optimizations were run until forces on relaxed atoms were lower than 2.10 -2 eV.Å -1 . All calculations were performed at the -point.

Surface models

The four surface orientations considered experimentally were investigated computationally (Figures 2 andS2). Multiple cells were considered each time to allow the simulation of low cobalt coverage. So as to mimic a strongly hydrated case, surface Al atoms were saturated by hydroxylation. For the R surface, the termination proposed by Trainor et al. [44], simulated by Tougerti et al. [45] with  OH = 23.9 nm -2 , was used as a starting point, knowing that the surface symmetry would remain the same if one would consider other kinds of terminations (such as the one called C2 in ref. [45]).

The M ( OH = 19.4 nm -2 ) and A ( OH = 16.8 nm -2 ) models were taken from ref. [31] but the lateral dimensions of the cells were adapted (M: 9.52x12.99 Ų, A: 8.24x12.99 Ų) so as to allow a gradual investigation of the coverage with cobalt. The initial OH coverages are 19.4 (μ 1 , μ 2 , μ 3 -OH, adsorbed water molecules) and 16.8 (μ 1 , μ 2 , μ 3 -OH) OH.nm -2 for the M and A surfaces respectively. Two types of hydroxylated 2x2 surface models were considered for the C orientation [46,47]. The first one exhibits μ 2 -OH only (24.3 OH.nm -2 ), the second combines μ 1 -OH and μ 2 -OH (16.2 OH.nm -2 ). This choice is motivated by the conditions that we wish to simulate (after drying at room temperature at least).

Interaction with Co II

Standard Gibbs free energy of adsorption Δ r G°a ds given by cobalt atom, corresponding to the chemical reaction depicted by equation 1, was quantified at 298 K by the method proposed in ref. [25] .

Alumina surf + m Co(OH) 2 (H 2 O) 2 = Alumina-m-Co surf + n H 2 O
Eq. ( 1)

The reference cobalt complex was Co(OH) 2 (H 2 O) 2 . Indeed, in the framework of the periodic boundary conditions calculations undertaken here, energetics cannot be accurately determined for charged systems. Moreover, within this constraint, the optimization of an isolated octahedral complex (Co(OH) 2 (H 2 O) 4 ) led to the decoordination of two water molecules from Co. For the evaluation of the Gibbs free energy of adsorption of the cobalt complex, we assume that the temperature dependence of the internal energies is due to the desorbed water molecules only and that the entropy difference of the surface with or without an adsorbed complex is zero, so that entropic terms are hold by water only. Results of the fit are given in Table S1. The close similarity of the Fourier-Transform in parallel polarization with that of Co(OH) 2 (a lamellar brucite-type structure) clearly indicates surface precipitation of the latter on the R(1 02) plane. Notably, as mentioned in section 2.1, no homogeneous precipitation is expected in the present experimental conditions, so that the observed Co(OH) 2 phase is likely connected to the surface at some point. The Co-O (2.08 Å)

RESULTS AND DISCUSSION

Co

and Co-Co (3.17 Å) distances are fully consistent with that for Co(OH) 2 (2.10 Å and 3.17 Å [32]). Such precipitation (Co 2+ + 2 OH -= Co(OH) 2 ) is also in line with the large drift of pH from the initial value of 8.5 to a final value close to 7 for long adsorption times (Figure 1).

Polarization-dependent spectra can yield another key information on a potential surfacedependent precipitation of the supported Co phase by comparing the spectra when the electric field vector of the synchrotron beam is set parallel or perpendicular to the surface of the single crystal wafer [30]. In the present case, the near absence of a Co-Co contribution in perpendicular polarization strongly suggests that the surface precipitation of Co(OH) 2 occurs mostly parallel to the surface, i.e the basal (001) plane of the brucite structure is aligned with the surface. The same phenomenon was previously described for the surface precipitation of Ni hydroxide on the same R surface [30]. The number of oxygen neighbors is also decreasing in the perpendicular polarization which can be explained by the complex dependence of the effective coordination number, determined in polarization-dependent measurements, with , the angle between the Co-X bond (directed along ) and the electric field vector [23] (Eq. ( 2))

Eq. ( 2)

For a perpendicular orientation,  tends toward 90° which leads to a decreased number of neighbors, both for Co and Oxygen. This motivates the simulation of the increase of Co coverage and of the condensation of cobalt (II) grafted complexes that shall result in i) the removing of such non-bridging water molecules, ii) the appearance of Co-Co proximity, as observed experimentally. Increasing the simulated Co coverage, several coordination modes were investigated at θ Co = 1.99 nm -2 : (i) the increase of the coverage in dispersed complexes (Figure 5-a), (ii) the epitaxial growth by grafting the second cobalt complex on surface sites, and with two shared hydroxyl ligands with the first Co complex (Figure 5-b), (iii) apical coordination of the second complex, leading to an out-of-surface growth mode (Figure 5-c). The Δ r G°a ds values show that the strength of the interaction is slightly depleted when increasing the coverage, and that all modes are competitive. The out-of-surface mode is slightly less stable than the two other ones, suggesting a preferred growth onto the surface. For the epitaxial growth mode (Figure 5-b), the Co-Co bond length in the dimer is of 3.02 Å, in reasonable agreement with experimental data (3.12-3.17 Å), together with an average Co-O bond length of 2.12 Å with a CN of 6. This bond length average remains slightly higher than the experimental one (2.04-

Å).

The coverage was increased more, confirming the trends observed at θ Co = 1.99 nm -2 .

At θ Co = 5.97 nm -2 (the highest surface coverage simulated on this surface, that already exceeds the coverage measured experimentally, see Figure 1), the most stable configuration obtained is depicted in Figure 5-d, and corresponds to the extension of the cobalt hydroxidelike chain, the third complexes of each chains (two chains per unit cell) being connected to the surface only thanks to the previous cobalt complex, plus an additional Co-O bond with a μ 1 -OH underneath. On this surface coordination structure, two thirds of the cobalt atoms are octahedral or nearly octahedral (one longer Co-ligand bond), the last third being four-fold coordinated due to the desorption of the two excess water molecules that had been put on it before geometry optimization. As they were ejected in the course of the optimization, a second structure minimization was performed by removing these from the system. A structure analysis leads to the average feature reported in Table S2. At such high coverage, the average Co-O length slightly decreases (2.10 Å) with respect to simulations performed at low coverage. This value is in good agreement with experimental observations. This confirms that water molecules are in minority in the coordination sphere of cobalt, hydroxyls being the dominant ligands. Notably, the calculations also suggest the presence of Co-Al proximity This is likely due to the mismatch between the Co-O and Al-O bond lengths, that leads to short Co-O bonds when epitaxial growth takes place. Such a feature is not appearing experimentally, suggesting that the next growth steps will take place without further anchoring of the cobalt complexes on the surface. Thus, cobalt hydroxide layers may be grafted only by a few anchoring points on the R surface, so that in average, the specificity of the cobalt environment at the grafting point is not detected.

To go further, an analogy between the growth scheme and the structure of Co(OH) 2 layers can be drawn, similar as what was inferred for γ-Al 2 O 3 [25]. Analyzing the symmetry of the surface deposit obtained at θ Co = 5.97 nm -2 , and assuming that the growth of the cobalt hydroxide layer will follow the growth orientation dictated by the surface deposit, one obtains the layer orientation depicted in Figure S3. The angle between the layer grown from the deposit and the alumina surface is expected to be 60°, which should result in a higher perpendicular versus parallel contribution in GI-EXAFS. The opposite trend is observed experimentally, from which it can be concluded that the main growth mode does not follow the orientation dictated by the most stable surface deposit, and that likely a curvature of cobalt hydroxide layer shall take place, probably promoted by physisorption onto the R alumina surface due to van der Waals and electrostatic interactions.

Comparison of the R, A and M surfaces

Comparison of EXAFS results obtained on the R(1 02) plane with those on the A(11 0) and M(10 0) planes is shown on Figure 6 for long adsorption times (6 days). Fitting results are shown in Table S3. From qualitatively (EXAFS FTs) and quantitatively (fitting results) considerations, it can be safely concluded that the Co deposition mode on these three planes is very similar and leads to an oriented surface precipitation of Co(OH) 2 parallel to the surface demonstrated by the large polarization dependence of the peak assigned to Co-Co 2 nd neighbors at about 2.7 Å on Figure 6 (uncorrected for phase-shift) which correspond to a distance of 3.14-3.17 Å. In a similar spirit as done for the R orientation, DFT calculations were performed to quantify the interaction strength and the geometry of possible surface deposits with the A and M surfaces, first for low Co coverage (Figure 4-b-c). Δ r G°a ds values are highly similar with what was observed on the R surface, as well as the coordination chemistry of the mononuclear grafted surface complexes. At higher coverage, the surface-driven growth according to our calculations mode follows a rather messy process on surface A, with many competitive grafting geometries, and Δ r G°a ds values between -220 and -230 kJ.mol Co -1 at θ Co = 5.60 nm -2 .

Regarding the M orientation, the investigation of a loading increase depicts the ability of this surface to saturate with cobalt, with energetic features that are well maintained whatever the coverage (Δ r G°a ds = -234 kJ.mol Co -1 at θ Co = 16.18 nm -2 ).

A structure analysis is proposed in Table S2 for the highest coverage modeled. In the coverage range investigated for the A and M surfaces, short (2.53-2.57 Å) and long (3.16-3.26, but also 3.67 Å) Co-Co distances co-exist. This disagreement with experiments is also reflected by the predicted orientations of the deposit-oriented growth mode, predicted at 25° and 30° for the A and M surfaces respectively, Figure S3, while experiments show that a parallel growth dominates. However, one notes that the perpendicular contribution of the M surface is graphically slightly higher than the one of the two other surfaces (despite a lower angle with respect to the R surface), which might be in link with the very good ability of this surface to reach high cobalt coverage with constant thermodynamic driving-force, as shown by DFT calculations.

Overall, the surface adsorption of Co(II) on the R (1 02), A(11 0) and M(10 0) planes appears to follow a common mechanism characterized by a surface-driven growth of Co(OH) 2 oriented with respect to the surface (with an angle ranging from 25 to 60° depending on the surface) followed by a curvature of the hydroxide layer promoted by physisorption onto the alumina surface with van der Waals and electrostatic interactions.

Specific behavior of the C surface

EXAFS results obtained for Co(II) adsorption on the C(0001) plane leads to different results (Figure 7, and Table S4 for fitting results of the parallel polarization) as compared to the surfaces investigated previously. At short adsorption times (1 hour), fast surface precipitation of Co(OH) 2 is observed (Figure 7-(a)), rather similarly as what was observed for the three previous surface orientations. The washing step does not appear to modify the nature of the surface precipitate except for a small decrease of the number of Co-Co 2 nd neighbors (from 6 to 4). For such short adsorption times, the surface precipitation of Co(OH) 2 is also oriented since the perpendicular polarization do not show any contribution from Co-Co 2 nd neighbors. These results differ from that obtained in a previous work [30] where the C(0001) plane appeared to be totally inert with respect to Ni(II) adsorption/precipitation. Such difference may be explained by a different adsorption pH (i.e. 7.0 for the Ni(II) case and 8.5

for the present case). A more basic pH will favor surface precipitation. (parallel and perpendicular) suggesting a large structural evolution with time from 1 h to 1 day. The oxygen coordination number is rather high for 1 day in parallel polarization (7.2) which can also be explained by the 3cos 2  dependence of the effective (measured) coordination numbers mentioned above. Hence, for a single neighbor, the coordination number can theoretically vary from 0 ( = 90°) to 3 ( = 0°). Hence, coordination numbers have to be taken here as qualitative and comparative guides among samples.

The most striking result on the C surface, with respect to other surface orientations, is the variation of the Co-Co distance as a function of time, accompanying the drop in second neighbor numbers. Whereas after 1 hour Co-Co is typical of cobalt hydroxide (3.17 Å), after 1 day it becomes much higher (3.89 Å), but it drops after 6 days (2.90 Å). This suggests the following mechanistic sequence:

-First, cobalt hydroxide layers are formed as precipitates parallel to the surface, -Then, they dissolve and give rise to a surface deposit in a dispersed state, where the distance between cobalt species becomes larger -Finally, a constrained deposit is formed, with a short Co-Co distance.

DFT calculations may give insight on the nature of such a constrained surface deposit obtained after long adsorption times. From a symmetry point of view, a significant difference of the C orientation with respect to all other investigated surfaces can already be seen from Figure 2. The expected epitaxial growth corresponds to an hexagonal symmetry, similar to the one of the cobalt hydroxide layers. Thus, the surface deposit-driven growth mode is expected to be parallel to the surface, but with possible direct coordination of cobalt (II) ions onto the surface, which is a major difference with respect to other surfaces. To quantify the feasibility of such an epitaxial cobalt hydroxide layer onto the C alumina surface, we simulated by periodic DFT the interaction of Co(II) precursors with the C-μ 1 -μ 2 and C-μ 2 at low coverage (θ Co = 2.00 nm -2 ). The behavior of these terminations appears to be very different: whereas the C-μ 2 termination exhibits poor affinity for the cobalt precursor (-155 kJ.mol Co -1 , Figure 4-e), the C-μ 1 -μ 2 interacts much more strongly with cobalt than any other investigated surface (-340 kJ.mol Co -1 , Figure 4-f). One specificity of the most favorable adsorption mode is to promote a dense network of hydrogen bonds between the three water molecules coordinated to cobalt, and the μ 1 -OH groups of alumina. The grafted cobalt atom occupies a site that is at the same height with respect to the aluminum atom holding the μ 1 -OH, although the geometry optimization slightly displaces it above the surface, due to longer Co-O bonds with respect to Al-O bonds. Conversely, on the C-μ 2 termination, cobalt lies on the row above all surface Al atom, weakening the interactions with the surface sites.

Thus, higher loadings were mainly computed for the C-μ 1 -μ 2 termination. Figure 8 reports some of the optimized structures, for θ Co = 8.11 nm -2 (4 Co per unit cell) and θ Co = 14.19 nm -2 (7 Co per unit cell). The former corresponds to the saturation of the epitaxial sites (see Figure 2-d), the later corresponds to the saturation of all possible grafting site, although a 8 th cobalt atom should have been hosted to fill all holes, but the Δ r G°a ds values obtained with 8

Co per unit cell are prohibitively positive (not shown), even upon variation of the hydration level. This shows that a full cobalt-hydroxide like layer, where Al are incorporated, is submitted to a strong strain at the alumina surface. This can be assigned to the mismatch between the Co-O and Al-O bond lengths. At θ Co = 8.11 nm -2 , where all the epitaxial sites are occupied, Δ r G°a ds equals -253 kJ.mol Co -1 , which reveals a much stronger interaction than any other surface in this range of cobalt coverage. This value is similar to the one obtained for R,

A and M at the lowest investigated coverage. At saturation (θ Co = 14.19 nm -2 ), the Δ r G°a ds value (-221 kJ.mol Co -1 ) is still competitive with the apical growth depicted in Figure 5-c. We can thus conclude that the filling of all epitaxial site is a favored process, with a clear driving force, and reaching saturation is feasible, which was not the case for other surface orientations (R, A and M). The structural analysis for the saturated surface model is proposed in Table S2.

A

CONCLUSIONS

In the present study, we investigate the interaction of cobalt (II) with alumina surfaces, which understanding is crucial for the optimization of a large set of industrially used heterogeneous catalysts. α-alumina is chosen as a model system, opening the possibility to experimentally investigate independently the behavior of various surface orientations, contrary to poorly crystalline supports such as -alumina.

A surface-dependent interaction is demonstrated by grazing-incidence EXAFS at the Co K-edge, complemented by periodic DFT calculations. R, A and surface orientations follow a similar behavior: the formation of cobalt hydroxide is observed, that grow parallel to the alumina surface. We propose that the Co(OH) 2 layers are grafted onto the surface by only a few anchoring points, that do not dictate the layer orientation with the surface at long distances (Figure 9). The C surface behaves very differently. Whereas at short contact times (one hour) the same Co(OH) 2 precipitate is formed, it then disappears (one day and more) and gives rises to a constrained surface deposit with short Co-Co distances (2.90 Å). The specific behavior of the C surface can be assigned to the surface symmetry of this surface plane, prone to host an epitaxial plane of Co(OH) 2 , interrupted by surface Al atoms that are not detected in EXAFS probably due to a large local surface disorder. The two kinetic regimes observed experimentally can be assigned as follows (Figure 9):

-First, at low contact time, the quicker process is the formation of cobalt hydroxide layers, that grow only with a few covalent anchorage points with the alumina support, but are likely in van der Waals plus electrostatic interaction with the surface, which explains a preferred parallel orientation detected in GI-EXAFS. This feature is common to all surface interactions. We propose the existence of grafting points although they are not experimentally detected, both on the basis of DFT calculations, and considering that in the present conditions, homogeneous precipitation of Co(OH) 2 is not expected to take place. These grafting points are, however, not as numerous as what our DFT calculations would suggest, because we did not simulate explicitly the stabilization (likely very high) of a cobalt hydroxide layer grown parallel to the surface.

-At longer contact time, dissolution of the hydroxide surface layer is observed on the C surface followed by coordination of cobalt, with a gradual cobalt coverage increase, leading to a decrease of Co-Co distances. The fact that such a coordination takes longer than the independent growth of cobalt hydroxide may be assigned to the higher entropy loss of the cobalt complex upon immobilization at the surface, likely to increase the preexponential factor of the rate constant. Another explanation can be that the C surface needs to be of the C-μ 1 -μ 2 type to be at the origin of a strong thermodynamic driving force for the coordination of cobalt. Should the C-μ 2 termination be more abundant, a reconstruction (possibly induced by cobalt itself) into the C-μ 1 -μ 2 type will be necessary. This is likely at the origin of kinetic limitations. The second step is not observed on the other surface orientations. DFT suggests that the grafting thermodynamic driving force is indeed lower, but we cannot exclude that kinetic limitations also contribute to the absence of evolution to the grafted state. In terms of possible consequences on catalytic performances, the C surface is thus the most likely to give rise to poorly reducible species due to an epitaxial relationship between the surface and the constrained Co deposit. Hardly reducible species are to be avoided for applications in hydrotreating and Fischer-Tropsch.

Finally, an analogy has made between the C surface of α-alumina [28] and the (111) surface of γ-Al 2 O 3 [48,49]. Should this analogy be followed in the case of interaction of alumina with cobalt (II) precursors, a specific interaction should be observed with the γ-Al 2 O 3 (111) surface, that was not considered in previous theoretical works [25]. This could be a motivation for further experimental and computational investigations, as well as a lever to monitor catalytic performances from the tuning of the alumina support morphology.

  as a function of final pH for three different adsorption times (1 hour, 1 day and 7 days). Results for 6 days are very close to 7 days. Co quantification yields results ranging from 1.4 to 4.5 Co atoms.nm -2 which would yield weight loadings of 2.7 to 8.1 wt% Co on a conventional -Al 2 O 3 of 200 m 2 .g -1 .

Figure 1 .

 1 Figure 1. Cobalt adsorption on the four crystal orientations (A(11 0), C(0001), M(10 0) and R(1 02)) as a function of final pH (starting pH is 8.5). Three sets of data are shown corresponding to different adsorption times: 1 hour (green), 1 day (red) and 7 days (black).

Figure 2 .

 2 Figure 2. Side (top) and top (bottom) views of surface models considered in the present work, with depiction of the missing cationic sites at the surface, expected to be preferred sites for epitaxial growth. Non-equivalent sites are depicted by different colours. (a) R surface, (b) A surface, (c) M surface, (d) and (e) C surfaces. For side views, only the outermost layers are depicted for the sake of clarity. Full sides views can be found in Figure S2.

  deposition on the four different α-alumina orientations, A(11 0), C(0001), M(10 0) and R(1 02), was followed with XAS as a function of the deposition time (1 hour, 1 day or 6 days). Short time experiments (1 hour) were conducted "in situ" as explained in the experimental section, i.e. the immersion of the wafer in the Co solution (and a following washing step in some cases) were automatically realized on the beamline with a motorized arm in order to avoid any delay between the experiment and the characterization. Long time experiments (1 or 6 days) were realized right before the XAS experiments. Results obtained on the R(1 02) plane will be detailed first. Then, DFT results will be discussed in line with these results, analyzing the relevance of the description of the observed phenomena in terms of grafting. The finding are then generalized for the other investigated surfaces, with a specific behavior of the C surface. 3.1. Behavior of the R surface EXAFS Fourier Transform (FT) (Figure 3) of Co(II)/-Al 2 O 3 on the R plane show two peaks after 6 days of Co adsorption that are assigned to Co-O and Co-Co neighbors.

Figure 3 .

 3 Figure 3. Fourier transforms (Δk = 3-8.5 Å -1 ) uncorrected for phase shift of k 3 -weighted Co K-edge EXAFS spectra for Co(II)/-Al 2 O 3 on the R (1102) plane. (a) Adsorption time of 6 days in parallel and perpendicular polarizations together with Co(OH) 2 as a reference. (b), (c) Different adsorption times (6 days, 1 hour) with or without a washing step in (b) parallel and (c) perpendicular polarizations.

  Changing the deposition time (1 h vs. 6 days) and including a washing step do not change this conclusion since FTs shown on Figure3-(b) (parallel polarization) and 3-(c) (perpendicular polarization) also demonstrate a surface-oriented precipitation of Co(OH) 2 . One can note a slight increase of the number of Co 2 nd neighbors with time in the parallel polarization (Figure 3-(b) and TableS1) suggesting a slight growth of the surface Co(OH) 2 phase from 1h to 6 days.The interaction of cobalt(II) precursors on the R(1 02) plane was modeled by DFT according to the method reported in section 2.3., first simulating the grafting of one cobalt per unit cell (surface R, θ Co = 1.0 nm -2 ). The epitaxial positions depicted in Figure2-a appeared to be favored, meaning that cobalt tends to occupy surface sites than should have been occupied by Al atoms in the bulk. The most favorable one (based on standard Gibbs free energy of adsorption Δ r G°a ds calculations) corresponds to the occupation of one of the sites depicted in green in Figure2-a. Its structure is depicted in Figure4-a. A Gibbs free energy of adsorption Δ r G°a ds of -259 kJ.mol Co -1 is calculated, in rather good agreement with the -234 kJ.mol Co -1 calculated on the (110) surface of γ-Al 2 O 3[25] , in spite of the slight variation of the level of theory (dispersion corrections). The Co II surface complex is octahedral, and its coordination sphere is composed of one framework μ 2 oxygen (becoming μ 3 upon interaction with cobalt, d Co-O = 1.95 Å), of two surface μ 1 OH groups (becoming μ 2 -OH, average d Co-O = 2.12 Å) and three water molecules (average d Co-O = 2.22 Å). The total Co-O average bond length is d Co-O = 2.14 Å, slightly higher than the experimental observations in EXAFS. As the longer Co-O contributions come from water coordinated molecules, this suggests that they are not dominant experimentally.

Figure 4 .

 4 Figure 4. Perspective view of the coordination complexes obtained upon grafting of a single cobalt complex per unit cell for all surfaces modeled, together with the standard Gibbs adsorption free energy.

( 2 .

 2 85 Å) but at low CN (1.3) with respect to oxygen (5.3), likely explaining why they cannot be invoked from the experimental fit. A corollary of the coverage increase on the R surface, according to the grafting modes found by DFT, is a dramatic decrease of the Co-Co average bond length down to 2.52 Å.

Figure 5 .

 5 Figure 5. Surface complexes on the R orientation, and corresponding standard Gibbs adsorption free energy average by cobalt unit, according to various configurations: (a) two dispersed complexes, (b) epitaxil growth, (c) apical growth, for θ Co = 1.99 nm -2 (two cobalt per unit cell). (d) Side (left) and top (right) views of the surface complexes on the R orientation for θ Co = 5.97 nm -2 (six cobalt per unit cell).

Figure 6 .

 6 Figure 6. Fourier transforms (Δk = 3-8.5 Å -1 ) uncorrected for phase shift of k 3 -weighted Co K-edge EXAFS spectra for Co(II)/-Al 2 O 3 on the R (1102), A(11 0) and M(10 0) planes for long adsorption times (6 days). (a) Parallel polarization, (b) perpendicular polarization.

Figure 7 .

 7 Figure 7. Fourier transforms (Δk = 3-8.5 Å -1 ) uncorrected for phase shift of k 3 -weighted Co K-edge EXAFS spectra for Co(II)/-Al 2 O 3 on the C(0001) plane for different adsorption times (1 hours vs. 6 days) with or without a washing step. (a) Parallel polarization, (b) perpendicular polarization.

Figure 8 .

 8 Figure 8. Top (top) and side (bottom) views of the surface structures optimized by DFT for the C-μ 1μ 2 termination at (a) θ Co = 8.11 nm -2 (b) θ Co = 14.19 nm -2 .

θ 1 θ

 1 Co = 8.11 nm -2 Δ r G ads = -221 kJ.mol Co -Co = 14.19 nm -2 Al O (framework) H O (from surface μ 1 -OH) Co O (Co complex)

Figure 9 .

 9 Figure 9. Scheme depicting the possible growth mode of cobalt deposits on the C surface from a cobalt hydroxide layer present on surfaces R, A et M whatever the adsorption time and on the C plane at short adsorption times only. After 1 day on the C Plane, a surface deposit in a dispersed state is formed

  short Co-Co distance is recorded (2.45 Å), although shorter than the experimentally measured one (2.90 Å) after 6 days of adsorption. It co-exists with longer distances in the model, suggesting that our proposed model is probably not exactly representing the true configuration. However, the energetic specificity of the C surface quantified by DFT, together with the observation of short Co-Co distances when epitaxial growth is modeled, makes us confident in the conclusion according to which the C surface is the single -alumina surface orientation (among those considered in the present work) able to lead to a constrained surface deposit stabilized by epitaxial growth at long contact times.

Acknowledgements. CC thanks Dominique Costa (Chimie Paris Tech) and Kim Larmier (IFP Energies nouvelles) for discussion about the alumina surface models. The DFT calculations were performed using HPC resources from GENCI (Grant A0020806134) and the ENER440 cluster at IFP Energies nouvelles. We also acknowledge SOLEIL for provision of synchrotron radiation facilities (proposal 20160216) and we would like to thank Guillaume Alizon for assistance in using beamline SAMBA. This work was supported by the French National Research Agency within the framework of the ANR-14-CE08-0019 SLIMCAT project.